Acute Sports Injuries - JOINTS

Joint stability depends on the interaction between the passive, active and neural subsystems. Muscles and tendons combine to form the active subsystem, which is controlled by the neural subsystem to provide dynamic joint stability. When the force applied to a joint exceeds the capabilities of the active subsystem or when the active and/or neural subsystems are compromised, loads are transferred to the passive subsystem. The passive subsystem consists of non-contractile connective tissues, including the bony anatomy, joint capsule, ligaments and fibrocartilage joint structures (e.g. labrum, volar plates, menisci).

Excessive load placed on the passive subsystem can cause the bones forming a joint to abnormally translate or luxate relative to one another. When the bones are forced to completely separate so the articulating surfaces are no longer in contact it is referred to as dislocation. When the bones shift relative to one another, but the articulating surfaces remain partially in contact, this is known as subluxation (partial dislocation).

Luxation of a joint will invariably result in damage to passive subsystem structures. The structures damaged and the extent to which they are damaged depends on the magnitude and direction of the luxation force and the inherent stability of the joint created by the passive subsystem. The hip joint is inherently stable as the large ball shape of the femoral head is well encased in a reciprocating socket-shaped acetabulum and the bones are supported by a robust joint capsule reinforced by ligaments. Considerably more force is needed to luxate the hip joint compared with a less stable joint such as the shoulder.

The shoulder lacks inherent stability as the large humeral head outsizes the small, shallow glenoid fossa and the joint possesses a thin, loose capsule minimally supported by ligaments. The heightened force required to luxate inherently stable joints (e.g. hip, elbow, ankle and subtalar joints) means that subluxation or dislocation of these joints is more likely to be associated with damage beyond that to the joint capsule and ligaments, including fractures and damage to cartilage, vessels and nerves.

A dislocated joint is readily identifiable by gross deformity with complete loss of joint function. The individual will present with significant, acute pain and will often cradle/hold the afflicted limb. Following a neurovascular screen to assess for nerve and vessel damage, the dislocated joint should be reduced as quickly as possible. In most cases, this can be performed by applying gradual and controlled distraction of the joint while simultaneously moving the joint through a passive range of motion. When distraction is unable to overcome the opposing muscle spasm and the joint does not readily reduce, use of an injected muscle relaxant or general anaesthetic may be required.

After reduction, all dislocated joints should be X-rayed for the presence of an accompanying fracture and the joint should be protected to allow the joint capsule and ligaments time to heal. However, early protected mobilisation is encouraged to promote functional healing. Training of the active and neural subsystems should start as early as possible and progress to functional activities, so these subsystems can compensate for the reduced stability afforded by the damage to the passive subsystem. Unfortunately, in some joints (e.g. shoulder), active and neural subsystem training is often insufficient to prevent re-dislocation or chronic subluxation, resulting in eventual surgical reconstruction of the damaged capsule and ligaments.

REFERENCES

Brukner, P., 2012. Brukner & Khan's clinical sports medicine. North Ryde: McGraw-Hill.