orthopaedic fixation

ORTHOPEDICS PROSTHESIS FIXATION FOR

PATRICK J. PRENDERGAST

Trinity Centre forBioengineeringDublin, Ireland

INTRODUCTION

The fixation of an orthopedic implant should secure itrigidly to the underlying bone. The ideal fixation willsustain high forces, pain free, for the remaining lifetimeof the patient. Difficulties in achieving this objective arisebecause (1)

1. The loads are often several times body weight in thelower extremity.The loads are fluctuating, or cyclic,and furthermore extremely high loads can occuroccasionally (2).

192 ORTHOPEDICS PROSTHESIS FIXATION FOR

Encyclopedia of Medical Devices and Instrumentation, Second Edition, edited by John G. WebsterCopyright # 2006 John Wiley & Sons, Inc.

2. The presence of the implant alters the stress transferto the underlying bone leading to bone remodelling orfibrous tissue formation at the bone/implant inter-faces. This can threaten the long-term mechanicalintegrity of the prosthetic replacement.

3. The range of materials that can be placed in contactwith bone is limited by biocompatibility issues.

The fixation of an orthopedic implant may be catagor-ized as either cemented fixation or biological fixation.

Cemented fixation involves securing the implant intothe bone with a ‘‘bone cement.’’ By far the most commonbone cement is based on the (polymer polymethylmetha-crylate (PMMA)). PMMA bone cement is polymerized insitu during the surgery. It contains radiopacificiers in theform of particles of barium sulphate (BaSO4) or zirconia(ZrO2), which make it visible in radiographs (3). It alsocontains an inhibitor (hydroquinone) to prevent sponta-neous polymerization and an initiator (benzoyl peroxide) toallow polymerization at room temperature. Antibiotics toprevent infection (e.g., gentimacin) may also be added.Table 1 lists typical components of bone cement and theirroles. Polymerization begins when a powder of the PMMApolymer is mixed with the MMA monomer liquid. Themixing can either be done by hand in a mixing bowl justbefore to its use in the surgery or a mechanical mixingsystem may be used; these have the advantage of reducingthe porosity of the bone cement and increasing its fatiguelife. The cement is applied in a doughy state to the bonebefore placement of the implant.

In biological fixation, the implant is secured to the boneby a process known as ‘‘osseointegration.’’ Osseointegra-tion occurs by bone ingrowth onto the surface of theimplant. The surface of the implant must have a structureso that, when the bone grows in, sufficient tensile andshear strength is created. Bone ingrowth requires amechanically stable environment and an osteoconductivesurface. An osteoconductive surface can be achieved byvarious treatments, e.g., plasma spraying with hydroxya-patite. Ingrowth occurs over approximately 12 weeks, andduring this period, implant stability is required: Initialstability can be achieved by press-fitting the implant intothe bone, or by using screws.

Hybrid fixation refers to the use of both cemented andbiological techniques for the fixation of a prosthesis. Forexample, a hip replacement femoral component may be

fixated using cement, whereas the acetabular cup may befixated into the pelvic bone by osseointegration.

Failure of prosthesis fixation is observed as looseningand pain for the patient. If loosening occurs without infec-tion it is called aseptic loosening. Loosening is a multi-factorial process and does not have just one cause.Loosening of cemented fixation often occurs by fatiguefailure of the bone cement, but loosening can have severalroot causes: fatigue from pores in the cement and stressconcentrations at the implant/cement interface, debondingat the prosthesis/cement interface or cement/bone inter-face, or bone resorption causing stresses to rise in thecement. Loosening of biological fixation occurs if the rela-tive micromotion between the bone and the implant is toohigh to allow osseointegration, i.e., if the initial stability ofthe implant is insufficient. Huiskes (4) proposed the con-cept of failure scenarios as a method for better under-standing the multifactorial nature of aseptic loosening.The failure scenarios are

1. Damage accumulation failure scenario: the gradualcracking of bone cement, perhaps triggered by inter-face debonding, pores in the cement, or increasedstresses due to peripheral bone loss.

2. Particulate reaction failure scenario: wear particlesemanating from the articulating surfaces or frommetal/metal interfaces in modular prostheses (fret-ting wear) can migrate into the interfaces causingbone death (osteolysis).

3. Failed ingrowth failure scenario: High micromotionof the implant relative to the bone can prevent boneingrowth, as can large gaps (> 3 mm). If the area ofingrowth is insufficient, then the strength of thefixation will not be high enough to sustain loadingwhen weight-bearing commences.

4. Stress shielding failure scenario: Parts of the bonecan be ‘‘shielded’’ from the stresses they would nor-mally experience because of the rigidity of theimplant. This can lead to resorption of the boneand degeneration of the fixation.

5. Stress bypass failure scenario: In biological fixation,ingrowth can be patchy leading to stress transferover localized areas. When this happens, some bonetissue is ‘‘bypassed,’’ and in these regions, bone atro-phy can occur because the stress is low.

ORTHOPEDICS PROSTHESIS FIXATION FOR 193

Table 1. Components of Bone Cement and Their Roles

Components Role Amount

Liquid 20 mLMethyl methacrylate (monomer) Wetting PMMA particles 97.4 v/oN,N,-dimethyl-p-toluidine Polymerization accelerator 2.6 v/oHydroquinone Polymerization inhibitor 75 þ 15 ppm

Solid powder 40 gPolymethyl methacrylate Matrix material 15.0 w/oMethyl methacrylate-styrene-copolymer Matrix material 75.0 w/oBarium sulphate (BaSO4), USP Radiopacifying agent 10.0 w/oDibenzoyl peroxide Polymerization initiator 0.75 w/o

From Park (3).

Note: v/o: % by volume; w/o: % by weight.

6. Destructive wear failure scenario: In some jointreplacement prostheses, e.g., hip and knee, wearcan occur to such a degree that the component even-tually disintegrates.

CEMENTED FIXATION

It is common to classify cementing techniques according to‘‘generation’’: The first generation involved hand-mixingand finger packing of the cement, and the second genera-tion improved the procedure by using a cement gun andretrograde filling of the canal, with a bone-plug to containthe cement within the medullary canal. This allows pres-surization of the cement and therefore better interdigita-tion of the cement into the bone. Third generation (calledmodern cementing) uses, in addition, mechanical mixingtechniques for the cement to remove pores and pulsativelavage to clean the bone surface of debris. The most com-mon mechanical mixing technique is ‘‘vacuum mixing,’’where the powder and monomer are placed together in amixing tube and the air is removed under pressure; oftenthe tube can then be placed into an injection gun fromwhich it can be extruded into the bone cavity. Anothermechanical mixing technique is centrifugation (i.e., spin-ning the polymerizing bone cement in a machine), which isfound to remove pores and increase the fracture strength(3). Precoating the implant with a PMMA layer or additionof a roughened surface strengthens the implant/bonecement interface.

Fixation strength using bone cement relies on an inter-digitation of the bone cement with the bone; i.e., it is amechanical interlock between the bone and the solidifiedcement that maintains the strength and not a chemicalbond. Good interdigitation requires that the bone bed berough. Creating a rough surface is done by appropriatebroaching during preparation of the bony bed; it alsorequires lavage to clean the bed of loose debris and marrowtissue. Mann et al. (5) found the strength of the bonecement/bone interface to be positively correlated withthe degree of interdigitation (Fig. 1). To achieve superior

interdigitation, it was thought useful to develop ‘‘low visc-osity’’ cements, and although higher penetration wasachieved, the clinical outcomes using low viscosity cementsin hips were not superior (6).

PMMA bone cement undergoes an exothermic polymer-ization reaction. This means that heat is produced onpolymerization and this can cause necrosis of the surround-ing bone tissue. Another consequence of heating is that thecement expands and contracts on cooling. As solidificationoccurs before to full cooling, residual stresses are generatedin the cement (7). This is one reason to minimizethe thickness of the cement layer. Also, metallic stems,because they conduct heat, can minimize the peak tem-perature transmitted to the bone, cooling the metallicimplant before implantation has also been suggested.Bioactive cements have also been proposed; see the reviewby Harper (8). These cements have filler particles added tocreate a bioactive surface on the cement; fillers can behydroxyapatite powder or fibers, bone particles, or humangrowth hormone. Alternatives to PMMA are bisphenol-a-glycidyl methacrylate (BIS-GMA) or poly(ethylmethacry-late) (PEMA)/n-butylmethacrylate (nBMA) cement. How-ever, these cements are not yet widely used.

The mechanical strength depends on the brand ofcement used and on the mixing technique (9). To preventthe damage accumulation failure scenario (see above),sufficient fatigue strength is required. This has been mea-sured as a function of mixing technique (Fig. 2) (10). Beinga polymer operating close to its melting temperature, bonecement is also subject to creep, i.e., viscoplasticity, and thecreep strain as a function of stress has been measuredunder dynamic loading (11). However, it is clear that thein vitro testing conditions may not account for many of theextremely complex in vivo conditions, so these resultsshould be interpreted with caution (12).

OSSEOINTEGRATION (CEMENTLESS FIXATION)

There is no simple definition of osseointegration, althoughAlbrektsson (13) advocates the following: Osseointegrationmeans a relatively soft-tissue-free contact between implant


3

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1.5

1

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00 100 200 300 400 500 600 700

App

aren

t str

engt

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app

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Figure 1. Interdigitation of the bone cement into the boneincreases the strength of the bone cement interface. qint is theproduct of the average value of the thickness of the interdigitatedregion and the density of the interface region measured using a CTscan. See Mann et al. (5) for details.

30

25

20

15

10

5

00 10 102 103 104 105 106

Cycles to Failure (Nf)

Str

ess

(MP

a)

Hand-mixed

Hand-mixedVacuum-mixed

Vacuum-mixed

Figure 2. A comparison of the fatigue strength of hand-mixed andvacuum-mixed bone cement. After Murphy and Prendergast (10).

and bone, leading to a clinically stable implant. Early inthe study of the osseointegration concept, Skalak (14)found osseointegration was promoted by a micro-roughsurface more so than a smooth one. Since then, manyanimal experiments investigating the effect of plasmaspraying the surface and various methods of creating aporous surface have been reported. For orthopedic fixation,porous surfaces with beads in one or more layers have beenused, as have wire meshes attached to surfaces, andplasma spraying the surface with hydroxyapatite.

Figure 3 shows bone ingrowth into a multilayer of aproximal part of a femoral hip prosthesis (15). It can beobserved that ingrowth is patchy; this is what is commonlyfound, even with successful implants retrieved at autopsy(16); it is evident, therefore, that ingrowth is not requiredeverywhere on the prosthesis for a successful fixation.Ingrowth is controlled by a combination of the mechanicalenvironment and the size of the pores; the spacing betweenthe pores should not be greater than the degree of micro-motion or else the new bone ingrowth path will be con-tinuously sheared as the tissue attempts to grow in. Inexperiments in dogs, Søballe (17) studied the relationshipbetween implant coating and micromotion and found thathydroxyapatite coating increased the rate of boneingrowth, and that a relative motion between implantand bone of 150 mm allowed osseointegration, whereas arelative motion of 500 mm inhibited it. The mechanobiolo-

gical consequences of these different shearing magnitudeswas analyzed by Prendergast et al. (18). The depth of theporosity will also affect the strength, with multilayerbeaded surfaces having the potential for greater tensilestrength (1).

FIXATION OF PROSTHESIS DESIGNS

Each implant design has specialized fixation features. Inthe following sections, examples are provided of the fixa-tion approaches used in the main orthopedic implant cate-gories.

Hip Prostheses

Although hip arthroplasty may involve replacement of thefemoral side only, total hip arthroplasty (THA) involvesreplacement of the proximal femur and the acetabularsocket. Both cemented and cementless fixation is usedfor both the femoral component (the ‘‘stem’’) and the acet-abular component (the ‘‘cup’’). Selection is a matter ofsurgeon choice, although there is some agreement thatthe cementless fixation is preferable in younger patientsbecause cementless implants are easier to revise thancemented where complete removal of the cement mantlemay be problematic.

Considering the femoral side first, cemented fixationtakes two categories: stem designs in which a bond isencouraged between the stem and the cement (referredhere as bonded stems) and designs that discourage a bond(referred here as unbonded stems). Stem bonding can beachieved through roughening of the stem surface to createa mechanical interlock between the metallic stem orcement or through use of a PMMA precoat to create achemical bond between the precoat/cement interface.Bonded stems usually contain a collar that rests on thebone surface preventing subsidence and often containingridges, dimples, and undercoats to provide additional inter-lock with the cement. As long as the bonded stems remainbonded, they have the theoretical benefit of reducing thestress levels in the cement. However, if the bonded stemsfail, the roughened surface could generate debris particlesand initiate a loosening process. In contrast to the bondedstems, unbonded stems discourage a bond betweenthe stem and the cement through use of a smooth, polishedstem surface in combination with a stem design thattypically has no collar or macrofeatures to lock into thecement. With the lack of a bond, the polished stems facil-itate some stem subsidence within the cement mantle andthereby allow wedging of the implant within the medullarycanal. Lennon et al. (19) compared the damage accumula-tion around polished with matt stems and did not find adifference in the damage accumulated in their cementmantles. Another point of comparison between cementedand cementless fixation is that cementless stems will havea larger cross-sectional area than cemented stems becausethey must fill the medullary canal; this will make cement-less hip prostheses stiffer and predispose them to the stressshielding failure scenario. Recognizing this, it is usual forthe osseointegration surface to be on the proximal part ofcementless stems to ensure proximal load transfer;


Figure 3. Bone ingrowth into a multilayer of a proximal part of afemoral hip prosthesis. After Eldridge and Learmonth (15).

furthermore, patches of osseointegrative surface may belimited to the posterior and anterior faces of the stem.

Considering the acetabular side, the cup is either madefrom ultra-high-molecular-weight polyethylene (UHMWPE),ceramic, or metal. UHMWPE cups may be metal-backed.As the head can be either ceramic (a modular head can beconnected to a metal femoral component using a Morsetaper) or metal (modular or monobloc), this means thatseveral combinations of bearing materials are possible.Polyethylene cups and metal heads are the most common,but the others, such as metal-on-metal, are advocated aswell. The selection of bearing materials is important forthe fixation because a high frictional torque predisposesto loosening of the cup or stem and because the wearparticles produced can provoke the particulate reactionfailure scenario. Polyethylene cups are cemented intothe acetabulum using bone cement. Metal-backing of thecup is designed to decrease stresses in the polyethylene‘‘liner,’’ which should lead to lower wear rates although itis also predicted to increase stress concentrations in thefixation at the periphery of the cup (20). Metal, ceramic,and metal-backed UHMWPE cups may be threaded on theoutside so that they can be screwed into the acetabelum, orthey may be fixated by osseointegration.

The interrelationship between design factors and fixa-tion of hip implants is complicated and involves maximiz-ing strength of the cement/metal interface, the cementitself, and the bone/cement interface. According to thedesign philosophy of polished stems, it is better to safe-guard the vital bone/cement interface by allowing thecement/metal interface to fail first and facilitating subsi-dence (21). Not only should the interfaces have therequired strength, but the stresses should be minimizedto ensure the most durable fixation (22)— the measures toachieve this are listed in Table 2.

Knee Prostheses

Total knee replacement involves femoral and tibial compo-nents, and a component for patellar resurfacing (a patellar‘‘button’’) is also often used. Both cemented and cementless

fixation is used in knee arthroplasty. The femoral compo-nent may be fixated with an intramedullary stem that maybe cemented, or it may have a porous surface for osseointe-gration with medial and lateral ‘‘posts’’ to aid initial sta-bility. The tibial component consists of a metal ‘‘tray’’ and apolyethylene insert; the tray may also be fixated with anintramedullary stem cemented into the tibia, perhapsaccompanied by medical and lateral posts/pegs for rota-tional stability. Figure 4 shows a design fixated by osseo-integration (23). Walker (24) gives a thorough descriptionof the options available for knee prostheses.

Upper Extremity and IVD Prostheses

Upper extremity prostheses include the shoulder, elbow,and wrist (1). Total shoulder arthroplasty (TSA) consists ofa humeral component with an intermedullary stem and a


Table 2. Measures that Maximize Strength and Minimize Stress in Total Hip Replacement Structures

Cement/MetalInterface Cement Cement/Bone Interface

Maximizestrength

Grit-blasted metalPMMA-coated metal

Optimal preparationPressurizationCement restrictor

Careful reamingPressurizationMinimal polymerization heatMinimal monomerBone lavageMinimal wear debris

Minimize stress Reduce patient weightReduce patient activityAnatomical reconstruction of the femoral headMinimal frictionNo impingement or subluxationBonded cement/metal interfaceOptimal implant and cement mantle designOptimal implant materialOptimal (reproducible) placement

Adapted from Huiskes (22).

Figure 4. A knee replacement prosthesis showing porous-coatingfor osseointegration and posts for fixation. From Robinson (23).

glenoid component inserted into the glenoid cavity of thescapula. The glenoid component is either all-polyethylene;in which case, it is cemented; or metal-backed; in whichcase, it may be fixated by osseointegration. Glenoid com-ponents may have several pegs, or they may have onecentral ‘‘keel’’ for fixation (25). Elbow prostheses consistof humeral, ulnar, and radial components, all which may befixated with or without cement. Wrist prostheses replacesthe radial head and the schapoid and lunate bones of thewrist and may be cemented and uncemented (1). Inter-vertebral disk (IVD) prostheses replace the degenerateddisk with a polymer; there are several strategies for fixa-tion: The endplates may be porous coated and plasmasprayed for osseointegration to the cancellous bone withvertical fins to increase stability. IVD prostheses may alsobe fixed to adjacent vertebral bodies with screws (26).

EVALUATION OF FIXATION AND FUTURE STRATEGIES

One of the key issues in orthopedic implant fixation iswhether to use cemented fixation or biological fixation,with surgeons on both sides of the debate (16,27). Cemen-ted fixation has the advantage of immediate postoperative,stability whereas concerns may be raised about the relia-bility of bone cement’s fatigue strength; furthermore,there is a school of thought that the exothermic polymer-ization reaction should be avoided if at all possible. Bio-logical fixation by osseointegration has the advantage ofavoiding the use of the PMMA cement but runs the risk ofthe failed ingrowth failure scenario; furthermore,immediate postoperative weight-bearing is not possible.Finally cementless implants are easier to revise ifthey fail.

Another key issue in orthopedic implant fixation is thatof preclinical testing and regulatory approval of new fixa-tion technologies. Considerable challenges exist in achiev-ing consensus around regulatory tests that safeguardpatients against ineffective devices while still allowinginnovation (4). Preclinical tests can use either (1) finiteelement models of the direct postoperative situation, e.g.,for the hip (28), knee (29), or shoulder (25), or computersimulations of a failure scenario, e.g., damage accumula-tion (30); (2) physical model ‘‘bench’’ testing with simula-tors (24,31); or (3) animal testing. Animal testing is notideal for testing the biomechanical efficacy of orthopedicimplant fixation because the implant geometry must bemodified to fit the animal skeleton. Furthermore, an impor-tant emerging concept is that of patient-specific implantsbased on computational analysis of a patient’s medicalimages (32).

One useful clinical method to assess implant fixation isthrough the use of radiostereometric analysis (RSA). Withthis approach, the migration of the implant relative to thebone can be determined and is used to determine designsthat may be at risk of early loosening. Retrospective andprospective clinical studies are also very useful to deter-mine designs or materials that have promising or poorclinical results. On a larger scale, implant registries per-formed in many countries in Western Europe can provideinformation on how designs, materials, and surgical tech-

niques rank in terms of risk of failure. All of these clinicaltools can aid in understanding the role of implant fixationin success of joint replacements.

A final issue is the degree to which broader technologicalinnovations in surgery and medicine will affect ortho-pedics. For example, minimally invasive therapy (33)requires special implants and associated instrumentation.Tissue-engineering and regenerative medicine also has thepotential to change the nature of orthopedics, not only byreducing the need for joint arthroplasty implants but byintegrating tissue engineering concepts with conventionalimplant technologies, for example, cell seeding intoimplant surfaces to promote biological fixation.

ACKNOWLEDGMENTS

Research funded by the Programme for Research in Third-Level Institutions, administered by the Higher EducationAuthority. Dr. A. B. Lennon and Ms. S. Brown are thankedfor their comments.

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See also BIOCOMPATIBILITY OF MATERIALS; BONE AND TEETH, PROPERTIES

OF; BONE CEMENT, ACRYLIC; HIP JOINTS, ARTIFICIAL; MATERIALS AND DESIGN

FOR ORTHOPEDIC DEVICES.

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orthopaedic fixation

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