Subacromial Bursa , abbreviatedSAB, are the largest Bursa in the human body, residing between the Acromion of Scapula , the Deltoid and the Rotator Cuff Tendons (SOURCE-1+2). These synovial sacs act as a friction-reducing buffer for the subacromial space. The Subacromial Bursa are anatomically variable which leads to inconsistencies in appropriate nomenclature. Often the parts of the SAB are described as their own distinct entities (SOURCE-1).
The oval-shaped Subacromial Bursa is consistently found beneath the Acromion, Deltoid and its associated Subdeltoid Fascia often described in three parts (SOURCE-1+3):
Subacromial- lines the underside of the Acromion and a portion of its lateral edge, often extended as far as the Acromioclavicular Joint
Subdeltoid- occasionally a distinct cluster of bursa line the deep surface of the Deltoid
Subcoracoid- lines a variable portion of the distal surface of the Coracoid Process of Scapula and the Conjoint tendon of Coracobrachialis and Short Head of Biceps
Additionally, ~50-70% of the Coracoacromial Ligaments undersurface is often lined with the Subacromial Bursa, particularly at its coracoid end (SOURCE-1+3). It has been postulated theSuperior Biceps Aponeurosisserves to compress the Bursa residing in the Subacromial Space to support gliding of the Supraspinatus Tendon (SOURCE-5).
The Subacromial Bursa receives nervous innervation from the Suprascapular Nerve , Lateral Pectoral Nerve and free nerve endings (SOURCE-2).
In the healthy shoulder the Subacromial Bursa do not typically articulate with the Glenohumeral Joint , rather they reduce friction of the contracting Supraspinatus Tendon with neighbouring structures under the Coraco-acromial Arch to promote quality movement of The Shoulder Girdle (SOURCE-3).
The Subacromial Bursae are highly vascularised and often described as a reservoir for immune cells, stem cells and growth factors. The precise role this reservoir plays is poorly understood and often appears conflicting. From one perspective, a high vascularisation density (~1.8-3.4%) that appears to increase in the presence of a Tear , nerve endings in close proximity to vessels and the increased expression of bursal Vascular Endothelial Growth Factor (VEGF) associated with conditions such as Subacromial Impingement or Rotator Cuff Tears indicates the Subacromial Bursa may serve a Pain generating and nociceptive role (SOURCE-2). This notion is supported by findings of bursal inflammation, necrosis, hypertrophy and oedema all correlated with pain in those with rotator cuff tears (SOURCE-2). Furthering this notion, the expression of VEGF in the Subacromial Bursa was associated with movement related pain in those with those with tears or impingement (SOURCE-2). These findings were distinguished by the fact that comparable inflammatory conditions of the Glenohumeral Joint Synovial Membrane were found not to contribute to pain related to Rotator Cuff pathology (SOURCE-2). Perpetuating the perspective that the Subacromial Bursa share a caustic relationship with their local environment, pro- Inflammation Cytokines within the bursa appear to heighten expression in pathologic states, leading some to postulate an impedance of rotator cuff recovery (SOURCE-2). Similarly, Bone Morphogenetic Proteins within Subacromial Bursa may induce ectopic Bone / Cartilage formation in local Tendons leading to a compromised state and eventually tears (SOURCE-2). These negative extrapolations however fail to gain harmonious support of the literature, with some challenging the Subacromial Bursa may instead play an integral tissue healing role.
The Subacromial Bursa are abundant in Mesenchymal Stem Cells (MSC) with levels comparable or greater than Bone Marrow, whose differentiation appears to favour a more tendinous lineage (SOURCE-2). Similarly, the localisation of progenitor potential cells within the Subacromial Bursa at sites closest to neighbouring Tendons suggests the Inflammation response may have tissue healing potential (SOURCE-2). This healing potential may also be heightened as a compensatory response to aging. While neo-angiogensis and inflammation decreases in Tendon s, these factors increase in the Subacromial Bursa with age (SOURCE-2). Additionally, cellular expression may be less abated by tissue pathology than originally thought. For one, BMP expression was reported as constant between pathologic states and healthy shoulders (SOURCE-2). While neither perspective refutes the other, it appears understanding the cellular role of the SAB is more nuanced than labelling it as negative or positive.
Along with the Rotator Cuff , the Subacromial Bursa are recognised as a primary Pain -producing tissue of The Shoulder Girdle (SOURCE-1). Pathology of the Subacromial Bursae may often arise through several mechanisms including direct trauma, repetitive overuse or local or systemic Inflammation (SOURCE-1). Pathology often occurs concomitantly with Subacromial Impingement and/ or Rotator Cuff Tendinopathy , two conditions which may also play a role in Subacromial Bursa pathogenesis (SOURCE-1+2).
As the Supraspinatus Tendon is distinguished from the Coracoacromial Ligament , Acromion of Scapula and Deltoid by the Subacromial Bursa, Inflammation of the bursa may increase subacromial pressure and tendon friction as the shoulder articulates (SOURCE-3). Left unchecked this pressure may predispose or exacerbate symptoms of Subacromial Impingement , which can eventually lead to Rotator Cuff pathology (SOURCE-1). Bursal pressure has also been shown to increase with arm elevation or when under load, leaving the Subacromial Bursae particularly vulnerable during overhead activities (SOURCE-1). Exacerbated pressure and friction may accumulate microtrauma, alter the local environment and lead to the deposition of Hydroxyapatite Crystals and consequent calcification of tendons (SOURCE-2). It is postulated these crystals may fragment into sharp acicular structures with the capacity to mechanically induce significant inflammation (SOURCE-2). This is supported by the findings that the region of the Subacromial Bursa closest to tendons is most subjected to this pathological change (SOURCE-2).
The substantial innervation and free nerve endings in the Subacromial Bursa provide a basis for the strong relationship observed between its Inflammation and shoulder Pain (SOURCE-2). Similarly, the increased expression of Vascular Endothelial Growth Factor within this Bursa has been found to be associated with painful motion of The Shoulder Girdle (SOURCE-2+4).
Other drivers behind pathology of the Subacromial Bursa include (SOURCE-2):
Infection
Autoimmune Disease
Shoulder Injury Related to Vaccine Administration (SIRVA)
The following pathologies are related to the Subacromial Bursa:
Subacromial Bursitis - Inflammation of the Subacromial Bursa and is a common cause of Shoulder Pain (SOURCE-2).
Subacromial Impingement - excessive inflammation has the capacity to reduce the narrow Subacromial Space. One hypothesis for the evolution of Subacromial Bursitis into Impingement attributes synovial folds, known asPlica. These Plica course through the Bursa and may increase friction of the Rotator Cuff (SOURCE-1).
Rotator Cuff Tendinopathy or Tear - the Subacromial Bursa share an intimate relationship with the Rotator Cuff, in particular the Supraspinatus Tendon on its course through the subacromial space. The Bursa display a tendon healing capacity, yet in pathologic states may lead to inflammation, impingment, calcification and a compromised rotator cuff.
Pain and/ or Inflammation of the Subacromial Bursa generally leads to restricted Shoulder - Active Range of Motion , in particular (SOURCE-1):
The following Shoulder - Special Tests have been validated for the diagnosis of Subacromial Bursitis :
Yocum’s Test - sensitivity 0.80, specificity 0.35
Cross Body Adduction Stress Test - sensitivity 0.25, specificity 0.79
Neers - sensitivity 0.85, specificity 0.49
Drop Arm Test - sensitivity 0.13, specificity 0.77
As pathology of the Subacromial Bursa and its potential sequelae are generally associated with Inflammation and/or a narrowing of the Subacromial Space, these factors become the main focus of treatment approach. Quality posture of The Shoulder Girdle both at rest and through movement should be promoted, in particular at the Glenohumeral Joint and Scapulothoracic Joint . Pain which is often the initial complaint with Subacromial Bursal pathology and a common symptom, may implicate nervous structures such as the Cervical Spine Nerve Roots and the Peripheral Nerves the extend from them, in particular those that arrise from C5 .
When in a shortened or excessively toned state, the following muscles have the capacity to alter shoulder mechanics, narrow the subacromial space and increase pressure on the bursa. Specific Stretching techniques are found on their respective pages:
Supraspinatus and Deltoid - both muscles have a superior draw on the Head of the Humerus
Pectorals - generally speaking, these muscles have the capacity to hinder function at serveral shoulder articulations, although through diverging mechanisms
Pectoralis Major - while unopposed contraction results in an anteromedial draw on the Humeral Head, excessive activity of the Pectoralis Major has been associated with a narrowing of the Subacromial Space and compression of the Subacromial Bursa
Pectoralis Minor - when shortened directly leads to a reduction of the Subacromial Space through restriction of Scapula posterior tilting
Serratus Anterior - while also often weak may be shortened, leading to disturbed Scapulothoracic Joint mechanics
Subscapularis - excessive tone has the capacity to draw the Humerus anteriorly, disturb Glenohumeral Joint mechanics and narrow the Subacromial Space
The following muscles may benefit from Strength training to improve relative tone and shoulder mechanics, with specific exercises found on their respective pages:
Rotator Cuff - the remaining rotator cuff muscles counter the superior draw the Supraspinatus has on the Head of the Humerus with an inferior bias (SOURCE-7). Both at rest and dynamically the Infraspinatus , Teres Minor and Subscapularis may be important areas of focus to increase subacromial space and reduce pressure on the bursa
Scapulothoracic Joint Muscle - the Middle and Lower Fibres of the Trapezius and Serratus Anterior form an upwards rotating force coupleing that elevates the acromion during movement. Weakness or other purtubance to this dynamic is a known predisposing factor for secondary impingement which could disrupt the Bursa (SOURCE-9)
Latissimus Dorsi - the ability of the Latissimus Dorsi to counter the superior bias of the Deltoid to maintain adequate subacromial space makes it a plausible treatment for Chronic Subacromial Bursitis (SOURCE-10)
Biceps Brachii - the Long Head of Biceps has been described to produce an inferior distraction force on the Humeral Head, thus weakness could predispose impingment (SOURCE-11)
Deep Cervical - Flexion Muscles - if Radiculopathy suspected
Myofascial Release of the following structures may be relevant in the treatment of Subacromial Bursal pathology: Muscle - same as those listed above in Stretching section Fascia - the never-ending nature of fascial continuities means many have the capacity to directly or indirectly increase pressure within the subacromial space. The following lists those connections that are more obvious:
Lateral Arm Chain - contains both the Deltoid and Trapezius which share an intimate relationship with the Head of the Humerus and capacity to narrow the Subacromial Space
Ventral Arm Chain - contains muscles that influence the positioning of the Humeral Head, in particular the Pectoralis Major
Dorsal Arm Chain - contains several muscles that influence Humeral Head positioning, including the Teres Minor , Infraspinatus , Latissimus Dorsi and Triceps Brachii
Superficial Front Arm Line - contains multiple muscles that influence Humeral Head positioning in the Glenohumeral Joint , in particular Pectoralis Major
Deep Front Arm Line - contains multiple muscles that influence positioning of the Humeral Head in the Glenohumeral Joint, in particular Pectoralis Minor
Superficial Back Arm Line - contains both the Deltoid and Trapezius which share an intimate relationship with the Humeral Head and capacity to narrow the Subacromial Space
Spiral Line - role in trunk motion, in particular Thoracic - Rotation which when restricted may lead to altered Scapulothoracic Joint mechanics and narrowing of the Subacromial Space
Superficial Front Line - excessive tightness may lead to Forward Head Posture , Thoracic Spine Kyphosis and anterior tilting of the Scapula
Lateral Line - restrictions or asymmetry may alter Scapulohumeral Rhythm
Regions
Cervical Fascia - has attachments to many joints of The Shoulder Girdle , including those made by the Upper Trapezius . Also contains muscles such as the Scalenes which have the capacity to entrap Nerve Roots relevant to the Subacromial Bursa
Clavipectoral Fascia - contains muscles such as the Pectoralis Minor and Serratus Anterior which influence the Scapulothoracic Joint
Pectoral/ Axillary Fascia - contains muscles such as the Pectoralis Major and Latissimus Dorsi which influence the Glenohumeral Joint . It is also continuous with the Deltoid fascia
The following Mobilisation techniques may be relevant to the Subacromial Bursa Joint Play
Scapulothoracic Joint - Scapula mechanics are integral for maintaining adequate subacromial space
Cervical Spine - with Cevical Spine pathology often occuring as a concomitant condition or predisposing factor to Subacromial Bursa pathology, mobililsations of the Cervical (in particular C5 ) may be relevant. PACVP , PAUVP and TVP pressures may be releavant as with the following techniques:
With specific technique procedures listed under their respective pages, Dry Needling of the following structures may be relevant to Subacromial Bursal pathology. Muscle
Same as those listed above in Stretching section
Middle Scalene - if C5 Entrapment suspected
Sternocleidomastoid - if Radiculopathy suspected, as SCM increases Forward Head Posture
Glenohumeral Joint Capsule - in particular the posterior fibres as they can lead to an anterosuperior translation to the Head of the Humerus
while at 8 week follow-up coricosteroid and combined with physiotherapy was shown to be more effective in the treatment of pain and range of motion than physiotherapy alone, the recurrence rate of the physotherapy only treatment was significantly lower (SOURCE-6)
Clinical anatomy of the Subacromial and related shoulder bursae_A review of the literature. (PDF).
Gray’s Anatomy - the anatomical basis for clinical practice (41st).
https://onlinelibrary.wiley.com/doi/abs/10.1016/S0736-0266(03)00102-5
‘Superior biceps aponeurosis’ – morphological characteristics of the origin of the short head of the biceps brachii muscle. (PDF)
Does supraspinatus initiate shoulder abduction. (PDF).
Kinesiology of the Musculoskeletal System_D. Neumann. (PDF).
Diagnosis and Management of the Painful Shoulder. Part 1: Clinical Anatomy and Pathomechanics. (PDF).
Complete Rupture of Both Heads of the Biceps Brachii Muscle Belly by Blunt Trauma - ScienceDirect