PMMA in Bone Allograft Fixation Utilizing Unicortical Versus Bicortical Screws: A Finite Element Analysis

Abstract

Bone grafts are widely used in skeletal reconstruction subsequent to tumor surgery, traumatic injuries, or in conjunction with a total joint procedure. As the graft site may be subjected to destabilizing mechanical forces, any motion at the interface between graft and the host tissue may impede or prevent healing. The application and the maintenance of the compression between screw threads and the bone is the most important factor in attaining rigid internal fixation by means of screws or screws & plates. This study is designed to assess various allograft fixation options with the intent of finding an option that is secure yet does minimal damage to the allograft. Utilizing computational modeling and analysis (SolidWorks), we compared the bending stiffness for diaphyseal bone constructs with a gap and stabilized with a dynamic compression plate, with screws placed in a unicortical or bicortical manner, with and without intramedullary PMMA. The model was validated by comparing the simulation results with the experimental results from literature for two unicortical screws application with PMMA in bending test. The study was then exteneded by looking at the stress distribution across the plates with the use of bicortical and unicortical screws with and without PMMA. For unicortical screws the use of PMMA reduces the displacement by 4:65% and reduces the stress by 5:69%. For bicortical screws, the use of PMMA reduces the displacement by 1:78% and reduces the stress by 9:45%. The bicortical screws with PMMA has a displacement which is 0:74% smaller but a stress which is 0:73% higher when compared to the unicortical screws with PMMA. From the results of total displacement and maximum stress on the plate, conclusion can be drawn that the two best arrangements are the use of unicortical screws with PMMA and bicortical screws with PMMA, thus allowing the use of fewer screws.

Surgical Fixation Method in Lower Extremity Intercalary Allograft Reconstruction After Oncologic Resection: A Comparison of Plates and Nails

“…Biomechanical studies strongly support the use of PMMA in decreasing fracture risk. 11,13 Gupta et al suggested that adding intramedullary cement led to fracture rates of only 4.3%, 1 and Frisoni et al 10 found a lower fracture rate when adding intramedullary cement to their plate fixation. These results contrast the current study, as we did not establish a difference in fracture rates between cemented and noncemented allografts.…”

“…Holes in an allograft function as stress risers and could contribute to the increased incidence of fracture. [11][12][13] Further study is needed to better characterize this correlation.…”