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Crankshaft Phenomenon After Definitive Spinal Fusion in Pre-teens: Should We Be Concerned with Pedicle Screw
Jake H Goldfarb1, Eshan S Sane1, Christian E Wright1
1Department of Orthopaedic Surgery, Washington University School of Medicine, St. Louis, MO, USA.
Background:
Because of remaining growth, early spinal fusion can lead to the crankshaft phenomenon, which occurs when continued growth of the anterior spine after posterior fusion causes curve progression. This project aims to explore the impact of early fusion on crankshaft.
Methods:
A retrospective analysis was conducted of idiopathic scoliosis patients aged 8-12 treated with posterior spinal fusion. Inclusion criteria were idiopathic scoliosis diagnosis, thoracic fusion, and ≥2 years of follow-up. Thoracic curve magnitude and Rib Vertebral Angle Difference (RVAD) were measured preoperatively and at the first and the final postoperative follow-ups, with crankshaft defined as a ≥10° change in thoracic angle and ≥20° change in the RVAD. Patient-Reported Outcome Measurement Information System (PROMIS) scores, screw density, and screw length were recorded. Significance was set at P < .05.
Results:
One hundred thirty-two patients were included, with a mean age of 12.0 years (range: 8-12.9) and mean follow-up of 4.7 years. The mean preoperative thoracic curve was 62° ± 17, with 70% of Risser 0 patients and 30% having open triradiate cartilage. 95.9% of the screws reached the anterior 50% of the vertebral body. No patients underwent anterior release. The thoracic curve minimally increased from the first to the final follow-up (mean difference: 1.54°, P < .01), as did the RVAD (mean difference: 4.08°, P < .01). Eight Risser 0 patients experienced crankshaft under the thoracic criteria and eight under the RVAD criteria. There was no significant association between RVAD and self-image, as measured by PROMIS Peer Relations.
Conclusions:
Pre-teen idiopathic scoliosis patients undergoing early spinal fusion may experience minimal crankshaft exhibited by worsening rib rotation. Surgeons may consider counseling families about small changes to the rib hump, with reassurance of no differences in outcome measures.
Key Concepts:
(1)The crankshaft phenomenon occurs at a low rate after posterior spinal fusion in pre-teens with idiopathic scoliosis.(2)Measuring crankshaft using the thoracic curve magnitude and Rib Vertebral Angle Difference (RVAD) produced similar results with respect to the crankshaft phenomenon.(3)Patients who met criteria for crankshaft did not demonstrate worsening Patient-Reported Outcome Measurement Information System scores.
Level Of Evidence:
III.
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