Investigation of the effects of allogeneic mesenchymal stem cells on bone union and regeneration in the necrosed bone by biomechanical test and the finite element method

Abstract

Since allogeneic mesenchymal stem cells can be acquired and reproduced easily under laboratory conditions today, they are used widely in many orthopedic applications. Many successful results have been obtained by using these cells on cartilage and tendon damages besides bone related fractures. In this study it is aimed to investigate the promoting effects of allogeneic mesenchymal stem cells, which are injected directly without any external support, on bone healing and regeneration in necrosis in terms of biomechanical and finite element evaluation. Fifty Wistar Albino rats were used in this study. 7 of those were used to derive allogeneic mesenchymal stem cells, while 3 rats were used for preliminary study. The remaining 40 rats were randomly divided into two groups: a treatment and a control group. Right femurs of all subjects were washed out intramedullary with 0.1 ml 10% formaldehyde before creating a fracture and the periosts were peeled off 10 mm towards proximal and distal, away from the region where a fracture would be created, and were removed. This procedure was carried out to lead to bone necrosis depending on formaldehyde, as well as avoiding the potential promoting effects of allogeneic mesenchymal stem cells on the union of fracture and bone regeneration. Then, a transverse fracture was created and detected intramedullary. On the second postoperative day, a solution including 0.1 ml allogeneic mesenchymal stem cells (10 7 units per ml) was administered to subjects, identified as group A, by direct injection into fracture site. 0.1 ml 0.9% isotonic NaCl solution was administered to group B by direct injection. The subjects were sacrificed after 60 days of follow-up. A biomechanical evaluation was performed as well with respect to stiffness, elasticity (Young's Modulus) and energy storage capacity. Then the finite element analysis was carried out to the rat femoral bone by constructing the model from images. Loads (as perpendicular to femoral head) and boundary conditions were applied similar to the experimental conditions. Cortical and trabecular structure of femoral rat were defined separately based on CT images. The finite element method has applied to the one constructed femoral bone and compared with biomechanical parameters that measured experimentally. Elasticity module was determined experimentally and was used as an input data for the FEM analysis. Poisson ratio was taken from the literature. When they are examined biomechanically a statistically significant difference was not found between their stiffnesses, elasticities and energy storage capacities. The axial displacement values u y and the normal stress obtained by using the finite element analysis are compared with the measured values. The amount of vertical displacement was measured as 2.2762 mm while it was found 2.6665 mm in the FEM results. The maximum normal stresses occurred in the critical sections of femoral bone in experimental test and the FEM were determined as 8.33 and 7.45 MPa, respectively. Mesenchymal stem cells have not increased the fracture healing in biomechanical evaluation. The vertical displacement and normal stress values obtained from the finite element analysis are agreed with experimental results. As a result it can be concluded that the results of the FEM are in good agreement with the experimental values. Non-invasive methods such as the FEM help us to determine the mechanical response of the femoral bone before any destructive tests applied to rats.