Fat infiltration in the gluteus minimus muscle in older adults

Fat infiltration (FI) in the gluteus minimus muscle (GMin) has been reported as being restricted to the anterior part. The aim of this study was to verify whether FI is localized only to the anterior part of the GMin through the dissection of large number of elderly cadavers and to clarify the underlying mechanism of FI localized to a specific region.

Thirty-eight hips from 20 embalmed elder cadavers were dissected for verification of FI localization. The samples of three regions - the fat infiltrated region of the anterior part (FI), the region without FI (non-FI), and the posterior region - were harvested from nine hips for the histological fiber type assessment.

The FI area was distributed at 2.6°-38° from the anterior border within the anterior part, of which function is responsible for hip extension and internal rotation and is likely reduced by disuse following aging. The number of type I fibers was lowest in FI, followed by non-FI, while the number of type I fibers was highest in the posterior. Meanwhile, the number of type II fibers was equivalent across the three regions. Atrophic type II fibers were observed in overall samples of the three regions, and grouped atrophy, which is evidence of denervation, was observed in FI and non-FI samples. Type grouping, which suggests to be reinnervated, was found in the non-FI and posterior samples but very little in the FI samples.

FI in the GMin in older adults restricted to deep within the anterior part is secondary to disuse and is strongly related to impaired denervation-reinnervation process.