The Role of the Collateral Ligaments in Stabilizing the Distal Interphalangeal Joint in Fingers With Small Mallet Fractures: A Biomechanical Study.

Abstract

In patients with mallet fractures, surgical fixation is traditionally recommended when over one-third of the distal phalanx articular surface is involved. Mallet finger patients with small-sized avulsion fractures, involving around 20% of the distal phalanx articular surface, can have instability at the distal interphalangeal (DIP) joint. This biomechanical study was conducted to determine whether damage to DIP joint collateral ligaments in mallet finger patients with small-sized avulsion fractures contributes to DIP joint instability causing subluxation in these patients. Nineteen cadaveric fingers were given a mallet finger-style avulsion fracture, ranging from 9% to 35% involvement of the distal phalanx articular surface. DIP joint stability was tested with the DIP collateral ligaments intact, with collateral ligaments partially damaged, and finally with full collateral ligament resection. Mixed models were used to study the relationship between stability and collateral damage. A receiver operating curve analysis along with Youden's J statistic was used to determine the avulsion fracture size at which the DIP joint becomes unstable for different degrees of collateral damage. The likelihood of subluxation differs by degree of collateral ligament damage, with subluxation probability increasing as collateral damage moves from intact to 50% cut to 100% cut. In fingers with 50% collateral resection, avulsion fractures involving >28% of the distal phalanx articular surface are more likely to experience subluxation. In fingers with 100% collateral resection, avulsion fractures >15% are more likely to subluxate. Collateral ligament injury contributes to instability at the DIP joint in a mallet fracture, increasing the likelihood of palmar subluxation. Collateral ligament injury contributes to DIP joint instability in mallet fractures involving <40% of the articular surface, supporting the need for assessing finger stability with a physical examination.