Cortical bone trajectory screws in the treatment of lumbar degenerative disc disease in patients with osteoporosis

Song Guo, Kai Zhu, Mei-Jun Yan, Xin-Hua Li, Jun Tan

Song Guo, Mei-Jun Yan, Xin-Hua Li, Department of Orthopedics Surgery, Shanghai General Hospital, Shanghai Jiao Tong University, Shanghai 200080, China

Kai Zhu, Department of Orthopedics Ⅱ, Qingdao No. 8 People’s Hospital, Qingdao 266121,Shandong Province, China

Jun Tan, Department of Orthopedics, United Family Healthcare, Shanghai 200336, China

Abstract Lumbar degenerative disc disease (DDD) in the elderly population remains a global health problem, especially in patients with osteoporosis. Osteoporosis in the elderly can cause failure of internal fixation. Cortical bone trajectory (CBT) is an effective, safe and minimally invasive technique for the treatment of lumbar DDD in patients with osteoporosis. In this review, we analyzed the anatomy, biomechanics, and advantages of the CBT technique in lumbar DDD and revision surgery. Additionally, the clinical trials and case reports, indications, advancements and limitations of this technique were further discussed and reviewed. Finally, we concluded that the CBT technique can be a practical, effective and safe alternative to traditional pedicle screw fixation, especially in DDD patients with osteoporosis.

Key Words: Lumbar degenerative disc diseases; Cortical bone trajectory screw; Anatomy;Biomechanics; Indications; Clinical trials and case reports; Advancements

INTRODUCTION

The incidence of lumbar degenerative disc diseases (DDDs) in the elderly has increased in recent years. The anatomy and curvature of the spine changes significantly with age. Hegazy and Hegazy[1] confirmed the change in morphology and dimensions of lumbar lordosis in aging adults, which suggested that the anatomy and curvature need to be given more attention during surgery[1]. Additionally, osteoporosis is quite common among elderly individuals. Therefore, another key treatment strategy for lumbar DDD in the elderly is the management of osteoporosis. Osteoporosis in the elderly can cause the loosening of internal fixation. A study showed that the rate of pedicle screw loosening in patients with osteoporosis was 12.8% to 25%. Additionally, the risks of proximal and distal junctional kyphosis also increase accordingly[2]. Therefore, the stability of internal fixation in osteoporosis patients should be enhanced during the operation by employing expansive pedicle screws, bone cement screws, or cortical bone trajectory (CBT) screws. Although expansive pedicle screws can increase the intensity of internal fixation, there are clear shortcomings, such as complicated placement, a high screw breakage rate, and limited clinical application. Bone cement screws have also been gradually applied in the treatment of DDD patients with osteoporosis. Zhanget al[3] showed that the loosening rate of bone cement screws is less than 4.3%, but intraoperative perfusion of bone cement increased the operation time and radiation exposure. Moreover, bone cement may leak into the spinal canal and blood vessels, leading to serious complications, such as neurological dysfunction and pulmonary embolism[3]. The CBT technique was proposed by Santoniet al[4] in 2009. Compared with the traditional technique, the CBT technique increases the contact surface between screws and cortical bone, and all screws used in the CBT technique are surrounded by the cortical bone. Therefore, this technique is more suitable for the treatment of lumbar DDD patients with osteoporosis[4]. Furthermore, in 2014, Mizunoet al[5] proposed the combination of this technique with lumbar posterior midline fixation and fusion in midline lumbar fusion (MIDLF) surgery. The CBT technique in MIDLF surgery has been widely used in lumbar DDD, adjacent vertebral diseases and postoperative revision due to its low invasiveness and high safety advantages[6].

ANATOMY AND BIOMECHANICS OF THE CBT TECHNIQUE

The CBT technique is performed at the intersection of the lateral isthmus of the pedicle and the lower edge of the transverse process. The entry point is at 5 o'clock on the left pedicle and 7 o'clock on the right pedicle. The ideal trajectory of placement is along the lower edge of the pedicle with a cranial incline of 25° to 30° and an external incline of 10° to ensure the maximum contact between the screw and cortical bone (Figure 1). Although the shape of the lumbar pedicles varies in different segments, the trajectory of placement remains unchanged. Four cortical bone surfaces are contacted using the standard CBT technique in the lumbar spine, namely, the isthmus, medial wall, lateral wall of the pedicle and anterior lateral wall of the vertebral body[7]. However, CBT screws are usually shorter and thinner than conventional pedicle screws. Matsukawaet al[8] measured the diameter and length of CBT in the adult lumbar spine using computed tomography (CT) and concluded that the diameter of the trajectory ranged from 6.2 ± 1.1 mm (L1) to 8.4 ± 1.4 mm (L5). The length of the trajectory at each vertebra was 36.8 ± 3.2 mm (L1), 38.2 ± 3.0 mm (L2), 39.3 ± 3.3 mm (L3), 39.8 ± 3.5 mm (L4), and 38.3 ± 3.9 mm (L5)[8]. Therefore, the biomechanical stability of the CBT technique has become a popular topic in research. Kojimaet al[7] found that the bone CT value around CBT screws was four times higher than that around traditional pedicle screws, which indicated that the bone-screw interface strength of the CBT technique was greater[9]. Liet al[10] also showed that the CBT technique had better fatigue resistance stability, especially in osteoporotic vertebrae[10]. However, the CBT technique is less effective against lateral bending and rotation than the conventional pedicle screw technique, which may result in a lower interbody fusion rate using CBT screw fixation than that using traditional pedicle screw fixation[11,12]. Therefore, a transverse connection could be used to improve the anti-lateral bending and anti-rotational stability during CBT screw placement.

Figure 1 Comparison of the cortical bone trajectory screw with the traditional pedicle screw trajectory. A: Axial view; B: Lateral view; C:Anteroposterior view.

CBT TECHNIQUE IN LUMBAR DDD

As the CBT technique is applied closer to the posterior midline than the traditional pedicle screw technique, vertebral muscles and adjacent segment joints are less harassed; therefore, the CBT technique has many advantages, including less blood loss and fatty infiltration, a shorter hospital stay and a lower incidence of adjacent segment degeneration (ASD). Additionally, the trajectory of the CBT technique is away from important nerve and vascular tissues, which further decreases the risk of injury. The CBT technique combined with MIDLF surgery is minimally invasive and safer and has been widely used in the treatment of lumbar diseases. Studies have shown that the CBT technique combined with MIDLF in the treatment of DDD patients with osteoporosis can achieve similar clinical decompression effects to those of the traditional pedicle screw technique combined with transforaminal lumbar interbody fusion (TLIF) technology[13]. Mizunoet al[5] proposed that CBT screws combined with MIDLF in the treatment of patients with single-level lumbar spondylolisthesis achieved good clinical outcomes. Lumbar decompression, fixation and fusion can be completed by the CBT technique at the same time, which is in line with the concept of minimally invasive surgery. Takenakaet al[14] compared the CBT technique and the traditional pedicle screw technique combined with lumbar posterior interbody fusion (PLIF) in the treatment of lumbar DDD. They concluded that the operation time, intraoperative blood loss, postoperative drainage volume, bed rest time and postoperative hospital stay time in the CBT technique group were significantly lower than those in the traditional pedicle screw technique group. CBT screws combined with PLIF surgery can achieve a more minimally invasive treatment effect for DDD patients with osteoporosis[14]. Kasukawaet al[15] compared the clinical efficacy of CBT and conventional pedicle screw internal fixation in TLIF. They concluded that the CBT technique can achieve better clinical results, smaller incisions and faster postoperative recovery than the conventional pedicle screw technique.

CBT TECHNIQUE IN LUMBAR REVISION SURGERY

Recently, the incidence of failed back surgery syndrome and ASD has increased with the extensive application of spinal internal fixation, leading to a high proportion of lumbar revision surgeries[16]. In revision surgery, the exposure risk of the nerve structure and blood vessels is significantly increased due to hypertrophic scar tissue and unclear spinal anatomy. Another advantage of the CBT technique is the reduction in exposure risk in revision surgery. During revision surgery, the internal fixation of the original operation usually needs to be replaced when the adjacent segment is decompressed and fixed. However, the replacement of internal fixation can not only increase the operation time and surgery risk but also result in more blood loss. Therefore, decompression, fixation and fusion on the adjacent segments without removing the internal fixation of the original surgery has become a key technique for the treatment of ASD. The CBT technique, which has a unique entry point and trajectory, can complete screw placement, decompression and fusion of adjacent segments through a small incision while retaining the original internal fixation, thereby avoiding extensive dissection and reducing the operation time and risk. In addition, the CBT technique can be used to place two groups of screws in the same vertebral body[17]. A study by Takataet al[18] showed that the CBT technique combined with MIDLF in lumbar revision surgery has the advantages of less soft tissue injury, fewer postoperative complications and better stability of internal fixation compared with traditional revision surgery[18].

CLINICAL TRIALS AND CASE REPORTS OF USING THE CBT TECHNIQUE

We performed an online database search on PubMed using the terms “cortical bone trajectory”, “clinical trials”, and “case reports”. Only papers published in English until June 10, 2022 were reviewed. Finally, seventeen articles were identified and included in Table 1[3,9,18-32]. There were thirteen retrospective cohort studies, two retrospective cohort comparative studies, and two prospective cohort studies. Most studies in this table indicated that the CBT technique offered good clinical outcomes with shorter incision length.

INDICATIONS FOR THE CBT TECHNIQUE

CBT screw fixation not only provides more stable internal fixation strength for patients with osteoporosis but can also be combined with a variety of minimally invasive procedures to reduce the risk of injury and intraoperative exposure. Especially for patients with obesity or diabetes, the application of CBT screw fixation can significantly reduce the incidence of postoperative complications. The following indications for CBT screw fixation were determined by comprehensive analysis of the anatomical characteristics, biomechanical characteristics and technical advantages of the CBT technique: (1) Lumbar disc degenerative diseases, especially combined with osteoporosis; (2) Obesity and high iliac crest; (3) ASD after traditional pedicle screw placement; (4) Salvage screw placement after failure of traditional pedicle screws; (5) Diseases mainly characterized by the destruction of the anterior and middle columns of the vertebral body, such as lumbar tuberculosis and intervertebral space infection; (6) Thoracolumbar fracture; and (7) Lumbar scoliosis correction and internal fixation with osteoporosis. However, CBT screw fixation is not suitable for bone destructive diseases with the absence of isthmus or spinal deformity characterized by rotation.

ADVANCEMENTS OF THE CBT TECHNIQUE

Although CBT screws are widely used in a variety of lumbar diseases, placing CBT screws is extremely demanding. Freehand techniques have a risk of exiting nerve root injury and lead to a high failure rate during screw placement. Because the anatomical entry point is hard and not evident, the placing instruments can easily slip and cause pedicle, isthmus and upper endplate injuries during surgery. The failure rate of the freehand placement technique is as high as 33.1%[33]. Second, the entry point and trajectory of the CBT technique are different from those of traditional screw placement. Surgeons cannot rely on tactile feedback to place screws, which will inevitably increase the amount of intraoperative Xray exposure and operation time. To reduce complications and ensure screw placement safety and accuracy, researchers have begun to use 3D-printed guide plates, navigation, and robots to assist in CBT screw placement. The 3D-printed guide plate, navigation and robot-assisted placement of CBT screws successfully compensated for the disadvantages of freehand screw placement and improved the safety and accuracy of CBT screw placement for internal fixation. Marengoet al[34] used a 3D-printed guide plate to assist in the placement of CBT screws, and they concluded that 85.2% of the screw entry points were within 2 mm of the planned entry points. Buzaet al[35] compared the surgical effects of MIDLF assisted by the Mazor spinal robot and free-hand MIDLF and found that the Mazor spinal robot improved the accuracy of CBT screw placement with less intraoperative blood loss and shorter hospital stays and operation times. Leet al[36] compared the free-hand CBT screw technique with the CBT screw technique assisted by the Tianji orthopedic surgery robot and found that the robot-assisted CBT screw technique reduced the incidence of adjacent segment facet joint injury. The accuracy of robot-assisted CBT screw placement was higher than that of the freehand group, and the acceptable screw placement in the robot-assisted group was 98.3%, which was significantly higher than that in the freehand group (84.5%). Additionally, the blood loss, operation time and radiation exposure dose of the robot-assisted group were significantly lower than those of the freehand group. Three-dimensional navigation technology is used to assist and monitor the trajectory of CBT screws in real time, which maximizes the contact between screws and the cortical bone interface and reduces the risks of screw placement (Figure 2). Navigation-assisted CBT screw placement can reduce the incidence of superior facet joint injury[17]. Khanet al[37] compared the accuracy of CBT screw internal fixation in the treatment of lumbar DDD with osteoporosis using a 3D guide plate, navigation and freehand[37], and they concluded that the accuracy rate of screw placement in the 3D-printed guide group and the navigation group was 100%. The accuracy rate of screw placement in the freehand group was only 87.5%. Although the application of spinal robotic and three-dimensional navigation technology has significantly improved the accuracy of CBT screw placement, there are still some shortcomings in this technology. Systematic errors are related to the patient's position change, image registration errors and screw skidding. Additionally, the angle and position of the screw may be shifted during implantation due to differences in surgeons' experience and learning curves. Therefore, it is necessary to probe the trajectory and perform intraoperative fluoroscopy to confirm the accuracy and safety of CBT screws as in the

Figure 2 Implantation of the cortical bone trajectory screw assisted by the navigation system. A: Feeling the entry point of the cortical bone trajectory (CBT) screw in L4 with the assistance of the navigation system; B: Awl of the CBT screw in L4 with the assistance of the navigation system; C: Tapping of the CBT screw in L4 with the assistance of the navigation system; D: Fluoroscopy showed the placement of CBT screws during the surgery; E: X-ray showed the implantation of CBT screws after surgery.

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FOOTNOTES

Author contributions:Guo S performed most of the writing; Zhu K performed data accusation, writing and prepared the figures and tables; Yan MJ provided the input in writing the paper; Li XH designed the outline and coordinated the writing of the paper; Tan J contributed to the conception of the study.

Supported byNational Natural Science Foundation of China, No. 82202694.

Conflict-of-interest statement:All the authors report no relevant conflicts of interest for this article.

Open-Access:This article is an open-access article that was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution NonCommercial (CC BYNC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is noncommercial. See: https://creativecommons.org/Licenses/by-nc/4.0/

Country/Territory of origin:China

ORCID number:Jun Tan 0000-0001-5206-6640.

S-Editor:Fan JR

L-Editor:A

P-Editor:Fan JR