Distribution of bone and tissue morphological properties related to subacromial space geometry in a young, healthy male population

PurposeParticular bone and tissue morphological features of the scapula and humerus often exist disproportionately in persons with subacromial impingement syndrome (SAIS) and/or rotator cuff pathology. However, the origins of morphological variation, genetic or mechanistic, remain unclear. This research evaluated the distribution of and correlation between several bone and tissue characteristics associated with these pathologies amongst a baseline cohort population consisting of young, healthy, males. As well, the predisposition to SAIS was estimated by calculating the ratio (occupation ratio) of subacromial tissue thickness to minimum subacromial space width (SAS).MethodsAnterior-posterior and trans-scapular radiographs and musculoskeletal ultrasound were used to measure morphological characteristics related to the subacromial space. Each bone morphological characteristic was classified as healthy or unhealthy based on previous definitions. Supraspinatus tendon and subacromial bursa thicknesses were used to calculate the occupation ratio from both radiographic and ultrasonic measures of the SAS.ResultsEach characteristic demonstrated considerable variability, with some participants having ‘unhealthy’ variants for each bone characteristic examined. The percentage of the population with bone characteristics classified as “unhealthy” ranged from 15 to 55 % across characteristics evaluated. The strongest correlation existed between the acromion index and the minimum subacromial space width (−0.59) suggesting that a larger lateral extension of the acromion may predispose an individual to SAIS. The average occupation ratio was 65.3 % with a 1–99 % confidence interval ranging from 21.6 to 108.9 %.ConclusionsThe distributions of both morphological characteristics and occupation ratios indicate that individuals within this healthy, baseline population have a highly differential predisposition for subacromial tissue compression solely based on inherent morphological variation. This suggests that while mechanistic and/or age-related degenerative changes may contribute to SAIS and eventual rotator cuff pathology, intrinsic predisposing geometry should not be discounted.