Thoracic Pedicle vs Pedicle/Rib Screw Fixation: A Biomechanical Study

Introduction: Segmental fixation for thoracic deformity has traditionally been achieved using transverse process, laminar, and pedicle hook instrumentation. Recent reports suggest that for thoracic deformity, pedicle screws constructs achieve superior correction. However, concern has been raised regarding the safety of standard 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 thoracic pedicle screw to an alternatively placed extra–pedicular screw placed using a “safer” lateral insertion technique.

Materials and Methods: Four fresh–frozen human cadaveric thoracic spines were harvested with the medial 5–6cm of rib, intercostal soft tissue and the overlying parietal pleura intact. All nonstructural soft tissue was removed. The intra–spinous, and costo–vertebral ligaments and facet capsules were carefully maintained. The specimens were instrumented on one side with intra–pedicular screws (standard pedicle screws) and on the opposite side using an extra–pedicular (pedicle/rib) technique. 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 using fluoroscopy. Fixed angle 5.5 mm x 45 mm, M–8 (Medtronic Sofamor Danekä) stainless steel pedicle screws were implanted. Fluoroscopy was used to verify the location of each screw after implantation. 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 that level. A loading rate of 50/N per 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 extrapedicular (pedicle/rib). The 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). The 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 insertion 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 thoracic intra–pedicular screws. The data from this study suggests that thoracic pedicle screws are significantly stronger than 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 deviations 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 screw 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.