Original ArticleThe association between degenerative hip joint pathology and size of the gluteus medius, gluteus minimus and piriformis muscles
Introduction
Osteoarthritis (OA) of the hip poses a considerable problem for modern society. As the incidence of OA of the hip increases with the aging population it has been declared by March and Bagga (2004) that ‘primary and secondary programs aimed at improving rehabilitation and physical activity are urgently required’ in the management of OA. Therapeutic exercise programmes designed to improve muscle function around the affected hip will only be maximally effective when we have further information available on both normal muscle function, and changes occurring in association with joint disease.
Hip abductor muscle function has been a primary focus of research due to the importance of these muscles in performing single leg function, the basis of human locomotion. Patients with OA of the hip have demonstrated a change in pelvic-femur alignment during gait depending on stage of pathology. Those with mild OA demonstrate increased hip adduction during stance (Watelain et al., 2001), while those with more advanced changes reduce adduction by increasing frontal plane trunk movement (Krebs et al., 1998). The specific changes in abductor muscle function occurring in association with OA are however unclear at this point. While some authors have demonstrated reduced electromyographic (EMG) activity in the gluteus medius (GMED) muscle in subjects with OA of the hip (Long et al., 1993), others have shown increased EMG activity during dynamic function (Angielczyk and Bronarski, 1982, Sims et al., 2002). EMG testing of the tensor fascia lata (TFL) muscle has shown similar inconsistency (Long et al., 1993, Sims et al., 2002). No EMG investigations of the other hip abductor muscles, upper gluteus maximus (UGM), gluteus minimus (GMIN) or piriformis (PIRI) muscles, in patients with OA of the hip, have been reported in the literature. Studies that have involved strength testing as a measure of hip abductor muscle function in subjects with OA of the hip, have used dynamometry to measure open chain isometric or isokinetic abduction strength, providing a global assessment of the abductor synergy (UGM, TFL, GMED, GMIN, PIRI). These studies have, like EMG studies, displayed considerable variability (Murray and Sepic, 1968, Teshima, 1994, Jandric, 1997, Arokoski et al., 2002, Sims et al., 2002). The body of literature to date thus provides an incomplete and unclear picture of hip abductor muscle dysfunction. More specific information on patterns of change within the abductor synergy is required.
The use of magnetic resonance imaging (MRI) provides an opportunity to assess each individual member of the abductor synergy simultaneously. One previous MRI study assessed cross sectional area (CSA) of the abductor muscles in subjects with OA of the hip, however most of the muscles were grouped together providing a global measure of abductor muscle size (Arokoski et al., 2002). In addition, single CSA measurements are unlikely to be as reflective of a muscle's morphology as a measurement of muscle volume. The research undertaken by the current authors used MRI to assess muscle volume of each individual member of the abductor synergy in subjects with OA of the hip. This has been presented as two papers with muscles divided on an anatomical basis. An initial study (Grimaldi et al., in press) investigated changes present in the superficial lateral musculature (UGM and TFL) that insert into the iliotibial band (Williams et al., 1989). The TFL was unaffected by the presence of joint pathology, while the UGM demonstrated asymmetry in subjects with advanced unilateral OA that appeared to be more closely related to hypertrophy of the unaffected side, than atrophy around the affected hip (Grimaldi et al., in press).
The main aim of the current study was to investigate in these same subjects, size of the muscles of the deep lateral stability mechanism of the hip, the GMED, GMIN, and PIRI muscles, that assert their effect via direct insertion into the greater trochanter. Subjects with either mild or advanced unilateral degenerative pathology of the hip were chosen for maximum clarity of effect. The specific aims were to examine i) if there was significant asymmetry in the deep abductor muscles across 3 groups (mild degenerative change, advanced degenerative change, control) and ii) if there were significant differences in actual muscle size among the pathology and control groups. This study also examined the association of both stage of pathology, and muscle size, with the factors of age, height, weight, pain, function and activity levels. Leg dominance was also tested as all of these factors were considered to have the potential to impact upon muscle size and symmetry.
The hypotheses of the study were that ia) there would be significant asymmetry in size of the GMED, GMIN, and PIRI in subjects with hip joint pathology, but not in controls, ib) asymmetry would be greater in subjects with advanced pathology, and ii) the GMED, GMIN and PIRI muscles would be smaller around the affected hip in those with advanced pathology compared to the matched hip of control subjects.
Section snippets
Subjects
Twelve subjects with degenerative hip joint pathology, and twelve age and sex matched control subjects were recruited for this study via medical practitioners and community advertisement. Control group subjects were required to be within 5 years of the age of their matched subject with joint pathology. Each group had equal numbers of males and females and all participants gave their informed consent to participate in this study after receiving detailed information on the study. Ethical approval
Side to side differences in muscle volumes within groups
There was no significant asymmetry in the control group for GMED, GMIN or PIRI muscle volume, although there was a trend for the GMIN muscle to be larger on the left side (p = 0.076, 9 of 12 control subjects larger on the left). No significant differences were observed for any of the muscles studied for the mild group. GMED and PIRI were both significantly smaller on the affected side for subjects with advanced pathology (t = 2.951, p = 0.008; t = 2.195, p = 0.03 respectively). Although comparisons of
Discussion
This study investigated the influence of degenerative hip joint pathology on size of the deep abductor muscles, GMED, GMIN and PIRI.
Conclusion
This study has shown that the deeper members of the hip abductor synergy, the GMED, GMIN, and PIRI muscles are smaller around the affected hip in subjects with advanced unilateral hip joint pathology. This atrophy was not measurable in subjects with mild pathology, however differing processes are likely in place associated with differing functional weight-bearing patterns. In subjects with mild pathology GMED muscle size was significantly larger on the affected side than control group subjects
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