Stress Analysis of Internal Fixation of Long Cortical Bone Fractures

The internal fixation of bone fractures with metal plates has evolved in recent decades, with a change of emphasis from mechanical to biological priorities. Internal fixation should restore continuous stiffness, eliminate pain and allow early mobility. Conventional metal plates may be too stiff and over-protect the bone and lead to osteopenia (decrease in bone mineral density) beneath the plate. New designs of plates and screws are required to have less contact between the plate and the bone, and to allow a better blood supply beneath the plate, thereby promoting healing.

ADINA was used in a study at the City University of New York to obtain insight into the physical factors that are important in a good internal fixation design. A finite element model of an assembly consisting of four screws, one plate and two segments of a cortical bone was set up (see the photo above). The complexities in the model include contact between the plate and the bone, between the bone and the screws, and between the screws and the plate, all of which contribute nonlinearity to the analysis. The discretized geometrical model allowed easy modification of the geometry to accommodate the different plate designs available in the market.

The deformation and stress results obtained from the analysis included the micro-motion of the bone implants which can determine the evolution of the interfacial tissue. The analysis was conducted for different types of loads, including cyclic external loads and static tensile preloads. We show in the above movie part of the model with the bending of the plate near its center.

These results will contribute to better designs of internal fixation schemes for bone fractures.

Courtesy of G. Gailani and A. Sadegh, City University of New York