Isolated thumb carpometacarpal (CMC) dislocations without any other concomitant injury are rare and account for <1% of all hand injuries.1These are high-energy injuries most commonly seen in young, active individuals, and the mechanism responsible is believed to be either a longitudinally-directed force along the axis of the thumb metacarpal with the CMC joint in full flexion, or a force driven into the first web space.2,3Thumb CMC dislocations in the dorsal direction are more common than volar dislocations, and because the volar ligaments are very strong, there may also be an avulsion of the thumb metacarpal base producing a Bennett's fracture.4 As with many other hand and wrist dislocations, these injuries are frequently missed or misdiagnosed upon initial presentation, which is why clinical suspicion must remain high. Although the optimal treatment approach for thumb CMC dislocations is still debated, it appears that closed reduction is indicated as the initial intervention for most acute cases that are closed and reducible. After reduction, thumb CMC joint stability is the main determining factor in whether to continue with conservative treatment or progress to surgical treatment.5,6
Definitions
- A thumb CMC joint dislocation occurs when the articular surface of the base of the thumb metacarpal is displaced off the articular surface of the distal end of the trapezium.
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 CMC joint dislocations are done by noting the direction of the displacement of the thumb metacarpal relative to the trapezium. The three possible directions of displacement are dorsal, lateral, and volar.7 Dorsal dislocations are the most common, while only a few cases of volar dislocations have been reported.5,6
- Dorsal dislocations are further divided into two subtypes: the hyperextension subtype, where the volar base of the metacarpal catches on the dorsal edge of the trapezium in an extended position, and the bayonet subtype, where the metacarpal base is displaced on top of the distal trapezium in a position parallel to its longitudinal axis.
- The degree of displacement of the thumb metacarpal further characterizes thumb CMC dislocations. In a true complete dislocation, the articular surface of the thumb metacarpal is no longer in contact with the articular cartilage of the distal trapezium. If there is partial contact of the cartilaginous surfaces, then this is not a true dislocation but rather a joint subluxation.7
O – Open vs closed
- The majority of thumb CMC joint dislocations are closed; the skin is intact, and there is no route for bacteria to contaminate the joint space.
- Open thumb CMC dislocations are extremely rare, but when present, require urgent irrigation, debridement, open reduction, pinning and ligament repair.
C – Complex vs simple
- Most thumb CMC 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 thumb metacarpal and trapezium joint surfaces.
- Complex (irreducible) thumb CMC joint dislocations are rare, but do occur on some occasions. These cases are clear indications for open surgical repair.5,7
S – Stability
- A stable thumb CMC 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 CMC joint dislocation is stable to stress testing of associated ligaments after reduction.
Related anatomy2,5
- Extensor pollicis longus
- Extensor pollicis brevis
- Flexor pollicis longus
- Flexor pollicis brevis
- Dorsal radial sensory nerve
- Abductor pollicis longus
- Opponens pollicis
- Anterior oblique ligament
- Intermetacarpal ligament
- Dorsoradial capsular ligament
- Posterior oblique ligament
- Osteology of the thumb metacarpal base and trapezium saddle joint
- The thumb CMC joint is supported by a thickened joint capsule composed of 16 ligaments, but its stability is primarily provided by 4 ligaments: the deep anterior oblique, intermetacarpal, dorsoradial capsular ligament, and posterior oblique ligaments. Dislocation may disrupt at least one of these ligaments.2,8
Overall incidence
- Isolated thumb CMC joint dislocations only account for <1% of all hand injuries.1As of 2014, fewer than 50 cases had been published in the literature.8,9
- Thumb CMC dislocations are more likely to occur with other concomitant injuries, like thumb metacarpophalangeal (MP) dislocation or fractures of the trapezium or distal radius.3,5
- Dislocations of the thumb CMC joint also appear to be less common than those occurring in the other CMC joints.10,11
- Complex thumb CMC joint dislocations are very uncommon.
Related Injuries/Conditions
- Fractures of the thumb metacarpal
- Fractures of the trapezium
- Dorsal ligament complex injuries
- Extensor tendon ruptures
- Flexor tendon ruptures
- Abductor pollicis longus tendon rupture
- Opponens pollicis tendon rupture
