A Closer Look At Bone Graft Substitutes
There are many instances when it is appropriate for the foot and ankle surgeon to use bone grafts in order to enhance a patient’s clinical outcome. When it comes to reconstructive osteotomies for angular realignment, arthrodeses and the repair of nonunions and cystic bone lesions, these are often best performed with procedures that take advantage of the many desirable features of bone grafts and, more recently, bone graft substitutes.
Before discussing the details of bone graft substitutes, it is helpful to review the bone graft options that are available to the surgeon. These options include: cancellous and/or cortical bone, allogeneic (typically freeze-dried) cadaver bone and autogenous bone.
Cancellous bone offers a porous, osteoconductive environment and ease of incorporation. However, it conveys limited structural strength and ability to retain fixation devices. Cortical bone, on the other hand, offers tremendous structural integrity, but it is slow to incorporate in comparison to cancellous bone. Allogenic bone implants offer ease of use, ready availability and provide a wide range of configurations that include cortical and cancellous bone in various proportions. However, despite rigorous measures related to the procurement and preparation of allogeneic tissue products, there remains a very small but real risk of disease transmission and hypersensitivity reaction.1,2
The autogenous corticocancellous bone graft, which is typically harvested from the iliac crest, remains the gold standard for comparison of all other grafting materials. In reconstructive foot and ankle surgery, the calcaneus offers a reliable source of autogenous bone graft material. Unfortunately, however, autogenous sources of corticocancellous bone are limited and there is distinct morbidity associated with harvesting autogenous bone.
Banwart et al., performed a meta-analysis of the related literature and observed that use of autogenous iliac crest bone grafts was associated with a 25 to 45 percent complication rate. They also noted that 40 percent of patients related harvest site pain up to five years postoperatively.3
For these reasons, the emergence of bone graft substitutes for enhancement or augmentation of bone grafting procedures, as well as the techniques of bone callus distraction, have proven to be of great service in recent years.
Understanding The Physiological Processes Of Bone Healing
In order to appreciate the useful features of bone graft substitutes, it is important to review some of the physiological processes underpinning bone healing in the presence of a bone graft. Bone graft incorporation occurs via a process referred to as “creeping substitution,” whereby the graft material is actually replaced by new bone formation. This desirable and complex event depends upon three underlying processes: osteoconduction, osteoinduction and osteogenesis.