The effect of body mass index on lumbar lordosis on the Mizuho OSI Jackson spinal table

Abstract

Study design: Prospective cohort study.

Clinical question: Does the patients’ body mass index (BMI) influence the degree of intraoperative lumbar lordosis in patients undergoing operative treatment on the Mizuho Orthopedic Systems Incorporated (OSI) Jackson spinal table?

Methods: Twenty-four consecutive patients undergoing posterior spinal instrumentation and fusion on the Jackson table, excluding those with sagittal malalignment, underwent standing preoperative and prone intraoperative lateral x-rays. Intervertebral body angle measurements were obtained from L1 – S1 using the modified method of Cobb. Changes in angle measurements were compared to BMI using linear regression and ANOVA.

Results: We found a mean lordosis of 52.6 ° in standing preoperative x-rays compared to a prone position mean lordosis of 61.5 ° on the Jackson table. The mean change was 8.88 ° with a range of 0° – 18 °. A linear association between lordosis and BMI was demonstrated (P < .0022). As BMI increased, so did lordosis (correlation coefficient, 0.59).

Conclusions:The current study is the first in which a correlation of patient body mass and use of the Jackson table has been evaluated. These data suggest that BMI influences lumbar lordosis on the Jackson table and that care must be used when dealing with a population with large BMI on the Jackson table.

References

• 1 Centers for Disease Control and Prevention .Health weight. It’s not a diet, it’s a lifestyle.  http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/; last accessed March 11 2010
• 2 Cochran T, Irstam L, Nachesmson A. Long term anatomic and functional changes in patients with adolescent idiopathic scoliosis treated by Harrington rod fusion.  Spine. 1993;  8 577-584
• 3 Hayes M, Tomkins S, Herndon W. et al . Clinical and radiographic evaluation of lumbosacral motion below fusion levels in idiopathic scoliosis.  Spine. 1988;  13 1161-1167
• 4 Davies A G, McMaster M J. The effect of Luque rod instrumentation on the sagittal contour of the lumbosacral spine in adolescent idiopathic scoliosis and the preservation of a physiologic lumbar lordosis.  Spine. 1992;  17 112-115
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• 6 Phillips W A, DeWald R L. A comparison of Luque segmental instrumentation with Harrington rod instrumentation in the management of idiopathic scoliosis.  Orthop. Trans ;  1985 9 437-438

Editorial staff perspectives

This is a CoE II prognostic study.

This is a novel study which provides a valuable perspective on the need to consider patient body habitus and its potential impact on maintaining appropriate lordosis. Certainly the finding that lumbar lordosis disproportionally increases in patients with higher BMI's when positioned prone on a Jackson spinal table, which leaves the abdomen freely suspended, is noteworthy for intraoperative consideration. To further evaluate this phenomenon and provide context for these findings, a few methodological points deserve consideration:

What is a „strong” correlation? A correlation of 0.59 may not be considered a „strong” correlation. The sample size is small and addition of a correlation line to the scatter plot would confirm that there is a lot of variation around it. Particularly in a cohort where potentially confounding factors (eg, age, sex) were not formally evaluated, the estimate of correlation (and R-squared value reported) should be interpreted cautiously.

Statistical methodology:

While authors report an R-squared based on linear regression, no information on the regression model is provided. Details of the model used to generate the R² and P-value should be described. Is this based on regression model that only has BMI in the model? If there were other variables in the model, it should be stated what was included (additional variables also influence R²). R² is probably not a clinically meaningful number. It tells you that for the particular model, a percent of the change is explained by the combination of factors in the model...and the rest is not explained by the model. R² is model dependent and there are number of other aspects of the model (and fit) that need to be considered.

Lack of comparators:

Although the mean BMI of the patients is high (33), the evaluation was not exclusively done in obese or overweight patients (there are some patients with BMI < 30) and there isn't a comparison of change in lordosis among obese with non-obese patients, including those considered normal with respect to BMI. While BMI is a commonly used indicator of obesity, it measures total body mass and doesn't take into account lean muscle mass, which varies for men and women and with age. Factors other than obesity may affect lordosis, such as trunk length and ligamentous laxity (eg, Marfan's syndrome and Ehlers-Danlos patients). These potential confounding factors should be considered in further studies. To the extent that clinical factors may influence both BMI and change in lordosis, these factors should be measured and evaluated.