High accuracy in knee alignment and implant placement in unicompartmental medial knee replacement when using patient-specific instrumentation.

PURPOSE: The influence of patient-specific instrumentations on the accuracy of unicompartmental medial knee replacement remains unclear. The goal of this study was to examine the ability of patient-specific instrumentation to accurately reproduce postoperatively what the surgeon had planned preoperatively.

METHODS: Twenty consecutive patients (20 knees) who suffered from isolated unicompartmental medial osteoarthritis of the knee and underwent medial knee replacement using newly introduced magnetic resonance imaging-based patient-specific instrumentation were assessed. This assessment recorded the following parameters: (1) the planned and the postoperative mechanical axis acquired through long-leg AP view radiographies; (2) the planned and the postoperative tibial slope acquired by means of standard AP and lateral view radiographies; and (3) the postoperative fit of the implanted components to the bone in coronal and sagittal planes. The hypothesis of the study was that there was no statistically significant difference between postoperative results and preoperatively planned values.

RESULTS: The study showed that (1) the difference between the postoperative mechanical axis (mean 1.9° varus ± 1.2° SD) and the planned mechanical axis (mean 1.8° varus ± 1.2° SD) was not statistically significant; (2) the difference between the postoperative tibial slope (mean 5.2° ± 0.6° SD) and the planned tibial slope (mean 5.4° ± 0.6° SD) was statistically significant (p = 0.008); and (3) the postoperative component fit to bone in the coronal and sagittal planes was accurate in all cases; nevertheless, in one knee, all components were implanted one size smaller than preoperatively planned. Moreover, in two additional cases, one size thinner and one size thicker of the polyethylene insert were used.

CONCLUSIONS: This study suggests that overall patient-specific instrumentation was highly accurate in reproducing postoperatively what the surgeon had planned preoperatively in terms of mechanical axis, tibial slope and component fit to bone.

LEVEL OF EVIDENCE: IV.