Recent advances in several different fields are significantly impacting anterior cervical spine fixation. Material engineering has developed several types of new materials to replace traditionally used bone products and titanium.
The new materials address several problems inherent in the use of bone or titanium. Secondly the revolution in molecular biology has finally reached spinal fusion. Bone morphogenetic protein (BMP) is a protein that stimulates bone growth and fusion. BMP potentially allows for higher anterior cervical spine fusion rates and its applications continue to evolve.
Before 1950, surgery in the cervical spine was primarily performed with a posterior approach. Bailey and Bagley performed the first anterior cervical spine stabilization with an onlay fusion in 1952. In 1955 Robinson and Smith published their operative technique using an interbody graft. In 1958, Cloward described the anterior cervical decompression and interbody grafting as it is generally performed to this day. The advantages to the anterior approach were quickly evident. The approach was easy to perform and provided a wide exposure. The anterior spine could be decompressed and stabilized in one operation.
Initially, most anterior cervical discectomies and fusions were done using a patient's own bone (autograft). A separate incision is made in the patient's anterior hip area and a tricortical piece of bone is harvested to be used to replace the removed disc. The fusion rates obtained with autograft are very high. The fusion rate is over 95% for one-level anterior cervical fusions with autograft. The problem with autograft is the complications associated in obtaining the graft. More than one-third of patients experience pain at the graft site after one year. Complications occur in up to 20% of patients and included wound hematomas, infection, nerve injury, and iliac fractures.
To avoid problems with autograft harvesting, the majority of surgeons switched to various freeze-dried bone from bone banks (allograft). Allografts are bone grafts obtained from cadavers and then sterilized. Sterilization prevents disease transmission and destroys all the endogenous bone growth stimulating molecules known as bone morphogenetic proteins. The bone bank donors are tested for Human Immunodeficiency Virus (HIV) and the freeze-drying process seems effective in destroying viruses. The fusion rates using allograft are not as high as with autograft. For single-level surgeries, the fusion rate using allograft is about 90% compared to 95% with autograft. For two-level surgeries, the allograft fusion rate is 72% versus 87% for autograft.
Most surgeons and patients have decided the advantages of allograft outweigh the disadvantages especially when allograft is combined with anterior cervical plate fixation. In order to avoid donor site complications, the use of allograft has increased substantially, resulting in a shortage of available allograft in the United States. Outside the United States, allograft has not been widely accepted for a variety of reasons.
Autografts and some allografts are also prone to a variable degree of collapse. Before fusion can occur, the grafts may weaken and can lose some of their height. Graft collapse can lead to a forward neck angulation known as kyphosis. There is an average of 20% loss in height at each level after autograft fusions. Different allografts have variable degrees of collapse, with iliac crest allografts exhibiting up to a 50% collapse in height. The use of anterior cervical plate fixation probably decreases the rate of graft collapse and reduces the risk of graft extrusion and spine angulation.