Local soft tissue thickness is a superior predictor of accuracy of implant alignment compared to body mass index in total knee arthroplasty.

BACKGROUND: Precise implant positioning is critical for optimizing outcomes and implant longevity in total knee arthroplasty (TKA). Although body mass index (BMI) has been linked to postoperative complications, its association with implant malalignment remains inconclusive. As local soft tissue thickness (LSTT) may serve as a more relevant risk factor, this study aimed to evaluate the relationship between LSTT and implant alignment deviation and to compare its predictive value to BMI in conventional, manual TKA. METHODS: A total of 122 consecutive patients who underwent primary TKA using the ATTUNE Knee System were retrospectively analyzed. Preoperative local soft tissue thickness was assessed at three anatomical levels: the ankle (ankle-adipose-index [AAI]), pretubercular region (pretibial subcutaneous fat [PSF]), and knee joint (knee-adipose-index [KAI]) using standardized radiographs. Postoperative implant positioning was measured by assessing coronal and sagittal alignment parameters. Group comparisons, correlation analyses, and receiver operating characteristic (ROC) curve analysis were performed to investigate associations between LSTT, BMI, and alignment deviations. RESULTS: PSF showed a statistically significant moderate positive correlation with deviation from target proximal tibial angle (PTA) (r = 0.422, P = 0.043) and sagittal femoral angle (SFA) (r = 0.431, P = 0.041). Patients classified as outliers with alignment deviations greater than 3° had significantly higher PSF values for PTA and posterior slope angle (PSA) (P < 0.001). ROC analysis of pooled data identified an optimal PSF threshold of 10.9 mm to predict outliers (AUC = 0.59, sensitivity 66.7%, specificity 50.4%). No significant associations were found between AAI, KAI, or BMI and implant positioning. CONCLUSION: PSF is a significant risk factor for component malalignment in both coronal and sagittal planes and provides a simple, radiograph-based measure that outperforms BMI in predicting alignment accuracy in conventional TKA. A PSF threshold of approximately 11 mm may help preoperatively identify patients at risk for malalignment, in whom navigation, robotics, or patient-specific instrumentation may be considered. Further prospective studies with larger cohorts and clinical outcomes are warranted to validate these findings. Video Abstract.