Experimental Study And Primary Clinical Application Of Patient-specific Navigational Template For The Thoracic Pedicle Screws Placement

ObjectiveThe aim of paper was to search for and develop a convenient and safe and available method for the thoracic pedicle screws placement.First of all,to design and generate a novel patient-specific navigational template by computer aided design and rapid prototyping and to observe the positioning accuracy of the template relative to the orientation of thoracic pedicle.Subsequently,to investigate feasibility of thoracic transpedicular screw placement with patient-specific drill template technique in the thoracic cadavers and further to validate the accuracy and safety by the primary clinical application.Methods1. Volumetric CT scan was performed on each of selected 12 complete dried thoracic vertebrae specimens and a 3-D reconstruction model was generated from the CT scan data. The optimal screw size and orientation were determined and a drill template was designed with a surface that is the inverse of the posterior vertebral surface. The drill template and its corresponding vertebra were manufactured using rapid prototyping technique. Subsequently, a diameter 2.0mm drill bit was inserted in the thoracic vertebrae specimen and its biomodel assisted by the drill template to simulate pedicle screw placement. The position and direction of drill bit in the pedicle and vertebral body was judged by visual observation. The accuracy of the rapid prototyping biomodel was measured by physical measurement With a vernier caliper of 0.02mm accuracy.2.20 thoracic cadavers were randomly divided into two groups, the navigational template group and the free hand group respectively 10 samples.All screws were implanted by the same orthopaedic surgeon with transpedicular screw placement experiences in the lumbar spines but no experiences in the thoracic spines. CT scanning were performed to to evaluate screw exit rates and orientation relative to the pedicle axis and further to measure perforation distance. Subsequently, all samples were dissected to validate the Screw positioning by visual observation. The relative position of the screw to the pedicle was graded.3. From July 2008 to September 2009,92 screws were inserted in the thoracic spines with patient-specific drill template technique in 11 patients undergoing spinal deformity correction surgeries using posterior pedicle screw instrumentation of the thoracic spine. After surgery, the positions of the pedicle screws were evaluated using CT scan and graded for validation.The complications involved in screws implant were recorded.Results1. The drill bits were completely in the pedicle and vertebral body and in the same direction with the thoracic pedicles. There was the same anatomy morphous between the thoracic biomodel and its corresponding thoracic vertebrae specimen. the contours was unanimous between the template and posterior anatomy morphous of the thoracic vertebrae specimen. The error between the thoracic vertebrae and its corresponding biomodel was 0.49mm±0.28mm.2.240 thoracic screws were implanted respectively thoracic transpedicular screw placement with patient-specific drill template technique in the navigational template group and the free hand group.224 screws (93.4%) in the navigational template group and 156 screws (65%) in the free hand group were placed completely within the pedicle respectively.The accuracy rate was statistically significant between the two groups (P＜0.05).The accuracy rate was higher in navigational template group than in the free hand group. The average piercing distance was 0.95mm±0.49mm in navigational template group and 2.40mm±0.10mm in the free hand group respectively. The average piercing distance was statistical significance (P＜0.05), The average piercing distance in navigational template group was less than the free hand group. The free hand group had an significant learning curve, learning curve of the navigational template group was not obvious.3. Total of 92 screws were placed from T2 to T12 in the 11 cases and postoperation CT scans were obtained in all 11 patients.83 screws were considered intrapedicular, while 9 screws were considered to have a 0-2 mm breach (2 medial,7 lateral in which 5 belonged to the planned in-out-in screws). No pedicle screw breach more than 2 mm, of these, overall screw accuracy (＜2 mm breach is safe) was 100%. No screws penetrated the inferior or superior cortex in the sagittal plane. The unintended perforation rate was 4.3%. There were no neurovascular and visceral related-complications by using this technique in the present study.Conclusion1. Accurate location and orientation of the thoracic pedicle can be obtained by the rapid prototyping patient-specific navigational template.2. Thoracic transpedicular screw placement with patient-specific drill template technique successfully reduced the perforation rate and insertion angle errors,demonstrating the clear advantage in safe and accurate pedicle screw placement.Learning curve of the technique was not obvious. The operative procedure is simple, without special experience requirements for surgeon.lt is especially implemental for beginners.3. Thoracic transpedicular screw placement with patient-specific drill template technique is a simple, safe, practical and feasible method.