ISB Keynote Paper — 1997
Strength requirements for internal and external prostheses

https://doi.org/10.1016/S0021-9290(98)00190-0Get rights and content

Abstract

Throughout the history of development of joint replacement implants and external prostheses there have been mechanical failures due to a discrepancy between material strength, cross-sectional characteristics and the loads developed in normal or abnormal function by the patient utilising the device. Particularly for internal prostheses attention is being paid at the present time to wear characteristics and the requirements for the articulating surfaces and the volume of wear particles produced during tests simulating the use of the device within the patient. The particular importance of the wear particles is that they seem to be associated with accelerated resorption of bone at areas essential for successful fixation of the implant within it. This article will consider joint replacements at the knee and hip and external prostheses for the leg. If failure due to external trauma is ignored the loads to be considered in testing standards correspond in implants to the muscular and ligamentous forces related to the forces developed between ground and foot and to the bending moments in the structure of leg prostheses. Generally it can be assumed that the treatment of the patient following trauma is more easily accomplished and more likely to be successful if the prosthesis has failed and not the bony structure of the patient. However, the author is unaware that these devices have ever been designed to have lower intrinsic strength than the anatomical structures to which they are connected; indeed in many cases particularly for implants they are much stronger than the bone to which they are connected. The major difficulty in rational design of prosthetic devices has been uncertainty about the importance of occasionally applied loads of a high value relative to those on a frequent basis and also to the frequency of application of these overloads. In this paper consideration is given to methods of determination of load systems relevant to the mechanical performance of implanted joint replacements at the hip and the knee and external prostheses for leg amputees. New data are presented relating to walking, other daily activities and the corresponding frequency of occurrence of these. Loading data on implants obtained by various biomechanical models is compared and related to the loads actually measured by implanted transducers. The philosophy of the standardised test load systems and the performance requirements is reviewed.

Section snippets

Leg prostheses

Concern about the strength of leg prostheses led to a series of meetings in North America and the United Kingdom terminating in ISPO (1978), the ‘Philadelphia Meeting’, which collated the data obtained in gait analysis laboratories on the force and moment actions transmitted at the ankle and knee of leg prostheses. The report of this meeting includes a large volume of results of such tests conducted largely in the Biomechanics Research and Development Unit (BRADU) at Queen Mary Hospital,

Implants

In the design of implants such as joint replacements the situation differs from leg prostheses in that incorporation of transducers into the device presents serious difficulties and has been achieved in only a few cases (Rydell, 1966; English and Kilvington, 1979; Brown et al., 1985; Davy et al., 1988; Bean et al., 1988; Bergmann et al., 1993; Taylor et al., 1997, Taylor et al., 1998) Most of the information relating to hip and knee joint force in gait have been obtained from calculations

Standards for strength requirements

The correctness of the values in a standard for testing mechanical strength of any device or component can only be assessed by a comparison of laboratory test results with performance of the device in normal service. The standard test should fail those components which are found to fail in service and should not produce failure in those which are found to be satisfactory in normal service. The value of test load for leg prostheses specified by the ISPO report of 1978 was found to cause failure

Wear

The experience of failure of all categories of joint replacement has indicated that the majority of failures are due to loss of fixation of the implant to the basic skeletal structures and also failure of the articulating surfaces due to wear. Wear is currently seen to be the major factor limiting the lifetime of the implant (Howie et al., 1988; Murray and Rushton, 1990; Schnalzreid et al., 1992). Two recent volumes of the Journal of Engineering in Medicine (vol. 210: H3 and vol. 211: H1) have

Test duration

Fatigue and wear tests are generally long duration activities and therefore expensive. They require complex testing machines and regular inspection. It is important therefore that the number of loading cycles specified in standards for should be realistic. They should be conducted long enough to give confidence that in the expected lifetime of the prosthesis of the joint replacement implant no adverse situation will arise. For leg prostheses regular maintenance and replacement of critical parts

Conclusions

Experimental measurement and calculation of the loads transmitted in leg prosthesis and joint replacements are vital in determining the form of strength tests which should be undertaken for these devices Cunningham and Brown, 1952. The amount of the calculated and experimental data and the restrictions on the circumstances on which it is acquired do not allow specification of test load values without feedback from national registers of failures. The variability in delivery and fitting of

Acknowledgements

The work reported here has been supported at various times by the Scottish Home and Health Department, the Medical Research Council and by studentships from the Medical Research Council and Science and Engineering Research Council. The author is greatly indebted to the University of Strathclyde for affording him the opportunity to undertake this work and also to his many colleagues and students who have all contributed to this report.

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