Abstract
Background
Three-dimensional (3-D) printing offers the opportunity to create patient-specific guides for pedicle screw placement based on CT-generated models. This technology might allow for more-accurate placement of pedicle screws in patients with severe congenital scoliosis who have rotated vertebrae and small pedicles, but to our knowledge, this premise has not been tested.
Questions/purposes
(1) Is the use of 3-D printing and pedicle guider technology as or more accurate than the use of the freehand technique for pedicle-screw placement in patients with severe congenital scoliosis? (2) Does surgical time differ with the use of these guiders? (3) Are complications less common in patients treated with this new approach to pedicle-screw placement?
Methods
A prospective controlled study was conducted of patients with severe congenital scoliosis (major curve ≥ 90°) from June 2016 to June 2018. During this period, we treated 93 patients with congenital scoliosis; 32 had severe scoliosis with a major curve ≥ 90°. The patients were divided into a pedicle guider group (n = 15) and a control group (n = 17) based on their willingness to use pedicle guider technology, which was considered a research technology. With the numbers available, there were no between-group differences in terms of age, sex, BMI, or parameters related to curve severity or flexibility, and all patients in both groups had severe curves. Preoperative and postoperative low-dose CT scans were performed in the two groups. In the pedicle guider group, custom software was used to design the pedicle guider, and a 3-D printer was used to print a physical spinal model and pedicle guiders. The pedicle guiders were tested on the surface of the physical spinal model before surgery to ensure proper fit, and then used to assist pedicle screw placement during surgery. A total of 244 screws were implanted with the help of 127 pedicle guiders (254 guiding tunnels) during surgery in the PG group. Five predesigned pedicle guiders were abandoned due to an unstable match, and the success rate of assisted screw placement using a pedicle guider was 96% (244 of 254). The freehand technique was used in the control group, which relied on anatomic localization to place pedicle screws. The accuracy of pedicle screw placement was evaluated with CT scans, which revealed whether screws had broken through the pedicle cortex. We compared the groups in terms of accuracy (defined as unanticipated breaches less than 2 mm), surgical time, time to place pedicle screws, and screw-related complications.
Results
A higher proportion of the screws placed using pedicle guider technology were positioned accurately than were in the control group (93% [227 of 244] versus 78% [228 of 291]; odds ratio, 3.69 [95% CI, 2.09–6.50]; p<0.001). With pedicle guider use, operative time (296 ± 56 versus 360 ± 74; 95% CI, -111 to -17; p = 0.010), time to place all screws (92 ± 17 versus 118 ± 21; 95% CI, -39 to -12; p = 0.001), and mean time to place one screw (6 ± 1 versus 7 ± 1; 95% CI, -2 to 0; p = 0.011) decreased. One patient in the pedicle guider group and four in the control group experienced screw-related complications; the sample sizes and small number of complications precluded statistical comparisons.
Conclusions
In this small, preliminary study, we showed that the accuracy of the surgical technique using spinal 3-D printing combined with pedicle guider technology in patients with severe congenital scoliosis was higher than the accuracy of the freehand technique. In addition, the technique using pedicle guider technology appeared to shorten operative time. If these findings are confirmed in a larger study, pedicle guider technology may be helpful for situations in which intraoperative CT or O-arm navigation is not available.
Level of Evidence
Level II, therapeutic study.
Introduction
Using a freehand technique in the surgical treatment of patients with severe congenital scoliosis is difficult and risky because of result of the presence of severely rotated vertebrae and small pedicles. Malposition of pedicle screw, neurologic injury, and other serious complications can occur [15, 18]. Ledonio et al. [8] systematically evaluated the safety of pedicle screws in pediatric patients. A total of 13,536 pedicle screws were placed in 1353 pediatric patients, and the accuracy of pedicle screw placement was 94.5%. The authors also stressed that the accuracy would decrease in spinal deformity with curves more than 90°, and the risk of nerve damage might increase. Reames et al. [17] found that the probability of neurologic complications was associated with the etiology of scoliosis based on data obtained from the Scoliosis Research Society Morbidity and Mortality database in reference to 19,360 children with scoliosis; congenital scoliosis had the highest risk of neurologic deficit (2.0%) followed by neuromuscular types (1.1%) and idiopathic scoliosis (0.8%).
Three-dimensional (3-D) printing technology in orthopaedics has undergone rapid development in recent years. A CT scan data of a spinal deformity can be reconstructed for rapid prototyping to print a one-to-one ratio polystyrene spinal model. A 3-D-printed spinal model provides a more accurate morphologic representation of a spinal deformity for use in both virtual and actual surgical procedures [2, 12, 19, 21]. Use of computer navigation technology has also increased substantially in the field of spinal surgery [14]. However, for CT navigation, O-arm navigation, or with surgical robots, there are limitations such as radiation exposure and a complicated operation during the image-matching process and economic factors that have restricted the popularization of these tools in developing countries [4, 13].
The application of 3-D printing technology in spinal surgery has spurred the development of a new technology, the pedicle guider, also known as a drill template [22]. Currently, this technology is primarily used in the initial stages of spinal deformity, and evidence of its safety and effectiveness is limited [9, 10, 16].
We therefore asked (1) Is the use of 3-D printing and pedicle guider technology as or more accurate than the use of the freehand technique for pedicle-screw placement in patients with severe congenital scoliosis? (2) Does surgical time differ with the use of these guiders? (3) Are complications less common in patients treated with this new approach to pedicle-screw placement?
Materials and Methods
Inclusion Criteria
After receiving approval from the Medical Research Ethics Committee of the First Affiliated Hospital of Zhengzhou University, patients with severe congenital scoliosis were recruited to participate in a prospective, nonrandomized controlled trial, which took place in our hospital from June 2016 to June 2018. During this period, we treated 93 patients with congenital scoliosis, and 32 of them were severe scoliosis with major curve ≥ 90°. Compared with severe congenital scoliosis, screw placement in moderate congenital scoliosis was relatively easy. These patients were not the focus of our study, and a freehand technique was used in the “easy” ones. Due to the high difficulty of screw placement in severe congenital scoliosis, we tried to adopt the pedicle guider technique to reduce screw-related complications.
Inclusion criteria were as follows: (1) age 10 to 18 years; (2) diagnosed with congenital scoliosis; (3) major curve ≥ 90°; (3) no previous spine-related surgery; (4) cardiopulmonary function sufficient to withstand spinal surgery; and (5) absence of neuropsychiatric or communication disorders. We excluded patients with idiopathic, neuromuscular, and syndromic deformities.
The patients with severe congenital scoliosis included in this study were fully informed of the purpose of the study and the potential advantages and disadvantages of pedicle guider technology before surgery as we understood them then. Based on the willingness of patients and their guardians to use pedicle guider technology, patients were divided into the pedicle guider group and the control group. Some conservative guardians did not choose pedicle guider technology because they felt that this immature technology was still in the research stage for severe congenital scoliosis, and it might not be safe compared with decades of freehand technique evidence. Patients in the pedicle guider group were treated with pedicle guider technology, and the freehand technique was the control group. All patients’ guardians were provided written informed consent.
A total of 32 consecutive patients with severe congenital scoliosis were included in the study, 15 in the pedicle guider group and 17 in the control group.
Comparison of Baseline Parameters Between the Two Groups
In the pedicle guider group, there were nine girls and six boys with a mean age of 12 ± 3 years. In the control group, there were 12 girls and five boys with a mean age of 14 ± 4 years. There were no differences in sex, age, height, weight, or body mass index between the two groups (Table 1). The major curve in the pedicle guider group was 113° ± 15° and 106° ± 14° in the control group. No differences were found between the two groups in the coronal and sagittal parameters.
Table 1.
Comparison of baseline parameters between the two groups
Production of the 3-D Spinal Model
A 64-row spiral CT750 HD (GE Healthcare, Milwaukee, USA) was used to scan the spine before and after operation for both groups (Fig. 1A), and model-based iterative reconstruction was used to lower radiation dose. The preoperative CTDIvol in the pedicle guider group was 2.0 ± 0.3 mGy, and 1.9 ± 0.4 mGy in the control group. The postoperative CTDIvol in the pedicle guider group was 1.8 ± 0.2 mGy, and 1.8 ± 0.3 mGy in the control group. There were no differences in preoperative and postoperative CTDIvol between the two groups.
Fig. 1 A-H.
A schematic diagram of pedicle guider technology is illustrated. (A) Three-dimensional image of spinal deformity was reconstructed based on preoperative CT scans. (B) The screw trajectory of the pedicle guider was designed by Mimics 14.0 software. (C) The design of one integrated pedicle guider was completed. (D) All pedicle guiders were designed corresponding to the predetermined screwing vertebral bodies. (E) Three-dimensional spinal model and pedicle guiders were printed with 3-D printing technology. (F) Pedicle guiders were matched with the spine model before surgery. (G) The Kirschner wire was drilled along the pedicle guider. (H) The Kirschner wire indicated the point and direction of pedicle screw insertion.
In the pedicle guider group, the Dicom format data were downloaded from the workstation and uploaded to Mimics 14.0 software (Materialise, Leuven, Belgium) to reconstruct a digital 3-D spinal model. The reconstructed 3-D spinal model could be observed and measured from any direction to assess various features such as pedicle length, transverse diameter, and vertebral rotation angle. The data obtained from the reconstructed 3-D spinal model were saved as a stereolithographic (STL) format file.
Based on the STL format file generated by the Mimics 14.0 software, we printed a 3-D spinal model with polystyrene using a MakerBot Replicator 2 printer (MakerBot, New York, USA). The polystyrene powder was solidified into a 3-D model by laser-melting technology. The 3-D spinal model was printed according to the actual measurements of the patient in a one-to-one ratio, and presentations such as hemivertebra and pedicle dysplasia could be readily observed. The 3-D physical spinal model was used in preoperative discussions to determine the osteotomy program and segments appropriate for fusion.
Design and Printing of the Pedicle Guider
We used Mimics 14.0 software to design the pedicle guider based on the virtual spinal model reconstructed by the software. The targeted vertebral body was displayed in a designated transparent color. We determined the ideal trajectory by repeatedly adjusting the insertion point and angle of the virtual rod. A hollow tunnel was designed according to the coordinates of the virtual rod to allow for passage of Kirschner wires during surgery. A connecting rod was designed to stabilize the guiding tunnels of the left and right pedicles. The substrate was supported by the laminae, spinous processes, and medial edges of the transverse processes, and it perfectly conformed to the local bone surface. The pedicle guider appliance consisted of the substrate, two hollow tunnels, and one connecting rod (Fig. 1B-D). The pedicle guider was labeled with the corresponding vertebral body and printed with a 3-D printer using polylactic acid material. To ensure proper fit, we tested each pedicle guider on the surface of the 3-D physical spinal model before surgery (Fig. 1E-F).
Pedicle Guider-assisted Screw Placement
After the patients were placed under general anesthesia, the paravertebral muscles were removed from the periosteum, and the laminae, the superior and inferior articular processes, and the spinous processes were revealed. In the pedicle guider group, the pedicle guider was carefully attached to the laminae and the articular bones, and a 3.0-mm K-wire was drilled into the pedicle through the guiding tunnel that provided the point and angle of insertion (Fig. 1G-H). After punching with a hole opener, the pedicle trajectory was enlarged with a pedicle probe, all bony borders were carefully palpated before and after tapping, then the pedicle screw was inserted. The freehand technique was used in the control group. A predetermined osteotomy or hemivertebral resection was performed according to the preoperative discussion.
Due to severe deformity of the spine and high rotation of the vertebral body, there was a possibility that the pedicle guider could not be placed stably. If this occurred, we abandoned the use of the pedicle guider and did not implant pedicle screws. A total of 244 pedicle screws were successfully inserted under the guidance of 127 designed pedicle guiders (254 guiding tunnels), and the success rate of assisted screw placement was 96% (244 of 254). Three pedicle guiders were difficult to place as a result of muscle occlusion at the concave side of the apical vertebra, and two others were unstable as a result of the extreme rotation of the vertebral body; therefore, the attempts of screw placement were abandoned in these patients. This problem did not exist in the control group because we used the freehand technique.
CT Classification and Accuracy of Pedicle Screw Placement
CT scans were conducted 2 weeks after surgery for the two groups. According to the relationship between the pedicle screw thread and pedicle cortex from the CT images, the degree of screw placement was classified as one of four grades: Grade 0 = the pedicle screw does not break through the pedicle cortex; Grade 1 = the pedicle screw breaks through the pedicle cortex 0 to 2 mm; Grade 2 = the pedicle screw breaks through the pedicle cortex 2 to 4 mm; and Grade 3 = the pedicle screw breaks through the pedicle cortex > 4 mm (Fig. 2A-D). A systematic review suggested that a safe range of the pedicle screw breaking through the pedicle cortex was ≤ 2 mm, whereas a breakthrough of > 2 mm was considered unsafe [1] . Therefore, in this study, Grades 0 and 1 were regarded as accurate fixation; Grades 2 and 3 were considered misplacement.
Fig. 2 A-D.
Evaluation of the accuracy of pedicle screw placement using CT scans is illustrated here. (A) In Grade 0, the pedicle screw does not break through the pedicle cortex. (B) In Grade 1, the pedicle screw breaks through the pedicle cortex 0 to 2 mm. (C) In Grade 2, the pedicle screw breaks through the pedicle cortex 2 to 4 mm. (D) In Grade 3, the pedicle screw breaks through the pedicle cortex > 4 mm. The arrows indicate the corresponding grades.
In the pedicle guider group, one of the advantages of pedicle guider technique was that lateral pedicle breaches could be identified preoperatively and anticipated. For the narrow pedicle, when designing the trajectory, we had to ensure that the screw did not breach the medial cortex, although a lateral breach was inevitable but predictable. Thus, the predicted lateral breaches were regarded as Grade 1 (accurate), as long as they could provide good distal fixation in the vertebral body.
To assess interobserver reliability, a senior spine surgeon (LX) examined 10 postoperative patients from this study, including 164 pedicles with screws implanted. Two fellowship-trained spine surgeons (ML, NNY) measured the breaches of pedicle cortex independently from the CT scans, and they graded the screws as 0, 1, 2, or 3, according to this classification system. The Kappa value of the interobserver agreement was 0.73. To establish intraobserver reliability, a fellowship-trained spine surgeon (ML) measured the 164 pedicles 10 days later, and the Kappa value of the intraobserver agreement was 0.78.
Two fellowship-trained spine surgeons (ML, NNY) independently measured 535 pedicles from 32 patients; differences were resolved by consensus with the participation of a senior spine surgeon (LX). Then, a fellowship-trained spine surgeon (ML) categorized pedicle screws as Grade 0 to 3 according to the classification system above. Grades 0 and 1 were regarded as accurate fixation.
Radiographic Parameters and Perioperative Data
The preoperative and postoperative radiographic parameters, including the major curve, bending, kyphosis, and coronal and sagittal balance, were measured from whole spine digital radiography. Two fellowship-trained spine surgeons (ML, NNY) independently measured radiographic parameters, and differences were resolved by consensus with the participation of a senior spine surgeon (LX). The perioperative data were carefully recorded, including operative time, time to place all screws, number of screws, and complications related to screw placement. The mean time to place one screw was calculated from the time to place all screws divided by the number of screws used in the procedure. Screw density was defined as number of screws per treated level.
Statistical Analysis
Statistical analysis was performed using IBM SPSS Version 21.0 software (IBM, New York, USA). There were no missing data on the three study endpoints (accuracy, time, and complications). We did not do an a priori sample size calculation because severe congenital scoliosis was very rare. It was not easy to predict how many patients would eventually be included in the study. We were also concerned with the accuracy of pedicle screw placement. The characteristics of each scoliosis patient were different, and it was difficult to predict the number of screws needed. The sample size included in similar evidence was less than 10. We were fortunate that there were 15 patients in the pedicle guider group with a total of 244 screws implanted, and 17 patients in the control group with a total of 291 screws implanted. We argue that the sample size could support our preliminary study. We used the independent-sample t-test for continuous variables between the two groups, such as major curve and operative time. The chi-square test was performed for categorical variables. For a categorical variable with a sample size of < 40 such as gender, we used a corrected chi-square test or Fisher's exact probability method. A p value < 0.05 was considered statistically significant.
Results
Is the Use of Pedicle Guider Technology as or More Accurate Than the Use of the Freehand Technique of Screw Placement?
A higher proportion of the screws in the pedicle guider technology group were placed accurately than were placed in the control group (93% [227 of 244] versus 78% [228 of 291]; odds ratio, 3.69; 95% CI, 2.09–6.50; p<0.001).With the numbers available, there were no differences between the groups in terms of the final curve (Table 2).
Table 2.
Comparison of radiographic outcomes between the two groups
Does the Operative Time Differ with the Use of These Guides?
With the use of pedicle guider technology, the operative time (296 ± 56 versus 360 ± 74; 95% CI, -111 to -17; p = 0.010), time to place all screws (92 ± 17 versus 118 ± 21; 95% CI, -39 to -12; p = 0.001), and mean time to place one screw (6 ± 1 versus 7 ± 1; 95% CI, -2 to 0; p = 0.011) decreased (Table 3).
Table 3.
Comparison of perioperative outcomes between the two groups
Complications
The sample sizes and small number of complications precluded statistical comparisons between the groups, and so complications will be presented descriptively. One patient of 15 in the pedicle guider group experienced a screw-related complication. On the third day after surgery, the patient experienced pain in the left lower extremity, and muscle strength was Grade IV. An emergency CT examination showed that the left pedicle of L3 was dysplastic, and that the pedicle screws had pierced the inner side of the pedicle cortex by 3 mm, but no obvious compression of the spinal cord or nerve root was found. The symptoms were relieved after intravenous injections of sodium aescinate, an antiinflammatory that removes free radicals, reduces exudation, and improves microcirculation; and mouse nerve growth factor, which reduces myelin edema and denaturation as well as promotes recovery of the injured nerve. Both agents have been approved by the China Food and Drug Administration. Four of 17 patients in the control group experienced screw-related complications. One patient had weakness of the right lower extremity on the first postoperative day, and muscle strength was Grade II. Emergency CT examination revealed that the right pedicle screw at T7 had penetrated the pedicle cortex by 4 mm, causing nerve compression. The right pedicle screw of T7 was removed immediately during emergency surgery. The muscle strength of the patient recovered to Grade IV 2 weeks after the revision surgery. In two patients, cerebrospinal fluid leakage occurred during the trajectory adjustment of screw insertion. A large amount of normal saline was replenished by intravenous injection, and elevation of the foot of the bed were implemented after surgery. These two patients did not report obvious discomfort. In one patient, the left pedicle screw of T6 broke through the anterior border of the vertebral body. This screw did not touch the thoracic aorta, and the patient was closely monitored (Table 3).
Discussion
Pedicle screw placement in severe congenital scoliosis is still a difficult problem due to severely rotated vertebrae and small pedicles. Pedicle guider technology seems promising but has not been fully tested. In this prospective controlled study, we compared the use of 3-D printing combined with pedicle guider technology with the traditional freehand technique of surgical correction in patients with congenital scoliosis. The results indicated that 3-D printing combined with pedicle guider technology appeared to improve the accuracy of screw placement in patients with severe congenital scoliosis and to shorten operative time.
This study has some limitations. First, although this study is a prospective controlled study, a randomized controlled study would provide stronger support for our results. We hope that these preliminary findings will stimulate such a study. Furthermore, a 3-D model of the spine was not created for patients in the control group to help the surgeon understand the curve and correction needed. However, we were trying to compare this newer approach to the existing practice of placing pedicle screws. In addition, the study sample size is small. We wanted to focus this study on patients with severe curves; however, the small sample size precluded statistical analysis of complications, and as a result of the small sample, we can make no claims about safety because uncommon complications were likely to go undetected. Another limitation was that we did not do reproducibility testing for the measurement of radiographic parameters. In this study, two fellowship-trained spine surgeons performed the measurements independently, and differences were resolved by consensus with the participation of a senior spine surgeon. Therefore, the results would be acceptable. The last limitation was that we only looked at two approaches (pedicle guider and freehand); future studies must compare pedicle guider technology and CT/O-arm navigation.
With pedicle guider technology, we found greater accuracy than we were able to achieve using the freehand technique. While we are excited by these results, we recognize that our study is small and preliminary; still, recent studies have reported conclusions similar to ours, and that tends to reinforce the message that this technology is promising. Another pilot study used pedicle guider technology in four patients with severe congenital scoliosis and found that 84% (64 of 76) of the screws were completely contained in the pedicle without perforation [16]. In a study of 48 screws implanted using drill templates in patients with severe scoliosis (major curve > 70°), the authors found that accuracy was 93.8% [10]. The accuracy we observed with the use of pedicle guider technology was comparable to that reported from the use of CT or O-arm navigation [5, 20]. A meta-analysis of 7533 pedicle screws showed that the median accuracy of CT navigation-assisted screw placement was 90.8% [20]. Another study of O-arm navigation-based pedicle screw insertion in patients with dystrophic scoliosis secondary to neurofibromatosis Type I found that the accuracy in the apical region was 79% [5]. The accuracy of the current study was similar to that reported in these two previous studies. However, CT navigation has disadvantages such as a complicated registration process and image drift caused by position changes in the patient [3]. O-arm navigation increased the radiation burden on pediatric patients, and it is expensive [7]. In contrast, pedicle guider technology does not have the disadvantages mentioned; use of this technology to assist with screw placement is not complicated, and there is no exposure to additional radiation. Although patients in this study underwent two CT scans in the short term, we used model-based iterative reconstruction to lower radiation dose without affecting quality. It was critically important that these be used in children to lower radiation exposure.
In this study, pedicle guider technology shortened overall operative time and per-screw surgical time. The severe rotation of the vertebral body and the narrow—or even missing—vertebral pedicle may necessitate use of the traditional freehand technique to need repeated intraoperative fluoroscopy and adjustment of the screw tract, which undoubtedly increases operative time for patients with severe congenital scoliosis. One multicenter study that included 4588 patients found that operative duration was an independent risk factor for postoperative complications of spinal surgery [6]. Therefore, shortening operative time may reduce the risk of infection and postoperative complications.
In this study, with the numbers we had, we could not detect a difference in the incidence of screw-related complications between the pedicle guider group and the control group. In the treatment of patients with severe congenital scoliosis, pedicle screw placement is challenging. The misplacement of a pedicle screw can cause nerve damage, and severe screw misplacements can cause paralysis [23] and a potentially life-threatening vascular injury. Lopera et al. [11] reported that seven patients sustained an aortic injury caused by improper pedicle screw position, including four instances of thoracic aortic injury and three occurrences of iliac artery injury. Of these seven patients, one patient died as a result of vascular rupture, one patient underwent amputation resulting from vascular embolization, three patients were managed with screw removal and placement of an artificial vessel stent, one patient had the pedicle screw replaced, and the remaining patient was observed conservatively.
In this small, preliminary study, we showed that the accuracy of the surgical technique using spinal 3-D printing combined with pedicle guider technology in patients with severe congenital scoliosis was higher than the accuracy of the freehand technique. In addition, the technique using pedicle guider technology appeared to shorten operative time. Further work is necessary to compare this option with others, such as the use of intraoperative CT or O-arm guidance, as well as to accurately assess the cost and radiation safety of these options.
Acknowledgments
We thank Weiwei Zhen for his help in operating the MakerBot Replicator 2 printer. We also thank the patients and their guardians in this study; without their participation and contribution, we could not have completed this study.
Footnotes
Each author certifies that neither he or she, nor any member of his or her immediate family, has funding or commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.
All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.
Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA approval status, of any drug or device before clinical use.
Each author certifies that his or her institution approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.
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