Estudo biomecânico do comportamento de diferentes espessuras de capa de cimento ósseo em novo modelo de prótese modular diafisária femoral em cão submetidas a ensaio de torção

Abstract

Background: The feasibility of modular cemented prosthesis in the femoral diaphysis was demonstrated in dogs, but several authors report loosening of bone-cement-implant interface as a major complication and have yet to reach a consensus on the ideal cement layer thickness for reducing postoperative morbidity. The objective of this study was to evaluate the stabilization of the prosthesis using different thicknesses of cement layer, subjected to torsion forces. Materials, Methods & Results: For this study 48 femurs from 24 dogs weighing over 15 kg were used. The animals used did not have any prior diagnosis of bone or nutritional diseases, which was an exclusion criteria for this study. All biological materials were obtained immediately after death or euthanasia of the animal, and then subjected to conventional storage in a freezer at a temperature of - 24 degrees C. The prosthesis was composed of a cylindrical part with rods for attachment to the intramedullary canal, made of alloy steel 316L. After thawing the femurs and subsequent stabilization at room temperature we performed osteotomies of the femoral diaphysis with an oscillating saw and the installation of the prosthesis. As preparatory measures for prosthesis fixation with cement, the intramedullary canals were cleaned, washed with saline and aspirated. They were separated in four groups, the first group with eight femurs used a cement mantle of 1.0 to 1.5 mm, the second group, with eight femurs, used a cement mantle of 2.0 to 2.5 mm and the third group, with eight femurs, used a cement layer of 3.0 to 3.5 mm. The femurs were submitted to destructive torsion tests using a universal testing machine with a load cell of 981 N (100 kg) and an essay speed of 22 mm / min. They were evaluated due to the values of torsional rigidity and maximum torque shown by the implant, bone and bone cement interface. The fourth group consisted of the remaining intact femurs, they were evaluated using the same torsion test used on the other groups. The results were analyzed using the Statistical Analysis System (SAS, 2001) and the normality of residuals was previously verified by the Shapiro-Wilk test. It was established to conduct the testing using an external rotation protocol in order to standardize, assuming a situation where the dog was in movement during a uniform curvilinear motion, trying to mimic a situation where the rotational forces were acting more significantly on the femur. Discussion: In our study the importance of cleaning up the intramedullary canal for proper bone cement penetration, a good reaming of the intramedullary canal and drying the area before the cement implantation was made clear. Statistical analysis showed that the thicknesses of the cement layer ranging from 1 mm to 2.5 mm, although bringing a gradual increase in maximum torque and torsional rigidity, are not sufficient to be statistically significant and may be considered equal in their biomechanical behavior assessed by this study. A similar result was found when we compared the 2 mm to 3.5 mm layers. The comparison that was statistically significant and can be considered in relation to the different biomechanical behavior of the cement layer was seen between the group I of 1 mm to 1.5 mm, and group III of 3 mm to 3.5 mm. Although some results were not statistically significant we must remember that, in absolute values, the torsional rigidity and maximum torque increased linearly with the increasing of the cement layer. From these results we can infer about the real advantage of using a thicker cement layer over an increment in the diameter of the intramedullary component, giving greater resistance to the prosthesis.