A biomechanical study of distal interphalangeal joint subluxation after mallet fracture injury.

PURPOSE: There is no consensus in the literature regarding the size of a mallet fracture fragment that may lead to subluxation of the distal interphalangeal (DIP) joint. The purpose of this study was to determine the relationship between the size of the dorsal articular fragment and DIP joint subluxation in a cadaveric mallet fracture model.

METHODS: Twenty-nine fresh-frozen fingers without evidence of DIP joint osteophytes were dissected to the metacarpal base. The mean age of the 17 donors at the time of death was 69 years (range, 46 to 89 years). Obliquely oriented fractures through the dorsal lip of the distal phalanx were randomly created with an osteotome (range, 27% to 69% of the joint surface). Each finger was fully flexed and extended 1,200 times by applying alternating tension to the flexor and extensor tendons. Fluoroscopic images were obtained and digitized for measurements of fracture fragment size and DIP joint subluxation.

RESULTS: Sixteen DIP joints remained reduced and 13 distal phalanges subluxated palmarward. Subluxation was not observed when the fracture fragment measured less than 43% of the joint surface, whereas subluxation consistently occurred when the defect measured greater than 52% of the articular surface. Subluxation averaged 18% +/- 7% of the overall joint surface in these specimens. There was no correlation between the amount of joint subluxation and the percentage of articular surface damage (p = .22).

CONCLUSIONS: This study supports the concept that a mallet fracture with a large articular fragment may be unstable. Palmar subluxation of a DIP joint without preexisting arthritic deformity is expected when more than one half of the dorsal articular surface is injured.