The cortical bone trajectory (CBT) has attracted attention as a new minimally invasive technique for lumbar instrumentation by minimizing soft-tissue dissection. Biomechanical studies have demonstrated the superior fixation capacity of CBT; however, there is little consensus on the selection of screw size, and no biomechanical study has elucidated the most suitable screw size for CBT. The purpose of the present study was to evaluate the effect of screw size on fixation strength and to clarify the ideal size for optimal fixation using CBT. A total of 720 analyses on CBT screws with various diameters (4.5–6.5 mm) and lengths (25–40 mm) in simulations of 20 different lumbar vertebrae (mean age: 62.1 ± 20.0 years, 8 males and 12 females) were performed using a finite element method. First, the fixation strength of a single screw was evaluated by measuring the axial pullout strength. Next, the vertebral fixation strength of a paired-screw construct was examined by applying forces simulating flexion, extension, lateral bending, and axial rotation to the vertebra. Lastly, the equivalent stress value of the bone-screw interface was calculated. Larger-diameter screws increased the pullout strength and vertebral fixation strength and decreased the equivalent stress around the screws; however, there were no statistically significant differences between 5.5-mm and 6.5-mm screws. The screw diameter was a factor more strongly affecting the fixation strength of CBT than the screw fit within the pedicle (%fill). Longer screws significantly increased the pullout strength and vertebral fixation strength in axial rotation. The amount of screw length within the vertebral body (%length) was more important than the actual screw length, contributing to the vertebral fixation strength and distribution of stress loaded to the vertebra. The fixation strength of CBT screws varied depending on screw size. The ideal screw size for CBT is a diameter larger than 5.5 mm and length longer than 35 mm, and the screw should be placed sufficiently deep into the vertebral body.

Biomechanical evaluation of fixation strength among different sizes of pedicle screws using the cortical bone trajectory: what is the ideal screw size for optimal fixation?

Introduction 
Pedicle screws are commonly employed to restore spinal stability and correct deformities. The Renaissance robotic system was developed to improve the accuracy of pedicle screw placement.


Purpose 
In this study, we developed an intraoperative classification system for evaluating the accuracy of pedicle screw placements through secondary registration. Furthermore, we evaluated the benefits of using the Renaissance robotic system in pedicle screw placement and postoperative evaluations. Finally, we examined the factors affecting the accuracy of pedicle screw implantation.


Results 
Through use of the Renaissance robotic system, the accuracy of Kirschner-wire (K-wire) placements deviating <3 mm from the planned trajectory was determined to be 98.74%. According to our classification system, the robot-guided pedicle screw implantation attained an accuracy of 94.00% before repositioning and 98.74% after repositioning. However, the malposition rate before repositioning was 5.99%; among these placements, 4.73% were immediately repositioned using the robot system and 1.26% were manually repositioned after a failed robot repositioning attempt. Most K-wire entry points deviated caudally and laterally.


Conclusion 
The Renaissance robotic system offers high accuracy in pedicle screw placement. Secondary registration improves the accuracy through increasing the precision of the positioning; moreover, intraoperative evaluation enables immediate repositioning. Furthermore, the K-wire tends to deviate caudally and laterally from the entry point because of skiving, which is characteristic of robot-assisted pedicle screw placement.

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Assessing the Intraoperative Accuracy of Pedicle Screw Placement by Using a Bone-Mounted Miniature Robot System through Secondary Registration

Osteoporosis, the most common form of metabolic bone disease, leads to alterations in bone structure and density that have been shown to compromise the strength of spinal instrumentation. In addition, osteoporosis may contribute to high rates of fracture and instrumentation failure after long posterior spinal fusions, resulting in proximal junctional kyphosis and recurrent spinal deformity. As increasing numbers of elderly patients present for surgical intervention for degenerative and traumatic spinal pathologies, current and future generations of spine surgeons will increasingly be faced with the challenge of obtaining adequate fixation in osteoporotic bone. The purpose of this review is to familiarize the reader with the impact of osteoporosis on spinal instrumentation, the broad variety of techniques that have been developed for addressing these issues, and the biomechanical and clinical evidence in support of the use of these techniques.

Surgical management of spinal conditions in the elderly osteoporotic spine

Study Design:This study is a retrospective cohort study using prospectively collected data.Objective:To compare the effectiveness of posterior lumbar interbody fusion (PLIF) using the cortical bone trajectory (CBT) and conventional pedicle screw (PS) techniques.Summary of Background Data:There are f

Clinical Outcomes After Posterior Lumbar Interbody Fusion: Comparison of Cortical Bone Trajectory and Conventional Pedicle Screw Insertion.

Study design Level III. Objective To report on the outcomes of midline cortical bone trajectory (CBT) pedicle screw surgical technique for posterior lumbar fixation in the outpatient surgery center (OSC) compared with traditional pedicle screws in the hospital. Summary of background data Traditional pedicle screws have been the gold standard for posterior lumbar fusion. Advances in spine surgery, including less invasive procedures have propelled the design of instruments and implants to achieve greater posterior spinal fixation, with decreased tissue destruction and higher safety margins. Biomechanical studies have validated the superior pullout strength of cortical screws versus the traditional pedicle screws and represent an opportunity to perform safe lumbar fusions in OSCs with same day discharge. Materials and methods The medical records of 60 patients with prospectively collected data were reviewed. Two matched cohort groups consisting of 30 patients each, CBT pedicle screws performed in OSC patients (group 1) was compared with traditional pedicle screws performed in hospital patients (group 2). Outcomes were assessed with self-reported Visual Analog Scale (VAS) scores, Oswestry Disability Index scores, and radiologic fusion rate. Results Totally, 33 males and 27 females, age range (28-75), average 58±3 years. Average body mass index was 29±1.15 kg/m. A total of 65% of surgeries were at L5-S1 level. Significant improvement noted in VAS back pain scores in the OSC group from 7.8±0.5 to 2.5±0.7, P=0.001. Comparing intergroup VAS back pain scores and Oswestry Disability Index scores, OSC group demonstrated significant improvement, P=0.004 and 0.027, respectively. Fusion rate at 2 years was similar, P=0.855 between groups. Conclusions We successfully transitioned our lumbar fusions from hospitals to OSCs using a midline CBT pedicle screw technique. Although traditional pedicle screw placement is effective and may be viable in an OSC, we see more advantages to use midline cortical screws over traditional pedicle screws.

Clinical Outcomes With Midline Cortical Bone Trajectory Pedicle Screws Versus Traditional Pedicle Screws in Moving Lumbar Fusions From Hospitals to Outpatient Surgery Centers.

The comparison of pedicle screw and cortical screw in posterior lumbar interbody fusion: a prospective randomized noninferiority trial.

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Jung Hwan Son

Papers

The comparison of pedicle screw and cortical screw in posterior lumbar interbody fusion: a prospective randomized noninferiority trial.

A Comminuted Spiral Fracture with Butterfly Fragment of Distal Humerus by Arm Wrestling -A Case Report-