From load-bearing to load-sharing: contact-driven transitions in locked plate constructs.

Locked plate osteosynthesis functions as an internal fixator, yet load-induced plate-bone and fragment-fragment contacts - key transitions from load-bearing to load-sharing - remain poorly characterized. This study quantified their effects on construct stiffness, plate strain, and interfragmentary motion under axial loading. Twenty-seven configurations were tested using epoxy-glass surrogate bone fixed with titanium LCP plates, varying fracture gap (3-9 mm), working length (49-121 mm), and plate-bone distance (0-3 mm). Digital image correlation measured strain and motion. Contact events were identified manually from characteristic transitions in the force-displacement response, supported by corresponding changes in plate-surface strain progression. Geometric parameters dominated initial stiffness (125-442 N/mm), decreasing with plate elevation and working length in elevated setups. Within the investigated load range up to 600 N, 16/27 configurations showed non-linear transitions associated with contact events: fragment-fragment contact caused pronounced stiffness increases and strain stagnation at the fracture gap, whereas plate-bone contact was associated with local plate-surface strain stagnation at the location overlying the contact while strain at the fracture gap continued to increase. Interfragmentary motion stagnated after contact, confirming contact-dependent shifts in load transfer. Response surface regression explained 81.9% of the variance in initial stiffness and 89.6% of the variance in axial interfragmentary motion, identifying outcome-specific associations with geometric parameters and selected interaction terms. In conclusion, load-dependent contact events were associated with distinct transitions in construct stiffness, plate-surface strain progression, and interfragmentary motion, complementing the observed associations between geometric parameters and initial mechanical behavior.