Posterior Thoracic Extrapedicular Fixation: A Biomechanical Study

Study Design. In vitro biomechanical testing of thoracic spine specimens using a standardized three-dimensional spine flexibility protocol. Objectives. To compare the mechanical stability of the intrapedicular and extrapedicular technique for pedicle screw placement. The hypothesis was that extrapedicular screw placement provides an equally rigid construct. Summary of Background Data. Pedicle screws provide rigid fixation of instabilities in the lumbar and lumbosacral spine. Anatomic considerations and the potential risk of neurologic complications are the main reasons to hesitate using pedicle screws in the thoracic spine. Extrapedicular fixation would allow safer insertion due to an increased distance to the spinal canal. Methods. Twelve human cadaveric thoracic spines (six intra-, six extrapedicular) were instrumented with the USS system, using computed tomography-based computer navigation to ensure accurate placement. The specimens were tested in flexion–extension, torsion, and lateral bending. The ROM was measured using an optoelectronic system, and the two methods were compared before and after implantation of the USS construct and before and after fatigue testing of the construct. Results. The ROM of the instrumented spine was reduced to less than 50% that of its original ROM. There were no statistically significant differences in the ROM reduction between the intra- and the extrapedicular technique. Cyclic fatiguing of the construct did not significantly increase the ROM. Conclusions. The extrapedicular technique provides a construct for stabilization of the thoracic spine that is as rigid as the conventional intrapedicular technique, but has the advantage of a safer surgical screw insertion.