biomaterials for orthopedics

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May 2004 Applications of Engineering Mechanics in Medicine, GED University of Puerto Rico, Mayaguez 1

Learn from yesterday, live for today, dream for tomorrow - - - Chicken Soup for the Soul

BIOMATERIALS FOR ORTHOPEDICS1

Brendamari Rodrguez, Annette Romero, Omar Soto and Oswaldo de Varorna 2

Abstract- Biomaterials deal with the materialaspects of the medical devices. Biomaterials

scientist are concerned with the physical and

chemical properties of materials and their

suitability for a particular device. They are

concerned how these properties are altered by the

biological environment and how the materials may

affect the body. Here we shall discuss selected

biomaterials for orthopaedics.

Keywords- Biomaterials, orthopedics, joints,

artificial joints, 316L stainless steel, Titanium,

Cobalt-Chrome, Zirconium, ceramics, calcium

phosphate, calcium sulfate, materials.

BACKGROUND

Biomaterials improve the quality of life for an everincreasing number of people each year. The range ofapplications is vast and includes such things as joint andlimb replacements, artificial arteries and skin, contactlenses, and dentures. This increasing demand arises froman aging population with higher quality of lifeexpectations. The biomaterials community is producingnew and improved implant materials and techniques tomeet this demand, but also to aid the treatment of youngerpatients where the necessary properties are even moredemanding. A counter force to this technological push isthe increasing level of regulation and the threat oflitigation. To meet these conflicting needs it is necessary

to have reliable methods of characterization of thematerial and material/host tissue interactions. The mainproperty required of abiomaterial isthat it does not illicitan adverse reaction when placed into service [4].

BIOMATERIALS CLASSIFICATIONS

Biomedical materials can be divided roughly into threemain types governed by the tissue response. In broadterms, inert (more strictly, nearly inert) materials illicit noor minimal tissue response. Active materials encouragebonding to surrounding tissue with, for example, newbone growth being stimulated. Degradable, or resorbable__________

1This review article was prepared on May 14, 2004 forthe course on Mechanics of Materials I. CourseInstructor: Dr Megh Goyal, Professor in BiomechanicalEngineering, Mayaguez Puerto Rico 00681-5984. Fordetails contact: [email protected] or visit at:http://www.ece.uprm.edu/~m_goyal/home.htm2 The authors are in the alphabetical order.3The numbers in the parentheses refer to references inthe bibliography.

materials are incorporated into the surrounding tissue, ormay even dissolve completely over a period of time.Metals are typically inert, ceramics may be inert, active orresorbable and polymers may be inert or resorbable. Table1 shows examples of biomaterials [4].

Table 1. Types given of biomaterials [4].

Metals Ceramics Polymers

316L stainlesssteel

Co-Cr Alloys

Titanium

Ti6Al4V

Alumina

Zirconia

Carbon

Hydroxyapatite

Ultra highmolecular

weightpolyethylene(UHMWPE)

Polyurethane(PE)

APPLICATIONS

The range of applications for biomaterials is large. Thenumber of different biomaterials is also significant.Applications of biomaterials are discussed below:

1. Orthopaedic Applications

Metallic, ceramic and polymeric biomaterials are used inorthopaedic applications. Metallic materials are normallyused for load bearing members such as pins and platesand femoral stems etc. Ceramics such as alumina andzirconia are used for wear applications in jointreplacements, while hydroxyapatite is used for bonebonding applications to assist implant integration.Polymers such as ultra high molecular weightpolyethylene are used as articulating surfaces againstceramic components in joint replacements.

Porous alumina has also been used as a bone spacer toreplace large sections of bone which have had to beremoved due to disease, [4].

2. Dental Applications

Metallic biomaterials have been used as pins foranchoring tooth implants and as parts of orthodonticdevices. Ceramics have found uses as tooth implantsincluding alumina and dental porcelains. Hydroxyapatitehas been used for coatings on metallic pins and to filllarge bone voids resulting from disease or trauma.Polymers, have are also orthodontic devices such as platesand dentures, [4].


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GENERAL REQUIREMENTS

Orthopaedics, like many specialties, has developedthrough a necessity to correct deformity, restorefunction and alleviate pain. Orthopaedic surgeonshave developed an ability to prevent major losses ofbodily function and indeed they can prevent otherwiseinevitable death. They seek perfection of their art, byensuring that the patient reaches optimal condition inthe shortest period of time by the safest possiblemethod.

History is very important to an orthopaedic surgeon.

The Orthopaedic surgeon has once again beenpresented with advancing technology. This technologymust be applied to the surgeon's practice, but it is bestapplied only when the surgeon has an underlyingknowledge of the history of his art. He must be awareof the way surgeons in the past have contributed toorthopaedics and more importantly, of the mistakesbut they have made in the process. The surgeon whomakes a mistake that was made by someone beforehim, is surely humbled and seen as poorly educated.So is he who states that he has developed a techniquethat no one has thought of before, because chances arethat it has been thought of in the past.

In order for orthopaedics to advance in an optimal

manner, it is clear that attention must be paid to ahistory of orthopaedics. The past is our foundation forfuture developments, we must build upon it so that wetoo can act as a stable foundation for futuregenerations.

Figure 1. Common sites of infection of bones and joints[16] .

Figure1 shows common sites of infection of bones andjoints. It includes pyogenic and tuberculous infection ofjoints, and osteomyelitis of bones, especially of the handsand feet, and of subcutaneous bones such as the tibiae. Incases when improvement cannot be gained throughphysical therapy, nonsurgical treatments, or surgicalrepairs, orthopedic surgeons often advised jointreplacement surgery in which the deteriorated joint isremoved and replaced with a man-made device. Figure 2shows a bone plate to assist in the healing of a fracture inthe bone. The plate is generally removed once the bone hashealed and the bone can support loads without refracturing.Artificial joints consist of a plastic cup made of ultrahighmolecular weight polyethylene (UHMWPE), placed in the

joint socket, and a metal (titanium or cobalt chromiumalloy) or ceramic (aluminum oxide or zirconium oxide) ballaffixed to a metal stem. This type of artificial joint is usedto replace hip, knee, shoulder, wrist, finger, or toe joints torestore function that has been impaired as a result ofarthritis or other degenerative joint diseases or trauma fromsports injuries or other accidents. Joint replacement surgeryis performed on an estimated 300,000 patients per year in

the U.S. In most cases, it brings welcome relief andmobility after years of pain (Figure 3). After about 10years of use, these artificial joints often need to be replacedbecause of wear and fatigue-induced delamination of thepolymeric component. Institute engineers are developingimproved materials to extend the lifetime of orthopedicimplants such as knees and hips


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Interpositional Implant (IPI). In theory, PTFE wouldseem an appropriate choice for an implant material, asit exhibits a low coefficient of friction and has beenused extensively as a bearing surface in otherengineering applications. However, of the more than25,000 PTFE TMJ implants received by patients,failed. The fibrillation and small particles arecharacteristic of an adhesive wear mechanism, which

can result in surrounding bone loss and the need forimplant replacement (Figure 5).

Figure 5.These micrographs, taken at a magnificationof 20,000X on a scanning electron microscope,illustrate the wear problem that occurs with anartificial joint implant component (socket) constructedof UHMWPE. At top is unworn UHMWPE. TheUHMWPE sample has undergone a friction and weartest versus cobalt chromium (artificial joint ballmaterial) [5].

A low coefficient of friction of PTFE is due toformation of a thin film of the material onto theopposing bearing surface. Although this transfer filmacts as a lubricant, it also, by virtue of its formation,subjects the material to an adhesive wear mechanism.In the case of the PTFE TMJ implants, surroundingtissues quickly became overwhelmed by wear debris,

and the immune system response result in osteolysis,causing massive destruction of the joint andsurrounding tissues. For those people who received theimplants, this was truly a tragedy; many sufferedsevere facial deformities, and most experiencedunbearable pain and were no longer able to chew,swallow, or sleep. At the time the IPI was developed,evidence did exist that PTFE was not an appropriateimplant material.

In the late 1950s, Dr. John Charnley, at WrightingtonHospital in the U.K. pioneered the first total hipreplacements using PTFE as the cup bearing surface.Dr. Charnley reported massive wear of the PTFE part

and early clinical failure as a result of asepticloosening. These findings, reported widely in the openliterature and in later reports from researchers testingthe IPI implant, should have been sufficient warningthat PTFE was not an appropriate material to use as aload-bearing surface in the body. Work at SwRI isaddressing the wear problem in UHMWPE total jointprostheses.

In collaboration with scientists at the University ofTexas Health Science Center at San Antonio funded

by the National Science Foundation, SwRI scientists andengineers are studying the wear process and biologicalresponses to wear debris. Results of these studies have ledto novel ideas for materials modification and development.The Institute is also developing new composite materials todefeat the fatigue-induced delamination observed in theUHMWPE component of knee implants. Studies of weardebris extracted from actual tissue samples of patients

whose implants failed as a result of aseptic loosening havegenerated significant information regarding wear particlesize, shape, and surface morphology. Institute scientistswere the first to use the atomic force microscope (AFM) toproduce detailed, high resolution images of wear particles.A few hundred nanometers in size, the UHMWPE weardebris studied at SwRI sometimes exhibits a cauliflower-like surface morphology. Scientists at the Health ScienceCenter will use similar particles to study the biologicalresponse elicited by the particles. By combining weardebris and cellular response studies, engineers andbiologists will be able to better understand implant failureand to re-engineer implants to prevent future problems, [ 5,16 ].

BONE GRAFT SUBSTITUTES

In many cases, the loss of bone due to surgery, accidents ornormal aging requires the substitution of bone in order tofacilitate the rehabilitation of the patient. Figure 6 showstwo cases in which bone substitution is required.

a. Collapsed disc. b. Non-Union

Figure 6. Examples where bone substitution is required[24]

Nowadays, the need for bone substitutes includesautograftings procedures, allograftings procedures orsynthetic bone substitutes. Autografting, which representsabout 58% of the current bone substitutes, involvesharvesting a bone from one location in the patients body,usually taken from the pelvic region, and transplanting itinto another part of the same patient. Using this procedure,when the autogenous grafts are available, typicallyproduces the best clinical results. This procedure hasobvious benefits, like the elimination of immunogenicityproblems. Autografting, however, has several associatedproblems including the additional surgical costs for theharvesting procedure, infection, pain at the harvesting siteand that the sample of the patients own bone that can betaken is very small, among other things. The allograftingprocedure consists in harvesting and processing bone froma live or deceased donor and then transplanting it to thepatient. These implants are acellular and are less successfulthan autografts implants for reasons attributed to


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immuogenicity and the absence of viable cells thatbecome osteoblasts. Another disadvantage ofallografting is relate with transmitted disease. Due tocomplications related to these procedures, bone graftsubstitutes made with synthetics materials arebecoming very important in bone substitutionsprocedures. The ideal bone graft substitute should beosteogenic, biocompatible, bioabsorbable, able to

provide structural support, easy to use clinically, andcost-effective. The bone grafts and their substitutescan be divided according to their properties ofosteoconduction, osteoinduction, and osteogenesis.Table 2 shows classifications of bone graft substitutes.The synthetic material belongs to the osteoconductivecategory. The osteoconductive synthetic grafts thatare used generally falls under the calcium sulphate andcalcium phosphate groups. They can be used as presetand injectable materials. The next sections willpresent a more detailed information about syntheticbone graft, calcium sulphate and calcium phosphatematerials, [6, 7].

Table 2. Classification of Bone Graft Substitutes

Based on Properties [6].

Description Classes

Osteoconduction Provides apassive porousscaffold tosupport of directbone formation

Calciumsulphate,ceramics,calciumphosphatecements,collagen,bioactive,glass, syntheticpolymers

Osteoinduction Induces adifferentiation ofstem cells intoosteogenic cells

Demineralizedbone matrix,bonemorphogenic,proteins,growth factors,gene therapy

Osteogenesis Provides stemcells withosteogenicpotential, whichdirectly laysdown new bone

Bone marrowaspirate

Combined Provides morethan one of theabove

Composites

1. Indications for Bone Substitutes

The main indications for bone substitutes will be inspinal fusion, bone defects, osteoporotic fractures,revision surgery and, recently, vertebroplasty(injecting a vertebra with synthetic material).Vertebroplasty using polymethylmethacrylate was firstintroduced in France more than 15 years ago byneurosurgeons, but its use is now spreading rapidly.

This mini-invasive procedure for the treatment of vertebralfractures in osteoporosis can reinforce fractured bone,alleviate chronic pain and prevent further vertebralcollapse. Vertebroplasty is performed under biplanarfluoroscopic control, CT or guided navigation (Figure 7)[6, 7].

Figure 7. Schematic drawing of a vertebral body, onwhich synthetic bone grafting (vertebroplasty) is performedusing the injection-suction method. Two needles are used,one for injecting the synthetic bone material and the otherfor developing an underpressure in the vertebral body. Thismethod reduces the risk of leakage into vessels or the

nerves in the spinal canal, [7].

Figure 8. Fracture treated with synthetic bone graft andinternal fixation [7].

2. Synthetic Bone Grafts

Synthetic materials can be made from different materialsthat are biologically compatible materials. The synthetics

materials exhibits the property of being osteoconductivesmaterials, which mean being bone-stimulating materials.Some of these materials can be mixed with bone marrowaspirate to obtain osteoinductive properties (bone forming).Some important characteristics of synthetic bone substitutesare:

- Its porosity (determines the amount of surfacearea expose to bone tissue ingrowth). Porosityalone is not adequate for bone ingrowth.


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- Resorption rate (ability to disappear so asto be replaced by new bone).

- Biocompatibility to prevent inflammatoryreactions, minimizing the interferencewith bone induction.

- Biodegradable so that the patients own

bone can replace the foreign substitute.

The osteoconductive synthetic grafts that are usedgenerally are made of calcium sulphate and calciumphosphate materials. Hydroxyapatite synthetic bone ,a derivate of calcium phosphate, is an importantmaterial due to its biocompatibility. Figure 8 showsan example of a fracture treated with injectablesynthetic bone and internal fixation [1, 6].

TITANIUM AND TITANIUM ALLOYS

Although Titanium has excellent heat and corrosionresistance capabilities, it is extremely difficult to formand machine into desired shapes. Also its extremechemical reactivity with air, combined with otherfactors, has caused the cost of titanium components tobe very high. The only economical applications ofthis material currently (until more efficient techniquesof working with it can be found) are in aerospaceapplications where weight and temperature resistanceare very important, and in military applications, theyprovide extreme corrosion resistance and durability.Titanium is also used in biomedical applications suchas prosthetics and implants ( Figure 9) due to itsbiological inertness.

There are several titanium alloys that have been

developed for use in the past four decades. Thesealloys include Ti-6Al-4V (an alloy of titanium,aluminum and vanadium), the most highly used alloyof titanium and Ti-4Al-4Mo-2Sn-0.5Si (an alloy oftitanium, aluminum, molybdenum, tin, and silicon),which was developed later and is used lessfrequently. Table 3 presents some properties of thetitanium alloy Ti-6Al-4V [17].

Figure 9. Example of a titanium biomedicalapplications [17].

Table 3. Properties of Ti-6Al-4V at 25C [22].

Property of the Ti-6Al-4V Values of the Ti-6Al-4V

Density 4430

Poisson's Ratio 0.34

Elastic Modulus

GPa

113.8

Tensile StrengthMPa

993

Yield StrengthMPa

924

Elongation%

14

Reduction in Area%

30

HardnessHRC

36

a. Physiological Behavior

These materials are classified as biologically inertbiomaterials or bioinert. As such, they remain essentiallyunchanged when implanted into human bodies. Thehuman body is able to recognize these materials asforeign, and tries to isolate them by encasing them infibrous tissues. However, they do not illicit any adversereactions and are tolerated well by the human body.Furthermore, they do not induce allergic reactions such ashas been observed on occasion with some stainless steels,which have induced nickel hypersensitivity insurrounding tissues. The surface of titanium is oftenmodified by coating it with hydroxyapatite. Plasmaspraying is the only commercially accepted technique fordepositing such coatings. The hydroxyapatite provides a

bioactive surface (i.e. it actively participates in bonebonding), such that bone cements and other mechanicalfixation devices are often not required [18].

b. Mechanical Suitability

Titanium and its alloys possess suitable mechanicalproperties such as strength, bend strength and fatigueresistance to be used in orthopaedics and dentalapplications. This is why they have been employed inload-bearing biomedical applications instead of materialssuch as hydroxyapatite, which displays bioactivebehavior. Other specific properties that make it adesirable biomaterial are density and elastic modulus. In

terms of density, it has a significantly lower density(Table 4) than other metallic biomaterials, implying thatthe implants will be lighter than similar items fabricatedout of stainless steel or cobalt chrome alloysHaving alower elastic modulus compared to the other metals isdesirable as the metal tends to behave a little bit more likebone itself, which is desirable from a biomechanicalperspective. This implies that the bone hosting thebiomaterial is less likely to atrophy and resorb [18].


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Table 4: Density and elastic modulus of selectedbiomaterials [18].

Material Density Elastic

Modulus

Cortical Bone ~2.0 g.cm

-3

7-30 GPaCobalt-Chrome alloy ~8.5 g.cm-3 230 GPa

316L Stainless Steel 8.0 g.cm-3 200 GPa

CP Titanium 4.51 g.cm-3 110 GPa

Ti6Al4V 4.40 g.cm-3 106 GPa

Figure 10. Implant components for a total hipreplacement (photo courtesy of Dr. Karlis Gross) [18].

c. Applications

Titanium is commonly used in orthopaedic implants suchas joint replacements and bone pins, plates and screws.Figure 10 shows the various components of a total hipreplacement. On the left is the femoral stem made of atitanium alloy. The long round section fits down into thethigh bone or femur. The white section is ahydroxyapatite coating to encourage bone bonding to theimplant. This section is also macrotextured to providesurface features for the bone to mechanically interlockwith. The ball on top of the femoral stem is called thefemoral head. It is made of zirconia ceramic and fits intothe hip joint in the pelvis.

The hemispherical item on the right is the acetabular cup,also made from titanium alloy. It is coated with porousalumina ceramic, to allow bone ingrowth for stabilisation.A ultra high molecular weight polyethylene (UHMWPE)liner fits inside the acetabular cup and provides thearticulating surface for the femoral head.

Figure 11 shows a prototype total knee replacementprosthesis, similar in design to many commercialimplants. It consists of titanium alloy upper and lowerstructural components. A zirconia wear surface hasbeen fabricated for the upper section. Similar to thehip prosthesis, this articulates against a UHMWPEinsert on the lower section. Other orthopaedicapplications for titanium-based materials include bonepins, plates and screws, used for repairing brokenbones etc [18].

Figure 11. A total knee replacement prosthesis (photocourtesy of Dr. Besim Ben-Nissan) [18].

CERAMICS

For many years, ceramic materials were only useful in the

making of pottery and other artwork. They have sinceevolved into one of the most important biomaterials usedtoday because of their beneficial properties. Ceramicmaterials are nonmetallic, inorganic compounds thatexhibit great strength and stiffness, resistance to corrosionand wear, and low density. These characteristics allowceramics to become prime candidates for a wide range ofbiomedical applications. Ceramics are used in severaldifferent fields such as dentistry, orthopaedics, and asmedical sensors. In dentistry, ceramics are commonly usedfor implants such as crowns and dentures. The orthopedicfield utilizes ceramics for joint and bone segmentreplacement and temporary bone repair devices. Ceramicsare also used as coatings for implants made of othermaterials to provide a biocompatible interface with the

body [9].

1. Bone graft substitutes: Calcium phosphate

The first applications of calcium phosphate salts werepowders. The ceramic form first became available in the1960s and was later evaluated as a bone graft substitute.The synthetic hydroxyapatite is one of the most commonlyused calcium phosphate ceramics. Synthetic ceramicsprovide an osteoconductive scaffold to which chemotactic,circulating proteins and cells can migrate and adhere, andwithin which progenitor cells can differentiate intofunctioning osteoblasts. Ceramics do not supply osteogeniccells as found in autograft. They do not have even the weak

osteoinductive potential found with allograft. However,ceramics are readily available and bypass the known risksof allograft-induced immunogenic response or diseaseconveyance, as well as surgical complications fromretrieving bone from an autogenous second site. Thechemistry, architecture, shape, and positioning of theceramic material influence the speed and extent ofremodeling. Its bioresorbability depends on the amount ofsurface area exposed, which is governed, in turn, by crystalsize, the form supplied, and density. A ceramic materialformed as a dense block exposes only a small surface area,thus slowing or confining surfaces accessible for


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Technology, Arlington, Tenn), standard or fast cure (5minutes setting time for fast cure kit compared to 20minutes for the standard kit), thus maximizing thesurgical options for adding antibiotics and fillingdefects with custom molded beads or shapes. Thechief advantage is that it can be used in the presenceof infection. Because it is bioabsorbable, it hasinherent advantages over other antibiotic carriers, such

as polymethylmethacrylate, which become a nidus forfurther infection after elusion of the antibiotics, thusrequiring a separate operation for removal from thesurgical site. When combined with the eradication ofdead space and the acidic environment created duringits resorption, the compound can be an effectivetreatment for acute bony infections with bone loss.

The disadvantages of calcium sulphates are their weakmechanical strength and rapid resorption within 612weeks. For clinical use, injectable osteoconductivegrafts should ideally be biphasic with a compressivestrength >25 Mpa. Their injection time should bebetween 2 and 6 min, with a setting time of less than10 min [3, 6].

3. Disadvantages of Ceramics

A shortcoming noted with ceramics used as stand-alone bone substitutes is the initial low resistance toimpact and fracture. Due to its brittle structure, use ofa ceramic material in conditions of torsional, impact,or shear stress is limited. However, cancellous bonegrafts likewise contribute little immediate structuralsupport prior to union with the host site andremodeling along lines of stress. Anotherdisadvantage found with ceramic implants is thedifficulty of radiographic assessment of the ingrowthinto the defect site until partial resorption has occurred

[1].

COBALT AND COBALT CHROME

Cobalt and Cobalt Chrome

Cobalt

Brandt discovered cobalt around 1735. It occurs in theminerals cobaltite, smaltite and erythrite and is oftenassociated with nickel, silver, lead, copper and ironores, from which it is most frequently obtained as aby-product. It is also present in meteorites. Cobalt is abrittle, hard metal white in appearance resemblingnickel (and iron) but with a bluish tinge instead of theyellow of nickel. It is rarer and more valuable thannickel. It is diamagnetic and has magneticpermeability approximately two thirds that of iron andthree times that of nickel. Cobalt exists as two

allotropes over a wide temperature range. The -forma close-packed hexagonal crystal is stable andpredominates below approximately 417C (782F),

and the -form a cubic crystal is stable andpredominates above this temperature until the meltingpoint. Although allied to nickel, it is more active

chemically than nickel. Cobalt dissolves in dilute sulphuricacid, nitric or hydrochloric acid and is slowly attacked byalkalis. The oxidation rate of pure cobalt is twenty fivetimes that of nickel. Cobalts ability as a whitening agentagainst copper alloys is inferior to that of nickel. However,small amounts in nickel-copper alloys will neutralise theyellowish tinge of the nickel and make them whiter. Cobaltimparts red-hardness to tool steels. It can harden alloys to

greater extent than nickel, especially in the presence ofcarbon and can form more chemical compounds in alloysthan nickel. Natural cobalt is cobalt 59, which is stable andnon-radioactive, but other isotopes 54 to 64 are allradioactive (table 6), emitting beta and gamma radiation.Other isotopes not listed in table 1 have short half-lives.[3]

Table 6. Cobalt isotopes and their half-lives. [3]

Isotope Half Life

Cobalt 60

Cobalt 58

Cobalt 57

Cobalt 56

5.3 years

72 days

270 days

80 days

Aplications:

Cobalt 60 has a number of applications. These include:

Radiographic inspection A gamma ray source A tracer A radiotherapeutic agent Irradiation of plastics A catalyst for the sulphonation of paraffin oil. In this

application the gamma rays emitted by the cobaltcause the reaction of sulphur dioxide and liquidparaffin.

Other uses for cobalt are:

In superalloys for aircraft gas turbine engines It is a key elemental ingredient in magnet steels, by

which it increases residual magnetism and coerciveforce and in nonferrous-base magnetic alloys

Cobalt is an important element in numerous glass-to-metal sealing alloys as well as low expansion alloys

Alloys for dental and surgical applications becausethey are not attacked by physiological fluids. Anexample of which is Vitallium which is used toreplace bone. Such alloys are ductile enough topermit anchoring of dentures on neighbouring teethand contain up to 65% cobalt

High-speed, heavy duty, high temperature cuttingtools, and dies

Gas turbine generators Electroplating.


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Cobalt salts are used as a source of brilliant permanentblue colour in porcelain, glass, pottery, tiles andenamels [22, 23].

Properties

Creep Resistance

One of the main attractions of cobalt-based alloys istheir excellent creep resistance. Materials creep due tothermally-activated movement of dislocations througha crystalline matrix. These alloys possess a matrix thatis resistant to this as cobalt has a good tolerance forother elements in solid solution. These elements caneffectively strengthen the matrix. Their ability to dothis depends on factors such as:

The difference in atomic size between cobaltand the solute

The effect of the solute on the stacking faultenergy

The diffusion rate of the solute into the cobaltmatrix

It has also been found that a matrix containing a largernumber of solutes is often better than one containing afewer number, hence the strengthening of the matrix isalso dependent on the amount of alloying elementsavailable to the go into solid solution, that have notformed carbides, or for that matter intermetallics. Thekey elements for this process are chromium, tungsten,niobium and tantalum. A second strengtheningmechanism also exists and involves the formation ofcarbides and carbonitrides forming with chromium(primarily), tungsten, molybdenum, niobium,tantalum, zirconium, vanadium and titanium. Carbides

formed include MC, M6C, M7C3, M23C6 andsometimes M2C3, with the amount of each dependingon factors such as availability of elements to formcarbides, carbon content and thermal history. It is alsopossible for nitrogen to substitute for carbon in thesestructures. Optimum properties are produced whencarbides precipitate both intergranularly andintragranularly. Intergranular precipitation preventsgross sliding and grain boundary migration and canform a skeleton if present in sufficient quantities,while intragranular precipitation strengthens thematrix by inhibiting the motion of dislocations.Carbide distribution by solidifaction parameters suchas pouring temperature and cooling rate. As cast alloysare rarely heat treated, carbides will generally only

form during prolonged exposure to operatingtemperatures. Wrought materials on the other handmay be hot worked. Further strengthening can beinduced by solution heat treatment between 1175-1230C and rapid cooling [22, 23].

Room Temperature Properties

RoomAs these alloys are generally used at elevatedtemperatures, the room temperature properties are not

relevant to the service conditions. They do however, play arole for manufacturers e.g. tensile strength and ductility caninfluence how much hot or cold working the material canwithstand and hardness influences machinablity. It shouldalso be noted that room temperature properties such aselongation can be effected by the thermal history of thematerial, i.e. amount of carbide precipitation, with moreprecipitation leading to lower ductility. Also increased

exposure to high temperatures increases the hardness ofhigher carbon alloys more so than lower carbon content

alloys.

It has been shown that cobalt-chrome alloys with veneeringcapacity, such as WirobondC, represent an alternative toalloys with a high gold content . As far as corrosion andbiocompatibility are concerned, both groups can bedesignated as equivalent. In mechanical terms (modulus ofelasticity, heat resistance, thermal conductivity), cobaltchrome is superior to alloys with a high gold content. Interms of price, the cobalt chrome alloys again have anadvantage. Both alloys and alloy types display adequatelyhigh shear bond strength. The experimental results show

higher values for the gold alloys, though whether this is ofclinical relevance is still a matter of debate [22, 23].

Properties

Cobalt 61.0% base metal

Casting temperature Higher, therefore cannotbe cast with all casting machines

Investment materials No investment materialscontaining plaster may be usedbecause the required preheating temperaturewould lead to decomposition of the investmentmaterial. The decomposition products resultingfrom this react strongly to the alloy melt flowingin.No investment materials containing graphite may

be used since chromium carbide would otherwisebe formed, leading to extreme hardening (HV 10> 700).

Finishing Increased work requirement andgreater wear of the equipment (grinding stones,milling units, etc.). Consequently the priceadvantage (Wirobond C: approx. DM 0.82/g; BioPontoStar: approx. DM 33.00/g) is reduced, butnot eliminated.

Tensile strength Higher than gold alloys

Modulus of elasticity Approximately double,which means that WirobondC has asignificantly higher load capacity with equalmodelling strength. This is of interest for long-

term stability. 0.2% ductile yield Comparable, measure for thepermanent deformation (important with clasps)

Elongation limit Lower, not of crucialimportance for the finishing capacity for crownand bridge alloys

Hardness Higher, difficult to finish

Coefficient of thermal expansion Comparable,see also veneering capacity

Heat resistance Significantly higher, particularlyin comparison to palladium-free gold alloys, thus


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more secure against distortion duringveneering process

Thermal conductivity Lower than withgold alloys, therefore greater wearingcomfort for the patient [3, 22].

Cobalt vs. Stainless Steel 316L

Stainless Sell 316L

Grade 316 is the standard molybdenum-bearing grade,second in importance to 304 amongst the austeniticstainless steels. The molybdenum gives 316 betteroverall corrosion resistant properties than Grade 304,particularly higher resistance to pitting and crevicecorrosion in chloride environments. It has excellentforming and welding characteristics. It is readily brakeor roll formed into a variety of parts for applications inthe industrial, architectural, and transportation fields.Grade 316 also has outstanding weldingcharacteristics. Post-weld annealing is not required

when welding thin sections.

Grade 316L, the low carbon version of 316 and isimmune from sensitisation (grain boundary carbideprecipitation). Thus it is extensively used in heavygauge welded components (over about 6mm). Grade316H, with its higher carbon content has application atelevated temperatures, as does stabilised grade 316Ti.

Characterised by high corrosion resistance in marineand industrial atmospheres, it exhibits excellentresistance to chloride attack and against complexsuphur compounds employed in the pulp and paperprocessing industries. The addition of 2% to 3% of

molybdenum increases its resistance to pittingcorrosion and improves its creep resistance at elevatedtemperatures. The low carbon content reduces the riskof intergranural corrosion (Due to carbideprecipitation) during welding, reducing the need forpost weld annealing. Finally it displays good oxidationresistance at elevated temperatures.

Stainless steel 316L cannot be hardened by thermaltreatment, but strength and hardness can be increasedsubstantially by cold working, with susequentreduction in ductility.

It is now available with improved machinability (by

calcium injection treatment), which has little effect oncorrosion resistance and weldability while greatlyincreasing feeds and/or speeds, plus extending toollife.

Typical uses are: Architectural Components, TextileEquipment, Pulp and Paper Processing Equipment,Marine Equipment and Fittings, PhotographicEquipment and X-Ray Equipment etc..

Material non magnetic in the annealed condition, but canbecome mildly magnetic following heavy cold working.Annealing is required to rectify if necessary [3, 22, 23].

Mechanical Properties

Table 7. Mechanical properties of 316 grade stainlesssteels.[3, 23]

HardnessGrade TensileStr

(MPa)min

YieldStr

0.2%Proof(MPa)min

Elong(% in

50mm)min

Rockwell B(HRB)

max

Brinell(HB)max

316 515 205 40 95 217

316L 485 170 40 95 217

316H 515 205 40 95 217

Note: 316H also has a requirement for a grain size ofASTM no. 7 or coarser.

Physical Properties

Table 8. Typical physical properties for 316 grade stainlesssteels. [3, 23].

Mean Co-eff of ThermalExpansion (m/m/C)

Grade Density(kg/m3)

ElasticModulus

(GPa) 0-100C 0-315C

0-538C

316/L/H 8000 193 15.9 16.2 17.5

Table 9. Possible alternative grades to 316 stainless steel[3].

Grade Why it might be chosen instead of 316?

316Ti Better resistance to temperatures of around 600-900C is needed.

316N Higher strength than standard 316.

317L Higher resistance to chlorides than 316L, but withsimilar resistance to stress corrosion cracking.

904L Much higher resistance to chlorides at elevatedtemperatures, with good formability

2205 Much higher resistance to chlorides at elevatedtemperatures, and higher strength than 316

Corrosion Resistance

Excellent in a range of atmospheric environments andmany corrosive media - generally more resistant than 304.Subject to pitting and crevice corrosion in warm chlorideenvironments, and to stress corrosion cracking above about60C. Considered resistant to potable water with up toabout 1000mg/L chlorides at ambient temperatures,reducing to about 500mg/L at 60C.

Stainless Steel 316 is usually regarded as the standardmarine grade stainless steel, but it is not resistant towarm sea water. In many marine environments 316 does


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exhibit surface corrosion, usually visible as brownstaining. This is particularly associated with crevicesand rough surface finish [22].

Heat Resistance and Temperature Properties

Good oxidation resistance in intermittent service to

870C and in continuous service to 925C. Continuoususe of 316 in the 425-860C range is notrecommended if subsequent aqueous corrosionresistance is important. Grade 316L is more resistantto carbide precipitation and can be used in the abovetemperature range. Grade 316H has higher strength atelevated temperatures and is sometimes used forstructural and pressure-containing applications attemperatures above about 500C.

316L displays good oxidation resistance in continuousservice up to 930 oC, and in intermittent service up to870 oC. Due to its low carbon content it is also lesssusceptable to carbide precipitation resulting inintergranular corrosion when heated or slow cooled

through the temperature range 430 oC - 870 oC eitherin service or during welding. There is however areduction in mechanical properties as temperatureincreases [22].

Applications

Typical applications include:

Food preparation equipment particularly inchloride environments.

Laboratory benches & equipment.

Coastal architectural panelling, railings & trim.

Boat fittings.

Chemical containers, including for transport.

Heat Exchangers.

Woven or welded screens for mining, quarrying& water filtration.

Threaded fasteners.

Springs [22, 23].

Thermal Properties

Thermal expansion properties are similar to those ofnickel-based alloys.

Thermal conductivity values for cobalt-based carbide-hardened alloys such as HS 21 are typically about 15% ofthose for pure cobalt [22, 23] .

Oxidation Resistance

This property is almost entirely dictated by the chromiumcontent. Chromium contents in the range 20-25% areusually sufficient to protect the alloy up to temperatures of1100C. Although the chromium is responsible for theformation of a protective oxide layer, it is susceptible toattack from elements such as sulphur, vanadium and alkalimetal halides or oxides. These commonly come fromcontaminatyed fuels and other sources. Sulphur penetrationcaqn lead to the formation of sulphides within the alloy,forming low melting point eutectics such as Co4S3 (meltingpoint 877C). Strengthening carbides may also bepreferentially attacked in some alloys [22, 23].

Cobalt Chrome:

Cobalt-chrome alloys are part of the group of non-precious

alloys, also referred to as preciousmetal-free alloys. The first cobalt-chrome alloy that wasintroduced in dentistry in the1930s was an alloy used inmedical implantology, where it had already proven itsclinicaleffectiveness. It was used in the partial denturetechnique and replaced steel in that field.

In dental usage the term steel became a synonym withpartial denture alloys consisting ofcobalt-chrome alloys.However, this designation is misleading since steel refers toiron alloyscontaining carbon. The frequently useddesignation chrome-cobalt alloy is also incorrectbecauseby definition this would involve alloys on a chromiumbase. What are meant arealloys on a cobalt base. Besidesbeing used as partial denture alloys, cobalt-chrome alloyssuch as WirobondC (BEGO) can be utilised as crown andbridge alloys for ceramic veneering. The acrylic veneeringof cobalt-chrome alloys generally displays more favourablebond values than with precious-metal alloys.

Non-precious alloys have a negative reputation amongsome dentists and dental technicians. Poor processability,inadequate chemical and biological properties are cited asreasons for this. This polarisation goes so far thatconsideration is only given to alloys with a high goldcontent, whose properties are applied to other precious-metal alloys (on palladium or silver base and to alloys withreduced gold content) without reflection, however. Thisresults in a distorted picture that does not accurately reflect

the non-precious alloys [3, 22, 23]. .

Composition

A carbon content of less than 0.02% ensures that no carbideprecipitation that would lead to brittleness of the marginalareas of the seam occurs during laser welding. This wouldthen result in an increased risk of fracture. Highly purebase metals are used to make alloys. However, there are no100% pure metals. For example, platinum ores containpalladium and sometimes also nickel impurities, cobalt isaccompanied by nickel (and conversely), etc. Complete


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separation of the elements is never possible. Therelevant standards stipulate a maximum nickelcontent of 0.1 %.

Concentrations of greater than 0.1% have to bedeclared. Alloys with less than 0.1% of nickelcan bedesignated as nickel-free. The claim that a cobalt-chrome alloy is absolutely nickel-free would be

objectively false and can only be understood on thebasis of marketing aspects. If a restoration made of acobalt-chrome alloy weighed 10 g (extremely largebridge, partial denture), the entire restoration wouldcontain a maximum of 0.07 g (= 70 mg) of nickel. Thelatter, however, is not only found on the alloy surface,but is spread homogeneously throughout therestoration. If one assumes that nickel is detachedfrom the alloy to the same extent as cobalt (which isprobable although nickel is nobler than cobalt), therelease of nickel will amount to approx. 0.00003mg/cm (0.03 g/cm) in the first week and constantlydecline thereafter. If one compares this to the dailyuptake in food, i.e. approx. 0.19 0.90 mg, (190 900g), toxicological or allergic stress appears very

improbable. In the case of alloys with veneeringcapacity, the available area is additionally reducedconsiderably due to the veneered ceramics [3].

Dental processing and mechanical values

Can be used for veneering crowns and bridges withceramics. The dental processing of cobalt-chromealloys is assessed as more unfavourable in comparisonto gold alloys. This is also reflected in the slightlyhigher costs for the required instruments. Thispartially offsets the price advantage of the alloy. Thisopinion must be qualified, however. In the veneeringof frames the difference in the required processing

between gold and cobalt-chrome alloys with veneeringcapacity is not very great. In the case of fully castcrowns, the more difficult processing of the cobalt-chrome alloys is a significant negative factor. It isrecommended, therefore, that the processinginstructions be followed. Each alloy has its specificfeatures that must be taken into account. This appliesto non-precious alloys as well as to precious-metalalloys [3, 22, 23]. .

Veneerability

Due to the higher melting interval, non-precious alloys

are generally more heat-resistant than gold alloys. Inparticular palladium-free gold alloys are sensitive heresince palladium is responsible for the heat resistance,among other things. Heat resistance refers to theability of an alloy not to deform even in the high-temperature range (slightly below the solidus point),i.e. not to distort under its own weight. With acoefficient of thermal expansion of 14.2 [10 6 * K.

Corrosion

As already explained in connection with the composition ofthe alloy, chromium and molybdenum are important forcorrosion resistance. The latter can be tested with animmersion test. Test objects are suspended in a solutionconsisting of sodium chloride and lactic acid (0.1 mol/leach) and the dissolved alloy components are determined

by means of a suitable analytical method (e.g. atomicabsorption spectrometry, AAS).

The ion quantities determined can then be compared toother alloys By comparing the corrosion rates ofcomparable and clinically proven alloys, conclusions canbe made concerning the behaviour of the alloy examined.This study method is therefore suitable as a pre-clinicalscreening test. It has been shown that cobalt-chrome alloysdisplay an ion release that is somewhat higher than that ofgold alloys, but is still on the same order of magnitude. It isknown that dental processing, such as casting, grinding orceramic veneering, may influence the corrosioncharacteristics of dental alloys. In the case of cobalt-chrome alloys, this influence are relatively small. This

means that such alloys are very rugged [3, 22, 23] .

Biocompatibility

The main components of cobalt-chrome alloys, cobalt,chromium and molybdenum, are essential elements].Therefore, they must be classified as more favorable inprinciple, as elements that have no function in the humanbody. For essential elements the human organism hasdiverse ways of decomposition and utilization. Thereappear to be certain threshold values, below which nointeraction takes place. However, these threshold values arevery individual and may be very low in specific cases sincethere is verification of allergies to cobalt, chromium and

molybdenum. In the relevant literature, however, there isno reference to the fact that cobalt-chrome alloys havecaused an allergy. This point is also supported by the use ofcobalt-chrome alloys for partial dentures for decades.Alloys of this type were positively assessed back in 1936.Thus, there is no clinical experience in this connection thatis older than that concerning gold alloys with veneeringcapacity.

Allergies are usually verified by means of the patch test. Itmust be emphasised here that this test itself is capable ofsensitising the subject. Therefore, in Norway, for example,it is only permitted if there is justified suspicion of anallergy. Furthermore, there is problem regarding suitable

test substances. For some elements there are still none, withothers the selection of unsuitable test substances may leadto incorrect statements. Chromium allergies (for the dentalfield), for example, should only be tested with chromates,in which chromium is found with the oxidation number+III. If one uses dichromate (here chromium has theoxidation number +VI and acts as a strong oxidant), onewill obtain in all likelihood incorrect results. Chromiumwith the oxidation number +III is released from dentalalloys due to corrosion processes. To avoid faultydiagnoses, the patch test should only be conducted byproperly trained persons (dermatologists, allergists)


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because the evaluation requires know-how andexperience [3, 22, 23] .

Table 10. Biomaterials Densities of Biomaterials andSome Other Related Materials [3].

Material Density

(g/cm3)

Material Density

(g/cm3)Amalgam 11.6 Palladium

basedalloys

~10.8

Alumina 3.85 Porcelaindental

~2.05

Bone - cortical ~2.0 Stainlesssteel 316L

8.0

Calciumhydroxidecement

~1.90 Titanium 4.51

Chromium 7.19 Ti6Al4V 4.40

Cobalt chromealloy

~8.50 Tooth dentine

2.14

Fluorapatite 3.22 Tooth -

enamel

2.97

Glass ionomercement

~2.10 UHMWPE 0.945

Gold 19.3 Vitreouscarbon

1.47

Hydroxyapatite 3.16 Zincphosphatecement

2.59

Methylmethacrylate

0.94 Zirconia 6.10

Mercury 13.5

TOTAL KNEE REPLACEMENT

1. History

Development of the total knee followed the success of thetotal hip replacement by Sir John Charnley in the 1960s.He pioneered the use of the polyethylene stainless steel

joints fixed to bone with polymethlmethacrylate (PMMA)plastic, often called "bone cement". Today most hip andknee prosthesis are made of cobalt chrome alloy or oftitanium. The use of PMMA is fading and many jointsare being fixed to bone with new techniques that involvebone ingrowth to the prosthesis. Bone cement still is usedwidely in the knee with newer techniques to reduce itsfailure rate. These include mixing under vacuum toprevent air bubbles in the plastic. Bone cement is the

same chemistry as Plexiglas (TM) except that bonecement is formed at room temperature and has bariumincluded allowing us to see it on x-rays.

2. Types

Hinge type prosthesis were used initially but had a highrate of failure due to loosening from bone. Theunicompartmental knees replace only one part of the jointand have not enjoyed the success of the threecompartments or total knee replacement. The modern

devices are minimally constrained. This term means theparts of the knee are not rigidly attached to one another asin a hinge. The successful designs use the ligaments ofthe knee to hold the knee in place and merely resurfacethe arthritic joint.

The figure 13 shows total condylar knee prosthesis as itappeared in the 1980s with cobalt-chrome alloy femoral

component and high density polyethylene tibialcomponent.

This knee was developed by Install-Burstein and was thestandard for total knee replacement for many years.

This knee is still in use with a metal tibial tray, not shownin this photo. The patellar button is also notshown and is round and more than an inch across.The patellar button is made of polyethylene plasticalso.

Figure 13. Total condylar knee prosthesis [19].

3. The Surgery

Total knee replacement is best done in a highly sterileoperating room. These are done as the first case in theday because activity in the room stirs up dust. The room

is cleaned thoroughly the day before. A clean airfiltration system removes airborne dust particles andkeeps the air movement horizontal. The surgical teamwears sterile gowns that cover the head. These "spacesuits" protect the patient from debris that could strike thesurgeon's face or head and fall back to the wound. Thesuits also protect the surgeon from contact with bloodymaterial from the bone saw used. Antibiotics are givenbefore surgery to reduce the risk of infection. Total kneereplacement requires about 90 minutes of time with thewound open. This means 3 hours in the operating room in


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most cases [19].

Figure 14. Surgical picture of a total knee before woundclosure [19].

Figure 15. Figure of the Bone Cutting at the kneesurgery [19].

ZIRCONIA

Zirconia as a pure oxide does not occur in nature but itis found in baddeleyite and zircon (ZrSiO4) whichform the main sources for the material. Of the two ofthese, zircon isby far the most widespread but it is less pure and

requires a significant amount of processing toyield zirconia. The processing of zirconiainvolves the separation and removal ofundesirable materials and impurities - in thecase of zircon - silica, and for baddeleyite, ironand titanium oxides. Typical properties ofzirconia are:

- High strength.

- High fracture toughness.- Excellent wear resistance.- High hardness.- Excellent chemical resistance.- High toughness.- Very refractory.- Good oxygen ion conductor.

The properties exhibited by zirconia ceramics depend upon

the degree and type of stabilisation and on the processingused. Table 11 shows some mechanical properties ofzirconia [3].

Table 11. Mechanical properties of zirconia [3].

Property Partiallystabilised

FullyStabilised

Partiallystabilised(plasmasprayed)

Density(g.cm-3)

5.7 - 5.75 5.56 - 6.1 5.6-5.7

Hardness -Knoop (GPa)

10-11 10-15

Modulus ofRupture(MPa)

700 245 6-80

FractureToughness(MPa.m-1/2)

8 2.8 1.3-3.2

Youngsmodulus(GPa)

205 100 -200 48

Poissons ratio 0.23 0.23-0.32 0.25

Thermalexpansion(10-6/K)

8-10.6 13.5 7.6-10.5

Thermal

Conductivity(W/m.K)

1.8-2.2 1.7 0.69-2.4

1. Limitations of Zirconia

To date, zirconias use has been limited by its loss ofstrength and its subsequent cracking when subjected to

temperatures of 100-600C in the presence of water aprocess known as hydrothermal degradation. Using state ofthe art techniques and working at the nanoscale, theresearch team has inhibited this process by adding tracequantities of materials such as alumina to the zirconia,without compromising its toughness. (One nanometer isone thousand millionth of a metre.) Targeting of the addedmaterials prevents degradation from progressing into thezirconia from its surface [3, 21].

2. Oxidized ZirconiumOrthopedic surgeons have traditionally delayed jointreplacement surgery in patients younger than 65 becausethey did not expect the materials used to withstand the wearplaced on them for longer than 10 to 15 years. Globalmedical device company Smith & Nephew Inc.'sOrthopaedics Division, in Memphis, TN, has developed


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Oxidized Zirconium in response to the medicalcommunity's concerns with wear. Smith & Nephewhas patented the material for orthopedic use, and theFDA has cleared Oxidized Zirconium for kneeimplants. Eleven years in development, the OxidizedZirconium knee is considered an industry-definingtechnology. Before this technology was developed,nearly 600,000 total knee replacements are performed

each year globally. The annual total global kneemarket is estimated to be $2 billion. Currently, mostknee implants are made from a cobalt-chrome alloythat slides against a plastic (polyethylene) bearing.The motion and friction caused by daily living candamage the implant's surface and cause metal andpolyethylene wear debris, ultimately causing bone lossand the need for another implant. Because OxidizedZirconium components are made of a metalliczirconium alloy that is heated to convert the surface toa ceramic (zirconia), the best of both worlds can beachieved. In addition, Oxidized Zirconium containsnondetectable traces of nickel, providing a solution forthe more than 20,000 candidates for total kneereplacement each year identified as acutely allergic to

this metal. Compared to cobalt chrome, OxidizedZirconium, in wear simulation testing, reduced the rate

of polyethylene wear by 85 percent[10].

3. Zirconium

Orthopedic surgeons at St. Anthony Central Hospitalare now pioneering a new knee-replacement prothesismade of zirconium. Through a special process, thismetal is heated, then bombarded in an oxygen-enriched environment to yield the second hardestmaterial known, exceeded only by diamonds. The neteffect is a femoral component that has the wearcharacteristics of ceramics without their downside

brittleness. Friction is reduced by as much as eighttimes, which means the replacement joint wears farlonger than previous metal models [14].

4. Wear Simulation Comparison of a Zirconia and

a Cobalt Chrome Femoral knee Implant

In recent years the major cause of long-term failure ofhip and knee total arthroplasties has been identified asoriginating with wear particles produced at theinterface in the synthetic articulating surfaces.Researchers have tested the hypothesis that a zirconia(zirconium oxide) femur would produce less wear ofthe counterfacing ultra-high molecular weight

polyethylene (UHMWPE) insert than a standard cobaltchrome molybdenum femur of similar design.

The results show a definite reduction in the averagesteady-state wear rate and the total wear in UHMWPEinserts articulating with the zirconia femurs comparedto those articulating with the cobalt-chrome femurs.We speculate that this reduction was due to theincreased hardness, scratchresistance and smoothnessof the zirconia femurs [20].

JOINT REPLACEMENTS

Total joint replacements of the hip and knee have beenperformed at St. Anthony Central Hospital since theirintroduction in the 1970s. During these surgical procedures,mechanical prostheses crafted of specialized metals,ceramics and plastics are used to replace joints irreparablydamaged by illnesses (such as rheumatoid arthritis and

osteoarthritis) or injury-related conditions (such as vascularnecrotitis and post-traumatic arthritis). As new and betterdesigns and materials have become available, outcomeshave improved remarkably. Many people having total jointreplacement surgery are able to enjoy active, full lives. St.Anthony Central Hospital nurses and therapists conduct atotal joint preparatory class twice each month so that thosehaving the surgery know what to expect before, during andafter the procedure. Topics covered include painmanagement, prevention of complications and earlymobility [14].

1. Reconstructive Implants

Unique metalworking capabilities and machiningtechniques, helped create an array of reconstructiveimplants including large joint replacements, spinalimplants, and neurocranial and maxillofacial meshes.

Hip Systems: First Product that actually isnear net shape of a forged hip (Figure 13)and has continued to improve forging andfinishing processes for Cobalt Chrome andTitanium hips and acetabular cups to meetthe needs for extending implant life.

Figure 13. Forged hip joint [15].

Knee Systems. The company known asTecomet's, has such an expertise in forgingTitanium, Cobalt Chrome, and Zirconiumwhich provides the solution to thechallenges set by complex designs offemoral and tibial components (Figure14). In June 2001, the company successfully


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manufactured the first ever forgedZirconium Femoral Component [15].

Figure 14. Femoral and tibial components [15].

2. Trauma Products

Metal implants remain the dominant form of traumafixation devices providing superior strength andbiocompatibility. Tecomet manufactures a variety ofquality trauma products to satisfy the medical

industry's need for versatile, cost effective designs.

Nail Systems. Trauma products for longbones include internal fixation devices (IMNails). When the orthopedic industryrequested strong but lightweight systems,Tecomet responded by developing aproprietary metal forming process toproduce hollow titanium fluted nails.

Plate Systems. From forged plates for longbone fixation to intricate photoetchedminiplates for hand surgery, maxillofacialand neurocranial applications, Tecomet's

manufacturing and design services supportdiverse product lines.

Maxillofacial and Neurocranial Mesh.Advanced photochemical etching providesthe foundation for creating a wide variety ofmetal reconstruction and fixation implants.Flexible and rigid configurations of fine andcoarse meshes allow applications specific tooral and maxillofacial surgery,otolaryngology, neurology, plastic surgeryand orthopedics, [15].

3. Capabilities

Tecomet excels as a technically strong problem-solving partner prepared to meet the toughestchallenges in manufacturing and product development.Through involvement at design inception they haveearned a reputation for providing engineered solutions,reducing product launch time and lowering cost.Tecomet's origins are in refractory metals such asmolybdenum, tungsten, tantalum and columbium.Extensive experience in forging, machining and thedevelopment of technologies to process high strength

materials such as titanium, zirconium, kovar, nickel andcobalt super alloys enables us to create products withdemanding performance requirements, [15].

4. Technologies and Procedures

The primary concern surgeons seem to have regardingmetal-on-metal implant procedures is elevated serumchromium levels. To date, there have been no reports ofcobalt toxicity in patients with elevated levels of serumcobalt in association with metal-on-metal total hipreplacements, says Josh Jacobs, MD, of Rush MemorialUniversity of Chicago Hospitals. However, Jacobs addsthat the literature is incomplete and the necessary studieshave not been conducted to determine whether theseelevated levels are a long-term concern. The advantage ofthe metal-on-metal implant is its longevity, whichminimizes the need for a later revision.

The ceramic-on-ceramic implants have enjoyed popularityoutside the United States for years; however, they are

associated with higher incidents of fracture. James T.Caillouette, MD, attending surgeon at Hoag MemorialHospital, Newport Beach, Calif, and an assistant clinicalprofessor at the University of California at Irvine, observes,There was a nexus of unfortunate events surrounding theFDA [approval] proceedings. They were trying to protectus. Under the circumstances, the incidents that fosteredconcern at the FDA were comparatively minute in terms ofthe number of procedures that occur withoutcomplications.

Metal-on-cross-linked polyethylene shows promise,according to orthopedic surgeon Richard A. Berger, MD, ofRush Memorial University of Chicago Hospitals. But thereis some evidence in the presence of abrasive particles thatwear may, in fact, be accelerated with cross-linkedpolyethylene. Some studies have indicated that the weardebris caused by repeated contact between the articulatingsurfaces has been a lingering clinical concern. The mostcost-effective procedure is cross-linked polyethylene on achrome cobalt head, says Berger. It's [the procedure]most [physicians and patients] are choosing across thecountry.

The latest FDA-approved innovation in hip arthroplasty isthe oxidized zirconium implant, a new material thatcombines the advantages of scratch resistance andextremely low wear but without the potential for fracture orhigh metal ion levels. Oxidized zirconium femoral heads

are made from zirconium metal that has been oxidized suchthat the surface of the implant is a ceramic zirconia. Theoxidized surface has proven to be extremely hard andabrasion resistant. These laboratory tests have validatedthat oxidized zirconium resists abrasion, minimizes wear,and has no demonstrable risk of breakage or delamination,[2].


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5. Biologically Compatible

Suboptimal alignment of hip prosthesesfor example,excessive vertical positioning of the acetabularcomponentincreases wear, especially near theperiphery of the component. Anatomic restoration ofthe hip center of rotation and offset and avoidance ofimpingement are associated with decreased wear.

Optimal surgical technique involves stable fixation tominimize interfacial motion and avoidance of residualparticles that could potentially contribute to third bodywear. But not all new techniques have built on thematerials and methods first pioneered in the 1960s.Recent studies of positive outcomes of hip proceduresusing noncemented tapered stems have made themamong the most favored of orthopedic physicians.Fresno, Calif-based orthopedist D. Kevin Lester, MD,who specializes in minimally invasive hip procedures,and is an assistant clinical professor at the Universityof California, San Francisco, is an enthusiasticproponent of the cementless tapered titanium femoralprosthesis. The cementless, collarless hip implant istotally compatible with a minimally invasiveprocedure, and has zero failures due to loosening, and12% chance of improvement, he says. With improvedmaterials and techniques have come less invasiveprocedures, which have aided in the success of hiparthroplasty,[2].

6.Orthopedic Biomaterials

Producing a material that can function intimately withliving tissue, with minimal adverse reaction, is quite achallenge for engineers and scientists. Biomaterialsare designed to perform specific functions in the bodyand, at times, are used to replace parts of livingsystems. Some common implants include knee andhip joint replacements, spinal implants, and bonereinforcement devices. Also popular are artificialheart valves, soft tissue replacements, and a variety ofdental implants. Each of these devices must beconstructed of special materials that are uniquelysuited for their respective tasks. Properties such asmechanical integrity, corrosion resistance, andbiocompatibility must be evaluated for anybiomaterials, [8].

7. Materials for knee replacements

Unlike hip replacement devices there is currently littlechoice in materials for knee replacements. A three

year European Community funded a program toexplore the use of ceramic materials in kneearthroplasty has demonstrated that ceramic onpolyethylene combinations reduce polyethylene wearcompared to existing metal on polyethylene bearings.Product features include:

Metal backed ceramic femoral component.

Zyranox zirconia on polyethylenebearing.

Existing, clinically proven fixation methods.

Patented braze technique for joining ceramic tometal.

Ceramic resurfacing available for any currentknee system [12].

8. Characteristics of Materials Used in Orthopedics

a. Fracture fixation :

i. Stainless steel

Iron based alloy containing chromium, nickel,molybdenum. Usually annealed, cold worked orcold forged for increased strength. A range ofstrength and ductilities can be produced.

Strong.

Cheap.

Relatively ductile therefore easy to alter shape.Useful in contouring of plates and wires during

operative procedures. Relatively biocompatible.

The chromium forms an oxide layer when dippedin nitric acid to reduce corrosion and themolybdenum increases this protection whencompared to other steels.

Can still undergo corrosion if carbon gets to thesurface.

High Youngs modulus - 200 GPa (10 that ofbone) so can lead to stress shielding ofsurrounding bone which can cause boneresorption.

Used in plates, screws, external fixators, I.M.nails.

Composition of 316L Stainless Steel: Iron- 60%,Chromium- 20% (major corrosion protection),Nickel- 14% (corrosion resistance),Molybdenum- 3% (protects against pittingcorrosion), Carbon- 0.03% (incr. strength),Manganese, Silicon,P,S,- 3% (controlmanufacturing problems).

ii. Titanium and its alloys

Excellent resistance to corrosion .

Youngs modulus approximately half that ofstainless steel, therefore less risk of stressprotection of bone, stress riser at end of plate or

nail. More expensive than stainless steels.

Poorer wear characteristics than others, thereforenot considered suitable as a load bearing surfacethese days.

Can be brittle i.e. less ductile than stainless steel,but more ductile titanium alloys being produced.

Can be as strong as stainless steels.

Used in plates, screws, I.M. nails, externalfixators. Useful in halos as more MR scancompatible than other metals.


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iii. Adhesives

Not common in orthopaedics but potentiallyuseful in small fragment fixation,controversial.

Need to 1. Have sufficient bond strength 2.Be able to bond to moist surfaces 3. Permithealing across the bond line 4. Besterilisable.

Bone cement does not count as an adhesive.

Cyanoacrylate-Experiments on rabbitsproduced poor results, long term fate notaddressed.

Fibrin-Conclusions of experiments are thatfibrin adhesives are only suitable for fracturefragments with considerable inherentstability or being non load bearing.

iv. Biodegradable polymers

Potential advantageso Hardware removal not necessary,

reducing morbidity and cost.o Stiffness of polymer decreases as

stiffness of fracture callusincreases.

o Can possibly be used in future forcontrolled release of antibiotics orstimulants to healing .

Requirementso Adequate mechanical strength for

the applicationo sufficient strength over a sufficient

period of time to maintain enoughstability for the fracture to healand prevent loss of reduction

o Degradability into products thatare mot harmful.

Exampleso Polyglycolic acido Polylactic acido Copolymers

Only about 1/20th the stiffness and strengthof stainless steel

Used in ankle fractures with poor results

Used in phalangeal fractures with betterresults

b. Materials Used in Joint Replacement Surgery

i. Stainless Steel

Now rarely used in new designs

Because Youngs modulus high, need to beinserted with a lower modulus polymercement for fixation, to prevent stressshielding of the surrounding bone.

ii. Cobalt Chrome

30-60% cobalt, 20-30% chromium,7-10%molybdenum + nickel.

Stronger and more corrosion resistant thanstainless steel.

Youngs modulus higher than stainless steel (250

cf 200 GPa). Stress shielding a theoretical risk.Usually fixed with cement.

iii. Titanium Alloys

Most common combination is Ti6Al4V

Strong and corrosion resistant

Youngs modulus 110GPa (less than cobaltchrome & stainless steel), therefore often used forcementless joint replacements.

Poorer wear characteristics.

Ultimate Strength: Stainless Steel > Titanium;Yield Strength (permanent deformation):Titanium > Stainless Steel

Ti13Zr13Nb is stronger and has lower Youngsmodulus.

Theoretically, may favour bone apposition andbone ingrowth more than cobalt chrome, but nodifference found clinically.

iv. Polyethylene

UHMWPE- Ultra high molecular weightpolyethylene. A polymer of ethylene.

Molecular weight 2-6 million.

90% success rates at 15 years with metal onpolyethylene (therefore the gold standard).

The weak link of any total joint replacement. Osteolysis produced due to polyethylene wear

debris causes aseptic loosening.

Submicron particles found in periprosthetictissues when polyethylene wear present.

Factors affecting polyethylene wear:o Material polymorphism - Ziegker

process used to producepolymerisation. Consolidationproduced by Ram extrusion orcompression moulded. Componentmachined from these blocks. Bankstonet al - linear wear rate 0.05mm /year forcompression moulded, 0.11mm / year

for ram extruded.o Gamma sterilisation in air produces

chain scission by oxidation. Companiesnow vacuum pack and sterilise theimplants.

o Thin polyethylene- increases wear, dueto increased fatigue wear if thicknessless than 6-8mm.

o Polyethylene should not directly touchbone in hip replacement- increaseswear.


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o Conformity- increased conformityreduces stresses ( particularlyrelevant in TKR

o Materials used- Titanium bearingsurface increases wear, ceramicsreduce wear.

o Size of femoral head:

Large femoral head,

causes increased slidingdistance at joint surfaceand so increasedadhesive wear.Therefore increasedvolumetric wear (Theorybehind Charnley's LFA22mm head).

Small femoral headcauses increased stresses(increased fatigue wearor penetrative wear)Trade off ideal femoralhead size is 28mm.

o Reduced offset of femoral

prosthesis causes increased fatiguewear, as joint reaction forcehigher.

o Three body wear:

Due to cement particles,metallic particles.

Can be reduced bycareful surgicaltechnique.

Can be increased withmodularity of implants.

v. Ceramics

Strong ionic bonds between the metallic andnonmetallic components.

Very strong.

Very stiff.

Very biocompatible.

Very hard, therefore good wearcharacteristics.

But very brittle.

Also difficult to process due to very highmelting points therefore expensive.

Bioinert e.g. Alumina, Zirconia, used forsurface replacement.

Bioactive e.g. hydroxyapatite and glass-used for coating joint replacements for osseointegration between bone and implant.

vi. Hydroxyapatite coating of THR

Ca10 (PO4)(OH)2 coated onto metal surface,usually onto a porous surface

Usually 50-150m thick. (Too thin can beresorbed, too thick can flake off during insertionof implant).

Is thought to be osteoconductive Not known how long it takes to resorb and how

stable the implant is after resorption

Good results at 5 years (99% survival) accordingto Norwegian Arthroplasty register

Some worry about increased three body wear onpolyethylene

The particles of HA may also stimulateosteolysis.

vii. Bone cement

Polymethylmethacrylate introduced 30 years ago

No other fixation principle has given better longterm results

Polymerised methyl methacrylates

mixed from powder polymer and liquid monomerin theatre usually in a vacuum to reduce porosityand increase strength. Powder also containscatalyst ( benzoyl peroxide).

Xray contrast medium (barium sulphate)

Colour (chlorophyll)

Stronger in compression than tension, weakest inshear

Exothermic reaction producing heat, can lead tobone necrosis

Leakage of monomer during polymerisation may

induce endothelial damage locally leading tothrombus formation and distal hypotensionthrough effects in pulmonary vascular bed

Cementing techniques have changed from fingerpacking to retrograde filling with a cement gununder maximal pressurisation - has not beenevaluated in randomised clinical studies

Controversy about type of bond between implantand cement- should it be maximal withroughening of implant surface or minimal with apolished surface [11].

COMPARISON OF PROPERTIES BETWEEN

STAINLESS STEEL 316L WITH OTHER

BIOMATERIALS

Table 12 shows a comparison of properties betweenstainless steel 316 L with the materials studied in thisresearch


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Table 12. Properties of biomaterials [22, 23].

Material Modulusofelasticity( GPa )

Shearmodulusofelasticity( GPa )

Poissonsratio

YieldstressMpa

Stainless

steel

200 82 0.27-0.30 min-

170Cobaltchrome

230 .30 413

Zirconium 200 70 0.22 2000

Titanium 100-120 39-45 0.33 760-1000

CalciumSulfate

18-21 6-10 .305 -------

CalciumPhosphate

18-21 7-13 .315 -------

SUMMARY

The most common materials used inorthopedics are: titanium, zirconium, cobalt-chrome,

calcium phosphate, and calcium sulfate and stainlesssteel 316-L. Titanium is used primarily for theloading faces which include the pin structure,fabrication of plates and femoral stems. The Modulusof Elasticity of Titanium is much lower than StainlessSteel 316-L, having a numerical difference in valuewhich ranges from 80-100 GPa. The Shear Modulusof Elasticity is also lower than the particular value ofStainless Steel 316-L, this difference is about 37 GPa.The difference in Poissons Ratio is just about 3decimal units, but the yield stress of Titanium is muchmore higher in comparison to that of Stainless Steel316-L. The difference expressed is more than 600MPa for the yield stress. In conclusion Stainless Steel316-L is much stronger, but that is not always good

because stress rises at end of plate or nail. Titaniumpossesses a lower ultimate strength than StainlessSteel 316-L but its yield strength is much more, this iswhat causes permanent deformation of the material,and Stainless Steel is easily expected to recover itsnormal state than Titanium and its Alloys.

Cobalt-Chrome which is a cobalt alloy has aModulus of Elasticity 230 GPa, when compared to theModulus of elasticity of Stainless Steel gives us adifference of 30 GPa. In this particular case cobaltchromes Modulus of elasticity is higher than StainlessSteel. The Poissons Ratio of both are very similar,they both are near 0.30. The yield stress that cobaltcan support is 413 Mpa. When compared to StainlessSteel 316-L the difference obtained is near to 243Mpa. In conclusion, this material seems to be betterthan Stainless Steel 316-L, but the only disadvantageis the price and the facility to find it.

Zirconiums Modulus of Elasticity is 230GPa which is very close to the Modulus of Elasticityof Stainless Steel. The Shear Modulus of Elasticity is70 GPa, when compared to Stainless Steel 316-L itgives a difference of 12 GPa. The Poissons Ration ofzirconium is 0.33 and the difference between both is

only from 3 to 4 units. The materials seem similar but theyield stress of both materials is different. Zirconium has a2,000 Mpa yield stress value and stainless steel has only170 MPa. This particular property gives the material is themaximum stress it could hold and return to its originalstate.

In the Bone Grafting face, the predominant

materials are calcium sulfate and calcium phosphate. TheModulus of Elasticity of calcium phosphate is in the rangefrom 18-21 MPa. This material compared to stainless steel316-L has a differential value of more that 180 MPa whichclearly states that is not as elastic as stainless steel, whichby definition makes it much more brittle. The ShearModulus of Elasticity of Calcium Phosphate is about 7-13GPa, and the differential value is more than 68 GPa. ThePoissons Ratio fluctuates in the same range. The yieldstress, which is the ability of the material to recover or toreturn to its normal state, has a value so low that is nottaken into consideration. The yield stress of the StainlessSteel 316-L is much more dominant.

Calcium sulfate has very similar properties as

calcium phosphate, which also makes it very brittle. TheModulus of Elasticity of calcium sulfate ranges from 18-21GPa which compared to Stainless Steel 316-L thedifference in values is very close to that of calciumphosphate which is in the range from 170-180 MPa. Bydefinition calcium sulfate is a very brittle material. Thedifference in numerical values of the Shear Modulus ofElasticity also fluctuates from 68 GPa and over. ThePoissons Ratio is almost in the same range, and has anumerical value of 0.315. Since the material is so brittlethe yield stress is not taken into much consideration, as inthe comparison with calcium phosphate, stainless Steel ismuch more dominant in this precise property.

As observed in bone grafting the materials used

are brittle but very strong and stiff, because of this they areused in joint replacement and osseo integration betweenbone and implant, some sort of a refilling process, and notas Stainless Steel which is used in other orthopedic surgeryprocesses.

ACKNOWLEDGEMENTS

Our thanks to Dr. Megh R. Goyal for his guidance.

REFERENCES

1. http://www.orthobluejournal.com.Szpalski, Marek and Gunzburg, Robert.Applications of Calcium Phosphate-BasedCancellous Bone Void Fillers in Trauma Surgery.

2. http://www.orthopedictechreview.com/issues/mayjune03/pg36.htm .Gordon II, MA, W.A. A Technical Evolution.


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3. http://www.azom.com.Azom-Metals, Ceramics ,Polymers,Composites An Engineer Resource. 2004. Ato Z of Material.

4. www.azom.comBiomaterials: An Overview.

5. http://www.suri.org/3pubs/ttoday/fall95/implant.htm

Blanchard, Cheryl R. Biomaterials: BodyParts of the Future.

6. http://www.orthobluejournal.comParikh, Shital N. Bone Graft Substitutes inModern Orthopedics.

7. http://www.karger.com/gazette/65/lidgren/art_5_0.htmLindgren, Lars. Bone Substitutes.

8. http://www.cheresources.com/newparts.shtml. Building New Body Parts.

9. http://www.biomed.tamu.edu10. http://www.goodsamdayton.com/knee.htm

Good Samaritan Hospital First in Area toIntroduce New Technology for KneeImplants.

11. http://www.orthoteers.co.uk/Nrujp~ij33lm/orthomat. htmImplants & Materials in Orthopedics.

12. http://www.bioceramics.com/knee1.htmKnee Replacements

13. Hasting,G.W and D.F. Williams.,1980.Mechanical Properties of Biomaterials.John Wiley & Sons.

14. http://www.stanthon4central.orgTotal Joint Replacement.

15. http://www.orthosupplier.com/players/4b.htm Tecomet

16. http://www.worldortho.com/history.htmlBrakoulias, Vlasios. The History ofOrthopedics.

17. 17.http://www.ae.msstate.edu/vlsm/materials/alloys/titanium.htm.Titanium Alloys and Their Classification.

18. www.azom.comTitanium and Titanium Alloys asBiomaterials.

19. http://www.ucbones.com/total_knee_replacement.htmTotal Knee Replacement

20. http://www.jbjs.org/ORS_2001/pdfs/1101Wear Simulation Comparison of a Zirconiaand a Cobalt Chrome Femoral KneeImplants.

21. www.azom.com

Zirconia (ZrO2) Is Zirconia a ViableAlternative to Steel.22. http://www.efunda.com23. www.matweb.com24. http://www.btec.cmu.edu/reFramed/tutorial/

mainLayoutTutorial.htmlThe Need for Bone Substitutes

GLOSSARY

Alignment - Positioning the femur and tibia so as to allowproper articulation at the knee joint.Allograft - A graft (occasionally bone) taken from a humanbeing and implanted in another.Alloy - A mixture of two or more metals.Anatomic - Relating to the structure of an organism. Often

used to describe a prosthesis, which closely resembles orduplicates the shape and size of a normal part of the body.Anatomic Axis - The axis formed by an imaginary linedown the center of the femoral canal. Usually 5-7 degreesoff the mechanical axis.Ankylosis - The fusion of a joint.Arthrodesis - Surgical fixation of a joint.Articulating Surface - Implant or bone surfaces whichtouch each other. Typically used in referring to thepolyethylene tibial surface or patellar surface.Autograft - A graft (sometimes bone) taken from a patientand reimplanted in another part of his/her own body.AVN - Avascular necrosis (particularly death of bonethrough lack of blood supply).

Biocompatibility - Referring to the degree of tissue orsystemic reaction caused by a foreign material in the body.Biomechanics - Relating to the forces that act on the joint,and their effect on the joint.Bone Ingrowth - The process of bone growing into thepores of a porous implant for enhanced fixation.Bone resorption - A remodeling of bone due to a lack ofstress through an applied load. A common result of stressshielding, where bone located in an area that is shieldedfrom stress is absorbed by the surrounding bone that isunder stress. Also called Stress Relief Osteoporosis.

Calcaneous - The heel boneCalcar - An area in the media region of the proximal femurwhich is characterized by very dense cortical bone.Caliper - An instrument used to determine thickness,diameter, or width.Cancellous Bone - Spongy bone composed of a looselatticework of bony traveculae and bone marrow within theinerspace found in the enlarged ends of long bone.Cannulated - An open-ended passageway through which awire or pin may be passed.Cast - The giving of shape to metal by pouring it in liquidform into a mold and allowing it to solidify.Closed Procedure - Done with the use of imageintensification but without the need of an incision at thefracture site.

Collateral Ligaments - The strong stabilizing ligamentslocated on both the medial and lateral sides of the knee.Comminuted Fracture - One in which there are severaldefinite disruptions in the bone, creating two or morefragmentsCompartment - A combination of two surfaces whicharticulate with each other.Compression - Adjustment of an external fixator toprovide closer bone-to-bone contact at a fracture site; or theapplication of centrally directed forces (forces appliedtowards the middle of the instrumented areas.Contracture - Abnormal shortening of muscle tissue,


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rendering the muscles highly resistant to stretching.Cortex - The outer surface of a bone or organ.

Countersink - Instrument used to form a flaringdepression around the top of a drilled hole. Insertionof an implant beneath the cortical surface of the bone.Cruciate ligaments - Two strong stabilizingligaments which cross between the condyles of theknee. The anterior cruciate ligament runs from the

back of the femur to the fro

biomaterials for orthopedics

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