Significance of the Pars Interarticularis in the Cortical Bone Trajectory Screw Technique: An In Vivo Insertional Torque Study

Abstract

Study design: Retrospective study.

Purpose: Cortical bone trajectory (CBT), a more medial-to-lateral and shorter path than the traditional one for spinal fusion, is thought to be effective for severely degenerated vertebrae because screws are primarily stabilized at the posterior elements. We evaluated the efficacy of this approach through in vivo insertional torque measurement.

Overview of literature: There has been only one prior in vivo study on CBT insertional torque.

Methods: Between January 2013 and April 2014, a total of 22 patients underwent posterior lumbar fusion using the CBT technique. The maximum insertional torque, which covers the radial strength needed for insertion, was measured for 113 screws, 8 of which were inserted for L5 spondylolysis. The insertional torque for cases with (n=8) and without (n=31) spondylolysis of L5 were compared using one-way analysis of variance (ANOVA). To evaluate vertebral degeneration, we classified 53 vertebrae without spondylolysis by lumbar radiography using semiquantitative methods; the insertional torque for the 105 screws used was compared on the basis of this classification. Additionally, differences in insertional torque among cases grouped by age, sex, and lumbar level were evaluated for these 105 screws using ANOVA and the Tukey test.

Results: The mean insertional torque was significantly lower for patients with spondylolysis than for those without spondylolysis (4.25 vs. 8.24 in-lb). There were no statistical differences in insertional torque according to vertebral grading or level. The only significant difference in insertional torque between age and sex groups was in men <75 years and women ≥75 years (10 vs. 5.5 in-lb).

Conclusions: Although CBT should be used with great caution in patient with lysis who are ≥75 years, it is well suited for dealing with severely degenerated vertebrae because the pars interarticularis plays a very important role in the implementation of this technique.

Keywords: Cortical bone trajectory; Insertional torque; Pars interarticularis; Spondylolysis; Vertebral degeneration.

Fig. 1. Photograph of the torque measurement device.

Fig. 2. Postoperative computed tomography demonstrating typical screw insertion in L5: sagittal view (A) and axial view (B) of L5 screws for a case with spondylolysis. Sagittal view (C) and axial view (D) for a case without spondylolysis.

Fig. 3. The mean±SEM for maximum insertional torque in cases with and without spondylolysis in L5 (p =0.040). SEM, standard error of mean.

Fig. 4. The mean±SEM for maximum insertional torque by age and sex group. Significantly greater torque was used by men <75 years than women ≥75 years (p=0.038). SEM, standard error of mean.

Fig. 5. The mean±SEM for maximum insertional torque classified by a semiquantitative method. The differences were not significant. SEM, standard error of mean.

Fig. 6. The mean±SEM for maximum insertional torque according to vertebral level. The differences were not significant. SEM, standard error of mean.