Original ArticleThe association between degenerative hip joint pathology and size of the gluteus maximus and tensor fascia lata muscles
Introduction
Therapeutic exercise has been cited as an important approach used in management of osteoarthritis (OA) of the hip (Hochberg et al., 1995, Altman et al., 2000, Smidt et al., 2005, National Collaborating Centre for Chronic Conditions, 2008, Zhang et al., 2008). There is however, a distinct scarcity of literature investigating the effectiveness of therapeutic exercise of the hip. Programmes have often been quite generalised with small to moderate short term effects and poorer long term effects (van Baar et al., 2001, Tak et al., 2005). Outcomes may be improved through the development of more specific programmes based on a greater understanding of muscle function and dysfunction around the hip joint. One of the most consistent findings in subjects with hip dysfunction is an inability to maintain adequate lateral control of the hip and pelvis in single leg stance (Hardcastle and Nade, 1985). Studies assessing hip abductor muscle strength in subjects with OA of the hip have found deficits of up to 31% (Murray and Sepic, 1968, Jandric, 1997, Arokoski et al., 2002), while others have found no significant losses in abductor strength (Teshima, 1994, Sims et al., 2002). These apparent inconsistencies may be associated with specific changes occurring within muscles of the abductor synergy, and the association of these changes with stage of pathology.
While strength testing provides information on global abductor muscle function, a resultant effect of all synergists, specific changes within the synergy will only become evident by addressing each muscle individually. Muscles of the abductor synergy providing lateral stability of the hip and pelvis could be divided into superficial muscles that provide their effect via insertion into the iliotibial band (ITB), and deeper muscles that act via insertion into the greater trochanter. Muscles of the superficial system include the tensor fascia lata (TFL) muscle and the gluteus maximus (GM) muscle. The deep system would include the gluteus medius (GMED), piriformis (PIRI) and gluteus minimus (GMIN) muscles. This paper will focus on the study of muscles of the superficial system, while the deep muscle system will be addressed in a further publication (Grimaldi et al., unpublished).
In clinical rehabilitation settings, the GM muscle has been targeted for strengthening exercises, due to its reported tendency to weaken and atrophy (Janda, 1983, Sims, 1999, Sahrmann, 2002). In contrast, the TFL muscle has been targeted for lengthening techniques, due to its reported tendency to become excessively active (Janda, 1983, Sims, 1999, Sahrmann, 2002). There has been little attention paid in either research or clinical settings, to the impact of the functional differentiation of the GM muscle on joint mechanics and the prescription of therapeutic exercise. The upper portion of the GM muscle (UGM) arises from the posterior iliac crest, while the lower portion of the GM muscle (LGM) arises from the inferior sacrum and upper lateral coccyx (Williams et al., 1989). Despite a lack of fascial separation in adult humans, studies on morphogenesis of the GM muscle have revealed that it arises from two muscle primordia with a loose connective tissue separation between cranial and caudal portions in the foetus followed by fusion in the prenatal period (Tichy and Grim, 1985). The UGM, acting above the centre of rotation of the hip, has a primary function of hip abduction, and does not have a role in hip extension. While both portions may externally rotate the femur, the lower portion of the GM muscle (LGM), acting below the centre of rotation of the hip, is the primary hip extensor (Stern, 1972, Stern et al., 1980, Lyons et al., 1983, Jaegers et al., 1992) playing an important protective role in absorbing ground reaction forces at heel strike during gait.
The role of the hip abductor synergy in joint protection is less clear. While hip abductor strengthening is generally considered as a priority in patients with hip pain, an in vivo study on joint loads during gait revealed that peak joint loads were associated with peaks in hip abductor muscle activity during stance phase rather than solely loads applied from body weight (Krebs et al., 1998). Contrary to common clinical belief, the authors from this study recommended that clinicians aiming to reduce joint load should reduce hip abductor activity.
Another important aspect that should be considered in the prescription of therapeutic exercise for patients with OA of the hip is the stage of pathology. While global atrophy of hip muscles may be present in end stage pathology, in the earlier stages of the condition, more specific changes in the muscles of the hip abductor synergy may occur. It has been proposed that these changes can result in alteration of the orientation of the resultant hip joint vector, and ultimately result in joint damage over time (Kummer, 1993, Sims, 1999). Further information pertaining to hip muscle dysfunction at different stages of pathology would be useful as it could be used in the development of more specific and possibly more effective conservative intervention or prevention programmes for those with degenerative hip joint pathology.
Imaging studies provide an excellent opportunity to analyse individual muscles of the hip. Only one study has measured muscle size in subjects with OA of the hip. Arokoski et al. (2002) used magnetic resonance imaging (MRI) to measure hip muscle cross sectional area (CSA) in men with and without hip OA. Two axial slices through the pelvis provided a single CSA for LGM and a combined CSA of all hip abductors, including the UGM. This measure unfortunately failed to provide specific information of individual muscles of the abductor synergy. Furthermore, volume measurements rather than single slice CSA measurements, may be more representative of the complex pelvic musculature. One study has reported muscle volume measurements of the hip muscles for three healthy subjects (Jaegers et al., 1992), but no volume measurements have been reported in subjects with hip OA.
The main aim of this study was to investigate size of the muscles of the superficial lateral stability mechanism of the hip, TFL and GM muscles, in subjects with either mild or advanced degenerative pathology of the hip. Subjects with unilateral pathology were selected in order to provide both within and between subject comparisons. The specific aims were to examine i) if there was significant side asymmetry in the superficial muscles across 3 groups (mild degenerative change, advanced degenerative change, matched controls), ii) if there were significant differences in actual muscle size among the pathology and control groups, and iii) if the functionally separate portions of the GM muscle, UGM, and LGM, display similar patterns of change in subjects with hip pathology. 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.
The hypotheses of the study were that ia) there would be significant asymmetry in size of the UGM, LGM, and TFL in subjects with hip joint pathology, but not in controls, ib) asymmetry would be greater in subjects with advanced pathology, ii) the affected side LGM muscle would be smaller that the comparable side in control subjects, based on clinical expectation (Sims, 1999, Sahrmann, 2002), and iii) changes in the UGM would more closely reflect changes in the TFL muscle based on their close functional relationship.
Section snippets
Subjects
Twenty-four subjects (12 subjects with hip joint pathology and 12 control subjects) were recruited for this study via community advertisement and via contact with medical practitioners. Control subjects were recruited to match each subject with pathology by sex and age. The age of the control subject was required to be within 5 years of the age of the matched subject with hip pathology. There was an equal distribution of males and females in each group. Subject details are listed in Table 1.
Side to side differences in muscle volumes within groups
There were no significant side to side differences in the control or mild pathology groups. While LGM size was smaller on the affected side in all but one subject in the group with mild joint changes, the asymmetry was not great enough to be statistically significant. In the group with advanced pathology there were significant between side differences in the GM but not the TFL muscle. The asymmetry was greater in the UGM muscle (mean difference 21%, p < 0.01) than the LGM muscle (mean difference
Discussion
This study investigated the influence of degenerative hip joint pathology on size of the GM and TFL muscles.
Conclusion
This study has demonstrated that the GM muscle should be considered as 2 functionally separate entities, the UGM a hip abductor and the LGM, a hip extensor, these muscles having differing responses to the presence of joint pathology. The UGM muscle like its functional counterpart, the TFL, appears unaffected on the side of joint pathology, while the LGM muscle demonstrates local atrophy. The lack of affect on the superficial hip abductors suggests that muscle weakness demonstrated in subjects
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