Medial expected gap (mEG): a quantitative, biology-guided predictor of coronal plane correction and knee balance in robotic functionally aligned total knee arthroplasty.

Osteophyte excision during total knee arthroplasty (TKA) influences gap balancing and deformity correction, but its effect on medial gap behavior remains poorly defined. This study evaluated the relationship between osteophyte burden, medial expected gap (mEG), and coronal correction in robotic assisted functional alignment TKA. This prospective study included 82 knees. Preoperative computed tomography images were used to measure medial tibial and femoral osteophytes. Following pilot development, mEG was calculated using an average osteophyte burden model with a non-linear physiological constraint capped at < 4 mm. Virtual gap balancing intentionally preserved the calculated mEG as medial tightness within predefined functional-alignment boundaries. Associations between osteophyte burden, mEG, coronal plane correction, and final knee balance were analyzed using correlation, regression, and group comparison statistics with bootstrap resampling for internal model validation. Raw medial osteophyte size demonstrated no significant association with lateral laxity severity (p = 0.357) or varus correctability (p = 0.302). Medial tibial (β = 0.12; p < 0.001) and medial femoral osteophytes (β = 0.16; p < 0.001) independently predicted mEG (R² = 0.47). Total osteophyte burden increased significantly across ascending mEG categories (p < 0.001), while final mEG values remained within a narrow physiological range. Mean coronal plane correction increased from 4.3° (mEG 1 mm) to 9.7° (mEG 4 mm) (p < 0.001). All knees achieved acceptable balance: 42 best balanced, 32 good, and 8 acceptable. Bootstrap internal validation supported the stability of the principal regression findings. The mEG provides a quantitative framework linking osteophyte burden to medial gap behavior, enabling preoperative planning of medial tightness and reproducible coronal correction during robotic assisted functional alignment TKA.Level of evidence: Level II, prospective cohort study.