Comparison of the Tibial Posterior Slope Angle Between the Tibial Mechanical Axis and Various Diaphyseal Tibial Axes After Total Knee Arthroplasty

BACKGROUND: The posterior tibial slope angle (PTS) is crucial for sagittal alignment after total knee arthroplasty (TKA). This study aimed to determine which PTS based on the lateral view of standard knee radiographs (LSKRs; 36 × 43 cm) reflects the PTS based on a full-length lateral tibial radiograph (FLTR). METHODS: A total of 290 patients (355 knees) who underwent primary TKA were retrospectively recruited. Cross-sectional views from the 3-dimensional digital model of the tibial prosthesis and bone complex in the sagittal plane were used as FLTRs and LSKRs. Considering the region 21.5 cm proximal to the site of FLTR as the spot for LSKR to determine the 5 tibial diaphyseal axes, the axis that simulates the PTS as determined by the tibial mechanical axis between the center of the tibial component and the ankle plafond in LSKR was determined and compared. RESULTS: PTS (α(5)) defined by the line connecting the midpoints of tibial width between the region 10-cm distal to the knee joint and the distal end of the tibia based on LSKR revealed the least mean difference (0.13° ± 1.00°) and the strongest correlation (P < .001, r = 0.948) with PTS based on FLTR (α(0)). The number of knees in α(5), indicating a difference of <2° from α(0), was 333 of 355 (93.8%). The equivalence test results indicated that α(0) and α(5) were statistically equivalent within a difference of 2° (P < .001). CONCLUSIONS: PTS (α(5)) can be used as a clinically reliable substitution of the true PTS on an FLTR for evaluating sagittal alignment after TKA.