The morphology of the subacromial and related shoulder bursae. An anatomical and histological study

Abstract

Shoulder bursae are essential for normal movement and are also implicated in the pathogenesis of shoulder pain and dysfunction. The subacromial bursa (SAB), within the subacromial space, is considered a primary source of shoulder pain. Several other bursae related to the subcoracoid space, including the coracobrachial (CBB), subcoracoid (SCB) and subtendinous bursa of subscapularis (SSB), are also clinically relevant. The detailed morphology and histological characteristics of these bursae are not well described. Sixteen embalmed cadaveric shoulders from eight individuals (five females, three males; mean age 78.6 ± 7.9 years) were investigated using macro-dissection and histological techniques to describe the locations, dimensions and attachments of the bursae, their relationship to surrounding structures and neurovascular supply. Bursal sections were stained with haematoxylin and eosin to examine the synovium and with antibodies against von Willebrand factor and neurofilament to identify blood vessels and neural structures respectively. Four separate bursae were related to the subacromial and subcoracoid spaces. The SAB was large, with a confluent subdeltoid portion in all except one specimen, which displayed a distinct subdeltoid bursa. The SAB roof attached to the lateral edge and deep surface of the acromion and coracoacromial ligament, and the subdeltoid fascia; its floor fused with the supraspinatus tendon and greater tubercle. The CBB (15/16 specimens) was deep to the conjoint tendon of coracobrachialis and short head of biceps brachii and the tip of the coracoid process, while the inconstant SCB (5/16 specimens) was deep to the coracoid process. Located deep to the subscapularis tendon, the SSB was a constant entity that commonly displayed a superior extension. Synovial tissue was predominantly areolar (SAB and SSB) or fibrous (CBB and SCB), with a higher proportion of areolar synovium in the bursal roofs compared to their floors. Blood vessels were consistently present in the subintima 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 approximately one-third of the samples, while encapsulated nerve endings were present in deeper tissue layers. The extensive expanse and attachments of the SAB support adoption of the term subacromial-subdeltoid bursa. Morphologically, the strong attachments of the bursal roofs and floors along with their free edges manifest as fixed and mobile portions, which enable movement in relation to surrounding structures. The presence of neurovascular structures demonstrates that these bursae potentially contribute blood supply to surrounding structures and are involved in mechanoreception. The anatomical details presented in this study clarify the morphology of the shoulder bursae, including histological findings that offer further insight into their potential function.

Keywords: anatomy; blood vessels; bursa; free nerve endings; shoulder; subacromial space; subcoracoid space; synovium.

FIGURE 1

Measurements of bursae and skeletal landmarks. A schematic diagram outlining measurements of bursae obtained during dissection. (a) Superior view. Key: (a) Acromial length. The midpoint of the acromion was defined as half of the acromial length; (b) coracoacromial ligament (CAL) length; (c) anterior‐posterior extent of the subacromial bursa (SAB) along the acromion and (d) coracoacromial ligament; (e) medial extent of the SAB from the anterolateral corner of the acromion; (f) medial extent of the SAB from the midpoint of the acromion, and in relation to the acromioclavicular joint—medial or lateral to the plane of the joint (descriptive only). Measurements ‘e’ and ‘f’ were aligned along the fibres of supraspinatus. (b) Anterior view. Key: Inferior extent of the SAB from the (g) anterolateral corner and (h) midpoint of the acromion; (i) distance from the inferior extent of the SAB to the axillary nerve (anterior branch). Measurements ‘h–i’ were aligned with the shaft of the humerus. Medial‐lateral and superior‐inferior dimension of the coracobrachial (CBB) (shaded purple) and subcoracoid (SCB) bursa (shaded orange), deep to the coracoid process (CP) and conjoint tendon (CT) of coracobrachialis and the short head of biceps brachii. Abbreviations: A, anterior; BT, biceps tendon (long head); I, inferior; L, lateral; M, medial; P, posterior; PM, pectoralis minor; S, superior

FIGURE 2

Extent of the subacromial bursa with the coracoacromial arch in situ (a–c) and after resection of the acromion (d–f). The subacromial bursa (injected with blue latex) is situated over the proximal humerus. Note (a, lateral view) its extent along the lateral edge of the acromion (blue pins) including the (b, anteromedial view) coracoacromial ligament anteriorly (black arrow) towards the coracoid process (CP, green pin). Posteriorly (c, posterior view) it extends towards the posterior aspect of the acromion. The red pin marks the midpoint of the acromion. (d–f) The acromion has been resected and the roof of the subacromial bursa has been opened and reflected to expose its floor. Note its (d, superolateral view) medial‐lateral and anterior‐posterior dimensions (blue pins), (e, posterior view) posterior extent (white pins) over the infraspinatus tendon and (f, superior view) medial extent (white pins, blue latex from injection in situ) overlying the supraspinatus tendon/muscle in the supraspinous fossa. Abbreviations: A, anterior; GT, greater tubercle; I, inferior; L, lateral; M, medial; P, posterior; S, superior. Scale bar marked in cm

FIGURE 3

Location and dimensions of the coracobrachial (a and b) and subcoracoid bursa (c) and the relationship between the superior extension of the subtendinous bursa of subscapularis and the subcoracoid bursa (d). Anterior view of the shoulder showing (a) the typical location of the coracobrachial bursa (unopened, injected with blue latex), deep to the tip of the coracoid process (CP), conjoint tendon (CT, reflected superiorly) and overlying the subscapularis tendon (white asterisk); and (b) a large, circular coracobrachial bursa (opened, latex removed) with its floor exposed, delineated with red pins. The distance between the anterior boundary of the subacromial bursa (white pins) and lateral border of the coracobrachial bursa is indicated by the dashed line. (c) shows an opened subcoracoid bursa (black asterisk) that extends deep to the coracoid process (CP, resected and reflected superiorly) and shares a wall (arrowhead) with a coracobrachial bursa (CBB). (d) shows a subcoracoid bursa (filled with blue latex) and superior extension of the subtendinous bursa of subscapularis (SSB, arrow), separated by a thin wall (arrowhead). The attachment of the subtendinous bursa of subscapularis to the deep surface of the subscapularis tendon is indicated by white pins. Abbreviations: BT, biceps tendon (long head); I, inferior; L, lateral; M, medial; S, superior. Scale bar marked in cm

FIGURE 4

Subtendinous bursa of subscapularis (a) and extent of its superior extension along the anterior surface of the subscapularis tendon (b). (a) Anterior view of the shoulder showing the subtendinous bursa of subscapularis (injected with blue latex). Its roof (indicated by red pins) attaches to the upper aspect of the deep surface of the subscapularis tendon and its floor (indicated by blue dotted line) fuses with the neck of the scapula and root of the coracoid process (CP, tip resected) as well as the anterior glenohumeral joint capsule. (b) Anterosuperior view of the shoulder showing the subtendinous bursa of subscapularis, overhanging the upper edge of the subscapularis tendon and attaching to its superficial surface (indicated by red pins). The close relationship between the upper edge of the subscapularis tendon along with its bursal attachment to the underside of the coracoid process (CP, tip resected and reflected) is also visible. White pins indicate the extent of the subacromial bursa, but are not relevant to this image. Abbreviations: I, inferior; L, lateral; M, medial; S, superior. Scale bar marked in cm

FIGURE 5

Types of bursal synovium (a–c) and its distribution in the various bursae (d). (a) Areolar and (b) fibrous synovium in the roof of the subacromial bursa, and (c) adipose synovium in the floor of the subacromial bursa; haematoxylin and eosin stain. (d) CBB/SCB: coracobrachial and subcoracoid bursa (combined values), n = 27 samples; SAB: subacromial bursa, n = 81 samples; SSB: subtendinous bursa of subscapularis, n = 31 samples

FIGURE 6

Immunoreactivity for von Willebrand factor showing blood vessels in the (a) subacromial bursa and (b) positive control tissue. Sections showing immunoreactivity to von Willebrand factor in (a) blood vessels in the subacromial bursa (arrowheads) and (b) blood vessels (arrows) in the positive control section within the sciatic nerve. The negative control section (c) shows no immunoreactivity to von Willebrand factor in blood vessels (arrowheads) or other non‐specific staining. Cell nuclei are stained blue (haematoxylin)

FIGURE 7

Immunoreactivity for neurofilament showing neural tissue in (a and b) bursal and (c) positive control tissue. Sections showing free nerve endings (arrowheads) within the (a) coracobrachial bursa floor and (b) subacromial bursa roof and (c) nerve bundles (arrowheads) in the positive control within the sciatic nerve. The negative control (d) shows no immunoreactivity to neurofilament or other non‐specific staining. Cell nuclei are stained blue (haematoxylin)

FIGURE 8

Neural structures in the subacromial and coracobrachial bursa deep to the subintima. Sections showing immunoreactivity for neurofilament contain large encapsulated neural structure resembling a (a, b) Pacinian corpuscle with typical onion‐like multi‐layered capsule. Central axons show immunoreactivity to neurofilament, (c) large nerve bundle and (d) Golgi‐Mazzoni corpuscles just deep to the subintima. Sections obtained from (a) subacromial bursa floor, (b, c) subacromial bursa roof, (d) coracobrachial bursa floor