The Imaging Anatomy And Biomechanics Studies Of Cortical Bone Trajectory(CBT) Screws Application In Middle-upper Thorax

Objective Through the two dimensional image measurement of the upper-middle thoracic spine,in vivo anatomical measurement,the establishment of a digital 3D model to determine the optimal trajectory and imaging measurement to determine the the feasibility of application of cortical bone trajectory screw in upper-middle thoracic spine.According to the results of 3D model simulation and anatomical imaging measurement,CBT screws were implanted in vitro.Then we verified the accuracy of screw placement.In vitro biomechanical test and finite element analysis evaluated the biomechanical stability.Finally,the individual minimally invasive screw 3D navigation template was established,and the accuracy of screw placement was verified.Methods The eight indexes of CT scanning data of upper middle thoracic spine(T3-T8)were measured in 80 adults:a:width of pedicle;b:height of pedicle;c:the transverse angle(the angle between the midline of the vertebral body and pedicle transverse axis);d:the sagittal angle(the angle between the upper endplate and vertebral pedicle sagittal axis);e:the maximum diameter of CBT screw(the maximum width of pedicle cancellous bone);f:the maximum length of CBT screw;f1:the length of CBT in body;f2:the length of CBT in pedicle;g:the cephaled angle of CBT screw(the angle between the upper endplate and CBT screw trajectory).Two dimensional CT data were imported into the Mimics software for 3D model reconstruction.On the establishment of a 3D model of the upper middle thoracic spine,the best cortical bone screw trajetory(screw radius is 2mm)was simulated.According to the result of simulation,the insertion point was determined.The distance between the insertion point and the midline of the superior articular process was measured.The distance between the insertion point and the midline of the transverse process was measured.A safe area was determined by Kirschner wire simulating the CBT screw placed in the cadaveric vertebral body.According to the results of two and three dimensional imaging measurement and anatomy measurement,the CBT screw was placed in cadaver specimen.The accuracy of screw placement was evaluated.12 fresh cadavers were divided into pedicle screw group and cortical screw group.The flexion extension,lateral bending and rotation range of motion were test in intact specimen,injured specimen and specimen with CBT screw and rod fixed.Then The fatigue test of the 1000 cycle was test.After test,the flexion extension,lateral bending and rotation range of motion were test again.Four groups of range motion were compared.Pollout strengths of pedicle screw group and cortical screw group were test and compared after the faigue test.At the same time,the finite element model was established.Digital simulation of pedicle screw and cortical screw were test in finite element model.Finally,an individualized navigation template was designed by 3D printing technology.The cortical screw was inserted with help of the template and the accuracy of the screw was verified on the C arm.Results Feasibility of the application of cortical bone screw were proved by anatomy and two and three dimensional imaging measurements.The insertion point and angle of CBT screw were determined.The insertion point of CBT screw in thoracic 3-4 was located at the midpoint of the midline of the transverse process and the superior articular process.The insertion point of CBT screw in thoracic 5-6 was located at the midpoint of the above 1/3 line of the transverse process and the superior articular process.The insertion point of CBT screw in thoracic 3-4 was located at the midpoint of the midline of the transverse process and the superior articular process.The length of CBT screw was 25-30mm,diameter of CBT was 4.0-5.Omm.The direction of the CBT screw was perpendicular to the lamina in sagittal and up about 15-20 degrees in coronal position.Under monitoring of C-arm machine,the CBT screw was place in specimens.The average excellent and good rate of screw was 95.4%.Biomechanical analysis showed that there was no significant difference between cortical screws and pedicle screws group in specimen's range of motion.But after fatigue test,the CBT screw groups had better screw insertion torque and pullout strength.The finite element analysis showed cortical screws had a better biomechanical stability.The stress distribution of Vertebral body and pedicle was less than the pedicle screw group.The corresponding navigation template is established by cutting irregular surface of lamina.After the screw placing by navigation template and examining the X-ray,it was found that the position of the screw and the location of the best trajectory in 3D model were almost the same.Conclusions The digital channels optimization cortical bone trajectory(CBT)screws application in middle-upper thorax was feasible which had stable biomechanical properties.The individualized 3D navigation template can be applied to the upper and middle thoracic spine more minimally invasive and safe.The cortical screw in middle and upper thoracic will be a new technique for spinal surgery,especially fo patients with osteoporosis and screw failure.