The Effect of the False Acetabulum on Femoral Proximal Medullary Canal in Unilateral Crowe Type IV Developmental Dislocation of the Hip

PURPOSE: To investigate the effect of the false acetabulum on femoral proximal medullary canal in unilateral Crowe type IV developmental dislocation of the hip (DDH) patients on anteroposterior radiographs. PATIENTS AND METHODS: We measured the following parameters of DDH and contralateral normal hip (CNH) of proximal morphology of femurs on 65 patients with unilateral Crowe type IV DDH (30 hips with no false acetabulum (type IVA) and 35 hips with a false acetabulum (type IVB)) in our hospital between September 2009 and July 2019 on anteroposterior radiographs: the widths of medullary canals at 20 mm above the center of lesser trochanter (CLT), 20 mm below the CLT and the isthmus. Canal flare index (CFI), metaphyseal canal flare index (MCFI), diaphyseal canal flare index (DCFI) were calculated. RESULTS: The values of CFI of DDH and CNH in unilateral type IVA patients were 2.8 and 4.4, respectively (p < 0.001), and those in type IVB patients were 3.9 and 4.6, respectively (p < 0.001). The MCFIs of DDH and CNH in type IVA group were 2.2 and 2.3, respectively (p = 0.032), and those in type IVB group were 2.4 and 2.4, respectively (p = 0.242). The DCFIs of DDH and CNH in type IVA group were 1.3 and 1.9, respectively (p < 0.001), and those in type IVB group were 1.7 and 1.9, respectively (p = 0.002). CONCLUSION: The false acetabulum stimulated the development of the proximal femur in Crowe type IV DDH. The variation of the femoral proximal medullary canal in type IVA DDH mainly occurred at the metaphyseal and proximal diaphyseal levels, and that in type IVB DDH mainly occurred at the proximal diaphyseal level.