Metacarpophalangeal (MP) joint dislocations are rare, mainly owing to the strong connective tissue support around the joints and their basal location in the hand.1 The typical mechanism of injury is a fall on the outstretched hand (FOOSH) that causes forcible hyperextension of the MP joint, but a combination of translational and hyperflexion forces may also contribute.2 The index finger is the most frequently involved of the non-thumb digits, followed by the little finger, while MP dislocations in the long and ring fingers are extremely rare.1,3 Most MP dislocations are simple, meaning there is no soft tissue within the joint and the injury can usually be reduced by closed reduction, while complex dislocations occur far less frequently but require surgical intervention in most cases.3
Definitions
- A MP joint dislocation occurs when the articular surface of the base of the proximal phalanx is displaced off the articular surface of the head of the metacarpal.
Hand Surgery Resource’s Dislocation Description and Characterization Acronym
D O C S
D – Direction of displacement
O – Open vs closed dislocation
C – Complex vs simple
S – Stability post reduction
D – Direction of displacement
- The primary description and characterization of MP joint dislocations are done by noting the direction of the displacement of the proximal phalanx relative to the head of the metacarpal.
- The three possible directions of displacement are dorsal, lateral, and volar.4
- Dorsal MP dislocations are the most common.
- Dorsal dislocations are further divided into two subtypes:
- The hyperextension subtype, where the volar base of the proximal phalanx catches on the dorsal edge of the metacarpal condyles in an extended position.
- The bayonet subtype, where the base of the proximal phalanx is displaced on top of the neck of the metacarpal in a position parallel to the longitudinal axis of the metacarpal neck.
- Volar dislocations can occur through either a hyperflexion or hyperextension injury.3
- The degree of displacement of the proximal phalanx further characterizes MP dislocations.
- In a true complete dislocation, the articular surface of the proximal phalanx is no longer in contact with the articular cartilage of the metacarpal head. If there is partial contact of the cartilaginous surfaces, then this is not a true dislocation but rather a joint subluxation.4
O – Open vs closed
- The majority of MP dislocations are closed; the skin is intact, and there is no route for bacteria to contaminate the joint space.
- Open MP joint dislocations are extremely rare and have been found to only account for 8% of these injuries, but when present, urgent irrigation, debridement, open reduction, and ligament repair are required.5
- As with closed MP dislocations, the usual mechanism of injury with open dislocations is hyperextension caused by a FOOSH, with the primary difference being the amount of force applied to the MP joint.1
C – Complex vs simple
- Most MP joint dislocations are simple, meaning that reduction is easily achieved under digital anesthetic block and is not blocked by soft tissue being interposed in the joint between the proximal phalanx and metacarpal joint surfaces.3
- Complex MP joint dislocations are rare but do occur, and most are dorsal dislocations.
- In dorsal complex dislocations, the mechanism of injury is forceful hyperextension that leads to the volar plate being drawn dorsally between the proximal phalanx base and metacarpal head, thus becoming interposed dorsally in the joint.3,6
- In volar complex dislocations, the dorsal capsule, distal insertion of the volar plate and/or collateral ligament can be avulsed and entrapped within the MP joint.3
S – Stability
- A stable MP joint dislocation can be reduced and then put through an active range of motion (ROM) test under a local anesthetic block without redislocating.
- Furthermore, a stable MP joint dislocation is stable to stress testing of the collateral ligaments in the radial/ulnar plane after reduction.
MP dislocation with special and complex features other than fractures
Complex (irreducible) MP dislocation
- Complex MP joint dislocations are very rare.
- The mechanism of injury involves forced MP joint hyperextension and torsional stresses that draw the volar plate into the MP joint.3
- The majority of these injuries are dorsal MP dislocations, while volar dislocations are even more rare.
- Complex dorsal MP dislocations most commonly occur in the index and little fingers because they lack the stabilizing deep transverse metacarpal ligament.7
- Complex volar MP dislocations frequently result from a proximal translational force acting on the proximal phalanx while the MP joint is in hyperflexion.7
- In addition to the dorsal capsule possibly being interposed in the joint, the distal insertion of the volar plate and/or collateral ligament can also be avulsed and entrapped within the MP joint.3
- Physical characteristics of a complex MP dislocation include a palpable metacarpal head, slight hyperextension of the proximal phalanx base, dimpling of the volar skin near the dislocated joint, and slight ulnar deviation of the affected digit.6
Imaging
- X-ray
- A true lateral view is most useful for visualizing these injuries.
- A widened joint space and presence of a sesamoid bone within this space may be indications of a complex MP dislocation.6
- MRI
Treatment
- Early diagnosis of complex MP joint dislocations is very important.
- Ideally, this is followed by open reduction and surgical anatomic repair of the collateral and tendon injuries.7
- Postoperatively, early motion with dynamic extension splint helps improve the post-injury function.
Complications
- Stiffness
- MP joint pain
- Persistent deformity
- Impaired ROM
- Digital nerve damage
- Osteoarthritis
Outcome
- Early diagnosis, surgical repair, and therapy will usually give a positive functional outcome, but some limited ROM is to be expected.
Related anatomy3
- Extensor tendon – central slip and lateral bands
- Flexor tendons
- Dorsal capsule
- Proper collateral ligament
- Accessory collateral ligament
- Volar plate
- Neurovascular bundle
- Transverse metacarpal ligament
- Abductor digiti minimi
- Natatory ligament
- Osteology of the head of the middle phalanx and base of the distal phalanx
Overall Incidence
- Traumatic dislocations of the MP joint are believed to be rare injuries and are less common than proximal interphalangeal (PIP) and distal interphalangeal (DIP) dislocations; however, some experts suspect that many cases are not reported and that the true incidence is actually higher.3
- The index finger is the most frequently involved non-thumb digit, followed by the little finger. MP dislocations in the long and ring fingers are extremely rare.8,9
- Complex MP joint dislocations are very uncommon.
Related Injuries/Conditions
- Fractures of the proximal phalanx
- Fractures of the metacarpal
- Collateral ligament injuries
- Volar plate injuries
- Central slip ruptures
Work-up Options
- Delayed and missed diagnoses are common in MP dislocations due to their low incidence and a lack of obvious radiographic signs, so clinical suspicion must be high.10
- X-ray
- The oblique view appears to be most helpful for diagnosing MP dislocations. On lateral and anteroposterior views, there may be an overlap of the adjacent joints or metacarpal head and proximal phalanx base, respectively.2
- Complex dislocations usually present with a widened joint space indicative of an interposed volar plate within the MP joint.3
- MRI
