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Repair of a Critical Porcine Tibial Defect via Allograft Revitalization
Christopher Runyan, M.D., Ph.D.1, Samantha Ali, BS1, Donna Jones, PhD1, David Billmire, MD1, Jesse Taylor, MD2.
1University of Cincinnati, Cincinnati, OH, USA, 2University of Pennsylvania, Philadelphia, PA, USA.

PURPOSE: This work seeks to provide an alternative to vascularized bone flaps for reconstruction of critical bony defects. We previously described the generation of large volume vascularized bone in a pig model, using a hemi-mandibular allograft scaffold, autologous adipose-derived stem cells (ASCs), recombinant human Bone Morphogenetic Protein-2 (rhBMP-2) and native periosteum. This study examines the efficacy of this technique, known as 'allograft revitalization', for repair of a critical tibial defect, with comparison to current modalities: vascularized autograft and structural allograft.
METHODS: Nine pigs had 3 cm defects created in their bilateral tibial diaphyses, for repair using one of three modalities. ‘Negative control’ animals had the ostectomized bone and surrounding periosteum removed, and replaced with allograft porcine tibia. To simulate repair using vascularized autograft, the osteotomized bone in ‘positive control’ animals was left in situ, with the posterior, native periosteum intact. ‘Experimental’ animals’ defects were repaired with allograft porcine tibia packed with autologous ASCs and rhBMP-2-soaked ACS (0.5 mg/ml rhBMP-2 solution, Medtronic, Memphis, TN), with the native periosteum intact. All constructs were secured with rigid plate and screw fixation. Each group contained three pigs (six tibias). Healing was assessed based upon clinical exam and serial radiography. Eight weeks post-grafting, each pig received an additional mid-graft osteotomy to assess their healing potential. This was performed on one side per pig to minimize morbidity (n=3 per group), and was followed by an additional 7-week incubation before sacrifice. Explants were further analyzed using micro-CT and standard histologic analyses.
RESULTS: Tibia healing was assessed with radiographs performed at 4, 7, 12 and 15 weeks after the initial operation. At 7 weeks, none of the allograft-only negative control (NC) tibias were healed radiographically (0/6) whereas all but one autograft positive control (PC, 5/6) and all experimental (EXP) group tibias had healed. A repeat osteotomy was performed at 8 weeks through the center of one graft site per animal, to assess graft ability to heal. As expected, radiographs of NC tibias showed non-union in all (3/3) the allograft sites at 15 weeks (7 weeks post-repeat osteotomy). However, all PC and EXP tibias had healed their repeat osteotomies by this time. Gross exam of the explanted tibias revealed large bony calluses around the osteotomy sites in NC pigs (59.6 ± 6.5 mm in width); in comparison, PC (44.1 ± 4.2 mm) and EXP (47.8 ± 2.5 mm) explants were well-remodeled and significantly narrower than NC explants (p = 0.028).
CONCLUSION: This is the first study demonstrating allograft revitalization for repairing a critical defect in a large animal model. No differences were observed in tibial defect healing between the vascularized autograft and revitalized allograft approaches. The ability of both of these grafts to fully heal following repeat osteotomy indicates they are both functionally vascularized. Although preliminary, these data provide strong evidence for 'revitalized allograft' as an alternative to vascularized bone flaps for critical defects.


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