title: A biomechanical investigation of dual growing rods used for fusionless scoliosis correction.
abstract: BACKGROUND
  The use of dual growing rods is a fusionless surgical approach to the treatment of early onset scoliosis which aims to harness potential growth and correct spinal deformity. The purpose of this study was to compare the in-vitro biomechanical response of two different dual rod designs under axial rotation loading.
  
  
  METHODS
  Six porcine spines were dissected into seven level thoracolumbar multi-segment units. Each specimen was mounted and tested in a biaxial Instron machine, undergoing nondestructive left and right axial rotation to peak moments of 4 Nm at a constant rotation rate of 8 deg. s(-1). A motion tracking system (Optotrak) measured 3D displacements of individual vertebrae. Each spine was tested in an un-instrumented state first and then with appropriately sized semi-constrained and 'rigid' growing rods in alternating sequence. The range of motion, neutral zone size and stiffness were calculated from the moment-rotation curves and intervertebral range of motion was calculated from Optotrak data.
  
  
  RESULTS
  Irrespective of test sequence, rigid rods showed a significant reduction of total rotation across all instrumented levels (with increased stiffness) whilst semi-constrained rods exhibited similar rotational behavior to the un-instrumented spines (P<0.05). An 11.1% and 8.0% increase in stiffness for left and right axial rotation respectively and 14.9% reduction in total range of motion were recorded with dual rigid rods compared with semi-constrained rods.
  
  
  CONCLUSIONS
  Based on these findings, the Semi-constrained growing rods were shown to not increase axial rotation stiffness compared with un-instrumented spines. This is thought to provide a more physiological environment for the growing spine compared to dual rigid rod constructs.
authors.affiliation: Paediatric Spine Research Group, Institute of Health and Biomedical Innovation, Queensland University of Technology and Mater Health Services, Brisbane, Australia. Electronic address: quickymark@gmail.com.
authors.forename: M E
authors.initials: ME
authors.lastname: Quick
authors.affiliation: Paediatric Spine Research Group, Institute of Health and Biomedical Innovation, Queensland University of Technology and Mater Health Services, Brisbane, Australia.
authors.forename: C A
authors.initials: CA
authors.lastname: Grant
authors.affiliation: Paediatric Spine Research Group, Institute of Health and Biomedical Innovation, Queensland University of Technology and Mater Health Services, Brisbane, Australia.
authors.forename: C J
authors.initials: CJ
authors.lastname: Adam
authors.affiliation: Paediatric Spine Research Group, Institute of Health and Biomedical Innovation, Queensland University of Technology and Mater Health Services, Brisbane, Australia.
authors.forename: G N
authors.initials: GN
authors.lastname: Askin
authors.affiliation: Paediatric Spine Research Group, Institute of Health and Biomedical Innovation, Queensland University of Technology and Mater Health Services, Brisbane, Australia.
authors.forename: R D
authors.initials: RD
authors.lastname: Labrom
authors.affiliation: Paediatric Spine Research Group, Institute of Health and Biomedical Innovation, Queensland University of Technology and Mater Health Services, Brisbane, Australia.
authors.forename: M J
authors.initials: MJ
authors.lastname: Pearcy
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D004195:Disease Models, Animal; D004867:Equipment Design; D006801:Humans; D066298:In Vitro Techniques; D016268:Internal Fixators; D009068:Movement; D019637:Orthopedic Procedures; D012399:Rotation; D012600:Scoliosis / Q000503:physiopathology* / Q000601:surgery*; D013131:Spine / Q000502:physiology* / Q000601:surgery*; D013552:Swine; D016474:Weight-Bearing
keywords: Biomechanical; Fusionless correction; Growing rod; In vitro; Intervertebral; Porcine; Range of motion; Scoliosis; Stiffness