Posterior Lumbar Interbody Fusion

Posterior lumber interbody fusion is an operation on the spine to remove bone from the back of the spine to relieve pressure on spinal nerves ( compression of the nerves can cause pain into your buttocks or pain into your legs) and then join two or more adjacent vertebra together. The part of the operation to remove the pressure from the nerves is called a decompression. The fusion portion of the operation is required where a surgeon thinks that the amount of bone that will have to be removed to get pressure off the nerves will lead to abnormal movement of the bones (instability), where the condition causing pressure on the nerves involves movement of one vertebra on the other (spondylolisthesis) or where narrowing of the disc between the two vertebra is leading to pressure on the nerves where they exit the site of the spine (the spinal foramen) and this pressure needs to be relieved by increasing the disc height to open up the foramen.

The operation is done under a general anaesthetic. Once you are asleep, a catheter is placed in the bladder. Electrodes are placed to various muscle groups in the leg and into the scalp to enable monitoring of nerve function during the surgery. You are then placed on your stomach on a special operating table. An incision is made in the middle of the spine. Muscles are taken off the back of the spine and the bones exposed throughout the planned levels of surgery. A separate stab incision is then made over the pelvis so that a pin can be placed in the bone. Attached to this pin is a reference frame. A special x-ray is then taken (like an intra operative CT scan). This allows special instruments to be used with the infrared navigation to place the screws in the spine with high level of accuracy. Once the screws are placed, bone is removed from the back of the spine. The bone is removed until the sac with all of the nerves and individual nerves are clearly free of pressure. Once this has been done, a retractor is placed pulling the nerve to the side. This exposes the back margin of the disc space. The back margin of the disc is cut open and spreaders of increasing size placed into the disc space to increase the disc height. The appropriate amount of opening is judged by feel. Once this has happened, special instruments are used to clean disc material from the bony ends of the vertebra. An implant is then placed into the disc space between the two vertebra to hold it open. The procedure is then repeated from the opposite side of the spine. Before a second implant is inserted, bone that has been removed from the back of the spine is ground up into fine gravel and packed into the disc space such that it sits between the two disc space implants. Often a chemical is added to the bone to stimulate bone healing. Once the second implant is inserted into the disc space, a rod is then placed between the screws on the back of the spine and fixed in place with locking nuts. The bones on the side of the spine are then roughened with a burr and some further bone graft placed along the side of the spine. An x-ray is taken to ensure the implants are nicely placed and the spine is well aligned as well as confirming the decompression of the nerves. A drain is usually inserted and the wound is sutured. You are then placed back onto the bed and taken to recovery.

NERVE ROOT INJURY

The surgery involves removal of bone and tissue from around the spinal nerves, and retraction of nerve roots to expose the disc space. This can result in stretching of a nerve. Less commonly the nerve can be partially cut or torn using instruments or implants used during the surgery. This can cause either pain, numbness or weakness ( or any combination of the above) in the distribution of a particular nerve in the leg.

DURAL TEAR

The dura is the sac lining the nerves and contains spinal fluid. This can be torn by instruments resulting in leakage of spinal fluid. Often this is simply repaired with a stitch. Some patients develop a headache for a few days, this would usually resolve. On rare occasions the leakage will continue and lead to a clear fluid discharge from the wound. If this occurs, further surgery to seal the leak may be required. In very rare cases it has been reported that some patients (usually elderly) have developed an unusual type of stroke after having had some leakage of spinal fluid. The mechanism for this is very poorly understood but in its simplest form is said to be due to pressure changes in the brain caused by loss of spinal fluid leading to shearing for some of the smaller blood vessels around the brain. Fortunately as above this is an exceptionally rare complication.

IMPLANT MALPOSITION OR FAILURE

Screws and wedges are placed into the spine, with small margins for error around nerves and other important structures. If the screws are placed too far towards the middle or bottom of the bone it can irritate a nerve running through the spine. If a screw is accidentally placed through the front of the spine the body’s major blood vessels (aorta and vena cava) can be damaged. This could potentially cause life threatening bleeding. It is possible that the screws can loosen in the bone or break requiring replacement. It is also possible the wedges between the vertebral body can sink into the bone with some resulting loss of position which at times can require further surgery.

PSEUDARTHROSIS

The aim of fusion is to get bone growing between the vertebral bodies. If bone does not solidly grow between the vertebral bodies this is called pseudarthrosis (false joint). If you are limited in the way of pain this is of no concern. If you have significant pain from the fusion not healing sometimes further surgery can be considered to promote bone healing.

ADJACENT SEGMENT DEGENERATION

A commonly held view is that spinal fusion will lead to early wear and tear on the joints either side and progressive arthritis. In reality this is a much more complicated process and depends upon age related progression of wear and tear changes in the spine, the type of surgery and the overall spinal alignment. If adjacent segment changes do develop over time resulting in stenosis or loss of alignment of the spine, extension of the surgery maybe indicated.

MEDICAL COMPLICATIONS

There are a variety of medical complications which can occur with major surgical procedures. The likelihood that these will occur to some extent is determined by your health before surgery and past medical history. Fortunately with modern anaesthesia and perioperative care, medical risks involved in elective surgery are comparatively low. Common surgical complications such as deep venous thrombosis, pulmonary embolism, pneumonia, urinary tract infection and skin pressure areas are best avoided with early mobilisation.

If you have any further questions regarding your procedure, please don’t hesitate to ask.

Rob Kuru, Spinal Surgeon