Like the hip, it is a ball-and-socket joint. It is retained in position much less by ligaments than by muscles, and, owing to the looseness of its capsule, as well as to all the other conditions of its construction and position, it is exceedingly liable to be displaced; on the other hand, it is sheltered from violence by the two projecting processes - the acromion and coracoid. The ligaments of the shoulder-joint are:
- Articular capsule
The articular capsule
The articular capsule is a loose sac, insufficient in itself to maintain the bones in contact. It consists of fairly distinct but not coarse fibers, closely woven together, and directed, some straight, others obliquely, between the two bones, a few circular ones being interwoven amongst them. At the scapula, it is fixed on the dorsal aspect to the prominent rough surface around the margin of the glenoid cavity, reaching as far as the neck of the bone. Superiorly, it is attached to the root of the coracoid process; anteriorly, to the ventral surface, at a variable distance from the articular margin, often reaching half an inch (12 mm.) upon the neck of the bone, and thus allowing the formation of a pouch; it may not, however, extend for more than a quarter of an inch (6 mm.) beyond the articular margin; inferiorly, it blends with the origin of the long head of the triceps. At the humerus, the superior half is fixed to the anatomical neck, sending a prolongation downward between the two tuberosities which attenuates as it descends, and covers the transverse humeral ligament. The lower half of the capsule descends upon the humerus further from the articular margin, some of the deeper fibers being reflected upward so as to be attached close to the articular edge, thus forming a kind of fibrous in- vestment for the neck of the humerus. This ligament is more uniform in thickness than that of the hip.
Gleno-humeral bands of the capsule
There are three accessory bands, known as the superior, middle and inferior gleno-humeral bands, which project toward the interior of the joint from the fore part of the capsule and are consequently best seen when the joint is opened from behind.
The middle band reaches from the anterior margin of the glenoid cavity along the lower border of the subscapularis tendon to the lower border of the lesser tuberosity, and the inferior band from the inferior part of the glenoid cavity to the inferior part of the neck of the humerus.
The superior band, known also as the gleno -humeral ligament, runs from the edge of the glenoid cavity at the root of the coracoid process, just medial to the origin of the long tendon of the biceps, and, passing laterally and downward at an acute angle to the tendon, for which it forms a slight groove or sulcus, is fixed to a depression, the fovea capitis humeri, above the lesser tuberosity of the humerus. It is a thin, ribbon-hke band, of which the superior surface is attached to the capsule, while the inferior is free and turned toward the joint. In the foetus it is often, and in the adult occasionally, quite free from the capsule, and may be as thick as the long tendon of the biceps.
The tendons of the supra- and infraspinatus, teres minor, and subscapularis muscles strengthen and support the capsule, especially near their points of insertion, and can be with difficulty dissected off from it. The long head of the triceps supports and strengthens the capsule below. The capsule also receives an upward sUp from the pectoralis major. The supraspinatus often sends a slip into the capsule from its upper edge.
The coraco-humeral ligament
The coraco-humeral ligament is a strong broad band, which is attached above to the lateral edge of the root and horizontal limb of the coracoid process nearly as far as the tip. From this origin, it is directed backward along the line of the biceps tendon to blend with the capsule, and is inserted into the greater tuberosity of the humerus.
Seen from the back, it looks like an uninterrupted continuation of the capsule, while from the front it looks like a fan-shaped prolongation from it overlying the rest of the ligament. At its origin there is sometimes a bursa between it and the capsule.
The glenoid ligament or lip [labrum glenoidale] is a narrow rim of dense fibro-cartilage, which surrounds the edge of the glenoid socket and deepens it. It is about a quarter of an inch (6 mm.) wide above and below, but less at its sides. Its peripheral edge is inseparably welded, near the bone, with the articular capsule. Its structure is almost entirely fibrous, with but few cartilage cells intermixed. At the upper part of the fossa the biceps tendon is prolonged into the glenoid ligament, the tendon usually dividing and sending fibers right and left into the ligament, which may wind round nearly the whole circumference of the socket. It may, however, send fibers into one side only, usually into the lateral.
The articular cartilage covering the glenoid fossa is thicker at the circumference than in the center, thus tending to deepen the cavity. It is usually thickest at the lower part of the fossa; over the head of the humerus the cartilage is thickest at and below the center.
The synovial membrane lines the glenoid ligament, and is then reflected over the capsule as far as its attachment to the humerus, from which it ascends as far as the edge of the articular cartilage. The tendon of the biceps receives a long tubular sheath, which is continuous with the synovial membrane, both at its attached extremity and at the bicipital groove, but is free in the rest of its extent. The synovial cavity almost always communicates with the bursa under the subscapularis, and sometimes with one under the infraspinatus muscle.
It also sends a pouch-like prolongation beneath the coracoid process when the fibrous capsule is attached wide of the margin of the glenoid fossa. A few fringes are seen near the edge of the glenoid cavity, and there is often one which runs down the medial edge of the biceps tendon, extending slightly below it and making a slight groove for the tendon to lie in.
The transverse humeral ligament is so closely connected with the capsule of the shoulder that, although it is a proper ligament of the humerus, it may well be described here. It is a strong band of fibrous tissue, which extends between the two tuberosities, roofing in the intertubercular (bicipital) groove. It is covered by a thin expansion of the capsule. It is limited to the portion of the bone above the line of the epiphysis.
The following muscles are in contact with the capsule of the shoulder-joint. In front, the subscapularis; above, the supraspinatus; above and behind, the infraspinatus; behind, the teres minor; below, the long head of the triceps and the teres major. In the interval between the subscapularis and the supraspinatus the subacromial bursa is close to the capsule and occasionally its cavity communicates with the cavity of the joint.
The axillary (circumflex) nerve and posterior circumflex artery pass beneath the capsule in the interval between the long head of the triceps, the humerus, and the teres major. When the arm is abducted, the long head of the triceps and the teres major are drawn into closer relation with the capsule and help to prevent dislocation of the humerus.
Vessels and nerves of the shoulder joint
The axillary vessels, the great nerves of the axilla, the short head of the biceps, and the coraco-brachialis are separated from the joint by the subscapularis, whilst the deltoid forms a kind of cap, which extends from the front to the back over the more immediate relations.
The arterial supply is derived from the transverse scapular (suprascapular), anterior and posterior circumflex, subscapular, circumflex scapular (dorsalis scapulae), and a branch from the second portion of the axillary artery.
The nerve-supply is derived from the suprascapular, by branches in both fosse; and from the axillary (circumflex) and subscapular nerves.
The movements of the shoulder-joint
Flexion is the swinging forward, extension the swinging backward, of the humerus; abduction is the raising of the arm from, and adduction depression of the arm to, the side. In flexion and extension the head of the humerus moves upon the center of the glenoid fossa round an oblique line corresponding to the axis of the head and neck of the humerus, flexion being more free than extension, and in extreme flexion the scapula follows the head of the humerus, so as to keep the articular surfaces in apposition. In extension, the scapula moves much less, if at all.
In abduction and adduction, the scapula is fixed, and the humerus rolls up and down upon the glenoid fossa; during abduction, the head descends until it projects beyond the lower edge of the glenoid cavity, and the greater tuberosity impinges against the arch of the acromion; during adduction, the head of the humerus ascends in its socket, the arm at length reaches the side, and the capsule is completely relaxed.
Rotation takes place round a vertical axis drawn through the extremities of the humerus from the center of the head to the inner condyle; in rotation forward (that is, medialward) the head of the bone rolls back in the socket as the great tuberosity and shaft are turned forward; in rotation backward (that is, lateral ward) the head of the bone glides forward, and the tuberosity and shaft of the humerus are turned backward, i. e., lateralward.
Great freedom of movement is permitted at the shoulder, and this is increased by the mobility of the scapula. Restraint is scarcely exercised at all upon the movements of the shoulder by the ligaments, but chiefly by the muscles of the joint.
In abduction, the lower part of the capsule is somewhat, and in extreme abduction considerably, tightened; and in rotation medialward and lateralward, the upper part of the capsule is made tense, as is also, in the latter movement, the coraco-humeral ligament.
The movements of abduction and extension have a most decided and definite resistance offered to them other than by muscles and ligaments, for the greater tuberosity of the humerus, by striking against the acromion process and coraco-acromial ligament, stops short any further advance of the bone in these directions, and thus abduction ceases altogether as soon as the arm is raised to a right angle with the trunk, and extension shortly after the humerus passes the line of the trunk.
Further elevation of the arm beyond the right angle, in the abducted or extended position, is effected by the rotation of the scapula round its own axis by the action of the trapezius and serratus anterior muscles upon the sterno-clavicular and acromio-clavicular joints respectively.
The acromion and coracoid process, together with the coraco-acromial ligament, form an arch, which is separated by a bursa and the tendon of the supraspinatus from the capsule of the shoulder. Beneath this arch the movements of the joint take place, and against it the head and tuberosities are pressed when the weight of the trunk is supported by the arms; the greater tuberosity and the upper part of the shaft impinge upon it when abduction and extension are carried to their fullest extent.
No description of the shoulder-joint would be complete without a short notice of the peculiar relation which the biceps tendon bears to the joint. It passes over the head of the humerus a little to the medial side of its summit, and lies free within the capsule, surrounded only by a tubular process of synovial membrane. It is fiat, with the surfaces looking upward and down- ward, until it reaches the intertubercular (bicipital) groove, when it assumes a rounded form. It strengthens the articulation along the same course as the coraco-humeral ligament, and tends to prevent the head of the humerus from being pulled upward too forcibly against the inferior surface of the acromion. It also serves the purpose of a ligament by steadying the head of the humerus in various movements of the arm and forearm, and to this end is let into a groove at the upper end of the bone, from which it cannot escape on account of the abutting tuberosities and the strong transverse humeral ligament which binds it down. Further, it acts Uke the four shoulder muscles which pass over the capsule, to keep the head of the humerus against the glenoid socket; and, moreover, it resists the tendency of the pectoralis major and latissimus dorsi muscles, in certain actions when the arm is away from the side of the body, to pull the head of the humerus below the lower edge of the cavity.
Muscles which act upon the shoulder-joint
Flexors or protractors. - Deltoid (anterior fibers), pectoralis major (clavicular fibers), coraco-brachialis, biceps (short head), subscapularis (upper fibers).
Extensors or retractors. - Latissimus dorsi, deltoid (posterior fibers), teres major, teres minor, infraspinatus (lower fibers).
Abductors. - Deltoid, supraspinatus, biceps (longhead).
Adductors. - Pectoralis major, latissimus dorsi, subscapularis, infraspinatus, teres major, teres minor, coraco-brachialis, biceps (short head), triceps (lower head).
Medial rotators. - Pectoralis major, latissimus dorsi, teres major, subscapularis, deltoid (anterior fibers).
Lateral rotators. - Deltoid (posterior fibers), infraspinatus, teres minor.
Circumductors. - The above groups acting consecutively.