Crankshaft Phenomenon in Congenital Scoliosis

Twin Cities Spine Center, Minneapolis, Minnesota, USA

INTRODUCTION: The crankshaft phenomenon of continued anterior growth with a solid posterior fusion is well recognized in idiopathic scoliosis, but the rate of occurrence in congenital scoliosis is not known. The purpose of this study was to identify the rate of crankshaft in congenital scoliosis and to identify possible risk factors.

METHODS: A retrospective chart and radiographic review was performed on all patients with congenital scoliosis who underwent fusion prior to maturity and with follow-up to full skeletal maturity. Sixty patients were identified. Pre-operative, post-operative and final follow-up radiographs were evaluated for changes in Cobb angle, rib-vertebral angle difference (RVAD), coronal balance, trunk shift and the need for a second procedure. The average age at surgery was 6.4 years with 56 posterior fusions and 4 anterior/posterior fusion. Forty-nine of the fusions were without instrumentation. Average follow-up was 11.6 years and the average age at final follow-up was 17.9 years.

RESULTS:

The crankshaft phenomenon of Cobb angle change occurred in one-third of the cases. The only factor that influenced the chance of crankshaft was the magnitude of the curve prior to surgery, with increased curve magnitude resulting in increased risk of crankshaft (Linear Regression, p=0.015). The age at surgery, number or percent of healthy discs in the fusion area and length of the fusion area did not affect the risk of crankshaft.

CONCLUSION: The crankshaft phenomenon occurs in one-third of the immature children fused posteriorly for congenital scoliosis but was difficult to predict. The only factor identified as increasing the risk of crankshaft was the magnitude of the curve prior to surgery.