Increasing Bending Stiffness of Antibiotic-impregnated Cement-covered Rod Constructs: A Biomechanical Study

Abstract

Introduction: The use of antibiotic-impregnated cement-covered rods (ACR) has been reported for the treatment of posttraumatic osteomyelitis and infection. Early reports of ACRs described using a 3-mm guidewire as the central core of the rod; however, certain clinical scenarios may require a more rigid construct. We describe the principals and technique of using a thicker metal core to increase stiffness of the ACR. A 40-Fr chest tube was filled with polymethyl methacrylate (PMMA) cement. Metal central rods (3 and 5.8 mm) were inserted before the cement cured. Bending stiffness was measured and compared with a conventional intramedullary nail. Increasing the core diameter of ACR constructs resulted in increased bending stiffness, but the ACR remained significantly weaker than conventional nails. Given the elution characteristics of antibiotic cement, and the relatively short duration of use of these rods, the relative decrease in volume of cement is unlikely to influence antibacterial effectiveness of the ACR. Purpose: The use of ACRs has been reported for the treatment of posttraumatic osteomyelitis and infection. Early reports of antibiotic rods described using a 3-mm guidewire as the central core of the rod; however, certain clinical scenarios may require a more rigid construct. The purpose of this study was to examine the use of a thicker metal core to increase stiffness of the ACR construct. Methods: A 40-Fr chest tube was filled with PMMA cement and metal central rods were inserted before the cement cured. Central cores utilized were a 3-mm intramedullary guidewire and a 5.8-mm fully threaded ring external fixator strut. Bending stiffness was then measured along multiple lengths of each specimen and compared with the bending stiffness of the bare metal inserts (without cement) and a conventional intramedullary nail (10.5-mm titanium). Results: The mean bending stiffness was 4.96±0.67 N m2 for the 5.8-mm-core cement rod and 3.07±0.28 N m2 for the 3-mm-core cement rod (P=0.0039). The bending stiffness for the 3-mm guidewire, 5.8-mm rod, and the intramedullary nail alone were 0.87, 3.1, and 31.7 N m2, respectively. Conclusions: Increasing core diameter of ACR constructs will predictably increase their bending stiffness; however, ACRs will remain significantly weaker than conventional intramedullary nails. Given the elution characteristics of antibiotic cement, and the relatively short duration of use of these rods, the relative decrease in volume of cement is unlikely to influence the antibacterial effectiveness of the ACR.