title: Repair of pars interarticularis defect utilizing a pedicle and laminar screw construct: a new technique based on anatomical and biomechanical analysis.
abstract: OBJECTIVE
  The theoretical advantage of pars interarticularis repair over spinal fusion to correct pars defects is that the treatment is a direct osteosynthesis that preserves motion at the involved functional spinal unit. Several techniques and constructs have been used to achieve greater rigidity, but these techniques may risk entry into the spinal canal, and adverse events are common. A pedicle and laminar screw construct placed entirely outside the spinal canal may offer greater stiffness and achieve higher pars defect healing rates. The purpose of this study was to biomechanically assess an intralaminar screw construct in cadaveric lumbar spines in comparison with other types of constructs typically used in pars repair and to quantify the sizes of screws that can be placed safely in both normal and spondylolytic vertebrae.
  
  
  METHODS
  The L-4 and L-5 laminae in patients with spondylolysis and in controls who underwent CT (n = 41, each group) were measured by analysis of conventional axial CT images and multiplanar reformations constructed on a Vitrea workstation to determine the feasibility of translaminar fixation with a 4.5-mm-diameter screw. Biomechanical tests for torsion and flexion-extension were performed on 8 fresh human cadaveric lumbar spines before and after modeling for bilateral spondylolytic defects. Three pars repair techniques were tested at each level and in the following sequence: pedicle screw-cable, pedicle screw-rod-hook, and pedicle screw-intralaminar screw.
  
  
  RESULTS
  The majority of laminae can accept 4.5 × 25-mm screws. The cable construct allowed the greatest motion and least stability across the defect in all biomechanical tests. The hook and laminar screw constructs performed similarly in all tests and exhibited no significant difference in stiffness.
  
  
  CONCLUSIONS
  A surgically placed intralaminar screw construct may be a safe and effective alternative to current pars repair methods.
authors.affiliation: Department of Orthopaedic Surgery, University of Michigan, Ann Arbor, MI 48109, USA. rakeshpa@med.umich.edu
authors.forename: Rakesh D
authors.initials: RD
authors.lastname: Patel
authors.forename: Humberto G
authors.initials: HG
authors.lastname: Rosas
authors.forename: Michael P
authors.initials: MP
authors.lastname: Steinmetz
authors.forename: Paul A
authors.initials: PA
authors.lastname: Anderson
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000502:physiology*; D001863:Bone Screws; D002648:Child; D003198:Computer Simulation; D005240:Feasibility Studies; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D012984:Software; D013123:Spinal Fusion / Q000295:instrumentation*; D013169:Spondylolysis / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D014057:Tomography, X-Ray Computed; D054159:Torsion, Mechanical; D016474:Weight-Bearing / Q000502:physiology; D055815:Young Adult