Keitaro Matsukawa

In vivo analysis of insertional torque during pedicle screwing using cortical bone trajectory technique.

Study Design. The insertional torque of pedicle screws using the cortical bone trajectory (CBT) was measured in vivo. Objective. To investigate the effectiveness of the CBT technique by measurement of the insertional torque. Summary of Background Data. The CBT follows a mediolateral and caudocephalad directed path, engaging with cortical bone maximally from the pedicle to the vertebral body. Some biomechanical studies have demonstrated favorable characteristics of the CBT technique in cadaveric lumbar spine. However, no in vivo study has been reported on the mechanical behavior of this new trajectory. Methods. The insertional torque of pedicle screws using CBT and traditional techniques were measured intraoperatively in 48 consecutive patients. A total of 162 screws using the CBT technique and 36 screws using the traditional technique were compared. In 8 of 48 patients, the side-by-side comparison of 2 different insertional techniques for each vertebra were performed, which formed the H group. In addition, the insertional torque was correlated with bone mineral density. Results. The mean maximum insertional torque of CBT screws and traditional screws were 2.49 ± 0.99 Nm and 1.24 ± 0.54 Nm, respectively. The CBT screws showed 2.01 times higher torque and the difference was significant between the 2 techniques (P < 0.01). In the H group, the insertional torque were 2.71 ± 1.36 Nm in the CBT screws and 1.58 ± 0.44 Nm in the traditional screws. The CBT screws demonstrated 1.71 times higher torque and statistical significance was achieved (P < 0.01). Positive linear correlations between maximum insertional torque and bone mineral density were found in both technique, the correlation coefficient of traditional screws (r = 0.63, P < 0.01) was higher than that of the CBT screws (r = 0.59, P < 0.01). Conclusion. The insertional torque using the CBT technique is about 1.7 times higher than the traditional technique. Level of Evidence: 2

Morphometric measurement of cortical bone trajectory for lumbar pedicle screw insertion using computed tomography

Study Design: A morphometric measurement of cortical bone trajectory (CBT) for the lumbar pedicle screw insertion using computed tomography (CT). Objective: The aim of this study was to conduct a detailed morphometric measurement of the CBT. Summary of Background Data: The CBT is a novel lumbar pedicle screw trajectory, which follows a caudocephalad path sagittally and a laterally directed path in the transverse plane. The advantage associated with this modified technique is increased cortical bone contact, providing an enhanced screw purchase. However, little is known about the possible screw size or detailed direction of the trajectory. Methods: The CT scans of 100 adults who underwent examination for spinal problems were studied. A total of 470 lumbar vertebrae excluding spondylosis, malformation, and tumor were observed. In this trajectory, the starting point was supposed to be the junction of the center of the superior articular process and 1 mm inferior to the inferior border of the transverse process. The CT images were analyzed using 3-dimensional reconstruction software. The diameter, length, lateral angle to the vertebral sagittal plane, and cephalad angle to the vertebral horizontal plane of the trajectory were measured. Results: The mean diameter gradually increased from L1 to L5 (from 6.2 mm at L1 to 8.4 mm at L5). The mean length from L1 to L5 were 36.8, 38.2, 39.3, 39.8, and 38.3 mm, respectively. The lateral angle from L1 to L5 were 8.6, 8.5, 9.1, 9.1, and 8.8 degrees, respectively. The cephalad angle from L1 to L5 were 26.2, 25.5, 26.2, 26.0, and 25.8 degrees, respectively. Conclusions: The morphology of the pedicle, such as shape and pedicle axis angle, differed over the lumbar levels, our measurements demonstrated similar data excluding the diameter of the trajectory. There were no significant differences between each level of the lateral and cephalad angles.

Cortical bone trajectory for lumbosacral fixation: penetrating S-1 endplate screw technique: technical note.

OBJECT A cortical bone trajectory (CBT) is a new pedicle screw trajectory that maximizes the thread contact with cortical bone surface, providing enhanced screw purchase. Despite the increased use of the CBT in the lumbar spine, little is known about the insertion technique for the sacral CBT. The aim of this study was to introduce a novel sacral pedicle screw trajectory. This trajectory engages with denser bone maximally by the screw penetrating the S-1 superior endplate through a more medial entry point than the traditional technique, and also has safety advantages, with the protrusion of the screw tip into the intervertebral disc space carrying no risk of neurovascular injury. METHODS In this study, the CT scans of 50 adults were studied for morphometric measurement of the new trajectory. The entry point was supposed to be the junction of the center of the superior articular process of S-1 and approximately 3 mm inferior to the most inferior border of the inferior articular process of L-5. The direction was straight forward in the axial plane without convergence, angulated cranially in the sagittal plane penetrating the middle of the sacral endplate. The cephalad angle to the sacral endplate, length of trajectory, and safety of the trajectory were investigated. Next, the insertional torque of pedicle screws using this technique was measured intraoperatively in 19 patients and compared with the traditional technique. RESULTS The mean cephalad angle in these 50 patients was 30.7° ± 5.1°, and the mean length of trajectory was 31.5 ± 3.5 mm. The CT analysis revealed that the penetrating S-1 endplate technique did not cause any neurovascular injury anteriorly in any case. The new technique demonstrated an average of 141% higher insertional torque than the traditional monocortical technique. CONCLUSIONS The penetrating S-1 endplate technique through the medial entry point is suitable for the connection of lumbar CBT, has revealed favorable stability for lumbosacral fixation, and has reduced the potential risk of neurovascular injuries.

Biomechanical evaluation of the fixation strength of lumbar pedicle screws using cortical bone trajectory: a finite element study.

OBJECT Cortical bone trajectory (CBT) maximizes thread contact with the cortical bone surface and provides increased fixation strength. Even though the superior stability of axial screw fixation has been demonstrated, little is known about the biomechanical stiffness against multidirectional loading or its characteristics within a unit construct. The purpose of the present study was to quantitatively evaluate the anchorage performance of CBT by the finite element (FE) method. METHODS Thirty FE models of L-4 vertebrae from human spines (mean age [± SD] 60.9 ± 18.7 years, 14 men and 16 women) were computationally created and pedicle screws were placed using the traditional trajectory (TT) and CBT. The TT screw was 6.5 mm in diameter and 40 mm in length, and the CBT screw was 5.5 mm in diameter and 35 mm in length. To make a valid comparison, the same shape of screw was inserted into the same pedicle in each subject. First, the fixation strength of a single pedicle screw was compared by axial pullout and multidirectional loading tests. Next, vertebral fixation strength within a construct was examined by simulating the motions of flexion, extension, lateral bending, and axial rotation. RESULTS CBT demonstrated a 26.4% greater mean pullout strength (POS; p = 0.003) than TT, and also showed a mean 27.8% stronger stiffness (p < 0.05) during cephalocaudal loading and 140.2% stronger stiffness (p < 0.001) during mediolateral loading. The CBT construct had superior resistance to flexion and extension loading and inferior resistance to lateral bending and axial rotation. The vertebral fixation strength of the construct was significantly correlated with bone mineral density of the femoral neck and the POS of a single screw. CONCLUSIONS CBT demonstrated superior fixation strength for each individual screw and sufficient stiffness in flexion and extension within a construct. The TT construct was superior to the CBT construct during lateral bending and axial rotation.

Incidence and Risk Factors of Adjacent Cranial Facet Joint Violation Following Pedicle Screw Insertion Using Cortical Bone Trajectory Technique.

Study Design. Retrospective study evaluating cranial facet joint violation (FJV) by pedicle screws. Objective. The aim of the study was to determine the incidence and risk factors of FJV following screw placement via cortical bone trajectory (CBT). Summary of Background Data. CBT is a new minimally invasive technique for lumbar pedicle screw insertion that minimizes muscle dissection. Inserting a screw from a more caudal entry point can reduce iatrogenic damage to the cranial facet joint; however, no previous reports exist describing the incidence of FJV secondary to the CBT technique. Methods. We reviewed 202 consecutive patients who underwent lumbar pedicle screw instrumentation using CBT from October 2011 to June 2015. Postoperative CT scans were obtained to determine the degree and incidence of FJV. Clinical and imaging data were analyzed to clarify the risk factors of FJV. The detailed positions of the proximal screws were also investigated and compared between the FJV and non-FJV groups. Results. The incidence of FJV by the proximal screws was 11.8% (48/404), with no occurrence of intra-articular FJV. Multiple logistic regression analysis revealed that age >70 years, vertebral slip >10%, and adjacent facet joint degeneration (Pathrias grade 2 or 3) were independent factors significantly affecting FJV. There were statistically significant differences between the FJV and non-FJV groups in the facet-screw distance (3.2 ± 1.0 vs. 8.1 ± 2.3 mm, P < 0.01), the cranial angle (25.8 ± 6.3° vs. 29.9 ± 7.6°, P < 0.01), and the lamina-screw head distance (5.6 ± 1.6 vs. 6.4 ± 1.9 mm, P < 0.01). Conclusion. Lumbar pedicle screw placement via CBT would reduce FJV; however, special care should be taken in patients with age >70 years, vertebral slip >10%, and facet degeneration. Level of Evidence: 3

Evaluation of the Fixation Strength of Pedicle Screws Using Cortical Bone Trajectory: What Is the Ideal Trajectory for Optimal Fixation?

Study Design. In vivo analysis of insertional torque of pedicle screws using cortical bone trajectory (CBT) technique. Objective. To investigate factors contributing to the fixation strength of CBT screws and to clarify the ideal cortical trajectory for lumbar fusion. Summary of Background Data. CBT has developed as a new minimally invasive technique of lumbar instrumentation. Despite biomechanical studies demonstrating the superior characteristics of CBT, no study has elucidated the most suitable path for optimal fixation or compared the fixation within variations of trajectory. Methods. The insertional torque of pedicle screws using CBT was measured intraoperatively in 72 consecutive patients. The detailed positions of a total of 268 screws were confirmed using postoperative reconstruction computed tomographic scans and were analyzed to identify factors contributing to the level of insertional torque. Investigated factors were as follows: (1) age, (2) bone mineral density of the femoral neck and lumbar vertebrae by dual-energy x-ray absorptiometry; (3) the pedicle width and height, (4) the length of the implant, (5) total screw length within the vertebra, (6) the screw length within the vertebral body, (7) the screw length within the lamina, (8) the cephalad and lateral angle of the trajectory, and (9) the distance from the long axis of the screw to the inferior and medial borders of the pedicle. Results. Multiple regression analysis showed that bone mineral density of the femoral neck, screw length within the lamina, and cephalad angle were significant independent factors affecting torque. Conclusion. The fixation of CBT screws varied depending on technical factors (cephalad angle and screw length within the lamina) as well as the individual patient factor of bone mineral density. The ideal trajectory was directed 25° to 30° cranially along the inferior border of the pedicle so as to obtain maximum contact with the lamina and sufficient length within the vertebral body. Level of Evidence: 2

Cortical Bone Trajectory for Thoracic Pedicle Screws: A Technical Note.

Study Design: A morphometric measurement of new thoracic pedicle screw trajectory using computed tomography and a biomechanical study on cadaveric thoracic vertebrae using insertional torque. Objective: To introduce a new thoracic pedicle screw trajectory which maximizes engagement with denser bone. Summary of Background Data: Cortical bone trajectory (CBT) which maximizes the thread contact with cortical bone provides enhanced screw purchase. Despite the increased use of CBT screws in the lumbar spine, no study has yet reported the insertional technique for thoracic CBT. Methods: First, the computed tomography scans of 50 adults were studied for morphometric measurement of lower thoracic CBT. The starting point was determined to be the intersection of the lateral two thirds of the superior articular process and the inferior border of the transverse process. The trajectory was straight forward in the axial plane angulated cranially targeting the posterior third of the superior endplate. The maximum diameter, length, and the cephalad angle were investigated. Next, the insertional torque of pedicle screws using this new technique was measured and compared with that of the traditional technique on 24 cadaveric thoracic vertebrae. Results: All morphometric parameters of thoracic CBT increased from T9 to T12 (the mean diameter: from 5.8 mm at T9 to 8.5 mm at T12; the length: from 29.7 mm at T9 to 32.0 mm at T12; and the cephalad angle: from 21.4 degrees at T9 to 27.6 degrees at T12). The mean maximum insertional torque of CBT screws and traditional screws were 1.02±0.25 and 0.66±0.15 Nm, respectively. The new technique demonstrated average 53.8% higher torque than the traditional technique (P<0.01). Conclusions: The detailed morphometric measurement and favorable screw fixation stability of thoracic CBT are reported. The insertional torque using thoracic CBT technique was 53.8% higher than that of the traditional technique.

Biomechanical evaluation of lumbar pedicle screws in spondylolytic vertebrae: comparison of fixation strength between the traditional trajectory and a cortical bone trajectory

OBJECTIVE In the management of isthmic spondylolisthesis, the pedicle screw system is widely accepted surgical strategy; however, there are few reports on the biomechanical behavior of pedicle screws in spondylolytic vertebrae. The purpose of the present study was to compare fixation strength between pedicle screws inserted through the traditional trajectory (TT) and those inserted through a cortical bone trajectory (CBT) in spondylolytic vertebrae by computational simulation. METHODS Finite element models of spondylolytic and normal vertebrae were created from CT scans of 17 patients with adult isthmic spondylolisthesis (mean age 54.6 years, 10 men and 7 women). Each vertebral model was implanted with pedicle screws using TT and CBT techniques and compared between two groups. First, fixation strength of a single screw was evaluated by measuring axial pullout strength. Next, vertebral fixation strength of a paired-screw construct was examined by applying forces simulating flexion, extension, lateral bending, and axial rotation to vertebrae. RESULTS Fixation strengths of TT screws showed a nonsignificant difference between the spondylolytic and the normal vertebrae (p = 0.31-0.81). Fixation strength of CBT screws in the spondylolytic vertebrae demonstrated a statistically significant decrease in pullout strength (21.4%, p < 0.01), flexion (44.1%, p < 0.01), extension (40.9%, p < 0.01), lateral bending (38.3%, p < 0.01), and axial rotation (28.1%, p < 0.05) compared with those in the normal vertebrae. In the spondylolytic vertebrae, no statistically significant difference was observed for pullout strength between TT and CBT (p = 0.90); however, the CBT construct showed lower vertebral fixation strength in flexion (39.0%, p < 0.01), extension (35.6%, p < 0.01), lateral bending (50.7%, p < 0.01), and axial rotation (59.3%, p < 0.01) compared with the TT construct. CONCLUSIONS CBT screws are less optimal for stabilizing the spondylolytic vertebra due to their lower fixation strength compared with TT screws.

Biomechanical Evaluation of Cross Trajectory Technique for Pedicle Screw Insertion: Combined Use of Traditional Trajectory and Cortical Bone Trajectory.

To introduce a novel double‐screw (cross trajectory) technique that combines use of the traditional trajectory (TT) and cortical bone trajectory (CBT) and to investigate its fixation strength quantitatively by finite element (FE) analysis.

Comparison of Pedicle Screw Fixation Strength among Different Transpedicular Trajectories: A Finite Element Study.

Study Design: Comparative biomechanical study by finite element (FE) method. Objective: To investigate the pullout strength of pedicle screws using different insertional trajectories. Summary of Background Data: Pedicle screw fixation has become the gold standard for spinal fusion, however, not much has been done to clarify how the fixation strength of pedicle screws are affected by insertional trajectories and bone properties. Materials and Methods: Three-dimensional FE models of 20 L4 vertebrae were constructed from the computed tomographic data. Five different transpedicular trajectories were compared: the traditional trajectory, the vertical trajectory, and the 3 lateral trajectories with different sagittal directions (caudal, parallel, cranial). For a valid comparison, screws of the same shape and size were inserted into the same pedicle in each subject, and the pullout strength were compared with nonlinear FE analyses. In addition, the pullout strength was correlated with bone mineral density (BMD). Results: The mean pullout strength showed a 3.9% increase for the vertical trajectory relative to the traditional trajectory, 6.1% for the lateral-caudal trajectory, 21.1% for the lateral-parallel trajectory, and 34.7% for the lateral-cranial trajectory. The lateral-cranial trajectory demonstrated the highest value among all trajectories (P<0.001). In each trajectory, the correlation coefficient between the pullout strength and BMD of the femoral neck (r=0.74–0.83, P<0.01) was higher than the mean BMD of all the lumbar vertebrae (r=0.49–0.75, P<0.01), BMD of the L4 vertebra (r=0.39–0.64, P<0.01), and regional BMD of the L4 pedicle (r=0.53–0.76, P<0.01). Conclusions: Regional variation in the vertebral bone density and the amount of denser bone-screw interface contribute to the differences of stiffness among different screw trajectories. BMD of the femoral neck is considered to be a better objective predictor of pedicle screw stability than that of the lumbar vertebra.