Biomechanics of porous ceramic implants used in long-bones segmental defect healing

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2002
Balçık, Cenk
Reconstruction of bone in segmental defect is a challenging orthopedic application in which nonunion is seen due to loss of tissue and atrophic bonding inability. Calcium and phosphate based ceramics are the new generation of bone grafts to be used in large segmental defects as they show excellent biocompatibility, biodegradability and osteoconductivity. Ideal ceramic composition, configuration and material property compatible to cortical bone is yet to be determined. It is hypothesized that long-bones segmental defects can be healed when appropriate ceramics will be used. In this study, easily applicable new segmental defect repair model of bone is evaluated by implanting two types of porous ceramic grafts. Composite ceramics, containing 40 % tricalcium phosphate and 60 % hydroxyapatite, and pure hydroxyapatite ceramics were implanted in the segmental defects of the rabbit tibiae. A 1 cm length of bone was resected from the mid 1/3 of the tibia and the ceramic was intramedullary fixed using Kirschner wire. mThe animals were allowed to move freely immediately after the operation. The healing progress with ceramic implants was evaluated at 6, 12 and 18 weeks by extracting ceramic-containing tibia specimens. X-ray, bone density measurements and mechanical tests performed on collected specimens indicated that the healing enhanced primarily with time. The healing progress determined by three-point bending was verified by the modal analysis of the specimens performed by dual channel frequency spectrum analyzer. Resonant frequencies yield comparable results with the three-point bending tests at the early stages of healing. As the flexural resonant frequency is proportional with the square root of the stiffness of the structure and the square of the length of the structure, it should be used with caution at the later stages of healing. X-ray, histology and mechanical analysis of composite ceramics yield a slight advantage over pure hydroxyapatite ceramics in the healing of segmental bone defect. At the end of 18 weeks of implantation period, the ceramic implanted tibiae gained their 30 % of intact strength. Inflammation was recorded in some specimens but the histology of ceramics did not reveal tissue reaction with the surrounding bone tissue. At the end of 12 weeks, the fracture callus surrounding defect zone formed a rigid union verified by the X-ray images and the vibration analysis. Finite element analysis of the segmental defect model determined the failure conditions of the ceramics in consideration. The finite element model signified that ceramic application is fully resistant to possible axial and torsion loading conditions. The critical loading condition, aside from unexpected excessive impact or shear, was the bending moment generated at the distal end of the tibia with increased lateral angle. Kirschner wire fixation gave support primarily to the bending conditions. The finite element analysis pointed out that excessive bending should be prevented in the segmental healing model until rigid callus union is achieved. Both ceramics had mechanical and osteoconductive properties suitable for bone grafting and showed good healing potential to be used in the repair of segmental defects.

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Citation Formats
C. Balçık, “Biomechanics of porous ceramic implants used in long-bones segmental defect healing,” Ph.D. - Doctoral Program, Middle East Technical University, 2002.