Pedicle screw fixation is an effective and reliable method for achieving stabilization in lumbar degenerative disease. The procedure carries a risk of violating the spinal and neural canal which can lead to nerve injury. This audit examines the accuracy of screw placement using intra-operative image guidance.

Retrospective audit of patients undergoing lumbar pedicle screw fixation using image guidance systems over an 18-month period. Case records were reviewed to identify complications related to screw placement and post-operative CT scans reviewed to study the accuracy of screw position.

Of the 98 pedicle screws placed in 25 patients, pedicle violation occurred in 4 screw placements (4.1%). Medial or inferior breach of the pedicle cortex was seen in 2 screws (2%). Nerve root injury as a consequence of this violation was seen in one patient resulting in irreversible partial nerve root dysfunction. Mean set up time for the guidance system was 42 minutes. The mean operative time was 192 minutes.

Violation of either the medial or inferior pedicle cortex during placement of fixation screws is a rare, but potentially serious complication bearing lasting consequences. Image-guided placement can be helpful and possibly improve accuracy; particularly in patients with distorted spinal anatomy.

Summary Statement. Incorrect pedicle screw placement can lead to neurological complications. Practice outside the operating room on realistic bone models, with force feedback, could improve safety. Pedicle forces in cadaveric specimens are reported, to support development of a training tool for residents. Introduction. Inserting screws into the vertebral pedicles is a challenging step in spinal fusion and scoliosis surgeries. Errors in placement can lead to neurological complications and poor mechanical fixation. The more experienced the surgeon, the better the accuracy of the screw placement. A physical training system would provide orthopaedic residents with the feel of performing pedicle cannulation before operating on a patient. The proposed system consists of realistic bone models mimicking the geometry and material properties of typical patients, coupled with a force feedback probe. The purpose of the present study was to determine the forces encountered during pedicle probing to aid in the development of this training system. Methods. We performed two separate investigations. In the first study, 15 participants (9 expert surgeons, 3 fellows, 3 residents) were asked to press a standard pedicle awl three times onto a mechanical scale, blinded to the force, demonstrating what force they would apply during safe pedicle cannulation and during unsafe cortical breach. In the second study, three experienced surgeons used a standard pedicle awl fitted with a one-degree of freedom load cell to probe selected thoracolumbar vertebrae of eight cadaveric specimens to measure the forces required during pedicle cannulation and deliberate breaching, in randomised order. A total of 42 pedicles were tested. Results. Both studies had wide variations in the results, but were in general agreement. Cannulation (safe) forces averaged approximately 90 N (20 lb) whereas breach (unsafe) forces averaged approximately 135–155 N (30–35 lb). The lowest average forces in the cadaveric study were for pedicle cannulation, averaging 86 N (range, 23–125 N), which was significantly lower (p<0.001) than for anterior breach (135 N; range, 80–195 N); medial breach (149 N; range, 98–186 N) and lateral breach (157 N; range, 114–228 N). There were no significant differences among the breach forces (p>0.1). Cannulation forces were on average 59% of the breach forces (range, 19–84%) or conversely, breach forces were 70% higher than cannulation forces. Discussion. To our knowledge, axial force data have not previously been reported for pedicle cannulation and breaching. A large range of forces was measured, as is experienced clinically. Additional testing is planned with a six-degree-of-freedom load cell to determine all of the forces and moments involved in cannulation and breaching throughout the thoracolumbar spine. These results will inform the development of a realistic bone model as well as a breach prediction algorithm for a physical training system for spine surgery. The opportunity to learn and practice outside of the operating room, including learning from deliberate mistakes, should increase the confidence and comprehension of residents performing the procedure, enhance patient safety, reduce surgical time, and allow faster progression of learning inside the operating room

Background. The overall incidence of neurological symptoms attributed to lumbar misplaced screws has been described to occur in 3.48% of patients undergoing surgery. These lumbar radicular neurological lesions are undetected with conventional intraoperative neurophysiological and radiological controls. The hypothesis of this study was that direct stimulation of the pedicle screw after placement in the lumbar spine may not work as well as for screws placed in the thoracic pedicles. A more suitable method for the lumbar spine could be the stimulation of the pedicle track with a ball-tipped probe. Methods. Comparative observational study on the detection of malpostioned lumbar pedicle screws using two different techniques in two different periods: t-EMG screw stimulation (2011–2012) and track stimulation (2013–2014). A total of 1440 lumbar pedicle screws were placed in 242 patients undergoing surgery for vertebral deformities in the last four years (2011–2014). In the first two years, 802 lumbar screws were neuromonitored using t-EMG during. In the last two years, 638 screws were placed after probe stimulation of the pedicle track. Standardised t-EMG conventional registration and fluoroscopy were afterwards performed in all cases. Results. Six patients (4.4%) in the t-EMG group without signs of screw misplacement developed radicular pain. After checking with CT scan, a caudal prominence of the screw at the inferior aspect of the pedicle was detected in 7 screws (0.9%) and they were removed. After removal, probe stimulation was performed at the middle track showing abnormal thresholds (3.9–9.7mA). In the second group (track stimulation), 11 cases (10.8%) had thresholds below 7 mA. In these cases, the intrapedicular route was changed. None of these 106 patients presented postoperative radiculopathy and CT scans showed that all screws were well positioned. Conclusions. The t-EMG stimulation of lumbar pedicle screws offer some false negatives cases. However, the record in the middle pedicle track is able to detect misplaced screws and prevent the development of lumbar radiculopathy. Therefore, systematic pedicle track stimulation is strongly recommended in the lumbar spine. Level of Evidence. Level III