Differential wedging of vertebral body and intervertebral disc in thoracic and lumbar spine in adolescent idiopathic scoliosis - A cross sectional study in 150 patients

Abstract

Background: Hueter-Volkmann's law regarding growth modulation suggests that increased pressure on the end plate of bone retards the growth (Hueter) and conversely, reduced pressure accelerates the growth (Volkmann). Literature described the same principle in Rat-tail model. Human spine and its deformity i.e. scoliosis has also same kind of pattern during the growth period which causes wedging in disc or vertebral body.

Methods: This cross sectional study in 150 patients of adolescent idiopathic scoliosis was done to evaluate vertebral body and disc wedging in scoliosis and to compare the extent of differential wedging of body and disc, in thoracic and lumbar area. We measured wedging of vertebral bodies and discs, along with two adjacent vertebrae and disc, above and below the apex and evaluated them according to severity of curve (curve < 30 degrees and curve > 30 degrees ) to find the relationship of vertebral body or disc wedging with scoliosis in thoracic and lumbar spine. We also compared the wedging and rotations of vertebrae.

Results: In both thoracic and lumbar curves, we found that greater the degree of scoliosis, greater the wedging in both disc and body and the degree of wedging was more at apex supporting the theory of growth retardation in stress concentration area. However, the degree of wedging in vertebral body is more than the disc in thoracic spine while the wedging was more in disc than body in lumbar spine. On comparing the wedging with the rotation, we did not find any significant relationship suggesting that it has no relation with rotation.

Conclusion: From our study, we can conclude that wedging in disc and body are increasing with progression on scoliosis and maximum at apex; however there is differential wedging of body and disc, in thoracic and lumbar area, that is vertebral body wedging is more profound in thoracic area while disc wedging is more profound in lumbar area which possibly form 'vicious cycle' by asymmetric loading to spine for the progression of curve.

Figure 1

Roentgen graphic assessment was performed with whole spine standing radiogram including pelvis in all patients.

Figure 2

Measurement of vertebral body and disc wedging. 1) The vertebral wedge (α) angle was measured with angle formed by two lines connecting upper endplate and lower endplate of vertebra. 2) The angle between the inferior end plate of the upper vertebra and superior end plate of the lower vertebra was measured to calculate the intervertebral disc wedge (β) angle.

Figure 3

Wedging in body and disc in thoracic spine. In thoracic spine, the vertebral wedging is significantly greater than the disc wedging. X-axis shows level of wedging measurement. 1 U: 1st upper body or disc. 2 U: 2nd upper body or disc. 1 L: 1st lower body or disc. 2 L: 2nd lower or disc. Y-axis shows wedging in of body or disc in degree.

Figure 4

Wedging in body and disc in lumbar spine. On the contrary, to thoracic spine, lumbar curve showed an exactly opposite result with greater disc wedging. X-axis shows level of wedging measurement. 1 U: 1st upper body or disc. 2 U: 2nd upper body or disc. 1 L: 1st lower body or disc. 2 L: 2nd lower or disc. Y-axis shows wedging in of body or disc in degree.

Figure 5

shows correlation of wedging of apical disc and body according to severity of curve in thoracic curves. using analysis of covariance. It pointed weak correlation between Cobb angle and wedging in apical disc and body in thoracic curves.

Figure 6

shows correlation of wedging of apical disc and body according to severity of curve in lumbar curves. using analysis of covariance. It also indicated weak correlation between Cobb angle and wedging in apical disc and body in lumbar curves.