This study is the largest series to date to examine hip abductor tendons in a cadaveric population. We demonstrated that the majority of tears involved the insertion of anterior/middle gluteus medius and the gluteus minimus. Microscopically, degeneration at both the tendon and enthesis underlies the observed macroscopic tears. Lastly, we found more advanced degeneration in the deeper layers of the tendon and enthesis, suggesting a potential pathomechanism of gluteus medius tears.
While many previous studies reported tears in the anterior gluteus medius, only a few have examined the gluteus minimus 26–28. In the present study, gluteus minimus tears were observed more commonly than gluteus medius tears. Furthermore, all complete tears of the gluteus medius were associated with a concurrent gluteus minimus tear. This finding has clinical importance, as both ultrasound and MRI are poor at identifying gluteus minimus tears when a gluteus medius tear was also present1, 26.
The pattern of disease identified also provides evidence that hip abductor tears may originate from the gluteus minimus. Previous electromyography and functional studies found increased loads in the gluteus minimus during the single stance phase of gait, particularly as the hip moves into adduction and extension29, 30. This “overload” phenomenon predominantly affects the gluteus minimus and not the gluteus medius31. Furthermore, the area of gluteus minimus deep to gluteus medius may experience increased compressive loads 8, 32. This combination of increased tensile and compressive forces is a known risk factor for the development of tendinopathy and tears32–34. Functionally, the gluteus minimus acts synergistically with the anterior gluteus medius to stabilise the pelvis during gait and facilitate forward contralateral rotation of the pelvis29, 30, 35, 36. This shared function may also help to explain the large number of tears observed in both the gluteus minimus and the adjacent anterior gluteus medius tendons.
We identified a strong association between complete tendon tears and fatty atrophy of the gluteus medius and minimus. This has been reported in a number of imaging studies37–39. Once established, muscle atrophy is often irreversible and therefore, repair of the tendon is not recommended in such cases40, 41. This raises the importance of early detection and repair of partial gluteus medius and minimus tears.
Microscopically, the present study has confirmed degeneration as the underlying pathology in hip abductor tendon tears. This is similar to histological findings in rotator cuff, Achilles and patella tendon tears24, 42–44. In the hip abductor tendons, a study by Fearon et. al examined tendon samples obtained intraoperatively at the time of gluteus medius repair. Given the small number of samples, they were only able to report qualitative degenerative change. Furthermore, the degree of degeneration was not specified and the enthesis was not examined1. Using semiquantitative scores, we found significant degeneration in not only the tendon but also the enthesis when tears were present in both the gluteus medius and minimus. Given the importance of the enthesis in load distribution, it is not surprising that degeneration in both tissue types contribute to tendon tears 21. Interestingly, we were unable to demonstrate significant degenerative changes in two small, partial tears in the posterior insertion of gluteus medius. However, a larger sample size is required for definitive conclusions.
An important finding of the study was that microscopically, degenerative changes are concentrated in the deeper layers of the tendon and enthesis. In both tissue types, samples with tears exhibited higher degeneration scores in the deep and middle layers compared to the superficial layer. This difference was not seen in samples without tears. However, two-way ANOVA did not find a significant interaction between tear presence and tendon layer degeneration. This is likely due to early degenerative changes that maybe be present in macroscopically normal tendons. Degeneration in the deeper layers of the tendon (articular side) have also been observed in rotator cuff tears44, 45. The hip abductor and the rotator cuff share a number of similarities in their anatomy and function 5, 8, 36. As such, the intrinsic mechanism of rotator cuff tendinopathy may apply to hip abductor tendon tears. The degeneration seen in the deeper layers may be secondary to the deeper fibres experiencing increased mechanical loads compared to the outer fibres, especially in adduction and small angles of abduction32. This theory is supported by both intraoperative findings in both the hip abductor and the rotator cuff 46–48. Histologically, rotator cuff tears exhibit significantly more advanced degeneration in the deeper layers of the tendon21, 44, 47, 48. This again mirrors the findings of our study. Given the similarities between hip abductor and the rotator cuff tears, novel surgical methods, materials and biological augments maybe equally applicable to both problems.
With the supero-posterior insertion of gluteus medius and the gluteus minimus insertion, we did not grade layers of the samples separately. This was due to the smaller cross-sectional area of these insertions compared to the lateral insertion of gluteus medius. Future studies evaluating these areas may employ longitudinal sectioning to better appreciate the difference in degeneration between the layers44.
A limitation of our study is that the tissue samples were obtained from cadavers with incomplete past medical histories. From patient discharge summaries and death certificates that were available, it was not possible to ascertain accurate information regarding the presence or absence of lateral hip pain and gait abnormalities. However, cadaveric studies have several advantages. Firstly, detailed macroscopic examination was possible as the area of interest was removed en-bloc. The insertion can be closely examined from both its superficial and deep surfaces without field of view limitations. Secondly, we were able to identify degenerative changes in both the tendon and enthesis by detailed histological examination of the entire tendon-bone unit. This provided greater insight into the pathological basis of disease which is not possible with imaging studies.
Our results have numerous implications for surgical repair. Firstly, we confirmed that the majority of gluteus medius tears involved the deep surface of the tendon. Therefore, this area must be specifically examined and addressed during repair. Secondly, we found higher than expected incidence of gluteus minimus tears that often co-exist with gluteus medius tears. Therefore, during surgical repair of the gluteus medius, the gluteus minimus should be carefully examined and repaired if torn. Lastly, the development of new biological materials for tendon repair requires improved understanding of the pathological features of abductor tendon tears. In our study, we have visualised and identified both tendon and enthesis degeneration as the key feature in hip abductor tendinopathy. Hence, novel materials and biologics must target both tissue types to improve healing following surgery.
In conclusion, we found degenerative changes are the underlying pathology in hip abductor tendon tears. Both the macroscopic and microscopic features of degeneration were predominantly observed in the deep layers of the tendons. These findings are important for guiding surgical repair techniques and to aid the development of novel materials and biologics in order to improve surgical outcomes.