Biomechanical analysis of push-pull principle versus traditional approach in locking plates for proximal humeral...

Proximal humerus fractures are usually treated with locking plates, which could present recurrence, screw penetration, joint varization. The push-pull principle was introduced to prevent these risks and showed promising results; a dedicated design was then developed and this feasibility study aims to compare the biomechanical performances of such dedicated push-pull plate with the traditional locking plate using finite elements.

The humerus geometry was obtained from Sawbone CT-scans; the geometries of a traditional locking plate and of the dedicated push-pull one were used. A fracture was added below the humeral head and the plates were virtually implanted. The wire pulling mechanism was simulated connecting the plate to the humeral head apex, considering two levels of tension. Three testing set-ups (axial, torsion and compression bending) were simulated. Stress distributions on bone, plate and screws were measured.

Stress distribution on the distal humerus was similar for both plates. Stress distribution on the proximal humerus was more homogeneous for the push-pull model, showing less unloaded sections (up to 78%). The different levels of tension applied to the wire returned slight differences in terms of stress values, but the comparison with the traditional approach gave similar outcomes.

More homogeneous stress distribution is found with the push-pull plate in all three testing set-ups, showing lower unloaded areas (and thus lower stress-shielding) compared to the traditional plate; the screws implemented returned to be all loaded in at least one of the set-ups, thus showing that they all contribute to plate stability.