Magnesium degradation-induced variable fixation plates promote bone healing in rabbits.

Background: Both initial mechanical stability and subsequent axial interfragmentary micromotion at fracture ends play crucial roles in fracture healing. However, the conversion timing of variable fixation and its effect on and mechanism of fracture healing remain inadequately explored.

Methods: A magnesium degradation-induced variable fixation plate (MVFP) for femurs was designed, and its conversion timing was investigated both in vitro and in vivo. Then, locking plates and MVFPs with and without a magnesium shim were implanted in rabbit femur fracture models. X-ray photography and micro computed tomography (micro-CT) were performed to observe the healing of the fracture. Toluidine blue and Masson's trichrome staining were performed to observe new bone formation. The torsion test was used to determine the strength of the bone after healing. Finally, reverse transcription-polymerase chain reaction (RT-PCR) and western blotting were used to detect the expression of osteogenesis-related genes in the three groups.

Results: The MVFP with sample 3 magnesium shim showed greater axial displacement within 15 days in vitro, and its variable capability was likewise confirmed in vivo. X-ray photography and micro-CT indicated increased callus formation in the variable fixation group. Toluidine blue and Masson's trichrome staining revealed less callus formation on the rigid fixation side of the locking plate, whereas the variable fixation group presented more callus formation, more symmetrical intraosseous calli, and greater maturity. The torsion test indicated greater torsional resistance of the healed bone in the variable fixation group. RT-PCR and western blotting revealed that the expression levels of BMP2 and OPG increased during early fracture stages but decreased in late fracture stages, whereas RANKL expression showed the opposite trend in the variable fixation group.

Conclusions: MVFP promoted faster and stronger bone healing in rabbits, potentially by accelerating the expression of BMP2 and modulating the OPG/RANKL/RANK signaling axis. This study offers valuable insights for the clinical application of variable fixation technology in bone plates and contributes to the advancement of both internal fixation technology and theory.

Level of evidence: level V.