Normal Shoulders, Unstable Shoulders
Normal Shoulder Structure
Your shoulder is one of the most complex joints in the human body. Three bones come together at the shoulder: the humerus, scapula, and clavicle. The upper end of the scapula has a socket (the glenoid cavity) into which the rounded head of the humerus fits. The head of the humerus and the socket of the scapula are enclosed by the synovial capsule which, under normal conditions, is relatively large and loose (Fig. 2a). This capsule is lined by a thin, smooth synovial membrane.
Altogether, there are eight ligaments that hold the shoulder together. Three of these ligaments (the upper, the middle, and the lower glenohumeral ligaments) are actually fibrous thickenings of the inner capsule wall. Fig. 2b shows the capsule is cut away to reveal the anterior band of the inferior (lower) glenohumeral ligament complex together with the superior and medial glenohumeral ligaments that normally lie within the capsule. The anterior band is more taut and therefore more supporting than the other ligaments, when the arm is raised 90° at the side and rotated outwards. Incidentally, this is the arm position most vulnerable to dislocation when a strong force is suddenly applied.
Fifteen muscles act on the shoulder joint. Of these, four short muscles (the teres major, supraspinatus, infraspinatus, and subscapularis), are particularly important to shoulder and arm movement. They cross the joint to insert into the humerus, thereby also helping to stabilise the shoulder joint. The tendons from these muscles form a rotator cuff which surrounds the capsule and blends with it (Fig. 2c). The interior of the shoulder joint also has sites for attachment for three other muscles required for arm movement: the triceps, biceps and deltoid.
In addition to the capsule, the shoulder has two bursae (subacromial and subscapularis bursae). These are small pockets of fibrous tissue lined with synovial membrane and filled with a fluid, which lubricates the joint to reduce friction.
Finally, there is an axillary nerve and associated blood vessels.
The diameter of the humeral head is nearly twice that of the shallow glenoid cavity in which it sits. Their proportions are like a golf ball on a tee. Because of this, the humeral head is only loosely held within the socket. This allows a wide range of shoulder movement but at some cost to its stability. That is the reason why the shoulder (often called the glenohumeral joint) is the most commonly dislocated joint in the body.
The edge of the shoulder socket is rimmed by the labrum, a fibrocartilage extension of the socket, which doubles its depth and normally contributes to shoulder stability (Fig. 2a). The uppermost parts of the shoulder ligaments and the biceps tendon attach to the labrum, which is firmly attached to the glenoid cavity along its lower border, but its upper border is more mobile and more loosely attached.
The Unstable Shoulder
Traumatic Anterior Instability
In 85% of all cases where traumatic instability has been sufficiently severe to warrant surgery, part of the anterior labrum has torn away. This is a Bankart lesion as shown in Fig 2d. Occasionally, a ligament detaches from the humerus and, rarely, ligaments detach from both the humerus and the labrum.
Some athletes who perform overhead activities, such as volleyball, swimming, throwing and tennis, apparently have shoulder pain resulting from instability. A current hypothesis is that repetitive shoulder movements at the extreme range of motion leads to gradual stretching of the anterior capsule, and tightening of the posterior capsule, with subsequent shifting in the position of the humeral head. Whilst at an extreme range of motion or during periods of shoulder muscle fatigue, this change in position in the humeral head may produce shoulder pain due to impingement in the form of friction and subsequent inflammation.
As for complications, fractures accompany about 24% of dislocations, particularly affecting middle-aged and older people. Technically speaking, almost 80% of dislocations are accompanied by a compression fracture or indentation along the humeral head (a "Hill-Sachs lesion"). However, this complication is usually minor and too small to influence the outcome. Approximately 5% of shoulder dislocation patients also suffer some associated nerve injury.
An estimated 5% of dislocations have a non-traumatic, "atraumatic", origin, i.e., minor incidents such as raising the arm or moving during sleep. Some people, especially certain females, can dislocate or sublux their shoulder by putting their arm into a certain position. This is called atraumatic instability. The exact cause of atraumatic instability is likely to be from excessive laxity (looseness) of the shoulder capsule. Interestingly, about 50% of people who have atraumatic shoulder instability are unaware that their shoulder is so loose that it sometimes subluxes (partially dislocates).
A person is said to have multidirectional instability (MDI) when the humeral head can be moved abnormally far in the glenoid cavity in more than one direction. This condition is due to having an excessively loose capsule and is often associated with labral tears or detachment. In a small number of cases, MDI is part of an inherited syndrome involving unusually loose ligaments throughout the body.
First Time Dislocation
First-time shoulder dislocations can occur at any age although they tend to occur most frequently in the second and sixth decades (Fig 2f). In young people, most dislocations occur within the shoulder capsule with the cartilaginous labrum detaching from the bone. In contrast, about 33% of older people have dislocations which protrude through the shoulder capsule.
An important complication of primary dislocation is that further dislocations tend to occur. About 70% of those who have already dislocated can expect to dislocate again within two years of the initial injury. Most of these recur in young males, possibly because they use their shoulders more vigorously than most other people.
Whether or not dislocation recurs strongly depends on one's age. Adolescents are most at risk (Fig 2g). Thus, dislocation recurs in 83-90% of people aged 20 years or under, 60-63% of people aged between 20-40, and 10-16% of people aged 40 years or older. For this reason, the average age of shoulder patients with primary dislocation is around 48 years while for patients with recurrent dislocation the average age is only about 23 years.
One contributing factor for this age difference is due to changes in collagen that occur as people grow older. In newborns, Type III collagen is produced and the fibres formed from this type of collagen are supple and elastic. With each passing decade, collagen-producing cells make less Type III collagen and progressively convert to synthesising another form of collagen (Type I) which has different characteristics (Fig 2h). Type I collagen contains sulfur groups that have a high tendency to cross-link and form bridges between the collagen filaments, causing the fibres they comprise to be relatively tough and non-elastic.
This changing ratio of collagen Types I and III throughout the body is so reliable that the chronological age of an individual can be determined by analysing the collagen Type III content of a skin sample according to the following equation :
collagen Type III (mg)/wet dermis (gm) = 1.3e-Age/23.5
Thus, the higher content of stretchy collagen in their tendons and ligaments can help to account for the observation that younger people who have already had a dislocated shoulder are much more prone to recurrent dislocation than older people. Once excessively stretched, the capsule and/or ligaments may be too loose to provide the secure and stable shoulder support required for maximum performance in athletes.
Warning: Remember, in the case of shoulder dislocation, always seek expert medical advice and aid.