The metacarpals are tubular bones structurally divided into a head, neck, shaft, and base. When viewed in the sagittal plane, the metacarpal head has an increasing diameter beginning dorsally and extending along the articular surface to the volar side. When viewed in the coronal plane, the metacarpal head is pear-shaped or dumbbell-shaped, with the volar surface extending out of each side. The metacarpal head is broader in volar orientation, which results in increasing bony stability as the joint is flexed. The volar plate, collateral ligaments, dorsal capsule, deep transverse intermetacarpal ligament, extensor tendon, and intrinsic tendons provide additional support and stability to the MCP joint.
The MCP joints are resistant to ligamentous injury and dislocation because of their inherent ligamentous structure, their surrounding supporting structures, and their protected position at the base of the fingers. The volar plate is a fibrocartilaginous structure that is attached firmly to the base of the proximal phalanx. It originates just proximal to the metacarpal head where it is thin and transparent. This allows for hyperextension of the MCP joint yet makes it vulnerable to injury during dorsal dislocations.
Side-to-side stability of the MCP joint is provided mainly by the collateral and accessory collateral ligaments, and to some extent by the lumbrical and the interossei muscles and tendons.8 The collateral ligaments originate from the mediolateral recesses in the metacarpal head and travel to insert onto the base of the proximal phalanx. The eccentric configuration of the metacarpal head and the relatively fixed length of the collateral ligaments cause it to tighten when the joint is in flexion (Figure 84-2). This accounts for the limited abduction and adduction of the MCP joint in flexion as compared to the laxity in extension. The collateral ligaments of the MCP joint are most vulnerable to injury from forces directed ulnarly and dorsally. 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 shape of the metacarpal head is eccentric, making the collateral ligaments tighter in flexion (A) than in extension (B).
The interphalangeal (IP) joints are hinges and motion only occurs as flexion and extension while the MCP joints are condyloid joints. The range of motion about the MCP joint includes 120° of flexion and 30° of hyperextension. It has up to 30° of additional mediolateral laxity and a small degree of rotational laxity to facilitate an efficient grasp.9 The clinical importance of these differences is that to minimize the development of contractures after joint injuries. The preferred position of immobilization of the IP joints is in extension, whereas the MCP joints are immobilized in flexion.
The opposable thumb is an essential structure for countless activities. Despite its strong ligamentous and capsular support, the exposed positioning of the thumb makes it a frequent site of dislocations and subluxations. The MCP joint of the thumb is similar to those of the fingers but has a stronger volar plate and collateral ligaments with less side-to-side mobility.2 The MCP joint of the thumb is also a condyloid joint with a quadrilateral rather than a spherical metacarpal head that allows mainly flexion and extension. However, it also permits some degree of abduction, adduction, and rotation. There is a large variability among individuals in its range of flexion and extension.
The metacarpal head is bicondylar, with the radial condyle being slightly larger. This structural difference provides the thumb proximal phalanx with a modest degree of pronation during flexion. Its range of motion consists of 15° to 20° of extension and 80° of flexion. However, the range of motion of the MCP joint of the thumb is restricted, especially in lateral motion with only 10° of adduction and abduction.
MCP joint dislocations are considerably less common than dislocations of the IP joints. Most MCP joint dislocations are dorsal. These were first described by Kaplan in 1957.10 A volar MCP joint dislocation is a rare finding.11 Dislocation of any of the MCP joints is possible from hyperextension injuries. The dorsal dislocation of the thumb MCP joint is the most common type of MCP joint dislocation.
Hyperextension of the MCP joint results in the rupture of the volar plate. Injuries are classified as simple (reducible with closed technique) or complex (irreducible with closed technique). Unless there is associated twisting of the finger, the collateral ligaments remain intact. Clinical and radiographic features can be used to differentiate simple from complex dislocations. In cases of simple dislocations, the joint usually hyperextends to approximately 90° and the volar plate is not interposed between the dislocated bones.12 The deformity is obvious with the finger in a claw position of extreme dorsiflexion at the MCP joint. In complex dislocations, the metacarpal and proximal phalanx usually lie more parallel to each other, with the metacarpal head causing a dimple on the volar skin and palpable underneath the skin. Clinical features that suggest a complex MCP dislocation include a proximal phalanx that is less acutely angulated than with a simple dislocation.2 The volar plate, sesamoids, flexor tendon, adductor tendons, extensor expansion, collateral ligaments, or capsule may become entrapped and prevent reduction.2 A widened joint space is seen on radiographs when the volar plate is interposed in the joint. Complex MCP dislocations occur more frequently in the index finger. They can be difficult to manage because reduction is often blocked by the interposed soft tissue.13
The MCP joint of the thumb is very mobile. Dislocations of the thumb MCP joint are more common than the MCP joints of the fingers. The MCP joint can be dorsally dislocated by a hyperextension injury or shearing forces that ruptures the volar plate, joint capsule, and at least part of the collateral ligament. The proximal phalanx will come to rest in a position dorsal to the first metacarpal. Displacement of the proximal phalanx varies from a subluxation to the complete dislocation. For the latter to occur, the volar plate and the collateral ligaments must completely tear. Volar dislocations are rare and result from extensive tearing of the dorsal capsule and the extensor pollicis brevis tendon, leaving the joint very unstable.14
A simple MCP joint dislocation can be converted into a complex one during prolonged or repeated reduction attempts, especially those in which axial traction is the primary component.2 This should not deter the EP from attempting a closed reduction for any MCP dislocation.
Radiographic evaluation of all hand injuries is relatively straightforward. It should include at least three views (i.e., anteroposterior, oblique, and lateral) of the injured area. The most important radiographic error in evaluating joint injuries of the hand is failing to get a true lateral view of the injured joint. This may result in missing a fracture or a loose body in the joint.2 Radiographic examination will show an obvious dislocation in the lateral view. Anteroposterior views may reveal widening of the joint space in complex dislocations. In addition, the sesamoid bones may be seen in the joint space, a pathognomonic sign for a complex MCP joint dislocation.