Measuring spinal canal size in lumbar spinal stenosis: description of method and preliminary results

Discussion

The lumbar spinal stenosis is the fairly common disease. Its incidence is about 1:1000 and this ratio has significantly increased in recent years due to the increase in life expectancy.^3–5 With introduce of new diagnostically methods into clinical practice, new radiological criteria of spinal canal assessment have been proposed, as well as areas and levels of measuring. In recent years MRI has become “gold standard” in diagnostic of the lumbar spinal stenosis, due to its possibility to visualize Roentgen-negative soft tissues. For quality research of one or another disease, sensitive and rational including criteria definition requires. However, for the lumbar spinal stenosis similar criteria still didn't develop.

Steurer et al.⁶ in their systematic review concluded about necessarily of well-defined, unambiguous radiological criteria of lumbar spinal stenosis to improve the quality of diagnostics and development of firm criteria for inclusion in clinical trials.

Genway et al.⁷ noted, that researchers use a variety of combinations of symptoms, clinical signs and radiological criteria for inclusion of patients in studies of lumbar spinal stenosis.

The North American Spine Society⁸ in their clinical recommendations noted, that medical visualization – a key method in diagnostic of lumbar spinal stenosis, but they didn't denote specific radiological criteria.

The goal of our study – to create a accuracy, informative, easy and objective evaluation method spinal canal narrowing degree in lumbar spinal stenosis. The essence of the method is that on MRI in segments L1 – L5 with measuring linear dimensions on axial scans cross sectional area of dural sac and lateral canals are calculated by the above formulas. We didn't include narrowing of spinal canal on L5-S1 level in our study, because of this part of spinal column significantly differs from lumbar spine in anatomical and functional aspects.

Variables used our study have been selected from 15 studies by CT and MRI. Criteria and their critical values for stenosis are listed below.

Anterio-posterior diameter of dural sac – distance between middle of vertebral body and middle of basis of processus spinosus at border of dural sac. This criteria has been described in 3 works ^9–11. Critical value varies due to different zones of measuring (<10 to <15 mm).

In certain study criteria “anterio-posterior diameter of spine canal” had been used, but the authors concluded that its diagnostic value was limited. Thus, Bolender et al.¹² suggested, that this parameter had lower value, than assessment of contrast zone of dural sac on mielogramms (20% vs 83% clear diagnosis). We think, that deformity of dural sac is more objective index of spinal stenosis, because compression neural elements itself causes clinical signs of stenosis. In the same time, narrowing spinal canal without deformity of dural sac doesn't mark of stenosis, because anatomy of spinal canal may vary in different patients.

Transverse diameter (size) of dural sac – distance between lateral borders of dural sac on the level of lateral canals. Criteria is evaluated in 2 studies.^{5, 13} Critical value for stenosis is <15 to <16 mm. Zones of measuring are different.

Cross-sectional area of dural sac – Criteria is evaluated in 7 studies.^{2, 4, 8, 12, 14–16} Zones of measuring are different. Critical value is <75 to <130 mm².

Intrafacet distance – distance between the inner surfaces of flaval ligaments on a line connecting the joint space of facet joints. Criteria is evaluated in 2 studies.^{9, 17} Critical values are <15 to <16 mm. Zones of measuring are different.

Depth of the lateral recess – distance between the superior articular facet and the top part of the pedicle. Criteria is evaluated in 2 studies.^{6, 18} Critical values are ≤ 3.6 to <2 mm. Zones of measuring are different.

Lateral recess angle - the angle between the lines parallel to the floor and the roof of the lateral recess. Criteria is evaluated in 1 study.⁶ Critical value is < 30°.

Furthermore, we proposed 3 own radiological criteria: level of measuring; cross-sectional area of left and right lateral canal and their summary. All measurements were performed by manual selection of evaluating area, which is a significant drawback of the methodology, due to the possibility of mistakes and subjective interpretation of images. To minimize the possibility of error, all measures were made for three times, after which arithmetic mean value has been calculated. Thus, this technique requires significant improvements to increase validity. This will be the purpose of our future studies.

As it's mentioned above, we used a fixed measurement level of vertebral canal: in the middle of the intervertebral disc and facet joints. Selecting the location of measurement is based on the position of the degenerative changes three-joint complex, that is defining stenosis in terms of its morphological substrate.¹³ In addition, in this zone yellow ligament can be visualized, and one can evaluate the degree of canal narrowing when prolapse or hypertrophy. From a mathematical point of view, the selected zone is the mid-distance between the centers of adjacent vertebral bodies. If the lumbar vertebrae is numbered from 1 to 5, the intermediate value measurement level L1 - L2 is equal to the number of 1.5, the level of L2 - L3 - number 2,5, etc. This parameter we include in regression equations as the independent variable, called “Level of measuring”.

In general, segmental division of cross-sectional area of spinal canal is well-founded. As well known, in lumbar spine the form of spinal canal is committed to triangular. Therefore, the degree of stenosis in various zones ambiguous in terms of relative values. Lee at al.⁴ described topographical anatomy of lateral canal, dividing it into 3 zones: enter zone, middle zone, and exit zone. As it has been proved, lateral canal is a complex structure and topographical anatomy in its various zones is not equal. Consequently, measuring the area of a lateral canal as an independent structure is justified and it's necessary to accurately assess the degree of stenosis of the spinal canal as a whole.

To assess the narrative validity of all the selected criteria we used analysis of linear bivariate correlation coefficients. The table above clearly shows that the strongest correlation is observed between linear values and area parameters. At the same time, the correlation between linear variables is weaker. However, there is no significant correlation of criteria “the angle of the lateral recess” with other linear variables and area parameters. As the result, this criteria was excluded from the study as uninformative.

Next, we found a mathematical relationship that revealed correlation interactions with regression equations. Area parameters were taken as dependent variables, because the greatest number of strong correlations with linear values. Accordingly, the linear values were taken as a function of the dependent variable. In fact, we obtained significantly adequate mathematical model of spinal canal stenosis at the level of the lumbar spine.

Due to small sample size to eliminate effect of parties narrowing of the lateral canal on correlation analysis, we introduced the parameter «the sum of the areas of lateral canals», thereby separated areas of lateral canals from the central area. This reduced the measurement error and gave an opportunity to assess narrowing of the lateral canals of a whole and to compare it with those of the central area.

Attempts to create such coefficient have been already taken elsewhere. So, Laurencin et al.¹⁴ introduced Coefficient of Stenosis as the ratio of the cross sectional area of the dural sac in motion segment to cross-sectional area of the dural sac in stable segment. This coefficient is rather subjective due to evaluation stability of the segment on static MRI in the horizontal (non-physiological) position. Moreover, it does not allow for condition of lateral canals.

We proposed a coefficient, that evaluating ratio of areas into spinal canal on one level. Its estimation doesn't require difficult mathematical operations and invasive diagnostic methods. In general, we believe, the conception of a “normal” cross sectional area is incorrect. The cross sectional of the spinal canal is always associated with individual anatomical and constitutional features. We believe that “Coefficient of Stenosis” is more objective parameter of spinal canal narrowing, due to not relative but absolute area values.

Another important aspect in our study is evaluating correlation between “Coefficient of stenosis” and clinical symptoms of disease. Currently, there're much numeric scales and questionnaires designed for objectifying clinical evaluation of the patients symptoms. Cleland et al.¹⁹ in their study compared psychometric properties of most frequently used numeric scales and questionnaires in diagnostic of lumbar spinal stenosis. Authors concluded, that most adequate psychometric properties had been seen for Oswestry Disability Index, Modified Swiss Spinal Stenosis Questionnaire and Patient Specific Functional Scale. Although authors concluded about necessarily of future studies for evaluation their specificity. The modified Swiss Spinal Stenosis Scale is considered a “gold standard” for evaluation of severity lumbar spinal stenosis.

In our study we used Oswestry Disability Index and Modified Swiss Spinal Stenosis Scale. We didn't use Patient Specific Functional Scale, because it assesses the effectiveness of treatment in certain time intervals, and it’s design isn't suitable for our study. For evaluation of multilevel lumbar spinal stenosis we used simple average value of “Coefficient of Stenosis”, we called “Mean Coefficient of Stenosis”. It was necessary to compare the questionnaires values with the spinal canal narrowing degree.

As mentioned above, we compared results of ODI and SSQ scales for two subgroups of patient based on “Coefficient of Stenosis” intervals. For evaluate differences between this two independent samples we used Mann – Whitney U-test (Table 3). As one can see, mean rank for ODI and SSQ is higher in Subgroup 1, so clinical picture in this group is more severe. In the same time, Mean Coefficient of Stenosis (mCS) value is higher for Subgroup 2, so spinal canal narrowing degree is less in this group. Z-criteria for all parameters is normalized with p < 0,05, so we can sure in significance of differences between this two subgroups.

This data confirm significant correlation of “Mean Coefficient of Stenosis” with Oswestry Disability Index and Modified Swiss Spinal Stenosis Scale, in other words - correlations between clinical symptoms and radiological findings.