Lumbar spinal stenosis, a narrowing of the spinal canal in the lower part of the back, is a relatively common medical problem, but optimal treatment for the condition is poorly understood. Lumbar spinal stenosis is commonly treated with decompression surgery (providing more space for the compressed nerve roots) with or without additional lumbar fusion surgery (connecting two or more vertebrae to eliminate instability). Orthopaedic surgeons are currently faced with the dilemma of whether or not to add fusion to a decompression procedure. To decide between these two surgical options, surgeons rely mostly on their personal experience. They have to subjectively gauge whether a level is unstable preoperatively or if a specific decompression procedure is likely to destabilize the spine. A valid and reliable test for spinal instability would facilitate research to determine whether spinal instability measurements can be used to choose the optimal surgical treatment for each level.
A reliable diagnostic assessment for spinal instability would ideally be validated using an existing "gold standard" test. Such a test should have a very high sensitivity and specificity and must be supported by clearly developed rationale and high-quality evidence. Unfortunately, such a test does not currently exist for spinal instability. However, abnormalities in intervertebral motion are often a hallmark for spinal instability. A commonly accepted way to examine intervertebral motion is by analyzing the patient's flexion-extension radiographs. Using these two radiographs, the spine's sagittal plane intervertebral motion can be measured. The relative motion between the two images can be measured at all relevant levels either through manual measurement or computer assisted methods. Ideally, a standardized flexion-extension radiograph imaging protocol accomplishes the following: the patient sufficiently stresses the spine in order to assess the integrity of intervertebral motion restraints (ligaments, annulus, facet capsule, etc.), it is easily implemented into the clinical work flow, and it is reliable and repeatable across subjects and clinicians.
A myriad of patient positioning protocols for the flexion-extension radiograph imaging procedure have been tested and deployed, although generally without well-validated success criteria. Despite the large amount of studies on the topic, no one flexion-extension radiograph imaging protocol has been widely accepted. This suggests that 1) the studies were not appropriately powered to influence change, 2) some protocols (methods, equipment, instructions, etc.) are not easily implemented or practical in routine clinical workflows, 3) acquiring radiograph images of the lumbar spine in sufficiently stressed positions is not widely accepted as clinically important, or 4) most ordering clinicians do not realize how often a patient fails to adequately stress the spine while undergoing a standard of care flexion-extension radiograph. One factor that can often cast doubt on the efficacy of the flexion-extension radiograph procedure is the level of patient effort or range of spinal motion achieved by the patient. For standing flexion-extension radiographs, more patient effort leads to more intervertebral motion. This is a fairly simple and intuitive concept. However, achieving maximum patient effort is not a simple task. It could be argued that symptomatic patients should not be expected to exert maximum effort because it can be painful or uncomfortable. This argument can be supplemented with evidence that fear avoidance can limit motion and intervertertebral motion is substantially increased after analgesic injections.
Despite some of the arguments that can be used against the implementation of standardized flexion-extension radiograph imaging protocols in assessing spinal instability, there is ample data that good patient effort can be obtained in symptomatic patients. In one large multi-site study of lumbar stenosis with the exact same patient inclusion/exclusion criteria, a large amount of variance can be seen in the average level of intervertebral rotation. This suggests that differences in the flexion-extension radiograph protocol could be the driving factor in this high amount of variance seen between sites. It is also possible that the physician and/or radiology technician helped to quell patient's fear of motion by explaining that the maximum flexion and extension will not injure their back and will provide the best data for a reliable diagnosis.
With this in mind, the need for a standardized flexion-extension radiograph imaging protocol to assess for spinal instability is evident. Such standardized protocols can influence the amount of intervertebral motion, both translational and rotational. Since intervertebral motion is often used as evidence for spinal instability, inadequate effort likely leads to false negatives for spinal instability. Thus, developing a standardized imaging protocol to reliably measure intervertebral motion during a flexion-extension exam is clinically significant, as it often influences the diagnosis of a patient.
