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.