PREDICTION OF THE CRANKSHAFT PHENOMENON BY PEAK HEIGHT VELOCITY

Study Design. Retrospective review. Objectives. To evaluate the relati on of the peak height velocity with the occurrence of the crankshaft p henomenon after posterior arthrodesis and instrumentation in idiopathi c scoliosis. Summary of Background Data. Although patients with closed triradiate cartilages are unlikely to exhibit the crankshaft phenomen on after a posterior spinal fusion and instrumentation, open triradiat e cartilages do not necessitate that crankshafting will occur. Less th an half of patients with idiopathic scoliosis and open triradiate cart ilages will exhibit the crankshaft phenomenon. Methods. The authors re viewed 43 patients with idiopathic scoliosis who were Risser 0 at the time of posterior spinal fusion, Twenty-three patients had open trirad iate cartilages and twenty had closed. The timing of peak height veloc ity was identified. Results. All patients with closed triradiate carti lages were beyond their peak height velocity at the time of surgery. A mong those with open triradiate cartilages, 8 were operated on before or during their peak and 15 were operated on afterward. All patients f used before or during the peak crankshafted. Two of the fifteen patien ts 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 pe ak 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 durin g the deceleration phase does not.