Introduction: Pediatric patients with Blount disease frequently demonstrate secondary adaptive deformities in the adjacent distal femur. This study evaluates adaptation of longitudinal and angular proportions of the ipsilateral healthy femur to progressive leg length discrepancy in unilateral cases.
Methods:  The study included 55 children with unilateral Blount disease. Preoperative radiographs were analyzed to characterize the condition as infantile or adolescent and measure femoral/tibial lengths and mechanical lateral distal femoral angles (mLDFA). 
Results: There were 26 patients with infantile and 29 with adolescent Blount disease. Adolescent patients were significantly older (14.4 ± 2.0 vs. 9.2 ±2.4; p<0.01). Black race was prevalent in both groups (69-79%). The adolescent group was predominantly male (25/29; 86%), while the infantile group was predominantly female (15/26; 58%, p<0.01). Leg length inequality in adolescent patients was significantly greater than in the infantile group (2.8 ± 2.0 vs.1.5 ± 1.1cm; p<0.01) with ipsilateral femoral shortening (1.8 ± 1.8 cm) accentuating tibial shortening (1.0 ± 1.1cm). Patients with infantile Blount disease had more pronounced tibial discrepancy (2.0 ± 1.1 cm; p<0.01) but modest overgrowth of the ipsilateral femur (0.5 ± 0.7; p<0.01) partially compensating ipsilateral tibial shortening. There was a significant difference in tibial:femoral ratios between the groups (p<0.01). The infantile group had on average normal mLDFA (88°), most adolescent patients had accentuating distal femoral varus deformity (96° ± 5°; p<0.01).
Conclusions: Patients with unilateral infantile and adolescent Blount disease demonstrated distinctly different adaptation of the ipsilateral femur. Concomitant ipsilateral femoral changes aggravate angular deformity and leg length discrepancy in adolescent Blount disease.