Quantitative analysis of bony and soft tissue contributions to varus deformity across knee osteoarthritis stages.

PURPOSE: To investigate the relationships among lower limb alignment parameters and quantitatively clarify the stage-dependent contributions of bony morphology and soft tissue laxity to varus deformity across knee osteoarthritis stages.

METHODS

We analysed 1346 knees of 673 participants in a community-based cohort. Radiographic parameters, including the hip-knee-ankle angle (HKAA), weight-bearing line ratio (WBLR), femorotibial angle (FTA), medial proximal tibial angle (MPTA), lateral distal femoral angle (LDFA) and joint line convergence angle, were measured using full-length standing radiographs. Spearman's correlation coefficients and linear regression analyses were performed for the overall cohort and stratified subgroups: early- (Kellgren-Lawrence Grade [KLG] 0-1), progressive- (KLG 2) and end-stage (KLG 3-4).

RESULTS

In the overall cohort, the HKAA exhibited robust correlations with WBLR (r = -0.99, p < 0.001) and FTA (r = 0.86, p < 0.001), validating FTA as a reliable global alignment surrogate. Stage-dependent analysis revealed a change in the relative associations among determinants of deformity. In the early- and progressive-stage groups, varus alignment was predominantly associated with bony parameters (MPTA and LDFA). Conversely, in the end-stage group, the correlation between the joint line convergence angle and varus deformity strengthened significantly (r = 0.61 vs. r = 0.20 in the early stage).

CONCLUSIONS

While bony deformity is a consistent factor, the contribution of soft tissue laxity (joint line convergence angle) becomes substantial in end-stage osteoarthritis. In end-stage osteoarthritis, the joint line convergence angle is a key alignment determinant. Surgical planning for joint-preserving procedures, including high tibial osteotomy, must account for this quantitative confirmation of stage-dependent differences to prevent reducible soft tissue component overcorrection.

LEVEL OF EVIDENCE: Level III.