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

Abstract

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.

Effect of positioning error on the Hilgenreiner epiphyseal angle and the head-shaft angle compared to the femoral neck-shaft angle in children with cerebral palsy

“…HSA was least affected by malpositioning; it was unaffected by abduction, was least affected by flexion, and was minimally impacted by internal/external rotations. This supports previous findings that found HSA was accurately measured within ±5° when the femur was positioned between 20° of internal rotation and 40° of external rotation [ 14 ]. The authors are unaware of literature investigating the impact of abduction/adduction on HSA.…”

“…HSA has been demonstrated to be greater in children with CP than in typically developing children and is associated with an increased risk of hip displacement [ 13 ]. Modeling studies have found that HSA was accurately measured within ±5° with varying degrees of internal rotation [ 13 , 14 ] with one noting that accurate measurement of HSA was unaffected by flexion [ 14 ]. However, these models were limited to two HSA angles and did not evaluate the impact of abduction/adduction.…”

Neck-shaft angle measurement in children: accuracy of the conventional radiography-based (2D) methods compared to 3D reconstructions

“…More dry bone, sawbone, cadaver or modelling study investigated to potential effect of malposition on NSA measuring. Wordie et al and Kay et al found quite big safe zone to measure NSA with acceptable accuracy (< 5°) 16 , 17 . In contrast, Bhashyam et al 19 stated that 10° rotational error with only minimal flexion or extension can cause more than 10° difference.…”

“…Although the effect of femoral torsion and rotation/flexion malpositioning on the measured NSA is widely known, most of the studies are based on ex vivo mathematical analysis or modelling of fully developed hips.Wordie et al examined the effect of rotation and flexion on the head-shaft angle using a dry bone model and found acceptable accuracy (< 5°) between 20° internal and 40° external rotation, as well as under 60° of flexion 16 . Kay et al used mathematical modelling and cadaver studies to define the safe zone to measure NSA.…”