Thoracic Pedicle vs Pedicle/Rib Screw Fixation

Michael F O’Brien MD,
Jeffrey Wood MD,
Thomas G Lowe MD,
Paul Alongi MD,
David Smith MSc,
David Fitzgerald BSME,
Lawrence G Lenke MD,
Stephen M Mardjetko MD

INTRODUCTION:
Recent reports suggest that for thoracic deformity, pedicle screw constructs achieve superior correction. Concerns have been raised regarding the safety of pedicle screw instrumentation in the thoracic spine because of the proximity to the spinal cord, the trajectory of the screws and the small margin for error.

PURPOSE:
To compare the biomechanical strength of a standard intra–pedicular pedicle screw to an alternative technique for thoracic “pedicle” screw fixation using a “safer” lateral insertion technique.

MATERIALS AND METHODS:
Four fresh–frozen human cadaveric thoracic spines were harvested with the medial 5 – 6 cm of rib, intercostal soft issue and the overlying parietal pleura intact. All nonstructural soft issue was removed. The spines were instrumented on one side with intra–pedicle screws (standard pedicle screws) and on the opposite side using an extra–pedicular technique (pedicle/rib). The extra–pedicular technique utilizes an insertion point lateral to that of a standard pedicle screw. The screw is inserted through the transverse process and directed to engage the lateral aspect of the pedicle and the medial aspect of the rib. Finally the vertebral body is engaged. Pilot holes were prepared and tapped for 5.5mm screws using fluoroscopy to insure accurate location of both the intra– pedicular and the extra–pedicular screws. Because of the small size of the specimens and the ability to easily manipulate them, complete visualization of the pedicle was possible in the AP and lateral projection using fluoroscopy. Fixed angle 5.5 mm x 45 mm, M–8 (Medtronic Sofamor Danek) stainless steel screws were implanted. Fluoroscopy was used to verify the location of each screw after insertion. The entire thoracic spine was then potted in DynaCastä epoxy. Biomechanical testing was performed on an MTS 809 servohydraulic biaxial biomechanical testing system. The screws were pulled out perpendicular to the longitudinal axis of the spine at each level. A loading rate of 50 N / second was utilized. Load versus displacement data were generated. Maximum load to failure and yield strengths were calculated.

RESULTS:
55 thoracic screws were placed. 29 screws were intra–pedicular (pedicle) and 26 screws were extra–pedicular (pedicle/rib). Intra–pedicular screws had a maximum load to failure of 1075 N ± 280 N (SE 55) and a yield strength of 772 N ± 220 N (SE 43). Extra– pedicular screws had a maximum load to failure of 719 N ± 338 N (SE 63) and a yield strength of 566 N ± 220 N (SE 41).

DISCUSSION:
Anatomic, radiographic and clinical studies have suggested that the use of thoracic pedicle screws is a practical technique. This study was undertaken to ascertain whether a potentially safer, extra–pedicular screw placement more laterally through the transverse process and engaging the lateral aspect of the pedicle and the medial aspect of rib would be a reasonable alternative to a thoracic intra–pedicular screw. The data in this study suggests that standard thoracic pedicle screws are significantly stronger than extra– pedicular (pedicle/rib) screws in pullout (p = 0.001). Additionally there is more variability in screw purchase with pedicle/rib screws as suggested by the larger standard deviation for both maximum load to failure and yield strength when compared to similar values for standard pedicle screws. However, thoracic pedicle/rib screws do achieve 70% of the biomechanical strength of standard thoracic pedicle screws. Extra–pedicular placement may be a useful salvage technique when intra–pedicular screw placement is not possible or when a more lateral approach is preferred for safety reasons and maximum fixation is not required.