The Formetric 4D dynamic system (Diers International GmbH, Schlangenbad, Germany) is a rasterstereography based imaging system designed to evaluate spinal deformity, providing radiation-free imaging of the position, rotation, and shape of the spine during the gait cycle. Purpose. This study was designed to evaluate whether repeated measurements with the Formetric 4D dynamic system would be reproducible with a standard deviation of less than +/？ 3 degrees. This study looked at real-time segmental motion, measuring kyphosis, lordosis, trunk length, pelvic, and T4 and L1 vertebral body rotation. Methods. Twenty healthy volunteers each underwent 3 consecutive scans. Measurements for kyphosis, lordosis, trunk length, and rotations of T4, L1, and the pelvis were recorded for each trial. Results. The average standard deviations of same-day repeat measurements were within +/？ 3 degrees with a range of 0.51 degrees to 2.3 degrees. Conclusions. The surface topography system calculated reproducible measurements with error ranges comparable to the current gold standard in dynamic spinal motion analysis. Therefore, this technique should be considered of high clinical value for reliably evaluating segmental motion and spinal curvatures and should further be evaluated in the setting of adolescent idiopathic scoliosis. 1. Introduction Adolescent idiopathic scoliosis (AIS) is a common condition affecting between 2 and 4 percent, or an estimated 6 million adolescents, in the United States [1]. Frequent assessment and monitoring of this patient population are necessary to determine an individual’s progression of spinal deformity. Healthcare providers most often use spinal radiographs as the standard-of-care for evaluation. X-rays currently offer the most reliable way to quantify the magnitude of the curve but have the disadvantages of exposing patients to harmful radiation. Nash et al. reported that over a three-year period, a group of teenage girls with AIS underwent an average of 22 radiographs [2]. Ronckers et al. found cancer mortality to be 8 percent higher than expected in patients with repeated radiographs for scoliosis, as well as a four times greater relative risk of breast cancer in female patients with spinal disorders [3]. Surface topography is the study of the three-dimensional shape of the surface of the back. Measurement systems using surface topography do not involve exposure to ionizing radiation and are therefore completely safe [4]. According to a study by Knott et al., if surface topography can deliver reliable results, then it should replace
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