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