Identifying Preoperative High-Risk Factors for Poor Functional Recovery: Achieving the Minimal Clinically Important Difference in the Forgotten Joint Score-12 After Medial Fixed-Bearing Unicompartmental Knee Arthroplasty

Technical Summary: Identifying Preoperative High-Risk Factors for Poor Functional Recovery in Medial Fixed-Bearing Unicompartmental Knee Arthroplasty

Problem Statement
While Unicompartmental Knee Arthroplasty (UKA) is recognized for its potential to achieve a "forgotten joint" (a state where the patient is unaware of the prosthesis), significant prognostic disparities exist among patients. A statistically significant improvement in scoring does not always equate to a substantial, patient-perceived benefit. The Minimal Clinically Important Difference (MCID) for the Forgotten Joint Score-12 (FJS-12) is generally defined as a 14-point improvement. However, a subset of UKA patients fails to cross this threshold postoperatively. Existing predictive models are often confounded by heterogeneity in surgical teams, alignment philosophies, and prosthesis designs, making it difficult to isolate the independent impact of inherent patient conditions from surgical technique variability. Furthermore, traditional modeling approaches often suffer from reduced statistical power due to random sample splitting and loss of nonlinear information through arbitrary dichotomization of continuous variables.

Methodology
This single-center, retrospective cohort study analyzed 206 patients (from an initial 240, with 34 lost to follow-up) who underwent primary medial fixed-bearing UKA between April 2024 and April 2025. To eliminate surgical heterogeneity, all procedures were performed by a single senior surgical team using the Zimmer fixed-bearing prosthesis system, conventional manual instruments, and strict mechanical alignment principles. No patellar resurfacing or lateral retinacular release was performed.

• Data Collection: Preoperative demographics, clinical scores (FJS-12, KSS, VAS, ROM), and radiographic parameters (HKA angle, Posterior Tibial Slope [PTS], lateral compartment KL grade, patellofemoral Iwano grade) were collected. A novel metric, anatomical deviation of PTS ($\Delta$PTS), was calculated to quantify osteotomy disturbance.
• Outcome Definition: The primary outcome was the achievement of the FJS-12 MCID (a 14-point increase) at one year post-operation. A dual-track validation strategy was employed, using both the anchor-based threshold (14 points) and a distribution-based threshold (6.4 points) for sensitivity analysis.
• Statistical Approach:
  • Full-Cohort Modeling: Instead of traditional 7:3 data splitting, the entire cohort ($N=206$) was used for modeling to maximize the event pool (78 non-achievers) and avoid overfitting.
  • Feature Selection: LASSO regression with 10-fold cross-validation was used for dimensionality reduction.
  • Nonlinearity Check: Restricted Cubic Splines (RCS) were utilized to test for nonlinear relationships in continuous variables (BMI, preoperative FJS-12).
  • Validation: Internal validation was performed via 1,000 optimism-corrected bootstrap resamples. Model performance was assessed via the C-index (AUC), calibration curves (Hosmer-Lemeshow test), and Decision Curve Analysis (DCA).
  • Sensitivity Analyses: Multidimensional sensitivity analyses were conducted using alternative endpoints (distribution-based MCID, absolute postoperative score $\ge$ 70, and a composite functional endpoint).

Key Results

• Achievement Rate: The overall FJS-12 MCID achievement rate was 62.1% (128/206).
• Independent Predictors: Multivariable logistic regression identified four independent preoperative risk factors for failing to achieve the MCID:
  1. Obesity: Defined as BMI $\ge$ 28 kg/m² (Chinese standard), OR = 2.46 ($P=0.005$).
  2. Preoperative Anterior Knee Pain: OR = 3.14 ($P<0.001$).
  3. Patellofemoral Degeneration: Higher Iwano grade (per 1-grade increase), OR = 2.18 ($P=0.017$).
  4. Lower Preoperative FJS-12 Score: OR = 0.91 per 1-point increase ($P=0.011$), indicating that lower baseline scores are a risk factor for not reaching the MCID threshold.
• Non-Associated Factors: The lateral compartment KL grade, $\Delta$PTS (anatomical deviation), and intraoperative patellar denervation status were not significant independent predictors.
• Model Performance: The full-cohort model demonstrated an optimism-corrected C-index of 0.768 (95% CI: 0.698–0.838) and excellent calibration ($P=0.52$ for Hosmer-Lemeshow). Decision Curve Analysis confirmed a net clinical benefit across threshold probabilities of 10% to 85%.
• Sensitivity: The identified predictors remained robust across all sensitivity analyses, including different MCID definitions and composite functional outcomes.

Key Contributions

1. Methodological Rigor: The study advances predictive modeling in orthopedics by utilizing full-cohort analysis with LASSO and bootstrap validation, avoiding the statistical pitfalls of sample splitting in moderate-sized datasets.
2. Risk Stratification Tool: The authors developed and internally validated a visual nomogram based on the full cohort to predict the probability of failing to achieve a "forgotten joint." This tool integrates patient traits (BMI, pain, degeneration grade, baseline score) rather than relying solely on surgical technique variables.
3. Specificity of Risk Factors: The study isolates specific patient-level determinants, highlighting that patellofemoral degeneration (Iwano grade) and anterior knee pain are critical determinants of subjective joint "forgetfulness," independent of surgical alignment precision.
4. Dual-Track Validation: By confirming results across both anchor-based and distribution-based MCID definitions, the study strengthens the clinical validity of its prognostic factors.

Significance and Claims
The paper claims that preoperative patient characteristics, particularly the severity of patellofemoral degeneration and the presence of anterior knee pain, are the critical determinants of achieving a "forgotten joint" following medial fixed-bearing UKA when surgical technique is standardized. The authors assert that the developed risk stratification framework provides an objective, data-driven tool for clinical decision-making, enabling surgeons to optimize patient selection, manage expectations, and implement personalized Enhanced Recovery After Surgery (ERAS) pathways.

The study posits that while minor mechanical alignment variations (within safe precision margins) do not preclude a forgotten joint, underlying biomechanical states of the patellofemoral joint and potential central sensitization (suggested by low baseline FJS-12 scores) play a more decisive role. The authors conclude that routine integration of Iwano grading and assessment of anterior knee pain into preoperative evaluations is essential for identifying high-risk populations who may not perceive substantial benefit from UKA.

Limitations Noted by Authors
The authors acknowledge the study is limited by its single-center, retrospective design and a 14.2% loss to follow-up (though sensitivity analysis suggested missing data was random). They note the lack of long-term imaging follow-up and the focus on 1-year functional recovery under ERAS protocols. Additionally, the high rate of patellar denervation (93.2%) limits the ability to fully quantify its independent impact, and the findings are strictly applicable to medial fixed-bearing UKA, not lateral or bilateral procedures. Future work is suggested to include external validation, long-term tracking, and the integration of psychosocial factors.