Use of Supercritical Sterilized Bone Allograft in Two Stage Revision ACL Reconstruction: A Histological and Histomorphometric Analysis

OBJECTIVES: Revision ACL-reconstruction can be compromised by bone loss as result of tunnel widening or poorly placed tunnels. Two-stage revision ACL consist of initial removal of the old fixation hardware and remaining ACL-graft tissue, followed by bone grafting of the tunnels. After a period of graft incorporation and bone remodeling, an ACL-reconstruction is performed. Our primary aim is to examine the use of supercritical carbon dioxide sterilized bone allograft for tunnel grafting in order to determine the bone quality, graft incorporation and remodeling, by using histology and histomorphometric analysis. Secondarily, we aimed to determine whether the histological findings correlate with the timing of the second stage revision procedure. METHODS: Case Series. 12 subjects underwent 2-stage revision ACL reconstruction. Femoral and tibial tunnels were bone grafted with supercritical carbon dioxide sterilized bone allograft (Australian Biotechnologies). Mean time from bone grafting to 2nd stage was 8.8 months (range, 5.6 to 21.3 months). Bone biopsies were taken at the time of the 2nd surgery and decalcified and embedded in paraffin. Sections were hematoxylin and eosin stained for microscopic analysis. RESULTS: The graft material was easily identified by its necrotic appearance with empty osteocytes lacunes within the lamellar trabecular bone. In all tissue samples predominately lamellar host bone apposition was seen on the surface of graft fragments known as creeping substitution. Separate bone graft fragments were bridged by newly formed woven bone. In the histological sections of 2 subjects some small islands of chondral cell differentiation were seen, which may relate to endochondral ossification. Active bone remodeling and resorption through combined osteoclastic and osteoblastic activity was present in 2 subjects (7.0 and 6.3 months post grafting), suggesting more advanced phases of graft incorporation. Mean bone volume was 68% over tissue volume (range 33-92%), and graft volume over bone volume was 41% (range 19-70%). Subgroup analysis for graft volume to time of second stage could not demonstrate a difference in 6 to 9 months (mean 44%, range 19-70 %) and >10 months (mean 34%, range 19-48%). CONCLUSION: The osteoconductive supercritical sterilized bone allograft acted as an effective structural framework, allowing for successful graft incorporation through creeping substitution. Although cancellous grafts may remodel completely with time, the initial bone apposition on and bridging of graft fragments provides early mechanical strengths to facilitate 2-stage revision ACL reconstruction. Bone volume and graft volume varied among subjects, but no beneficial effect was demonstrated for graft incorporation and bone remodeling in delaying the 2(nd) stage procedure for more than 6 months.