To our knowledge, this is the first study to examine the relationship between hip muscle CSA and hip pain and functional outcomes in community-based individuals with mild-to-moderate hip OA. There was a positive association between greater CSA of hip adductors and better function (activity of daily living and sport and recreation) and quality of life. There was also a non-significant positive association between greater CSA of hip flexors and better quality of life. These findings suggest that targeting the hip adductor and flexor musculature in treatment may lead to improved hip function and quality of life among individuals with mild-to-moderate hip OA.
We found that greater CSA of hip adductors and flexors was associated with better functional outcomes in those with mild-to-moderate hip OA. The most significant and consistent associations were found for the hip adductors, including adductor longus and magnus. The hip adductor muscles play an important role in balancing the pelvis during standing and walking and for overall hip stability and injury prevention. Previous studies reported decreased strength of adductor muscles in people with pain or injury, with one study reporting a lower hip adduction/abduction strength ratio in soccer players with groin pain during hip adduction testing compared with those with a pain-free test (13) and another study showing that hip adductor strength was decreased both preceding and during the onset of groin pain in football players (14). Although these studies investigated adductor strength rather than CSA, the findings indicate the importance of hip adductor muscles when considering pain and recovery for better function.
There was a trend that larger CSA of hip flexors would be associated with better quality of life in our study. Hip flexors are used for a variety of everyday functional activities such as advancing the lower extremity during gait, running, or lifting the leg when going up steps. However, no previous studies have examined the relationship between hip flexors and hip pain or functional outcomes. One study found a weak negative correlation between gluteus medius muscle fiber CSA and hip pain (15). In contrast, our study did not find an association between CSA of gluteus medius and minimus muscle and hip pain or function. The gluteus medius and gluteus minimus muscles were measured together in our study as they perform the similar function of abduction; as a result, the association between gluteus medius and hip pain may have been overlooked.
In those with hip OA, larger CSA of hip muscles may result in greater muscle strength (7) which would allow efficient force distribution within the joint and improvement of hip stability, resulting in better function. It is also plausible that better function facilitates more use and therefore larger CSA of the hip muscles. Our results suggest that there are potential muscles that could be targeted in those with hip OA to improve functional outcomes. This will need to be tested in clinical trials. Clinical guidelines for the treatment of hip OA recommend education, strength training and exercise programs (21–23). A systematic review of 13 cross-sectional studies suggested the need to target muscle weakness in the clinical management of hip OA (8). The greatest reduction in muscle strength of the affected leg was seen for hip flexors and extensors, with less consistent data for hip adductors and abductors (8). Adding to the literature, our study found beneficial associations of larger CSA of hip adductors and flexors with better function and quality of life in hip OA. Taken together, these data suggest that targeting these hip muscles may have significant implications for reducing the burden of hip OA in the community, where pain, disability and impaired quality of life are growing concerns in the current population.
This study had limitations. It is a cross-sectional study with a moderate sample size. Whether there is a temporal relationship between hip muscle CSA and functional outcomes could not be investigated. The moderate sample size limited the power of the study to detect an association between CSA of some hip muscles and pain and functional outcomes. Due to the lack of demarcation between muscles, some specific muscles could not be segmented individually and instead were grouped for CSA measurement, such as the adductors. Identifying specific muscles could shed further light on which muscles contribute to better hip outcomes. Consistency with regards to anatomical positions for muscle CSA measurement was another issue. The slice thickness may have surpassed some regions due to the difference in terms of where the initial slice began or the difference in body size among participants. However, this measurement error was at random and would have resulted in underestimation of the magnitude of observed associations. We were only able to adjust for limited numbers of confounders in the statistical analyses, and we have controlled for the difference in body size by adjustment for gender. The strengths of this study include the high reproducibility of the MRI measurement of hip muscle CSA. A full-length scan from the iliac crest to the knee allowed the CSA measurement of hip muscles of different functional groups. HOOS is validated and well accepted in the OA scientific literature and clinical settings (19). It is easy to use within clinical practice to follow patients with hip OA over time and is suitable to use in research as a disease-specific questionnaire (19).
This study found that greater CSA of hip adductors and flexors was associated with better function and quality of life in individuals with mild-to-moderate hip OA. These findings, while need to be tested in clinical trials, suggest that targeting hip adductor and flexor muscles may have a beneficial effect on improving function and quality of life in those with mild-to-moderate hip OA.