the development of artificial wrist joint replacements_final

Literature Research Project December 2008

THE DEVELOPMENT OF ARTIFICIAL WRIST JOINT

REPLACEMENTS

Imperial College London

Mechanical Engineering Department

Year 3

Ambrose Tey

Supervisor: Professor Andrew Amis

Associate supervisor: Dr. Ulrich Hansen


The Development of Artificial Wrist Joint Replacements | Abstract 2

ABSTRACT

The wrist joint is extremely complex and there remain many problems with current wrist joint

prostheses. Consequently, artificial wrist joint replacements have not achieved similar success

compared to knee or hip joint replacements. This paper gives an overview of the development of

wrist joint replacements including the problems and issues encountered with and aims to examine

possible alternatives or solutions to address these issues. It was concluded that the development of

artificial wrist joint replacements will continue to depend on future research and there remains

much scope for improvement.


The Development of Artificial Wrist Joint Replacements | <Table of Contents 3

TABLE OF CONTENTS

Abstract ........................................................................................................................................................... 2

Table of Contents ............................................................................................................................................ 3

1. Introduction ............................................................................................................................................. 4

2. The Wrist .................................................................................................................................................. 5

2.1 Anatomy ......................................................................................................................................... 5

2.2 Biomechanics .................................................................................................................................. 6

2.2.1 Motion ........................................................................................................................................ 6

2.2.2 Forces ......................................................................................................................................... 7

3. Artificial Wrist Joint Replacements ........................................................................................................... 7

3.1 Swanson ......................................................................................................................................... 7

3.2 Meuli ............................................................................................................................................... 8

3.3 Volz ................................................................................................................................................. 9

3.4 Universal Total Wrist System ........................................................................................................ 10

3.5 Biaxial ........................................................................................................................................... 11

3.6 Trispherical ................................................................................................................................... 11

4. Design Requirements ............................................................................................................................. 12

4.1 Design Objectives ......................................................................................................................... 13

4.2 Mobility ......................................................................................................................................... 13

4.3 Materials ....................................................................................................................................... 14

4.4 Fixation ......................................................................................................................................... 14

5. Discussion .............................................................................................................................................. 15

5.1 Future Design Requirements ......................................................................................................... 15

5.1.1 Design Objectives ..................................................................................................................... 15

5.1.2 Mobility..................................................................................................................................... 16

5.1.3 Materials ................................................................................................................................... 17

5.1.4 Fixation ..................................................................................................................................... 18

5.2 Future Artificial Wrist Joint Replacements ..................................................................................... 18

5.2.1 Design Components .................................................................................................................. 20

5.2.2 Advantages ............................................................................................................................... 21

5.2.3 Proposed Modification* ............................................................................................................ 21

6. Conclusion ............................................................................................................................................. 22

7. Acknowledgements ............................................................................................................................... 23

8. References ............................................................................................................................................. 23


The Development of Artificial Wrist Joint Replacements | Introduction 4

1. INTRODUCTION

The wrist is an essential joint of the upper extremity and plays a significant role in maintaining a

normal daily life. Unlike the hip and knee, the wrist is one of the most complex joint of the body and

the kinetics and kinematics of the wrist has yet been thoroughly understood. Normal wrist motion

is achieved through complex intercarpal articulations involving the ligaments as well as the carpal,

radius and ulna bones. Wrist joint disorders, rheumatoid arthritis in particular, often result in

deformity, severe pain and ultimately loss of proper wrist function which introduces a substantial

degree of disability and renders the sufferer incapable of accomplishing many standard activities.

Today there exist two primary solutions to wrist joint disorders: arthrodesis and arthroplasty.

Arthrodesis remains the primary solution to painful wrists as recommended by doctors and

orthopedists alike. It however removes all functional motion of the wrist. Arthroplasty on the other

hand aims at preserving this motion while removing pain or any other problems associated with the

wrist joint. A recent review of total wrist arthroplasty compared with total wrist arthrodesis (1)

concluded that ‘although arthroplasty may be a more appealing treatment... function may not be

significantly better than for arthrodesis, and current evidence does not support the widespread

implementation of this procedure.’

The follow sections will give an overview of the human wrist and current developments in wrist joint arthroplasty. Different aspects of current prostheses will then be critically examined to evaluate

possible solutions and the future of wrist joint replacements.


The Development of Artificial Wrist Joint Replacements | The Wrist 5

2. THE WRIST

2.1 ANATOMY

The wrist joint, as shown in Figure 1, comprises of the eight carpal bones, the ulna and the radius. The carpal bones are separated into two rows, namely the proximal and distal. The proximal row is

made up of the pisiform, scaphoid, triquetrum and lunate. The distal row is formed by the

trapezoid, hamate, trapezium and capitate.

Figure 1: Bony anatomy of the wrist, also showing the main movements of the wrist:

flexion/extension, radial/ulnar deviation.(2)

The wrist joint can be divided into three different parts, the radiocarpal joint, intercarpal joint and

the distal radioulnar joint. Most movement of the wrist occurs at the radiocarpal joint, which is a

synovial articulation composed by the distal end of the radius and the schaphoid, lunate and triquetrum bones.(3) The triangular fibro-cartilage complex connects the distal end of the ulna to

the proximal carpal bones.

The radius, ulna and carpal bones are stabilised by numerous ligaments and tendons. Ligaments are

tissue structures that connect bones to bones and the twenty eight ligaments surrounding the

carpal bones combine to form a joint capsule, which is a watertight sac containing lubricating fluid

called synovial fluid. Tendons on the other hand connect muscles to bone. The flexor and extensor

tendons travel across the wrist anteriorly and dorsally respectively (2,4-6). Wrist joint disorders such

as rheumatoid arthritis are attributed to the damage of these supporting structures, often causing

pain, stiffness and deformation of the joint.

This complex interaction between the radius, ulna, carpal bones and the stabilising structures is

crucial to the proper motion control of the thumb, fingers and wrist. Although there have been

extensive research into the anatomy of the wrist and its separate structures, the biomechanics of

the wrist has yet been thoroughly understood and remains one of the main problem in the

mechanical replication of the wrist. This will be assessed in the following section.


The Development of Artificial Wrist Joint Replacements | The Wrist 6

2.2 BIOMECHANICS

To successfully model and design a wrist joint replacement, it is important to first acquire a sound

understanding of the biomechanics of the human wrist joint. Ideally, prostheses should be able to

attain normal human wrist motion and approximate its functions as closely as possible. The stress

distribution and force transmission of prostheses should also be accurately analysed and improved

through experimentation before its implementation. Several studies (7-9) have examined cases of

prosthesis failure and suggest that metacarpal perforation, fracture, joint loosening and dislocation

were the main modes of failure. It is therefore essential to anticipate and reduce these failures in

prosthesis design.

2.2.1 MOTION

It has been universally accepted that the wrist has two primary range of motion. The first degree of

freedom involves flexion and extension, while the second degree of freedom is the radial and ulnar deviation of the wrist. Rotation of the first two degrees of freedom occurs in the head of the

capitates. There exists a third degree of freedom that governs a minute amount of wrist rotation

between the radius and the carpal bones. It should also be noted that these movements altogether

combine to perform other modes of motion such as circumduction motion, which ‘is an elliptical

motion in which the hand starts in radial deviation, moves down into flexion, over into ulnar

deviation and up into extension’ (3). Figure 2 further illustrates these three degrees of freedom.

Figure 2: Palmar view of the right wrist showing anatomical directions and rotations.(10)

It was found that the range of motion for flexion-extension movement is a total of 151°, with 75° for

extension and 76° for flexion. Additionally, the average wrist deviates 36° and 22° for ulnar and

radial deviation respectively, adding up to a total of 58° for radial-ulnar deviation (11). Also, simple

daily activities can still be accomplished within a smaller range of motion. Shepherd et al (3)

investigated the functional range of wrist movements examined by several authors proving that daily

activities could be achieved with as little as 6° of extension, 5° of flexion, 6° of ulnar and 7° of radial

deviation.


The Development of Artificial Wrist Joint Replacements | Artificial Wrist Joint Replacements 7

2.2.2 FORCES

The wrist joint is subjected to many different sources of stress such as ligament forces, viscoelastic

forces of soft tissues, muscle forces and tendon forces. These forces, both tensile and compressive,

help to dynamically balance the wrist joint, especially with external loading. In essence, the joint

acts to transmit forces from the hand to the arm and on to the body through the shoulder. Hence,

careful considerations have to be given when determining the materials of different parts of the

replacement such that it will be able to withstand the tensile and compressive forces experienced

by the joint. Ensuring adequate tensile or compressive strength of the prosthesis-bone interface as

well as the prosthesis itself to avoid fracture failure will greatly increase its reliability and durability.

A variety of studies have been done in the past three and a half decades to evaluate the forces

acting through the wrist. Maximum forces along the muscles in the wrist are known to exceed 500N

in strenuous activities (5,6,12). However, much lesser forces pass through the wrist under normal

conditions. An average of 200N of force is sufficient to perform ordinary daily activities (13). Several

other studies also confirm that forces are well below 500N during normal conditions and that these

forces vary between 118N and 143N (14-18).

3. ARTIFICIAL WRIST JOINT REPLACEMENTS

There are various designs of artificial wrist joint replacements with different materials, each having

its own benefits and disadvantages in relation to the others. Current wrist joint replacements are

still unable to replicate the full physiological motion and performance of normal wrists and have not

been very successful due to a high revision rate. This section will review different past and

contemporary prosthesis designs to evaluate the associate issues and problems after implementation.

3.1 SWANSON

Swanson (19) designed the first generation of artificial wrist joint replacements. It is currently

manufactured by Wright Medical Technology and the Swanson II wrist joint implant is shown in Figure 3. It is a one-piece intramedullary stemmed implant fabricated from flexible silicone

elastomer. The proximal stem extends into the intramedullary canal of the radius while the distal

stem is directed through the capitate and into the third metacarpal. Grommets are also used

together with the implant in certain cases to protect the silicone elastomer from shearing or tearing

due to sharp bone edges. It acts as a thin shield to protect the implant from contiguous bones and is

made of unalloyed titanium for surgical application. The purpose of the Swanson implant is to

support the development of a new capsuloligamentous system by maintaining an adequate joint

space (between the carpal bones and the radius) and alignment, which is crucial to the restoration

of wrist motion.

Although the Swanson implant is often referred to as the ‘gold standard’ in wrist joint arthroplasty,

it has a few problems and disadvantages due to the very nature of its design and material. Firstly,

fracture of the implant is a common cause of failure in the Swanson implant, resulting in the need

for revision surgery. It was designed as a one-piece implant without bearings or any articulating

parts such that the implant is subjected to three degrees of motion in the central area. For this

reason, any excessive motion or over-activity of the wrist will ultimately lead to fracturing of the

silicone elastomer. Secondly, particle formation may be caused by wearing of the implant with

abrasion against bony surfaces. The grommets only protect the silicone elastomer at the root of

the proximal and distal stem and do not extend to the entire length. Thence it only prevents wear at crucial areas of the implant and abrasion is still inevitably present. These wear particles could



potentially lead to silicone synovitis, which is an immune response to the silicone debris leading to

inflammation of the synovial membrane. If untreated, silicone synovitis will cause pain, swelling and

loss of motion of the wrist, which is a major disadvantage for this implant. After all, the main

purpose of artificial wrist joint replacements is to restore functional motion of the wrist.

Figure 3: Swanson II wrist joint implant with grommets. (20)

3.2 MEULI

Meuli developed the first version of this wrist implant in 1971 after Swanson. It has gone through

several revisions since then and this section will examine the third and latest version, MWP III,

illustrated in Figure 4. The general design and objectives of the implant are unchanged despite

drastic improvements to its material and functionality.

Figure 4: Meuli MWP III wrist implant. (21)(3)

The Meuli implant is a ball joint and consists of 3 components, the metacarpal, radial component

and spherical head. The ball joint allows for all three degrees of motion and also slight translations. Both the radial and metacarpal stems are made from malleable titanium alloy with a composition of

Ti-6Al-7Nb and consist of two prongs to improve stability. The radial prongs fit into the radius while

Metacarpal Component

Radial Component

Spherical Radial Head

Distal Stem

Grommet Proximal Stem



the metacarpal prongs fit into the second and third metacarpals. They are fixed within the

medullary canals with bone cement. The spherical head is fitted onto the radial component and is

made of high molecular weight polyethylene with titanium nitride coating to increase wear

resistance. The cup of the metacarpal component complements the shape of the spherical head and

articulation between the two serve to imitate normal wrist motion. (21)

Unlike the Swanson implant, this implant was designed upon a ball joint and hence eliminating the

problem of fracturing at the central area. Problems with this implant are mainly associated with the

metacarpal component, particularly metacarpal perforation and loosening of the stem.

3.3 VOLZ

Volz (22) paid particular attention to cases of severe bone deformation in most patients seeking

wrist joint arthroplasty and developed an implant to address this issue. He suggests that:

‘The design of a total wrist prosthesis based upon the premise that the implant would merely

provide for a resurfacing of defective interfaces between the radius and proximal carpal row

was viewed as unsatisfactory. In many disease processes, especially those of inflammatory

nature, such as rheumatoid arthritis, diffuse destructive changes to the carpus are observed...

a wrist prosthesis should allow for usage when extremes of deformity in far-advanced disease

prevail. Not only should the implant replace such destroyed articulations, but it should also permit only those planes of motion which are normally seen at the radial carpal complex.’

As such, the implant was designed with an interface possessing a ‘hemispherical configuration with

2 different radii (a torroidal sector)’ (22). Only two degrees of freedom were intended, 90° of flexion

and extension and 50° of radial-ulnar deviation with negligible amount of rotation. The Volz implant

is shown in Figure 5 and consists of a carpal and radial component.

Figure 5: Volz wrist joint prosthesis. (23)

Both the carpal and radial components are made of cobalt chrome alloy and are designed for intramedullary stem fixation. The metacarpal component is similar to the Meuli implant and fits into

the medullary canal of the second and thrid metacarpals while the radial component is seated

within the medullary canal of the distal radius. It is however secured to the carpus and radius with

methylmethacrylate cement (24). In addition, the concave surface of radial component is made of

polyethylene as it articulates against the hemispherical surface of the metacarpal component.

Metacarpal

component

Radial component

Polyethylene

bearing



As cited by Sheperd (3), the main reported causes of failure were perforation and loosening of the

metacarpal component. Dislocation of the prosthesis was also reported. Volz (22) reported that the

most common postoperative problem with the prosthesis was ulnar-deviation of the wrist.

3.4 UNIVERSAL TOTAL WRIST SYSTEM

The Universal Total Wrist System was first developed Menon and the current version, Universal2

Total Wrist System, is now produced by Kinetikos Medical Inc (KMI). Illustrated in Figure 6, the

carpal plate is made of titanium alloy and consists of a fixed central peg and two variable angle

screws to enhance stability through intercarpal fusion. The carpal plate is fixed onto an ellipse-shaped polyethylene carpal component which articulates against the concave surface of the radial

component. The radial component is contoured to complement the normal distal radius anatomy

and is made from cobalt chrome alloy. Beaded porous coating is applied on both the carpal plate

and radial stem to assist osteointegration. Furthermore, the radial component and the central peg

are fixed, using bone cement, to the radius and carpal bones respectively. (25)

Figure 6: Components of the Universal2 Total Wrist System.

This implant was designed to improve and rectify the problems of other earlier generation of

implants. The variable screws helped increase stability and correct the problem of metacarpal perforation and loosening, which were common in other implants. The presence of volar offset in

both radial and carpal component significantly improved joint stability and wrist extension.

Articulation of the ellipse-shaped interface between the carpal and radial component also allowed a

functional range of motion with high stability.

According to Menon (26), dislocation with loosening of the radial component was the most

common complication of this implant. It also has a relatively smaller functional range of motion, to

improve stability, compared to other implants. As expected, with the use of variable screws and

volar offset, there were no reported problems with the carpal component. The Universal2 Total

Wrist System implant can be considered one of the safest implant currently available with a relatively low revision rate.

Variable angle

screws

Polyethylene Carpal

Component

Beaded porous

coating on radial

stem and carpal plate



3.5 BIAXIAL

The biaxial prosthesis, displayed in Figure 7 and Figure 8, consists of a carpal component with an

ellipsoidal head that articulates against the polyethylene concave surface of the radial component.

The articulating interface was ellipsoidal shaped to reproduce a more physiologic type of motion

similar to the normal wrist. The carpal component and the stem of the radial component are made

of cobalt chrome alloy. The long stem of the carpal component fits into the medullary canal of the

third metacarpal while the radial stem is inserted into the radius, both using bone cement. Both

stems also have porous-coated surfaces to assist osteointegration. There is also a small stud on the

carpal component designed to fit into the trapezoid bone to enhance fixation and stability as well as to stabilise rotation. (27)

As with all other implants which consist of a single stem that fits into the medullary canal of the

third metacarpal, the problems with the biaxial total wrist implant are dislocation, loosening (27,28)

and metacarpal perforation of the carpal stem (29,30).

3.6 TRISPHERICAL

The Trispherical wrist prosthesis (Figure 9 and 10) operates similarly to a hinge mechanism and the

radial and carpal components are made from titanium alloy. The carpal component of the

prosthesis consists of a long stem that fits into the third metacarpal and a shorter stem for the

second metacarpal. The radial component fits into the radius and bone cement is used for fixation

of both components. Articulation of the implant occurs between a spherical head and a

polyethylene bearing, which are pinned together to create an axle constraint to prevent dislocation of the implant. Unlike other implants, the bearing surface is attached to the carpal component while

the spherical head is fitted onto the radial component.

Figure 7: Radiograph of the biaxial

total wrist implant. (28)

Carpal

component

Polyethylene

surface

Radial component

Porous-coated

surfaces

Figure 8: Components of the biaxial total wrist

implant.


The Development of Artificial Wrist Joint Replacements | Design Requirements 12

The main advantage of this Trispherical implant is that it prevents dislocation between the radial

and carpal components. However, with the implementation of the pinned joint, free articulation of

the implant is restricted. This restriction could potentially induce additional stresses in the wrist

joint, especially in the stem-bone cement interface. Reported failures include loosening of the carpal component and metacarpal perforation (7,31). O’Flynn et al (32) also reported a case with

failure of the hinge mechanism.

4. DESIGN REQUIREMENTS

Various aspects of the natural wrist joint have to be taken into consideration when designing an artificial wrist joint replacement. Additionally, the physical and clinical conditions of each patient

undergoing arthroplasty are different. Such differences include bone sizes, cause of disease,

severity of bone deformation and destroyed supporting tissues, previous injuries, medical allergies,

physical activities of the patient and much more. Hence, it is impossible for there to be a one-size-

fits-all wrist joint prosthesis. Even current designs consist of different sizes and allow customization

to a certain extent so as to accommodate different circumstances and needs of the patient.

However, engineering a unique design for each case will be too expensive and a compromise has to

be reached between the adaptability and affordability of the implant. Ideally, it will be best for an

implant to be designed in such a way that its dimensions and materials or even the mechanism in which it works can be easily changed according to different requirements of the patient. For

example, the design should be able to accommodate the requirements of a pianist accordingly with

an implant of a larger radial-ulnar deviation at the cost of a reduced load capacity or a smaller

flexion-extension range of the wrist.

This section will seek to determine the important objectives of current artificial wrist joint

replacements and further translate these to define various different engineering aspects of these

replacements.

Figure 9: Radiograph of a well-placed

Trispherical implant. (31)

Carpal component

Radial component

Figure 10: Schematic of the Trispherical

wrist implant. (3)



4.1 DESIGN OBJECTIVES

Contemporary artificial wrist joint replacements, as seen in Section 3, were designed with similar

objectives despite their differences. There are a number of main design objectives to be met:

Relief of pain

Sufferers of wrist joint disorders usually seek medical help due to severe pain of the wrist. This is the

most important objective of wrist joint arthroplasty and hence, an implant should provide a pain-

free solution to the patient.

Functional range of movement

The implant should provide a functional range of movement of the wrist joint and it should be

approximated as closely as possible to the characteristics of the range of movement in normal wrist.

After all, the purpose of arthroplasty is to replicate a normal wrist joint and allow the patient to

perform daily activities.

Strength

The wrist joint is constantly subjected to different forces, both tension and compression, in different

directions. The implant should be able to withstand these forces without fail. However, the tissues

and bones of the affected wrist have been severely damaged, which greatly reduces its strength. The implant should therefore be strong enough to sustain daily activities while prohibiting

overloading and overactivity of the wrist joint.

Sustainability

The implant should allow the patient to return to normal daily life and therefore it should be able to

function within the wrist for a long time. The materials of each component must not cause any

adverse effect to the wrist or the general well-being of the patient. Many different chemicals run

through the body system and the material of choice should be inert to these chemicals. Corrosion or

any chemical reaction must be prevented. They should be able to function harmoniously with the body and immune system. Failure to consider this aspect of designing the implant system will lead

to problems such as silicone synovitis met by the Swanson implant.

Stability

The implant should be stable in all circumstances and be able to function in situ within the wrist.

This means that the positioning of the implant should be stable and undisturbed. Fixation methods

must be carefully chosen and considered to ensure stability. In addition, the design of the implant

must not potentially harm other structures of the wrist. For example, the shape and length of the

carpal stems should not create unnecessary abrasion or deterioration of the metacarpal or carpal

bones.

These objectives help determine different aspects of engineering a wrist joint prosthesis. Section

4.2 to 4.4 will discuss how current implants are designed to meet the objectives above.

4.2 MOBILITY

As mentioned in Section 2.2.1, the normal range of healthy wrists are 75° of extension, 76° of

flexion, 36° of ulnar deviation and 22° of radial deviation. Current implants are still unable to

successfully replicate the normal range of the wrist and they differ from one another. Table 1

summarises and compares the range of motion of various implants after arthroplasty as reported by

several authors.



The major factor which determines the range of motion is the articulation mechanism of an implant.

This mechanism can generally be categorised as constraint or non-constraint. The Swanson and

Trispherical implants are constrained while the Meuli, Volz, Universal and Biaxial implants are not

constrained. In non-constraint implants, the geometry of the articulating surfaces largely affects the

range of motion. There is a trade off between range of motion and the susceptibility of the implant

to dislocation; the larger the range is, the more susceptible the implant is to dislocation of the joint.

Hence, careful considerations of this relationship have to be made in future designs.

Table 1: Range of motion of normal wrist and various wrist implants.

Flexion (°) Extension (°)

Radial

deviation (°)

Ulnar

deviation (°)

References

Normal wrist 76 75 22 36 (11)

Swanson 39 6 -2 21 (33)

Meuli 30 40 10 10 (34)

Volz 32 17 2 23 (35)

Universal 41 36 7 13 (26)

Biaxial 29 36 10 20 (27)

Trispherical 50 Total (Flex + Ext) 10 10 (31)

4.3 MATERIALS

The material of each component in wrist implants is crucial and determines the durability, function

and feasibility of the component. With the exception of the Swanson implant, all other implants are

generally divided into four different parts, each with its own material composition. These four parts

are the radial stem, the carpal stem(s) and the articulating surfaces of the radial and carpal

component. In addition, porous coating is also applied on a few implants.

Cobalt chrome alloy and titanium alloy have been used for the radial and carpal stems in most

implants. Swanson used silicone elastomer, which proved to be unfeasible with the problem of

silicone synovitis. The use of cobalt chrome alloy and titanium alloy has shown successful clinical results and the body system does not has any adverse reaction to these material.

Material selection for the articulating surfaces has to be considered for both the radial and carpal

part. All of the implants, except Swanson, employ a metal-on-polymer articulation. Furthermore,

the Meuli implant has a polyethylene spherical head with titanium nitride coating to improve wear

resistance. The use of a metallic-polyethylene interface proves to be successful and there is no

report on the failure of implants due to this combination of materials, further confirming its

suitability for wrist joint implants.

4.4 FIXATION

There are 2 fixation methods have been used in current implants, specifically bone cement and

titanium screws. The Swanson implant does not employ a fixation method as the radial and carpal

stems are not fixed but instead allowed to move within the medullary canals.

The most reported causes of implant failure are loosening and metacarpal perforation, showing that

the use of bone cement is not very suitable for the fixation of wrist implants. This is probably

because the strength and stiffness of the bone structure are reduced in some wrist disorders due to

osteoporosis. This deterioration of the bone is common in severe rheumatoid arthritis (36).


The Development of Artificial Wrist Joint Replacements | Discussion 15

Therefore the use of bone cement is most likely incompatible for patients with reduced bone

strength as it creates a rigid fixation of the implant to the weak radial and carpal/metacarpal bones.

On the other hand, fixation of implants with variable titanium alloy screws, such as the Universal

Total wrist system, has shown much better results in comparison to bone cement and it has been used in patients suffering from advance rheumatoid arthritis where the results suggests that the

titanium screws remained well integrated into the bone (37).

5. DISCUSSION

The development of artificial wrist joint replacements in the past three and a half decade has been relatively slow and less successful compared to other joint replacements such as the hip and knee. A

number of reasons contribute to its slow development. Firstly, research into the mechanics of

healthy wrist joint as well as its implants has not been as extensive due to low levels of demand.

Secondly, the wrist joint is small and its working mechanism is much more complicated than other

joints as its movement involves complex interaction between 10 different bones, more than any

other joints in the body. Lastly, unlike other joints such as the hip and elbow, the natural movement

of the wrist does not function similarly to a hinge or ball joint. The geometry of the elliptical

articular surface of the radio-carpal joint is difficult to replicate. Hence it is almost impossible for

implants to copy this articulation geometry exactly and alternative methods have to be used.

It may be advantageous for future implant designs and design requirements to undertake a

different approach towards the problem of total wrist arthroplasty. The benefits and disadvantages

of several implants have been evaluated in Sections 3 and 4, and this information will be useful in

the development and evaluation of new designs and concepts.

5.1 FUTURE DESIGN REQUIREMENTS

Current wrist implants have been able to meet various design criteria despite its slow development.

With the advancement in technology, demands for implants with improved performance will

inevitably increase. Coupled with the fact that there remains much room for research and

development, it will be futile to take on a new approach towards designing future wrist implants.

5.1.1 DESIGN OBJECTIVES

Design objectives of future implants are similar to previous implants in some ways but not all. An

implant with the potential of meeting these objectives will be able to provide higher performance

and satisfaction to the patient.

Relief of pain

As with all other implants, future implants should still provide a pain-free solution to the patient. Relief of pain remains the most important objective.

Functional range of movement

Future implants should be able to provide a spectrum of functional range of movement depending

on the patient’s requirements. There is a trade off between the range of movement and other

qualities, such as stability and strength. Future implants should therefore satisfy the patient’s needs

at an optimum level and yet remain stable and strong enough to prevent failure.

Strength



Depending on each patient’s circumstances, the implant should be sufficiently strong. For example,

wrist joints in males will generally be subjected to larger forces compared to females. A technician

will require a stronger wrist joint than an accountant.

In addition, the medical condition of every patient requiring wrist joint arthroplasty differs from others, which will determine the amount of forces the wrist joint is able to withstand. For example,

osteoarthritic patients have stronger bones compared to patients with rheumatoid arthritis, which

will greatly affect the compatibility of different implant designs and material strength (38).

Sustainability

Future implants should still be able to function within the wrist for a long time. The materials of

each component must not cause any adverse effect to the wrist or the general well-being of the

patient. Many different chemicals run through the body system and the material of choice should

be inert to these chemicals. Corrosion or any chemical reaction must be prevented. They should be

able to function harmoniously with the body and immune system.

Stability

Future implants should be stable to a certain extent and be able to function within the wrist in situ.

The stability of the implant should be able to be modified according to the circumstances of each

patient. As seen in Section 4, a rigid fixation of the implant to the bone might not be entirely

advantageous and implant failures due to this have arisen in several cases. An optimum index of

stability should be determined for each patient and the implant should be modified accordingly to

satisfy this requirement.

Similarly, the implant design must not potentially harm other structures of the wrist. For example, the shape and length of the carpal stems should not create unnecessary abrasion or deterioration of

the metacarpal or carpal bones.

5.1.2 MOBILITY

As described in the previous section, future implants should be designed to accommodate different

needs and provide a suitable range of motion to the patient. After diagnosis and testing of the

diseased wrist, the maximum limit of wrist motion can be evaluated and the implant can be

customized accordingly.

With knowledge of the maximum limit of motion for the artificial wrist joint, the optimum working

mechanism and articulation geometry can then be decided. This will not only help prevent implant

failure due to excessive wrist motion but also improve the compatibility of the design to each

particular patient.

Table 2 below illustrates normal and functional wrist motions. Several studies were done to

evaluate the minimum wrist motion required to perform simple daily activities and have yielded

various results. As mentioned previously, few extensive researches have been done in the sphere of

total wrist arthroplasty and these studies employ different methods to evaluate the functional

range of motion. Therefore, further research needs to be done to confirm the minimum range of

wrist motion to accomplish simple daily activities.

Table 1 shows that current implant designs have allowed patients to regain wrist motion of

approximately 30° to 40° flexion/extension and 10° to 20° radial/ulnar deviation. Future implants

should therefore aim to provide a maximum range that is equal to normal healthy wrists and a minimum functional range to accomplish basic tasks. Based on previous studies, the maximum



functional range of motion amongst these studies was 40° flexion/extension, 12° radial and 28°

ulnar deviation. According to these values, future implants should then be able to provide a

spectrum of motion of 40°-76° flexion, 40°-75° extension, 12°-22° radial and 28°-36° ulnar deviation.

In other words, future implants should be able to accommodate a maximum range of motion of at

least 40° flexion/extension, 12° radial and 28° ulnar deviation. On the other hand, although current

implants have not been able to achieve these values, reports have shown that the patients were

able to successfully complete basic tasks. Therefore, it is very likely that the functional range is

actually lower than these values. Future implants should then be designed to accommodate a lower

functional range.

By designing future implants that can incorporate various different joint mechanisms

interchangeably, it will be possible for a single design to accommodate a variety of range of motion

and the optimum joint mechanism can be chosen according to the requirements.

Table 2: A comparison of normal and functional wrist motions.

Flexion (°) Extension (°)

Radial

deviation (°)

Ulnar

deviation (°)

References

Normal range 76 75 22 36 (11)

Fu

nct

ion

al

ran

ge

40 40 12 28 (39)

10 35 - - (40)

5 30 10 15 (41)

5 6 7 6 (42)

Future implants 40-76 40-75 12-22 28-36 -

5.1.3 MATERIALS

Radial and Carpal Stems

Although current implants consist mostly of metallic stems with many desirable properties, there

could be other materials that are more suitable for future implants. Metallic alloys such as cobalt

chrome and titanium alloys have higher density weight, hardness and strength compared to human

bones. It may however be more advantageous for the stems to be made of materials with physical

and properties that are similar to human bones, especially the Young’s Modulus of Elasticity, yield

strength and hardness. Such materials could be biocompatible polymers or even composites.

Furthermore, rheumatoid bones have various physical properties that are different from healthy

bones and such properties can be estimated through density scans. If future implants are to be made from materials with similar properties to the bone, they will have to be designed to be

manufactured from different choices of materials with varying properties. For example, the radial

stem of an implant for a patient with rheumatoid arthritis will be manufactured from softer

materials compared to a patient with osteoarthritis. It can easily be done with metallic alloys by

altering the alloy composition to achieve different ductility and strength.

Lastly, studies have shown that silicone polyurethane possesses many desirable properties for

artificial wrist implants. It has a relatively high biocompatibility, durability and strength but further

research still needs to be done before it can be used for future implants (43,44).

Articulating surfaces

Although there have been no known problems with metal-on-polymer interface, there exist other

options of materials combinations such as, metal-on-ceramic, metal-on-metal, ceramic-on-ceramic



and polymer-on-ceramic. Some of these material combinations have also been used for hip joint

replacements with promising results (45).

In addition, current wrist implants have not been very successful and hence a relatively large

percentage of patients undergo revision surgery or eventually arthrodesis. Hence, these implants were not used for a sufficiently long time for wear debris to accumulate and cause any problems. If

the revision rate of future implants is reduced and the implants can be used for a much longer time,

problems due wear debris may arise and articulating surfaces with improved mechanical properties

will have to be considered.

5.1.4 FIXATION

As mentioned in section 4.4, studies have suggested that titanium alloy screws are a better option

compared to bone cement. The use of variable screws in the Universal implant has produced

satisfactory results and future implants should continue to use this method of fixation or one with a

similar concept. A major advantage in using variable screws is that they provide sufficient support

to the implant but does not induce additional pressure to the bones. This concept should be the

basic guideline in designing future implants to provide support as and when it is needed.

5.2 FUTURE ARTIFICIAL WRIST JOINT REPLACEMENTS

Recently, a new design concept was proposed by Johnstone and Shepherd (2) for total wrist

arthroplasty to overcome the disadvantages of other artificial wrist joint implants. It is a

combination of the articulating mechanism of a constraint implant and a non-constraint implant.

The implant is illustrated in Figure 11 and consists of four different components, a radial

component, carpal component, a plate and a flexible part.



Figure 11: Components of the new design: (a) radial part; (b) carpal part, (c) plate; (d) flexible part;

(e) exploded view of the assembly. (2)



5.2.1 DESIGN COMPONENTS

Radial and carpal components

The radial and carpal components, shown in Figures 11(a) and (b), both comprise of a tapered stem,

a bearing surface and a tapered hole. The stems of the radial and carpal components fit into the

radial and carpal/metacarpal bones respectively. The convex radius on the bearing surface of the

radial component provides flexion and extension motions of the wrist. The convex bearing surface

of the carpal component allows radial and ulnar deviation of the wrist. Moreover, both components

are to be made from ultra high molecular weight polyethylene (UHMWPE).

Plate

The plate is regular-shaped and has small edges on the carpal side to prohibit rotation of the plate,

as shown in Figure 11(c). It is to be made from cobalt chrome molybdenum alloy (CoCrMo) and a

hole extends through the centre of the plate.

Flexible part

The flexible part, in Figure 11(d) was suggested to be circular in cross-section and tapered at both

ends. It should be made from a flexible material with good fatigue strength. The material chosen

was ‘Elast-Eon’ or silicone polyurethane as it has a high biostability, durability and tear resistance.

Assembly

The different components are assembled together as illustrated in Figure 11(e) and 12. Flexion-

extension, radial-ulnar deviation and rotation are achieved through articulation between the plate

and the bearing surfaces of the radial and carpal components. The flexible part extends through the

plate and into the tapered holes of the radial and carpal components at each end.

Figure 12: Assembly of the new design with sectioned views. (2)



5.2.2 ADVANTAGES

This main characteristic of this design is that it combines the design of an elastomer implant with

that of an articulating surface implant and the authors hope that this combination will eliminate

many disadvantages of each system. By sheathing the flexible part within the articulating

prosthesis, the elastomer will be protected from sharp bone edges while exhibiting its strong

fatigue strength. Johnstone and Shepherd (2) also states that the flexible part can also ‘act as an

internal ligament’ to prevent dislocation and loosening of the carpal/metacarpal stem.

Considering the fact that most patients require artificial wrist joint replacements due to rheumatoid arthritis, it was also proposed that this design will employ an interference fit for the fixation of the

stems to the bones. Moreover, the stems are made from UHMWPE, which is considerably more

compatible to the soft rheumatoid bone than metallic alloys.

5.2.3 PROPOSED MODIFICATION*

In the light of this new design concept, several modifications can be made to achieve a couple of

design objectives in Section 5.1.

Bearing radii

The bearing radius determines the maximum angel of flexion, extension, radial and ulnar deviation

by which the implant can produce and it can be changed according to the needs of each patient.

The bigger the radius is, the smaller the maximum angle will be. If a patient requires an implant with

a reduced range of motion, the maximum contact stress experienced by the bearing surfaces and the flexible part will thus be lesser. The patient will then be able to exert a higher force. Through this

modification, the maximum forces and range of motion of the implant can be balanced to suit the

patient’s requirements.

Fixation

Although interference fit of the implant is suggested for the rheumatoid bone, alternatives can be

considered. Variable screws can be used for fixation of the components, especially for the

carpal/metacarpal stem. This is because the metacarpal bones are prone to perforation and variable

screws reduce the level stresses in these bones due to the implant. The use of variable screws

instead of interference fit may potentially reduce the likelihood of metacarpal perforation and hence increasing the usability of the wrist joint.

*Note that the proposed modifications are theories based on certain assumptions and no actual

test or studies have been done to ascertain these assumptions.


The Development of Artificial Wrist Joint Replacements | Conclusion 22

6. CONCLUSION

The evolution of artificial wrist joint replacements has seen the emergence of multiple distinct

concepts, designs and materials despite its slow development. It was found that there is insufficient

research into the bio-mechanism of the wrist, which leaves plenty of room for further development

of wrist implants. The advantages and disadvantages of previous prostheses were evaluated to help

determine feasible options, such as designs and materials, for future development of wrist joint

replacements.

Several options were suggested to improve the efficiencies of future implants. These suggestions include the use of materials with physical properties similar to human bones, the concept of

providing sufficient support and stability when needed and the moderation of the range of motion

to accommodate different needs.

Amongst the suggested ways of improving future implants, the main concept is that it is currently

impossible to design an artificial wrist joint that would perform identically to normal wrist joint.

Rather than aiming to replicate the normal wrist, future designs should seek to eliminate or reduce

the problems faced by current designs. They should adopt an open concept system whereby certain

designs, parts, materials or functions of the implant can be chosen depending on the condition of

each patient.

So far there have yet been one universal design that is very successful and the compatibility of each

implant varies between patients. An implant that could be manufactured with various different

choices of materials, fixation methods, interchangeable parts and articulating mechanism will be

able to accommodate different wrist disorders and conditions. However, such designs will inevitably

be more complex than existing designs as it involves more variables.

In conclusion, it is currently possible to design an open system implant but it will require a long time

to identify all variable factors before it can be successfully adopted. The development of artificial

wrist joint replacements will continue to rely on future research and there remains much scope for improvement.


The Development of Artificial Wrist Joint Replacements | Acknowledgements 23

7. ACKNOWLEDGEMENTS

Special thanks to Professor Andrew Amis and Dr. Ulrich Hansen for guidance and advice with

literature research as well as writing this report.

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