Cortical Bone Trajectory for Lumbar Pedicle Screw Placement
There have been a number of developments in screw design and implantation techniques, including proposal of an alternative trajectory for screw fixation aimed at increasing purchase of pedicle screws in higher density bone. Cortical bone trajectory (CBT) screw insertion follows a lateral path in the transverse plane and caudocephalad path in the sagittal plane. This technique has been advocated because it is reportedly less invasive, improves screw-bone purchase and reduces neurovascular injury; however, these claims have not been supported by robust clinical evidence. The available evidence was therefore reviewed to assess the relative merits of CBT and highlight areas for further research. To this end, a search of relevant published studies reporting biomechanical, morphometric or clinical outcomes after use of CBT screws in patients with spinal pathologies was performed via six electronic databases 1).
A major drawback to the use of cortical bone trajectory pedicle screws (CBTPS) with traditional posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) grafts is that traditional graft insertion trajectories require a wider posterior exposure. This wider exposure, beyond the limits otherwise required for CBTPS placement, negates a primary benefit of CBTPS fixation.
To define an alternative surgical technique for interbody graft placement that, when used in conjunction with CBTPS fixation, permits both minimal soft tissue dissection and optimal graft placement.
A team of neurosurgeons specializing in the treatment of spinal pathologies developed a surgical technique for insertion of bilateral PLIF grafts that complements the principles of CBTPS fixation. This technique is illustrated in a patient undergoing lumbosacral decompression, CBTPS fixation, and 3-column arthrodesis. RESULTS:
The described technique utilizes a divergent trajectory of bilateral PLIF grafts rather than the traditional parallel or convergent trajectories.
A divergent trajectory affords numerous benefits: first, by aiming medially to laterally with the interbody graft, one recapitulates many advantages of CBTPS, including avoidance of wide tissue dissection, greater intergraft volume available for bone grafting, and greater graft coverage of the hypophyseal ring. The prospective collection of outcome data for patients who undergo lumbosacral fusion using the divergent PLIF technique is ongoing 2).
Santoni et al. were the first one to report the cortical bone trajectory (CBT), in which screws follow a lateral path in the axial plane and caudocephalad path in the sagittal plane. In contrast to conventional pedicle screw fixation, CBT screws do not penetrate the vertebral body trabecular space 3).
Several biomechanical studies have demonstrated the favorable mechanical properties of the cortical bone trajectory (CBT) screw. However, no reports have examined surgical outcomes of posterior lumbar interbody fusion (PLIF) with CBT screw fixation for lumbar degenerative spondylolisthesis (DS) compared with those after PLIF using traditional pedicle screw (PS) fixation. The purposes of this study were thus to elucidate surgical outcomes after PLIF with CBT screw fixation for DS and to compare these results with those after PLIF using traditional PS fixation.
Ninety-five consecutive patients underwent PLIF with CBT screw fixation for DS (CBT group; mean followup 35 months). A historical control group consisted of 82 consecutive patients who underwent PLIF with traditional PS fixation (PS group; mean follow-up 40 months). Clinical status was assessed using the Japanese Orthopaedic Association (JOA) scale score. Fusion status was assessed by dynamic plain radiographs and CT. The need for additional surgery and surgery-related complications was also evaluated. RESULTS The mean JOA score improved significantly from 13.7 points before surgery to 23.3 points at the latest follow-up in the CBT group (mean recovery rate 64.4%), compared with 14.4 points preoperatively to 22.7 points at final follow-up in the PS group (mean recovery rate 55.8%; p < 0.05). Solid spinal fusion was achieved in 84 patients from the CBT group (88.4%) and in 79 patients from the PS group (96.3%, p > 0.05). Symptomatic adjacent-segment disease developed in 3 patients from the CBT group (3.2%) compared with 9 patients from the PS group (11.0%, p < 0.05).
PLIF with CBT screw fixation for DS provided comparable improvement of clinical symptoms with PLIF using traditional PS fixation. However, the successful fusion rate tended to be lower in the CBT group than in the PS group, although the difference was not statistically significant between the 2 groups 4).