Posterior tibial slope after total knee arthroplasty is not reproducible or consistent

Our aim was to compare posterior tibial slope (PTS) values, distribution, deviation from manufacturer's recommended value and restoration of this value in total knee arthroplasty (TKA) with tibial component placement using intramedullary (IM) and extramedullary (EM) methods, and to analyze factors affecting these values.

A total of 151 knees operated with Zimmer-Biomet NexGen® LPS Fixed Bearing Knee TKA between 2022 and 2024 were retrospectively investigated. Knees were grouped by method of tibial component placement (IM or EM). Pre-operative descriptive data, postoperative PTS, scatteredness of PTS, difference between pre-operative and postoperative PTS, deviation from 7° which was claimed by the manufacturer and rate of outliers were compared between groups. Factors that might affect postoperative PTS in the EM group were analyzed.

Ninety-five knees (62.9%) in the EM group and 56 knees (37.1%) in the IM group were analyzed. Mean PTS decreased from 8.8 ± 3.5° to 6.7 ± 2.7° (P < 0.001) in the EM group, and from 10 ± 4.1° to 4.9 ± 2.6° (P < 0.001) in the IM group. Postoperative PTS was significantly lower in the IM group (P < 0.001). Mean deviation from 7° claimed by the manufacturer was 2.4 ± 1.4° for the EM group and 2.8 ± 1.8° for the IM group (P = 0.14). In 15 of 151 patients (9.9%), postoperative PTS was above 10°. Linear regression analysis revealed a moderate effect of pre-operative PTS and weight in the EM group.

Although not statistically significant, the EM method is closer to the manufacturer's recommendation for sagittal alignment. However, both groups had a wide range of PTS and a large number of outliers. Both methods are not reliable in terms of PTS reproducibility and consistency.