Fixation of proximal humeral Salter-Harris type 1 fractures in a 3-dimensionally printed skeletally immature canine bone model: a biomechanical assessment of fully threaded headless cannulated screws.

FTHCS combined with a K-wire provides superior biomechanical stability for canine proximal humeral physeal fracture fixation.

This in vitro biomechanical study compared four fixation techniques for proximal humeral Salter-Harris type 1 physeal fractures in skeletally immature dogs using 3D-printed bone models derived from CT imaging of a 6-month-old Labrador Retriever. Twenty-four identical models were divided into four groups (n=6 each): pin and tension band wire (Group A), partially threaded cannulated screw (Group B), fully threaded headless cannulated screw (FTHCS) alone (Group C), and FTHCS combined with a 1.6-mm Kirschner wire (Group D). Tensile loading at 105° simulated supraspinatus muscle traction. Group D demonstrated significantly superior resistance at both 1-mm (663.95 N) and 2-mm (1,080.33 N) displacements compared to all other constructs, with statistically significant mean differences versus Groups C, A, and B at the 1-mm threshold. Maximum failure loads did not differ significantly among groups. Notably, the partially threaded cannulated screw (Group B) failed via proximal subhead bone crushing and subsidence, raising concerns about bone collapse in immature patients with soft physeal cartilage. FTHCS constructs (Groups C and D) maintained proximal bone integrity, failing distally instead. The authors conclude that FTHCS augmented with a K-wire provides superior biomechanical resistance to early displacement and may reduce the risk of subhead collapse, offering a clinically viable fixation strategy that could support early rehabilitation in skeletally immature canine patients.

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