A novel zone-based union classification for hybrid lateral closing wedge high tibial osteotomy: radiographic validation and comparative clinical observations with medial opening wedge high tibial osteotomy

To introduce a zone-based union classification for hybrid lateral closed-wedge high tibial osteotomy(Hybrid LCW-HTO) and, as a secondary comparison, to describe bone union timing and serial alignment changes over 12 months relative to medial open-wedge high tibial osteotomy (MOW-HTO). Retrospective cohort of 161 knees (81 Hybrid LCW-HTO; 80 MOW-HTO), operated between 2018 and 2021. A zone-based classification was introduced to grade union progression for Hybrid LCW-HTO. Alignment parameters were measured pre-operatively, immediately under anaesthesia, on post-operative day 1, and at 1, 3, 6, and 12 months. CT and plain radiographs confirmed bone union. Multivariate regression identified independent predictors of union time. The zone-based classification showed good inter- and intra-observer reliability (intraclass correlation coefficient(ICC) 0.80–0.94). In this cohort, Hybrid LCW-HTO union occurred later than MOW-HTO (5.38 vs. 3.59 months; p < 0.001), with delayed union more frequent (16.0% vs. 5.0%; p = 0.043). On multivariate analysis, surgical technique (Hybrid LCW-HTO) and lower bone mineral density were independently associated with longer union time. Neither group showed clinically meaningful alignment loss. But, under anaesthesia, the joint line convergence angle (JLCA) fell immediately in Hybrid LCW-HTO (3.35°→1.59°), but not in MOW-HTO (3.43°→3.08°); from POD1 onward, both groups were statistically identical. This study introduces and radiographically validates a reliable zone-based union classification for Hybrid LCW-HTO. In this cohort, Hybrid LCW-HTO was associated with slower union and a higher delayed-union rate than MOW-HTO, although all knees ultimately achieved union by one year. Both techniques maintained stable alignment over one year, but had different timing of JLCA change, with Hybrid LCW-HTO achieving its final JLCA correction immediately under anaesthesia. III, Retrospective cohort study.