The Shoulder is an enarthrodial or ball-and-socket joint. The bones entering into its formation, are the large globular head of the humerus, received into the shallow glenoid cavity of the scapula, an arrangement which permits of very considerable movement, whilst the joint itself is protected against displacement by the strong ligaments and tendons which surround it, and above by an arched vault, formed by the under surface of the coracoid and acromion processes, and the coraco-acromial ligament.
The articular surfaces are covered by a layer of cartilage : that on the head of the humerus is thicker at the centre than at the circumference, the reverse being observed in the glenoid cavity.
The ligaments of the shoulder are, the Capsular. Coraco-humeral. Glenoid. * The Capsular Ligament completely encircles the articulation ; being attached, above, to the circumference of the glenoid cavity beyond the glenoid ligament; below, to the anatomical neck of the humerus, approaching nearer to the articular cartilage above, than in the rest of its extent.
It is thicker above than below, remarkably loose and lax, and much larger and longer than is necessary to keep the bones in contact, allowing them to be separated from each other more than an inch, an evident provision for that extreme freedom of movement which is peculiar to this articulation. Its external surface is strengthened, above, by the Supraspinatus ; above and internally, by the coraco-humeral ligament; below, by the long head of the Triceps ; externally, by the tendons of the Infraspinatus and* The long tendon of origin of the Biceps muscle also acts as one of the ligaments of this joint.
Teres minor; and internally, by the tendon of the Subscapularis. The capsular ligament usually presents three openings: one at its inner side, below the coracoid process, partially filled up by the tendon of the Subscapularis; it establishes a communication between the synovial membrane of the joint and a bursa beneath the tendon of that muscle. The second, which is not constant, is at the outer part, where a communication sometimes exists between the joint and a bursal sac belonging to the Infraspinatus muscle.
The third is seen in the lower border of the ligament, between the two tuberosities, for the passage of the long tendon of the Biceps muscle. The Coraco-humeral or Accessory Ligament is a broad band which strengthens the upper and inner part of the capsular ligament. It arises from the outer border of the coracoid process, and descends obliquely downwards and outwards to the front of the great tuberosity of the humerus, being blended with the tendon of the Supraspinatus muscle. This ligament is intimately united to the capsular in the greater part of its extent.
The Glenoid Ligament is a firm fibrous band attached round the margin of the glenoid cavity. It is triangular on section, the thickest portion being fixed to the circumference of the cavity, the free edge being thin and sharp. It is continuous above with the long tendon of the Biceps muscle, which bifurcates at the upper part of the cavity into two fasciculi, which encircle its margin, and unite at its lower part. This ligament deepens the cavity for articulation, and protects the edges of the bone. It is lined by the synovial membrane.
The Synovial Membrane lines the margin of the glenoid cavity and the fibrocartilaginous rim surrounding it; it is then reflected over the internal surface of the capsular ligament, covers the lower part and sides of the neck of the humerus, and is continued a short distance over the cartilage covering the head of that bone. The long tendon of the Biceps muscle which passes through the joint, is enclosed in a tubular sheath of synovial menbrane, which is reflected upon it at the point where it perforates the capsule, and is continued around it as far as the summit of the glenoid cavity.
The tendon of the Biceps is thus enabled to traverse the articulation, but is not contained in the interior of the synovial cavity. The synovial membrane communicates with a large bursal sac beneath the tendon of the Subscapularis, by an opening at the inner side of the capsular ligament; it also occasionally communicates with another bursal sac, beneath the tendon of the Infraspinatus, through an orifice at its outer part. A third bursal sac, which does not communicate with the joint, is placed between the under surface of the deltoid and the outer surface of the capsule.
The Muscles in relation with the joint are, above, the Supraspinatus; below, the long head of the Triceps; internally, the Subscapularis; externally, the Infraspinatus, and Teres minor; within, the long tendon of the Biceps. The Deltoid is placed most externally, and covers the articulation on its outer side, as well as in front and behind. The . Arteries supplying the joint, are articular branches of the anterior and posterior circumflex, and suprascapular. The Nerves are derived from the circumflex and suprascapular. Actions.
The shoulder-joint is capable of movement in every direction, forwards, backwards, abduction, adduction, circumduction, and rotation. Shoulder dislocation The most common causes of a shoulder dislocation occur by a forceful blow to the front of shoulder when the arm is outstretched or overhead. Such a blow can occur during a fall to the ground, or a collision with an object or another player, during a tackle. Dislocation is common in American football, rugby, wrestling, and sking. When a persons arm is stopped and the body continues to move forward the tremendous force is created across the shoulder joint.
This force can result in humeral head(ball) slipping out of the glenoid fossa(socket) which is part of the scapula, this results in a shoulder dislocation. Athletes with long history of participation in sports involving repetitive overhead motions or throwing, such as swimming, volleyball, or baseball, are more prone to suffering a shoulder dislocation. The repetitive stretching of the shoulder capsule and ligaments that occurs over time causes the shoulder to become loose or unstable. The way a shoulder dislocation is identified mostly by a person complaining of immediate pain with inability to move the shoulder or arm.
The person might report that the shoulder has popped out of place. A deformity might be visible, with prominence of the acromion(the upper part of the scapula that forms the roof of the shoulder) and a depression in the skin beneath it suggesting a dislocation. During a shoulder dislocation, the shoulder capsule and glenohumeral ligaments, which hold the shoulder in place, are torn and stretched. There might also be detachment of the labrum(anchorpoint of the shoulder capsule and ligaments) from the glenoid fossa.
Occasionally, other structures around the shoulder, such as the rotator cuff muscles or the surrounding nerves, are injured. Associated fractures can also occur during a shoulder dislocation, especially in older athletes. Greater tuberosity fractures have been reported to occur in up to a third of anterior shoulder dislocation. The vast majority of dislocation are anterior dislocations in which the humeral head slips out through the front, but depending on the position of the arm at the time of the trauma, the humeral head might slip from the gleniod fossa through the back, producing a posterior dislocation.
The initial treatment of an acute shoulder dislocation requires puting the shoulder back into place by placing the humeral head back into the glenoid fossa, also known as shoulder reduction. Shoulder reductions can often be done on the scene of incident in sports by a experienced physician. When the shoulder cannot be reduced on the scene, the arm and shoulders must be immobilized while the person is transported to an emergency room, where xrays can rule out an associated fracture and ensure that the shoulder is placed back into normal anatomical position.
Once the shoulder dislocation is reduced, the arm and the shoulder should be immobilized in a sling or brace for three to four weeks to allow for adequate healing. A shorter period of immobilization is recomended for older people over 40 to prevent joint stiffness and developement of an adhessive capsulitis(frozen shoulder). For anterior dislocation, recent research that immobilization in braces that keep the arm and shoulder rotated away from the body might lead to better healing of the shoulder structures and decrease the likelihood of future dislocations.
Associated greater tuberosity fractures are ussually treated conservatively with immobilization in a sling for four weeks. However when a fractures shows significant displacement greater than 5mm from its normal anatomic position, surgery is recomended. Following an appropriate period of immobilization, the individual will begin physical therapy to restore range of motion and strength in preparation for return to regular activity.
Strengthening in rotator cuff muscles supraspinatus, infraspinatus, teres minor and subscapularis is critically important in treating all shoulder disorders, and this is especially true for shoulder instability. These muscles are dynamic stabilizers of the shoulder joint and help prevent recurrent dislocations by holding the humeral head in place within the glenoid fossa. The muscles along the spine, the paraspinal muscles, play a role in shoulder function, and strengthining geared toward these muscle, are also recomended.