Characterizing the shape of the lumbar spine using an active shape model: reliability and precision of the method.

STUDY DESIGN Analysis of positional magnetic resonance images of normal volunteers. OBJECTIVE To compare the reliability and precision of an active shape model to that of conventional lordosis measurements. SUMMARY OF BACKGROUND DATA Characterization of lumbar lordosis commonly relies on measurement of angles; these have been found to have errors of around 10 degrees . METHODS T2 weighted sagittal images of the lumbar spines of 24 male volunteers in the standing posture were acquired using a positional magnetic resonance scanner. An active shape model of the vertebral bodies from S1 to L1 was created. Lumbar lordosis was also determined by measuring the angles of the superior endplates. All measurements were performed twice by one observer and once by a second observer. RESULTS The shape model identified 2 modes of variation to describe the shape of the lumbar spine (mode 1 described curvature and mode 2 described evenness of curvature). Significant correlations were found between mode 1 and total lordosis (R = 0.97, P < 0.001) and between mode 2 and mean absolute deviation of segmental lordosis (R = 0.80, P < 0.001). Intra- and interobserver reliability was higher for the shape model (intraclass correlation coefficients, 0.98-1.00) than for the lordosis angle measurements (intraclass correlation coefficients, 0.68-0.99). The relative error of the shape model (mode 1 = 4%; mode 2 = 9%) was lower than the conventional measurements (total lordosis = 10%). CONCLUSION The shape of the lumbar spine in the sagittal plane can be comprehensively characterized using a shape model. The results are more reliable and precise than measurements of lordosis calculated from endplate angles.