Enhancing spinal fusion.

The strategies for enhancing bone fusion are myriad. Each, in a way, uses a “trick of its own on mother nature” to improve the spine fusion environment. Such “tricks” are designed to foil the all too-often unfavorable surgical outcome of pseudarthrosis. Many of these strategies are discussed in the following pages. Each should be considered either currently or in the future, in selected circumstances or perhaps routinely. The strategies designed to enhance spinal fusion can be broken down into two separate and distinct categories: alternatives to autograft and physiology enhancement or augmentation techniques. Fundamentally, traumatized bone, whether disrupted through an overt act of trauma or through surgical intervention, heals best when bone healing-enhancing parameters are optimized. These parameters include: 1) surgical technique; 2) the use of bone graft substitutes that take the form of bone graft expanders or bone fusion enhancers through growth factors or augmentation techniques; and 3) bone physiology enhancement or augmentation strategies. Each of these strategies attempts to emulate or augment the existing attributes of autograft. Therefore, it is emphasized that autograft is the unequivocal “gold standard” strategy used for the acquisition of bone fusion. It provides the osteogenic, osteoinductive, and osteoconductive components required to achieve bone fusion. No other current strategy accomplishes such. The bone healing process, whether it follows trauma or surgical intervention, involves multiple phases of healing (Fig. 7.1). First, a hematoma forms around the trauma or surgical site. An inflammatory phase associated with granulation tissue formation, fibrin matrix formation, and cellular and vascular infiltration ensues. It is the inflammatory phase that is particularly critical to the subsequent bone healing process and the acquisition of a solid arthrodesis. Of significant note, this phase is particularly sensitive to toxic extrinsic factors such as nicotine (and other factors associated with tobacco abuse), steroid use, radiation, chemotherapy, and so on. Therefore, it is emphasized that such clinical strategies should be delayed until the inflammatory phase is complete (approximately 3 weeks after trauma or surgery). The phase of callus formation follows in which cartilage and fibrous tissue (primitive woven bone) is laid down. Finally, the callus is remodeled with the deposition and resorption of lamellar bone. The latter is remodeled to achieve the final state of a solid and strong healed fracture or bone fusion. Surgical Technique Surgical technique obviously plays a major role in clinical outcome, but unfortunately is difficult to characterize. Regarding bony fusion, “carpentry” (e.g., the careful crafting of the bone fusion beds and the insertion of a carefully crafted graft or strut) plays a major role in enhancing and optimizing the environment for fusion. Surface area of contact optimization, the assurance of appropriate (neither excessive nor too little) loading of the fusion site, the use of carefully selected implants that do not excessively load nor “stress shield” the fusion site (Fig. 7.2), and the selective employment of the strategies discussed in the following pages are surgeon-dependent and often dictate the fate of a surgical procedure.3,5 The optimization of the surface area of contact between fusion surfaces was initially espoused by Cloward8 and others as a strategy to enhance bony fusion. Cloward9 suggested that a bone fusion surface area of contact that involved 80% of the surface area of the lumbar intervertebral endplate was optimal (Fig. 7.3). Most do not heed such advice today, and thus, many operations fail.