Influence of orthotopic placement on the incorporation and mechanical strength of a loaded structural cortical graft: an experimental study in rabbits.

The biological and mechanical behavior of loaded cortical autografts were evaluated in an experimental study using 40 adult male New Zealand rabbits divided into 4 groups. A double osteotomy was performed on all animals at the mid-diaphysis of the right ulna. The bone segment (5 mm) in between the osteotomies was considered a cortical autograft and fixed with an intramedullary Kirschner wire. The graft was fixed in an orthotopic manner in groups A and C, and rotated 180 degrees in groups B and D. After the animals were sacrificed (at 2 months for groups A and B and at 4 months for groups C and D), bone mineral density, graft cross-sectional geometry, and SSI index were assessed. Mechanical testing of the grafted area was carried out using a three-point bending configuration, and the parameter fracture load was assessed. Graft union and incorporation also was studied in histologic sections. In group C (orthotopic graft placement--4 months), bone specimens showed statistically significantly higher values for fracture load, total cross-sectional area, volumetric total bone mass and density, and polar SSI (P values < or = .05) compared to the other groups. In the same group, union and incorporation of the graft was complete, the cortex showed low porosity, and the collagen fibers were mature and properly oriented. Structural cortical bone autografts placed in an orthotopic manner recognize the new mechanical environment as optimal and thus their biological and mechanical behavior are enhanced. Loaded structural cortical grafts should be placed in an orthotopic manner when used for the management of cortical bone defects.