Assessment of the iliocapsularis muscle on magnetic resonance imaging

Morphologic features of Iliocapsularis (IC) may aid clinical decision-making in the symptomatic hip. The relationship between IC muscle size and underlying hip pathology is emerging; however, research is limited in the imaging literature. The purpose of this study was to determine the reliability and reproducibility of measurements of the IC muscle and its MRI appearances. It also looked for any association between IC dimension and axial levels, side, gender and bony features of hip instability.

Retrospective study of 37 MRI scans were assessed by four observers. MRI axial T1 images were used to define the IC anatomy, measure the IC and rectus femoris at the femoral head centre (FHC) and adjacent levels and calculate the iliocapsularis-to-rectus femoris (IR) ratio. Measurements were repeated at least 2 weeks later. Radiographic assessment of the lateral centre edge angle, acetabular index and femoral neck-shaft angle were also conducted.

The IC was always present, but was well-defined in only 4% of cases with fair agreement. The intraclass correlation coefficient for reliability and reproducibility was the highest for IC width 0.94 (0.91-0.96). No significant correlation was identified between the IR ratio and radiographic parameters.

Iliocapsularis is visible and reliably measured on MRI despite observers considering the muscle to be not well-defined. Despite gender differences in muscle size, the IR ratio was unchanged. There is a significant difference in the IR ratio above and below the FHC; therefore, clinicians need to be aware of how this may impact the clinical use when utilising the IR ratio.