Traditional T1-S1 Measurement of the Spinal Length on X-ray Images Does Not Correlate With the True Length of the Spine

Abstract

Background: The T1-S1 distance to evaluate spinal length is traditionally measured as a straight line on an anteroposterior radiograph. However, this method may not reflect the true 3-dimensional (3D) spinal length. The objective of the study was to evaluate the difference between the traditional T1-S1 measurement and a 3D reconstruction from standard x-ray imaging.Methods: Radiological assessment and 3D reconstruction of spinal length in pediatric patients with various spine deformities. The 3D reconstruction derived from standard biplanar spine x-ray images using a specialized but free available software and calibration device. Direct comparison of length, intraobserver variance for repeated measurements, as well as interobserver correlation for both measurement methods and between different levels of training were evaluated. Furthermore, the influence on spinal length by the degree of spinal deformity as well as other factors was analyzed.Results: A total of 39 x-ray images from 35 patients at a mean age of 15.4 years (8.9-26.8 years) were evaluated. There was excellent agreement for intra-and interobserver correlation for both measurement techniques. Spinal length assessed using 3D reconstruction was significantly longer compared with the traditional T1-S1 distance, on average 2.7 cm (0.5-6.1 cm). There was also a significant positive correlation between the maximum extent of the deformity and the difference in spinal length.Conclusions: Traditional T1-S1 distance significantly underestimates the true length of the spine. A 3D measurement reflects the real length of the spine more adequately.Clinical Relevance: Such information is relevant to the treating spine surgeon when planning or assessing therapeutic measures, especially in advanced deformities.