Prevalence of Intra- and Extra-articular Anterior Subspine Impingement in FAI Patients with Low Femoral Torsion Using 3D CT-Based Impingement Simulation

Purpose: It is unclear if decreased femoral torsion (FT) causes anterior intra- or extra-articular femoroacetabular impingement (FAI). Therefore, we analyzed symptomatic patients with FAI and with decreased femoral version (FV), using computed tomography (CT)-based virtual three-dimensional (3D) impingement simulation and compared this group with patients with normal FT and with asymptomatic hips. Our aim was to investigate (1) the osseous range of motion (ROM), (2) the osseous femoral and acetabular impingement zones, and (3) if hip impingement is extra- or intra-articular in symptomatic hips with FAI.

Methods and Materials: A retrospective comparative analysis approved by our institutional review board was performed on a total of 84 hips. Of these, 37 hips in 24 symptomatic patients with FAI had decreased FT (< 5 degrees). These hips were compared with 21 hips of 18 symptomatic patients with anterior FAI with normal FT (10–25 degrees) and 26 asymptomatic hips with no FAI and normal FT. All patients with FAI were symptomatic and had anterior hip pain and a positive anterior impingement test. They underwent pelvic CT scans to measure FT and for 3D reconstruction. The 37 hips with decreased FT were both with and without cam and pincer FAI. All 84 hips were evaluated using CT-based 3D models and a validated 3D ROM and impingement simulation. Asymptomatic hips were contralateral normal hips of patients undergoing total hip arthroplasty.

Results: Hips with FAI combined with decreased FT had a significantly (p < 0.001) lower mean flexion (114 versus 125 degrees) and internal rotation (IR) at 90 degrees of flexion (18 versus 32 degrees; p < 0.001) compared with the asymptomatic control group. In a subgroup analysis, we found a significantly (p < 0.001) lower IR in 90 degrees of flexion in hips with FT < 5 degrees combined with mixed-type FAI compared with hips with FT < 5 degrees without a cam- or pincer-type deformity. The maximal acetabular impingement zone for hips with decreased FT was located at the 2 o’clock position and ranged from 1 to 3 o’clock. In hips with decreased FT, most of the impingement locations were intra-articular, but 32% of hips had combined intra- and extra-articular FAI in internal rotation in 90 degrees of flexion. During the anterior impingement test performed in 10 and 20 degrees of adduction, a significantly (p < 0.001) higher prevalence (68% and 84%, respectively) of extra-articular subspine FAI was detected in hips with decreased FV compared with normal hips.

Conclusion: Hips with FAI and decreased FT had less flexion and internal rotation in 90 degrees of flexion compared with the asymptomatic control group. Most of the hip impingement due to low FT was intra-articular, but a third of patients had combined intra- and extra-articular subspine FAI. Anterior extra- and intra-articular hip impingement can be present in patients who have FAI with decreased FT. This could be important for patients undergoing hip arthroscopy and could change surgical decision making.

Fig. 1 Anterior impingement test. FAI, femoral acetabular impingement; FV, femoral version.

Fig. 2 Anterior test with 10-degree adduction. FAI, femoral acetabular impingement; FV, femoral version.

Fig. 3 Anterior test with 20-degree adduction. FAI, femoral acetabular impingement; FV, femoral version.

No conflict of interest has been declared by the author(s).

Publication History

Article published online:
17 December 2020