Excerpt from Hand Dislocation

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Background

Dislocations in the hand are common injuries in sports and in occupational settings, often appearing to be minor. If the athlete, trainer, or coach has already reduced the dislocation, it appears unimpressive compared to a major knee injury or a shoulder dislocation. However, these injuries have real potential for long-term disability in sports and other areas of life if adequate reduction is not performed, if associated injuries are not identified and appropriately treated or referred, and if potential complications of the injury and its treatment are not foreseen. The judgment of the initial treating physician can be critical in determining the long-term outcome.

Frequency

United States

The annual incidence of all types of dislocations in the hand is approximately 67,000 in the United States. Most hand dislocations are sports or occupational injuries, with a lesser number sustained in falls and traffic collisions (sometimes associated with airbag deployment). Most of these are dislocations at the proximal interphalangeal (PIP) joint because the greater range of motion (ROM) of the PIP joint makes it more vulnerable to injury. Of the PIP dislocations, most are dorsal. Volar dislocations of the PIP joint are much less common, are more difficult to reduce, and are associated with more complications. Distal interphalangeal (DIP) joint dislocations also are uncommon, almost always dorsal, and often open.

In addition to PIP and DIP dislocations in order of decreasing frequency, metacarpophalangeal (MCP) joint dislocations and carpometacarpal (CMC) dislocations also occur. The MCP joint of the 4 fingers usually dislocates posteriorly (simple type) but can, on the rare occasion, become entrapped between the palmar fascia and the palmar plate and become irreducibly dislocated. The CMC joint dislocation is a disabling injury, which is usually dorsal, and may be associated with fractures of the bases of the metacarpals.

Functional Anatomy

The bony anatomy of the PIP joint consists of medial and lateral condyles on the proximal phalanx, with matching concavities on the associated distal phalanx. This allows for a wide ROM in flexion and extension, but it is relatively rigid in abduction and adduction, making it a hinge (ginglymus) joint functionally. DIP joint bony anatomy is similar but with surrounding soft tissue providing more restriction in flexion. The extrinsic flexors across both joints are at least 4 times stronger than the extensors, allowing flexion contractures to develop very rapidly, especially with immobilization in flexion. Adequate ROM, especially at the PIP joint, is critical for normal hand function.

Both types of interphalangeal joints are supported by similar soft tissue structures on all 4 sides, which includes the volar plate on the palmar side (the integrity of this structure is essential to a stable reduction), collateral ligaments on the radial and ulnar sides, and the extensor complex (central slip, lateral bands, and hood) dorsally. These structures attach to and reinforce the joint capsule. For a dislocation to occur, at least 1, often 2, and sometimes 3 of these structures must be significantly injured.

The volar plate is a roughly triangular structure with its base oriented distally, attaching to the volar base of the middle phalanx with its tip attaching to the distal aspect of the proximal phalanx. The volar plate functions largely in limiting hyperextension. Thus, it is nearly inevitably injured in dorsal dislocations.

The collateral ligaments restrict the joint from opening to varus or valgus stress and are also commonly injured in dorsal dislocation. Injury to the radial collateral ligament is about 6 times more common than injury to the ulnar collateral ligament. The extensor complex over the PIP joint consists of the central slip, which attaches to the base of the middle phalanx; the lateral bands, which run dorsolaterally on each side; and the transverse retinacular ligament, which connects these structures and extends laterally. These structures help to limit volar movement of the base of the middle phalanx and are thus commonly injured in volar dislocations at the PIP joint, with the middle phalanx either tearing the central slip from its insertion or buttonholing through the transverse retinacular ligament between the central slip and one of the lateral bands.

The MCP joint is thought to be an ellipsoid joint. The head of the metacarpal consists of medial and lateral condyles and is narrower on its dorsal surface than on its palmar surface; it fits into the concavity of the base of the proximal phalanx. The true collateral ligament attaches to a recess created by the junction of the shoulder and head. The collateral ligament is composed of the following 2 parts: (1) a dorsally placed cord portion and (2) a fan-shaped volar portion or accessory collateral ligament, which extends from the metacarpal to the sides of the volar plate. To accomplish flexion and extension at the MCP joint, the anterior and posterior parts of the capsule must be lax. When the joint is extended, the phalanges have considerable lateral play in abduction and adduction and, therefore, this joint avoids frequent injury; however, if the ligament is torn, dislocation occurs.

The bony anatomy of the CMC joint consists of the 5 metacarpal bases that articulate with the trapezoid, trapezium, capitate, and hamate (in that order) from the radial to the ulnar aspect of the hand. The CMC joint is a relatively fixed joint segment because of the articular congruity of the joint surfaces, with the metacarpal bases acting like concave receptacles to the distal carpal row, and because of the strong interosseous and extrinsic ligament complex. The palmar and dorsal ligaments are distinct, with the palmar ligaments being stronger. The scaphoid acts as a link between the proximal and distal carpal rows. The extensor and flexor tendons pass over this articular area but add no strength to the CMC joint because the bases of the metacarpals dislocate dorsally relative to the distal carpal row.

The MCP joint of the thumb has radial and ulnar collateral ligaments, which are loose when the joint is extended and tight when flexed. When the joint is extended, the proximal phalanx has the lateral play achieved by the action of the interosseous muscles. When the thumb is flexed and in a functional position, as in the case of many sports situations (eg, skiing, falls on a gloved hand), the ulnar collateral ligament is the structure at risk and can be ruptured. The ulnar collateral ligament can then be displaced so that the adductor aponeurosis is interposed between the ruptured end of the ligament and its site of bony attachment.

The first CMC joint or the first metacarpotrapezoid joint is a very mobile saddle joint, with articular surfaces that are reciprocally concavoconvex. The most important soft tissue support for this first CMC joint is the deep ulnar or anterior oblique ligament that runs from the volar beak of the metacarpal to the tubercle of the trapezium. This ligament can be ruptured, but it tends to be avulsed with a piece of bone (Bennet fracture dislocation).

Sport Specific Biomechanics

Dislocations of the interphalangeal joints of the hand probably occur most commonly in basketball and football. In basketball, the usual mechanisms of injury include being struck by the ball, catching a finger on the rim, or contact with another player. In football, the finger may be caught on a jersey, slapped against a helmet, or crushed between some combination of other players, their equipment, and the ground. Linemen and defensive players are at highest risk for hand injuries. In both of these sports, return to play almost always requires that the injury can be splinted stably to allow for a power grip.

Dislocations of the MCP and basilar CMC joints occur most commonly with falls on the outstretched hand or the flexed supinated wrist. With this extension vector, the forces are transmitted up through the carpus.

Injuries and dislocations of the thumb, MCP joint, and the CMC basilar joint can commonly occur in falls, with the thumb in abduction. Examples of this type of injury include falls on the gloved hand in baseball or an abduction force applied to a flexed thumb while grasping an object, such as in skiing injuries when the pole impacts the proximal phalanx tearing the radial collateral ligament. This occurs when the wrist is extended at the time of the injury.

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