As our human ancestors evolved to become bipedal, the scapulohumeral complex evolved to comply with the specific demands that arise from an orthograde posture and to facilitate prehension. The inherent osseous articular congruity required for weight-bearing activities were sacrificed in the upper extremities for soft tissue stability to afford a greater degree of mobility at the glenohumeral joint.
Our shoulder joint provides a greater range of motion than any other joint in the body and also has the greatest risk of instability.
Shoulder movements are a synthesis of motion at four articulations:
sternoclavicular, acromioclavicular, glenohumeral, and scapulothoracic
The sternoclavicular (SC) joint is a spheroidal joint between the medial end of the clavicle and both the manubrium and the first costal cartilage. An intraarticular fibrocartilaginous disk stabilizes the joint and prevents medial displacement of the clavicle. The joint capsule is reinforced by the anterior and posterior SC ligaments.
The acromioclavicular (AC) joint is a spheroidal joint between the lateral end of the clavicle and the acromion process of the scapula
The AC and SC joints augment the range of shoulder movements, particularly abduction and rotation. The joints also allow slight axial rotation of the clavicle, as well as elevation/depression and forward/backward thrusting of the shoulder.
The glenohumeral (GH) joint is formed by the humeral head and the glenoid surface of the scapula. Their geometric relationship allows a remarkable range of motion.
The so-called scapulothoracic articulation is not a true joint but functions as an integral part of the shoulder complex. The scapula, which is connected to the posterior aspect of the chest wall by the axioappendicular muscles, provides the origin for the rotator cuff muscles and deltoid, and the trapezius inserts into its superior aspect.
The shoulder capsule is large and has twice the surface area of the humeral head. It typically accepts approximately 28 to 35 mL of fluid, with a greater amount in women than in men. However, in pathologic conditions, this amount varies. For example, in patients with adhesive capsulitis, the shoulder capsule accepts only 5 mL or less of fluid, whereas in patients with considerable laxity or instability it can accept larger volumes of fluid.
The capsule is lined by synovium and extends from the glenoid neck (or occasionally the labrum) to the anatomic neck and the proximal shaft of the humerus to varying degrees.The capsule often extends and attaches to the coracoid process superiorly and on either side of the scapular body (via the anterior and posterior recesses).
The joint capsule blends with ligamentous structures that arise on nearby bony landmarks and contains within its substance the glenohumeral ligaments, including the inferior glenohumeral complex. All of these structures show great variation in size, shape, thickness, and attachment. A cadaveric study demonstrated diminished tensile behavior of the glenohumeral capsule in patients with anterior instability, highlighting the importance of addressing this capsule with more than plication and suturing in symptom control for this population.
On all sides of the shoulder capsule, except for the inferior portion, the capsule is reinforced and strengthened by the tendons of the rotator cuff muscles, that is, the supraspinatus, infraspinatus, teres minor, and subscapularis. The tendons blend into the capsule over varying lengths and average approximately 2.5 cm. The most prominent of these is the anterior tendinous portion of the subscapularis. Together, they form the musculotendinous or capsulotendinous cuff.
The glenohumeral ligaments are collagenous reinforcements to the shoulder capsule that are not visible on its external surface. They are best appreciated in situ arthroscopically without distension by air or saline . Their function depends on their collagenous integrity, their attachment sites, and the position of the arm.
Bursae & recesses
Several bursae are present in the shoulder region, and a number of recesses are found in the shoulder capsule between the glenohumeral ligaments. Two bursae in particular have clinical importance: the subacromial bursa, and the subscapular bursa. Although uncommon, another bursa may be present between the infraspinatus muscle and the capsule. Other synovial recesses are usually located in the anterior portion of the capsule. The number, size, and location of these recesses depend on topographic variations in the glenohumeral ligaments.
Six common variations or types of recesses in the anterior capsule are described:
Type 1: has one synovial recess above the middle glenohumeral ligament
Type 2: has one synovial recess below the middle glenohumeral ligament
Type 3: has one recess above and one below the middle glenohumeral ligament
Type 4: has one large recess above the inferior ligament, with the middle glenohumeral ligament being absent.
Type 5: the middle glenohumeral ligament is manifested as two small synovial folds
Type 6: has no synovial recesses, although all the ligaments are well defined
Regardless of the arrangement in which they are found, the recesses show extreme variability.
Nerve Supply to the Shoulder Joint:
The superficial and deep structures of the shoulder are profusely innervated by a network of nerve fibers that are mainly derived from the C5, C6, and C7 nerve roots (the C4 root can also make a minor contribution).
The innervation of the joint itself follows Hilton’s law, which states that nerves crossing a joint give off branches to the joint, providing its innervation. Therefore the nerves supplying the ligaments, capsule, and synovial membrane of the shoulder are medullary and nonmedullary fibers from the axillary, suprascapular, subscapular, and musculocutaneous nerves. Occasional contributions are made from small branches of the posterior cord of the brachial plexus.
The brachial plexus begins to divide into its branches from the medial, lateral, and posterior cords at the level of the coracoid, anterior to the inferior glenoid. The axillary nerve arises from the posterior cord of the brachial plexus and passes through the quadrilateral space to supply the deltoid, teres minor, and long head of the triceps muscles. The posterior cord also splits into the upper and lower subscapular nerves, which supply the subscapularis muscle, and the thoracodorsal nerve, which supplies the latissimus dorsi. The posterior cord ultimately forms the radial nerve. The lateral cord gives rise to the musculocutaneous nerve, which supplies the coracobrachialis, brachialis, and biceps brachii muscles. It also joins a branch of the medial cord to form the median nerve. The medial cord also gives rise to the medial cutaneous nerves of the arm and forearm and ultimately the ulnar nerve. Lateral and medial pectoral nerves also branch off the lateral and medial cord, respectively, to supply the pectoralis major and minor muscles.