Determination of neutral tibial rotational alignment in rotating platform TKA.

UNLABELLED: Use of a fixed anatomic landmark to set rotation of the tibial component may lead to rotational malalignment. Post wear in stabilized components, backside wear in any conforming modular system, and patellar maltracking may result from tibiofemoral rotational incongruence. We aimed to quantify tibial rotational alignment in 109 primary rotating platform TKAs. After trial components were inserted with the knee properly balanced, we recorded the neutral point of the rotating tibial insert, in extension, relative to the most medial aspect of the tibial tubercle. We hypothesized that all neutral points would lie within 10 degrees of the mean. Divergence of the neutral point was recorded as being internal or external to the medial border of the tibial tubercle to the nearest 5 degrees increment. Our results showed a mean divergence of 5 degrees +/- 5 degrees external to the medial border of the tubercle. Five percent of knees, however, had neutral points > or = 10 degrees from the mean. Surgeons who use fixed-bearing modular components with any rotational constraint must be cautious in choosing a fixed anatomic tibial landmark to determine the rotational alignment of the tibial component. Doing so may create tibiofemoral rotational malalignment in full extension that may lead to suboptimal outcomes. LEVEL OF EVIDENCE: Diagnostic study, Level II-3. See the Guidelines for Authors for a complete description of levels of evidence.