Development of a novel radiographic measure of lumbar instability and validation using the facet fluid sign

Abstract

Background: Lumbar spinal instability is frequently referenced in clinical practice and the scientific literature despite the lack of a standard definition or validated radiographic test. The Quantitative Stability Index (QSI) is being developed as a novel objective test for sagittal plane lumbar instability. The QSI is calculated using lumbar flexion-extension radiographs. The goal of the current study was to use the facet fluid sign on MRI as the "gold standard" and determine if the QSI is significantly different in the presence of the fluid sign.

Methods: Sixty-two paired preoperative MRI and flexion-extension exams were obtained from a large FDA IDE study. The MRI exams were assessed for the presence of a facet fluid sign, and the QSI was calculated from sagittal plane intervertebral rotation and translation measurements. The QSI is based on the translation per degree of rotation (TPDR) and is calculated as a Z-score. A QSI > 2 indicates that the TPDR is > 2 std dev above the mean for an asymptomatic and radiographically normal population. The reproducibility of the QSI was also tested.

Results: The mean difference between trained observers in the measured QSI was between -0.28 and 0.36. The average QSI was significantly (P = 0.047, one-way analysis of variance) higher at levels with a definite fluid sign (2.3±3.2 versus 0.60±2.4).

Conclusions: Although imperfect, the facet fluid sign observed may be the best currently available test for lumbar spine instability. Using the facet fluid sign as the "gold standard" the current study documents that the QSI can be expected to be significantly higher in the presence of the facet fluid sign. This supports that QSI might be used to test for sagittal plane lumbar instability.

Clinical relevance: A validated, objective and practical test for spinal instability would facilitate research to understand the importance of instability in diagnosis and treatment of low-back related disorders.

Keywords: Instability; QSI; fluid sign; lumbar spine; radiographic.

Fig. 1

TPDR is calculated as the sagittal plane translation, between flexion and extension, of the posterior inferior corner of the superior vertebra in a direction defined by the superior endplate of the inferior vertebra. The left half of an L5 vertebra is shown, reconstructed from CT. The left half of the L4 vertebra is shown in the flexed and extended position relative to L5. In this example, L4 rotated 13° between flexion and extension. The posterior corner of L4 translated 26% of the L5 endplate width, so the TPDR is 26/ 13 = 2% endplate width/degree. Average TPDR in the asymptomatic population was 0.53±0.14, so QSI = (2.0 – 0.53)/0.14 = 10.5. This would be considered very abnormal.

Fig. 2

An out-of-plane (OOP) index was calculated for each level based on the distance between the apparent widest separation between the left-most and right-most edges of the vertebral endplate (white line with arrows on either end) divided by the anterior height of the vertebral body (black dashed line). Ideally, the OOP index would be zero. The most OOP endplate at the level where QSI was measured was used for this calculation. The calculation was made for the worst OOP in the flexion or extension radiographs.

Fig. 3

The average QSI at the L4-5 level at preoperation for patients with no evidence of fluid in the facet joint, possible fluid in the joint, and with a definite fluid sign. The error bars show the standard error.