Finite element and in vitro biomechanical analysis of a novel magnesium degradation-induced variable fixation plate

Magnesium degradation-induced variable fixation plates (MVFPs) offer different fixation modes during fracture healing, but their biomechanical reliability is not well established.

CT images of femurs from volunteers were used to build a model, and Abaqus software simulated deformation, stress, and relative displacement under various stress conditions. Mechanical tests including vertical loading, four-point bending, torsion, and fatigue were conducted using femur simulation models and suitable magnesium shims were screened.

Finite element analysis showed that under 700N vertical loading, MVFP exhibited 83%-116% of the total deformation, 88%-120% of the maximum stress, and 86%-121% of the average relative displacement compared to locking plate (LP). Under 250N four-point bending, these were 76%-186%, 73%-183%, and 61%-170%, respectively. Under 10Nm torsional moment, they were 102%-109%, 114%-118% (for implants), and 110%-113%, respectively. In vitro biomechanical tests showed that MVFP had greater total and relative displacements but lower axial, four-point bending, and torsional stiffness (81.5%, 68.5%, and 63.9% of LP, respectively). Fatigue testing indicated both LP and MVFP samples endured 100,000 cycles of 700N vertical load without failure. The MVFP with a 0.5 mm shim exhibited superior stiffness and offered greater space for elastic deformation compared to the 1 mm shim.

Although MVFP's stiffness slightly decreases compared to LP after shim degradation, it improves interfragmentary micromotion and reduces stress shielding while maintaining good fatigue resistance. MVFP with 0.5 mm axial micromotion shows promise for further development and clinical application.