Pathophysiology

Shoulder dislocations

• Often caused by trauma, and 25% are associated with concurrent humeral fractures. [20]

• Hill-Sachs and Bankart's lesions are possible sequelae of the dislocation if the humerus dislocates anteriorly.

• Rotator cuff tears are often found after dislocations, particularly in older adults. [21] Some studies have shown up to 100% incidence of cuff tears in patients over 70 years of age. [22]

Finger dislocations

• Dorsal DIP dislocations occur following failure of the volar plate (i.e., the restraint to dorsal displacement of the digit) with dislocation of the distal phalanx. Occasionally, this volar plate can become entrapped in the joint, making the DIP irreducible. Volar dislocations may become irreducible as the extensor tendon or sesamoid bones become wrapped around the head of the middle phalanx. [10] [23] [24]

• Dorsal PIP dislocations involve axial stress and hyperextension and result in disruption of the volar plate. Occasionally, the volar plate can become entrapped in the joint, making the PIP irreducible. On rare occasions these dislocations can be in a volar direction. This dislocation is often irreducible because of interposition of the dorsal plate, the central slip, or the lateral bands of the finger. [10] [21] [25]

• Dorsal MCP dislocations are uncommon because of the ligamentous stability afforded at the base of the fingers. Dislocations at this level imply disruption of the volar plate, collateral ligaments, and the joint capsule. [10] [17]

Patellar dislocations

• The primary restraint to lateral translation of the patella is the medial patellofemoral ligament (MPFL). The incidence of tears to the MPFL during an acute patellar dislocation varies from 75% to 98%. As this structure fails, the main stabiliser is disrupted, which allows the patella to translate laterally. [8] [18] [19]

Elbow dislocations

• In elbow dislocations, there is a predictable 3-stage pattern of soft tissue disruption. In stage 1, the lateral collateral ligament is injured. In stage 2, the remaining lateral structures and the anterior/posterior capsular attachments become involved. In stage 3, the anterior band of the medial collateral ligament is disrupted. Throughout this continuum of injury, there is a potential for associated fractures to the intra-capsular structures including the radial head and coronoid process. [23]