Surgical Techniques
Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation
I
Paul Justin Tortolani, MD D. Alexander Stroh, MD
From MedStar Union Memorial Hospital, Baltimore, MD. Dr. Tortolani or an immediate family member has received royalties from Globus Medical, is a member of a speakers’ bureau or has made paid presentations on behalf of Globus Medical, serves as a paid consultant to Globus Medical, Innovasis, and Spineology, and has received research or institutional support from Globus Medical and Spineology. Neither Dr. Stroh nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this article. The video that accompanies this article is online at http://links.lww.com/ JAAOS/A11. J Am Acad Orthop Surg 2016;24: 755-761 DOI: 10.5435/JAAOS-D-15-00597 Copyright 2016 by the American Academy of Orthopaedic Surgeons.
nstrumented posterior spinal fusion in the lumbar spine may be compromised by poor bone quality. The cortical bone trajectory (CBT) for pedicle screw placement is a novel technique that may improve fixation by traversing a pathway with greater amounts of cortical bone. This technique is useful for posterior instrumentation in all patients, especially in those with osteopenia. The technique focuses on maximizing screw pullout strength while minimizing surgical exposure. The CBT for placing lumbar pedicle screws is a technique used to improve fixation during instrumented fusion of the lumbar spine. In comparison with traditional trajectory (TT) for pedicle screws, CBT screws (otherwise known as pars screws or cortical screws) have a more medial starting point and are aimed in a medial to lateral, caudal to cranial direction. First reported in 2009 as a method to increase the purchase of lumbar pedicle screws within osteoporotic bone,1 the use of CBT recently has been proposed for use in thoracic pedicles.2 The more medial starting point obviates the need for lateral dissection, thereby reducing the risk of blood loss. Early reports suggest that the rate of screw misplacement or breach with CBT is at least as low as that of TT.3 CBT screws typically are shorter and have a smaller diameter and a smaller pitch for purchase in cortical bone.4 Several major device manufacturers have developed systems specifically for CBT screw placement. Screws placed with CBT have been evaluated biomechanically, showing 30% greater pullout strength and
equivalent or improved resistance to toggle compared with TT screws.1,5 The insertional torque for CBT screws is more than 1.5 times greater than that of TT screws.6 Single-level rod-screw constructs exhibit similar stability when used with CBT and TT screws independent of the presence of lateral or transforaminal interbody devices.7 Although largescale trials using CBT screws are still in progress, we routinely use the technique in single-level instrumented posterior fusions of the lumbar spine. In this article, we review our indications and technique for the placement of CBT screws and briefly present our experience gained through learned pearls and pitfalls.
Indications and Contraindications We believe that the most appropriate indication for CBT screws is singlelevel lumbar degenerative spondylolisthesis requiring posterior spinal fusion above L5-S1. Although the pars interarticularis region can be cannulized at virtually any thoracic or lumbar level, we believe the safest levels for screw placement are at L3L4 and L4-L5 because of the wider pars diameter in these vertebrae. We have not performed this technique at the S1 level because of the absence of a pars and because of the relatively cancellous bone in this region. Although this technique is designed to improve fixation in osteoporotic bone, it may be used in patients with healthy bone. In our experience, it is appropriate for all patient body habitus types and it may ease the exposure in deep incisions. In patients
November 2016, Vol 24, No 11
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755
Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation
Figure 1
Figure 2
AP (A) and lateral (B) fluoroscopic projections show a grade I spondylolisthesis in which a single-level decompression and fusion is indicated. These projections are achieved prior to incision, and the positions of the C-arm are recorded for future projections. On the AP image, the end plates of the vertebrae of interest should be sharp in focus, and the spinous process should bisect the distance between the pedicles. On the lateral image, the end plates should be sharp in focus and the pedicles should overlap.
with a substantial bleeding risk, the reduced exposure also may reduce the risk of blood loss. CBT is not suitable for use in patients who require extensive lateral decompression near the pars or who have a preexisting spondylolysis, insufficient bone stock, or an aberrant anatomy that may preclude accurate screw placement or increase the risk of a pars fracture during screw insertion. We have found this technique to be suitable for the reduction and stabilization of grade I and II spondylolistheses. We avoid the use of CBT screws in higher slip grades, however, especially those requiring extensive reduction. Although some manufacturers have created specific reduction instruments, we use these instruments with caution because we believe the vector force placed on the CBT screws (oblique to the pedicle axis) during reduction is not parallel to the vector required for reduction (in line with the true pedicle axis) and therefore may increase the risk of bone-implant failure. Finally, CBT may not be appropriate for use in revision cases,
756
in which the retention of existing instrumentation placed via TT is planned. In cases of adult spinal deformity in which a single-level decompression and fusion are indicated, we found that CBT is less disruptive to the muscular envelope. Careful positioning of the patient and the use of intraoperative imaging helps facilitate safe screw insertion in these cases (Figure 1).
Surgical Technique Positioning Standard prone positioning can be achieved with any choice of an appropriate table. Our preference is an OSI Jackson table (Mizuho OSI) with an open abdominal frame. Prior to preparing the patient, we adjust the patient position in a way that rotates the vertebrae of interest neutrally on the AP and lateral fluoroscopic views. Standard sterilization and draping is performed. If a unilateral interbody procedure is planned, the primary
Illustration demonstrating the typical exposure for the cortical bone trajectory (CBT) technique, which extends no further laterally than the lateral border of the facet joints (dotted lines). Cranial to caudal exposure extends to the inferior aspect of the facet joint above the most cranial level to be fused and the entire pars region of the most caudal level to be fused. The CBT screw starting points are represented by the two dots.
surgeon is positioned on the same side. Otherwise, surgeon positioning may be interchanged freely.
Exposure At a minimum, the incision should extend from the pars of the cranial level to be fused to the inferior aspect of the spinous process of the caudal level to be fused (see Video, Supplemental Digital Content 1, http:// links.lww.com/JAAOS/A11, Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation). A standard posterior midline approach is taken. Typically, an incision of 5 to 7 cm is sufficient for a single-level fusion. Subperiosteal dissection of the paraspinal muscles aids in identifying the pars and in ascertaining an appropriate starting point. The lateral exposure of the deep muscle compartment should proceed to,
Journal of the American Academy of Orthopaedic Surgeons
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
Paul Justin Tortolani, MD, and D. Alexander Stroh, MD
but not beyond, the lateral edge of the facet and pars regions of interest. The proximal-distal exposure should allow a clear and unimpeded view of the two pars regions of interest. In an L4-L5 instrumentation, for example, we expose the inferior aspect of the L3-L4 facet, the L4 pars region, the L4-L5 facet, and the L5 pars region. Care is taken to preserve the multifidus attachments to the facets as well as the facet capsules and supraspinous and infraspinous ligaments of the regions not involved in the fusion. Two angled cerebellar retractors adequately expose the surgical field, obviating the need for larger or more aggressive retractors on the paraspinal muscles. The typical exposure is shown in Figure 2.
Decompression Necessary decompression of the central canal, lateral recess, and foramen proceeds as required by the observed pathology. As the decompression proceeds laterally, the surgeon needs to remain cognizant of the importance of preserving the bone stock in the region of the pars. We prefer to use Kerrison rongeurs, rather than Leksell rongeurs or osteotomes, as we approach the pars region because inadvertent removal of too much pars bone or the creation of stress risers may adversely affect the safety or stability of screw insertion.
Figure 3
Illustration demonstrating the starting point for cortical bone trajectory (CBT) on an idealized clock face, which represents the cross-section of the pedicle. On a right pedicle, the starting point is at 7 o’clock, whereas for the traditional trajectory, the starting point is at 3 o’clock. The CBT screw starting points are represented by the two dots. The circles represent the position of the pedicle as viewed from the dorsal aspect of the lamina. TT = traditional trajectory.
Figure 4
Pilot Hole Preparation A trajectory from medial inferior to lateral superior is planned for screw placement. Visually, the trajectory of right side pedicle instrumentation moves from 7 o’clock to 1 o’clock (Figure 3). A safe starting point for the upper level of the construct is generally 2 mm medial to the lateral border of the pars and just inferior to the superior facet complex. Sharp, 1inch Kirschner wires are inserted through the cortex using needle
AP (A) and lateral (B) fluoroscopic projections depict 1-inch Kirschner wires (Kwires) inserted through the cortex. The K-wires should be checked fluoroscopically to ascertain the proper trajectory. A, The K-wires indicate that the right L4 trajectory is too lateral and the left L4 trajectory is not lateral enough. The starting points are excellent, however, and corrections to the trajectory can be made during probing. B, The starting point is at the superior border of the foramen, with the Kwires aimed toward the halfway point of the body. L = left, R = right.
drivers after a small pilot hole is created with a 2-mm burr tip. The intended starting hole and trajecto-
ries are estimated using these wires (Figure 4). In the axial plane, the medial to lateral trajectory typically
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Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation
Figure 5
Illustrations demonstrating the axial (A) and sagittal (B) trajectories of the cortical bone trajectory (CBT) screws. The axial trajectory of the CBT screw is medial to lateral, not lateral to medial. The sagittal trajectory of the CBT screw is aimed caudal to cranial, not the more cranial-to-caudal trajectory of a traditional screw. The overall trajectory is from 7 o’clock to 1 o’clock for a right pedicle. TT = traditional trajectory.
diverges 10° to 15° from the midline and is aimed at the lateral cortex of the vertebral body (Figure 5). In the sagittal plane, the caudal-to-cranial trajectory is approximately 20° from the plane parallel to the superior end plate, aimed toward the superior end plate at the midpoint of the vertebral body (Figure 5). Fluoroscopy is used to confirm the starting point and the trajectories. After the starting hole and the trajectory are acceptable, a 2-mm drill burr is used to elongate the pilot hole and establish the first several millimeters of the screw pathway. We prefer to create the pilot holes before completing the lateral decompression to avoid removing too much bone in this region or creating a fracture inadvertently. In patients in whom the inferior aspect of the facet is removed during the decompression (eg, the inferior aspect of the L4 facet in the case of L4-L5 decompression and fusion), we generally use a TT for screw insertion at the caudal level. This technique requires less use of fluoroscopy than does CBT because the starting hole is readily visible using anatomic landmarks, and no
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additional retraction is necessary. Thus, for a typical L4-L5 degenerative spondylolisthesis, the L4 screw is inserted using a CBT and the L5 screw is inserted using a TT.
Creating the Screw Trajectory Using fluoroscopy, the cortical and cancellous bone is traversed with a long, 2-mm pedicle probe. We prefer to use a probe with a slight curve that allows subtle changes in trajectory as needed. As with traditional pedicle screws, the leading edge of the screw should not traverse the center of the pedicle axis before advancing to half of the planned screw length (Figure 6). In this trajectory, we typically use shorter 35- to 40-mm screw lengths with a smaller diameter to prevent hoop stress-related fractures of the prepared cortical canal.8
Tapping and Placing Screws Prior to tapping, a fine ball-tipped probe is used to palpate the pathway for breaches and to measure screw length. We prefer to use an undersized tap initially, proceeding in a
slow methodical manner, because the resistance through the dense cortical region is substantial. Each device manufacturer has a slightly different tap and final screw thread design. Therefore, the instructional manual specific to the design should be followed to avoid stripping the cortical threads when performing the tapping step. After the desired screw length and diameter are known, the screw is inserted by hand after probing each screw tunnel for breaches. The tactile sensation of traversing different bone densities may be more apparent in healthy bone than that found in osteoporotic bone. The final screw position should be checked with perfect AP and lateral projections that parallel the end plates (Figure 7).
Transforaminal Interbody Fusion and Reduction With Cortical Bone Trajectory Screws We use distracting instruments placed on the screws to permit ligamentotaxis, to allow expansion of the foramen, and to provide more working space for disk space preparation and interbody device placement. When using an articulating table, bending the patient into lumbar kyphosis assists in distracting the interbody space and in increasing visibility during disk space preparation. When a reduction is desired, we perform this bend after the interbody space has been prepared, the decompression has been completed, and the screws have been inserted. Any kyphosis that was created with the table is reduced to neutral, and an appropriately sized rod is affixed securely to the caudal screw. A reduction tower is attached to the cranial screw, and gradual reduction of the screw to the rod can be achieved. After this step is performed on the side opposite the transforaminal interbody fusion (TLIF), we reexamine
Journal of the American Academy of Orthopaedic Surgeons
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Paul Justin Tortolani, MD, and D. Alexander Stroh, MD
Figure 6
Illustrations demonstrating the ideal screw progression from AP (A) and lateral (B) views. Looking into the surgical field in the coronal plane, the leading thread should not pass the middle of the pedicle in the cranial to caudal dimension or in the medial to lateral dimension. Fluoroscopic image (C) demonstrating that, while tapping or probing, the instrument should be aimed toward the midpoint of the end plate on the lateral view.
the interbody space to ensure that no further disk material needs to be removed to accommodate an interbody device. The reduction often allows a clearer evaluation of the disk space. We typically choose to combine CBT screw placement with interbody fusion through a single-side TLIF approach; however, bilateral TLIF or posterior lumbar interbody fusion is certainly feasible. To increase the visibility for the TLIF, we generally tap and measure the CBT tunnels, place the TLIF, and then insert the screws. After both rods are attached with set screws, compression instruments allow the preloading of the interbody device and facilitate lordotic alignment.
Bone Graft and Closure Autologous bone graft from the spinous processes and laminae is milled and combined with blood from the surgical field. Typically, the morcellized graft is packed into the interbody space before device insertion to create the largest possible surface area for bony union. Remaining bone graft is packed into the facets. Bone graft extenders or additional biologics generally are not needed for standard single-
Figure 7
AP (A) and lateral (B) fluoroscopic images demonstrating the final construct for L4-L5 instrumentation. A, Note the medial-to-lateral cortical bone trajectory screws at L4 and the hybrid, more traditional pedicle screws at L5. A transforaminal interbody fusion device already has been placed. B, Note the greater inclination of the cortical bone trajectory screws and the finishing point near the halfway point of the superior end plate.
level cases. Closure is achieved in a layered fashion over a Hemovac drain.
Postoperative Management Patients are permitted to bear weight as tolerated. Physical therapy is initiated the day of surgery to teach spinal precautions, such as no bending, lifting, or twisting. The Hemovac drain remains in place
until its output falls below 30 mL in a 12-hour period.
Outcomes In general, there is a paucity of published results on posterior instrumented fusion using the CBT. Glennie et al9 reported on a case series of eight patients with a mean age of 67 years (range, 40 to 87
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Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation
years) who underwent decompression and posterior fusion with instrumentation using the CBT. At a mean follow-up of 16 months, two of eight patients required revision for pseudarthrosis and caudal adjacent segment disease, and one patient was offered revision for persistent back pain but declined because of advanced age. Most spines (six of seven patients) were fused from L4 to L5, one was fused from L3 to L4, and one was fused from L3 to L5. Six of eight patients showed signs of screw loosening, and four of eight showed a loss of reduction. Three of the four cases that lost reduction had not received an interbody fusion device. Lee et al3 performed a prospective randomized noninferiority trial comparing CBT screws with TT screws for posterior lumbar interbody fusion at L4-L5 or L5-S1 in 77 younger, nonosteoporotic patients. At a mean follow-up of 12 months, fusion rates were 87% in the TT group and 92% in the CBT group. Visual analog pain scores were substantially lower for the CBT group at 1 week postoperatively, but after this time, scores were not markedly different. Mean blood loss (450 mL versus 360 mL), surgical time (2.6 hours versus 2.1 hours), and incision length (10.7 cm versus 7.3 cm) were all lower in the CBT group. Anecdotally, we observed that patients who underwent CBT fusion tolerated early mobility better than those who underwent TT fusions where exposure was extended to the transverse processes. All but the most elderly and frail patients are permitted to bear weight as tolerated with the use of a lumbar girdle brace. Many patients leave the hospital ambulating at their previous functional level without a new need for an assistive device. We also observed that patients who underwent TT fusion had occasional difficulties with postoperative ileus, urinary reten-
760
tion, and pain control, but we have not noted these concerns in those who underwent CBT fusion. Most CBT patients tolerate regular diets well on postoperative day zero, void easily following indwelling catheter removal, and experience well-controlled pain with lower doses of narcotics.
Pearls and Pitfalls As with TT screws, the greatest risk when using CBT screws is neurologic injury from screw malposition. Inferior and medial breaches are the most worrisome and are avoided best by ensuring that the starting hole remains proximal to the foramen (Figure 2). When starting the screw trajectory, we think of the pedicle as a clock face. Rather than starting outside the clock circumference, we prefer to start inside the clock circumference. We believe this technique provides a greater probability that the entirety of the screw will remain inside the pedicle. For right-side pedicles, therefore, we start at the 7 o’clock position and aim for 1 o’clock, whereas for left-side pedicles we start at the 5 o’clock position and aim for 11 o’clock. In addition, we prefer to use a 2-mm round burr to drill the initial 5 mm of bone in the pars interarticularis because we believe this technique provides the best control of the trajectory. A sharp awl or pedicle-seeking probe may start to skive in the dense bone under hand control alone, and once the trajectory through the pars is set, it is challenging to revise. The inability to redirect the screw trajectory through the cortical pars region is a drawback of this technique. The reduced exposure with CBT precludes the placement of bone graft in the far lateral intertransverse process region. Graft placement further from the midline
may enhance construct stiffness by increasing the moment arm of the resultant callus from the coronal spine axis. In addition, the suitability of CBT is uncertain for long primary or revision constructs.10 Lateral vertebral body breaches or pars blowout are not likely to have neurologic implications but may compromise fixation strength. Lateral breaches can be avoided by ensuring that the divergence trajectory is not too extreme. To ensure that the vertebrae of interest are rotated neutrally, we check patient positioning with AP fluoroscopy before preparing and draping the patient. This step is of particular importance because patients with spondylolisthesis often exhibit some degree of vertebral rotation. Pars blowouts may occur even with perfect screw trajectory because of the substantial hoop stresses generated during tapping and screw insertion. We generally tap with the smallest tap available and gradually increase the tap size until we are satisfied with the degree of purchase. For this reason, extra caution is needed when using CBT screws .5.5 mm in diameter at L4 or above. Superior breaches through the vertebral end plate may injure the intervertebral disk. To maximize screw length and fixation and avoid this complication, we attempt to start the screw in the more cranial aspect of the pars and proceed with a shallower, less superiorly angulated trajectory through the pedicle and vertebral body as visualized on a lateral fluoroscopic image. Therefore, we aim toward the anterior half of the superior end plate and tap well shy of the end plate, usually at a distance of approximately 30 mm. As with thoracic pedicle screws, obtaining an accurate pilot hole may be the most important step in the technique because the corridor for a safe CBT screw position is much narrower than that of TT screws. If
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Paul Justin Tortolani, MD, and D. Alexander Stroh, MD
the starting hole is inaccurate, it will be difficult to achieve a safe final screw position. We spend considerable time cleaning the pars bone of all soft tissue so that we can see “around the corner” to the starting point of the transverse process and proximally to the bony confluence with the superior articular process. Commonly, facet osteophytes overhang into the pars region and may obscure it completely. In such cases, we use a 4-mm oval burr to perform an inferior “facetplasty,” in which we remove the osteophytes gradually and uncover the entire pars region. A narrow Leksell rongeur can be used to accomplish the same goal. In cases in which a foraminotomy of the proximal level (eg, L4 in an L4-L5 construct) is required, we prefer to create the CBT starting hole before performing the foraminotomy. This sequence helps to avoid inadvertent removal of too much bone. In these instances, we favor using curved foraminotomy Kerrison rongeurs that allow the surgeon to reach up and laterally into the foramen without removing more pars bone. In certain cases, the pars region may have to be removed to achieve a satisfactory decompression. In such cases, or in any case in which the pars is breached by the screw, we abort the technique and use traditional TT screws.
Summary CBT provides a method of minimizing dissection and achieving stable fixation in posterior spinal fusion. Although especially useful in patients with osteoporotic bone, the technique generalizes to all patients in whom wide exposure is not necessary. Although CBT screw fixation strength is proven through biomechanical studies, only preliminary reports exist from clinical use.
Acknowledgment We thank Lyn Camire, MA, ELS, of our department for editorial assistance. We also thank Jenn Udan, PAC, and Lauren Quattro, RN, for ongoing clinical and research support.
References Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, reference 3 is a level I study. Reference 6 is a level II study. References 8 and 9 are level IV studies. Reference 10 is level V expert opinion. References printed in bold type are those published within the past 5 years.
2. Matsukawa K, Yato Y, Hynes RA, et al: Cortical bone trajectory for thoracic pedicle screws: A technical note. Clin Spine Surg 2016. 3. Lee GW, Son JH, Ahn MW, Kim HJ, Yeom JS: The comparison of pedicle screw and cortical screw in posterior lumbar interbody fusion: A prospective randomized noninferiority trial. Spine J 2015;15(7):1519-1526. 4. Ueno M, Sakai R, Tanaka K, et al: Should we use cortical bone screws for cortical bone trajectory? J Neurosurg Spine 2015; 22(4):416-421. 5. Baluch DA, Patel AA, Lullo B, et al: Effect of physiological loads on cortical and traditional pedicle screw fixation. Spine (Phila Pa 1976) 2014;39(22): E1297-E1302. 6. Matsukawa K, Yato Y, Kato T, Imabayashi H, Asazuma T, Nemoto K: In vivo analysis of insertional torque during pedicle screwing using cortical bone trajectory technique. Spine (Phila Pa 1976) 2014;39(4): E240-E245. 7. Perez-Orribo L, Kalb S, Reyes PM, Chang SW, Crawford NR: Biomechanics of lumbar cortical screw-rod fixation versus pedicle screw-rod fixation with and without interbody support. Spine (Phila Pa 1976) 2013;38(8):635-641. 8. Matsukawa K, Yato Y, Nemoto O, Imabayashi H, Asazuma T, Nemoto K: Morphometric measurement of cortical bone trajectory for lumbar pedicle screw insertion using computed tomography. J Spinal Disord Tech 2013;26(6): E248-E253. 9. Glennie RA, Dea N, Kwon BK, Street JT: Early clinical results with cortically based pedicle screw trajectory for fusion of the degenerative lumbar spine. J Clin Neurosci 2015;22(6):972-975. 10.
1. Santoni BG, Hynes RA, McGilvray KC, et al: Cortical bone trajectory for lumbar pedicle screws. Spine J 2009;9(5):366-373.
Mobbs RJ: The “medio-latero-superior trajectory technique”: An alternative cortical trajectory for pedicle fixation. Orthop Surg 2013;5(1):56-59.
November 2016, Vol 24, No 11
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
761
Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation
I
Paul Justin Tortolani, MD D. Alexander Stroh, MD
From MedStar Union Memorial Hospital, Baltimore, MD. Dr. Tortolani or an immediate family member has received royalties from Globus Medical, is a member of a speakers’ bureau or has made paid presentations on behalf of Globus Medical, serves as a paid consultant to Globus Medical, Innovasis, and Spineology, and has received research or institutional support from Globus Medical and Spineology. Neither Dr. Stroh nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this article. The video that accompanies this article is online at http://links.lww.com/ JAAOS/A11. J Am Acad Orthop Surg 2016;24: 755-761 DOI: 10.5435/JAAOS-D-15-00597 Copyright 2016 by the American Academy of Orthopaedic Surgeons.
nstrumented posterior spinal fusion in the lumbar spine may be compromised by poor bone quality. The cortical bone trajectory (CBT) for pedicle screw placement is a novel technique that may improve fixation by traversing a pathway with greater amounts of cortical bone. This technique is useful for posterior instrumentation in all patients, especially in those with osteopenia. The technique focuses on maximizing screw pullout strength while minimizing surgical exposure. The CBT for placing lumbar pedicle screws is a technique used to improve fixation during instrumented fusion of the lumbar spine. In comparison with traditional trajectory (TT) for pedicle screws, CBT screws (otherwise known as pars screws or cortical screws) have a more medial starting point and are aimed in a medial to lateral, caudal to cranial direction. First reported in 2009 as a method to increase the purchase of lumbar pedicle screws within osteoporotic bone,1 the use of CBT recently has been proposed for use in thoracic pedicles.2 The more medial starting point obviates the need for lateral dissection, thereby reducing the risk of blood loss. Early reports suggest that the rate of screw misplacement or breach with CBT is at least as low as that of TT.3 CBT screws typically are shorter and have a smaller diameter and a smaller pitch for purchase in cortical bone.4 Several major device manufacturers have developed systems specifically for CBT screw placement. Screws placed with CBT have been evaluated biomechanically, showing 30% greater pullout strength and
equivalent or improved resistance to toggle compared with TT screws.1,5 The insertional torque for CBT screws is more than 1.5 times greater than that of TT screws.6 Single-level rod-screw constructs exhibit similar stability when used with CBT and TT screws independent of the presence of lateral or transforaminal interbody devices.7 Although largescale trials using CBT screws are still in progress, we routinely use the technique in single-level instrumented posterior fusions of the lumbar spine. In this article, we review our indications and technique for the placement of CBT screws and briefly present our experience gained through learned pearls and pitfalls.
Indications and Contraindications We believe that the most appropriate indication for CBT screws is singlelevel lumbar degenerative spondylolisthesis requiring posterior spinal fusion above L5-S1. Although the pars interarticularis region can be cannulized at virtually any thoracic or lumbar level, we believe the safest levels for screw placement are at L3L4 and L4-L5 because of the wider pars diameter in these vertebrae. We have not performed this technique at the S1 level because of the absence of a pars and because of the relatively cancellous bone in this region. Although this technique is designed to improve fixation in osteoporotic bone, it may be used in patients with healthy bone. In our experience, it is appropriate for all patient body habitus types and it may ease the exposure in deep incisions. In patients
November 2016, Vol 24, No 11
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
755
Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation
Figure 1
Figure 2
AP (A) and lateral (B) fluoroscopic projections show a grade I spondylolisthesis in which a single-level decompression and fusion is indicated. These projections are achieved prior to incision, and the positions of the C-arm are recorded for future projections. On the AP image, the end plates of the vertebrae of interest should be sharp in focus, and the spinous process should bisect the distance between the pedicles. On the lateral image, the end plates should be sharp in focus and the pedicles should overlap.
with a substantial bleeding risk, the reduced exposure also may reduce the risk of blood loss. CBT is not suitable for use in patients who require extensive lateral decompression near the pars or who have a preexisting spondylolysis, insufficient bone stock, or an aberrant anatomy that may preclude accurate screw placement or increase the risk of a pars fracture during screw insertion. We have found this technique to be suitable for the reduction and stabilization of grade I and II spondylolistheses. We avoid the use of CBT screws in higher slip grades, however, especially those requiring extensive reduction. Although some manufacturers have created specific reduction instruments, we use these instruments with caution because we believe the vector force placed on the CBT screws (oblique to the pedicle axis) during reduction is not parallel to the vector required for reduction (in line with the true pedicle axis) and therefore may increase the risk of bone-implant failure. Finally, CBT may not be appropriate for use in revision cases,
756
in which the retention of existing instrumentation placed via TT is planned. In cases of adult spinal deformity in which a single-level decompression and fusion are indicated, we found that CBT is less disruptive to the muscular envelope. Careful positioning of the patient and the use of intraoperative imaging helps facilitate safe screw insertion in these cases (Figure 1).
Surgical Technique Positioning Standard prone positioning can be achieved with any choice of an appropriate table. Our preference is an OSI Jackson table (Mizuho OSI) with an open abdominal frame. Prior to preparing the patient, we adjust the patient position in a way that rotates the vertebrae of interest neutrally on the AP and lateral fluoroscopic views. Standard sterilization and draping is performed. If a unilateral interbody procedure is planned, the primary
Illustration demonstrating the typical exposure for the cortical bone trajectory (CBT) technique, which extends no further laterally than the lateral border of the facet joints (dotted lines). Cranial to caudal exposure extends to the inferior aspect of the facet joint above the most cranial level to be fused and the entire pars region of the most caudal level to be fused. The CBT screw starting points are represented by the two dots.
surgeon is positioned on the same side. Otherwise, surgeon positioning may be interchanged freely.
Exposure At a minimum, the incision should extend from the pars of the cranial level to be fused to the inferior aspect of the spinous process of the caudal level to be fused (see Video, Supplemental Digital Content 1, http:// links.lww.com/JAAOS/A11, Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation). A standard posterior midline approach is taken. Typically, an incision of 5 to 7 cm is sufficient for a single-level fusion. Subperiosteal dissection of the paraspinal muscles aids in identifying the pars and in ascertaining an appropriate starting point. The lateral exposure of the deep muscle compartment should proceed to,
Journal of the American Academy of Orthopaedic Surgeons
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
Paul Justin Tortolani, MD, and D. Alexander Stroh, MD
but not beyond, the lateral edge of the facet and pars regions of interest. The proximal-distal exposure should allow a clear and unimpeded view of the two pars regions of interest. In an L4-L5 instrumentation, for example, we expose the inferior aspect of the L3-L4 facet, the L4 pars region, the L4-L5 facet, and the L5 pars region. Care is taken to preserve the multifidus attachments to the facets as well as the facet capsules and supraspinous and infraspinous ligaments of the regions not involved in the fusion. Two angled cerebellar retractors adequately expose the surgical field, obviating the need for larger or more aggressive retractors on the paraspinal muscles. The typical exposure is shown in Figure 2.
Decompression Necessary decompression of the central canal, lateral recess, and foramen proceeds as required by the observed pathology. As the decompression proceeds laterally, the surgeon needs to remain cognizant of the importance of preserving the bone stock in the region of the pars. We prefer to use Kerrison rongeurs, rather than Leksell rongeurs or osteotomes, as we approach the pars region because inadvertent removal of too much pars bone or the creation of stress risers may adversely affect the safety or stability of screw insertion.
Figure 3
Illustration demonstrating the starting point for cortical bone trajectory (CBT) on an idealized clock face, which represents the cross-section of the pedicle. On a right pedicle, the starting point is at 7 o’clock, whereas for the traditional trajectory, the starting point is at 3 o’clock. The CBT screw starting points are represented by the two dots. The circles represent the position of the pedicle as viewed from the dorsal aspect of the lamina. TT = traditional trajectory.
Figure 4
Pilot Hole Preparation A trajectory from medial inferior to lateral superior is planned for screw placement. Visually, the trajectory of right side pedicle instrumentation moves from 7 o’clock to 1 o’clock (Figure 3). A safe starting point for the upper level of the construct is generally 2 mm medial to the lateral border of the pars and just inferior to the superior facet complex. Sharp, 1inch Kirschner wires are inserted through the cortex using needle
AP (A) and lateral (B) fluoroscopic projections depict 1-inch Kirschner wires (Kwires) inserted through the cortex. The K-wires should be checked fluoroscopically to ascertain the proper trajectory. A, The K-wires indicate that the right L4 trajectory is too lateral and the left L4 trajectory is not lateral enough. The starting points are excellent, however, and corrections to the trajectory can be made during probing. B, The starting point is at the superior border of the foramen, with the Kwires aimed toward the halfway point of the body. L = left, R = right.
drivers after a small pilot hole is created with a 2-mm burr tip. The intended starting hole and trajecto-
ries are estimated using these wires (Figure 4). In the axial plane, the medial to lateral trajectory typically
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Cortical Bone Trajectory Technique for Posterior Spinal Instrumentation
Figure 5
Illustrations demonstrating the axial (A) and sagittal (B) trajectories of the cortical bone trajectory (CBT) screws. The axial trajectory of the CBT screw is medial to lateral, not lateral to medial. The sagittal trajectory of the CBT screw is aimed caudal to cranial, not the more cranial-to-caudal trajectory of a traditional screw. The overall trajectory is from 7 o’clock to 1 o’clock for a right pedicle. TT = traditional trajectory.
diverges 10° to 15° from the midline and is aimed at the lateral cortex of the vertebral body (Figure 5). In the sagittal plane, the caudal-to-cranial trajectory is approximately 20° from the plane parallel to the superior end plate, aimed toward the superior end plate at the midpoint of the vertebral body (Figure 5). Fluoroscopy is used to confirm the starting point and the trajectories. After the starting hole and the trajectory are acceptable, a 2-mm drill burr is used to elongate the pilot hole and establish the first several millimeters of the screw pathway. We prefer to create the pilot holes before completing the lateral decompression to avoid removing too much bone in this region or creating a fracture inadvertently. In patients in whom the inferior aspect of the facet is removed during the decompression (eg, the inferior aspect of the L4 facet in the case of L4-L5 decompression and fusion), we generally use a TT for screw insertion at the caudal level. This technique requires less use of fluoroscopy than does CBT because the starting hole is readily visible using anatomic landmarks, and no
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additional retraction is necessary. Thus, for a typical L4-L5 degenerative spondylolisthesis, the L4 screw is inserted using a CBT and the L5 screw is inserted using a TT.
Creating the Screw Trajectory Using fluoroscopy, the cortical and cancellous bone is traversed with a long, 2-mm pedicle probe. We prefer to use a probe with a slight curve that allows subtle changes in trajectory as needed. As with traditional pedicle screws, the leading edge of the screw should not traverse the center of the pedicle axis before advancing to half of the planned screw length (Figure 6). In this trajectory, we typically use shorter 35- to 40-mm screw lengths with a smaller diameter to prevent hoop stress-related fractures of the prepared cortical canal.8
Tapping and Placing Screws Prior to tapping, a fine ball-tipped probe is used to palpate the pathway for breaches and to measure screw length. We prefer to use an undersized tap initially, proceeding in a
slow methodical manner, because the resistance through the dense cortical region is substantial. Each device manufacturer has a slightly different tap and final screw thread design. Therefore, the instructional manual specific to the design should be followed to avoid stripping the cortical threads when performing the tapping step. After the desired screw length and diameter are known, the screw is inserted by hand after probing each screw tunnel for breaches. The tactile sensation of traversing different bone densities may be more apparent in healthy bone than that found in osteoporotic bone. The final screw position should be checked with perfect AP and lateral projections that parallel the end plates (Figure 7).
Transforaminal Interbody Fusion and Reduction With Cortical Bone Trajectory Screws We use distracting instruments placed on the screws to permit ligamentotaxis, to allow expansion of the foramen, and to provide more working space for disk space preparation and interbody device placement. When using an articulating table, bending the patient into lumbar kyphosis assists in distracting the interbody space and in increasing visibility during disk space preparation. When a reduction is desired, we perform this bend after the interbody space has been prepared, the decompression has been completed, and the screws have been inserted. Any kyphosis that was created with the table is reduced to neutral, and an appropriately sized rod is affixed securely to the caudal screw. A reduction tower is attached to the cranial screw, and gradual reduction of the screw to the rod can be achieved. After this step is performed on the side opposite the transforaminal interbody fusion (TLIF), we reexamine
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Figure 6
Illustrations demonstrating the ideal screw progression from AP (A) and lateral (B) views. Looking into the surgical field in the coronal plane, the leading thread should not pass the middle of the pedicle in the cranial to caudal dimension or in the medial to lateral dimension. Fluoroscopic image (C) demonstrating that, while tapping or probing, the instrument should be aimed toward the midpoint of the end plate on the lateral view.
the interbody space to ensure that no further disk material needs to be removed to accommodate an interbody device. The reduction often allows a clearer evaluation of the disk space. We typically choose to combine CBT screw placement with interbody fusion through a single-side TLIF approach; however, bilateral TLIF or posterior lumbar interbody fusion is certainly feasible. To increase the visibility for the TLIF, we generally tap and measure the CBT tunnels, place the TLIF, and then insert the screws. After both rods are attached with set screws, compression instruments allow the preloading of the interbody device and facilitate lordotic alignment.
Bone Graft and Closure Autologous bone graft from the spinous processes and laminae is milled and combined with blood from the surgical field. Typically, the morcellized graft is packed into the interbody space before device insertion to create the largest possible surface area for bony union. Remaining bone graft is packed into the facets. Bone graft extenders or additional biologics generally are not needed for standard single-
Figure 7
AP (A) and lateral (B) fluoroscopic images demonstrating the final construct for L4-L5 instrumentation. A, Note the medial-to-lateral cortical bone trajectory screws at L4 and the hybrid, more traditional pedicle screws at L5. A transforaminal interbody fusion device already has been placed. B, Note the greater inclination of the cortical bone trajectory screws and the finishing point near the halfway point of the superior end plate.
level cases. Closure is achieved in a layered fashion over a Hemovac drain.
Postoperative Management Patients are permitted to bear weight as tolerated. Physical therapy is initiated the day of surgery to teach spinal precautions, such as no bending, lifting, or twisting. The Hemovac drain remains in place
until its output falls below 30 mL in a 12-hour period.
Outcomes In general, there is a paucity of published results on posterior instrumented fusion using the CBT. Glennie et al9 reported on a case series of eight patients with a mean age of 67 years (range, 40 to 87
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years) who underwent decompression and posterior fusion with instrumentation using the CBT. At a mean follow-up of 16 months, two of eight patients required revision for pseudarthrosis and caudal adjacent segment disease, and one patient was offered revision for persistent back pain but declined because of advanced age. Most spines (six of seven patients) were fused from L4 to L5, one was fused from L3 to L4, and one was fused from L3 to L5. Six of eight patients showed signs of screw loosening, and four of eight showed a loss of reduction. Three of the four cases that lost reduction had not received an interbody fusion device. Lee et al3 performed a prospective randomized noninferiority trial comparing CBT screws with TT screws for posterior lumbar interbody fusion at L4-L5 or L5-S1 in 77 younger, nonosteoporotic patients. At a mean follow-up of 12 months, fusion rates were 87% in the TT group and 92% in the CBT group. Visual analog pain scores were substantially lower for the CBT group at 1 week postoperatively, but after this time, scores were not markedly different. Mean blood loss (450 mL versus 360 mL), surgical time (2.6 hours versus 2.1 hours), and incision length (10.7 cm versus 7.3 cm) were all lower in the CBT group. Anecdotally, we observed that patients who underwent CBT fusion tolerated early mobility better than those who underwent TT fusions where exposure was extended to the transverse processes. All but the most elderly and frail patients are permitted to bear weight as tolerated with the use of a lumbar girdle brace. Many patients leave the hospital ambulating at their previous functional level without a new need for an assistive device. We also observed that patients who underwent TT fusion had occasional difficulties with postoperative ileus, urinary reten-
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tion, and pain control, but we have not noted these concerns in those who underwent CBT fusion. Most CBT patients tolerate regular diets well on postoperative day zero, void easily following indwelling catheter removal, and experience well-controlled pain with lower doses of narcotics.
Pearls and Pitfalls As with TT screws, the greatest risk when using CBT screws is neurologic injury from screw malposition. Inferior and medial breaches are the most worrisome and are avoided best by ensuring that the starting hole remains proximal to the foramen (Figure 2). When starting the screw trajectory, we think of the pedicle as a clock face. Rather than starting outside the clock circumference, we prefer to start inside the clock circumference. We believe this technique provides a greater probability that the entirety of the screw will remain inside the pedicle. For right-side pedicles, therefore, we start at the 7 o’clock position and aim for 1 o’clock, whereas for left-side pedicles we start at the 5 o’clock position and aim for 11 o’clock. In addition, we prefer to use a 2-mm round burr to drill the initial 5 mm of bone in the pars interarticularis because we believe this technique provides the best control of the trajectory. A sharp awl or pedicle-seeking probe may start to skive in the dense bone under hand control alone, and once the trajectory through the pars is set, it is challenging to revise. The inability to redirect the screw trajectory through the cortical pars region is a drawback of this technique. The reduced exposure with CBT precludes the placement of bone graft in the far lateral intertransverse process region. Graft placement further from the midline
may enhance construct stiffness by increasing the moment arm of the resultant callus from the coronal spine axis. In addition, the suitability of CBT is uncertain for long primary or revision constructs.10 Lateral vertebral body breaches or pars blowout are not likely to have neurologic implications but may compromise fixation strength. Lateral breaches can be avoided by ensuring that the divergence trajectory is not too extreme. To ensure that the vertebrae of interest are rotated neutrally, we check patient positioning with AP fluoroscopy before preparing and draping the patient. This step is of particular importance because patients with spondylolisthesis often exhibit some degree of vertebral rotation. Pars blowouts may occur even with perfect screw trajectory because of the substantial hoop stresses generated during tapping and screw insertion. We generally tap with the smallest tap available and gradually increase the tap size until we are satisfied with the degree of purchase. For this reason, extra caution is needed when using CBT screws .5.5 mm in diameter at L4 or above. Superior breaches through the vertebral end plate may injure the intervertebral disk. To maximize screw length and fixation and avoid this complication, we attempt to start the screw in the more cranial aspect of the pars and proceed with a shallower, less superiorly angulated trajectory through the pedicle and vertebral body as visualized on a lateral fluoroscopic image. Therefore, we aim toward the anterior half of the superior end plate and tap well shy of the end plate, usually at a distance of approximately 30 mm. As with thoracic pedicle screws, obtaining an accurate pilot hole may be the most important step in the technique because the corridor for a safe CBT screw position is much narrower than that of TT screws. If
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the starting hole is inaccurate, it will be difficult to achieve a safe final screw position. We spend considerable time cleaning the pars bone of all soft tissue so that we can see “around the corner” to the starting point of the transverse process and proximally to the bony confluence with the superior articular process. Commonly, facet osteophytes overhang into the pars region and may obscure it completely. In such cases, we use a 4-mm oval burr to perform an inferior “facetplasty,” in which we remove the osteophytes gradually and uncover the entire pars region. A narrow Leksell rongeur can be used to accomplish the same goal. In cases in which a foraminotomy of the proximal level (eg, L4 in an L4-L5 construct) is required, we prefer to create the CBT starting hole before performing the foraminotomy. This sequence helps to avoid inadvertent removal of too much bone. In these instances, we favor using curved foraminotomy Kerrison rongeurs that allow the surgeon to reach up and laterally into the foramen without removing more pars bone. In certain cases, the pars region may have to be removed to achieve a satisfactory decompression. In such cases, or in any case in which the pars is breached by the screw, we abort the technique and use traditional TT screws.
Summary CBT provides a method of minimizing dissection and achieving stable fixation in posterior spinal fusion. Although especially useful in patients with osteoporotic bone, the technique generalizes to all patients in whom wide exposure is not necessary. Although CBT screw fixation strength is proven through biomechanical studies, only preliminary reports exist from clinical use.
Acknowledgment We thank Lyn Camire, MA, ELS, of our department for editorial assistance. We also thank Jenn Udan, PAC, and Lauren Quattro, RN, for ongoing clinical and research support.
References Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, reference 3 is a level I study. Reference 6 is a level II study. References 8 and 9 are level IV studies. Reference 10 is level V expert opinion. References printed in bold type are those published within the past 5 years.
2. Matsukawa K, Yato Y, Hynes RA, et al: Cortical bone trajectory for thoracic pedicle screws: A technical note. Clin Spine Surg 2016. 3. Lee GW, Son JH, Ahn MW, Kim HJ, Yeom JS: The comparison of pedicle screw and cortical screw in posterior lumbar interbody fusion: A prospective randomized noninferiority trial. Spine J 2015;15(7):1519-1526. 4. Ueno M, Sakai R, Tanaka K, et al: Should we use cortical bone screws for cortical bone trajectory? J Neurosurg Spine 2015; 22(4):416-421. 5. Baluch DA, Patel AA, Lullo B, et al: Effect of physiological loads on cortical and traditional pedicle screw fixation. Spine (Phila Pa 1976) 2014;39(22): E1297-E1302. 6. Matsukawa K, Yato Y, Kato T, Imabayashi H, Asazuma T, Nemoto K: In vivo analysis of insertional torque during pedicle screwing using cortical bone trajectory technique. Spine (Phila Pa 1976) 2014;39(4): E240-E245. 7. Perez-Orribo L, Kalb S, Reyes PM, Chang SW, Crawford NR: Biomechanics of lumbar cortical screw-rod fixation versus pedicle screw-rod fixation with and without interbody support. Spine (Phila Pa 1976) 2013;38(8):635-641. 8. Matsukawa K, Yato Y, Nemoto O, Imabayashi H, Asazuma T, Nemoto K: Morphometric measurement of cortical bone trajectory for lumbar pedicle screw insertion using computed tomography. J Spinal Disord Tech 2013;26(6): E248-E253. 9. Glennie RA, Dea N, Kwon BK, Street JT: Early clinical results with cortically based pedicle screw trajectory for fusion of the degenerative lumbar spine. J Clin Neurosci 2015;22(6):972-975. 10.
1. Santoni BG, Hynes RA, McGilvray KC, et al: Cortical bone trajectory for lumbar pedicle screws. Spine J 2009;9(5):366-373.
Mobbs RJ: The “medio-latero-superior trajectory technique”: An alternative cortical trajectory for pedicle fixation. Orthop Surg 2013;5(1):56-59.
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