Author and graduation date: Michelle C. MARKS, November, 2001

Committee members: Peter Francis (Chair), Roger Simmons, Tom Impelluso, Edward Quigley

Thesis title: Static and Dynamic Sagittal Spinal Balance in Normal Subjects

Sagittal spinal alignment can be evaluated from a segmental, regional, or global perspective. Global sagittal spinal alignment is described as the sagittal vertical axis (SVA), and is measured on a lateral x-ray by the horizontal distance from the posterior superior comer of the sacrum to the plumb line dropped from the center of the body of the C7 vertebra.

Though the primary deformity in Adolescent Idiopathic Scoliosis (AIS) can be observed in the coronal plane, it is also associated with a decrease in the normal thoracic kyphosis. Careful analysis of the sagittal plane curvature and balance in this patient population is becoming more routine. However, the measure of SVA can change with subtle variations in posture, such as head, arm, or lower extremity movements. Visualization of the spine with a lateral x-ray requires arm positioning away from the trunk, or flexed forward. This positioning may demonstrate an SVA measure that is not representative of a functional position. The purpose of this study was to compare the differences in the Sagittal Vertical Axis (SVA) during relaxed standing and walking to SVA during positions commonly assumed during x-ray acquisition.

This study used 15 healthy females, ages 10-14 years. Markers were attached to the skin overlying the spinous processes of C7 and L4, and on the right heel. Marker positions were captured by an 8-camera Motion Analysis System (Motion Analysis Corp., Santa Rosa, CA) during simultaneous acquisition of a lateral radiograph SVA calculation, using motion data, was matched to S V A measurement obtained by x-ray using anthropometric corrections. This allowed for measurement of SVA during experimental positions without subsequent exposure to radiation. Six standing positions with varying shoulder and knee flexion and inspiration and expiration were used. Mean S VA values for each position and the mean during gait were compared using a repeated measures ANOVA.

The mean SVA during relaxed standing (shoulders at 0 degrees and knees at 0 degrees) and during gait were positive indicating forward positioning of the C7 vertebrae over the posterior aspect of the sacral endplate. Positions that involved 45 degrees flexion of the shoulders exhibited a negative SVA, and were significantly different from relaxed standing as well as the mean SVA during gait (p = 0.00) .

Forward flexion of the shoulders during x-ray acquisition allows for visualization of the spine, but changes the SVA to an abnormally negative position. Flexion of the knees has a positive influence on SVA, but does not compensate for the negative shift induced by shoulder flexion. Current x-ray positioning (arms flexed forward) does not provide for an estimate of sagittal balance that is reflective of a relaxed or functional position.

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