Tibial tubercle–trochlear groove distance and angle are higher in children with patellar instability

PURPOSE: The purpose of this study was to assess the variations in tibial tubercle-trochlear groove distance and angle as a function of age and gender in a population of children without patellar instability (PI) compared with those with PI. METHODS: A retrospective review of 869 children's knee MRIs, ages 5 to 15 years, were evaluated using a control group (792 children) without evidence of PI and a group with PI (77 children). Tibial tubercle-trochlear groove distance (TT-TGd) and angle (TT-TGa) were measured twice by two readers to assess intra- and inter-observer reliability and compared between PI and control groups. In both groups, functions of age and gender on TT-TGd and TT-TGa values were evaluated. RESULTS: Both TT-TGd and TT-TGa measurements showed excellent intra- and inter-observer reliability. The mean TT-TGd for the PI group was 17.2 mm (SD 6.6) and significantly higher than the mean TT-TGd for the control group (10.4 SD 3.8 mm, P = 0.001). The mean TT-TGa for the PI was 20.8° (SD 8.3°), which was also significantly higher than the mean TT-TGa for the control group (12.5° SD 4.6°, P < 0.001). Control group revealed a positive correlation between age and TT-TGd measurements (r = 0.243, P < 0.001). The mean TT-TGa for girls (13.3° SD 4.7°) was higher than the mean TT-TGa for boys (11.9° SD 4.4°) in the control group (P < 0.001). CONCLUSION: TT-TGa and TT-TGd are reliable and can be used for the evaluation of the extansor mechanism alignment in children with and without PI. However, it must be considered that TT-TGd is increasing in growing patients. Soft-tissue procedures may be prone to failure, since bony procedures for patellar alignment cannot be done until skeletal maturity. LEVEL OF EVIDENCE: III.