Summary

In TKA, patellofemoral joint pressure is influenced by implant design, rotation placement, and patellar shape, but there is limited research on the relationship between the femoral-tibial position in the sagittal plane. The results suggest that changes in femorotibial posterior overhang and posterior tibial slope in TKA may affect patellofemoral osteosclerosis.

Abstract

Background

Patellar sclerosis is often observed after total knee arthroplasty (TKA), mainly due to changes in patellofemoral joint (PFJ) pressure. PFJ pressure post-TKA can be affected by factors like implant design and individual patellar morphology. While femoral rotational alignment is known to influence PFJ pressure, the impact of femur-tibia positioning is not well understood. This study investigates the relationship between changes in femorotibial posterior overhang (FTPO) and posterior tibial slope (PTS) in the sagittal plane postoperatively and the occurrence of patellar sclerosis after TKA.

Methods

The study included 80 knees undergoing patellar non-resurfacing TKA for medial osteoarthritis between January 2017 and January 2023. The cohort had 12 males and 47 females, with a mean age of 76 years (range 58 to 84). All cases received the JOURNEY II BCS (Smith & Nephew) prosthesis. Patellar non-resurfacing TKA was performed if the preoperative patellar facet angle (PFA) on the skyline X-ray was ≥ 132°, indicating a shallow facet angle. Exclusion criteria were severe patellofemoral joint deformities, significant anterior knee symptoms, lateral knee osteoarthritis, trauma history, prior surgeries, and rheumatoid arthritis.
Participants were divided based on the presence or absence of postoperative patellar sclerosis. FTPO and PTS were measured on lateral knee radiographs in full extension, both preoperatively and one year postoperatively. FTPO was defined as positive when the femur was posterior to the tibia, and PTS as the angle between the medial tibial plateau and the posterior cortex of the proximal tibia. Pre- and postoperative values and changes in these parameters were compared between groups to explore their relationship with patellar sclerosis. Statistical analyses included t-tests, Wilcoxon's rank sum test, and chi-square test, with significance set at p<0.05.

Results

Patellar sclerosis was found in 23 knees (28.7%) postoperatively, while 57 knees (71.3%) showed none. The sclerosis group had a significantly higher preoperative FTPO (-0.3±4.3 mm) than the non-sclerosis group (-3.3±4.7 mm) (p<0.01). Postoperatively, the FTPO was smaller in the sclerosis group (-3.0±3.7 mm) than in the non-sclerosis group (0.3±3.3 mm) (p<0.01), indicating a larger anterior shift of the femur (sclerosis: -2.7±3.7 mm, non-sclerosis: 3.6±4.9 mm, p < 0.01). The preoperative PTS was also greater in the sclerosis group (6.9±2.2°) compared to the non-sclerosis group (3.6±2.5°) (p<0.01), but no significant postoperative difference was noted. The change in PTS from pre-to-postoperative was more pronounced in the sclerosis group (-2.2±0.4°) than in the non-sclerosis group (1.3±0.3°), showing a greater anterior tilt (p<0.01). A weak positive correlation was observed between changes in FTPO and PTS (r=0.4, p<0.01).

Conclusion

Changes in FTPO and PTS post-TKA may contribute to patellar sclerosis development. Knee joint alignment in the sagittal plane, linked to changes in PF pressure after TKA, could be crucial for predicting postoperative outcomes.