Orthobiologic interventions have been specifically designed to target the various stages of fracture repair. This article reviews each broad category of intervention in terms of the desired effect these materials have on specific stages of bone healing and incorporation.

Autologous bone graft
For bone augmentation, the gold standard is autologous iliac crest bone graft. Fresh cancellous autograft provides the quickest and most reliable type of bone graft. Its open structure promotes rapid revascularization, in a mechanically stable environment; a 5-mm graft may be totally revascularized in 20 to 25 days. These grafts depend on ingrowth of host vessels and perform best in well-vascularized beds. The large surface area of harvested autograft enables the survival of numerous graft cells. It is estimated that approximately 30 mL of graft can reliably be harvested from an anterior iliac crest.

The ability to obtain substantial amounts of autogenous graft material appears to be advantageous for treating critical-sized defects (total defects greater than 2 cm to 3 cm in length). The intermedullary bone graft harvesting offers a technique to achieve substantial amounts of graft volumes, up to 80 cc, for the treatment of larger segmental defects. Numerous basic science studies have demonstrated the biologic potential of intramedullary bone graft. Investigators have documented high levels of osteoinductive growth factors in this graft material.

Allograft and demineralized bone matrix
Allograft bone has been tested and processed, typically by tissue banks, and is available in many forms including morselized chips, demineralized bone matrix (DBM), and structural grafts. New technologies have resulted in the creation of specialized machine-made products that combine cancellous and cortical bone.

As an alternative to autografts, allografts have been proven effective in the repair of defects and nonunions and in joint arthrodesis. Like autografts, allografts require revascularization and benefit from a stable environment and a vascular bed. The efficacy of allografts, however, is generally slightly less than that of autografts.

DBM is formed by acid extraction of the mineralized extracellular matrix of allograft bone. It contains type-1 collagen, noncollagenous proteins, and osteoinductive growth factors. DBM is highly osteoconductive, due to its particulate nature, and has a large surface area and three-dimensional architecture, which serves as a site of cellular attachment. 

Several DBM formulations are available, based on refinements in the manufacturing process. DBM is available as a freeze-dried powder, as granules, as a gel, as putty, or in strips. DBM formulations are also used in combination products with other materials such as allogeneic bone chips and calcium sulfate granules. Evidence indicates that DBM preparations may have different potencies based on the manufacturer and manufacturing process, carrier properties, and sterilization techniques. Sterile processing of the bone may also affect the protein effectiveness of these materials.

Most human clinical trials on the use of DBM as a graft extender or graft enhancer report high fusion rates when DBM is used. Few prospective randomized controlled trials have been performed comparing DBM to autologous iliac crest bone graft in spine fusion or for the management of nonunions. Although many animal and human studies demonstrate comparable efficacy of DBM when combined with autograft or compared to autograft alone, additional high level of evidence studies are required to clearly define the indications for its use and the appropriate patient population that will benefit from DBM. In general, DBM formulations should not be used in isolation as a solitary graft material.

Osteoconductive graft substitutes
Creating osteoconductive matrices using nonbiologic porous structures implanted into or adjacent to bone has generated considerable interest. The graft must mimic the cancellous bony architecture and have very specific surface kinetics to facilitate the migration, attachment, and proliferation of resident progenitor cells.

In orthopaedics, most relevant materials are categorized as calcium ceramics. They include the following specific materials: calcium sulfate, calcium phosphate, synthetic tricalcium phosphate, beta tricalcium phosphate, and coralline hydroxyapatite. The porosity of these materials is the primary factor in determining their ability to foster ingrowth and osteointegration. Although they have variable rates of incorporation, all use two major pathways—chemical dissolution or a slow cell-mediated response (creeping substitution)—to osteointegrate. These materials have been specifically evaluated as bone graft substitutes in the management of subchondral bone defects associated with tibial plateau fractures, pilon, and distal radius fractures.

Because calcium-based bone substitutes lack osteoprogenitor cells and have low osteoinductive potential, composite grafts have been developed to accelerate bone formation. A composite graft is created by adding an osteoinductive factor to an osteoconductive calcium phosphate matrix, which theoretically increases bone formation. Many clinical series use composite ceramic grafts, which combine the scaffolding properties of these materials with biologic elements, to stimulate cell proliferation and differentiation.

Patient-derived cellular therapies
New options include methods for harvesting and transplanting progenitor cells, combined with bioactive scaffold matrix materials, and delivering bioactive molecules that enable these stem and progenitor cells to differentiate into bone.  One such cell-based strategy is to transplant autologous progenitor cells derived from bone marrow or other tissues.

Bone marrow aspirate
The concept of composite grafts—combining marrow elements with other conductive and or inductive substrates—has become a major area of interest based on early test results that document the superiority of using composite grafts compared to grafting with marrow aspirates alone. A potentially stable environment for cell growth is provided by concentrating marrow elements and then loading cells into an osteoconductive substrate with a microporous structure.

Because osseous regeneration is dependent on the number of cells available to participate in bone synthesis, grafts with fewer cellular precursors will typically have a poorer healing response, leading to potential nonunion. A very specific aspiration technique is used to maximize the number of effective progenitor cells per unit volume.

Clinical series have demonstrated excellent results when these factors have been controlled. In those series in which correct aspiration and concentration methods were combined with use of appropriate composite grafting techniques, the results were comparable to those achieved with autogenous iliac crest autograft. These composites avoid the attendant morbidity associated with autograft harvest.

Platelet-rich plasma and growth factors
After an acute fracture or a surgical intervention, thrombin and subendothelial collagen activate platelets. During activation, the alpha granules within platelets fuse with the platelet plasma membrane and release some of their protein contents to the surroundings (degranulation). The alpha granules in platelets contain more than 30 bioactive proteins, many of which have a fundamental role in hemostasis and/or tissue healing. These proteins include platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-ß), vascular endothelial growth factor, epidermal growth factor, platelet-derived endothelial growth factor, insulin-like growth factor, fibrinogen, and many others.

The clinical use of platelet-rich plasma (PRP) to facilitate graft augmentation has been reported for a wide variety of clinical applications, primarily maxillofacial, spine, and foot-and-ankle fusion procedures. Collectively, these studies provide variable support for the clinical use of PRP. However, many reports are anecdotal, and few Level 1 studies with control group comparison are available to definitively determine the role of PRP for hard-tissue augmentation. At present, platelet gel appears to function best as a physiologic carrier for other graft materials.

As a possible alternative to autologous bone graft, recombinant human PDGF applied to calcium phosphate matrix (rhPDGF/TCP) has been tested in many preclinical studies. The data have consistently indicated that rhPDGF-BB treatment ameliorates the effects of diabetes on fracture healing by promoting early cellular proliferation that ultimately leads to more bone formation.

Inductive substrates
The bone morphogenetic proteins (BMPs) belong to the TGF-ß superfamily of growth and differentiation factors. Unlike DBM, which is a mixture of BMPs and immunogenic noninductive proteins, the BMPs are true “osteoinducers.” As they are released, they feed back onto undifferentiated progenitor cells (stem cells), changing them directly into osteoprogenitor cells.

Conclusion
Comprehensive indications and strategies for the application of these new technologies include the acceleration of fracture healing, treatment of nonunions, enhancement of fusion mass, and the treatment of massive segmental bone loss.

The ideal bone-graft substitute is biocompatible, bioresorbable, osteoconductive, osteoinductive, structurally similar to bone, easy to use, and cost effective, and at minimal risk for material-based complications.

J. Tracy Watson, MD, chairs the AAOS Biologic Implant Committee, and acknowledges the committee’s contributions to this article.

Definitions
The biology of bone grafts and their substitutes requires an understanding of the bone formation processes of osteogenesis, osteoinduction, and osteoconduction.

Graft osteogenesis: The cellular elements within a donor graft, which survive transplantation and synthesize new bone at the recipient site.

Graft osteoinduction: New bone realized through the active recruitment of host mesenchymal stem cells (MSCs) from the surrounding tissue. The MSCs differentiate into bone-forming osteoblasts, a process facilitated by the presence of growth factors within the graft, principally bone morphogenetic proteins.

Graft osteoconduction: The facilitation of a bone-healing process into a defined passive trellis structure.