The education of residents in the proper placement of pedicle screws is key to the safety of the surgery. The more experienced the surgeon, the more accurately the pedicle screws tend to be placed. A physical bone model, with properties and tactile feel similar to human bone, was developed with the intention of using the bone model to train residents in pedicle screw placement. The purpose of this study was to test whether the model improves the performance of orthopaedic residents when cannulating spinal pedicles, as judged by the number of breaches, and to gain feedback from the residents on their experiences. Six orthopaedic residents were recruited, with ethics approval. Prior to testing, the residents were given an instructional video describing the correct cannulation of a lumbar vertebra. The residents were each provided with 12 bones mounted in holders: 3 for initial skills assessment, 6 for free practice, and 3 for final skills assessment. In the pre- and post-practice sets, the 3 bone models had different properties: weak, normal and strong. The residents were asked to complete both pre and post-testing questionnaires. The number of breaches was counted in initial and final bone testing. The forces for each bone model were compared using an ANOVA; these were followed by post-hoc t-tests if significant (p<0.05).Introduction
Materials and Methods
Inserting screws into the vertebral pedicles is a challenging step in spinal fusion and scoliosis surgeries. Errors in placement can lead to neurological complications. The more experienced the surgeon, the better the accuracy of the screw placement. A physical training system would provide 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. We performed two separate investigations: [1] 15 participants (9 expert surgeons, 3 fellows and 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; [2] 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. A total of 42 pedicles were tested. 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), 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 between 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. To our knowledge, these axial force data are the first available 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.
