Anatomical and clinical study of a new mallet fracture classification method

Mallet fracture is avulsion of the terminal extensor tendon from the base of the distal phalangeal bone with a bony fragment. This study was performed to evaluate the anatomical characteristics of mallet fractures, investigate a new mallet fracture classification system using anatomical and imaging methods, and discuss the treatment schemes for different types of mallet fracture.

Sixty-four fresh cadaveric fingers were divided into four groups, and models of different types of mallet fracture with distal interphalangeal joint instability were established by dissecting 25%, 50%, 75%, and 100% of the bilateral collateral ligaments. The effect of mallet fractures on the stability of the distal interphalangeal joint was then observed. The lateral radiographs of mallet fractures in 168 patients were analyzed and classified according to the involvement of the joint surface in the fracture, the thickness of fracture, the untreated time after injury, and the complication of distal interphalangeal joint palmar subluxation. Forty-seven patients were surgically treated by reconstruction of extensor tendon insertion, the Ishiguro method, or single Kirschner wire fixation.

The established mallet fracture model showed that the distal interphalangeal joint was stable when the bilateral collateral ligaments were cut off by 25% (t = -0.415, P = 0.684) and significantly unstable when this range was ≥50% (50% transection: t = -6.363, P < 0.001; 75% transection: t = -17.036, P < 0.001; 100% transection: t = -30.977, P < 0.001, respectively). The mallet fractures were divided into Types I, II, and III (fracture involving <20%, 20%-50%, and >50% of the joint surface, respectively). Type II was further divided into Types IIa and IIb according to whether the course of injury was < or ≥2 weeks, respectively. The mean post-operative flexion of the distal interphalangeal joint was 63.4° ± 7.9°, and the mean extension lag was 6.7° ± 4.6°.

The lateral collateral ligament is the main factor that maintains the stability of the distal interphalangeal joint. Classification that combines the involvement of the joint surface in the fracture, the thickness of the fracture, and the untreated time after injury is reasonable and will help to choose an appropriate operational method.