Prediction of the crankshaft phenomenon by peak height velocity

Study Design. Retrospective review. Objectives. To evaluate the relation of the peak height velocity with the occurrence of the crankshaft phenomenon after posterior arthrodesis and instrumentation in idiopathic scoliosis. Summary of Background Data. Although patients with closed triradiate cartilages are unlikely to exhibit the crankshaft phenomenon after a posterior spinal fusion and instrumentation, open triradiate cartilages do not necessitate that crankshafting will occur. Less than half of patients with idiopathic scoliosis and open triradiate cartilages will exhibit the crankshaft phenomenon. Methods. The authors reviewed 43 patients with idiopathic scoliosis who were Risser 0 at the time of posterior spinal fusion. Twenty-three patients had open triradiate cartilages and twenty had closed. The timing of peak height velocity was identified. Results. All patients with closed triradiate cartilages were beyond their peak height velocity at the time of surgery. Among those with open triradiate cartilages, 8 were operated on before or during their peak and 15 were operated on afterward. All patients fused before or during the peak crankshafted. Two of the fifteen patients fused after the peak crankshafted. In one, it was low grade. In the other, it appears that the fusion blunted the peak height velocity to a point at which it was unidentifiable. Conclusions. In patients with open triradiate cartilages, surgery performed before or during the peak height velocity is a strong predictor of the crankshaft phenomenon, and later surgery is a strong negative predictor of the crankshafting (P = 0.000009). Isolated posterior fusion before the height velocity decelerates results in the crankshaft phenomenon, whereas fusion during the deceleration phase does not.