Five-Year Outcomes of UKA Versus TKA in Elderly Patients With Severe Varus Deformity in Knee Osteoarthritis: The Modifying Effect of Preoperative Extensor Muscle Radiomics Quality

Technical Summary: Five-Year Outcomes of UKA Versus TKA in Elderly Patients With Severe Varus Deformity

Problem Statement
The role of unicompartmental knee arthroplasty (UKA) in elderly patients with severe varus deformity (>15°) remains controversial. While UKA offers potential benefits in preserving bone and ligaments, conventional indications often caution against its use in severe varus due to risks of residual malalignment, bearing wear, and lateral compartment degeneration. Existing literature has primarily focused on alignment correction and implant survival, largely overlooking patient-specific soft-tissue factors. Specifically, it is unclear whether preoperative extensor muscle quality modifies the early functional advantage of UKA over total knee arthroplasty (TKA) in this specific demographic.

Methodology
This single-center, retrospective cohort study analyzed 176 elderly patients (≥60 years) with medial compartment knee osteoarthritis and severe varus deformity (>15° and ≤20°) who underwent surgery between February 2016 and April 2019. Patients were divided into a TKA group (n=95) and a UKA group (n=81).

• Surgical Protocols: All procedures were performed by a single experienced surgical team. TKA involved standard anterior midline incisions and total joint replacement. UKA utilized a medial parapatellar mini-incision (10–12 cm) with preservation of the anterior cruciate ligament and lateral compartment.
• Muscle Quality Assessment: Preoperative MRI was used to evaluate extensor muscle quality.
  • Conventional Metrics: Cross-sectional area (CSA), intramuscular adipose tissue (intra-MAT) CSA, and fat infiltration percentage were measured at the superior pole of the patella.
  • Radiomics Analysis: A subset of 146 patients underwent MRI radiomics analysis. Images were resampled, normalized, and bias-field corrected. Morphological, first-order, and texture features (e.g., GLCM, GLRLM) were extracted. Stable features (ICC >0.80) were selected via mRMR and LASSO regression to construct an extensor Rad-score. A higher Rad-score indicates poorer muscle quality.
• Outcomes: Perioperative metrics, complications, and clinical scores (VAS, HSS, WOMAC, ROM) were assessed at 1 month, 1 year, and 5 years. Radiographic alignment (HKA, mLDFA, MPTA, JLCA) was measured at 5 years.
• Statistical Analysis: Multivariable linear regression models were employed to assess outcomes, adjusting for age, sex, BMI, preoperative alignment, and baseline scores. Crucially, an interaction term between surgical procedure and extensor Rad-score was tested to determine if muscle quality modifies the treatment effect.

Key Results

• Perioperative Outcomes: Compared to TKA, UKA resulted in significantly less intraoperative blood loss, shorter operation time, smaller incisions, and lower postoperative drainage volume (all P < 0.001). Hospital stay duration was comparable.
• Clinical Outcomes:
  • Early Recovery: The UKA group demonstrated superior recovery at 1 month and 1 year, with higher HSS and ROM scores and lower WOMAC and VAS scores.
  • Five-Year Outcomes: At 5 years, VAS, HSS, and ROM scores were comparable between groups. However, the UKA group maintained a slightly lower (better) WOMAC score (9.4 ± 1.9 vs. 10.1 ± 1.6, P = 0.010).
• Radiographic Alignment: TKA achieved more complete correction of mechanical axis alignment (HKA, mLDFA, MPTA) and joint line convergence compared to UKA. Despite this radiographic superiority, TKA did not translate to superior 5-year clinical function (VAS, HSS, ROM).
• Muscle Quality and Interaction:
  • Higher extensor Rad-scores (poorer muscle quality) were independently associated with lower early HSS and ROM scores and higher WOMAC scores.
  • Interaction Effect: A significant interaction was found between surgical procedure and extensor Rad-score for 1-month WOMAC scores (β = 0.74, P = 0.032). This indicates that poorer extensor muscle quality attenuates the early functional benefit of UKA relative to TKA.
  • Stratified analysis showed that while UKA offered an early advantage in both muscle-quality subgroups, the magnitude of this advantage was greater in patients with better preoperative muscle quality.

Key Contributions

1. Validation of UKA in Severe Varus: The study demonstrates that in carefully selected elderly patients with varus deformity between 15° and 20°, UKA yields comparable 5-year clinical outcomes to TKA, despite less complete radiographic alignment correction.
2. Radiomics Integration: The study introduces the use of MRI-derived extensor Rad-scores to quantify muscle quality, capturing tissue heterogeneity beyond simple CSA or fat fraction.
3. Identification of Effect Modification: The research establishes that preoperative extensor muscle quality is a significant modifier of early recovery. Poor muscle quality reduces the early functional advantage typically associated with UKA, suggesting that muscle reserve is a critical factor in surgical decision-making.

Significance and Claims
The paper claims that for carefully selected patients with severe varus medial knee OA, UKA is a viable alternative to TKA, offering less surgical trauma and faster early recovery without compromising mid-term clinical function. The study posits that the traditional concern regarding severe varus deformity as an absolute contraindication for UKA may be refined by considering muscle quality.

The authors suggest that preoperative extensor muscle radiomics quality can help stratify recovery potential. Specifically, patients with poor extensor muscle quality may not experience the full early benefit of UKA and could be candidates for intensified prehabilitation (e.g., quadriceps strengthening, neuromuscular training) or tailored rehabilitation planning. The study concludes that incorporating muscle quality assessment into surgical decision-making supports more individualized procedure selection, rather than relying solely on anatomical alignment and compartment involvement.

Limitations Acknowledged
The authors note the study's limitations, including its single-center retrospective design, potential selection bias, and the fact that all surgeries were performed by a single expert team, which may limit generalizability. Additionally, the 5-year follow-up is insufficient to assess long-term revision risks specific to UKA (e.g., lateral compartment progression), and the radiomics model requires external validation before clinical implementation. The study also lacked objective postoperative muscle function tests (e.g., isokinetic strength).