The development of non-resorbable bone allografts: Biological background and clinical perspectives.

Bone grafts are typically categorized into four categories: autografts, allografts, xenografts, and synthetic alloplasts. While it was originally thought that all bone grafts should be slowly resorbed and replaced with native bone over time, accumulating evidence has in fact suggested that the use of nonresorbable xenografts is favored for certain clinical indications. Thus, many clinicians take advantage of the nonresorbable properties/features of xenografts for various clinical indications, such as contour augmentation, sinus grafting, and guided bone regeneration, which are often combined with allografts (e.g., human freeze-dried bone allografts [FDBAs] and human demineralized freeze-dried bone allografts [DFDBAs]). Thus, many clinicians have advocated different 50/50 or 70/30 ratios of allograft/xenograft combination approaches for various grafting procedures. Interestingly, many clinicians believe that one of the main reasons for the nonresorbability or low substitution rates of xenografts has to do with their foreign animal origin. Recent research has indicated that the sintering technique and heating conducted during their processing changes the dissolution rate of hydroxyapatite, leading to a state in which osteoclasts are no longer able to resorb (dissolve) the sintered bone. While many clinicians often combine nonresorbable xenografts with the bone-inducing properties of allografts for a variety of bone augmentation procedures, clinicians are forced to use two separate products owing to their origins (the FDA/CE does not allow the mixture of allografts with xenografts within the same dish/bottle). This has led to significant progress in understanding the dissolution rates of xenografts at various sintering temperature changes, which has since led to the breakthrough development of nonresorbable bone allografts sintered at similar temperatures to nonresorbable xenografts. The advantage of the nonresorbable bone allograft is that they can now be combined with standard allografts to create a single mixture combining the advantages of both allografts and xenografts while allowing the purchase and use of a single product. This review article presents the concept with evidence derived from a 52-week monkey study that demonstrated little to no resorption along with in vitro data supporting this novel technology as a "next-generation" biomaterial with optimized bone grafting material properties.