The use of electronic conductivity devices can effectively reduce radiation exposure in vertebral body tethering

Vertebral body tethering (VBT) presents new challenges in respect to radiation exposure, as screws cannot be placed free-hand and the lateral positioning of the patients increases scattered radiation. To reduce radiation exposure, we introduced the use of electronic conductivity device (ECD). These are drilling probes send an audio signal when cortical bone is breached. Thus, anterior, bicortical screws can be placed without multiple fluoroscopic controls. ECD has been used for all VBT procedures at our institution starting April 2020. The aim of this study was to test the safety of ECD and its efficacy in radiation reduction in comparison with the current standard, the fluoroscopic guidance. All patients who underwent VBT between August 2019 and December 2020 were retrospectively reviewed and divided into two groups according to whether ECD had been used or not. The radiation exposure per procedure and per screw was compared among the two groups, overall and separately for thoracic, lumbar and bilateral procedures. The rate of misplaced screws was calculated. Data from 62 patients and 825 screws were obtained (397 with ECD). No screw misplacement was observed. Radiation reduction with ECD reached up to 41%. A significant reduction was observed in the radiation per procedure in bilateral instrumentation (from 9.16 to 5.52 mGy*m2), and in the analysis per screw overall (from 9.16 to 5.52 mGy*m2) and for lumbar curves (from 0.54 to 0.32 mGy*m2). ECD can safely and effectively reduce the radiation exposure for VBT procedures.


Introduction
The exposure to ionizing radiations represents a central issue during the diagnostic, clinical follow-up and surgical treatment of patients with adolescent idiopathic scoliosis (AIS), as it leads to a higher rate of cancer compared to the general population [1]. Next to the radiation exposure of the patients, the intraoperative exposure of orthopedic surgeons and surgical staff should not be forgotten [2][3][4].
Vertebral body tethering (VBT) is gaining popularity among spine surgeons for the treatment of selected AIS patients. It allows for maintenance of spine mobility, shorter recovery time and reduced adjacent segment degeneration [5][6][7][8]. However, VBT comes with the disadvantage of an increased radiation exposure for the patient and the surgical staff, as the patient is positioned in lateral decubitus [9]. Furthermore, free-hand screw positioning [10] has not yet been developed for VBT and fluoroscopic guidance is used for bicortical screw placement.
In an effort to reduce radiation exposure during VBT procedures, we introduced the use of a handheld electronic conductivity device (ECD) in our practice in April 2020 for the placement of 6.5mm screws. ECDs have been originally developed to improve the accuracy of pedicle screw placement in scoliosis surgery, but proved to be efficient in limiting radiation exposure as well [11,12]. These devices present a sharp tip, the drilling probe, which is provided with an electronic conductivity sensor. As cancellous bone is characterized by a lower resistance in comparison with the surrounding cortical bone and soft tissue, the sensor measures the local electronic conductivity and immediately translates it into an audio signal that informs the surgeons when cortical bone has been breached [12]. In the case of VBT, bicortical screws are inserted in the lateral, convex aspect of the vertebral body. While the entry point and trajectory of the screw are determined by anatomical landmarks, ECD alerts the surgeon that the cortical bone on the concave side has been breached without necessity of subsequent X-rays.
The aim of this study was to test the safety of hand-held ECD in the setting of VBT and assess its efficacy in respect to radiation exposure.

Patient selection
The present study was conducted according to the Strengthening the Reporting of Observational Studies in Epidemiology: the STROBE Statement [13]. A retrospective review was conducted, of all consecutive patients who underwent VBT at our institution between August 2019 and December 2020. Only patients with a complete record of intraoperative radiation exposure were included. The study was approved by the local Ethics Committee (EK 130/19).

Outcomes of interest
Demographic data, along with radiographic measurements and intraoperative radiation exposure, were collected. The number of misplaced screws in postoperative, first standing radiographs were also recorded. A screw was considered misplaced when it would breach the discal or neural space as seen in the postoperative X-rays, or if systemic symptoms would be suggestive of organic or vascular damage through a misplaced screw, which would then be confirmed in CT scans.
Patients were divided into two groups according to whether or not ECD had been used in the procedure. All surgeries performed prior to April 2020 were done without ECD, while all surgeries performed after this date were done with ECD. However, the ECD was only used for the placement of the 6.5-mm screws. These screws are used for the posterior lumbar instrumentation in the case of a double tether, as well as for the thoracic spine. For the supportive anterior tether, 5.5-mm screws and no ECD was used. An example of the use of the ECD from the intraoperative image intensifier views is shown in Fig. 1.
All surgeries were performed by one author (**) as previously described [14] and the same fluoroscope was used for all surgeries. Thoracic curves down to L1 were approached with video-assisted thoracoscopic surgery (VATS) technique, while a mini-open access was used for lumbar curves. In procedures performed without ECD, a manual, sharp probe was used to drill the channel for the screw and trajectory and depth of the screw were controlled under fluoroscopic guidance.
Of note, the ECD cannot distinguish between the contralateral cortical breach or a possible breach in the cranial or caudal disk spaces or toward the spinal cord. Thus, even when using the ECD, anteroposterior and lateral X-rays are still required at the beginning of the threading to ensure that the trajectory is correct and runs parallel to the endplates. When the ECD gives the auditive signal that indicates the breach of the contralateral cortex, one more a.p. view is taken to confirm the correct trajectory and measure the length of the respective screw.
In summary, we hypothesized that the ECD reduces the radiation by avoiding multiple and frequent fluoroscopic anteroposterior controls until the contralateral cortex is reached. Fig. 1 Example of the intraoperative use of the ECD (anteroposterior X-ray). The device is threaded into the vertebra (left) and, once the contralateral cortex is breached (right), the auditive signal for dynamic surgical guidance is sent The entire peri-and intraoperative radiation exposure was considered, including the preoperative X-rays required for patient positioning and marking. The fluoroscope automatically records the dose area product (DAP), which is then copied in the patients' surgical records as per local law requirements. One C-arm was used and was swung underneath the operating table for anteroposterior and lateral views, the radiation field was reduced as much as possible to limit radiation exposure while offering a sufficient view. The thoracic images were not taken during expiration or during breath-holding. The same fluoroscope with flat-bed detector was used for all surgeries. The records of all patients who underwent VBT in the observation period were accessed to retrieve the data on radiation exposure.

Statistical analysis
The statistical analysis was performed with the software R. After performing a Shapiro-Wilk test to confirm the normal distribution of the variables, a two-tailed, paired T-Test was performed to compare radiation exposure in the groups with and without ECD. The parameters were expressed as mean ± standard deviation and the confidence interval was set at 95% for all comparisons. The comparison was conducted both per procedure and per screw, dividing the patients into three subgroups according to the kind of VBT procedure performed (lumbar, thoracic or bilateral). Radiation per screw was calculated dividing the radiation exposure of a given procedure by the number of screws used.

Results
During the observation period, VBT was performed on 62 consecutive patients (one male and 61 females). The required radiation exposure protocols were completed for all the subjects, so that all 62 patients could be enrolled in the study. The mean age of the included patients was 14.3 ± 2.4 years and the mean age was comparable among the two groups (14.6 years in the ECD group, 14 years in the group without ECD, p = 0.2).
Overall, 17 thoracic, 16 lumbar and 29 bilateral VBT procedures were performed and 825 screws were placed, for 397 (48%) screw an ECD was used. The mean preoperative Cobb angle was 56.8 ± 10.2° at thoracic level and 51.9 ± 12.5° at lumbar level.
The radiation exposure per procedure is shown in Table 1, while the radiation exposure per screw is shown in Table 2.
No screw malpositioning was observed in either of the groups.

Discussion
The main finding of the present study is that the use of ECD can effectively reduce the radiation exposure (up to 40%) in VBT procedures, while maintaining the same level of safety as the current standard, which is represented by fluoroscopy-guided screw placement.
During VBT procedures, fluoroscopy is required for patient positioning, screw placement and curve correction. In particular, in the screw placement phase both lateral and anteroposterior fluoroscopy is required. At our institution, we first insert the staples in all vertebrae that can be instrumented with one given incision, perform a lateral fluoroscopy to control the correct positioning of the staple (and thus the entry point of the screw) in the vertebral body, and then proceed with screw placement. As the screws are placed bicortically, the standard technique requires multiple successive X-rays to confirm that the drill has breached the cortical wall on the convex side, but has also not reached too far on the contralateral side as to pose a damage to organs or blood vessels, and to measure the length of the screw. When ECD is employed, audio guidance is sufficient to safely drill the vertebra without fluoroscopic control. However, one anteroposterior x-ray is still required to determine the length of the screw. Theoretically, it would be possible that radiation exposure was influenced by the learning curve [15] and thus that the data of our cohort may have been biased. At the beginning of the observation period, the surgeon performing all interventions had already conducted over 50 VBT procedures, so that any possible effect of the learning curve on the presented results is highly unlikely. The pathologic effect of radiation exposure can be either deterministic, and thus dose-dependent, or stochastic, or non-dose dependent. Deterministic damage reflects on different organs and tissues and is for example connected to the development of cataract [16], while stochastic damage is associated to cancerogenesis and teratogenesis [16].
While only the comparison for bilateral curves per procedure and per screw and the overall comparison per procedure yielded significant results, the mean values of all comparisons showed a lower radiation exposure when ECD was used. Considering that the pathologic effect of the exposure to ionizing radiation can be not only deterministic, and thus dose-dependent, but also stochastic, every effort toward the reduction of radiation exposure is valuable to protect the patients' health.
Radiation exposure, however, involves both patients and surgical staff. The use of protective leaded equipment and maintaining a safe distance from the source of radiation reduce the health hazard to surgical personnel [17], but the exposure to scattered radiation may still potentially be damaging. One report showed that the risk of developing cancer among the members of the Scoliosis Research Society (SRS) was increased by 13% [18]. However, the available literature only concerns posterior fusion in the adult population [19,20] and it is known that lateral patient positioning increases scattered radiation [9]. As anterior spine surgery is gaining popularity not only for AIS, but also in degenerative settings [21], further studies will be required to assess the radiation exposure for patient and surgical staff in this new setting, both in the adult and pediatric population.
The main limitation of the present study is represented by its retrospective nature. Thus, for procedures in which both 6.5-and 5.5-mm screws were used, the effect of ECD may have been underestimated; 5.5-mm screws are placed with the use of a fluoroscope.

Conclusion
The present study shows that the use of ECD can effectively reduce the radiation exposure while maintaining the same level of safety achieved with the standard technique, namely the use of fluoroscopic guidance.
Funding Open Access funding enabled and organized by Projekt DEAL.
Data availability Data can be made available in anonymized form and upon reasonable request.

Consent to participate and publication
Due to the retrospective nature of the study, consent to participate or for publication was not required.
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