Spinal fusions are indicated for patients with low back and/or leg pain due to degenerative disc disease or instability that is refractory to conservative treatments. Why a degenerated disc produces symptoms is somewhat unclear. As the disc loses water (which appears as darkness on the MRI), it begins lose height which produces redundancy in its covering (annulus, see anatomy).
This may produce chronic tearing of the disc covering and pain. As the vertebra settle due to disc height loss, contact pressure on the posterior joints (facets) increases which incites inflammation and pain. Currently, spinal fusion is the only proven surgical techniques which improves the pain of degenerative disc disease.
In the past, most fusions were performed posteriorly utilizing autologous bone and commonly supplemented with fixation devices such as pedicle screws and rods. Posterior fixation devices have been shown to enhance fusion rates and facilitate postoperative patient mobilization. Wide exposure of the posterior spinal elements is required for implantation of these devices with resultant damage to the paraspinous musculature. Often times this muscular damage may limit the ultimate benefits of the fusion operation.
In response to the shortcomings of posterior spinal fusion, new anterior fusion techniques have been developed. As one of the Food and Drug Administration investigators for these new techniques, I would like to share my experience with an anterior fusion fixation device which I feel will revolutionize our current thinking in spine surgery.
An anterior subumbilical incision measuring approximately 2 inches long is made horizontally in the abdominal wall between the rectus musculature. No muscle is cut or distorted during the spinal approach. A abdominal retroperitoneal approach to the lumbar spine is achieved. Very little bleeding is encountered during exposure of the spine. The device is packed with autologous bone and inserted, usually in 30 minutes.
Postoperatively the patients are allowed to ambulate the night of surgery and are discharged on postoperative day two. This is compared with 5-7 hospital days with standard posterior fusions and 4-5 hours of operative time. Blood loss is minimal and patients rarely require transfusions.
In 1995, I was asked by the cage manufacturer to do a safety and efficacy study on anterior interbody fusion devices for the purpose of gaining FDA approval. 65 patients were done on the initial study. All patients had only one level fused. All patients fused, and 98% of patients reported good and excellent results. All patients had relief of preoperative symptoms. There were no infections and only one patient was returned to the operating room for minor bleeding.
Since that time, I have done approximately 400 of these procedures with similar results. What has been learned is that if more than one level is fused with an anterior interbody fusion technique, the results degrade proportional to the number of levels attempted. Two level fusions are successful in select patients, but three level anterior interbody fusions without posterior augmentation, should rarely, in my opinion, be attempted.
Two main categories of devices are available. Cages made of titanium and allograft (cadaver bone) are FDA approved and implanted in the United States. I have used both successfully and have come to appreciate the relative benefits of each device. Allografts have the theoretical advantage of healing primarily to the vertebra whereas titanium can only act as a carrier for bone placed within it. Allografts degrade postoperative MRI scans less than metal cages which is important if the patient should need studies in the future. Both cages types require CT scans to conclusively assess complete fusion within the device. Allografts have a theoretical disadvantage as being a carrier of infection or target of rejections but these complications, as far as I know, have not been reported.
Theoretically, as a patient becomes older and relatively osteoporotic, a metallic cage could collapse into the adjacent vertebra. Because allografts are incorporated, this probably would not happen. Before surgery, I present both options to my patients and allow them to decide the best choice for them.
Many patients ask about laparoscopic placement of cages. I do not do it for the following reasons. My operative time with open procedures is 60-90 minutes. Laparoscopy, in anyone’s hands, takes much longer. The limiting factor in incision size is the size of the device for both procedures and since the device is equal size, in my hands, the incisions are about the same. The published complication rates are much higher for laparoscopic cases even in the most experienced hands. Finally, since most insurance plans will currently allow patients to stay in the hospital for 1-2 days following one of these procedures, the theoretical advantage of allowing patients to be discharged sooner with laparoscopy, may benefit all parties except the patient.