The iliocapsularis muscle: an important stabilizer in the dysplastic hip

Abstract

Background: The iliocapsularis muscle is a little known muscle overlying the anterior hip capsule postulated to function as a stabilizer of dysplastic hips. Theoretically, this muscle would be hypertrophied in dysplastic hips and, conversely, atrophied in stable and well-constrained hips. However, these observations have not been confirmed and the true function of this muscle remains unknown.

Questions/purposes: We quantified the anatomic dimensions and degree of fatty infiltration of the iliocapsularis muscle and compared the results for 45 hips with deficient acetabular coverage (Group I) with 40 hips with excessive acetabular coverage (Group II).

Patients and methods: We used MR arthrography to evaluate anatomic dimensions (thickness, width, circumference, cross-sectional area [CSA], and partial volume) and the amount of fatty infiltration.

Results: We observed increased thickness, width, circumference, CSA, and partial volume of the iliocapsularis muscle in Group I when compared with Group II. Additionally, hips in Group I had a lower prevalence of fatty infiltration compared with those in Group II. The iliocapsularis muscle typically was hypertrophied, and there was less fatty infiltration in dysplastic hips compared with hips with excessive acetabular coverage.

Conclusion: These observations suggest the iliocapsularis muscle is important for stabilizing the femoral head in a deficient acetabulum. This muscle serves as an anatomic landmark when performing a periacetabular osteotomy. Additionally, preoperative evaluation of morphologic features of the muscle can be used as an adjunct for decision making when treating patients with borderline hip dysplasia or femoroacetabular impingement.

Fig. 1

An axial MR image shows the iliocapsularis muscle (dashed line) anterior of the hip. Five parameters were measured to describe morphologic features of the muscle: thickness (a), width (b), circumference (dashed line), cross-sectional area (dashed line), and fatty infiltration according to Goutallier et al. [11]. Muscle thickness (a) was measured along a radial line (dotted line) passing through the femoral head center (F). The width (b) was measured perpendicular to the thickness (a).

Fig. 2

The cadaver dissection shows the anterior aspect of the hip. The iliocapsularis muscle (IC) originates from the anterior-inferior iliac spine (AIIS) and the anterior hip capsule (C). Its insertion is located just distal to the lesser trochanter (LT).

Fig. 3A–B

An (A) MR image and (B) three-dimensional drawing show the two locations for measurement of muscle morphology and fatty infiltration: 4 cm below the AIIS (Location A), and on the first section inferior to the femoral head (Location B)

Fig. 4A–D

(A) A drawing and an axial MR arthrography slice from a dysplastic hip (Group I, at 4 cm below the AIIS [Location A]) shows increased size (thickness, width, circumference, CSA) of the iliocapsularis muscle (IC) with decreased fatty infiltration compared with (C) the drawing and (D) axial MR arthrography slice of the iliocapsularis muscle in a hip with excessive acetabular coverage (Group II). F = femoral head, A = acetabulum, PA = psoas major muscle, PI = psoas minor muscle, I = iliacus muscle, R = rectus femoris muscle, S = sartorius muscle, TF = tensor fascia muscle, MI = gluteus minimus muscle, ME = gluteus medius muscle, MA = gluteus maximus muscle, O = obturator internus muscle, PE = pectineus muscle, A = femoral artery, V = femoral vein, N = femoral nerve, C = hip capsule.