title: The ventral lamina and superior facet rule: a morphometric analysis for an ideal thoracic pedicle screw starting point.
abstract: BACKGROUND
  With the increasing popularity of thoracic pedicle screws, the freehand technique has been espoused to be safe and effective. However, there is currently no objective, definable landmark to assist with freehand insertion of pedicle screws in the thoracic spine. With our own increasing surgical experience, we have noted a reproducible and unique anatomic structure known as the ventral lamina.
  
  
  OBJECTIVE
  We set out to define the morphologic relationship of the ventral lamina to the superior articular facet (SAF) and pedicle, and describe an optimal medial-lateral pedicle screw starting point in the thoracic spine.
  
  
  METHODS
  We conducted an in vitro fresh-frozen human cadaveric study.
  
  
  METHODS
  One hundred fifteen thoracic spine vertebral levels were evaluated. After the vertebral body was removed, Kirschner wires were inserted retrograde along the four boundaries of the pedicle. Using digital calipers, we measured width of the SAF and pedicle at the isthmus, and from the borders of the SAF to the boundaries of the pedicle. We calculated the morphologic relationship of the ventral lamina and the center of the pedicle (COP) to the SAF.
  
  
  RESULTS
  Two hundred twenty-nine pedicles were measured, with one pedicle excluded because of fracture of the SAF during disarticulation. The ventral lamina was clearly identifiable at all levels, forming the roof of the spinal canal and confluent with the medial pedicle wall (MPW). The mean distance from the SAF midline to the MPW was 1.36±1.23 mm medial. The MPW was lateral to SAF midline in 34 pedicles (14.85%) and, on average, was a distance of 0.52±0.51 mm lateral. The mean distance from the SAF midline to the COP was 2.17±1.38 mm lateral. The COP was medial to SAF midline in only 11 pedicles (4.80%).
  
  
  CONCLUSIONS
  The ventral lamina is an anatomically reproducible structure located consistently medial to the SAF midline (85%). We also found the COP consistently lateral to the SAF midline (95%). Based on these morphologic findings, the medial-lateral starting point for thoracic pedicle screws should be 2 to 3 mm lateral to the SAF midline (superior facet rule), allowing screw placement in the COP and avoiding penetration into the spinal canal.
authors.affiliation: Department of Orthopaedic Surgery and Rehabilitation, Walter Reed National Military Medical Center, 8901 Wisconsin Ave., Bethesda, MD 20889, USA. Electronic address: armyspine@yahoo.com.
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Lehman
authors.affiliation: Department of Orthopaedic Surgery and Rehabilitation, Walter Reed National Military Medical Center, 8901 Wisconsin Ave., Bethesda, MD 20889, USA.
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Kang
authors.affiliation: Department of Orthopaedic Surgery, Washington University School of Medicine, 660 South Euclid Ave, Campus Box 8233, St. Louis, MO 63110, USA.
authors.forename: Lawrence G
authors.initials: LG
authors.lastname: Lenke
authors.affiliation: Department of Orthopaedic Surgery and Rehabilitation, Walter Reed National Military Medical Center, 8901 Wisconsin Ave., Bethesda, MD 20889, USA.
authors.forename: Rachel E
authors.initials: RE
authors.lastname: Gaume
authors.affiliation: Department of Orthopaedic Surgery and Rehabilitation, Walter Reed National Military Medical Center, 8901 Wisconsin Ave., Bethesda, MD 20889, USA.
authors.forename: Haines
authors.initials: H
authors.lastname: Paik
mesh_terms: D001863:Bone Screws; D006801:Humans; D013123:Spinal Fusion / Q000379:methods*; D013904:Thoracic Vertebrae / Q000033:anatomy & histology* / Q000601:surgery; D021801:Zygapophyseal Joint / Q000033:anatomy & histology* / Q000601:surgery
keywords: Starting point; Superior articular facet; Superior facet rule; Thoracic pedicle screw; Ventral lamina