Comparative failure analysis of allograft constructs stabilized with compression plates utilizing unicortical versus bicortical screws with or without PMMA

Abstract

Orthopaedic internal fixation is subject to high stress concentration around screw holes leading to screw loosening. We compared the screw pullout strength (n=8), and bending and torsional stiffness (n=6) for cadaveric (young adult donors age 45years or less) diaphyseal bone constructs with a gap and stabilized with Zimmer 4.5 narrow stainless steel dynamic compression plates with 4.5mm cortical stainless steel screws placed in a unicortical or bicortical manner with and without intramedullary PMMA. The pullout strength for unicortical fixation with PMMA (4.3±.6kN) was not significantly different with bicortical fixation with PMMA (5.3±1.5kN), n=8, P>.5; and for the group with bicortical fixation without PMMA (4.3±1.1kN), n=8, P>.5. The difference in bending stiffness for the group with two unicortical screws fixation with PMMA (324.1±59.7kN/m) was also not statistically significant when compared with the groups with four bicortical screws fixation without PMMA (278.7±89.6kN/m) and two bicortical screws fixation with PMMA (347.9±66.7kN/m), n=6, P>.1. The difference in torsional stiffness for the group with two unicortical screws fixation with PMMA (173±44Nm/deg) was not statistically significant when compared with the groups with four bicortical screws fixation without PMMA (190±22Nm/deg), two bicortical screws fixation with PMMA (179±36Nm/deg), and two bicortical screws fixation with PMMA (193±31Nm/deg), n=6, P>.5. The use of PMMA in allograft fixation appears to allow the use of fewer screws, and perhaps unicortical fixation without significant compromise of strength. Clinically, two unicortical screws with PMMA may replace the use of two bicortical screws without PMMA, or two bicortical screws with PMMA may replace four bicortical screws. The clinical relevance would be the ability to use fewer screws and reduce violation of the far cortex of the allograft.