Problem: Unless properly recognized and treated, this intraarticular fracture subluxation may result in an unstable, arthritic joint with secondary loss of motion and pain. Because the thumb CMC joint is critical for pinch and opposition, this injury may severely affect function.

Frequency: The thumb is a highly mobile, border digit. For that reason injury to this ray is common.

Etiology: Axial loading of a partially flexed thumb metacarpal causes this injury.

Pathophysiology: Thumb CMC joint stability is maintained by five ligaments and the articular contours. The most critical of these stabilizers is the volar oblique ligament. This ligament courses from the volar lip of the trapezium to the volar ulnar corner of the thumb metacarpal base. The injury occurs when an axial force is transmitted through a partially flexed thumb metacarpal. The portion of the metacarpal onto which the volar oblique ligament inserts remains in anatomic position and the remainder of the articular base subluxates in a dorsal, radial, and proximal direction due to the pull of the abductor pollicis longus.

Closed reduction and thumb spica cast immobilization can be effective in the treatment of some Bennett's fractures. Generally, cases characterized by small avulsion fractures, and minimal articular incongruity and instability can be managed in this fashion. These patients must be followed carefully with serial radiographs. The strong pull of the abductor pollicis longus (APL) frequently leads to displacement. As a result, open or closed reduction combined with internal fixation frequently is required. More than 1 mm of articular incongruity after closed reduction is an indication for operative intervention. This degree of articular incongruity is associated with an increased rate of articular degeneration in the thumb carpometacarpal (CMC) joint over time.

Relevant Anatomy: The thumb affords prehensile abilities that were essential in human evolution. The bony anatomy of the thumb consists of 2 phalanges and a metacarpal, which articulates with the trapezium bone in the distal carpal row. The metacarpal is actually a primordial phalanx.

The CMC joint consists of an articulation between the trapezium and the metacarpal base composed of 2 reciprocally interlocking saddles with perpendicular longitudinal axes. Ligamentous stability at the trapeziometacarpal joint is maintained by the anterior (volar) and posterior oblique ligaments, the anterior and posterior intermetacarpal ligaments, and the dorsal radial ligament. The anterior (volar) oblique ligament originates on the trapezium and inserts into the volar, ulnar beak of the thumb metacarpal. This is the most important ligament in maintaining CMC stability. The dorsal ligament is not as strong as the volar ligament but is reinforced by the APL.

Contraindications: Contraindications to closed treatment include an open fracture, an unstable fracture, failed closed reduction with residual articular incongruity greater than 1 mm or instability and joint subluxation.

Imaging Studies:

• Radiographs

• Obtain standard posteroanterior, lateral, and oblique radiographs in patients with suspected fractures or dislocations of the thumb. Traction radiographs may be used to assess the degree of comminution in appropriate fractures (eg, Bennett, Rolando, comminuted metacarpal base fractures).

• Radiographs of the CMC joint are obtained by placing the hand's palmar surface flat on the imaging plate for a true lateral view, allowing accurate assessment of the CMC joint. Pronate the hand and wrist approximately 20-30° and direct the imaging beam obliquely at 15° in a distal to proximal direction centered over the trapeziometacarpal joint. A broken "V" sign may be present on the lateral radiograph, indicating disruption of the normal "V" that is formed by the radial aspect of the trapeziometacarpal articulation. This may indicate undetected CMC joint subluxation.

• Computed tomography and/or tomograms: These studies help define the degree of comminution within a fracture as well as suspected impaction of the articular surface.

Surgical therapy: Generally, closed reduction utilizing the technique described above followed by percutaneous K-wire fixation is successful. Two 0.045" K-wires are drilled through the dorsal, radial thumb metacarpal base into the reduced volar, ulnar fragment. If the fragment is very small reduction may be maintained by placing the K-wire from the thumb metacarpal into the trapezium or the index metacarpal. Maintaining thumb abduction is key to preserving the first web space.

If adequate reduction can't be achieved utilizing this percutaneous technique, open reduction and internal fixation is performed. An L-shaped incision is made over the subcutaneous border of the thumb metacarpal. The incision is carried down radially to allow for subperiosteal reflection of the thenar musculature and direct visualization of the joint. Towel-clip forceps are extremely valuable in obtaining and temporarily maintaining reduction. Fixation is achieved using either K-wires or mini screws (2.0 mm).

Follow-up care: A well-molded thumb spica cast is utilized for 2-6 weeks depending on the stability obtained at surgery. Once the cast is discontinued, a thermoplastic splint is fabricated and a protected mobilization program initiated until fracture healing is complete.

Displaced intraarticular fractures predispose the patient to arthritis and loss of motion within the affected joints. Unfortunately, even after restoration of articular congruity, some patients develop post-traumatic arthritis secondary to the osteocartilaginous injury sustained as a result of the initial trauma.

Loss of motion also occurs following prolonged immobilization. Rigid fixation enables patients to initiate movement sooner postoperatively minimizing this problem.

Other potential post-operative complications include loss of reduction with recurrent joint subluxation and instability, infection, and sensory nerve injury.