The effect of end screw orientation on the stability of anterior instrumentation in cyclic lateral bending.

BACKGROUND CONTEXT: Screw pullout at the proximal or distal end of multilevel anterior instrumentation can occur clinically. Previous laboratory studies have shown that angulation of vertebral body screws increases screw pullout strength and stability in toggling. PURPOSE: To determine the effect of end screw angulation on instrumentation construct stability after cyclic, lateral bending. STUDY DESIGN: A biomechanical study in calf spines comparing two anterior spinal instrumentation constructs, one with parallel polyaxial screws and the other with angled polyaxial end screws. METHODS: Sixteen instrumented constructs were made from eight thoracic (T8-T12) and eight lumbar calf spines (L1-L5). Eight (four lumbar specimens and four thoracic specimens) had five bicortical screws inserted mid-body and parallel to the end plates. The other eight specimens had two screws angled toward the superior end plates of the top two vertebrae; the middle vertebra had a mid-body screw parallel to the end plate, and the bottom two vertebrae had screws angled towards their inferior end plates. The constructs were then cycled in lateral bending, and the displacements of the two instrumentations with a 10 N-m bending load were compared. RESULTS: After 10,000 cycles, constructs with parallel end screws exhibited twice the average displacement than those with angled screws: 5.4 mm versus 2.9 mm (p=.031). CONCLUSION: The use of angled screws at the ends of anterior constructs demonstrated increased construct stability after cycling compared with traditional transverse screws. Although angled screw insertion is technically more difficult and is possible only with specific screw designs, its use might increase instrumentation longevity.

Using triggered electromyographic threshold in the intercostal muscles to evaluate the accuracy of upper thoracic pedicle screw placement (T3-T6).

STUDY DESIGN: A prospective clinical study of high thoracic pedicle screws monitored with triggered electromyographic (EMG) testing. OBJECTIVE: To evaluate the sensitivity of recording intercostal muscle potentials to assess upper thoracic screw placement. SUMMARY OF BACKGROUND DATA: Triggered EMG testing from rectus muscle recordings, which are innervated from T6 to T12, has identified medially placed thoracic pedicle screws. No clinical study has correlated an identical technique with the intercostal muscle for upper pedicle screws placed in the upper thoracic spine (T3-T6). METHODS: A total of 311 high thoracic screws were placed in 50 consecutive patients. Screws were placed from T3 to T6 and were evaluated using an ascending method of stimulation until a compound muscle action potential was obtained from the intercostal muscles. Screw position was then evaluated using computed tomography and results were compared with evoked EMG threshold values. RESULTS: Fifteen screws (5%) showed penetration on postoperative computed tomography scans. Eleven screws showed medial cortical breakthrough (3.6%), 6 had stimulation thresholds 20 mA. Of the 296 screws with thresholds between 6 and 20 mA, 285 (91%) were within the vertebra. No postoperative neurologic complications were noted in any of the 50 patients. CONCLUSION: In this series, cortical violation is highly unlikely in patients whose stimulation threshold lies between 6 and 20 mA with values 60% to 65% decreased from the mean (98% negative predictive value). Although verification of screw placement should not depend solely on stimulation thresholds, pedicle screw stimulation provides rapid and useful intraoperative information on screw placement during procedures involving the use of thoracic pedicle screws.

Comparing the clinical and radiological outcomes of pedicular transvertebral screw fixation of the lumbosacral spine in spondylolisthesis versus unilateral transforaminal lumbar interbody fusion (TLIF) with posterior fixation using anterior cages.

STUDY DESIGN: This study retrospectively compares the clinical and radiologic outcomes of unilateral transforaminal lumbar interbody fusion (TLIF) with those of transvertebral screw fixation of the lumbosacral spine in high-grade spondylolisthesis. OBJECTIVE: To examine the outcome and perioperative complications of unilateral TLIF and compare those results with Transvertebral Screw Fixation in the treatment of high-grade spondylolisthesis. SUMMARY OF BACKGROUND DATA: High-grade spondylolisthesis has been associated with a high complication and failure rate regardless of the method of treatment. We compare 2 techniques to improve success rates. METHODS: Forty patients were divided into 2 groups: group A, unilateral TLIF, and group B, transacral screws. The mean age was 33 years (range, 19-48 years), and the mean follow-up was 35 months (range, 24-48 months). The mean grade of spondylolisthesis measured by Meyerding grading was 3.6 (range, 3-5). A Scoliosis Research Society outcome score was obtained on all patients. Fusion was determined by both radiograph and computed tomography scan. RESULTS: Group A: 100% fusion. The slip angle improved from 38.6 degrees (range, 24-78 degrees ) before surgery to 23.8 degrees (range, 12-38 degrees ) after surgery. Group B: 95% of patients evidenced solid fusion by the 6-month follow-up. The slip angle, improved from 38.2 degrees (range, 22-78 degrees ) before surgery to 23 degrees (range, 9-36 degrees ) after surgery. There was no significant improvement in the percentage slip or the sacral inclination in any of the groups. COMPLICATIONS: A: 7 unintended durotomies and 3 wound infections. B: 1 unintended durotomy, 1 pseudarthrosis, 2 wound infections, and 1 implant failure. There were no neurologic complications in any of the groups. The Scoliosis Research Society outcome instrument demonstrated good postoperative pain control, function, self-image, and satisfaction in both groups. CONCLUSION: No significant differences in radiologic and clinical outcome were found, in either group. Both procedures appear to be safe and effective surgically and radiographically.