Experiment On Screw Pull-out Strength Of The Transarticular Screw Of Thoracic Spine

Objective To compare the screw pull-out strength between two transarticular instrumentations and transpedicular instrumentation and to evaluate their biomechanical effects.Methods Five fresh thoracic specimens (T1～T12) were harvested and disarticulated into6segments(T1～T2, T3～T4, T5～T6, T7～T8,T9～T10,T11～T12) for one specimen from the following lever:intervertebral disk,facet articulation and inferior costovertebral articulations,and reserving partly ribs(4～5mm),there are30segments in all. For every segment, transarticular pedicle screws were placed on one side, and pedicle screws and transarticular rib screw were placed on the other side. The screw insertion technique at each segment was randomized for right or left. The starting point for transarticular pedicle screws insertion was located at the inferior facet of superior vertebrae and through the facet articulation and pedicle into vertebral body, the screw diameter is4.5mm;It is the same as the transarticular pedicle screws for transarticular rib screw in starting point and diameter of screw, through inferior facet articulation of superior vertebrae into costal head of the inferior costovertebral articulation; pedicle screws were placed according to transverse process and lamina, the screw diameter is5mm. All the screws were placed under direct visualization. After the length of the screw trajectory were measured., each set of vertebral bodies were mounted in a custom jig for axial pullout testing using a servohydraulic testing machine. The load-displacement curves were obtained for each screw pullout.Results The mean pullout strength for the transarticular pedicle screws was (535.3±241.4) N. This compares with (504.7±218.4) N for the pedicle screws, but there was not significant difference statistically (P=0.595). The mean pullout strength for the transarticular rib screws was (469.7±173.7) N,less than transarticular pedicle screw and pedicle screw, there were significant differences statistically (P=0.005,0.020). The length for the transarticular pedicle screws trajectory was (43.3±5.7)mm., longer than (41.4±5.8) mm for the pedicle screws, but there was not significant difference statistically (P=0.145). The mean length for the transarticular rib screws trajectory was(26.1±3.4)mm, shorter than transarticular pedicle screw and pedicle screw, there were significant differences statistically (P=0.000)Conclusion The mechanical strength of transarticular pedicle screws have an advantage than the pedicle screws, and the transarticular pedicle screw fixation has the biomechanical feasibility of clinical application.