The Rule of Fours-Clinical and Radiographic Parameters for Trans-articular Distal Interphalangeal Joint Kirschner Wire Insertion.

Background: Distal phalangeal and interphalangeal joint injuries are common, and confer a significant burden to the individual, healthcare system and society. Operative intervention (when required) may involve retrograde trans-articular Kirschner wire (K-wire) fixation. Safe wire passage through the center of the distal interphalangeal joint (DIPJ) and associated phalanges is key in maintaining alignment and reducing complications. There is little evidence to guide optimal wire entry point and passage.

Purpose: The aim of this study was to determine soft tissue and radiographic landmarks to guide optimal trans-articular k-wire placement at the DIPJ.

Methods: A retrospective cohort study was conducted at a single institute, with 100 uninjured lateral phalangeal radiographs with a clear sagittal projection assessed by 3 independent assessors. Each assessor drew a line of ideal insertion, traversing the isthmus of the middle and distal phalanges, and the midline of the DIPJ, with soft tissue and bony parameters identified.

Results: The mean distance from the dorsal aspect of the nail plate to the line of ideal insertion was 3.86 mm, with a disparity between sexes noted. The distance from the dorsum of the soft tissue to the line of ideal insertion was expressed as a proportion of the total soft tissue diameter-the line of ideal insertion traversed approximately 40% of total width at the DIPJ and DIPJ soft tissue crease.

Discussion: The results suggest that a simple 'rule of fours' can be utilized to allow expedient and optimal passage. The entry point should be midline in the coronal plane, approximately 4 mm volar to the dorsal surface of the nail plate and aimed at a point 40% volar to the dorsal aspect of the soft tissue envelope at the level of the DIPJ crease. These guidelines are easily replicable and conveyable; additionally, they can guide insertion in the absence of fluoroscopy.