Clinical anatomy of the subacromial and related shoulder bursae
Kennedy, Marion Sigrid
The shoulder is complex from both functional and biomechanical perspectives, with shoulder disorders affect a large proportion of adults at some stage in their life and are the most common musculoskeletal complaint in people over 65 years of age. The bursae associated with the shoulder are essential for normal movement and are also implicated in the pathogenesis of shoulder pain and dysfunction. The subacromial bursa (SAB), situated in the subacromial space, is the largest of the shoulder and considered a primary source of pain. Several other bursae, including the coracobrachial (CBB), subcoracoid (SCB) and subtendinous bursa of subscapularis (SSB), related to the nearby subcoracoid space, are also clinically relevant in terms of shoulder pathology. However, the detailed morphology of all of the shoulder bursae has not been well studied. Narrowing of the subacromial and subcoracoid spaces, the spaces in which the bursae reside, with subsequent impingement of the interposed soft tissues is considered important in the pathogenesis of shoulder pain. The acromiohumeral (AHD) and coracohumeral (CHD) distances are commonly used as a proxy to measure these spaces using ultrasound. However, it is unclear if AHD and CHD are influenced by age and if other variables such as thoracic kyphosis and shoulder muscle strength are related to these measures in a population without shoulder pain. The main purpose of this thesis was to: (1) systematically review the literature describing the morphology of the subacromial and related shoulder bursae, and describe their associated spaces, including the AHD and CHD; determine the (2) detailed gross and (3) microscopic morphology of the SAB and related shoulder bursae; and (4) examine AHD and CHD in a population without shoulder pain across the lifespan and investigate if certain factors are associated with these measurements. The systematic review of the main shoulder bursae (SAB, CBB, SCB and SSB) revealed that some morphological aspects of these structures were still unclear. Specifically, further information is required regarding precise bursal location and dimensions, attachments, constancy, and communication between the bursae. In addition, comprehensive histological descriptions, including the type of synovium and the distribution and density of neurovascular structures are lacking. Increased severity of rotator cuff pathology is related to reduced AHD and CHD. However, no normative data are available across the lifespan and little is known regarding the relationship of these measures with factors such as thoracic kyphosis and shoulder strength in people without shoulder pain. In the anatomical study, the shoulder bursae were examined in 16 embalmed cadaveric shoulders from eight individuals (five female, three male; mean age 78.6 ± 7.9 years) using macro-dissection and histological techniques. Up to four separate bursae were related to the subacromial and subcoracoid spaces. The SAB (5.6 ± 1.6 cm, anterior-posterior; 6.0 ± 1.6 cm, medial-lateral) was a large, constant, separate bursa with a confluent subdeltoid portion in all except one specimen, which displayed an additional subdeltoid bursa. Roof attachments of the SAB included the lateral edge and inferior surface of the acromion and coracoacromial ligament and the subdeltoid fascia, and its floor was fused with the underlying supraspinatus tendon and greater tubercle. The extensive expanse and attachments of this bursa supports adoption of the term subacromial-subdeltoid bursa. The CBB (2.6 ± 0.9 cm, superior-inferior; 2.3 ± 1.0 cm, medial-lateral) was present in all but one specimen and located deep to the tip of the coracoid process and the conjoint tendon of coracobrachialis and short head of biceps brachii. The SCB (1.6 ± 0.5 cm, superior-inferior; 1.5 ± 0.5 cm, medial-lateral) was inconstant (5/16) and positioned deep the coracoid process. The SSB (2.6 ± 1.1 cm, superior-inferior; 2.8 ± 1.0 cm, medial lateral) was always present and located deep to the subscapularis tendon. The SSB commonly displayed a superior extension. This projected underneath the root of the coracoid process and often overhung the upper edge of the subscapularis tendon anteriorly coming into close proximity with the CBB or SCB. The synovium was predominantly areolar in the SAB and SSB, and fibrous in the CBB and SCB. There was a higher proportion of areolar synovium in all the bursal roofs compared to their floors. Blood vessels were identified in all sections (n = 43), with a median density of 3% of the tissue surface area, being greatest in the SSB and SAB roofs (4.9% and 3.4%, respectively) and least in the SAB floor (1.8%) and CBB roof and floor (both 1.6%). Nerve bundles and free nerve endings were identified in the subintima in over a third (15/43, 34.8%) of the samples; few specialised mechanoreceptors were present in deeper tissue layers. The median density was 1.6 per mm2, varying between 0.8 per mm2 in the SSB roof and 7.1 per mm2 in the CBB roof. In the ultrasound study, volunteers (n = 104) without a history of shoulder pain were recruited into five stratified groups between 18-80 years. Measurements of AHD and CHD and tendon thickness were taken in various shoulder positions using ultrasound. In general, intra- and inter-rater reliability of all sonographic outcome variables were good to excellent. The relationship between the outcome variables (AHD and CHD) with several primary (age and thoracic kyphosis index) and secondary (isometric shoulder strength, height, weight, body mass index and level of shoulder activity) independent variables were examined using uni- and multivariable linear regression analysis. When all independent variables were controlled for, age and thoracic kyphosis were not associated with AHD and CHD. Positive, yet small, relationships were evident between isometric abduction strength and AHD (in neutral) and between height and CHD (in internal rotation and flexion), but not for the remaining variables. Morphologically, the strong attachments of the bursal roof and floor along with their mobile edges imply that that there are fixed and mobile portions of the SAB and related shoulder bursae, which enable movement in relation to the surrounding structures. The presence of neurovascular structures demonstrates that these bursae have the potential to contribute blood supply to surrounding structures and while they are likely involved in nociception, their role in proprioception is less certain. Acromiohumeral distance and CHD do not appear to change across the lifespan in a population without shoulder pain and are not influenced by normal thoracic kyphosis. The anatomical details presented in this research clarify the location, size, and relationships of the shoulder bursae, supplemented by histological findings that offer further insight into their potential function. Novel, normative data on AHD and CHD provide an essential basis for interpreting research focussed on these ultrasonographic measurements in people with shoulder pathology and pain.
Advisor: Woodley, Stephanie J.; Nicholson, Helen D.
Degree Name: Doctor of Philosophy
Degree Discipline: Anatomy
Publisher: University of Otago
Keywords: anatomy; blood vessels; free nerve endings; histology; shoulder; subacromial space; subcoracoid space; subcoracoid bursa; coracobrachial bursa; synovium; subscapularis bursa; subtendinous bursa of subscapularis; subacromial impingement; coracoid impingement; bursa; subacromial bursa; subdeltoid bursa; acromiohumeral distance; coracohumeral distance; ultrasonography; reliability; aging; thoracic kyphosis; isometric shoulder strength; physiotherapy; rotator cuff; supraspinatus tendon; subscapularis tendon; musculoskeletal system; immunohistochemistry; morphology; subcoracoid impingement
Research Type: Thesis