The glenohumeral joint is the most movable in our body, with a range of motion of almost 360°. It is composed of the humeral head and the glenoid cavity. The shoulder joint is mainly guided and stabilized by soft tissues (labrum, ligaments, rotator cuff), which on the one hand, allows this extraordinary range of motion, but on the other hand, makes it more prone to instability and/or dislocation. Unlike other joints, such as the hip joint, it is relatively little ‘secured’ by bony structures.
Once dislocation or subluxation has occurred, it can often be accompanied by other soft tissue and bone structure injuries. Ligaments, labrum, or tendons may be involved. Bony structures, such as the glenoid cavity or humeral head, may show abrasions. Depending on the extent of the associated injuries, surgical intervention may be considered.
With so many different terminologies, it can be easy to lose track of what’s going on. Therefore, all essential terms concerning shoulder instability are briefly explained here:
Shoulder instability: The inability to keep the humeral head centered in the socket.
Subluxation: The head of the humerus and the glenoid cavity have partial loss of contact under load. Spontaneous reduction is possible.
Shoulder luxation: the humeral head and glenoid cavity permanently lose contact.
Chronic-recurrent shoulder dislocation: the sequential condition following an initial traumatic shoulder dislocation.
Voluntary dislocation: The glenohumeral joint can be dislocated at will and in a controlled manner and repositioned by itself.
Laxity: The regular and physiological joint mobility required to perform physiological movements
Hyperlaxity (hypermobility): The increased mobility of a joint beyond the physiological level, which can lead to clinical symptoms
Shoulder instability is a broad issue because therapies and follow-up treatments must also be selected and adapted accordingly depending on the type.
A common classification for shoulder instability is that according to Gerber. Here, a distinction is made between instability and hyperlaxity (hypermobility) and indirectly between traumatic (unidirectional) and atraumatic (multidirectional) instability, whereby hyperlaxity may be combined with instability but primarily has no pathological value.
Here, I will essentially discuss the types of instabilities (cat. 2 Unidirectional instability without hyperlaxity), which can occur after a traumatic event.
Basically, 3 types of post-traumatic shoulder instability can be distinguished:
Static instability means that the humeral head is not centered in the glenoid cavity. It may have migrated either upward, backward, forward, or downward. Since it is not a complete dislocation, this is also called a subluxation.
Dynamic instability is a completely dislocated shoulder joint that has not been repositioned. Sometimes, affected individuals may perceive pain, but a limited range of motion is still present. Something like this can be observed in older patients who have had experienced, for example.
There are instabilities after a dislocation that tend in one direction (anterior, posterior, superior, inferior). Especially people who are hyperflexible can be prone to subluxations.
Voluntary instability is understood to be the controlled dislocation of the shoulder joint. Some people can dislocate and retract the shoulder at will (e.g., contortionists in the circus).
Younger people have an increased risk of recurrent dislocation because the connective tissue is usually more flexible than older people. Although older people are not as susceptible to recurrent dislocation, damage to the surrounding tissues (rotator cuff, ligaments, tendons, cartilage, …) may occur after injury.