Association between abnormal native tibial torsion and poorer clinical outcomes following robotic-assisted total knee arthroplasty: follow-up of at least two years

Technical Summary: Association between Abnormal Native Tibial Torsion and Clinical Outcomes in Robotic-Assisted TKA

Problem Statement
While rotational alignment is a critical determinant of success in total knee arthroplasty (TKA), the influence of preoperative native tibial torsion (TTA) on postoperative outcomes remains under-investigated, particularly in the context of robotic-assisted TKA (RA-TKA). Although excessive native tibial torsion (>30°) is traditionally an indication for tibial derotation osteotomy (TDO) to address biomechanical dysfunction, most patients with end-stage knee osteoarthritis (KOA) and abnormal torsion undergo TKA alone. It is unclear whether preoperative TTA variations influence postoperative functional scores, patient satisfaction, or the incidence of patellofemoral symptoms (specifically anterior knee pain [AKP] and patellofemoral crepitus) when a functional alignment (FA) strategy is employed with robotic precision.

Methodology
This retrospective cohort study analyzed 382 patients treated between May 2022 and November 2023 for end-stage varus KOA using RA-TKA with a functional alignment strategy. All procedures were performed by a single surgeon using the Mako robotic platform.

• Cohort Definition: Patients were stratified based on native TTA measured via preoperative axial CT scans. An abnormal tibial torsion (ATT) group was defined as TTA > 30°, and a normal tibial torsion (NTT) group as TTA ≤ 30°.
• Matching: A 1:1 propensity score matching process was applied to control for confounding variables, including age (±5 years), sex, body mass index (±5 kg/m²), surgical side, prosthesis type (Stryker Triathlon CR or PS), and mechanical hip-knee-ankle angle (mHKA ±5°). This resulted in a final analyzed cohort of 77 patients per group (154 total).
• Surgical Technique: The study utilized a cemented CR or PS prosthesis via a standard medial parapatellar approach without patellar resurfacing. Tibial component rotation was guided by the anteroposterior axis defined by a line connecting the center of the posterior cruciate ligament (PCL) attachment and the medial third of the tibial tubercle.
• Outcomes: Clinical outcomes were assessed at a minimum 2-year follow-up (24–35 months). Metrics included the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Oxford Knee Score (OKS), Knee Society Score (KSS) function, and the Forgotten Joint Score (FJS). Patellofemoral symptoms (AKP and crepitus) and patient satisfaction were also recorded.
• Statistical Analysis: Continuous variables were compared using Student's t-test or Wilcoxon rank-sum tests, while categorical variables were analyzed via chi-square tests. A sample size of 77 per group was calculated to detect a minimal clinically important difference (MCID) of 13.7 points in FJS with 80% power.

Key Results

• Functional Scores: Both groups demonstrated significant improvements from preoperative to postoperative assessments in WOMAC, OKS, and KSS function scores. No statistically significant differences were found between the ATT and NTT groups for these metrics at the 2-year follow-up.
• Forgotten Joint Score (FJS): The ATT group exhibited a significantly lower postoperative FJS compared to the NTT group (75.5 ± 23.5 vs. 83.4 ± 18.4; P = 0.045). However, the authors note that this difference did not reach the widely accepted MCID of 13.7 points.
• Patellofemoral Symptoms: The ATT group had significantly higher incidences of patellofemoral crepitus (25% vs. 10%; P = 0.020) and anterior knee pain (19% vs. 8%; P = 0.035) compared to the NTT group.
• Patient Satisfaction: Numerically higher satisfaction was observed in the NTT group (94%) compared to the ATT group (84%), but this difference was not statistically significant (P = 0.072).

Significance and Claims
The study concludes that while RA-TKA with a functional alignment strategy improves clinical outcomes for patients with both normal and abnormal native tibial torsion, those with ATT are more likely to experience lower FJS scores and higher rates of patellofemoral symptoms at the 2-year mark.

The authors posit that these findings provide clinically relevant evidence suggesting that additional tibial derotation osteotomy (TDO) may not be necessary for patients with ATT undergoing primary RA-TKA, given that the overall functional outcomes (OKS, WOMAC, KSS) remained comparable between groups. The study highlights that while ATT is associated with specific patellofemoral symptoms, the robotic-assisted functional alignment approach appears to mitigate major functional deficits.

Limitations Acknowledged by the Authors
The paper explicitly notes several constraints:

1. Generalizability: The cohort was restricted to varus KOA patients without patellar resurfacing, limiting applicability to valgus deformities or different alignment strategies.
2. Demographics: The population consisted exclusively of Asian patients, whereas native tibial torsion norms may differ in Caucasian populations.
3. Follow-up Duration: The 2-year follow-up period is relatively short, necessitating longer-term studies to assess outcome trajectories.
4. Implant Design: The study did not specifically examine whether ATT affects CR and PS implants differently, despite matching for prosthesis type.
5. Measurement Precision: Postoperative CT assessment of component rotational alignment was not performed, limiting the ability to correlate exact implant-tibia rotational relationships with outcomes.
6. Symptom Grading: AKP and crepitus were assessed as binary (yes/no) self-reports without severity grading.
7. Femoral Rotation: Variations in femoral component rotational alignment were not evaluated, though prior literature suggests they have less impact on outcomes than tibial rotation.