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MMI Investigates Spinal Instability Using the Sagittal Plane Shear Index (SPSI)

October 11, 2023
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In a recently published research letter in Spine, MMI collaborated with a multicenter group of researchers to investigate the sagittal plane shear index (SPSI) as an objective measure of instability in lumbar spinal stenosis patients with degenerative spondylolisthesis.

Key Highlights:

  • Traditionally, spine motion has been described as separate measurements of rotation and translation per level. There are currently no universally accepted thresholds for defining spinal instability using these two parameters.
  • SPSI is a standardized metric that not only accounts for the natural mechanical coupling between sagittal plane rotation and translation, but also the variation in motion across different levels of the spine using a normative reference dataset.
  • SPSI was demonstrated to be different between treatment and nontreatment levels in lumbar spinal stenosis patients with spondylolisthesis, suggesting that the objective metric has potential to distinguish those levels requiring treatment.

The question at hand: Can we objectively quantify intervertebral motion in a manner that enables us to reliably detect abnormal motion patterns indicative of spinal instability?

With two decades of spine experience and as a leader in the spine clinical trial space, MMI is proud to continue the advancement of spine research in the newly published research letter, “Evaluating the Prevalence of Motion Abnormalities at Treatment Levels and Nontreatment Levels in Lumbar Stenosis and Spondylolisthesis Patients”, co-authored by MMI’s Diagnostics Enablement Manager, Trevor Grieco, Ph.D.

Lumbar spinal stenosis is a condition where changes in the spine causes impingement of the spinal cord, and it affects upwards of 11% of older adults in the United States. Often associated, spondylolisthesis is a disorder where a vertebra slips over another. These often progressive conditions can lead to debilitating pain, leading to surgical and non-surgical interventions. A variety of methods exist to manage symptoms or directly treat the issue, but a lack of objective understanding of the nature of spinal instability can cloud the correct treatment decision. Traditional methods of assessing intervertebral motion include measurements of rotation and translation in the sagittal plane. Unfortunately, there are no universally accepted thresholds for these two metrics that are often used to diagnose spinal instability.

The Sagittal Plane Shear Index (SPSI) is a standardized metric based on the translation per degree of rotation (TPDR) measured at a spinal level. This metric was designed to provide clinicians with a biomechanical signal that could be objectively used to compare a patient’s TPDR to normative reference data, thereby enabling a quick and easy identification of levels that are within normal limits and those that are outside of normal limits.

In a retrospective pilot investigation, MMI used our proprietary technology, Quantitative Motion Analysis (QMA®), to accurately and reproducibly determine SPSI from standard-of-care flexion/extension X-rays in single-level lumbar spinal stenosis patients with spondylolisthesis that were treated with spinal fusion. Preoperative SPSI at the levels that were subsequently operated on (treatment levels) were pooled and compared to the preoperative SPSI from non-operative levels (nontreatment levels) to investigate the prevalence of abnormal TPDR in this patient population. Treatment levels had an average SPSI value near the upper limit of normal, with more than 54% of levels exhibiting abnormal motion. In contrast, only 17% of nontreatment levels showed abnormal motion. In this letter, the researchers surmise that this difference “suggests that the normative database generalizes in a manner that may be clinically useful”. With additional research on an appropriate and clinically relevant threshold, SPSI has the potential to become an objective metric of spinal instability to drive treatment decisions.

Access the full letter here.

Medical Metrics, Inc. is an ISO 9001:2015-certified provider of independent imaging core lab services and a leading core lab for spine and orthopedics clinical trials and related consulting. We assist sponsors with designing, implementing, and executing the imaging strategy of their global trials through our responsive trial management team, robust operating infrastructure, and world-class imaging expertise. Learn more about our Spine expertise and our Services.

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