Crankshaft phenomenon occurs in the scoliotic spine when the posterior part of the spine is fused.
A solid spinal fusion stopped the longitudinal growth in the posterior elements, but the vertebral bodies continued to grow anteriorly. The anterior growth causes the vertebral bodies and discs to bulge laterally toward the convexity and to pivot on the posterior fusion, causing loss of correction, increase in vertebral rotation, and recurrence of the rib hump.
The term crankshaft phenomenon was coined by Dubousset.
Thus in cases where there is significant growth potential remaining, the anterior part of the spine grows normally and posterior fused part acts as tethering. This causes the spine to curve and rotate.
This phenomenon of progression of curve and rotation of curve resulting due to linear growth of the anterior part of the spine is termed as crankshaft phenomenon.
A crankshaft, sometimes abbreviated to crank, is responsible for conversion between reciprocating motion and rotational motion
Patients at greatest risk for this problem are of age less than 10 years because they have growth potential left.
Following patients would have increased risk for crankshaft phenomenon
- Open triradiate cartilages [cartilage in the pelvis, open cartilage indicates growth potential.
- Girls younger than 11 years;
- Boys younger than 13 years
The phenomenon is suspected noted with more than 10-degree increase of the Cobb angle or the rib-vertebral angle with latter being more sensitive.
Neither thoracic kyphosis nor the magnitude of the rib curve, seem to be associated with the development of crankshaft phenomenon. The crucial risk factor is skeletal immaturity.
The patients younger than 11 years that present open triradiate cartilage may have a significantly greater risk of developing the crankshaft phenomenon.
To avoid crankshaft phenomenon, both the front and back of the spine (anterior-posterior surgery) may be fused in patients with large growth potential or surgery may be revised to accommodate the growth.