The Sagittal Plane Shear Index (SPSI) for planning whether to fuse
after decompressing a stenotic lumbar level

Lumbar spinal stenosis is a relatively common medical problem, the clinical
outcomes are often suboptimal, and the patient-specific, optimal treatment for
the condition is poorly understood, in part due to a lack of validated
diagnostic tests . Decompression surgery with or without additional fusion
surgery are common surgical treatments for lumbar spinal stenosis. The
decompression part of the surgery is generally accepted as essential to
achieving clinical benefit by relieving the physical source of stenosis. Fusion
surgery, if indicated, is typically performed as part of an expanded surgical
procedure (decompression plus fusion).

There are two generally accepted hypotheses for justifying fusion surgery in
addition to decompression surgery. First, fusion is justified if the level
being decompressed was unstable pre-operatively. Unstable is typically defined
as intervertebral motion above the range of motion expected in healthy and
asymptomatic spines. Second, fusion surgery is justified if needed to prevent
complications from iatrogenic instability that might be created by the
decompression surgery. The first justification is supported by studies
documenting an association between abnormal intervertebral motion and symptoms.
Correcting a known potential source of symptoms (abnormal intervertebral
motion) during the same surgery used to correct the primary source of symptoms
(stenosis) is rational. The challenge with the first justification has been the
lack of a validated, objective test to diagnose preoperative instability.

Neither of these two justifications for supplemental fusion surgery has been
validated by rigorous scientific methods, partly due to the lack of a validated
diagnostic test for spinal instability. Surgeons are currently faced with the
dilemma of whether or not to add fusion to a decompression procedure. Surgeons
currently rely mostly on their experience of interpreting X-ray images to
decide if a level is unstable preoperatively or if a specific decompression
procedure is likely to destabilize the spine.

An objective metric called the Sagittal Plane Shear Index (SPSI) has been
recently developed by Medical Metrics Diagnostics, Inc. Based on two X-ray
images, a flexion and extension image of the spine, the software calculates
intervertebral motion. More specifically, the SPSITM metric quantifies the
sagittal plane intervertebral translation-per-degree-of-rotation (TPDR), and is
reported as the number of standard deviations from the average found at
radiographically normal levels in asymptomatic volunteers.

The metric provides for an objective diagnostic indicator for spinal
instability defined as a specific, well-defined intervertebral motion metric
that is outside the 95% confidence interval established for asymptomatic
volunteers. This gives the clinician objective evidence of abnormal motion and
this may help to determine whether fusion should be completed in addition to
the decompression surgery.

To assess the proportion of lumbar spinal stenosis surgical treatment plans
that change when an objective measurement of spinal stability is included and
applied following a simple treatment algorithm

This is a prospective, multi-center, single arm clinical investigation

Flexion- and extension radiographs are performed at the hospital, and will be
send to Sponsor for calculation of the SPSI. Based on the SPSI, the surgeon
will adjust the surgical plan as required. The planned surgery is performed
according to standard of care of the hospital. At the 12-month follow-up
flexion- and extension radiographs are performed to calculate post-surgical
SPSI. Additionally, during the follow-up visit at 12- and 24-months the
clinical outcome will be evaluated using the Oswestry Disability Index (ODI)
and Numerical Rating Scale - Leg Pain (NRS-LP) questionnaire. Information
regarding reoperation, analgesic use and adverse events is also obtained.

If the measurement of preoperative SPSI is the reason that a surgeon does not
fuse a level that should have been fused, or does fuse a level that should not
have been fused, then it is possible that the patient will have worse clinical
outcomes or a higher probability of reoperation.

Investigators in the clinical investigation can choose not to implement a
change in the surgical plan that the per-protocol use of preoperative SPSI
would require. For example, if the SPSI metrics for the stenotic level suggest
that there is abnormal intervertebral motion and that fusion should be added to
the decompression surgery, but the investigator is certain that the level is
stable or that there is another reason not to add fusion surgery, then the
investigator is free to do what they believe is best for the subject. In that
scenario, the surgeon would record the specific reason for not following the
clinical investigation plan. The experience and clinical judgement of the
investigator will help to mitigate risk.

Anticipated clinical benefits of the use of the device would be improved
reduction in preoperative symptoms and lower probability of reoperation. In
addition, there may be a reduced need for unnecessary fusion surgery if the
SPSI reports document normal motion where the surgeon had suspected abnormal
motion.