Another type of spine surgery is spinal fusion. The diseased disc and lamina are first removed. Pieces of bone are removed from your hip (donor) and are placed along the spine and between the vertebrae. This is called bone grafting. When the bone heals, this is called a bone fusion and the vertebrae no longer move separately. This fusion takes three months to heal.
A new type of fusion is a BAK cage fusion. First, the diseased disc is removed. Two BAK cage devices are placed in the opening where the disc has been removed.
The BAK cage device is a hollow threaded cylinder with holes and is made of titanium. The devices are then filled with bone taken from the lamina. Bone grows through the holes to fuse your vertebra above with your vertebra below.
Indications for Spinal Fusion
When a disc ruptures, the hydraulic effect of the disc is disrupted. The facet joints (the joints between two vertebrae), muscles, and surrounding ligaments are required to take over the job of the disc. If the disc does not heal, it is said to be degenerative. A degenerative disc is not able to support the weight of the body and the space between vertebra narrows. When the space between two vertebra narrows, so do the holes (or foramen) that the nerves pass through. This causes the nerve to be pinched and results in leg and/or back pain. Over time the facet joints become arthritic, get larger, and develop bone spurs.
This is called spondylolisis and narrows the formen even further. Finally, as the facet joints become arthritic and lose their cartilage, they begin to slide on one another. This allows one vertebrae to “slip” on the other, narrowing the hole even more. This kind of slipping and narrowing is a dynamic process and is worse when sitting or riding in a car and is called spondylolisthesis.
When a nerve is pinched by a ruptured disc, the disc material can be removed to relieve pressure on the nerve (laminectomy and discectomy). When the disc is degenerative and the nerve is pinched by bone (from narrowing of the disc space and foramen, spondylolisis, and slipping or spondlylolisthesis), spinal fusion is indicated to relieve pressure on the nerve and keep the vertebra from slipping.
With the development of fusion cages, it is now much easier to relieve pressure on a pinched nerve, keep the vertebra from slipping, and getting the fusion to heal. The fusion cages can be put in from the back or from the front. We prefer to put our cages in from the back because, the nerves can be seen better and protected better during surgery and the holes (foramen) can be made bigger allowing the nerve more room.
The BAK cage allows the patient to be up and about without a hard plastic brace. However, the BAK fusion cage depends on the bone healing from one vertebra through the cage to the other vertebra. It is imperative that the patient not smoke. Smoking decreases blood supply (because of the nicotine). Spinal fusions require a good blood supply to heal and the process can take up to 4 months.
Once the pinch on the nerve is removed, the patients legs feel better and stronger almost immediately. The patient will experience moderate back discomfort, however. A lumbar corset can be worn for support. The intense back pain resolves quickly (2-3 days), but the residual nagging back ache lasts up until the fusion is healed.
In active, younger individuals, the demand placed on the lumbar spine may be greater than what cages alone can support. Secondly, in patients who have already had back surgery, it is very difficult to create enough room for two titanium cages without risk of nerve injury. This is due to scarring around the nerves from the previous surgery.
For these reasons, a 360 fusion is used. The 360 fusion is more stable than the cages by themselves and requires less retraction of the nerves to implant. In a 360 fusion, only one implant (titanium, bone, or carbon fiber) is placed between the vertebral bodies on an angle. Strength testing has shown that one cage on an angle can be just as strong as two cages straight in, once the bone heals. However, one cage cannot stand by itself. It needs to be supported with screws and rods. So, the advantage of 360 fusion is that less room is needed to place the implant, but the disadvantage is that screws and rods are needed for support. With screws and rods in place, more twisting, bending, and lifting is permitted once the fusion heals.
Some surgeons do a 360 fusion in two parts. The cage is put in from the front, then the patient is turned over and the screws are put in from the back. We do this procedure entirely from the back, requiring only one incision. Also, the carbon fiber cage is gaining popularity with a lot of surgeons (us included). With the carbon fiber, you can see the bone inside better on x-ray and it is easier to determine if a fusion is successful. With the titanium cage, it is much more difficult to see the bone and serial CT scans are required.
360 Fusion pictures courtesy of Depuy/Acromed.
Pedicle screws can be used alone or in conjunction with other implants for a 360 fusion (see above). These titanium screws are placed from the back into the pedicles (strong, bony bridges from the spinal column in the back to the vertebral body in front). Each patient’s pedicles are of different size, so the screws are available in different diameters. A drill is used to prepare the bone before the screw is placed. Two screws are placed into each bone (one in each of two pedicles). For a one level fusion then, four screws would be needed, two in each bone. Rods are then attached to the screws and the disc space is spread apart. Once the disc space is distracted (to allow more room for the nerve), the rods are secured to the screws to hold the spine in its new position. Bone graft is then placed outside the pedicle-screw-rod assembly (away from the nerves) to cause the two bones to heal or “fuse” together. The screws and rods hold the bones in place until the bone graft fuses, effectively forming one larger bone from two.
When used in conjunction with other implants (BAK, carbon fiber, etc.), the implant is placed in between the vertebral body to give another point at which the bones can fuse together. The stabilizing device also gives each bone a third point of stability (the implant in front, and the two screws in back) forming a triangle.
Pedicle screws have some significant disadvantages. This is quite a bit of metal and acts a bit like a barometer. Even years later patients can have mild discomfort with weather change. Additionally, the process of placing the screws is technically demanding. Pedicle breakage, while not common, can occur. Also, the placement of screws infrequently creates excess scar tissue in some patients. This scar tissue can irritate the nerves, producing symptoms similar to what a pinched nerve does.
In smoking patients especially, there is the possibility that the bone graft does not heal. While this is not ideal, the screws will hold the spine together for a long time. Without a fusion however, the screws have to take all of the stress of supporting the spine and over time can work loose, or even worse, break.
Your surgeon can help you decide if screws are right for you. In a younger, working patient who has to do a lot of rotation, the additional support of screws may be necessary for that patient to successfully return to work. Generally, the younger the patient and the more degenerative the disc, the more likely screws will be recommended.
Older patients can benefit from screws, too. However, an assessment of your bony quality is important to insure that your bone is strong enough for screws to hold onto. Older women (> 70-75 yrs), especially with early osteopenia (leading to osteoporosis), are not good screw candidates, because their bone is softer. Soft bone is not dependable and those patients will be recommended fusion using different techniques.