Last Updated on December 11, 2023
The crankshaft phenomenon refers to a rotational and angular spinal deformity that can occur after posterior spinal surgery in children with idiopathic, congenital, and neuromuscular scoliosis. It is mostly seen in cases of infantile and juvenile idiopathic scoliosis. However, it can also occur in adolescents who have immature skeletons at the time of spinal fusion.
It occurs due to the continued growth of anterior elements of the spine after the posterior spinal fusion has been done. The spinal fusion stops the longitudinal growth in the posterior elements. However, the growth of vertebral bodies continues to grow on the anterior aspect.
This causes an increase in anterior curvature (anterior side becomes convex) due to increased vertebral body height and bulging of the discs towards the anterior convexity. This curvature pivots on posterior fusion resulting in loss of correction, an increase in rotation of vertebra, and recurrence of corrected rib hump.
Thus in cases where there is significant growth potential remaining, the anterior part of the spine grows normally and the posterior fused part acts as tethering. This causes the spine to curve and rotate.
This phenomenon of progression of curve and rotation resulting from linear growth of the anterior part of the spine is termed the crankshaft phenomenon.
Dubousset coined the term crankshaft phenomenon.
What is a Crankshaft?
A crankshaft, sometimes abbreviated to crank, is responsible for the conversion between reciprocating motion and rotational motion. Thu it converts linear motion into rotational motion. It is an essential part of the engine.
The phenomenon is named after it because on serial X-rays over the period, the spine gradually turns and deforms like the anterior part of the spine which continues to grow around the axis of the fusion mass just like the automotive crankshaft. [ It appeared that the spine gradually rotated along the length of the fusion because of the continued anterior spinal growth.]
Why Crankshaft Phenomenon Occur?
Most of the patients who undergo correction of deformity for adolescent idiopathic scoliosis are skeletally mature and the growth that remains may not affect the anterior spine leading to crankshaft.
But this is not so in immature children. There, the growth potential is immense and this may lead to a worsening of deformity due to unabated growth of the anterior spine while posterior growth is stopped by the fusion.
Dubousset noticed that the entire spine and torso gradually turn and deform like the front of the spine, which continues to grow and rake around the axis of the fusion mass (like the automotive crankshaft).
The incidence has decreased during the past years as patient selection has become better (avoiding immature patients for surgery) and also due to the advent of pedicle screws as they maintain correction by traversing all three columns.
Recognition and Assessment
Clinical appearance is very subtle. So radiological confirmation is required. The phenomenon is said to be present if
- Increase in the Cobb angle >10°
- Increase in rib-vertebra angle difference >10° [considered more sensitive]
- Any decrease in the apex-rib thoracic distance
- Increase in the vertebral apical lumbar translation
However, the crankshaft phenomenon is not only in a change in the angles and other aspects of this should be considered as well. These are
- Worsening of spinal imbalance
- Worsening of deformity above and below the fusion.
Who is at Risk for Crankshaft Phenomenon?
Patients at greatest risk for this problem are less than 10 years old because they have growth potential left.
Following patients would have an increased risk for the crankshaft phenomenon
- Open triradiate cartilages [cartilage in the pelvis, open cartilage indicates growth potential.]
- Risser sign– 0-2
- Girls < 11 years
- Boys < 13 years
Neither thoracic kyphosis nor the magnitude of the rib curve, seem to be associated with the development of the crankshaft phenomenon. The crucial risk factor is skeletal immaturity.
Patients younger than 11 years who present open triradiate cartilage may have a significantly greater risk of developing the crankshaft phenomenon.
Thus more immature the skeleton, the higher the risk. Therefore, patients with infantile and juvenile idiopathic scoliosis are at the highest risk. But the phenomenon is seen in adolescent scoliosis too.
The condition has also been reported in the setting of newer, so-called growth-friendly posterior distraction-based spinal instrumentation.
There is no way to stop or control the progression. A surgery for correction of the deformity may be needed.
To avoid the 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.
Full screws constructs can reduce the risk of crankshafting by traversing 3 columns of the spine but the risk is not eliminated.
The following points may aid in making a better patient selection for surgery
- Avoiding fusion in patients with open triradiate cartilage
- Using braces and other conservative measures till the fusion age is reached.
- Use of growth-friendly systems for correction of deformity
- Growing rods
- Vertebral body tethering
- Dubousset J, Herring JA, Shufflebarger H. The crankshaft phenomenon. J Pediatr Orthop 1989 [Link]
- Murphy RF, Mooney JF, 3rd. The Crankshaft Phenomenon. J Am Acad Orthop Surg 2017;25:e185-93. [Link]