Ligament-dependent joint gap behavior reveals limitations of gap-based tissue balancing in total knee arthroplasty

Soft tissue balancing is a key determinant of stability in total knee arthroplasty (TKA), yet the biomechanical interpretation of intraoperative gap assessment remains unclear. This study developed a subject-specific finite element model of a TKA knee to evaluate how collateral ligament release and tightening conditions influence joint gap behavior during simulated intraoperative laxity tests. The medial collateral ligament (MCL) and lateral collateral ligament (LCL) were modeled using nonlinear elastic elements, and ligament pretension was systematically varied under graded release and tightening conditions. Varus and valgus moments (5 Nm) were applied at 0°, 60°, and 90° flexion to quantify medial and lateral tibiofemoral gap changes. Comparison with in vivo fluoroscopic measurements from CAMS-Knee subject K5R showed comparable magnitudes of postoperative knee translation and previously reported biomechanical trends. Joint gap response was direction-specific, flexion-dependent, nonlinear, and partially coupled between compartments. At 90° flexion, extensive MCL release increased the medial gap by up to 85% under valgus loading, while extensive LCL release increased the lateral gap by up to 75–80% under varus loading. In contrast, sensitivity remained minimal (< 10%) in full extension. Ligament release produced disproportionately larger gap changes than equivalent tightening conditions. The findings indicate that joint gap measurements do not provide uniform or unique information about ligament behavior and are most informative in mid to deep flexion. Overall, the results highlight inherent limitations of gap-based tissue balancing and support a more structured, flexion-dependent interpretation of intraoperative gap assessment in TKA.