The effect of femoral orientation on the measurement of the head shaft angle: an ex-vivo study.

This laboratory study evaluates head shaft angle (HSA) reliability using ranges of simulated femoral orientation often seen in children with cerebral palsy. A dry femur was mounted in a jig that enabled the bone to be positioned in a range of internal and external rotation (-40° to +40°) and flexion (0°-60°), alone or in combination. A metal wire was placed as a surrogate physis to give two HSA angles of 140° and 160°. Radiographs were taken of the femur in differing combinations of rotation, flexion and the two HSA angles. The HSA was measured by four independent observers on two separate occasions. Intraclass correlation coefficients (ICCs) were used to assess interobserver and intraobserver reliability. The HSA was accurately measured within ±5° when the femur was positioned between 20° internal rotation and 40° external rotation. Flexion up to 60° did not affect the accuracy of the measurement. The interobserver reliability for the HSA was excellent with an ICC of 0.9970 [95% confidence interval (CI): 0.9995-0.9983] for the first measurement and 0.9988 for the second (95% CI: 0.9979-0.9993, all P<0.01). The intraobserver reliability was also excellent with an ICC of not less than 0.990 for all four observers (95% CI: 0.9806-0.9986, all P<0.01). There was excellent interobserver and intraobserver reliability when measuring the HSA in an experimental model provided femoral rotation lay within 20° internal and 40° external rotation and less than 60° of flexion.