Iatrogenic coronal-sagittal coupling driven by a 12.4° rotational mismatch in manual total knee arthroplasty and precise decoupling with robotic assistance: a radiographic retrospective cohort study

BACKGROUND: Achieving precise reconstruction of both the coronal and sagittal planes is pivotal in total knee arthroplasty (TKA). However, conventional manual TKA (M-TKA) may induce an unintended "coronal-sagittal coupling" effect, where adjustments in one plane inadvertently alter the outcome in the other. This study aimed to quantify this iatrogenic coupling effect in M-TKA and to evaluate the potential benefits of robotic-assisted TKA (R-TKA) in decoupling these two planes and preserving sagittal Combined Flexion (CF). METHODS: This retrospective study consecutively enrolled 360 patients who underwent primary TKA between October 2023 and October 2025 (199 in the manual group and 161 in the robotic group). Pre- and postoperative coronal and sagittal parameters were measured on standing full-length lower extremity radiographs and standard lateral radiographs. The "Tibial Cutting Guide Varus/Valgus Angle" (TCVA) was defined to quantify the deviation in cutting guide placement. The study adopted a systematic stepwise analysis strategy: initially detecting potential inter-planar coupling via correlation analysis, subsequently identifying independent predictors of sagittal posterior tibial slope using linear regression models, and finally calculating the equivalent rotational mismatch angle driving this coupling from regression coefficients based on stereometric projection principles. RESULTS: In the M-TKA group, ΔMPTA showed a significant negative correlation with ΔPTS (r = - 0.209, p = 0.003), an effect not observed in the R-TKA group. TCVA was a significant independent predictor of postoperative PTS in M-TKA, and geometric analysis revealed an equivalent rotational mismatch of approximately 12.4° between the cutting guide rotational axis and the prosthesis placement axis. The CF preservation rate was significantly higher in the R-TKA group than in the M-TKA group (60.9% vs. 22.6%, p < 0.001). Additionally, ΔDFF and ΔPTS showed a significant negative correlation within the R-TKA group, demonstrating a unique intra-sagittal compensatory mechanism. CONCLUSION: Conventional M-TKA exhibits an iatrogenic coronal-sagittal coupling effect, whereby coronal plane correction errors are projected onto the sagittal plane through inherent rotational mismatch and cutting guide tilt, with error magnitude positively correlated with deformity severity. R-TKA effectively decouples these two planes and better preserves the native sagittal geometric alignment through femoral-tibial synergistic adjustment. LEVEL OF EVIDENCE: Level III, a retrospective cohort study.