There are three essential aspects to understanding whether one is a good candidate for spinal fusion to treat their low back pain:
A patient history entails reviewing when the pain occurs, where it is located, how it began, the previous treatment and the extent to which it limits the patient's activities. Additionally, the physician will try to determine if other factors (such as depression) may be contributing to the patient's back pain. An individual's general health will also be reviewed as it can influence the role of spine surgery (e.g. heart or lung disease).
A physical examination is done to determine whether there is evidence for any neurologic (nerve-related) injury.
There are a number of diagnostic studies that are available to investigate the etiology (medical cause) of the pain. The most common study is an X-ray of the low back, which can show if there is some boney instability or deformity to the spine. It can also image such things as a fracture, or in advanced stages, it can show tumors of the spine. Although done frequently, they usually fail to demonstrate the cause of the back pain.. Almost all individuals need additional studies to establish an anatomic diagnosis.
The gold standard is to follow X-rays with magnetic resonance imaging (an MRI scan). MRI scans provide very precise anatomic information about the health of the discs as well as presence of any tumors or compression on the nerves.
The difficulty with this study, as with many others, is that "abnormalities" that show up on an MRI scan may not be the cause of the pain. Most often the disc degeneration identified on an MRI is a normal finding, secondary to aging, which occurs in more than 50% of people in their 40's.
For a small percentage of people an MRI scan cannot be safely performed, e.g., if the patient has a pacemaker. In these cases a CT scan with myelogram may be done. The anatomic information from a CT scan with myelogram is very similar to that of an MRI scan. A CT myelogram is also sometimes ordered as an adjunctive study to a MRI scan as it can show very subtle nerve root compression and also images out in the foramen better.
A discogram, which is a more controversial study, may also be performed before spine fusion surgery. This study involves inserting a needle into the disc and injecting dye. If this process causes the patient's normal pain to occur, it is presumed that the specific disc is the anatomic cause for the pain.
The difficulty with discography is that it is not entirely objective, and its effectiveness is quite dependent upon the person doing the discogram. Also, it is clearly an unpleasant test to undergo. Unfortunately there is probably a high incidence of false positive discograms, although the true rate is not known. Currently, however, it probably is the most reliable test available to add anatomic precision to the diagnosis of mechanical low back pain.
Many factors can result in pain production during a dicograms, not only the true anatomic source of the problem. A discogram is a test to confirm the pathology, rather than one to "find the cause of the pain." When done properly, a painless "control" level should be performed.
Other studies may also be performed before a spine fusion surgery, such as an electromyography (EMG), particularly in situations where there is a large amount of leg pain. This study involves placing needles within the legs to determine whether the nerves are working properly and to help identify which nerve is compromised.
Another study to try to determine the specific nerve that is a problem is a procedure to block that specific nerve. A selective nerve root block (SNRB) can be quite helpful, particularly in situations where there is evidence on other studies of compression of many nerves.
Piecing together all of the indications to determine the potential need for a spine fusion surgery is obviously quite complex. It involves compiling results from history, physical exam and diagnostic studies. While the information does not always produce a clear indication for spine fusion surgery, certainly the best opportunity for improvement with a spine fusion occurs when all the pieces of the puzzle fit neatly together.