To understand what a shoulder instability is, what it involves and how it is treated it is important to have some notions of the anatomy of the shoulder. The shoulder is a joint of simple design, but complex anatomy. It is the joint of the body with the greatest range of motion, which makes it the most susceptible to instability. The shoulder joint involves two bony ends, the head of the humerus, shaped like a hemisphere with two tuberosities for anchoring the musculature, and the glenoid which is a part of the scapula that forms a pear-shaped cavity. Both surfaces are covered with hyaline cartilage to facilitate gliding with minimal friction.
Anatomy of the shoulder
The humeral head pivots on the glenoid as the arm moves, but as the surface of the glenoid is very small compared to the size of the humeral head it tends to pull out, to lose contact. For this we have other structures that contribute to stabilize the humeral head: at the top, as a roof of the joint, there is the acromion (another portion of the scapula) and the coracoacromial ligaments, the labrum or glenoid surround that circumferentially increases the diameter of the glenoid, and radially, hugging the head against the glenoid we have the tendons of the rotator cuff (supraspinatus, long portion of the biceps and subscapularis), and the glenohumeral ligaments and the joint capsule. Superficially we have the large muscles: deltoid, trapezius, latissimus dorsi and pectoralis major and minor, which shape the shoulder and contribute to its stability.
What is shoulder instability?
Shoulder instability is a condition that occurs when the structures that stabilize the shoulder (muscles, tendons and ligaments) do not properly perform their function, which is to keep the humeral head in contact with the glenoid socket. When the shoulder becomes unstable, the humeral head may become partially displaced out of place but maintain some contact between the head and the glenoid, which is known as a shoulder subluxation. When it is completely displaced, losing contact between the head and the glenoid, it is a shoulder dislocation.
How does instability occur?
Shoulder instability can occur by two different mechanisms:
- The most frequent one appears as a sequel of a sudden trauma that originated a shoulder dislocation which, after being reduced, the soft parts that were injured did not heal adequately and did not develop their function properly, leaving the shoulder unstable. The probability of having an unstable shoulder after a first episode of shoulder dislocation is directly related to age. In young people under 20 years of age, the risk of the shoulder becoming unstable after a dislocation is 90%, a risk that decreases to 70% in those under 35 years of age, while in those over 40 years of age the risk of instability does not exceed 10%.
- The second mechanism, less frequent, is the instability produced by repeated minor trauma, which, without causing a true shoulder dislocation, distends the joint capsule and ligaments and destabilizes the shoulder. It is frequent in athletes who perform throwing sports or overhead movements such as handball, basketball, swimming or tennis. The instability that originates is more complex because it is multidirectional, the shoulder generally does not dislocate, it subluxates in different directions causing pain.
In 80% of the cases the instability of the shoulder is anterior, by damage in the capsuloligamentous structures of the anterior part of the joint. The shoulder tends to dislocate anteriorly or anteroinferiorly. In 10% of the cases the instability is posterior, due to a defect in the posterior structures. In the remaining 10%, the instability is multidirectional, with the shoulder being able to subluxate in any direction, rarely dislocating.
Diagnosis of shoulder instability
Shoulder instability causes two types of symptoms:
- Acute symptomatology derived directly from repeated episodes of dislocation (severe pain and inability to move the arm). This is what we call a recurrent shoulder dislocation. Frequently, when the episodes of shoulder dislocation are very continuous, the dislocation tends to reduce spontaneously or the patient learns to reduce it with small maneuvers. In these cases the pain is usually not important.
- Chronic, continuous symptoms, derived from the lack of stability: chronic pain, generally in the anterior aspect of the shoulder, limitation of mobility, feeling of anxiety, fear that the shoulder may come out, feeling of shoulder displacement and lack of strength due to muscle atrophy.
The patient’s account of the symptoms is usually sufficient to diagnose shoulder instability. Clinical examination at the consultation, together with additional diagnostic tests will help to confirm the diagnosis, establish the extent and location of the injury and plan the most appropriate treatment.
Currently, the diagnostic test indicated to evaluate shoulder instability is Arthroresonance or Magnetic Resonance Imaging with intrarticular contrast injection. This test consists of injecting 20 cc of saline with a paramagnetic contrast (Gadolinium) into the joint. If there is a rupture of the ligaments or the joint capsule, the serum with the contrast will leak through the defect, identifying its location and size. Likewise, if the joint capsule and the ligaments are distended (“given of themselves”), not fulfilling their function, the contrast will accumulate in greater quantity than usual.