Summary

Individuals with FAI are unable to balance and stabilize their hips during tasks that require high degrees of combined hip extension and internal rotation.

Abstract

Cam-type femoroacetabular impingement (FAI) is characterized by an abnormal extension of the femoral head that disrupts normal joint biomechanics; and is a leading cause of early hip osteoarthritis. Further research is needed to explore whether symptomatic FAI affects other aspects of functional performance, particularly joint stability and single-leg balance during tasks that demand postural control. The aims were to examine biomechanical differences between FAI patients – comparing their symptomatic-affected and asymptomatic-unaffected hips – and healthy controls during the hurdle step task and the star excursion balance test distances both in stance and swing phases.

Seventeen patients with cam FAI (n = 17, m:f = 8:9; age = 27 ± 7; BMI = 24 ± 5) and seventeen healthy control participants (n = 17, m:f = 7:10; age = 25 ± 7; BMI = 24 ± 4), as confirmed on MRI, were recruited. Biomechanical assessments were conducted in our motion capture laboratory to capture 3D hip kinematics and kinetics. Each participant performed the hurdle-step task, keeping one foot on the ground and stepping over a hurdle bar (at knee height) to tap their toe and then return to a double-leg stance. This task was averaged over three trials on each side, alternating between swing and stance phases. Additionally, participants completed the star excursion balance test (SEBT) by performing single-leg squats and reaching as far as possible in eight clockwise directions (anterior, anterolateral, lateral, posterolateral, posterior, posteromedial, medial, anteromedial) when reaching with the right side and anticlockwise with the left (reach distance normalized by leg length). The Wilcoxon signed-rank test or paired-sample t-tests were used to compare each FAI patient’s affected and unaffected hips, while the Mann-Whitney U test or independent sample t-tests were used to compare the control group with each FAI group.

During hurdle-step, both the affected and unaffected FAI groups exhibited similar hip flexion angles during the swing phase compared to controls but required increased abduction to cross and return over the hurdle. In the stance phase of hurdle-step, the FAI group demonstrated decreased hip flexion and internal rotation compared to both the control group and their unaffected side throughout the entire task. During SEBT, when the FAI group’s affected side was standing, the swinging hips reached similar distances in the anteromedial, medial, and lateral directions compared to their contralateral, unaffected hip and the control group. However, the FAI group’s affected side had a shorter reach in the posterolateral direction (76% leg length) compared to the control group (85% leg length, p = 0.031).

The most important finding was that the FAI group’s affected side was unable to balance and stabilize their hips during tasks that required high degrees of combined hip extension and internal rotation. This altered biomechanics suggest that FAI patients adopt compensatory patterns affecting both the affected and unaffected sides. These findings highlight the complexities of reduced hip mobility and altered compensatory patterns in FAI patients. The inability to maintain stability during reach tasks, especially those requiring postural control and hip stabilization, indicates significant impairment in dynamic balance.