Issues in pre-clinical testing of implants

Reconstructive surgery usually requires a prosthetic implant to replace or augment tissues that have failed. In the case of load-bearing implants such as orthopaedic, cardiovascular, or dental implants, a pre-clinical testing procedure is required to ensure that the implant is efficacious and safe. Biomechanical pre-clinical testing procedures include: (a) computational modelling (numerical models), and (b) laboratory bench testing (physical models). Pre-clinical testing is becoming more important as alternatives to animal testing become an ethical requirement and as patients become more informed about involvement in clinical trials. In this paper, issues in pre-clinical testing of implants for reconstructive surgery are analysed. A generalized methodology for development of pre-clinical testing protocols is illustrated in detail for one of the most commonly used implants — a hip prosthesis. To develop a pre-clinical test, the first requirement is to determine a mechanical variable associated with the progress of failure. The second requirement is to design an experimental or computational procedure (or preferably both together) to monitor the progression towards failure under conditions that represent sufficiently well the in vivo environment. The failure measure chosen for hip prostheses is relative motion of the implant and bone under cyclic loading, termed migration of the implant. The higher the magnitude of implant migration, the greater is the likelihood of early failure [Karrholm et al., J. Bone Jt. Surg. B 76 (1994) 912–916]. An experimental protocol for pre-clinical testing of hip prosthesis under this system is described.