Dynamic gap analysis of valgus knees shows large inter-individual variability of gaps

The purpose of this study was to investigate the dynamic gap widths of valgus knees in extension and flexion to evaluate the influence of deformity on gap differences and to find out whether different ligamentous subtypes in valgus knees exist.

Dynamic gap widths of 1000 consecutive total knee arthroplasty (TKA) patients were measured at different flexion angles by applying a computer-assisted surgery (CAS) technique. 198 knees showed a valgus deformity and were assessed regarding its degree of fulfillment of the following criteria of valgus knee: 1. Medial extension gap greater than lateral; 2. Medial flexion gap greater than lateral; 3. Flexion gap greater than extension gap. A single-factor ANOVA subgroup analysis was performed, based on the amount of deformity. The effect of other patient factors (age, gender, weight) on gap differences was investigated.

The medial extension gap (3.7 ± 2.2 mm) was significantly (p < 0.01) larger than the lateral extension gap (1.1 ± 2.9 mm). The amount of deformity correlated highly with gap difference in extension (r² = 0.67) but not in flexion. In 92.4% (183), the flexion gap (6.2 ± 3.1 mm) was significantly (p < 0.01) larger than the extension gap (2.4 ± 2.3 mm). Only 29.3% (58) of patients met all three criteria, this was mainly due to the fact that in flexion the medial gap was larger than the lateral in only 35.4% (70). Patient factors showed no significant influences (NS) on the gap widths.

Valgus knees show large variability in terms of gap widths. The extent of deformity correlates highly with gap difference in extension, but not in other flexion angles. Vast majority of valgus knees were valgus in extension only. Because of this variability, it should be aimed for an individualized balancing technique based on intraoperative gap sizes.

Level III.