Morphometric measurement of the lumbosacral spine for minimally invasive cortical bone trajectory implant using computed tomography

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.

Evaluating the contact anatomy and contact bone volume of spinal screws using a novel drilled surface image

Intraoperative navigation systems have been widely applied in spinal fusion surgery to improve the implantation accuracy of spinal screws using orthogonal tomographic and surface-rendering imaging. However, these images contain limited anatomical information and no information on bone volume contact by the implanted screw, which has been proven to affect the stability of implanted screws. This study proposed a novel drilled surface imaging technique that displays anatomical integration properties to calculate the contact bone volume (CBV) of the screws implanted along an implantation trajectory. A cylinder was used to represent the area traversed by the screws, which was manually rotated and translated to a predetermined implantation trajectory according to a vertebra model obtained using computed tomography (CT) image volumes. The drilled surface image was reconstructed by interpolating the CT numbers at the predefined sampling points on the cylinder surface. The anatomical integration property and CBV of the screw implanted along the transpedicular trajectory (TT) and cortical bone trajectory (CBT) were evaluated and compared. The drilled surface image fully revealed the contact anatomical structure of the screw under the trajectories, improving the understanding of the anatomical integration of the screw and surrounding tissues. On average, the CBV of the CBT was 30% greater than that of the TT. The proposed drilled surface image may be applied in preoperative planning and integrated into intraoperative navigation systems to evaluate the anatomical integration and degree of bone contact of the screw implanted along a trajectory.

The treatment efficacy of cortical bone trajectory (CBT) pedicle screws for lumbar degenerative disease in the Chinese Han population

Purpose

To provide reference data on CBT pedicle screws from CT measurements of L1 to L5 in the Chinese Han population and to assess the treatment efficacy of CBT pedicle screws in cases of lumbar degenerative disease.

Methods

In total, 100 patients were identified from the CT database for CBT morphometric measurement of the lumbar spine. According to sex and age, patients were divided into four groups. The diameter, length, and angle of the vertebral pedicle and trajectory were measured. Then, a total of 36 patients with lumbar degenerative disease were included in this study for clinical and radiographic evaluation. Demographic characteristics, health-related quality of life (HRQOL), and extent of intervertebral disc herniation and spondylolisthesis were evaluated.

Results

The mean diameter and the mean length varied from L1 to L5 in Groups I to IV. The lateral angles ranging from L1 to L5 were 8.9 to 9.2°, 8.7 to 12.2°, 8.7 to 11.2°, and 9.2 to 10.1° in Groups I to IV, respectively. The cephalad angles from L1 to L5 were 23.5 to 28.6°, 24.7 to 26.6°, 25.0 to 28.2°, and 24.7 to 27.9° in Groups I to IV, respectively. In the case series, all patients’ neurological function and HRQOL were significantly improved at the final follow-up (p < 0.0001), and 75% of patients achieved satisfaction.

Conclusions

The morphology of the lumbar vertebral pedicle varied from L1 to L5, and the trajectory was feasible and safe. CBT pedicle screws were effective in treating lumbar degenerative diseases and benefited the patients.

Análise tomográfica dos parâmetros anatômicos para inserção dos parafusos de trajeto cortical

Objective  To study the parameters related to the insertion path of cortical screws and to describe this technique.

Methods  Computed tomography (CT) scans of 30 patients, as well as the measurements from the L1 to the L5 vertebrae, were studied. A second observer evaluated ten randomly-selected exams. The parameters studied included the lateral angle (LA) and the screw diameter (SD) as axial variables, and the cranial angle (CA) and screw length (SL) as sagittal variables.

Results  We studied 15 male patients (mean age: 31.33 years) and 15 female patients (mean age: 32.01 years). The LA varied between 13.8° and 20.89°, with a tendency to increase in the proximal to distal direction. The CA varied from 17.5° to 24.9°, with a tendency to decrease in the caudal direction. The SD ranged from 2.3 mm to 7.2 mm, with a tendency to increase as we progressed from proximal to distal. The SL varied from 19 mm to 45 mm, with a tendency to decrease as we proceeded from proximal (L1) to distal (L5). No statistical difference was observed between the genders or in the interobserver agreement regarding the values studied when comparing the sides.

Conclusion  The path of insertion of the cortical screw shows a variation in different populations. Therefore, we recommend a preoperative imaging study to reduce the surgical risks related to the technique.

Alternatives to Traditional Pedicle Screws for Posterior Fixation of the Degenerative Lumbar Spine

BACKGROUND

Traditional pedicle screws are currently the gold standard to achieve stable 3-column fixation of the degenerative lumbar spine. However, there are cases in which pedicle screw fixation may not be ideal. Due to their starting point lateral to the pars interarticularis, pedicle screws require a relatively wide dissection along with a medialized trajectory directed toward the centrally located neural elements and prevertebral vasculature. In addition, low bone mineral density remains a major risk factor for pedicle screw loosening, pullout, and pseudarthrosis. The purpose of this article is to review the indications, advantages, disadvantages, and complications associated with posterior fixation techniques of the degenerative lumbar spine beyond the traditional pedicle screws.

METHODS

Comprehensive literature searches of the PubMed, Scopus, and Web of Science databases were performed for 5 methods of posterior spinal fixation, including (1) cortical bone trajectory (CBT) screws, (2) transfacet screws, (3) translaminar screws, (4) spinous process plates, and (5) fusion mass screws and hooks. Articles that had been published between January 1, 1990, and January 1, 2020, were considered. Non-English-language articles and studies involving fixation of the cervical or thoracic spine were excluded from our review.

RESULTS

After reviewing over 1,700 articles pertaining to CBT and non-pedicular fixation techniques, a total of 284 articles met our inclusion criteria. CBT and transfacet screws require less-extensive exposure and paraspinal muscle dissection compared with traditional pedicle screws and may therefore reduce blood loss, postoperative pain, and length of hospital stay. In addition, several methods of non-pedicular fixation such as translaminar and fusion mass screws have trajectories that are directed away from or posterior to the spinal canal, potentially decreasing the risk of neurologic injury. CBT, transfacet, and fusion mass screws can also be used as salvage techniques when traditional pedicle screw constructs fail.

CONCLUSIONS

CBT and non-pedicular fixation may be preferred in certain lumbar degenerative cases, particularly among patients with osteoporosis. Limitations of non-pedicular techniques include their reliance on intact posterior elements and the lack of 3-column fixation of the spine. As a result, transfacet and translaminar screws are infrequently used as the primary method of fixation. CBT, transfacet, and translaminar screws are effective in augmenting interbody fixation and have been shown to significantly improve fusion rates and clinical outcomes compared with stand-alone anterior lumbar interbody fusion.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Cortical Trajectory Screw Fixation in Lumbar Spine Surgery: A Review of the Existing Literature

Posterior lumbar fusion is a safe and effective surgical method for diseases, such as lumbar stenosis, spondylolisthesis, lumbar instability, spinal deformity, and tumor. Pedicle screw (PS) fixation was first introduced by Bouche and has been adopted as the gold standard for posterior lumbar fusion. Santoni and colleagues introduced a new methodological screw insertion technique that uses a cortical bone trajectory (CBT), described as that from a medial to lateral path in the transverse axial plane and caudal to the cephalad path in the sagittal plane through the pedicle for maximum contact of the screw with the cortical bone. Owing to the lower invasiveness, superior cortical bone contact, and reduced neurovascular injury incidence, the CBT technique has been widely used in posterior lumbar fusion; however, these advantages have not been proven in clinical/radiological and biomechanical studies. We designed the present study to review the existing evidence and evaluate the merit of CBT screw fixation. Six electronic databases were searched for relevant articles published in August 2020 using the search terms "cortical bone trajectory," "CBT spine," "CBT fixation," "cortical pedicle screws," and "cortical screws." Studies were analyzed and divided into the following groups: "biomechanics investigation," "surgical technique," and "clinical/radiological studies." Most studies compared CBT and PS fixation, and the CBT screw fixation method showed better or similar outcomes.

MORPHOMETRY OF THE POSITIONING OF CORTICAL TRAJECTORY PEDICLE SCREWS IN BRAZILIANS

ABSTRACT Objective Morphometric study of the positioning of the cortical trajectory pedicle screw in the lumbar spine of Brazilian patients of different sexes and ages, through the use of computed tomography images, in order to obtain more reliable data about cortical screw insertion and the variations observed, providing assistance for a safer, more effective approach with fewer complications. Methods Selection of 100 patients from a database, alternating by sex, measuring the length, diameter, cephalic angulation, and lateral angulation of the vertebrae from L1 to L5. Results Statistically significant measurements were obtained for the four different parameters in relation to sex. The mean age was 56, with a minimum of 20 and a maximum of 87 years. The L4 and L5 screws showed a reduction in relation to the other levels, while the width showed a progressive increase starting at L3. Lateral angulation was the parameter with the least variation among the levels, while there was greater variation and a reduction from L4 to L5 in cephalic angulation. Conclusion Statistically significant results were obtained for length, diameter, lateral and cephalic angulation. Sex was a significant factor in spine surgery instrumentation using the cortical trajectory pedicle screw technique. Level of evidence I; Diagnostic study (investigation of an examination for diagnosis).

A simple formula for predicting diameter of safely inserted cortical bone trajectory screws for fixation of the lower lumbar spine

BACKGROUND

Cortical bone trajectory (CBT) screws are popular for spinal fixation, but their ideal diameter has not been determined. Studies using postoperative computed tomography (CT) have revealed ample bone marrow space around 5.5-mm screws, which are commonly used. However, evidence indicates that a larger screw diameter provides a greater fixation strength. This study aimed to develop a generalizable formula for computing the diameter of CBT screws that could be inserted safely for fixation of the lower lumbar spine.

METHODS

Records of 44 consecutive patients who had undergone posterior fusion with CBT screws for single-level degenerative lumbar spondylolisthesis were retrospectively reviewed. We estimated the maximum diameter for conventional pedicle screws by the minimum diameter of the pedicle using preoperative CT (PSD). We measured the minimum endosteal diameter of the pedicle on the reconstructed plane of the postoperative CT which passed through the cannula used for the screw and estimated the maximum diameter for the CBT screws that could be inserted within the bone marrow space of the pedicle (CBTD).

RESULTS

Among the 176 pedicles measured, there were 151 (85.8%) with a PSD of 8.5 mm and 13, 7.5 mm. Because of a slight pedicle wall breach, 13 screws were excluded from the sample. There were 64 (39.3%) screws with a CBTD of 8.5 mm; 45, 7.5 mm; and 40, 6.5 mm. Of 163 screws, 156 (95.7%) had PSD minus CBTD ≤2 mm for each pedicle. PSD minus the minimum outer cortical diameter was ≤1 mm for each pedicle in 155 (95.1%) screws.

CONCLUSION

Our results show that CBT screws with a diameter 1 mm smaller than the endosteal diameter of the pedicle were inserted safely.

STUDY

Design: Clinical study.

Validation of a freehand technique for cortical bone trajectory screws in the lumbar spine

OBJECTIVE

The cortical bone trajectory (CBT) technique for pedicle screw placement has gained popularity among spinal surgeons. It has been shown biomechanically to provide better fixation and improved pullout strength compared to a traditional pedicle screw trajectory. The CBT technique also allows for a less invasive approach for fusion and may have lower incidence of adjacent-level disease. A limitation of the current CBT technique is a lack of readily identifiable and reproducible visual landmarks to guide freehand CBT screw placement in comparison to the well-defined identifiable landmarks for traditional pedicle screw insertion. The goal of this study was to validate a safe and intuitive freehand technique for placement of CBT screws based on optimization of virtual CBT screw placement using anatomical landmarks in the lumbar spine. The authors hypothesized that virtual identification of anatomical landmarks on 3D models of the lumbar spine generated from CT scans would translate to a safe intraoperative freehand technique.

METHODS

Customized, open-source medical imaging and visualization software (3D Slicer) was used in this study to develop a workflow for virtual simulation of lumbar CBT screw insertion. First, in an ex vivo study, 20 anonymous CT image series of normal and degenerative lumbar spines and virtual screw insertion were conducted to place CBT screws bilaterally in the L1-5 vertebrae for each image volume. The optimal safe CBT trajectory was created by maximizing both the screw length and the cortical bone contact with the screw. Easily identifiable anatomical surface landmarks for the start point and trajectory that best allowed the reproducible idealized screw position were determined. An in vivo validation of the determined landmarks from the ex vivo study was then performed in 10 patients. Placement of virtual "test" cortical bone trajectory screws was simulated with the surgeon blinded to the real-time image-guided navigation, and the placement was evaluated. The surgeon then placed the definitive screw using image guidance.

RESULTS

From the ex vivo study, the optimized technique and landmarks were similar in the L1-4 vertebrae, whereas the L5 optimized technique was distinct. The in vivo validation yielded ideal, safe, and unsafe screws in 62%, 16%, and 22% of cases, respectively. A common reason for the nonidealized trajectories was the obscuration of patient anatomy secondary to severe degenerative changes.

CONCLUSIONS

CBT screws were placed ideally or safely 78% of the time in a virtual simulation model. A 22% rate of unsafe freehand trajectories suggests that the CBT technique requires use of image-guided navigation or x-ray guidance and that reliable freehand CBT screw insertion based on anatomical landmarks is not reliably feasible in the lumbar spine.

Novel Placement of Cortical Bone Trajectory Screws in the Lumbar Spine: A Radiographic and Cadaveric Study

STUDY DESIGN

This is a radiographic and cadaveric study.

OBJECTIVE

The aim of this study was to conduct a detailed comparison about the parameters between the ideal cortical bone trajectory (CBT) and practical CBT screw.

SUMMARY OF BACKGROUND DATA

CBT screws have recently become popular in spinal surgery, mainly because of their improved fixation while minimizing soft tissue dissection. However, to our knowledge, no previous study has provided a detailed comparison on the parameters between the ideal CBT and practical CBT screws.

MATERIALS AND METHODS

The 3-dimensional computed tomography scans of 126 spinal vertebrae separated from 40 lumbar spines were studied. After determining the CBT, the maximal screw length, maximal screw diameter, lateral angle (LA), and cephalad angle (CA) were calculated. Thereafter, CBT screws were inserted into these cadaveric lumbar spines. Subsequently, the lateral angle of screw (LAs) and cephalad angle of screw (CAs) of the screws were measured using 3-dimensional computed tomography.

RESULTS

As for the ideal CBT, the maximal screw length gradually increased from L1 (32.0 mm) to L4 (35.3 mm) and then decreased at L5 (34.8 mm). Moreover, the maximal screw diameter increased from L1 (4.5 mm) to L5 (7.5 mm). The LA from L1 to L5 were 9.2, 9.4, 9.9, 11.2, and 12.0 degrees, respectively. The CA from L1 to L5 were 25.8, 25.8, 26.3, 26.8, and 26.0 degrees, respectively. As for the actual CBT screw, the LAs from L1 to L5 were 12.7, 14.1, 13.2, 12.4, and 13.0 degrees, respectively. The CAs from L1 to L5 were 25.4, 27.5, 29.2, 30.1, and 30.5 degrees, respectively. Significant differences were observed between the angles of the ideal trajectory and the actual screw.

CONCLUSIONS

Parameters of the actual CBT screw are slightly larger than the optimal trajectory measurements. The CAs and LAs are ∼25-30 and 10-16 degrees, respectively. L1 and L2 should be given considerable attention when using the CBT technique. And the trajectory measurements can only be used as a guide in clinical practice.

Lumbar pedicle screw fixation with cortical bone trajectory: A review from anatomical and biomechanical standpoints

Over the past few decades, many attempts to enhance the integrity of the bone-screw interface have been made to prevent pedicle screw failure and to achieve a better clinical outcome when treating a variety of spinal disorders. Cortical bone trajectory (CBT) has been developed as an alternative to the traditional lumbar pedicle screw trajectory. Contrary to the traditional trajectory, which follows the anatomical axis of the pedicle from a lateral starting point, CBT starts at the lateral part of the pars interarticularis and follows a mediolateral and caudocranial screw path through the pedicle. By markedly altering the screw path, CBT has the advantage of achieving a higher level of thread contact with the cortical bone from the dorsal entry point to the vertebral body. Biomechanical studies demonstrated the superior anchoring ability of CBT over the traditional trajectory, even with a shorter and smaller CBT screw. Furthermore, screw insertion from a more medial and caudal starting point requires less exposure and minimizes the procedure-related morbidity, such as reducing damage to the paraspinal muscles, avoiding iatrogenic injury to the cranial facet joint, and maintaining neurovascular supply to the fused segment. Thus, the features of CBT, which enhance screw fixation with limited surgical exposure, have attracted the interest of surgeons as a new minimally invasive method for spinal fusion. The purpose of this study was: 1) to identify the features of the CBT technique by reviewing previous anatomical and biomechanical literature, and 2) to describe its clinical application with a focus on the indications, limitations, surgical technique, and clinical evidence.

Review of Cortical Bone Trajectory: Evidence of a New Technique

This article summarizes recent evidence on the cortical bone trajectory (CBT) obtained from published anatomical, biomechanical, and clinical studies. CBT was proposed by Santoni in 2009 as a new trajectory that can improve the fixation of pedicle screws in response to screw loosening in osteoporotic patients. Recently, research interest has been growing with increasing numbers of published series and frequent reports of new applications. We performed an online database search using the terms “cortical bone trajectory,” “pedicle screw,” “CBT spine,” “CBT fixation,” “MISS CBT,” and “traditional trajectory.” The search included the PubMed, Ovid MEDLINE, Cochrane, and Google Scholar databases, resulting in an analysis of 42 articles in total. These covered three aspects of CBT research: anatomical studies, biomechanical parameters, and clinical cases or series. Compared to the traditional trajectory, CBT improves pullout strength, provides greater stiffness in cephalocaudal and mediolateral loading, and shows superior resistance to flexion/extension; however, it is inferior in lateral bending and axial rotation. CBT seems to provide better immediate implant stability. In clinical studies, CBT has shown better perioperative results for blood loss, length of stay in hospital, and surgery time; similar or better clinical postoperative scores; and similar comorbidity, without any major fixation system complications due to instrumentation failure or screw misplacement. In addition, advantages such as less lateral exposure allow it to be used as a minimally invasive technique. However, most of the clinical studies were retrospective case series or case-control studies; prospective evidence on this technique is scarce, making a definitive comparison with the traditional trajectory difficult. Nevertheless, we can conclude that CBT is a safe technique that offers good clinical results with similar biomechanical and perioperative parameters to those of the traditional trajectory. In addition, new applications can improve its results and make it useful for additional pathologies.

Minimally invasive cortical bone trajectory screws placement via pedicle or pedicle rib unit in the lower thoracic spine: a cadaveric and radiographic study

PURPOSE

To evaluate the feasibility of cortical bone trajectory (CBT) screws fixation via pedicle or pedicle rib unit in the cadaveric thoracic spine (T9-T12).

METHODS

Computed tomography (CT) images of 100 patients are analyzed by multiplanar reconstruction. Ten cadaveric thoracic spines are used to insert 4.5 × 35.0 mm CBT screws at all levels from T9 to T12.

RESULTS

Maximal screw length obtained by CT has a tendency to gradually increase from T9 (29.64 mm) to T12 (32.84 mm), and the difference reaches significant level at all levels except T9 versus T10 (P < 0.01). Maximal screw diameter increases from T9 (4.92 mm) to T12 (7.47 mm) and the difference reaches significant level among all levels (P < 0.01). Lateral angle increases from T9 (7.37°) to T12 (10.47°), and the difference reaches significant level among all levels except T11 versus T12. Cephalad angle from T9 to T12 are 19.03°, 22.10°, 25.62° and 27.50° (P < 0.01), respectively. The percentage of the inner and outer pedicle breakage are 2.5 and 22.5 %, respectively. The violation of lateral pedicle wall occurs at T9 and T10, especially for women at T9.

CONCLUSIONS

Both radiographic and cadaveric studies establish the feasibility of CBT screws placement via pedicle or pedicle rib unit in the lower thoracic spine (T9-T12). Furthermore, our measurements are also useful for application of this technique.