Testing and evaluation of implant materials Several surgical implant materials have gained acceptance over the.years, often not because they were ideal, but because there was nothing else. As biomaterials study becomes more of a science (which we hope to promote by this journal) it is now pertinent to ask whether well tried, but not always well-loved materials can be replaced. Is there now available sufficient evidence to exclude, for example, certain metallic alloys on the basis of inadequate fatigue life? Have new alloys coming into surgical use been evaluated with the same care given to those used for aircraft production?

It would appear that there are at least two areas which need particular attention: first the development of a methodology of implant material testing which will not only evaluate a material persebut will also examine its function as an implant; secondly, a systematic study of all factors associated with biocompatibility.

Testing of implant materials has several stages. There is firstly the examination of the material in its initial ‘as manufactured’ condition, usually for quality control purposes. This is followed by f testing in its final condition, for example, after heat treatment or cold working is completed. Finally, there is evaluation as a finished implant, which may in fact coincide with the second stage referred to. It is in this completed form that there is debate about adequacy or even validity of test methods. Are the tests intended solely for quality control purposes, as are most Standards proposals, in which case the test method adopted may be completely unrelated to the intended end use, or alternatively, are the tests intended to give functional assessment of, say, a prosthetic joint replacement, in order to test design features and to give information to the surgical user?

If functional assessment is the purpose then the methods required will be more complicated and costly. The answers obtained by some present methods are necessarily tentative, because in viva conditions are not completely known. Long-term evaluation to establish long-term reliability is a priority in biomaterials development. The importance of this is illustrated by the fact that it is reported from some centres that up to 11% of total joint replacement surgery may be associated with replacement of failed prostheses. Not all these result from failure of materials, and it has been said that the poor surgeon will always win against the good materials scientist. This introduces another dimension to the problem but it is one that can be changed dramatically by the provision of adequate training schemes for surgical specialists. Part of the long-term evaluation should include study of implants removed from patients for various reasons and this study should extend beyond merely failure analysis.

The assessment of reliability based upon sound scientific study is a research area that needs to be taken seriously by biomaterials scientists. This is the end-point of any testing programme encompassing all aspects from quality control to performance assurance.

The second major area for consideration is that of biocompatibility testing. This must include consideration of many factors, for example, the in viva reactivity of a metal must be dependent on the reactivity of the alloying elements present, but before their behaviour in an alloy can be really understood, information is needed about completely pure metals. The effect of molecular structure of polymers on surface reactivity is only beginning to be studied and this is a research area of considerable potential.

It is now realised that no material can be classified as inert. There is always a reaction or interaction and for this reason reference has been made to a ‘benign reactivity’ as an acceptable level of reaction. The question of developing reactive materials which would be actively engaged in the processes of repair or regeneration has been discussed by this author and others have since referred to ‘bioactive’ and ‘biopassive’ materials to differentiate between levels of biocompatibility.

Laboratory and in vivo studies will need to be combined in a very systematic basic study if the problems posed are to be solved. It is timely that the British Science Research Council have produced a report which deals, amongst other items, with issues referred to in this Editorial. We are pleased to publish the report in this issue to assist in the widest possible discussion of these and other items essential to the further progress of biomaterials research. Some papers already published in Biomaterials are relevant to these themes. The publication of the report and the future development of this journal will represent the emergence of a recognized scientific discipline of biomaterials science.

G.W. Ha&in@

122 Biomaterials 1980, Vol 1 July