Joint dislocation

Anterior shoulder dislocations in those over 40 years of age are usually the result of a fall or a direct blow to the shoulder, whereas in younger patients they often occur as a result of high-impact activities (e.g., sports and motor vehicle collisions).[6][7] The injury results from abduction, external rotation, and extension, followed by a posterior-directed force. Posterior dislocations are usually caused by a blow to the anterior shoulder and axial loading of the adducted internally rotated arm. They may also be a result of violent muscle contractions (e.g., seizures, electrocution).[21] Inferior dislocation (luxatio erecta) is caused by hyperabduction or axial loading on an abducted arm.[22]

Distal interphalangeal and proximal interphalangeal finger dislocations are usually dorsal in direction and associated with axial loading, hyperextension, and ball-catching sports.[15][23] Metacarpophalangeal finger dislocations are relatively rare and are often associated with accompanying fractures.[15][24]

Primary acute patellar dislocations usually occur in young patients engaged in athletic activities. The patella is typically dislocated laterally and subsequently causes rupture of the medial patellofemoral ligament in about 90% of patients.[25] Patients will often state that when the injury occurred the affected leg was planted while they attempted to pivot. Several anatomic risk factors for first-time and recurrent lateral patellar dislocation have been described, including trochlear dysplasia, patella alta (an abnormally high patella in relation to the femur), increased tibial tuberosity to trochlear groove (TT-TG) distance, patellar tilt, external tibial torsion (i.e., tibia rotates outward), and a high Q angle (a measurement of the angle between the quadriceps and the patellar tendon).[26][27][28][29][30] Trochlear dysplasia appears to be the most significant contributor to instability of the patellofemoral joint.[16][31][32]

Elbow dislocations usually occur in young patients, and nearly 50% are sports-related.[18] They are most commonly the result of a fall on an outstretched arm, resulting in an externally rotated, valgus, and axially directed load to the elbow.[33][34] Direct high-energy impact can also be a cause.[35]

Acquired hip dislocations are either native dislocations or dislocations after total hip replacement. Axial loading of the femur toward the acetabulum is the classically described mechanism for native hip dislocation. It is commonly observed with impact of the bent knee with a dashboard in a motor vehicle crash.[36] The direction of the dislocation is dependent on the position of the femur and the acetabulum at impact, and the direction of the force vector applied.[37] An adducted femur that is flexed at the hip will sustain a posterior hip dislocation. A hip that is abducted and externally rotated is more likely to sustain an anterior dislocation.

Dislocation after total hip replacement usually occurs within the first 3 months following surgery. It occurs when the patient reaches the extremes of the prosthetic range of motion and the femoral neck levers on the acetabular cup, allowing the femoral head to leave the acetabulum. Other common conditions that can lead to postoperative dislocations include laxity or soft tissue incompetence surrounding the hip joint, incorrect positioning of prosthetic components, and neuromuscular disorders (e.g., Parkinson disease).[38]

Those with loose ligaments may experience sprains or dislocations more frequently. Loose ligaments may be generalized or confined to a few joints; the trait is usually hereditary. Affected patients have joints with a wide range of movement (e.g., "double-jointed" people). Ehlers-Danlos syndrome is a rare inherited condition characterized by unusually flexible joints, very elastic skin, and fragile tissues. It may be the cause of widespread laxity of connective tissue.

Shoulder dislocations

Often caused by trauma; 25% are associated with concurrent humeral fractures.[39]
Hill-Sachs and Bankart lesions are possible sequelae of the dislocation if the humerus dislocates anteriorly.[40]
Rotator cuff tears are often found after dislocations, particularly in older adults.[41] The overall frequency of rotator cuff tears after an anterior dislocation ranges between 7% and 32% and rises with advancing age.[42] Some studies have shown up to 100% incidence of cuff tears in patients over 70 years of age.[43]

Finger dislocations

Dorsal distal interphalangeal (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, the 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.[15][44][45]
Dorsal proximal interphalangeal (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.[15][41][46]
Dorsal metacarpophalangeal 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.[15][24]

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 stabilizer is disrupted, which allows the patella to translate laterally.[13][31][32]

Elbow dislocations

The most common direction of elbow dislocation is posterior or posterolateral; however, the exact mechanism has been the subject of debate.[47]
O'Driscoll et al proposed a sequential 3-stage disruption of soft tissue structures from lateral to medial, termed the "Horii circle":[48]
- the lateral collateral ligament is injured (stage 1)
- the remaining lateral structures and the anterior/posterior capsular attachments become involved (stage 2)
- the anterior band of the medial collateral ligament is disrupted (stage 3).
Other studies have proposed that the soft tissue injury sustained during elbow injury begins, and is more severe, on the medial side.[34][47][49][50][51]
Common to all proposed mechanisms is an extended elbow with the forearm in supination, subjected to an axial load and a posterolateral, or valgus, force, and resulting in a spectrum of soft tissue injuries.[47] Throughout this continuum of injury, there is a potential for associated fractures to the intracapsular structures including the radial head and coronoid process.[44]

Hip dislocations

Dislocation of the hip is an uncommon event, as the hip is a confined ball and socket joint that is considered to be extremely stable.[52]
Significant force applied through the femoral head can lead to a dislocation of the joint combined with a fracture to the posterior acetabular wall, the acetabulum in general, the femoral head, and rarely the femoral neck.[52][53]
Cartilage damage is also commonly documented as a result of a shearing injury across the cartilage of the acetabulum and the femoral head. Resulting loose bodies from fragmented bone and cartilage can be left behind, in and around the hip joint.[4]

Clinical definitions

Shoulder dislocations:

Types are based on direction of dislocation: anterior, posterior, or inferior (i.e., luxatio erecta).

Can also be characterized as primary (i.e., first dislocation) or recurrent.[1]

Finger dislocations:

Characterized by the joint involved: metacarpophalangeal (MCP), proximal interphalangeal (PIP), or distal interphalangeal (DIP).
Also described by the direction of displacement (i.e., dorsal or volar in respect of position of distal part).

Patellar dislocations:

Characterized by direction of dislocation, the majority being lateral.
Can also be described as primary (i.e., first dislocation) or recurrent.

Elbow dislocations:

Characterized as simple or complex.
Simple elbow dislocation is defined as acute dislocation that occurs without fracture.[2]
Complex dislocations involve concomitant fractures of the proximal radius, ulna, or distal humerus. They can also be classified based on the direction of displacement of the radius and ulna with respect to the humerus (i.e., anterior or posterior). A posterior dislocation of the elbow associated with a fracture of the coronoid process and fracture of the radial head is described as the "terrible triad."[3]

Hip dislocations:

Characterized by direction of dislocation (i.e., anterior or posterior) and severity as per the Stewart-Milford classification system:[4]
- Type I: simple dislocation without fracture
- Type II: dislocation with one or more rim fragments, but with sufficient socket to ensure stability after reduction
- Type III: dislocation with fracture of the rim producing gross instability
- Type IV: dislocation with fracture of the head or neck of the femur.

Pathophysiology

Classification

Clinical definitions