Trapezoid Fractures

What it is. A trapezoid fracture is a break of the trapezoid, the second bone of the distal carpal row and the smallest bone of that row (the pisiform is the smallest carpal bone overall). It is conventionally described as the least commonly fractured carpal bone, quoted at under 1% of carpal fractures.

Why that figure carries an asterisk. The incidence is very low in every published series, but the classical figures come from the plain-radiograph era. The denominators usually cited are unselected hand-fracture audits from the 1990s in which the trapezoid essentially never appeared, yet trapezoid fractures are routinely invisible on plain films because of carpal overlap. An injury that the imaging of the day could not see will be counted as absent, not as missed.

What the authors themselves say. Nijs and colleagues, reporting two occult undisplaced fractures found only on cross-sectional imaging, concluded that the lesion "has often been missed in the past and modern diagnostic tools will make its diagnosis far more frequent." Treat "under 1%" as the best available estimate from an era that could not reliably detect the injury, not as a measured biological rate.

More dislocations than fractures. In the published literature, dislocations of the trapezoid outnumber fractures. Nijs counted around 30 reported dislocations and noted that fractures were rarer still, the reverse of what happens at almost every other site. It is a consequence of the same detection problem: a displaced bone is visible on a plain film, and a crack in a recessed bone is not.

Why it is rare. The trapezoid is recessed within the distal carpal row and protected by the bones around it. Strong ligamentous attachments hold it, and it is the keystone of the rigid second carpometacarpal (CMC) joint.

The bone. The trapezoid sits in the distal carpal row between the trapezium radially and the capitate ulnarly. It is wedge-shaped, broad dorsally and narrowing to an apex palmarly, and it articulates on four sides:

• Proximal: the scaphoid, and through it, indirectly, the radius
• Distal: the base of the second metacarpal, the key articulation
• Radial: the trapezium
• Ulnar: the capitate

The fixed pillar. The second and third CMC joints, of the index and middle rays, are essentially immobile and form the fixed central pillar of the hand. Around that rigid unit move the thumb, whose first CMC is a saddle joint, and the ulnar rays, the fourth with about 10-15° and the fifth about 20-30° of flexion-extension, for grip and cupping. The second CMC is the most stable CMC joint of all.

The keystone. Recessed and mortised between the trapezium, capitate, scaphoid and second metacarpal, the trapezoid locks the index ray into the carpus and gives it the rigid, stable base needed for precision and power grip. A trapezoid fracture or a second-CMC fracture-dislocation destabilises this fixed pillar, threatening grip strength and risking second-CMC arthrosis. That is why displaced injuries are anatomically reduced and the CMC stabilised.

Blood supply. Vessels enter from both the dorsal and the palmar surfaces, and AVN is rare.

Mechanism. Axial load through the second metacarpal, from a punch or a fall on the flexed wrist, compresses the trapezoid against the scaphoid and can fracture it.

Why it dislocates dorsally. The palmar (volar) intercarpal ligaments are stronger than the dorsal ones. An axial force on the index metacarpal, resisted by those palmar ligaments, levers the wedge dorsally out of its mortise and tears the weaker dorsal capsule, so dorsal dislocation is the usual pattern reported in the trapezoid-dislocation literature. A palmar dislocation is much less common: it needs higher energy, because the strong palmar ligaments must fail, and it is typically seen in polytrauma.

Fracture pattern. The working description is by displacement and by what came with the fracture, and CT is necessary to classify it accurately.

• Non-displaced - no significant articular step-off
• Displaced - significant displacement or articular incongruity
• With dislocation - an associated second CMC or intercarpal dislocation
• Comminuted - multiple fragments, often from high energy

Associated injuries. The second CMC fracture-dislocation is the most common association, and the entire carpus is always assessed. The others to look for:

• Perilunate injury, as a greater arc injury
• Scaphoid fracture, from the same axial load
• Multiple carpal fractures, with high energy

History. Ask about the mechanism: a punch, a fall on the flexed wrist, or another axial load. The pain is in the dorsal wrist, over the base of the second metacarpal.

Examination. The trapezoid is tender dorsally, between the bases of the first and second metacarpals, with dorsal wrist swelling and pain on axial loading of the second metacarpal. Wrist flexion and extension are painful, and neurovascular status is usually intact.

Keep it in the differential. Because the injury is rare it may be overlooked. Maintain a high suspicion in anyone with dorsal wrist pain after an axial load.

Radiographs. PA, lateral and oblique views may show the fracture, but it is often missed because the carpal bones overlap. What there is to see is a subtle cortical irregularity, overlapped by the scaphoid and capitate.

CT is essential for diagnosis: if the injury is suspected, get one. It clearly delineates the fracture, defines the pattern and displacement, shows articular involvement and is the planning study for surgery.

MRI is rarely needed. It is kept for the occult fracture or for soft-tissue assessment.

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The trapezoid fracture presents as dorsal wrist pain at the index-ray base after axial load — overlapping with several commoner injuries. CT is the discriminator.

The decision. Displacement and the second CMC decide it. A non-displaced fracture is treated in a cast; a displaced fracture, or one with a dislocation, is reduced and fixed.

Algorithm

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Non-displaced fractures. A short arm cast or thumb spica for 4-6 weeks, with repeat radiographs or CT at 4-6 weeks to confirm healing and range-of-motion exercises once the cast is off. Most non-displaced fractures heal well.

Operative indications. Surgery is indicated for:

• Displaced fracture
• Articular incongruity
• Associated dislocation

Approach. A dorsal longitudinal incision centred over the base of the second metacarpal, with a capsulotomy between the second and third extensor compartments. The structures at risk:

• Radial artery, in the anatomical snuffbox
• Extensor tendons: EPL, ECRL and ECRB
• Superficial branch of the radial nerve

Reduction. Three steps, in order:

1. Expose the trapezoid through the capsulotomy
2. Reduce it with a joystick, a K-wire in the fragment or the metacarpal
3. Assess articular congruity under direct vision

Fixation. Headless compression screws are preferred when the fragment is large enough, K-wires are used for comminuted or small fragments, and transarticular K-wires are added if the CMC is unstable. Any ligamentous injury is repaired, and the reduction is confirmed with fluoroscopy before closure.

With a second CMC dislocation. Reduce the CMC joint under fluoroscopy and confirm articular congruity, then hold it with transarticular K-wires from the second metacarpal to the carpus, with or without a headless screw for a large trapezoid fragment. The capsule and intercarpal ligaments are repaired.

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Outcome. Non-displaced fractures have a good prognosis with immobilisation. Articular fractures may lead to second-CMC arthrosis, and displaced or dislocated injuries carry that risk if they are not anatomically reduced.

After a cast. Range-of-motion exercises begin at 4-6 weeks, when the cast comes off, and strengthening at 6-8 weeks.

After fixation. A splint for the first 2 weeks, then a cast or removable splint, for 4-6 weeks of immobilisation in all. Range of motion begins once healing is confirmed, at 4-6 weeks. K-wires, if used, come out at 6-8 weeks, and strengthening follows at 8-12 weeks.

There are no condition-specific society guidelines or arthroplasty-registry data for trapezoid fractures given their rarity; practice is governed by general carpal-injury and hand-trauma principles.

Global epidemiology

• Carpal bones contribute roughly 18% of hand fractures, of which the scaphoid dominates (~60% of carpal fractures); the trapezoid is consistently the least frequently fractured carpal bone (classically under 1%).
• Reported cases cluster in young men after high-energy axial mechanisms (punch injury, falls on the flexed wrist, motor-vehicle trauma).

Side-by-side guideline principles

High- vs limited-resource practice variation

• Well-resourced settings: Ready CT access enables early detection and ORIF of displaced fractures and associated 2nd CMC dislocations; this is the main reason reported incidence is rising.
• Limited-resource settings: Reliance on plain radiographs means occult and undisplaced trapezoid fractures are frequently missed; immobilisation of the clinically suspicious wrist with delayed re-imaging is a reasonable strategy where CT is unavailable.
• Referral: Displaced fractures and fracture-dislocations warrant referral to a hand/upper-limb surgeon wherever the resource pathway allows.

Related pages: Metacarpal Fractures matters more here than the carpal pages do - the trapezoid is injured by axial load transmitted down the index ray, and the associated second-CMC fracture-dislocation is the injury you are actually looking for. Perilunate Dislocations is the other carpal injury defined by being missed on the initial films, and it teaches the same discipline of reading the carpal arcs rather than hunting a fracture line. Scaphoid Fractures is the opposite end of the same frequency table and the reason a suspicious wrist gets imaged at all. Trapezium Fractures, Capitate Fractures, Hamate Fractures, Lunate Fractures and Pisiform Fractures complete the rest of that table - and each is subject to the same caveat about plain-film-era denominators. Systematic Approach to Wrist and Hand Imaging is where the carpal cascade lines and the 'missing carpal sign' belong, and Carpal Instability (DISI and VISI) covers the ligamentous consequences if the distal row is disrupted. Radial-Sided Wrist Pain is the presenting complaint under which an undisplaced trapezoid fracture will usually hide.

The literature on trapezoid fractures is limited to case reports and small series, so several questions remain unresolved:

• Imaging threshold: There is no consensus on which patients with a "normal" radiograph warrant CT. Most authorities favour a low threshold for cross-sectional imaging given how often the injury is missed, but the cost-effectiveness of routine CT after axial index-ray trauma is undefined.
• Screw vs K-wire fixation: No comparative data exist for displaced fragments. Headless compression screws are preferred for fragments large enough to accept them; K-wires are used for comminuted or small fragments. Choice remains surgeon preference.
• Management of the associated 2nd CMC instability: Whether transarticular K-wiring alone, ligament repair, or temporary CMC fixation gives the best long-term result is unknown — the rarity of the injury precludes trials.
• True incidence: Historical "less than 1%" figures predate routine CT. The real incidence of occult, undisplaced trapezoid fractures is probably higher than classically reported, as several authors have argued.
• Late presentation: Optimal treatment of a missed/chronic trapezoid fracture or dislocation (delayed ORIF vs excision vs limited fusion) is not established.