Bony mallet finger injuries: assessment of stability with extension stress testing

Bony mallet injuries with a large dorsal fracture fragment may sublux giving a poor outcome. The hypothesis that was tested was that subluxation could be predicted by extension stress lateral radiographs. It was anticipated that the main distal fracture fragment would glide and be stable or pivot and be unstable. There were 32 bony mallet injuries with dorsal fracture fragments of >1/3 in 31 patients. There were three patterns shown on lateral extension stress radiographs: gliding, pivoting, and tilting - a combination of the former two. Defining stability as congruence or subluxation ⩽1 mm at final radiographs and instability as subluxation >1 mm, there was a very strong association with pivoting and subluxation, and gliding and congruence (p < 0.001). Tilting gave mixed results. There was significant difference in the size of the fracture fragment in stable (mean 49%) and unstable injuries (54%) (p = 0.044). Extension stress testing has a sensitivity of 89% and a specificity of 100%. Extension stress testing highlights that instability is not just a function of fracture fragment size and is a more reliable method of predicting subluxation than any previously described.

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