title: What is the best way to optimize thoracic kyphosis correction? A micro-CT and biomechanical analysis of pedicle morphology and screw failure.
abstract: METHODS
  A human cadaveric biomechanical analysis.
  
  
  OBJECTIVE
  The purpose of this study was to evaluate the bone density/trabecular width of the thoracic pedicle and correlate that with its resistance against compressive loading used during correction maneuvers in the thoracic spine (i.e., cantilever bending).
  
  
  BACKGROUND
  As surgeons perform cantilever correction maneuvers in the spine, it is common to have pedicle screws pullout or displace while placing corrective forces on the construct. Currently, surgeons either compress against the cephalad aspect of the pedicle or vice versa. We set out to establish which aspect of the pedicle was the most dense and to determine the optimal direction for screw compression during kyphosis/deformity correction.
  
  
  METHODS
  Fifteen fresh-frozen cadaveric vertebrae (n = 15) were examined by micro-computed tomography to determine percent bone volume/total volume (%BV/TV) within the cephalad and caudad aspects of the pedicle. Specimens were sectioned in the sagittal plane. Pedicles were instrumented according to the straightforward trajectory on both sides. Specimens were then mounted and loading to failure was performed perpendicular to the screw axis (either the cephalad or the caudad aspect of the pedicle).
  
  
  RESULTS
  Mean failure when loading against the caudad aspect of the pedicle was statistically, significantly greater (454.5 ± 241.3 N vs. 334.79 1 ± 158.435 N) than for the cephalad pedicle (P < 0.001). In concordance with failure data, more trabecular and cortical bones were observed within the caudad half of the pedicle compared with the cephalad half (P < 0.001).
  
  
  CONCLUSIONS
  Our results suggest that the caudad half of the pedicle is denser and withstands higher forces compared with the cephalad aspect. In turn, the incidence of intraoperative screw loosening and/or pedicle fracture may be reduced if the compressive forces (cantilever bending during deformity correction) placed upon the construct are applied against the caudad portion of the pedicle.
authors.affiliation: Integrated Department of Orthopaedic Surgery and Rehabilitation, Walter Reed National Military Medical Center, Washington, DC, USA. armyspine@yahoo.com
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Lehman
authors.forename: Melvin D
authors.initials: MD
authors.lastname: Helgeson
authors.forename: Anton E
authors.initials: AE
authors.lastname: Dmitriev
authors.forename: Haines
authors.initials: H
authors.lastname: Paik
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Bevevino
authors.forename: Rachel
authors.initials: R
authors.lastname: Gaume
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Kang
authors.forename: Lawrence G
authors.initials: LG
authors.lastname: Lenke
mesh_terms: D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D019245:Compressive Strength; D004868:Equipment Failure; D006801:Humans; D020007:Implants, Experimental; D007738:Kyphosis / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D008422:Materials Testing; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery* / Q000648:ultrastructure; D014057:Tomography, X-Ray Computed / Q000379:methods*; D016474:Weight-Bearing