Influence of fracture morphology and working length on shear motion and construct stiffness in osteosynthesis...

Construct stability is a key factor in fracture healing and is influenced by fracture morphology, working length, and fixation strategy. While osteotomized fracture models are widely used for biomechanical testing, their relevance to real, interdigitated fracture patterns remains unclear.

This study compared the axial stiffness, torsional stiffness, and interfragmentary shear motion of synthetic distal femur models with osteotomized and realistic fractures. All constructs were tested under axial and torsional loading while progressively reducing the number of diaphyseal screws from five to two, thereby increasing the working length. Realistic fractures with a gap were analyzed in both an "open" state (prior to contact) and a "contact" state (after fragment contact. Shear displacements were quantified as resultant vectors derived from 3D motion tracking.

Fracture morphology and screw number significantly affected construct stiffness and shear motion. Osteotomized fractures showed higher axial stiffness (up to 997 N/mm in OC) compared to realistic fractures (up to 792 N/mm in RC), while realistic fractures without a gap exhibited superior torsional stability (up to 7.4 Nm/° in RC). Increasing working length reduced axial stiffness by up to -24% and torsional stiffness by up to -51%. Shear displacement increased with reduced screw number, particularly in constructs with a fracture gap.

Realistic fractures exhibit complex and direction-dependent stabilization mechanisms that are not captured by osteotomized models. Working length strongly influences construct behavior across all configurations. This study highlights the biomechanical differences between osteotomized and realistic fractures. Osteotomized models remain valuable as reproducible worst-case scenarios, whereas realistic fractures provide complementary insights by capturing stabilizing mechanisms such as fragment interlocking. Both approaches should therefore be combined in biomechanical research. Clinically, the results underline the importance of anatomical reduction and fixation planning to maximize construct stability.