title: Risk of Implant Loosening After Cyclic Loading of Fusionless Growth Modulation Techniques: Nitinol Staples Versus Flexible Tether.
abstract: METHODS
  Biomechanical evaluation using porcine spines.
  
  
  OBJECTIVE
  Compare the fixation strength of two currently used fusionless adolescent idiopathic scoliosis correction techniques following cyclic loading using porcine spines.
  
  
  BACKGROUND
  The ability of fusionless implants to control or correct scoliosis in a growing patient requires such implants to maintain spinal fixation. Because they cross the disc, motion may weaken fixation over time.
  
  
  METHODS
  Eight pig spines were divided into cycled segments (T10-T13) and uncycled segments (T7-T8, L2-L3). Initial range of motion (ROM) was determined in torsion, flexion-extension, and lateral bending (0.5°/s to 1.75 N·m).Staple group (n = 4): 6 mm parallel staples were inserted on the right anterolateral spine across each intervertebral disc. Cycled segments received six staples (three adjacent discs) and uncycled segments received four staples (two separate discs).Tether group (n = 4): 5.35 × 35 mm right anterolateral vertebral body screws were placed into each vertebra. Cycled segments received four screws and uncycled segments received four screws. Screws in cycled segments were connected with a flexible tether tensioned to straight alignment.ROM of instrumented cycled segments was measured, and then segments were loaded to the measured ROM in flexion-extension (2000 cycles), lateral bending (1000 cycles), and axial rotation (2000 cycles). Implants were axially loaded to failure. Parametric tests compared pre- to postimplant ROM; nonparametric tests compared staple to screw pullout strength; P < 0.05 was significant.
  
  
  RESULTS
  There were no differences in ROM before instrumentation between groups. ROM was not changed except tethers decreased left lateral bending (-6.2°). Although staple pullout was less than screw pullout for cycled and uncycled segments (P < 0.05 and P = 0.057, respectively), there was no difference in pullout strength with and without cyclic loading for either group (P = 0.4).
  
  
  CONCLUSIONS
  Tethers decreased lateral bending away from the tether. Screws had almost five times greater load to failure than staples. Five thousand cycles did not result in loosening of either staple or tether screws.
  
  
  METHODS
  N/A.
authors.affiliation: Division of Orthopedics, Rady Children's Hospital-San Diego, San Diego, CA.
authors.forename: Burt
authors.initials: B
authors.lastname: Yaszay
authors.affiliation: Division of Orthopedics, Rady Children's Hospital-San Diego, San Diego, CA.
authors.forename: Joshua D
authors.initials: JD
authors.lastname: Doan
authors.affiliation: Department of Orthopedic Surgery, University of California, San Diego, CA.
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Parvaresh
authors.affiliation: Division of Orthopedics, Rady Children's Hospital-San Diego, San Diego, CA.
authors.forename: Christine L
authors.initials: CL
authors.lastname: Farnsworth
mesh_terms: D000497:Alloys; D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D001863:Bone Screws / Q000592:standards*; D016268:Internal Fixators / Q000592:standards*; D007403:Intervertebral Disc / Q000473:pathology / Q000601:surgery; D008159:Lumbar Vertebrae / Q000473:pathology / Q000601:surgery*; D008297:Male; D016059:Range of Motion, Articular / Q000502:physiology; D013123:Spinal Fusion; D013537:Sutures / Q000592:standards*; D013552:Swine; D013904:Thoracic Vertebrae / Q000473:pathology / Q000601:surgery*; D016474:Weight-Bearing / Q000502:physiology
chemical_list: D000497:Alloys; C013616:nitinol