Complex Metacarpophalangeal Joint Dislocation

A complex dorsal metacarpophalangeal dislocation is a true orthopaedic emergency in the hand. The index finger is involved in over 50 percent of cases (53.8% in the largest surgical series, PMID 41906838). Misdiagnosis as a simple dislocation leads to repeated failed reduction attempts, cartilage damage, and ultimately an open reduction that could have been planned from the outset; recognising the pathognomonic skin dimple allows immediate preparation for surgery and avoids iatrogenic injury from forceful traction.

Where it sits. The wider set of digital injuries is covered in finger dislocations; the systematic films that accompany any of them are in imaging of the wrist and hand, and the sensory mapping that localises the nerve at risk is in hand examination and clinical localisation.

Mechanism. A hyperextension force at the MCP joint:

• Fall on the outstretched hand with the MCP hyperextended
• Direct blow - dorsal force driving the proximal phalanx into hyperextension
• Sports injury - ball sports, gymnastics, martial arts
• High-energy trauma - road traffic accidents, with associated fractures

Why it matters. Volar plate interposition makes the joint irreducible without surgery. The radial digital nerve is stretched over the metacarpal head, prolonged dislocation abrades the articular cartilage, and delayed treatment leads to permanent loss of motion and pain.

The joint. The metacarpal head is cam-shaped, with a wider dorsal articular surface. Dorsally the sagittal bands hold the extensor tendon centred over the head, the proper and accessory collateral ligaments flank it, and volar to the joint lies the volar plate, tethered to its neighbours by the deep transverse metacarpal ligament, with flexor digitorum profundus and superficialis passing beneath it through the A1 pulley. In a complex dislocation the metacarpal head is trapped among these structures.

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The volar plate. A thick fibrocartilaginous structure on the palmar surface of the joint, containing the sesamoids in the index and middle fingers, with two very different attachments. Distally, onto the base of the proximal phalanx, it is firm, fibrocartilaginous and strong; proximally, onto the metacarpal neck, it is thin, membranous and weak, and this is the critical zone. The asymmetry is the whole pathology: it is what lets a hyperextension force produce an irreducible joint. The same volar-plate mechanics govern the interphalangeal injuries in finger dislocations.

What happens in hyperextension. The base of the proximal phalanx rides over the metacarpal head and the plate avulses or ruptures at its weak proximal origin while staying firmly attached to the phalanx. As the phalanx displaces dorsally the freed plate is carried with it and flips to lie dorsal to the metacarpal head, buttonholed between the head and the phalangeal base, with the sesamoids adding a bony wedge to the block. This is the Kaplan lesion, the classic complex dislocation: the metacarpal head buttonholed through the volar structures, with the volar plate, sesamoids and flexor sheath as the irreducible block. It is the anatomical basis of the simple-versus-complex distinction.

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The noose. The deep transverse metacarpal ligament tethers the plate to its neighbours, which is why it cannot simply slide back once it has flipped, and with the lumbrical on the radial side and the flexor tendons on the ulnar side it completes a constricting noose around the metacarpal neck. Traction increases the tension on these structures, so the proximal phalanx stays locked in hyperextension however hard it is pulled. Forceful or repeated attempts add cartilage abrasion and soft-tissue swelling, converting a surgical case into a more difficult one.

The structures at risk. The radial digital nerve is stretched over the prominent metacarpal head in index finger dislocations; the ulnar digital nerve is less commonly at risk. Both must be identified and protected during any volar approach. The radial collateral ligament is frequently torn and should be repaired after reduction. The destroyed-joint endpoint of a missed injury is the subject of MCP joint arthritis.

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By digit. The index finger predominance relates to its position at the radial border and the presence of a radial sesamoid that contributes to the bony block.

By direction. Dorsal is the pattern this topic is about. Volar dislocations are uncommon and usually require open reduction because the extensor mechanism becomes interposed.

History. The answers that change management:

• Mechanism - a hyperextension injury, typically a fall onto the outstretched hand
• Timing - acute presentation, or a delayed and missed injury
• Previous attempts - how many closed reductions, and how forceful
• Associated injuries - other hand trauma, open wounds, nerve symptoms

Examination. The MCP sits hyperextended, the finger slightly extended and ulnarly deviated, with the metacarpal head prominent and tender in the palm, so the deformity looks far less dramatic than the radiograph. Test the radial and ulnar digital nerves and capillary refill before anything is manipulated. A single gentle reduction attempt, under digital block or sedation, is part of the assessment; the technique has its own section below.

The skin dimple. Examine the palm with the MCP in maximal hyperextension. A transverse or oblique dimple at the proximal palmar crease, directly over the metacarpal head, means the volar plate has buttonholed and is interposed: the dimple is pathognomonic of a complex, irreducible dislocation. Its absence does not completely exclude complexity, but makes a simple dislocation more likely. Document it before any reduction attempt, because its presence changes management from attempted closed reduction to planned open reduction.

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Differential. The main differential for a locked digit without trauma is trigger finger.

Associated injuries. Metacarpal head fracture, proximal phalanx base fracture, collateral ligament avulsion and sesamoid fracture all change the post-reduction protocol and may require internal fixation. The metacarpal-head osteochondral fracture is the one that should influence the choice of approach, for the reason given under the dorsal approach below.

Radiographs. PA, true lateral and oblique of the affected ray, before and after any reduction attempt. The lateral confirms dorsal versus volar displacement and guides approach planning; overlap on the AP alone can hide the dislocation. Look for:

• Direction of dislocation
• Associated fractures of the metacarpal head or proximal phalanx base
• Sesamoid displacement - a sesamoid sitting within the joint space is pathognomonic
• Joint incongruity

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CT when an intra-articular fracture or a comminuted metacarpal head is suspected, or to assess sesamoid integrity. Fragments involving greater than 20 percent of the articular surface or with greater than 2 mm of step-off warrant consideration of ORIF, and the scan helps decide between volar plate repair and excision of comminuted fragments.

MRI for the chronic or missed case: a delayed presentation with fibrosis, where cartilage status and volar plate integrity decide between joint salvage and arthroplasty. It shows interposed volar plate scarring, chondral loss and possible osteochondral defects.

The decision is clinical. Radiographs confirm the diagnosis and exclude fracture, but open reduction is decided on the skin dimple and a failed closed attempt. Do not delay surgical exploration waiting for advanced imaging in an acute irreducible dislocation.

A single gentle closed attempt is appropriate for an acute dorsal MCP dislocation before complexity is confirmed: it is curative for a simple dislocation or subluxation, which has an excellent prognosis, and does no harm when done correctly. If a skin dimple is present, or the one attempt fails, proceed to open reduction and do not repeat it. The technique matters because the common instinct, longitudinal traction, is exactly what locks a complex injury.

1. Anaesthesia and relaxation. Wrist block or sedation, never an awake, guarding hand. Flex the wrist and the IP joints to slacken the long flexors.
2. Direction of force. Accentuate the hyperextension first, then push the base of the proximal phalanx distally and volarly so that it rolls over the metacarpal head into flexion, keeping the base in contact with the head throughout. Do not pull along the axis of the finger: traction tightens the volar plate, flexor and lumbrical noose and can convert a reducible subluxation into an entrapped complex dislocation.
3. One attempt. Then reassess. Repeated forceful attempts abrade cartilage and worsen the interposition.
4. After reduction. Confirm concentric reduction on radiographs, test stability and apply an extension-block splint, for 3 weeks in a simple dislocation. Nobody is discharged without a post-reduction film and a stability check.

Irreducible without a dimple. A dislocation that will not reduce but has no skin dimple may have the radial collateral ligament interposed; it is approached dorsally, and stability is confirmed after reduction as for any other.

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Timing and planning. Open reduction within 24 hours if possible, before swelling peaks, aiming for a concentric reduction with minimal articular damage and stable motion afterwards. Consent covers stiffness, nerve injury, the possible need for collateral repair and infection. Equipment: hand table, fluoroscopy, fine rongeurs, 3-0 or 4-0 nonabsorbable suture.

Choosing the approach. Neither is proven superior, but know the anatomical argument, because it runs the opposite way to what most candidates say. The interposed plate lies dorsal to the metacarpal head, so it is the dorsal approach that looks straight at it. Becton's two stated objections to the volar (Kaplan) approach are that it gives a limited view of the plate and that the digital nerves are easily damaged during the exposure; the comparative series bears the second out (see Complications). In that same series, the only head-to-head comparison, the dorsal approach favoured motion and the volar approach favoured grip.

Dorsal approach (Becton).

1. Direct dorsal longitudinal incision through skin and extensor tendon over the MCP joint
2. The entrapped plate, trapped over the dorsal aspect of the head, comes into full view; this is the anatomical reason Becton proposed the approach
3. Split the plate longitudinally; the dislocation reduces spontaneously as the flexor tendons and lumbrical slip past the metacarpal head, with no forceful manipulation
4. Reduce and fix a metacarpal-head osteochondral fracture, which frequently accompanies this injury, through the same exposure
5. Repair the extensor tendon split and any sagittal band rent to restore stability

Volar approach (Kaplan).

1. Zigzag or Brunner incision centred over the MCP joint, extended proximally and distally as needed
2. Identify and protect both digital neurovascular bundles before dividing any volar structure: the radial digital nerve is stretched over the metacarpal head, can be mistaken for a fibrous band, and is the structure at risk in this approach
3. Divide the A1 pulley if needed and identify the interposed plate and sesamoids
4. Divide the plate longitudinally, retract the lumbricals, and reduce the phalanx by flexing it and pushing the base over the head
5. Once the plate has been incised and extracted from the joint, repair it above the metacarpal head

The trade-off is a limited view of the plate and exposed nerves against direct access to the sesamoids, lumbricals and collateral ligaments, and the better grip strength in the one comparative series.

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Before closure. Test passive MCP extension under fluoroscopy. If the joint tends to redislocate in extension, repair the radial collateral ligament and consider temporary K-wire stabilisation in 30 degrees of flexion for 3 weeks. Document stability before closure; it decides the splint position.

Rehabilitation.

• Extension block splint at 20-30 degrees of flexion for 2-3 weeks
• Protected active range of motion from week 2-3 under therapist supervision
• Grip strengthening from week 6, return to sport at 10-12 weeks

Missed or chronic (greater than 3 weeks). The goal is to restore motion and stability while addressing fibrosis and cartilage damage, and the extensor hood may also contribute to the block in a chronic or missed case. Through a volar approach the additional steps are:

1. Extensive release of volar plate scar and contracted collateral ligaments
2. Possible excision of fractured or arthritic sesamoids
3. Check the metacarpal head for osteochondral defects requiring microfracture or grafting
4. Consider a temporary spanning external fixator if stiffness is severe

Chronic cases often require more aggressive soft-tissue release and have higher rates of residual stiffness.

Salvage when the joint is destroyed. Severe cartilage loss ends any attempt at joint preservation. Silicone arthroplasty suits the low-demand, older patient and arthrodesis the young labourer with high demands; vascularised joint transfer is rare, for the young patient with a single digit involved.

Read the percentages as conventional teaching ranges, not measured rates. No cohort of this injury is large enough to support precise complication rates: the published literature is case reports, technique descriptions and small series, the largest comparative series has 13 patients, and no registry collects MCP dislocations. The figures in the table are the ranges conventionally quoted in hand-surgery teaching. Quote them as orders of magnitude with the uncertainty attached; "stiffness is the commonest problem and is the main thing we are trying to prevent with early motion" is both truer and more useful than a spuriously precise percentage.

The trend. Earlier reduction does better, consistently across every published series, and that is the point the examiner wants. The percentages in the table are conventional teaching estimates rather than measured figures, since no cohort of this injury is large enough to generate them, and "functional ROM" is not a defined endpoint; quote the trend and the mechanism rather than the numbers if pressed on where they come from.

Prognostic factors. The best results follow acute presentation, a single gentle reduction attempt, concentric reduction, a repaired collateral ligament and a compliant early-motion protocol. Diagnosis delayed beyond 3 weeks, multiple failed closed attempts, chondral damage on the metacarpal head and non-compliance with therapy predict poorly: outcomes decline significantly after the 3-week window, with higher rates of stiffness and post-traumatic arthritis, and the conventional figure in the complications table has arthritis common once the delay exceeds 6 weeks.

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