Lumbar spinal stenosis is a relatively common medical problem, but optimal treatment for the condition is poorly understood. Lumbar spinal stenosis is commonly treated with decompression surgery with or without additional fusion surgery. 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 experience to conclude if 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.~Recently, an objective metric called the Sagittal Plane Shear Index (SPSI) has been developed and described in the scientific literature. SPSI quantifies the magnitude of sagittal plane translation-per-degree-of-rotation (TPDR) of the posterior-inferior corner of a vertebra in a direction defined by the superior endplate of the immediately inferior vertebra. The TPDR calculations are based on flexion-extension radiographs: one (flexion) radiograph during which a patient bends forward, and one (extension) radiograph during which a patient bends backwards.~The TPDR is reported as the number of standard deviations from the average found at radiographically normal levels in asymptomatic volunteers. The resulting SPSI metric informs the orthopaedic surgeon about whether the spine is stable in and of itself (necessitating decompression surgery only) or whether there is spinal instability (necessitating decompression and additional fusion surgery). Because additional fusion procedures can add substantial expense and morbidity to the surgery, it is important to avoid fusion when possible. As such, knowledge about spinal (in)stability is critical for selecting the most appropriate surgical procedure for treatment of lumbar spinal stenosis.~The SPSI metric can be obtained in routine clinical practice using translation and rotation measurements obtained with a validated computer-assisted method named Quantitative Motion Analysis (QMA®). Recently, a fully automated version of this method (Spine CAMP) has been developed, which provides the same SPSI metric analysis using neural networks and coded logic without any human intervention. SPSI metric values > 2 informs a clinician that the TPDR is > 2 standard deviations above the average TPDR in the radiographically normal, asymptomatic population. This 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 radiographically normal, asymptomatic volunteers. If SPSI is > 2 at a level where surgical treatment for stenosis is planned, then the clinician has objective evidence of abnormal motion and this may help to determine whether fusion should be added to the decompression surgery and what type of fusion surgery should be performed. SPSI is intended only to help with treatment planning and the clinician is required to interpret the metric in context of all other relevant clinical factors.~The QMA® method has previously been validated to have the accuracy and reproducibility required for measuring the small translations that occur in a healthy spine. Spine CAMP has been documented to provide rotation and translation measurements equivalent to QMA®. A clinical investigation is also currently underway in The Netherlands to test the potential clinical efficacy of the SPSI in diagnosing abnormal motion, in order to use this diagnosis in the decision process on whether to add fusion to decompression of a stenotic lumbar level. However, evidence supporting the repeatability of SPSI measurements, and equivalence of SPSI from Spine CAMP versus QMA®, is lacking and therefore needed.
Orthopaedic surgeons are often faced with the clinical dilemma of whether or not to add fusion to a decompression procedure. To decide between these two surgical options, surgeons rely mostly on their experience to conclude if a level is unstable preoperatively or if a specific decompression procedure is likely to destabilize the spine. Recently, the Sagittal Plain Shear Index (SPSI) has been developed as a valid test for determining the degree of spinal (in)stability. The SPSI metric, which can be calculated using flexion-extension radiographs of the lumbar spine, informs the orthopaedic surgeon about whether the spine is stable in and of itself (necessitating decompression surgery only) or whether there is spinal instability (necessitating decompression and additional fusion surgery). The SPSI metric can be calculated using both the validated semi-automated QMA® and more recently developed fully automated Spine CAMP software platforms. The concurrent validity between these two software platforms, as well as the reliability of both of these objective diagnostic indicator for spinal instability have not yet been evaluated. This study will investigate if SPSI-metrics/values obtained with Spine CAMP are equivalent to measurements from QMA®, and will also investigate the repeatability of two measurements of the SPSI taken one hour apart ('test-retest reliability').