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.