Practice Essentials
Sprains and dislocations of the metacarpophalangeal (MCP) joint of the finger are relatively rare due to the protected position of this joint in the hand. [1, 2, 3, 4] Injuries to the MCP joint of the thumb are more common, although these usually consist of collateral ligament injuries rather than dorsal or palmar dislocations. [5, 6]
MCP joint dislocation is seen in the image below.
Metacarpophalangeal joint dislocation of the small finger. Posteroanterior radiograph demonstrates loss of joint space.
MCP joint dislocations are usually caused by forced hyperextension of the digit. Radial collateral ligament injuries occur during forced ulnar deviation of a flexed MCP joint.
Functional Anatomy
The bony anatomy of the finger MCP joint provides greater laxity in extension, with the shallow articular surface of the proximal phalanx resting on the spherical metacarpal head. The metacarpal head is wider in palmar orientation, which leads to increasing bony stability as the joint approaches maximal flexion. Soft-tissue constraints, including the volar plate, accessory and true collateral ligaments, dorsal capsule, extensor tendon and sagittal band, and intrinsic tendons provide additional stability to the MCP joint. This results in an arc of motion from 30º of hyperextension to 120º of flexion, 30-40º of mediolateral laxity, and a small degree of rotational laxity.
The volar plate is a fibrocartilaginous structure firmly attached to the base of the proximal phalanx. Its origin, just proximal to the metacarpal head, is thin and diaphanous; this allows hyperextension of the MCP joint, but it is also the part of the joint most susceptible to injury during dislocations. The deep transverse metacarpal ligaments further stabilize the volar plates of the neighboring MCP joint.
The collateral ligaments originate from mediolateral depressions in the metacarpal head and travel in a distal-palmar direction to insert onto the base of the proximal phalanx. The elliptical shape of the metacarpal head causes these ligaments to loosen in extension and tighten in flexion. The accessory collateral ligament spans from the true collateral ligament to the volar plate, providing additional joint stability in extension. The central extensor tendon and sagittal band augment the thin dorsal capsule. The tendons of the palmar and dorsal interossei add a small degree of dynamic stability.
The MCP joint of the thumb is a condyloid (hinged) joint, with a quadrilateral rather than spherical metacarpal head. The capsule and ligaments of this joint are similar to those of the finger MCP joint. Additionally, the volar plate of the thumb MPJ usually contains 2 sesamoids that articulate with the metacarpal head. The insertion of the thenar muscles into the sesamoids contributes to joint stability. These bony and ligamentous constraints allow less motion than in the MCP joint of the fingers, especially in lateral motion and rotation; abduction and adduction average 10º and a slight amount of pronation occurs during flexion.
Sport-Specific Biomechanics
Dorsal MCP joint dislocations have been described as simple or complex. Simple dislocations are those in which no soft tissue is interposed in the joint. These are usually reduced easily with an appropriate closed technique. In a classic article published in 1957, Kaplan elegantly described the anatomic features of the complex MCP joint dislocation. [7] A metacarpal head displaced in palmar orientation sits between the lumbrical muscle radially and the flexor tendons ulnarly. The volar plate, still firmly attached to the base of the proximal phalanx, is displaced into the MCP joint. Longitudinal traction only further tightens these already taut soft tissues, trapping the metacarpal head. Complex dislocations usually require open reduction.
Prognosis
The prognosis is good for MCP joint injuries that are recognized early and treated appropriately. Following operative intervention for nonreducible MCP joint dislocation, full recovery of range of motion typically occurs within 4 to 6 weeks. [8]
Complications
Soreness and swelling may persist for many months after the capsule and ligaments have healed. Inadequate immobilization or early return to high-stress activities may result in ligamentous laxity or recurrent instability. Excessive immobilization or severe soft-tissue damage may lead to some joint stiffness, which is common after many of these injuries. Traumatic open reductions or late reductions can result in osteonecrosis of the metacarpal head. [8] Posttraumatic arthritis may occur after multiple closed reductions or unrecognized (chronic) dislocations. Digital nerve injury may occur during the volar surgical approach to the MCP joint.
Patient Education
The patient needs to be educated regarding his or her particular injury and the methods of proper treatment to ensure adequate healing. Joint protection is a key concept for the patient to understand. Once healing has taken place through immobilization, the patient should be instructed in a progressive range-of-motion program to regain mobility of the injured joint(s).
For patient education resources from eMedicineHealth, see Finger Sprain, Finger Dislocation, Broken Finger, and Sprains and Strains.
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Metacarpophalangeal joint dislocation of the small finger. Posteroanterior radiograph demonstrates loss of joint space.
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Metacarpophalangeal joint dislocation of the small finger. Oblique radiograph best demonstrates dorsal dislocation of proximal phalanx.
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Metacarpophalangeal joint dislocation of the small finger. Dorsally dislocated proximal phalanx is obscured by other digits. Note that the small finger metacarpal sits palmar to the other metacarpals.
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Closed reduction
