Fascias and spaces of the shoulder girdle region
IntroductionThe shoulder girdle is better thought of as a joint complex of multiple bones which aims to connect the upper limb with axial skeleton. It allows the movement of the upper arm and shoulder in different directions and also supports and protects the neurovasculature that lies beneath. This article will discuss the fascia and bursae of this region and provide clinical content to contextualize the information.
The thickness of subcutaneous fascia of the upper limb varies from person to person and depends on the degree of obesity. The deep fascia lies beneath which sends several intermuscular septa to the humerus medially and laterally dividing the upper arms muscles in different compartments. The fascial relations of shoulder girdle muscles will be covered in the following section.
The deltoid fascia is a fibrous membrane that surrounds the deltoid muscle. It covers the muscle on the outside and unifies with the fascia of the chest (pectoral fascia) at its frontal most edge. Superiorly it is attached to the clavicle, as well as the nearby acromion and the crest of the scapular spine. Inferiorly it is continuous with the brachial fascia that continues down into the arm. At the back it is continuous with the strong posterior infraspinatus fascia. From the deep surface of the fascia several septa arise which penetrate within the muscles and forms fasciculi.
This section of fascia covers the subscapularis muscle. This muscle attaches to the anterior aspect of the scapula and inserts onto the lesser tubercle of the humerus. It is responsible for protraction of the scapula. The overlying fascia is relatively thin aponeurosis and is attached to the entire circumference of the subscapular fossa. The deepest surface of the fascia gives rise to some of the most superficial fibers of subscapularis.
This fascia covers the supraspinatus muscle. It has a dense structure and completes the osseofibrous compartment in which supraspinatus is attached. It is thick medially, but thinner laterally under the coracoacromial ligament. It is attached to the scapula around the outer circumference of the attachment of supraspinatus.
The Infraspinatus fascia is attached at the edges of the infraspinatus fossa, and is continuous anteriorly with the deltoid fascia and along the overlapping posterior border of the deltoid muscle.
This is a thin layer of fascia that covers pectoralis major and extends between its fasciculi. Medially it is attached to the sternum, superiorly it is attached to the clavicle and inferolaterally it is continuous with the fascia of the thorax, shoulder and axilla. The fascia does become thinner over the pectoralis major muscle, however it is thicker between this muscle and latissimus dorsi. At its most lateral section it forms the floor of the axilla as the axillary fascia.
The axillary fascia is relatively dense, especially in the central third of the shoulder and below the deltoid muscle. It divides at the lateral margin of the latissimus dorsi muscle into two layers, which ensheaths the muscle and are attached posteriorly to the spinous processes of the thoracic vertebrae. As the fascia leaves the lower edge of pectoralis major to cross the axilla, a layer ascends under cover of the muscle and splits to envelop pectoralis minor. At the upper edge of pectoralis minor it becomes the clavipectoral fascia.
The classic bowl like appearance of the armpit results primarily via the action of this fascia in tethering the skin to the floor of the axilla, and it is frequently referred to as the suspensory ligament of the axilla. The axillary fascia is pierced by the tail of the breast (aka the tail of Spence).
The clavipectoral fascia is a strong fibrous sheet posterior to the clavicular part of pectoralis major. It fills the gap between pectoralis minor and subclavius, and covers the axillary vessels and nerves. It splits around subclavius and is attached to the clavicle both anterior and posterior to the groove for subclavius. The posterior layer fuses with the deep cervical fascia that connects omohyoid to the clavicle and also with the sheath of the axillary vessels. Medially it blends with the fascia over the first two intercostal spaces and is attached to the first rib, medial to subclavius. Laterally, it is thick and dense, and is attached to the coracoid process, blending with the coracoclavicular ligament.
Between the first rib and coracoid process the fascia often thickens to form a band, the costocoracoid ligament. Below this the fascia becomes thin, splits around pectoralis minor and descends to blend with the axillary fascia and laterally with the fascia over the short head of biceps. The cephalic vein, thoraco-acromial artery and vein, and lateral pectoral nerve pass through the fascia.
Bursae are fluid filled sacs that roll over themselves and serve to lubricate the points of possible friction between the muscles and the joint capsule. They are found all over the body, although mainly near the large joints i.e. the knee, hip and shoulder. The major bursae in the shoulder include the subacromial bursa (deep to the superior attachments of the deltoid muscle as well as the acromion process), the subdeltoid bursa (usually continuous with the subacromial bursa) the subscapular bursa (deep to the tendon of subscapularis) and the subcoracoid bursa (inferior and deep to the coracoid process).
The Subacromial bursa
Sandwiched between the rotator cuff muscles and the outer layer of large bulky muscles is a structure known as the subacromial bursa. This can commonly become inflamed and cause pain (bursitis). It is a common site for injections of cortisone to help decrease the pain in the shoulder.
Spaces of the shoulder include the quadrangular space (bordered by the teres major inferiorly, the long head of the triceps medially, the lateral head laterally, and the teres minor superiorly. The axillary nerve and the posterior circumflex humeral artery exit through this opening.
The triangular interval lies below the quadrangular space and is bordered medially by the long head of triceps and laterally by the lateral head of triceps, with the teres major superiorly. The radial nerve exits through this opening, as does the profunda brachii.
The triangular space lies inferomedially to the quadrangular space and is bordered by the teres major inferiorly, teres minor superiorly and the mong head of triceps laterally. This is where the circumflex scapular artery passes through.
Spread of Infection
The local arrangement of the shoulder fascia determines the precise course of the pus or infectious material spreads within the axillary region. The infectious spread (suppuration) may be superficial or deep to the clavipectoral fascia, and may be between the pectoral muscles or sometimes posterior to the pectoralis minor.
If it occurs between the pectoral muscles, an abscess would appear at the edge of the anterior axillary fold or the groove between deltoid and pectoralis major. If it occurs posterior to pectoralis minor, pus usually travels to surround vessels and nerves and ascend into the neck which is the direction of least resistance.
The infectious material may also track along the vessels into the arm. When an axillary abscess is incised, the scalpel should enter the axillary ‘base’, midway between the anterior and posterior margins and close to the thoracic wall, so as to avoid vital structures such as the lateral thoracic, subscapular and axillary vessels on the anterior, posterior and lateral walls respectively.
Infection of any of the bursa (usually the subacromial bursa) surrounding the shoulder joint will lead to pain known as bursitis. Pain mainly occurs upon movement especially abduction when the bursa becomes compressed between the tendons of the rotator cuff muscles and the acromion process. Treatment includes anti-inflammatory medication, drainage, steroid injection (if non infectious) and more aggressive treatment e.g. intravenous antibiotic therapy or even arthroscopic joint wash out.