The detailed anatomy of the hip abductor muscles and their role in lateral hip pain

Abstract

Lateral hip pain (LHP) is a non-specific, clinical condition, characterised by pain and tenderness over the greater trochanter (GT) of the femur. Primarily, females between the ages of 40 and 70 years are affected, but differential diagnosis of the pathology causing symptoms is difficult. In the past, trochanteric bursitis has been thought to be the primary cause of pain, but recently pathology and atrophy of the hip abductor muscles, gluteus medius (GMed) and gluteus minimus (GMin), has been observed in these individuals. Little information is available on the anatomy of the hip abductor muscles, despite inference of their roles in specific movements of the hip joint and their association with LHP. A lack of comprehensive understanding of the normal anatomy of these muscles may explain the problems with accurately diagnosing pathologies implicated in LHP. With regards to treatment of lower limb dysfunction, the posterior part of GMed is often targeted, but there is little anatomical evidence to support the compartmentalisation of this muscle, and hence the effectiveness of these interventions. To further inform clinical practice it is necessary that the detailed morphology of the hip abductor muscles be described. The main purposes of this thesis are (1) to determine the detailed morphology of GMed, GMin and tensor fascia lata (TFL), including fibre type composition and volumes of these muscles in cadavers; (2) compare these data with that obtained from living individuals, and (3) investigate the differences in muscle volume, atrophy and radiological diagnoses of pathologies in the hips of patients with LHP compared to age and sex-matched controls. In the dissection component of this study, the three hip abductor muscles were examined in 12 cadaveric specimens. Details pertaining to attachment sites, fascicle orientation, fascicle and muscle architecture and volumes, tendon dimensions and innervation patterns were collected. Based upon these data, compartmentalisation of the muscles was determined. This study revealed that the anatomy of the hip abductor muscles is more complex than portrayed in the literature. It was found that GMed could be anatomically compartmentalised into either three or four distinct parts, but GMin and TFL were deemed homogenous structures. Also, the boundary between the musculotendinous junction (MTJ) and the free tendon within the tendinous complex of GMed was defined; at the level of the apex of the greater trochanter (GT). Such information is significant when considering the diagnoses and treatment of LHP. The second part to this thesis utilised the contralateral limb from six of the cadaveric specimens used for dissection, in order to determine the fibre type composition of GMed, GMin and TFL, using immunohistochemistry. It was found that GMed was comprised of approximately even amounts of type I and type II muscle fibres. GMin had the lowest percentage of type II muscle fibres, while TFL had the greatest percentage of type II fibres. These preliminary results support the proposed functions of these muscles, as observed within the literature. No differences were identified across the different muscle regions of GMed and GMin, suggesting that there were no specific parts of these muscles involved in preferentially phasic or postural movements. Finally, magnetic resonance (MR) images taken from a sample of individuals with LHP (n=10) and age and sex-matched controls (n=10) were utilised to determine muscle volumes of the hip abductor muscles in a living population. GMed was the largest in volume, followed by GMin and TFL. MR imaging volumes were approximately twice the size of muscle volumes determined from cadaveric specimens. This highlights the importance of supplementing cadaveric data with that from a living population. As muscle atrophy has been observed within the hip abductor muscles of patients with LHP, it was hypothesised that the muscle volumes of these individuals would be less than those found in age and sex-matched controls. Using the MR images, comparisons were made across the study groups to see if muscle volume loss, was associated with LHP. There were no differences in muscle volume between those women with LHP and age and sex-matched controls. It is speculated that the extent and duration of symptoms within the cohort used for the current study were not severe enough to detect volumetric differences, should atrophy be associated with LHP. A radiological assessment of the pathological changes within the hip region was performed for each participant. Specific structures were focused upon, and pathological changes such as distention of the trochanteric bursae, pathology of the GMed and GMin tendons, osteoarthritis (OA) and muscle atrophy were noted. According to radiological diagnoses, the only identifiable difference between symptomatic hips of individuals with LHP and matched controls was the dual presence of tendon pathology and bursitis in symptomatic hips. This indicates that radiological definitions for the diagnosis of LHP may need to be addressed, and MR imaging may not be an ideal modality to use for the differential diagnosis of pathologies underlying LHP. It is believed that anatomical details such as compartmentalisation, tendon dimensions and muscle volumes of the hip abductor muscles as reported in the current study will aid in the clinical diagnosis of LHP. In addition, these reports may help the radiological diagnoses of the pathologies associated with LHP, and subsequently, improved treatment strategies can be developed accordingly.