How to Choose the Appropriate Posterior Slope Angle Can Lead to Good Knee Joint Function Recovery in Total Knee Arthroplasty?

Total knee arthroplasty (TKA) is a highly effective treatment for severe osteoarthritis, significantly alleviating pain and restoring knee function. While TKA boasts high success rates, with prosthesis utilization exceeding 90% after 15 years, some patients experience unsatisfactory functional recovery necessitating revision surgery. Factors such as postoperative knee varus, poor coronal plane force lines leading to tibial tray settlement, and rotational malalignment of the prosthesis have been implicated. The posterior slope angle, specifically, is a critical parameter influencing TKA outcome. Previous research, albeit with limitations in sample size and follow-up, suggested that reducing the posterior slope angle while preserving the posterior cruciate ligament can improve functional rehabilitation. This study aimed to investigate the long-term effects of changes in the posterior slope angle on postoperative knee function recovery in a large cohort of patients undergoing TKA with posterior cruciate ligament preservation, while also evaluating the incidence of complications such as periprosthetic femoral fracture, prosthetic loosening, or pad wear.

The literature highlights the impact of various factors on TKA outcomes, including prosthesis type, patellofemoral joint pressure, postoperative knee motion, and alignment. Studies have shown that postoperative knee varus can decrease knee joint function, while rotational malalignment, particularly excessive internal rotation of the femoral prosthesis, can cause patellofemoral problems. Finite element analysis has demonstrated the influence of poor force lines on tibial tray settlement. However, optimal posterior slope angle remains debated, with significant discrepancies between preoperative planning and postoperative reality reported. Differences in surrounding soft tissues also affect patellofemoral joint pressure and quadriceps femoris strength. Previous smaller studies suggested a correlation between posterior slope angle reduction and improved functional rehabilitation by preventing paradoxical anterior translation; however, these studies were limited in scope. This study aimed to address these gaps by using a larger sample size and extended follow-up period.

This retrospective study analyzed data from 240 patients who underwent TKA between September 2012 and September 2015, with a three-year follow-up. All surgeries were performed by the same senior surgeon using a consistent technique and CR prosthesis (Waldemar Link GmbH & Co. KG). Preoperative and postoperative posterior tibial slope (PTS) angles were measured using lateral radiographic images and a computer program. Patients were divided into five groups based on the change in PTS: Group 1 (>5°), Group 2 (3°-5°), Group 3 (0°-3°), Group 4 (-3°-0°), and Group 5 (< -3°). Three years post-surgery, knee function and pain were assessed using the Knee Society Clinical Rating System (KSS), the Western Ontario and McMaster Universities Arthritis Index (WOMAC), and the Visual Analogue Scale (VAS). Statistical analysis using SPSS 21.0 was performed, comparing scores across groups with the Kruskal-Wallis H-test.

Preoperative WOMAC and KSS scores showed no significant differences between groups. Postoperatively, significant differences were observed across groups in all three outcome measures (WOMAC, KSS, and VAS). Group 2 (3°-5° change in PTS) exhibited the lowest WOMAC and VAS scores and the highest KSS scores, indicating the best functional recovery. Groups 1 and 5 (with the greatest changes in PTS, either increase or decrease), showed significantly poorer functional recovery compared to Group 2. No patients experienced periprosthetic femoral fracture, prosthetic loosening, or pad wear during the three-year follow-up period.

The findings demonstrate a significant relationship between changes in the posterior slope angle following TKA and postoperative knee function recovery. Group 2, showing a small reduction in posterior slope, achieved superior outcomes, supporting the hypothesis that a moderate decrease in the angle, in the context of posterior cruciate ligament-preserving prostheses, optimizes knee function. Conversely, excessive increases or decreases in the angle resulted in poorer outcomes. The absence of major complications may be attributed to the relatively short follow-up duration, warranting further long-term investigations. The study highlights that while restoring the physiological posterior slope angle is important, it may not guarantee optimal function when using a posterior cruciate ligament preserving prosthesis, potentially due to changes in biomechanical balance.

This study confirms that the change in posterior tibial slope angle during TKA significantly affects postoperative knee function rehabilitation. A moderate decrease in the angle, within the range of 3° to 5°, appears to be associated with optimal outcomes. Excessive alteration of the angle, in either direction, negatively impacts recovery. Future research should investigate the long-term effects, explore the interplay with other surgical parameters, and examine the impact on different prosthesis types.

This study is retrospective, limiting causality determination. Other factors not considered in this analysis may also influence functional outcomes after TKA. The relatively short follow-up period (3 years) may not capture long-term complications. Further research with a prospective design and a longer follow-up period is necessary to strengthen the conclusions and explore the long-term impact of posterior slope angle changes on TKA outcomes.