The Accuracy of Transpedicular Thoracic Screw Placement in Vivo

CPT Philip J. Belmont, Jr., MD,
LTC William R. Klemme, MD,
CPT Aman Dhawan, MD and
LTC David W. Polly, Jr., MD
* · (a– Medtronic Sofamor Danek, DePuy Acromed)
Walter Reed Army Medical Center, Washington, DC, USA

INTRODUCTION:
Previous studies have outlined the importance of properly placed transpedicular thoracic screws to prevent neurologic and vascular injury. To our knowledge, the accuracy of thoracic pedicle screws has not been documented in vivo.

METHODS:
Thirty–five consecutive patients underwent posterior stabilization utilizing 218 titanium transpedicular thoracic screws. All screws were inserted using anatomical topography under fluoroscopic control and intra–operative pedicular sounding with a blunt tipped probe. Laminotomies were not performed. Screws were inserted throughout the thoracic spine and regionally grouped for analysis (T1–T4: 25 screws, T5–T8: 63 screws, T9–T12: 130 screws). Post–operative CT scans were used to assess the transverse and sagittal placement of screws relative to the pedicle and the anterior vertebral body. Cortical perforations were graded in 2 mm increments. The transverse screw angle as measured relative to the midline of the vertebral body was also determined.

RESULTS:
Of the 218 pedicle screws studied, 126 screws (58%) were totally contained within the confines of the pedicle. Ninety–two pedicle screws (42%) were found to have penetrated either the medial (15%) or lateral cortex (27%). Among the subset of screws with a medial breech, 31 screws (14%) had 2 mm or less of canal intrusion. Another 1% (2 screws) were inserted with 2.1–4 mm of canal encroachment. Among those screws with a lateral cortical breech, 33 screws (15%) had perforated 2 mm or less, while 24 screws (11%) were measured between 2.1 – to 4 mm and 2 screws (1%) between 4.1–6 mm. There were no superior or inferior perforations of the pedicle. Thirteen screws (6%) penetrated the anterior vertebral cortex by an average of 1.8 mm (range, 1–4 mm). The mean transverse angle for screws localized within the pedicle was 15 degrees. For those screws with medial or lateral cortical perforation, the transverse angle averaged 18 and 11 degrees respectively. The percentage of totally contained intrapedicular screws varied from 28% in the upper thoracic spine (T1–T4) to 73% in the lower thoracic spine (T9–T12). Although there were no neurologic or vascular complications, 2 screws in close proximity to the aorta were revised to prevent any long–term sequelae.

CONCLUSIONS:
In the present study, although 42% of screws were associated with cortical penetration, only 15% of screws unintentionally penetrated the medial cortex. We believe that the subset of screws with £ 2 mm of cortical penetration represent well–positioned screws accompanied by cortical expansion and medial wall fracture. Importantly, in only 2 instances (1% of screws) was there significant canal encroachment of greater than 2 mm. The parameters linked to satisfactory transpedicular thoracic screw placement in the transverse plane included the transverse screw angle and the thoracic region of the instrumented vertebral level.

* · If noted, the author indicates something of value received. The codes are identified as: a research or institutional support, b–miscellaneous funding, c–royalties, d–stock options, e–consultant. For full information, refer to page 3.
** The FDA has not cleared a drug and/or medical device for the use described in this presentation. (i.e., the drug or medical device is being discussed in an “off–label" use).