The iliocapsularis muscle is a little known muscle that originates in part from the inferior border of the anterior-inferior iliac spine, but the main origin arises from an elongated attachment of the anteromedial hip capsule and inserts just distal to the lesser trochanter. Nevertheless, this muscle is an important landmark for exposure of the anteromedial hip capsule and psoas tendon interval during performance of the Bernese periacetabular osteotomy. Speculations about the function of this muscle as a tightener of the hip capsule and femoral head stabilisator have grown when an apparent hypertrophy of this muscle was encountered in patients with dysplasia of the hip. The aim of this study was to compare the morphology of the iliocapsularis muscle between patients with decreased (developmental dysplasia of the hip) and increased acetabular coverage (pincer-type of femoroac-etabular impingement) using Arthro-MRIs.

Dysplasia of the hip (Group I) was defined as an LCE angle of less than 25° with a minimal acetabular index of 14° and pincer type of FAI (Group II) was defined as and LCE angle exceeding 39° on conventional radiographs. This resulted in 37 hips in Group I and 45 hips in Group II. The morphology of the iliocapsularis muscle was measured on axial slices of Arthro-MRIs. The parameters were muscle thickness, width and cross section at 4cm distal of the spina iliaca anterior inferior and also distal of the femoral head. Additionally, the volume of the muscle from its origins to the cross section distal of the femoral head was computed.

All parameters were significantly increased in Group I compared to Group II (p< 0.05). In Group I the mean thickness was 20 ± 4.5 (range, 12 – 29) mm, width 25 ± 5.2 (range, 17 – 37), and cross section 281 ± 10.7 (range, 139 – 591) mm2 compared to Group II with a mean thickness of 17 ± 4.4 (range, 10 – 27) mm, width 22 ± 5.0 (range, 11 – 31), and cross section 235 ± 10.3 (range, 90 – 535) mm2. The muscle volume in Group I was 6.8 ± 2.9 (range, 2.2 – 13.0) cm3 compared to Group II with 8.7 ± 3.7 (range, 3.4 – 18.1) cm3. The results differed more when corrected for gender with the largest differences found for women.

In hip dysplasia the anterior acetabular coverage is decreased. Because of the iliocapsularis muscle’s origination on the hip capsule, contrition of the muscle theoretically can tighten the anterior hip capsule, thus helping to stabilized the femoral head within the dysplastic acetabular. Although the true function of the iliocapsularis muscle remains unknown, constant use of this muscle in attempting to stabilize the femoral head in hip dysplasia theoretically would explain the apparent hypertrophy of this muscle.