title: Computed tomography and biomechanical evaluation of screw fixation options at the cervicothoracic junction: intralamina versus intrapedicular techniques.
abstract: METHODS
  In vitro cadaveric biomechanical analysis.
  
  
  OBJECTIVE
  Define the T1 and T2 anatomic lamina size and evaluate the bone-screw interface strength of various pedicle screw options and intralamina techniques.
  
  
  BACKGROUND
  Transpedicular instrumentation is well accepted, but salvage techniques in the proximal thoracic spine are limited. Intralamina fixation has been described at C2 with favorable biomechanical characteristics. In addition, this technique has been introduced clinically in the proximal thoracic spine. However, the biomechanical potential has not been evaluated.
  
  
  METHODS
  Fourteen fresh-frozen cervicothoracic cadaveric specimens were scanned using dual-energy radiograph absorptiometry for bone mineral density, imaged under computed tomography, and then instrumented in the following configuration: (1) Right-sided pedicle screws in a straight-forward trajectory, (2) "salvage anatomic trajectory pedicle screws, and (3) "salvage" intralamina screws into the contralateral lamina. Insertional torque (IT) was recorded with each revolution and screws were pulled out in-line (POS) with the screw axis to simulate intraoperative failure of fixation.
  
  
  RESULTS
  Lamina screws as a salvage technique generated statistically greater peak IT (P = 0.002) and relative POS (P < 0.05) in comparison with straight-forward transpedicular screws as the initial fixation type. Furthermore, lamina screws, when compared to the salvage anatomic trajectory pedicle screws, had a significantly greater peak IT (P = 0.011). The peak IT showed a stronger correlation with POS in lamina screws than straight-forward or anatomic pedicle screws with a similar trend noted in mean IT. Bone mineral density correlated with POS in all methods of fixation. The mean lamina width measured on computed tomography at the thinnest point was 5.9 +/- 0.7 mm (range, 4.9-7.9).
  
  
  CONCLUSIONS
  Our results suggest that lamina screws, used as a salvage technique in the proximal thoracic spine, provide stronger fixation than transpedicular screws when using standard 4.5-mm cervical screws. In-tralamina screws appear to be a biomechanically sound salvage technique in the region, and appear to be a safe, effective technique for instrumenting the proximal thoracic spine.
authors.affiliation: Department of Surgery, Walter Reed Army Medical Center, Washington, DC 20307, USA. mtcardoso@comcast.net
authors.forename: Mario J
authors.initials: MJ
authors.lastname: Cardoso
authors.forename: Anton E
authors.initials: AE
authors.lastname: Dmitriev
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Lehman
authors.forename: Melvin
authors.initials: M
authors.lastname: Helgeson
authors.forename: Patrick
authors.initials: P
authors.lastname: Cooper
authors.forename: Michael K
authors.initials: MK
authors.lastname: Rosner
mesh_terms: D015502:Absorptiometry, Photon; D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D002102:Cadaver; D006801:Humans; D008422:Materials Testing; D011474:Prosthesis Design; D011475:Prosthesis Failure; D013123:Spinal Fusion / Q000295:instrumentation*; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging* / Q000601:surgery*; D014057:Tomography, X-Ray Computed; D019415:Torque