Biomechanical and histological evaluation of the application of biodegradable poly-l-lactic cushion to the plate internal fixation for bone fracture healing

Background: Internal plate fixation of fracture can provide favorable mechanical environment for fracture fragments. However, osteoporosis under the plate is often found, and refracture may occur after the plate is removed. There are two different opinions about the bone loss beneath the plate: first is the stress shielding effect brought by the rigid fixation, and the second is the insufficient blood supply of the bone caused by the placement of the plate. In this paper, we tried to achieve a favorable condition for the fracture healing by inserting a kind of biodegradable cushion, through which the stress shielding effect and the interruption of the bone blood supply could be relieved at the same time. Methods: Animal models of internal fixation for tibia diaphyseal fracture with the placement of the poly-l-lactic (PLLA) cushion between the plate and the bone were established; a series of in vitro investigations and finite element (FE) analysis were performed to evaluate the effect of this new internal fixation system. Findings: During both the initial and 50% healing periods, the extent of stress shielding of the fracture zone decreased due to the use of the PLLA cushion. Especially for the 50% healing stage, the insertion of the PLLA cushion on alleviating the stress shielding of the bone tissue between the inner two screws directly under the plate is more apparent than that at the initial healing period. Meanwhile, radiological and histological coloration results demonstrated sooner callus growth and better trabecular rearrangement of the fracture zone in the PLLA group with the degradation of the PLLA cushion during the healing periods. Interpretations: This study showed that the use of the PLLA cushion at the initial period did not impair the stability of the whole system, which provides a favorable mechanical environment for the following fracture healing. On the other hand, its superiorities on alleviating stress shielding effect and interruption with the blood supply of the bone tissue beneath the plate contributed to the favorable fracture healing conditions in PLLA group with the degradation of the materials.