Dolph Houben

81 Outcomes of vascularized bone allotransplantation 4 X-Ray At 1, 2, 4, 6, 10 and 20 weeks after transplantation, lateral and anterior/posterior radiographs of the tibia were obtained. All images were digitally developed and scored using the radiographic evaluation of osseous healing and transplant incorporation system [24] . This scoring system quantifies periosteal bridging, callus remodeling, union and transplant appearance at both of the host/transplant interfaces. Therefore, this system is reliable and suitable scoring system for bone healing after reconstruction with allografts or transplants [19, 24] . Transplant appearance was scored as either no reaction of the transplant (0 points), resorption (-1 point) or periosteal reaction from the transplant (1 point). Scores ranged between 0-4 for each other evaluation point, with a maximum of 25 points. The scoring was done by an independent orthopedic surgeon at our institution. Micro-Computed Tomography (micro-CT) The experimental and contralateral tibias were both scanned using Micro-Computed Tomography (Inveon PET CT, Siemens Medical Solutions USA, Inc., Malvern, PA) at the termination of the experiment 20 weeks post-surgery. We used settings of 80 kV and 500uA, obtaining 180 projections with 188.5 μ m thick sections at low magnification with a current version of the imaging software PMOD (PMOD Technologies, Zurich, Switzerland). The scans were used to quantify bone mineral density (BMD), visualize bony bridging and confirm AV-bundle patency. All measurement and imaging data calculations were performed using the current version of the measurement program AnalyzePro (Analyze, Mayo Clinic, Rochester, MN). Bone mineral density calculations were made after calibration with a hydroxyapatite phantom. Biomechanical testing Axial compression test Bone quality was evaluated on a macro scale by axial compression testing to calculate the elastic modulus of the allotransplant and normal bone [25] . The elastic modulus, defined as the ratio of stress to strain, is a representation of material stiffness. It serves as an indication of bone quality and its resistance to fracture. After harvesting the allotransplant was wrapped in a moist saline gauze and a 5mm transverse section obtained using a water-cooled Exakt saw (Exakt Technologies. Inc. Oklahoma City, OK). It was subsequently stored at -20⁰C for later testing [17, 26] . Before testing, heights at three locations around the perimeter of each specimen were measured with a digital caliper (Absolute Digimatic, Mitutoyo, Kanagawa, Japan) and averaged to yield the specimen height. Specimen cross-sections were imaged on a digital scanner (CanoScan LIDE 100, Canon) at a resolution of 300 pixels/inch (PPI). Cortices were segmented by converting the scan to a binary image and cross-sectional area calculated using Image J (NIH, Bethesda, Maryland). Mechanical testing was conducted on a servo-hydraulic test frame (Model 312, MTS Systems, Eden Prairie, MN) instrumented with a 2500-kg capacity load cell. Specimen were loaded between two flat plates at a constant displacement rate of 1 mm/minute until reaching a maximum, sub-failure compressive load of 5000 N. The modulus of elasticity was determined from the slope of the linear region of the stress-strain curve.