The effect of growth on the correlation between the spinal and rib cage deformity: implications on idiopathic scoliosis pathogenesis

Abstract

Background: Numerous studies have attempted to quantify the correlation between the surface deformity and the Cobb angle without considering growth as an important factor that may influence this correlation. In our series, we noticed that in some younger referred children from the school-screening program there is a discrepancy between the thoracic scoliometer readings and the morphology of their spine. Namely there is a rib hump but no spinal curve and consequently no Cobb angle reading in radiographs, discrepancy which fades away in older children. Based on this observation, we hypothesized that in scoliotics the correlation between the rib cage deformity and this of the spine is weak in younger children and vice versa.

Methods: Eighty three girls referred on the basis of their hump reading on the scoliometer, with a mean age of 13.4 years old (range 7-18), were included in the study. The spinal deformity was assessed by measuring the thoracic Cobb angle from the postero-anterior spinal radiographs. The rib cage deformity was quantified by measuring the rib-index at the apex of the thoracic curve from the lateral spinal radiographs. The rib-index is defined as the ratio between the distance of the posterior margin of the vertebral body and the most extended point of the most projecting rib contour, divided by the distance between the posterior margin of the same vertebral body and the most protruding point of the least projecting rib contour. Statistical analysis included linear regression models with and without the effect of the variable age. We divided our sample in two subgroups, namely the younger (7-13 years old) and the older (14-18 years old) than the mean age participants. A univariate linear regression analysis was performed for each age group in order to assess the effect of age on Cobb angle and rib index correlation.

Results: Twenty five per cent of patients with an ATI more than or equal 7 degrees had a spinal curve under 10 degrees or had a straight spine. Linear regressions between the dependent variable "Thoracic Cobb angle" with the independent variable "rib-index" without the effect of the variable "age" is not statistical significant. After sample split, the linear relationship is statistically significant in the age group 14-18 years old (p < 0.03).

Conclusion: Growth has a significant effect in the correlation between the thoracic and the spinal deformity in girls with idiopathic scoliosis. Therefore it should be taken into consideration when trying to assess the spinal deformity from surface measurements. The findings of the present study implicate the role of the thorax, as it shows that the rib cage deformity precedes the spinal deformity in the pathogenesis of idiopathic scoliosis.

Figure 1

A drawing of a lateral spinal radiograph describing the rib-index. The rib-index is the ratio d1/d2. d1 is the distance between the posterior margin of the vertebral body and the most extended point of the most projecting rib contour. d2 is the distance between the posterior margin of the same vertebral body and the most protruding point of the least projecting rib contour.

Figure 2

The only linear association was the one between Thoracic Cobb Angle and rib-index in the age group of 14–18 years. (Predicted Thoracic Cobb Angle = - 6.357 + 7.974 × (Rib-Index).