title: Anatomic parameters: direct intralaminar screw repair of spondylolysis.
abstract: METHODS
  Analysis via computed tomography imaging software.
  
  
  OBJECTIVE
  To determine parameters for ideal intralaminar screw trajectory and the feasibility of screw placement at L3, L4, and L5 laminae for pars defect fixation.
  
  
  BACKGROUND
  To our knowledge, no studies provide anatomic parameters for ideal intralaminar screw trajectory for treating spondylolysis.
  
  
  METHODS
  We used advanced imaging software for 3-dimensional interactive viewing to analyze 20 randomly selected normal adolescent lumbar computed tomographic scans. The ideal intralaminar screw trajectory was drawn from the inferior lamina, a point chosen to maximize cortical diameter at the isthmus of the lamina and bisect the pedicle. We measured and evaluated ideal trajectory parameters for percutaneous screw placement for pars defect fixation at the L3 to L5 laminae.
  
  
  RESULTS
  The ideal pathway was the thick portion of the lamina between the inferior edge of the lamina and the pedicle. This area was close to the inferior articular process (axial plane), becoming more so at progressively caudal levels. At the ideal trajectory, the mean (standard deviation) coronal angle slightly decreased (L3-L5): 7.3° (5.1°), 6.6° (3.7°), and 4.2° (2.5°), respectively. The trajectory distance increased from cranial to caudal. These parameters increased (L3-L5): mean distance (transverse plane) between the starting point and middle of the spinous process, 1.2 (0.18 cm), 1.3 (0.2 cm), and 1.6 (0.3 cm), respectively; mean screw sagittal angle with respect to the posterior skin, 15.5° (5.0°), 24.3° (6.5°), and 43° (5.8°), respectively; and mean distance for guide wire entry, 28.8 (10.6 cm), 20.1 (5.4 cm), and 11.9 (2.1 cm), respectively.
  
  
  CONCLUSIONS
  At the ideal screw trajectory, pars fixation by intralaminar screw is uniformly feasible at L3 to L5 laminae, where most patients can accommodate a 4.5-mm screw.
  
  
  METHODS
  2.
authors.affiliation: *Department of Orthopaedic Surgery and the †Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University, Baltimore, MD.
authors.forename: Emmanuel N
authors.initials: EN
authors.lastname: Menga
authors.forename: Amit
authors.initials: A
authors.lastname: Jain
authors.forename: Khaled M
authors.initials: KM
authors.lastname: Kebaish
authors.forename: Stefan L
authors.initials: SL
authors.lastname: Zimmerman
authors.forename: Paul D
authors.initials: PD
authors.lastname: Sponseller
mesh_terms: D000293:Adolescent; D001863:Bone Screws; D002648:Child; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging* / Q000601:surgery*; D008297:Male; D019060:Minimally Invasive Surgical Procedures / Q000379:methods; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D013169:Spondylolysis / Q000000981:diagnostic imaging* / Q000601:surgery*; D014057:Tomography, X-Ray Computed / Q000379:methods