Depth of vertebral screw insertion using a cortical bone trajectory technique in lumbar spinal fusion: radiological significance of a long cortical bone trajectory

A 3D-CT Study of the Cortical Bone Trajectory Screw Placement Parameters Based on Lumbar CT

OBJECTIVE

The cortical bone trajectory (CBT) technology is an effective substitute for traditional pedicle screw (PS) technology. However, there is still controversy about the CBT screw technology placement strategy. The objective of this study was to simulate cortical screw placement with the help of three-dimensional (3D) software, to discuss the differences in screws between genders and vertebral segments, and to explore a safer and more efficient strategy for cortical screw placement.

METHODS

Mimics Medical software was used to construct a 3D model of the lumbar spine, and the placement of CBT screws was simulated. The volume of each vertebral body from L1 to L5, the pedicle isthmus height (IH), the pedicle isthmus width (IW), and the sagittal vertebral distance (SAVD) were measured. The transverse distance (TD) and the longitudinal distance (LD) between the ideal starting point (SP) and the clinical SP (the intersection Q of the midline of the superior articular process and the horizontal line 1 mm below the transverse process) were measured. The cephalad angle (CA), lateral angle (LA), maximum screw diameter (MSD), maximum screw length (MSL) of each trajectory of the L1 to L5 vertebral bodies, and the percentage of the screw insertion depth (PSID) into the vertebral body were measured. Data were statistically analyzed using Student's t-test, one-way analysis of variance (ANOVA), and Tukey's test.

RESULTS

Vertebral anatomical parameters and CBT screw parameters differed between males and females. Female patients had lower IH, IW, SAVD, CA, LA, MSD, and MSL than males. IH was greatest in L1 (male, 17.81 mm; female, 16.12 mm) and the smallest in L5 (male, 14.11 mm; female, 13.05 mm). IW was smallest in L1 (male, 8.89 mm; female, 7.37 mm) and greatest in L5 (male, 16.59 mm; female, 15.43 mm). The MSD of males was smallest in L1 (6.05 mm) and greatest in L3 (7.06 mm); the MSD of females was smallest in L1 (5.13 mm) and greatest in L4 (6.64 mm). MSL was greatest at L3 (male, 33.63 mm; female, 32.28 mm) and smallest at L5 (male, 31.25 mm; female, 29.97 mm). CA was smallest in L1 (male, 22.80°; female, 21.92°) and greatest in L3 (male, 25.29°; female, 24.33°). LA was smallest in L1 (male 12.37°, female 11.27°) and greatest in L5 (male 13.56°, female 12.96°). Among the males, TD was smallest at L1 (-0.51 mm) and greatest at L5 (1.37 mm), while LD was greatest at L2 (3.46 mm) and smallest at L5 (2.40 mm). In females, TD was greatest at L1 (0.12 mm) and smallest at L3 (-0.51 mm), while LD was greatest at L1 (3.69 mm) and smallest at L5 (2.08 mm). In the overall sample, the incidence of SAVD and PSID gradually increased from L1 to L5.

CONCLUSION

The optimal screw placement strategy for CBT screws varies significantly according to sex and vertebral body segments, particularly noting the specificity of screw placement at L5. The optimal screw placement strategy should be selected based on the patient's sex and segment characteristics before surgery to maximize the safety and accuracy of CBT screw placement.

Comparison of Safety and Perioperative Outcomes Between Patient-specific Template-Guided and Fluoroscopic-Assisted Freehand Lumbar Screw Placement Using Cortical Bone Trajectory Technique

STUDY DESIGN

Non-randomized prospective controlled study.

OBJECTIVES

To compare the safety and perioperative outcomes between patient-specific template-guided and fluoroscopic-assisted freehand techniques in transforaminal lumbar interbody fusion (TLIF) using cortical bone trajectory (CBT).

METHODS

The subjects consisted of 94 consecutive patients who underwent single-level TLIF using CBT. The standard trajectory was set so as to start from the pars interarticularis, pass the inferior border of the pedicle, and end around the middle of the vertebral endplate. Template guide technique was performed in 66 patients (Guide group), and fluoroscopic-assisted freehand technique was performed in 28 patients (Freehand group). Intraoperative parameters, screw placement accuracy, and complications were compared between the two techniques.

RESULTS

The Guide group had significantly shorter operative and radiation exposure times than the Freehand group (operative time 84.6 ± 16.7 vs 93.0 ± 15.0 minutes; P = .023, radiation exposure time 7.0 ± 6.0 vs 20.4 ± 11.8 seconds; P < .001, respectively). The screw diameter and the screw insertion depth in the vertebra in the Guide group were significantly greater than those in the Freehand group. The degree and incidence of facet joint violation were comparable between the two groups, while the accuracy of screw placement was significantly different, with no perforation rate of 97.7% in the Guide group vs 82.1% in the Freehand group (P < .001). No significant difference was found in the rate of clinically relevant complications between the two groups.

CONCLUSIONS

The template-guided technique provided a safe and highly accurate option for CBT screw placement.

Radiation Exposure During Lumbar Interbody Fusion Surgery Can Be Reduced by Using a Three-Dimensional Patient-Specific Template Guide

BACKGROUND

The use of posterior lumber interbody fusion (PLIF) using cortical bone trajectory (CBT) with a patient-specific 3D template guide is increasingly widespread. To our knowledge, no studies have extensively evaluated the reduction of radiation exposure when using patient-specific drill template guides. The purpose of this study is to compare the intra-operative radiation dose and surgeon's exposure to radiation in CBT-PLIF when using a patient-specific drill guide with that in traditional minimally invasive (MIS)-PLIF.

METHODS

In this observational study, we retrospectively compared data from five patients who were treated with single-level CBT-PLIF using a patient-specific drill guide (G group) and five patients who were treated with single-level traditional MIS-PLIF (M group). We compared the surgical time, surgeon's exposure to radiation, and intra-operative radiation time and dose between the two groups of patients.

RESULTS

The mean age of the patients was 67.0 years in the M group and 74.2 years in the G group. The average surgical time was 242.8 min in the M group and 189.6 min in the G group (p = 0.020). The surgeon's exposure to radiation was 373.7 µSv in the M group and 81.75 µSv in the G group at chest level outside the protector (p = 0.00092); 42.0 µSv (M group) and 3.6 µSv (G group) at chest level inside the protector (p = 0.0000062); and 4.33 µSv (M group) and 1.20 µSv (G group) at the buttocks of the surgeon (p = 0.0013). Radiation time was 269.8 s (M group) and 56.6 s (G group) (p = 0.0097), and radiation dose was 153.7 mGy (M group) and 30.42 mGy (G group) (p = 0.00057).

CONCLUSION

The patient-specific drill template guide is an invaluable tool that facilitates the safe insertion of CBT screws with a low radiation dose from the outset.

Influence of Pedicle Screw Insertion Depth on Posterior Lumbar Interbody Fusion: Radiological Significance of Deeper Screw Placement

STUDY DESIGN

Retrospective case series.

OBJECTIVES

To investigate the influence of screw size on achieving bone fusion in posterior lumbar interbody fusion (PLIF).

METHODS

In total, 137 consecutive patients with L4 degenerative spondylolisthesis who underwent single-level PLIF at L4-L5 were evaluated. Factors investigated for their contribution to bone fusion included: 1) age, 2) sex, 3) body mass index, 4) bone mineral density, 5) intervertebral mobility, 6) screw diameter, 7) screw length, 8) screw fitness in the pedicle (%fill), 9) screw depth in the vertebra (%depth), 10) screw angle, 11) facetectomy, 12) crosslink connector, and 13) cage material.

RESULTS

Bone fusion was confirmed in 88.2% of patients. The comparison between fusion (+) and fusion (-) groups showed no significant differences in screw size. The %fill and %length were significantly greater in the fusion (+) group than in the fusion (-) group (%fill: 58.5% ± 7.5% vs 52.3% ± 7.3%, respectively, P = .005; %depth: 59.8% ± 9.7% vs 50.3% ± 13.8%, respectively, P = .025). Multivariate logistic regression analysis revealed that %fill (odds ratio [OR]= 1.11, P = .025) and %depth (OR = 1.09, P = .003) were significant independent factors affecting bone fusion. Receiver operating characteristic curve analyses identified a %fill of 60.0% and a %depth of 54.2% as optimal cutoff values for achieving bone fusion.

CONCLUSIONS

Screw size should be determined based on the screw fitness in the pedicle (%fill > 60%) and screw insertion depth in the vertebral body (%depth > 54.2%) according to individual vertebral anatomy in L4-L5 PLIF.

Biomechanical comparative study of midline cortical vs. traditional pedicle screw trajectory in osteoporotic bone

INTRODUCTION

In lumbar spinal stabilization pedicle screws are used as standard. However, especially in osteoporosis, screw anchorage is a problem. Cortical bone trajectory (CBT) is an alternative technique designed to increase stability without the use of cement. In this regard, comparative studies showed biomechanical superiority of the MC (midline cortical bone trajectory) technique with longer cortical progression over the CBT technique. The aim of this biomechanical study was to comparatively investigate the MC technique against the not cemented pedicle screws (TT) in terms of their pullout forces and anchorage properties during sagittal cyclic loading according to the ASTM F1717 test.

METHODS

Five cadavers (L1 to L5), whose mean age was 83.3 ± 9.9 years and mean T Score of -3.92 ± 0.38, were dissected and the vertebral bodies embedded in polyurethane casting resin. Then, one screw was randomly inserted into each vertebra using a template according to the MC technique and a second one was inserted by freehand technique with traditional trajectory (TT). The screws were quasi-static extracted from vertebrae L1 and L3, while for L2, L4 and L5 they were first tested dynamically according to ASTM standard F1717 (10,000 cycles at 1 Hz between 10 and 110 N) and then quasi-static extracted. In order to determine possible screw loosening, there movements were recorded during the dynamic tests using an optical measurement system.

RESULTS

The pull-out tests show a higher pull-out strength for the MC technique of 555.4 ± 237.0 N compared to the TT technique 448.8 ± 303.2 N. During the dynamic tests (L2, L4, L5), 8 out of the 15 TT screws became loose before completing 10,000 cycles. In contrast, all 15 MC screws did not exceed the termination criterion and were thus able to complete the full test procedure. For the runners, the optical measurement showed greater relative movement of the TT variant compared to the MC variant. The pull-out tests also revealed that the MC variant had a higher pull-out strength, measuring at766.7 ± 385.4 N, while the TT variant measured 637.4 ± 435.6 N.

CONCLUSION

The highest pullout forces were achieved by the MC technique. The main difference between the techniques was observed in the dynamic measurements, where the MC technique exhibited superior primary stability compared to the conventional technique in terms of primary stability. Overall, the MC technique in combination with template-guided insertion represents the best alternative for anchoring screws in osteoporotic bone without cement.

Application of the cortical bone trajectory technique in posterior lumbar fixation

The cortical bone trajectory (CBT) is a novel technique in lumbar fixation and fusion. The unique caudocephalad and medial-lateral screw trajectories endow it with excellent screw purchase for vertebral fixation via a minimally invasive method. The combined use of CBT screws with transforaminal or posterior lumbar interbody fusion can treat a variety of lumbar diseases, including spondylolisthesis or stenosis, and can also be used as a remedy for revision surgery when the pedicle screw fails. CBT has obvious advantages in terms of surgical trauma, postoperative recovery, prevention and treatment of adjacent vertebral disease, and the surgical treatment of obese and osteoporosis patients. However, the concept of CBT internal fixation technology appeared relatively recently; consequently, there are few relevant clinical studies, and the long-term clinical efficacy and related complications have not been reported. Therefore, large sample and prospective studies are needed to further reveal the long-term complications and fusion rate. As a supplement to the traditional pedicle trajectory fixation technique, the CBT technique is a good choice for the treatment of lumbar diseases with accurate screw placement and strict indications and is thus deserving of clinical recommendation.