Metacarpophalangeal (MP) joint dislocations are rare injuries, which is primarily due to the strong connective tissue surrounding these joints and their basal location in the hand.1The index MP joint is most commonly affected, with the thumb MP joint ranking second in incidence.2,3 Thumb MP dislocations usually occur in young, active individuals and the mechanism of injury in most cases is a fall on an outstretched hand (FOOSH) that causes forcible hyperextension of the MP joint.4,5 Most thumb MP dislocations are simple, meaning there is no soft tissue within the joint and the injury can usually be reduced by closed reduction.6 Complex dislocations occur less frequently but are more likely when the dislocation is in the volar direction, often from hyperflexion or a direct blow to the dorsum of the proximal phalanx. Volar, complex, and open thumb MP dislocations are all indications for surgery.7
Definitions
- A thumb MP joint dislocation occurs when the articular surface of the base of the thumb proximal phalanx is displaced off the articular surface of the head of the thumb 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 thumb MP joint dislocations are done by noting the direction of the displacement of the thumb proximal phalanx relative to the head of the thumb metacarpal. The three possible directions of displacement are dorsal (most common)9, lateral, and volar.8
- Dorsal dislocations of the thumb MP joint are further divided into two subtypes:
- the hyperextension subtype (most common) 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.5
- Volar dislocations are far less common than dorsal dislocations. These injuries are typically complex (irreducible) and caused by hyperflexion of the thumb MP joint or a force to the thumb proximal phalanx in flexion.7,10
- The degree of displacement of the proximal phalanx further characterizes thumb MP dislocations. In a true complete dislocation, the articular surface of the thumb proximal phalanx is no longer in contact with the articular cartilage of the thumb metacarpal head. If there is partial contact of the cartilaginous surfaces, then this is not a true dislocation but rather a joint subluxation.8
O – Open vs closed
- The majority of thumb MP dislocations are closed; the skin is intact, and there is no route for bacteria to contaminate the joint space.
- Open thumb MP dislocations are rare and have only been found to account for 8% of these injuries. When present, urgent irrigation, debridement, open reduction, and ligament repair are required.11
C – Complex vs simple
- Dorsal thumb MP dislocations may be classified with respect to their ease of reduction into 3 categories: incomplete, simple complete, and complex.12
- Most thumb 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.6
- Complex MP joint dislocations are complete, irreducible dislocations that require a surgical approach for reduction and proper alignment. They are rarely seen in the thumb but do occur, and the majority are dorsal dislocations.
- The volar plate is the most commonly interposed structure in dorsal thumb MP dislocations, but other possible obstacles include the flexor and adductor tendons, the extensor expansion, the collateral ligaments, the joint capsule, and the sesamoid bones.5
S – Stability
- A stable thumb 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 thumb MP joint dislocation is stable to stress testing of the collateral ligaments in the radial/ulnar plane after reduction.
Thumb MP dislocation with special and complex features other than fractures
Complex (irreducible) thumb MP dislocation
- Complex thumb MP joint dislocations are very rare. The majority of these injuries are dorsal MP dislocations, while volar dislocations are even less common.13
- The mechanism of injury typically involves forced MP joint hyperextension and torsional stresses that draw the volar plate or another soft-tissue structure into the thumb MP joint. The volar plate usually ruptures from its weakest proximal attachments to the metacarpal bone, remains attached to the base of the proximal phalanx, and then resides within the joint space trapped between the base of the proximal phalanx and the head of the metacarpal.12
- Other structures that may be interposed include the flexor and adductor tendons, the extensor expansion, the collateral ligaments, the joint capsule, and the sesamoid bones.5,6
- 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.14
Imaging
- X-ray
- In simple dislocations, the MP joint usually hyperextends to ~90°, while in complex dislocations, the metacarpal and proximal phalanx usually lie more parallel to each other. 5
- In complex dislocations, radiographs typically show the MP joint to be hyperextended with the proximal phalanx dorsal and sometimes lying in a "bayonet" apposition to the metacarpal. There may be increased space between the metacarpal head and proximal phalanx suggestive of interposed soft tissue.
- Interposition of the sesamoids between the metacarpal head and proximal phalanx is strong evidence of a complex irreducible dislocation.3
- MRI
Treatment
- Early diagnosis of complex thumb MP joint dislocations is very important.
- Ideally, this is followed by open reduction and surgical anatomic repair of the interposed soft-tissue structure(s), as the likelihood of success for closed reduction in these cases is low.5,12
- Whether a dorsal or volar approach should be used is a matter of debate, and the choice is usually based on surgeon preference.
- Advantages of the dorsal approach include lower risk of injury to the neurovascular bundles, full visualization of a dorsally entrapped volar plate, and if present, a better management of associated osteochondral fractures; however, it requires vertical splitting of the volar plate to reduce it and the metacarpal head, which may impair long-term MP joint stability.5,12
- A percutaneous approach is another viable option that may avoid the risks associated with an open approach.5
- If there is still instability post reduction, consider direct ligament repair, pull out suture or bone anchor repair or K-wires to stabilize the joint preferably in approximately 25 degrees of flexion.3,12
- Postoperatively, early motion with a dynamic extension splint helps improve the post-injury function, but there is also no consensus regarding the duration of immobilization: some authors recommend an early mobilization protocol, while others prefer immobilization for 3-4 weeks postoperatively.5,12
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 anatomy6
- Extensor tendons – central slip and lateral bands
- Flexor tendons
- Dorsal capsule
- Proper collateral ligament
- Accessory collateral ligament
- Volar plate
- Neurovascular bundle
- Abductor pollicis brevis tendon
- Opponens pollicis tendon
- Transverse metacarpal ligament
- Osteology of the head of the thumb metacarpal and base of the thumb proximal phalanx
- The thumb MP joint is similar to the MP joints of the other fingers, but due to unique articular surfaces, it is more hingelike than multiaxial. The sesamoids lie anterior to the metacarpal head and articulate with it because they are embedded in the volar plate.6
Overall incidence
- Thumb MP dislocations rank second to index MP dislocations in incidence amongst the digits. Dislocations of the little MP joint rank third, while dislocations of the long and ring MP joints are extremely rare.2,3
- Complex thumb MP joint dislocations are very uncommon.
- According to one study, only 8% of all MP joint dislocations are open injuries.11
- Dorsal dislocations of the thumb MP joint are most commonly seen in young, active individuals.5
- As of 2017, only 26 cases of volar dislocations of the thumb MP joint have been reported in the English literature.9
Related Injuries/Conditions
- Fractures of the thumb proximal phalanx
- Fractures of the thumb metacarpal
- Collateral ligament injuries
- Volar plate injuries
- Central slip ruptures
