Research progress on the safety of nail placement in adolescent idiopathic scoliosis surgery

YANG Xue-jian, CHEN Huan-xiong, LI Guo-jun, PENG Qiu-yu, HUANG Tao, MENG Zhi-bin

Department of Spinal Surgery, the First Affiliated Hospital of Hainan Medical University, Haikou 570102, China

Keywords:

ABSTRACT Adolescent idiopathic scoliosis is a spinal deformity with unknown etiology and high incidence, especially in adolescent females.If scoliosis patients do not get effective treatment in the early stage, the degree of scoliosis will continue to worsen with the growth of age or the rapid development of scoliosis in a short time, and the appearance deformities such as high and low shoulders and razor back will gradually appear, which can affect the cardiopulmonary function and even shorten life expectancy.It has a great impact on the physical and mental health of the patients.Mild scoliosis is often well controlled by bracing, and moderate to severe scoliosis deformity often requires scoliosis corrective surgery.In adolescent idiopathic scoliosis correction, pedicle screws offer better stability and firmness, stronger orthopedic force, and lower incidence of related complications caused by instruments than traditional hook and conical internal fixation instruments.At the same time, scoliosis patients have uneven development of the left and right sides of the vertebral body, narrow diameter of concave pedicle and varying degrees of rotation of parietal vertebra, resulting in changes in the position of important anatomical structures such as spinal cord and thoracic aorta in the spinal canal.It makes it difficult to place pedicle screws, resulting in a high misplacement rate.How to improve the safety of screw placement, reduce the misplacement rate and the risk of spinal cord, nerve root, blood vessels, viscera and so on, has always been the focus of spinal surgeons.This paper summarizes the related literature on the effects of new techniques such as pedicle anatomy, complications of nail placement and computer navigation, and 3D guided template on the accuracy of screw placement in adolescent idiopathic scoliosis patients, and the safety of nail placement.In order to provide reference for clinical practice.

1.Introduction

Adolescent idiopathic scoliosis (AIS) is a complex threedimensional deformity with unknown pathogenesis, accounting for about 75% / 80% of all scoliosis[1].The main manifestations are coronal imbalance, sagittal physiological bending abnormalities and vertebral rotation.Severe scoliosis can lead to rib deformity,thoracic deformation and compression to important anatomical organs such as heart and lung.affect the appearance and even reduce the life expectancy of patients[2].In order to control the progression of scoliosis, improve the appearance of deformities and eliminate the psychological disorders of patients, surgical treatment is often needed.at present, the mainstream surgical method is posterior pedicle screw fixation and orthopedic surgery.Boucher[3] first used pedicle screws in spinal fusion in 1959, and it was not until the 1980s that pedicle screws were widely used.Compared with traditional hook instruments, pedicle screws have the advantages of three-column fixation, higher pullout strength and better rotation control, and stronger ability to correct scoliosis.Patients with scoliosis are often accompanied by vertebral rotation, thinning of vertebral pedicle, abnormal anatomical structure around vertebral body, development of vertebral deformity and so on.This makes it difficult to place nails in orthopedic surgery.Studies have shown that the incidence of screw misplacement in scoliosis correction is high and may lead to catastrophic complications[6].Misplacement of AIS screws can cause damage not only to the spinal cord and nerve root, but also to the surrounding organs, including aorta,azygos vein and esophagus[7-9].How to improve the safety of nail placement and avoid catastrophic complications has always been the focus of spinal surgeons.Related studies have shown that compared with the traditional C-arm assisted nail placement, the use of computer navigation and 3D printing guide plate technology can improve the accuracy of nail placement, reduce the rate of wall breaking and complications and improve the safety of nail placement.By summarizing the characteristics of spinal anatomical structure development, the clinical experience of instrument-related complications and the advantages and disadvantages of various nail placement methods in adolescent idiopathic scoliosis, it is hoped that it can provide a reference for operators to further improve the safety of nail placement in the correction of adolescent idiopathic scoliosis.2 Relationship between anatomical structure and safety of adolescent scoliosis

2.1 Anatomical characteristics of adolescent idiopathic scoliosis

The width of the pedicle of the spine affects the placement of the pedicle screw to a great extent.some studies have shown that the width of the lateral and medial cortex of the pedicle is significantly related to the distance from the spinal cord to the medial wall of the pedicle.The narrower the pedicle, the greater the risk of screw injury.especially the concave side has a greater risk of pedicle screw injury[10].However, there is a great difference in the anatomical structure of the pedicle between AIS patients and normal people.Panjabi et al.[11] found that the narrowest pedicle is located in the T3-T8 segment,with an average diameter of 6.3 mm.Zindrick et al.[12] found that the narrowest pedicle was located at T5.Generally speaking, most researchers agree that the narrowest part of the pedicle is in the T4-T6 segment.However, there are also differences in pedicle width among people of different races and regions.Tan et al.[13] found that the width of pedicle in Asian is relatively narrow.Many studies have found that the pedicle width of scoliosis is narrower on the concave side of the thoracic curve.Sarwahi[14] and Watanabe et al[15] found that the concave pedicles of AIS patients were thinner and had a higher rate of deformity, most of which were located in the upper thoracic vertebrae.Gao et al.[16] the width of the concave side of the pedicle of the AIS group (4.99 ±1.87 mm) was smaller than that of the convex side (6 ±1.66 mm) and the normal group (6 ±1.45 mm), while the width of the convex side of the AIS group was not significantly different from that of the normal group.At the same time, it was also found that the pedicle variation was the highest in the parietal vertebra region on the scoliosis structure curve.In the study of Czerwein et al.[17], it was found that the incidence of pedicle of type C (width < 4 mm, no cancellous bone channel) and type D (width < 2 mm, no cancellous bone channel) was 4.4% and 3.2%, respectively.The puncture rate of misplaced screws in the abnormal pedicle was as high as 28.1%.To sum up, in adolescent idiopathic scoliosis, the width of concave pedicle is relatively narrow and the incidence of pedicle deformity is higher, and the difficulty of pedicle screw placement and the probability of screw misplacement are higher.it is of great significance to summarize the morphological and anatomical characteristics of pedicle in patients with scoliosis and to optimize the starting point and trajectory of screw path during screw placement.

2.2 injury of azygos vein and esophagus caused by misplaced screws

In order to reduce the occurrence of catastrophic complications caused by screw misplacement, many scholars have studied and analyzed the relationship between pedicle and surrounding organs in patients with scoliosis.Studies have shown that misplacement of AIS screws can cause damage not only to the nervous system, but also to the surrounding organs, including azygos veins and esophagus.Jiang and other studies have found that during the implantation of AIS pedicle screws, the risk of azygos vein injury in the left thoracic vertebra (T7-T10) is higher.The risk of left azygos vein injury is 72% at T10 and 85% at T8.In patients without scoliosis, no azygos vein injury on both sides was observed.[8].Takeshita et al found that in T4 vertebrae, the risk of esophageal injury increased with the increase of screw length (concave 30.5 mm and convex 29.4 mm)[18].

2.3 damage to the nervous system caused by misplaced screws

In AIS orthopedic surgery, misplaced screws piercing the medial cortex of the pedicle are likely to damage the spinal cord, especially in the upper thoracic region.Sarlak et al found that during the placement of pedicle screws in AIS orthopedics, the risk of spinal cord injury caused by T2-T3 and T5-T9 concave screws was the highest[19].Spinal cord injury is often catastrophic, the postoperative recovery effect is very poor, the higher position of spinal cord injury may lead to respiratory and cardiac arrest, endangering the life and safety of patients.When the misplaced screw penetrates the upper and lower endplate of the vertebral body, the nerve root may be damaged, which may cause sensory numbness and motor dysfunction in the corresponding region.During the correction of scoliosis, the nervous system injury is easy to occur during the correction of scoliosis.The neuroelectrophysiological real-time monitoring of SEP and MEP can detect the nervous system injury during the operation, which can play an early warning role in the operation of the scoliosis, prevent the risk of aggravation caused by the continued operation and provide help for finding the cause of the injury in time.It can minimize the risk of surgery-related nerve injury, and is the right arm of spinal surgeons to protect spinal orthopedic surgery[20].

2.4 injury of aorta caused by misplacement of screws

At present, there are few reports on aortic injury caused by misplacement of pedicle screws in AIS orthopedics at home and abroad.Jiang[7] and Liu[21] found that the highest risk of aortic injury was at T10 and T11, followed by T4 and T5.The Sarlak study found that the risk of aortic injury was increased by T4-T8 concave screws and T11-T12 convex screws[19].Hicks et al[22] made a comprehensive analysis of the complications caused by misplacement of pedicle screws in scoliosis surgery, and found that 6 of the 8147 screws were adjacent to the aorta, with an incidence of 0.07%.With the increase of the patient’s age, the vascular fragility increases with the gradual hardening of the aorta.These adjacent pedicle screws are a huge safety hazard for patients with ruptured aorta.Iyer et al.[15] advocated the use of transesophageal echocardiography to identify aortic injury caused by misplaced screws.When misplaced screws are adjacent to the aorta, it indirectly increases the risk of long-term aortic injury.The use of screw tip elimination and transesophageal ultrasound to prevent and identify aortic injury caused by misplacement of screws can improve the long-term quality of life of patients and reduce safety risks to a certain extent.By summarizing a large number of clinical examples of instrument-related complications in patients with scoliosis, it has important reference significance to increase the experience of surgeons in nail placement and improve the safety of operation.

3.Evaluation criteria of misplaced screws

3.1 X-ray and CT evaluation

Reviewing the research on misplaced screws at home and abroad,the literature reports on the cortical penetration rate of pedicle screws are quite different, and the large-scale change of screw penetration rate is affected by the evaluation method.The penetration rate of study screws using X-ray assessment was very low, ranging from 1.2% to 3.0%[23-26].The screw penetration rate of the study assessed by CT was as high as 12% to 17%[22].This is because CT evaluation can determine the high-quality images with the greatest degree of penetration from multiple scanning levels, and the error caused by poor quality of X-ray images is larger, so the use of thin-layer CT evaluation is more objective and reliable for the evaluation of broken pedicle screws.

3.2 grading of pedicle wall breaking

The standard of wall-breaking classification also has a great influence on the evaluation of misplaced screws.Samdani[28] and Liu [27] used CT to evaluate the penetration rate of misplaced pedicle screws, which were 12.1% and 32.0%, respectively.The main reason for this difference is due to the use of different wall-breaking criteria of pedicle screws.Samdani only included pedicle perforation > 2 mm as misplaced screws, while Liu included wall-broken screws >0mm.After removing the difference in inclusion criteria, the screw misplacement rate studied by Liu and others was reduced to 15.4%,and the gap between the two was significantly narrowed.In the systematic evaluation of pedicle screw complications in scoliosis surgery, Hicks[22] used CT to evaluate the screw misplacement rate as high as 15.7%.In 2016, Sarwahi et al.[29] considered misplaced screws to be dangerous screws when they pierced the medial wall >4 mm or through the anterolateral wall to the visceral edge < 1 mm.In his study, the misplacement rate of dangerous screws was 1.06%.The misplacement rate of pedicle screws is closely related to the evaluation method and the standard of wall-breaking classification.The evaluation of pedicle penetration rate by CT is more objective and reliable than that by X-ray.At the same time, standardized and unified grading criteria have important clinical significance for the safety evaluation of nail placement.

4.the safety of different nail placement methods.

4.1 traditional methods of nail placement

At present, there are a variety of pedicle screw placement techniques in scoliosis correction, among which the traditional techniques include freehand technique, fluoroscopy assistant technique and funnel technique.Kim et al.[30] reported that the overall penetration rate of pedicle under unarmed technique was 6.4%.[31] the study found that the penetration rate of screw placement assisted by fluoroscopy was only 1.2%.However, most of the patients in the above studies were evaluated only by X-ray so that the penetration rate was low.Yingsakmongkol et al.[25] reported the use of funnel technique and found that the total pedicle penetration rate in 14 patients was 65.0%.In 2016, Liu et al.[27] reported the accuracy of unarmed techniques in AIS patients (assessed by CT) and found that the total penetration rate was 32.0%.However, after excluding lateral pedicle puncture, the rate of cortical penetration decreased to 12.1%.Kwan et al.[32] reported that the funnel technique was used in 140patients with AIS.After evaluating the location of 2020 pedicle screws by CT, it was found that the overall screw penetration rate was 18.3%, but the overall penetration rate decreased to 6.5%after excluding the lateral penetration of screws.They believe that most of the perforations are due to lateral punctures in the thoracic region, especially at the T3-T5 level on the right, which is caused by the surgeon’s subconscious lateral screw placement in this area, and these lateral punctures at the thoracic level (pedicle extrapedicular technique) are considered safe given that the screws are confined to the rib-vertebral junction.Xie et al.[33] used the five-step nail placement method in the operation of severe spinal deformity, and the failure rate of nail placement was reduced from 17.8% to 0.4%.Although the pedicle penetration rate of traditional screw placement is high, the incidence of instrument-related complications caused by screws is low, and its reliability has been widely recognized by scholars.Although various nail placement techniques have developed rapidly in recent years, the traditional nail placement technology is still the most widely used nail placement method.In practical application, the operator will flexibly choose the appropriate method according to the actual situation.After screw placement, C-arm fluoroscopy can be used to evaluate the screw position in real time to adjust the screw and improve the accuracy of screw placement.The main advantage of traditional nail placement method is that compared with navigation and robot technology, it can effectively reduce the economic burden of patients and hospitals, and reduce the preoperative system registration and navigation preparation time.The main disadvantage is that the learning cycle is relatively long.In the process of orthopedic surgery for scoliosis, it is impossible to obtain three-dimensional images of the spine in real time.The anatomical structure of vertebral pedicle in patients with scoliosis is a great test for the operator, which requires high surgical skills and clinical experience.With the rapid development of digital technology in medical and health, computer navigation and orthopaedic robot have been greatly applied and widely recognized in the clinical practice of spinal surgery.The use of these high and new technologies can fill many shortcomings of traditional methods.

4.2 computer navigation assisted nail placement

On the basis of intraoperative fluoroscopy, a computer-aided navigation technology is developed.The navigation system includes C-arm X-ray machine perspective two-dimensional image navigation, CT preoperative image navigation, Iso-C intraoperative 3D navigation, intraoperative MRI navigation, intraoperative 3D CT navigation technology and robot auxiliary system, etc.[34].According to the principle types, navigation is also divided into optical navigation, magnetic navigation and ultrasonic navigation, among which optical navigation is the most widely used and most mature.Up to now, many studies have shown that the use of computer navigation assistance in AIS surgery can improve the accuracy of pedicle screw placement, and reduce the placement time and radiation dose[35].Compared with the traditional C-arm fluoroscopyassisted nail placement, navigation-assisted nail placement can provide real-time three-dimensional images in the process of locating the surgical area, preparing the nail insertion point and determining the nail placement depth, which can effectively improve the overall nail placement accuracy.reduce the penetration rate of the inner and lateral cortex, and at the same time, the navigation can also monitor the direction and angle of the nail placement.It reduces the difficulty of pedicle screw placement and the probability of nerve and vascular injury in spinal orthopedic surgery.Although the accuracy of nail placement can be improved by using computer navigation in the research of many scholars, there are also many problems in the process of using computer navigation, the most fatal one is navigation drift.Once the navigation is inaccurate, it will be disastrous if it is not detected and calibrated in time.Especially for long segment orthopedic surgery, the probability of navigation drift will be greatly increased.In the use of navigation, there are many factors that lead to navigation drift.The spine itself is an elastic body in the process of nail placement, the relative displacement between the spine and the reference frame will lead to navigation misalignment, followed by the deformation of surgical instruments and blood stains on the navigation ball can lead to navigation drift.During the operation, the respiratory movement of patients is easy to produce artifacts during C-arm fluoroscopic scanning, which affects the navigation accuracy.Young doctors should be familiar with pedicle screw placement skills and navigation principles when using navigation screws, and should not blindly insert screws according to the navigation results.If the navigation offset is found, the cause should be found in time to avoid serious consequences caused by misplacement of screws.In the process of navigation, we should pay attention to select the relatively fixed ilium to place the reference frame to avoid light path occlusion, and avoid excessive violence in the process of nail placement to avoid spinal deformation.highspeed grinding drill can be used to assist the preparation of nail path.So far, few clinical studies on navigation drift have been reported.Chen et al.[38] found that the positioning needle navigation realtime calibration technology can further improve the nail placement accuracy and monitor the angle and direction of navigation offset in real time.Compared with navigation-assisted AIS nail placement,navigation real-time calibration technology can immediately monitor the direction and angle of navigation drift, effectively improve the overall nail placement accuracy of AIS, reduce the rate of wall breaking and reduce the occurrence of complications.With the continuous progress of science and technology and the arrival of the robot era, spinal robot has become the latest way of surgical navigation.Fan et al.[39] analyzed the accuracy and curative effect of different nail placement methods.There were 176 nails for spinal robot (group 1), 134 nails for navigation template (group 2), 234 nails for O-arm navigation (group 3) and 346 nails for fluoroscopy guidance (group 4).The placement rates of clinically acceptable screws were 94.32%, 95.52%, 90.60% and 78.03%, respectively.Compared with navigation template or O-arm system, robot-assisted technology has no obvious advantage in the accuracy of screw implantation, but it significantly reduces adverse events, fluoroscopy time per screw, postoperative hospital stay and blood loss.The use of robots provides higher accuracy in pedicle screw placement than navigation surgery, unarmed, and fluoroscopic guidance techniques.Navigation technology provides a new means to improve the accuracy of nail placement, but it also has its disadvantages:preoperative navigation systems need to be registered to increase surgical preparation time; their high cost and space constraints; long learning navigation curves; navigation drift leads to catastrophic complications.The emergence of 3D printing guide plate technology fills the gap that hospitals are unable to obtain high-precision navigation.Compared with navigation and robot, the cost of guide template technology is lower, and the accuracy of nail placement is also reliable.

4.3 Guide template nailing

With the rapid development of 3D printing technology, guide template technology has been fully applied and developed.The principle of the use of the guide template is first to scan the spine of the surgical patients with thin CT before operation, and then 3D reconstruction of the spine by digital software, and then design the screw path according to the shape of the vertebral body, and make the guide template that can be attached to the surface of the vertebral body.During the operation, the guide plate was directly attached to the patient’s spine, the pedicle was drilled through the guide plate,and then the pedicle screw was placed[40].In the study of Liu[33] and Pan [34], it is found that the guide template is more accurate than the traditional nail placement, and that the use of guide plate will not affect the operation time, and there is no significant difference in scoliosis correction rate.In the study of Fan et al.[41], it was found that there was no significant difference in screw placement accuracy and satisfaction between the navigation group and the navigation group, but the operation time was significantly lower than that of the navigation group[41].Chen et al.[36] believe that the use of 3D printing technology can facilitate spinal orthopedic surgery.They studied that with the help of 3D printing technology, 10 patients with a total of 173 nails were implanted, and the excellent and good rate of CT reexamination was as high as 97.1%.These studies have confirmed the effectiveness and safety of the guide plate technology.When expensive medical devices such as intraoperative navigation and robot systems are not available, 3D print-oriented template technology can become an effective substitute for computer navigation and robot.Compared with the traditional nail placement method, the advantage of 3D printing navigation template in AIS orthopedic surgery can effectively shorten the nail placement time,significantly reduce the radiation dose and blood loss caused by X-ray fluoroscopy frequency, at the same time reduce the difficulty of nail placement and improve the accuracy of nail placement[42].In addition, the navigation template can not take into account the adjustment and direction of the entry point in the process of screw placement, and at the same time realize the individualization and accuracy of treatment plans and measures, which is especially suitable for inexperienced young doctors to complete pedicle screw placement.The disadvantage is that in order to make the template adhere to the bone surface better and facilitate the accurate placement of nails, it is often necessary to peel off more bony structure and soft tissue, which increases the trauma caused by surgery to a certain extent.In the process of making the guide template, there are often some errors after many times of data transformation coupled with the differences in the production process; in addition, high temperature disinfection may also cause deformation.Production needs a certain cycle, which indirectly increases the hospitalization time and economic burden of patients[43].Guide template technology has the accuracy of nail placement with the aid of navigation system,but there is still a lot of room for improvement in material design and practical application.

5.Prospect

Compared with traditional spinal internal fixation instruments,pedicle screws have better biomechanical advantages and can be transformed into higher scoliosis correction rate in scoliosis surgery.However, the technique is at risk of injury to the nervous system as well as to the aorta, azygos vein and esophagus.Therefore,orthopedic surgeons should have a full understanding of the morphology of spinal deformities and their surrounding visceral structures to prevent catastrophic complications.In addition, only reasonable, efficient, accurate and personalized nail placement concept will be integrated into the traditional technology, navigation technology and guided template technology.In order to maximize the reduction of screw penetration rate and improve the safety of nail placement, and ultimately make patients more benefit and improve the long-term quality of life, reduce the occurrence of postoperative complications.At present, great progress has been made in the research on the safety of nail placement in adolescent idiopathic scoliosis.The related research results show that the use of computer navigation, robot and guide template technology can effectively improve the accuracy of nail placement, reduce the wall-breaking rate and improve safety.The use of intraoperative neuroelectrophysiological monitoring can reduce the risk of nervous system injury and protect patients in real time.In the future research, we should focus on the development of standardized screw penetration classification, the development of computer navigation system and robot, and the research of pedicle screw implantation guide plate material [44,45].

6.Contribution of the author

The first author: Yang Xuejian: collect relevant literature and paper writing; newsletter author: Meng Zhibin: article project conception and review; other authors participate in literature collection and analysis.