Dorsal Dislocations of the MCP Joint

Assessment and Closed Reduction

CAPT Edward R. McDevitt, MD

William O. Roberts, MD
Department Editor

THE PHYSICIAN AND SPORTSMEDICINE - VOL 27 - NO. 7 - JULY 1999

Dislocations of the metacarpophalangeal (MCP) joint are not common, but when they do occur, they are usually dorsal (the proximal phalanx is displaced dorsally relative to the metacarpal), and typically involve the index finger. Prompt recognition of this injury is important to ensure the best possible outcome.

In particular, it is important to determine whether a dislocation is simple or complex, because simple dislocations can be treated with closed reduction, whereas complex dislocations usually require surgery. Further, closed reduction must be done properly to avoid converting a simple injury into a complex one.

MCP Joint Anatomy
The anatomy of the MCP joint protects it from injury. The joint is in a protected position at the base of the finger, and the volar diameter of the distal metacarpal head is significantly larger than the dorsal diameter, making the joint less vulnerable to axial blows. A thick, strong volar plate (figure 1) protects the joint and is also the attachment site for the flexor tendon sheath, which keeps the flexor tendons tight to the bone, preventing bowstringing. The deep transverse metacarpal ligament further reinforces the volar plate.

The joint is protected laterally by the collateral ligaments and the inter-volar-plate ligaments. The lateral bands (the lumbrical and interosseous muscles) also act as partial checkreins to lateral instability. Dorsally, the capsule, the extensor tendons, and the sagittal bands secure the joint.

Injury Mechanism
Dorsal MCP dislocations typically occur in football and basketball. The mechanism of injury is usually forced hyperextension of a digit in a fall on an outstretched hand (1). As the joint is forced into hyperextension, the large volar metacarpal head tears through the proximal aspect of the volar plate. If the distal portion of the torn volar plate becomes interposed between the proximal phalanx and the head of the metacarpal (figure 2), the dislocation is complex (also called complete or irreducible); if it is not interposed, the dislocation is simple.

As the metacarpal head continues volarly, it becomes trapped between the lumbrical radially and the flexor tendon ulnarly. The flexor tendons are still tethered to the flexor sheath that is attached to the volar plate. The tethering causes the flexor tendon to be drawn tightly across the neck of the metacarpal. If the volar plate has been displaced dorsally with the proximal phalanx, as in the case of a complex dislocation, the tethering will be even tighter, helping to make the complex dislocation impossible to reduce without surgery.

Clinical Signs
The clinical signs of simple and complex dislocations are distinctly different. In a simple dislocation, in which the torn volar plate is not trapped in the MCP joint, the finger is hyperextended about 60° above the horizontal.

In a complex dislocation, with the volar plate trapped between the metacarpal head and proximal phalanx, the digit is hyperextended about 15° above the horizontal. Further, the displaced metacarpal head forms a prominence in the palm, and the adjacent skin is usually puckered, a clear sign of a complex dislocation.

An athlete who has suffered a dorsal dislocation of the MCP joint during a game or competition may ask for the injury to be reduced so he or she can resume activity right away. This temptation must be resisted. Instead, the hand should be given a thorough examination, including a neurologic exam. Look for the sign of a complex dislocation—volar puckering or dimpling of the skin.

Closed Reduction and Follow-Up
If the injury is a simple dislocation of the MCP joint, a closed reduction must be done carefully to avoid complication. For a complex dislocation, closed reduction under a local block may be attempted once.

Before reduction, a wrist or metacarpal block using 3 mL of 1% lidocaine hydrochloride solution without epinephrine is advised. To reduce the joint, flex the wrist to relax the flexor tendons. Then grasp the affected metacarpal between the index finger on the palmar side of the patient's hand and the thumb on the dorsal side. With the thumb, apply slow and steady proximal-to-distal pressure to the base of the dorsally displaced proximal phalanx (figure 3).

Do not hyperextend the MCP joint while applying distal traction. This may work effectively in reducing proximal interphalangeal joint dislocations, but it can convert a simple dorsal MCP joint dislocation into a complex one.

Obtain postreduction anteroposterior and lateral radiographs. If reduction is complete, place the athlete in a dorsal extension block splint for 4 weeks. This prevents extension beyond neutral but allows the patient to gently flex the finger 5 to 10 minutes four times per day.

Reference

1. Dray GJ, Eaton RG: Dislocations and ligament injuries in the digit, in Green DP (ed): Operative Hand Surgery, ed 3. New York City, Churchill Livingstone, 1993, vol 16, pp 777-779

Dr McDevitt is the chief of orthopedic surgery and sports medicine at the US Naval Academy in Annapolis, Maryland, and a member of the American Orthopaedic Society for Sports Medicine. Dr Roberts is a family physician at MinnHealth SportsCare in White Bear Lake, Minnesota, and medical director of the Twin Cities Marathon. He is a fellow of the American College of Sports Medicine and an editorial board member of The Physician and Sportsmedicine.