Biomechanical comparison of single- and dual-lead pedicle screws in cadaveric spine.

OBJECT: The pedicle screw (PS) is the cornerstone of spinal instrumentation, and its failure often entails additional surgery. Screw pullout is one of the most common reasons for screw failure, particularly in the elderly population. In this study the authors undertook a biomechanical comparison of the maximum pullout force (MPF) required for single- and dual-lead PSs in cadaver vertebrae. METHODS: Radiographs of 40 cadaveric vertebrae (T11-L5) were obtained, and bone mineral density (BMD) was measured in the lateral plane using dual-x-ray absorptiometry with a bone densitometer. One screw of each design was implanted for side-by-side comparison. Vertebrae were potted and mounted on an MTS test frame for accurate measurement of MPF. A total of 80 PSs were tested, 40 each of single- and dual-lead design types. RESULTS: The average MPF for dual-lead screws (533.89 +/- 285.7 N) was comparable to that of single-lead screws (524.90 +/- 311.6 N) (p = 0.3733). The BMD had a significant correlation with MPF for both dual-lead (r = 0.56413, p < 0.0001) and single-lead screws (r = 0.56327, p < 0.0001). CONCLUSIONS: Barring the effect of BMD, this in vitro biomechanical test showed no significant difference in MPF between single- and dual-lead PSs. Dual-lead PSs can be used to achieve a faster insertion time, without compromising pullout force.

In vitro biomechanical comparison of transpedicular versus translaminar C-2 screw fixation in C2-3 instrumentation.

OBJECT: In instrumentation of the upper cervical spine, placement of pedicle screws into C-2 is generally safe, although there is the potential for injury to the vertebral arteries. Owing to this risk, translaminar screws into C-2 have been used. The aim of this study was to compare the stability of the in vitro cadaveric spine using C-2 laminar compared with C-2 pedicle screws in C2-3 instrumentation. METHODS: Eight fresh frozen human cadaveric cervical spines (C1-6) were potted at C1-2 and C5-6. Pure moments in increments of 0.3 Nm to a maximum of 1.5 Nm were applied in flexion, extension, right and left lateral bending, and right and left axial rotation. Each specimen was tested sequentially in three modes: 1) intact; 2) C2 pedicle screw-C3 lateral mass fixation; and 3) C2 laminar screw-C3 lateral mass fixation. The sequence of fixation testing was randomized. Motion was tracked with reflective markers attached to C-2 and C-3. RESULTS: Spinal levels with instrumentation showed significantly less motion than the intact spine in all directions and with all loads greater than 0.3 Nm (p < 0.05). Although there was no significant difference between C2 pedicle screw-C3 lateral mass fixation and C2 laminar screw-C3 lateral mass fixation, generally the former type of fixation was associated with less motion than the latter. CONCLUSIONS: When pedicle screws in C-2 are contraindicated or inappropriate, laminar screws in C-2 offer a safe and acceptable option for posterior instrumentation.