Abstract
Aim: Investigate the influence of various types of allograft (from the tibia, femur, and fibula) through finite element analysis to evaluate the best clinical configuration.
Methods: A non-linear 3D finite element model of a lumbar spine L3–L5 was used as a physiologic model (Noailly, 2003). The model was modified with the insertion of a transpedicular instrumentation (Surgival SA, Spain) and the removal of the L4 body and two adjacent discs. CT scans of a femur, tibia and fibula from the same patient were performed. Fragments of each bone were reconstructed and inserted within the model. Four configurations of allografts were investigated: one femur fragment, one tibial fragment, three fragments of fibula, six fragments of fibula. Four types of loadings were applied: compression (1000N), flexion, extension, and rotation (15Nm). Strain and stresses were calculated in large displacement (MARC, MSC Software).
Results: Von Mises stresses within the internal fixator are well below the Yield stress and the fatigue limit and therefore no fracture of the fixator is foreseen. The use of a fixator to create fusion of the two vertebras makes the lumbar spine much stiffer. The geometry and configuration of the allografts have a large influence on the strain and stresses within the adjacent vertebrae with a reduction of strains and stresses. The use of fragments of fibula gives the most stable configuration. However, this is also the configuration that changes most the maximal principal strains within the vertebrae. Results obtained with the femur or the tibia are very similar between each other. However, due to its ellipsoidal geometry, the allograft in tibia gives more asymmetric deformations than the femur.
Conclusion: Allografts harvested from the femur seems to be more reliable and change least the strain and stress distributions within the lumbar spine compared to allografts from the tibia or fibula.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
