Longitudinal migration and inducible displacement of the Mobility Total Ankle System

Abstract

Background and purpose RSA can be used for early detection of unstable implants. We assessed the micromotion of the Mobility Total Ankle System over 2 years, to evaluate the stability of the bone-implant interface using radiostereometric analysis measurements of longitudinal migration and inducible displacement.

Patients and methods 23 patients were implanted with the Mobility system. Median age was 62 (28–75) years and median BMI was 28.8 (26.0–34.5). Supine radiostereometric analysis examinations were done from postoperatively to the 2-year follow-up. Standing examinations were taken from the 3-month to the 2-year follow-up. Migrations and displacements were assessed using model-based RSA software (v. 3.2).

Results The median maximum total point motion (MTPM) for the implants at 2 years was 1.19 (0.39–1.95) mm for the talar component and 0.90 (0.17–2.28) mm for the spherical tip of the tibial component. The general pattern for all patients was that the slope of the migration curves decreased over time. The main direction of motion for both components was that of subsidence. The median 2-year MTPM inducible displacement for the talar component was 0.49 (0.27–1.15) mm, and it was 0.07 (0.03–0.68) mm for the tibial component tip.

Interpretation The implants subside into the bone over time and under load. This corresponds to the direction of primary loading during standing or walking. This statistically significant motion may become a clinically significant finding that would correspond with premature implant failure.

MD was the principal investigator and contributed to the study design, acquisition and analysis of the data, and writing of the manuscript. JF and DW contributed to analysis of the data and writing of the manuscript. AH contributed to the manuscript. PF contributed to the recruitment of patients and to the manuscript. MG was the operative surgeon, and contributed to the study design, acquisition and analysis of the data, and writing of the manuscript.

This study was funded by an unrestricted research grant from DePuy, a Johnson and Johnson Company. Halifax Biomedical Inc. was consulted for its experience with RSA, and assisted in some of the preliminary data collection and analysis for this study. Medis Specials provided access to and technical support for the model-based RSA (v. 3.2) software used in this study.

Although this study was sponsored by DePuy, it was initiated by the authors and DePuy was not involved in the planning of the study, in data collection, in interpretation of the results, or in writing of the manuscript. The authors were not influenced by this grant and we have remained scientifically objective in the collection, analysis, and presentation of the data.