The risk factors associated with subluxation of the distal interphalangeal joint in mallet fracture.

Surgical fixation is recommended when a mallet fracture involves more than one-third of the articular surface of the distal phalanx. This recommendation originates from the idea that involvement of more than one-third of the base of the distal phalanx causes subluxation of the distal interphalangeal (DIP) joint. Eighty-six fingers of 85 patients with a mallet fracture involving more than one-third of the articular surface of the distal phalanx were enrolled in this study. Patients were allocated on the basis of subluxation of the DIP joint into a group with no subluxation or a group with subluxation. These two groups were compared with respect to age, sex, fracture size, fracture displacement, time to finger immobilizer application, and initial extensor lag of the DIP joint. Backward stepwise multiple logistic regression analysis was performed to identify the risk factors of DIP joint subluxation, and receiver operating curve analysis was used to calculate the optimal cut-off point for the risk factors. Half of our patients with a mallet fracture involving > one-third of the articular surface of the distal phalanx showed subluxation of the DIP joint. A significant intergroup difference was found for fracture size and time to application of a finger immobilizer, but no significant difference was observed for other parameters. The risk factors of DIP joint subluxation were fracture size and time to application of finger immobilizer. The optimal cut-off values for the development of DIP joint subluxation were 48% for the fracture size and 12.5 days for time to finger immobilizer application.