Intramuscular fatty infiltration and its correlation with muscle composition and function in hip osteoarthritis.

BACKGROUND: Hip osteoarthritis patients display higher levels of fatty infiltration (FI) in the gluteus minimus (GM) compared to other hip muscles. We investigated specific histological factors such as fiber type composition and collagen deposition, and functional outcomes like muscle strength and activation associated with FI in these patients. METHODS: In twelve men (67 ± 6 y) undergoing total hip replacement (THR), hip and knee muscle strength and activation (electromyography, EMG) were assessed bilaterally. Magnetic resonance imaging (MRI) was used to compare the relative FI area and muscle cross sectional area (CSA) of the GM, rectus femoris (RF), tensor fascia latae (TFL) and vastus lateralis (VL). Adipocyte content, fiber type composition, grouping, fiber size, centrally nucleated fiber frequency, collagen deposition, satellite cell density and capillarization were assessed in intraoperative biopsies of the four muscles. Differences between GM and other muscles were assessed with repeated-measures one-way ANOVA followed by Dunnett's post-hoc test. Pearson coefficients were calculated for the correlations between FI measurements and the other histological and functional parameters. RESULTS: Strength was lower in the affected limb. Knee extensor weakness was accompanied by lower VL muscle activation. Muscle CSA and FI did not differ between sides. In the affected limb, GM displayed larger relative FI area (MRI) compared to RF and VL. Biopsy adipocyte content was higher in GM than RF and TFL. Compared to the other hip muscles, GM displayed higher type 1 fiber content while its type 2X fiber content was lower. Fiber grouping levels were higher in GM than the other muscles. Collagen content was higher in GM than TFL and VL.  FI in GM was associated with type 1 (r = 0.43) and type 2X (r = -0.34) fiber content, fiber grouping (r = 0.39), and collagen deposition (r = 0.37). FI in VL was negatively associated with maximal knee extension strength (r = -0.65). CONCLUSIONS: In patients undergoing THR, the higher FI levels of GM compared to other hip muscles were associated with fiber type composition and grouping, and with higher collagen deposition. Experimental studies exploring these associations could potentially uncover new targets for the treatment of intramuscular FI and related impairments in muscle function. TRIAL REGISTRATION: KEK number: 2016-01852, date of registration: 12-4-2017.