Pelvic reference selection in patients with unilateral Crowe type IV DDH for measuring leg length inequality.

We identified the presence of deformities in the affected pelvis of unilateral Crowe type IV DDH patients, and if present, whether the teardrop and ischial lines were parallel with the sacral base line. We also verified whether the sacral base line provided a better pelvic landmark than the teardrop line for determining leg length inequality (LLI). After leveling the pelvis by using a block to lift the short leg, standard anterior-posterior full-length radiography was performed on 10 patients and 10 healthy volunteers as controls. The ratio of pelvic heights on each side of the pelvis, the angles formed by the sacral base line and the other 2 lines between 2 groups were measured. LLI were measured by sacral base line and teardrop line respectively. The ratio between the pelvic heights was lower in the patient group than in the control group (0.95 versus 0.99). The angles between the teardrop and ischial lines and the sacral base line in the patient group were both greater than in the control group (6.08° versus 0.92° and 7.13° versus 0.97°). LLI measured from the sacral base line was larger than from the teardrop line in the patient group (5.55 cm versus 4.36 cm). There was pelvic asymmetry and the sacral base line was not parallel with the other 2 lines in unilateral Crowe type IV DDH. The leveled sacral base line was perpendicular to the longitudinal axis of the body, and may be a better choice for accurate LLI measurement in this situation.