A joint is dislocated when the articular surfaces of the bones that normally meet at the joint are completely out of contact with one another. This is distinct from subluxation, a condition in which the articular surfaces are only partially out of contact.
The urgency of treating dislocated joints is based on several factors. One is the potential for neurologic or circulatory compromise. The neurovascular bundle passing close to the affected joint is typically "kinked" around the deformity associated with the dislocation. Persistence of this condition can result in a neurologic or vascular deficit that may be temporary if the deformity is reduced promptly but irreversible if treatment is delayed.
Another consideration is that, the longer a joint has been dislocated, the more difficult it may be to reduce and the more likely it is to be unstable after reduction. This is probably due at least in part to edema, muscle spasm, and other tissue changes that increase over time.
Dislocation of the hip carries its own particular urgency in addition to those mentioned above: the danger of avascular necrosis of the femoral head. Avascular necrosis occurs because much of the blood supply to the femoral head is delivered through vessels that emerge from the acetabulum. When the joint is dislocated, circulation to the femoral head is disrupted. At some point, the vascular insult becomes irreversible, and bony necrosis is the ultimate result. Although aseptic necrosis may occur despite the physician's best efforts, its likelihood increases with the time delay until reduction.
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