Pedicle Screw Placement for Treating Adolescent Idiopathic Scoliosis: Comparison
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Posterior spinal fusion (PSF) and segmental spinal instrumentation (SSI) using pedicle screws (PS) is the most used procedure to correct adolescent idiopathic scoliosis. Computed navigation, robotic navigation, and patient-specific drill templates are available, besides the first described free-hand technique. None of these techniques are recognized as the gold standard. 

  • pedicle screw
  • adolescent idiopathic scoliosis
  • CT navigation

1. Introduction

PSF and SSI using pedicle screws through a single posterior approach is the most commonly used surgical technique to treat adolescent idiopathic scoliosis (AIS) patients. Basques et al. even found it the second most frequently performed pediatric orthopedic surgical procedure in the USA [1]. The rationale behind the popularity of segmental PS constructs is that, in comparison to earlier-developed procedures like the single Harrington rod constructs, the Luque sublaminar wire constructs and the hook constructs, it allows for better coronal and sagittal deformity correction; improves chest wall deformity correction, making thoracoplasty unnecessary for most of the cases; obviates the need for anterior release even for severe deformity cases; reduces the number of levels fused; and lowers the rate of implant-related surgical revisions [2][3][4][5][6].
Despite these recognized advantages, there are also concerns about using PS constructs related to potential screw misplacements and associated neurologic, vascular, and visceral complications that might even be lethal. The literature primarily focusses on the overall rate of screw misplacements to define the safety of PS instrumentation. In 2011, the Scoliosis Research Society and the Pediatric Orthopaedic Society of North America task force stated that screw placement accuracy might be a surrogate for complications and safety data. In a systematic review, they reported an overall PS accuracy rate of 94.9%, considering 17 studies, including 13,536 PS placed in 1353 pediatric patients, of whom the majority had AIS [7]. They did not analyze the used technique for PS placement in the included studies nor report the potential clinical impact of screw misplacement.
Currently, there is neither general agreement on the optimal technique for PS placement for treating AIS patients nor the definition of PS misplacement. The presearch nt narrative review therefore aims to present and compare relevant classification systems and assess PS placement accuracy; assess the clinical relevance of screw misplacements; discuss risk factors associated with PS misplacement; and present the available PS placement techniques, including free-hand with fluoroscopic assistance, computed navigation using 3D fluoroscopy, an O-arm (Medtronic, Dublin, Ireland) or a multidetector computed tomography (CT), robotic-assisted navigation techniques, and patient-specific drill templates with pre-planned trajectory, and compare their benefits and disadvantages in terms of accuracy and safety, including patient radiation exposure, and additionally describe the authors’ preferred PS placement technique.

2. PS Placement Accuracy Classifications

There is no generally accepted standardized method for determining the PS placement accuracy or defining PS misplacements. In 2007, Kosmopoulos and Schizas published a meta-analysis regarding PS placement accuracy [8]. They surprisingly identified 35 different PS placement assessment methods relying either on the analysis of radiographs or CTs, respectively, on dissection for the included cadaveric studies within the 130 included articles. More recently, Aoude et al. published a systematic review on the same subject and found less heterogenic results [9]. Out of the 68 included articles, 37 (54%) had a comparable methodology, representing the most used method for PS placement accuracy assessment.
Although simple to use with an intraoperative or postoperative CT, the most widely reported PS accuracy grading systems relying either on 2 mm breach increments or on the differentiation of screws being “in” or “out” fail to point out the clinical relevance of the breaches, which should actually be primarily focused on. This shortcoming was addressed by Sarwahi et al., who reported a novel classification system aimed at recognizing potentially clinically significant PS misplacements and considering the location of the breach and the remaining distance between the PS and the anatomical structures of concern [10]. This PS accuracy classification system defines the following gradings: accurately placed (AP); benign misplacement (BMP); indeterminate misplacement (IMP); and screw at risk (SAR). AP screws are, per definition, completely contained by the pedicle. BMPs breach the cortical wall but do not place any structure at risk; IMPs breach either medially by 2–4 mm or laterally respectively anteriorly, while >1–2 mm distance is kept between the screw tip and the anatomic structures at risk; and SARs either breach medially more than 4 mm or breach laterally or anteriorly and impinge (<1 mm distance between the screw tip and organ) on anatomic structures of concern, such as the aorta, the trachea, or the esophagus. The latter classification integrates the facts that medial breaches of up to 2 mm are generally accepted as safe and that transient or permanent neurologic deficit occurrence has been exclusively reported for medial breaches of at least 4 mm [8][9][10][11][12]. It also considers the risk associated with anterolaterally misplaced screws, as the definition of SAR encompasses PS, with an anterolateral breach impinging on an anatomic structure of concern. However, it is unknown if the chosen cut-off of <1 mm between the screw tip and the organ at risk for defining SAR due to anterolateral breaching is optimal. Foxx et al. gave some insights into the risk of vascular complication due to PS misplacement by reporting 9 cases summarizing 33 screws being in contact with a great vessel (aorta, iliac artery, iliac vein) within their studied cohort of 182 cases having undergone posterior spinal fusion (surgical indication not mentioned) [13]. Despite 33 SARs in their series, no vascular injury, including pseudoaneurysm, happened during the 9-year follow-up period. The analysis of this case series is reassuring; however, due to the relatively small size of the studied population and the presence of some case reports describing pseudoaneurysm formation or erosion of the aorta secondary to impinging PS, the risk of dramatic vascular complication occurrence is real and cannot be ignored [14][15].
In summary, a standardized method to assess the PS placement accuracy is needed to gain more evidence about the PS safety. The optimal classification system should be reproducible and simple and point out the clinical relevance of the breach. Because of the advantages of Sarwahi’s accuracy classification system presented here above, this method might fulfill the requirements for a standardized assessment method to be generally used in the future, if its reproducibility proves to be high and if the chosen cut-off (<1 mm distance between screw tip and organ) to define anterolaterally misplaced SAR proves to be clinically relevant.

3. Patient-Related Risk Factors for Screw Misplacement

Liljenqvist et al. first reported the morphology of pedicles in AIS patients [16]. They analyzed 337 pedicles in 29 surgically treated AIS patients with CT and observed that the endosteal transverse pedicle width was significantly smaller in the apical region of thoracic curves (T7–T10 and T12) on the concave side (mean 2.5–5.2 mm) in comparison to the convex side (mean 4.1–5.9 mm). They, therefore, considered that PS instrumentation on the concavity in the apical region of thoracic curves is critical. Watanabe et al. stated that these imaging findings were not correlated to the practical feasibility of navigating a probe down a thoracic pedicle into the vertebral body and consequently reported a pedicle channel classification describing the osseous anatomy experienced during free-hand probe insertion into the pedicle channel in a prospective series of 53 consecutive scoliosis patients (including 38 AIS cases) [17]. They defined the following four types of pedicle channels: “Type A”, a pedicle probe smoothly inserted without difficulty, the morphology described as a “Large Cancellous Channel”; “Type B”, a pedicle probe inserted snugly with increased force, described as a “Small Cancellous Channel”; ”Type C”, a pedicle probe cannot be manually pushed but must be tapped with a mallet down the pedicle into the body, described as a “Cortical Channel”; and “Type D”, a pedicle probe cannot locate a channel, thus necessitating a “juxtapedicular” screw position, described as a “Slit/Absent Channel”. The distribution of pedicle channel types varied according to the diagnosis leading to scoliosis and was as follows for the included 38 AIS cases operated using 474 PS: 254 PS (53.6%) Type A, 157 PS (33.1%) Type B, 39 PS (8.2%) Type C, and 24 PS (5.1%) Type D. Thus, 87% of the pedicles (Types A and B) had cancellous channels, and 13% (Types C and D) had none. Watanabe et al. considered pedicles with a cancellous channel as relatively safe for free-hand probe insertion and PS instrumentation, and those without as needing more challenging techniques such as tapping the probe with a mallet, using a small high-speed drill or burr to perform a juxtapedicular placement. This prospective clinical channel classification was further correlated to the pedicle size measured in preoperative CT by subset data analysis to help surgeons preoperatively foresee PS placement technical challenges based on preoperative CT performance. However, Watanabe et al. did not correlate the pedicle channel grades to a corresponding PS placement success rate. Akazawa et al. revised the hereabove-presented channel grading system and defined the following grades based on preoperative CT measurements: grade 1 for a “large cancellous channel” with an inner diameter of ≥4 mm; grade 2 for a “moderate cancellous channel” with an inner diameter of ≥2 mm and < 4 mm; grade 3 for a “small cancellous channel” with an inner diameter of ≥1 mm and <2 mm; and grade 4 for a “cortical channel” with an inner diameter of <1 mm [18]. They used this revised pedicle channel grading system to describe 810 pedicles that were planned for PS placement and were probed for AIS correction in a series of 55 patients using an O-arm-based intraoperative navigation, a technology which basically consists of two units: a motorized circular fluoroscopy, able to rotate 360° around the region of interest to generate a series of projections used to generate 3D CT-like volume datasets based on the acquired 2D projections; and an additional navigation unit holding two infrared cameras used for matching and to localize the navigated instruments by geometrical triangulation. Azakawa et al. found that a higher pedicle channel grade was a significant risk factor for PS placement failure (significant difference for each increment). PS placement was considered a failure if perforation while probing occurred and was followed by PS placement discontinuation, if the PS was intraoperatively removed due to malposition on imaging, or if a PS deviation of at least 2 mm was seen in postoperative CT. There were 61 failures among the 810 probed pedicle channels. The respective failure rate breakdown was as follows for grades 1 to 4: 0.5%, 2.9%, 12%, and 31.5% (p < 0.001). They concluded that PS placement should be avoided in grade 4 pedicles (<1 mm inner diameter). Interestingly, although their series included 89 grade 4 pedicles (number of concerned patients unknown) with a failure rate of 31.5% for PS placement, no patient had neurologic or vascular complications. Another study by Akazawa et al. reported a significant PS misplacement rate increase for grade 4 pedicles (22.2%, p = 0.008) relative to other grades when using robotic navigation to treat AIS [19].

4. PS-Misplacement-Related Morbidity and Mortality

The literature mainly focusses on the overall rate of PS misplacements to define the safety of PS. The Scoliosis Research Society and the Pediatric Orthopaedic Society of North America task force have correspondingly stated that the PS placement accuracy might be considered as a surrogate for complication and safety data [7]. However, as previously reported, the overall rate of PS misplacement primarily measures the surgeons’ technical skills and is an unsatisfactory indicator of patient safety [11]. This manner of reporting PS misplacements indeed underestimates the patient’s incurred risk, as the overall rate of misplaced screws is much lower than the rate of concerned patients. This is particularly true for deformity surgeries, as the number of used PS per patient is high and as the deformity renders the PS placement more complex. The discrepancy between the overall and the per-patient rate of PS misplacement is well-illustrated by a series of 127 pediatric patients (89 with AIS) reported by Sarwahi et al., in which a total of 2724 PS were placed [11]. They had an overall PS misplacement rate of 1.1%, a per-patient PS misplacement rate of 14.2% when using their definition of SAR to define misplaced PS. In their series, 18 patients had SAR. Among them, 10 had medial misplacements, 6 had PS impinging on the aorta, 1 had screws impinging on the trachea, and 1 had a medial misplaced screw and another screw impinging on the aorta. Two patients had transient neurologic injury, which needed intraoperative PS removal due to loss of signals and a later surgical revision after neurological recovery. All PS impinging on the aorta were asymptomatic. Additionally, Suk et al. reported a series of 462 spinal deformity cases, including 330 idiopathic scoliosis patients corrected using a total of 4604 thoracic PS [20]. They found an overall PS misplacement rate of 1.5% and a per-patient PS misplacement rate of 10.4%, leading to one transient paraparesis and three dural tears. However, no significant PS-related neurologic or visceral complication adversely affected the long-tern outcome. In their systematic review concerning PS fixation complications in scoliosis surgery, Hicks et al. reviewed 21 studies totalizing 14,570 PS implanted in 1666 pediatric patients and found an overall PS misplacement rate of 4.3%, while the rate of patients with misplaced PS, which was reported only in a minority of the included studies, was about 11% [21]. The average revision rate for PS misplacement was 0.8% when considering 11 studies, including a total of 1436 patients. One pulmonary effusion resulting from an intrathoracic misplaced PS was reported. Four studies reported dural leaks during PS placement and found an incidental durotomy rate of 0.4% per inserted PS. The only reported neurologic complication was a transient paraparesis due to an epidural haematoma secondary to a medially misplaced PS [20]. No irreversible neurologic complications and no major vascular complications were recorded.
From these case series, it can be assumed that PS misplacements occur at least in 10% of the pediatric deformity patients and that despite this high frequency, at least the short-term related morbidity is low. Additionally, even though the PS-misplacement-related mortality rate is unknown, it must be low, as the total (any cause) mortality rate for AIS cases was found to be 2.8 deaths per 1000 cases within 60 days of surgery in a large review based on the Scoliosis Research Society Morbidity and Mortality database [22]. However, the exact clinical relevance of PS misplacements remains unclear because most of them are at least initially asymptomatic, and little is known about their long-term natural history.

References

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  18. Akazawa, T.; Kotani, T.; Sakuma, T.; Minami, S.; Tsukamoto, S.; Ishige, M. Evaluation of Pedicle Screw Placement by Pedicle Channel Grade in Adolescent Idiopathic Scoliosis: Should We Challenge Narrow Pedicles? J. Orthop. Sci. 2015, 20, 818–822.
  19. Akazawa, T.; Torii, Y.; Ueno, J.; Umehara, T.; Iinuma, M.; Yoshida, A.; Tomochika, K.; Ohtori, S.; Niki, H. Accuracy of Computer-Assisted Pedicle Screw Placement for Adolescent Idiopathic Scoliosis: A Comparison between Robotics and Navigation. Eur. Spine J. 2023, 32, 651–658.
  20. Suk, S.I.; Kim, W.J.; Lee, S.M.; Kim, J.H.; Chung, E.R. Thoracic Pedicle Screw Fixation in Spinal Deformities: Are They Really Safe? Spine 2001, 26, 2049–2057.
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