Geometric and electromyographic assessments in the evaluation of curve progression in idiopathic scoliosis

Study Design. The natural history of patients with idiopathic scoliosis was analyzed radiographically and electromyographically in a prospective longitudinal study. Objectives. To identify changes in geometric variables and the sequence in which these changes occur during curve progression in the natural history of patients with idiopathic scoliosis. In addition, to study the relationship between several geometric variables and electromyographic (EMG) measurements to determine their predictive value as risk factors to curve progression of the scoliotic deformity. Summary of Background Data. The main area of concern in treating children with adolescent idiopathic scoliosis is the unpredictability of curve progression during the early development of the deformity. Methods. The changes in radiographic geometric and EMG variables between the first presentation and consecutive 4-6-month follow-up periods were analyzed in 105 patients with idiopathic scoliosis. Statistical analyses were performed to elucidate in more detail how spinal geometry evolves during curve progression. Results. Curve severity was associated with remaining growth potential expressed as an increasing spinal growth velocity (SGV). With increasing SGV, an enhanced EMG activity at the lower part on the convex side of the curve expressed as EMG ratio was found. High EMG ratio was associated with increased axial rotation and diminished kyphosis before the rapid increase in Cobb angle. Lateral deviation, wedge angle, and axial rotation all increased during periods of progression. Changes in tilt angle and lordosis were not associated with curve progression. Conclusions. In the natural history of idiopathic scoliosis, SGV and EMG ratio at the lower end vertebra are prominent risk factors of curve progression. The asymmetric muscle activity is associated with increased axial rotation, which in its turn is associated with increasing Cobb angle and diminishing kyphosis. The combination of these variables provides insight in the physiologic and 3-dimensional biomechanical evolution of the natural history of curve progression in idiopathic scoliosis.