Measurement of the migration of a cemented hip prosthesis in an in vitro test

Objective. To develop a method to measure the migration of a cemented hip p rosthesis in an in vitro experimental test. Design. A device to measure prosthesis movement relative to bone was design ed and fabricated. It was tested using a Lubinus prosthesis (W. Link, Germa ny) implanted in a composite femur. Background. Clinical studies using radiostereophotogrammetry have shown tha t those cemented hip prosthesis that migrate rapidly in the first two post- operative years are the ones that require early revision. If migration be u sed as a basis for a pre-clinical test, then it should be possible to scree n-out inferior designs before implantation in animal or clinical trials. Methods. The micromotion measurement device consisted of a 'target' of thre e spheres arranged in a cruciform structure. Six linear variable displaceme nt transducers were aligned with the spheres so that motion of the prosthes is relative to the bone could be measured. Results. The displacement and rotation of the prosthesis relative to the co mposite femur was recorded for two million cycles. Relative rapid initial m igration was found, followed by a period of steady-state migration. Distal migration (called 'subsidence' in this paper) of up to 0.1 mm was measured; however the variability in absolute prosthesis migration was very high des pite efforts to ensure that all extraneous factors were minimised. In the m ajority of cases, the prostheses migrated medially, distally and anteriorly . The absolute subsidence, and its variability, were similar to that record ed clinically. Conclusions. A method has been designed and tested which measures prosthesi s migration in an experimental test. It provides a basis for a pre-clinical testing standard.