|Year : 2021 | Volume
| Issue : 2 | Page : 163-169
In-vivo clinical validation of perpendicular to superior articular process as thoracic pedicle trajectory: A retrospective case series of 60 pediatric scoliosis
Aziz Ahmad1, Chadi Ali2, Oliver Stokes3
1 Trauma & Orthopaedics, Ipswich Hospital, Ipswich, UK
2 Trauma and Orthopaedics, Royal National Orthopaedic Hospital, Brockley Hill, UK
3 Exeter Spine Unit, Princess Elizabeth Orthopaedic Centre, Royal Devon and Exeter NHS Foundation Trust, Exeter, UK
|Date of Submission||29-Jul-2020|
|Date of Decision||26-Aug-2020|
|Date of Acceptance||02-Feb-2021|
|Date of Web Publication||10-Jun-2021|
Trauma & Orthopaedics, Ipswich Hospital, Heath Road, Ipswich, Suffolk IP4 5PD.
Source of Support: None, Conflict of Interest: None
Study Design: This is a retrospective case series study. Objective: Thoracic pedicle screw insertion can be technically challenging because of narrow pedicles. Placement of thoracic pedicle screws in pediatric scoliosis and adult deformity surgeries, due to three-dimensional rotation of vertebrae, is even more challenging because the usual landmarks are less evident, and the sagittal trajectory is more difficult to correctly orientate due to the vertebral rotation. We describe a variation of freehand technique to guide sagittal trajectory of thoracic pedicle screw. Materials and Methods: The inferior articular process of cranially adjacent vertebrae is osteotomized using a Capener Gouge to expose the superior articular process (SAP) of the thoracic vertebrae to be instrumented. An O’Connell dissector is then placed flush on the SAP. The main shaft of the dissector is at right angle to the base plate; pedicle finder is placed parallel to the shaft and follows the same sagittal trajectory as the shaft. Results: A total of 390 pedicle screws were identified in a consecutive series of 60 scoliosis patients inserted using this technique. Only one screw was revised for lateral breach. There was no intra-operative complication or neurological sequelae in any of our patients. Conclusion: Freehand pedicle screw placement remains a very common technique, used particularly by pediatric scoliosis surgeons. One of the drawbacks of previous reports of the freehand technique is that the sagittal trajectory is not clearly defined. Our technique fills this gap, and this series demonstrates that the technique produces a reliable and consistent result.
Keywords: Freehand, sagittal trajectory, thoracic pedicle screw
|How to cite this article:|
Ahmad A, Ali C, Stokes O. In-vivo clinical validation of perpendicular to superior articular process as thoracic pedicle trajectory: A retrospective case series of 60 pediatric scoliosis. Indian Spine J 2021;4:163-9
|How to cite this URL:|
Ahmad A, Ali C, Stokes O. In-vivo clinical validation of perpendicular to superior articular process as thoracic pedicle trajectory: A retrospective case series of 60 pediatric scoliosis. Indian Spine J [serial online] 2021 [cited 2022 May 20];4:163-9. Available from: https://www.isjonline.com/text.asp?2021/4/2/163/318122
| Introduction|| |
Thoracic pedicle screw insertion can be technically challenging because of narrow thoracic pedicles and the sequelae of the misplaced screw are significant due to the presence of the spinal cord. Freehand pedicle screw insertion relies on tactile feedback of the surgeon and the use of anatomical landmarks. Placement of thoracic pedicle screws in pediatric scoliosis and adult deformity surgeries, due to three-dimensional rotation of vertebrae, is even more challenging because the usual landmarks are less evident, and the sagittal trajectory is more difficult to correctly orientate due to the vertebral rotation.
The pedicle screw insertion technique was first described in 1950. Subsequently, pedicle screws have been widely adopted by spinal surgeons, primarily because of their powerful three-dimensional corrective force in the management of spinal deformity. Pedicle screws were initially used only in lumbar vertebrae, but as experience and confidence grew, they are increasingly also used in the thoracic spine.
Pedicle screws are the most commonly deployed instrumentation in pediatric scoliosis corrections, and it has been shown that they can be safely and effectively placed even with significant rotation of vertebrae. The risks of mal-placing thoracic pedicle screws remain high, particularly in the concavity of thoracic curves, in which the spinal cord drapes over the concave pedicles, which are frequently narrow and dysplastic secondary to the compressive force of the deformity during growth.
To avoid screw misplacement, the reported maximum permissible translational error is less than 1 mm, and the rotational error should be less than 5° at the normal midthoracic spine due to small pedicle diameter. To achieve this level of accuracy, using the freehand technique several years of training are required as misplaced screws can result in catastrophic complications including paralysis, nerve damage, and damage to the lungs and great vessels.
Fluoroscopically guided pedicle screw insertion was developed in an attempt to increase the safety of screw placement. More recently, navigation has been used to increase the ease and safety of screw insertion. Most commonly, this utilizes intra-operative 3-dimensional (3-D) fluoroscopy or computerized tomography (CT). Such technologies increase radiation exposure to the patient and the surgical team. The freehand pedicle screw placement remains a common modality of fixation for various thoracic pathologies, such as trauma, degenerative spinal disease, scoliosis, and tumors.,
We describe a variation of freehand technique using anatomical landmarks and readily available instruments to guide sagittal trajectory of thoracic pedicle screws.
- Remove the inferior articular process (IAP) from the cranially adjacent vertebra, with a Capener Gouge. This permits visualization of the superior articular process (SAP) of the vertebra which is to be instrumented [Figure 1] and [Figure 2].
- The thoracic transverse processes (TP) offer an initial approximation of the sagittal pedicle trajectory. Therefore, remove only the leading edge of the TP. Place the mouth of the bone rongeur at the junction of the TP and the lamina [Figure 3]. Removal of the bone is normally followed by a flush of cancellous bone which is a further approximation of the entry point to the pedicle.
- Place the flat end of an O’Connell dissector on the SAP [Figure 4]. The long axis of the O’Connell dissector is perpendicular to the flat end and provides the sagittal direction of the thoracic pedicle.
- Place a thoracic pedicle finder into the starting point, which has been selected using steps 1–3, and ensure that it is parallel to the O’Connell dissector [Figure 5] and [Figure 6].
- Use tactile feedback to ensure that there is a cancellous bone in front of the pedicle finder, cannulate the pedicle, and check the track with a ball-tipped feeler. An AO depth gage is used to determine screw length.
- The pilot hole can be expanded using a larger diameter pedicle finder or tap as required.
- Carefully insert the pedicle screw.
- At the end of the procedure, we obtain fluoroscopic images before deformity correction and each pedicle screw is individually assessed, in case of any doubt whether position screw is removed and the track is rechecked with a ball-tipped feeler.
|Figure 2: Removal of IAP of cranially adjacent vertebra to expose superior articular process|
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|Figure 4: O’Connell dissector flush with SAP to give sagittal trajectory|
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| Method and Result|| |
Inclusion and Exclusion
Data were analyzed from the senior author’s consecutive scoliosis case series. All consecutive pediatric scoliosis cases (age range 10–18 years) performed by a senior author between October 2015 and October 2019 were included. All other cases including degenerative scoliosis, tumor, and trauma were excluded.
In our institution, there is a robust data collection, and all operated cases are entered into the British Spine Registry and there is an independent administrator who collects data, including patient feedback and complications. Data were retrieved from registry from October 2015 to October 2019; all intra-operative and postoperative complications and returns to theater were noted. These data were further cross-referenced with hospital case notes review and electronic clinic letter review, and the intra-operative and postoperative radiological images were also evaluated. The follow-up period ranged from 8 to 60 months. Postoperative radiographs were assessed independently by two reviewers to identify any mal-positioned screw and metal work failure. Assessment was based on the technique described by Learch et al. Each vertebra was divided into four equal vertical quadrants with spinous process in the center in antero-posterior (AP) plane; in lateral view, pedicle was divided in three equal horizontal quadrants. A screw position was deemed adequate if it was placed in zone 2 in both AP and lateral view. In case of any discrepancy, the two reviewers with the senior author jointly assessed the position.
We identified 390 thoracic pedicle screws inserted in a total of 60 scoliosis patients using our technique, all except one was found to be inadequate. This screw was revised due to lateral breach in one patient. The patient complained of pain and the radiograph showed loosening. CT scan confirmed that there was a lateral breach, and the screw was subsequently revised as it was causing symptoms. No intra-operative complication was noted in the series and there was no medial breach or neurological deficit in any of the patients.
| Discussion|| |
Thoracic pedicle screws can be inserted either under fluoroscopic guidance via navigation or using a freehand technique; each method has its advantages and limitations.
Fluoroscopically guided techniques are associated with radiation and although surgeons and theater staff can take precautionary measures, patients are exposed to the long-term effects of radiation. It has been reported that this technique is associated with risks of fatal cancers and genetic defects in 115 and 4 per million patients treated, respectively.
Although the literature supports that spinal navigation improves the accuracy of pedicle screw placement as shown in a meta-analysis by Tian et al., there is no significant evidence that it improves clinical outcomes. Meta-analysis by Verma et al. does not show that spinal navigation improves functional outcome after surgery. Wagner et al. showed that there was no difference in complications after spinal navigated surgery, and the study failed to show any clinical benefit of spinal navigation.
The freehand technique utilizes posterior bony landmarks and rotation of the vertebrae to guide the trajectory of the screw. The freehand technique of thoracic pedicle screw placement performed in a stepwise, consistent manner is an accurate, reliable, and safe method of insertion to treat a variety of spinal disorders, including spinal deformity. Rajan and Murugan reported that freehand thoracic pedicle screw placement is 2 min faster per screw when compared to the fluoroscopically assisted technique, but techniques have similar accuracy which implies that freehand technique is superior as it is faster and there is less radiation exposure to both the surgeon and patient. Although Pan et al. have shown that drill guide template to be more accurate to freehand technique in their study, they did highlight many shortcomings of the technique such as using the template based on supine CT and applying this to a prone patient which can render the template inaccurate if spine is flexible. Furthermore, the template should be well fitted to avoid motion between template and bone and as such a very careful preparation of soft tissue needs to be carried out to have an exact fit.
Despite being widely used, there are issues with freehand technique and clinical and cadaveric studies have shown that 15–25% of thoracic screws placed using freehand technique may violate the pedicle cortex., The breach rates for thoracic pedicle screw placement vary from 1.5% to 58%. The large variability, in published breach rates, is explained by imaging modality chosen to assess the breach. If the modality is plain radiographs, there could likely be an underestimation of the breach rate with Suk et al. reporting a breach rate of 1.5%, whereas Belmont et al. reported a breach rate of 58% using CT. Samdani et al. used postoperative CT to evaluate pedicle breach and found an incidence of 12.7% pedicle breach; however, in their cohort, none of these patients had any neurological, vascular, or visceral complications. Belmont et al. also came with the definition of “acceptable” when compared with “fully contained” screw placement. Acceptable screw placement included generous lateral cortex violation (6 mm) and minor medial cortex breech (2 mm). A systematic review by Hicks et al. has concluded that most of the referenced authors agreed that a 2-mm encroachment of spinal canal is acceptable. This also corresponds to the thickness of the blade of sublaminar hooks that have been inserted for years in the spinal canal, with an extremely low incidence of neurologic injuries.
Long-term adverse sequala of asymptomatic thoracic pedicle breach is not common; therefore in our institution, routine postoperative CT is not undertaken to minimize radiation risk to patients.
Freehand pedicle screw placement remains a very common technique, used particularly by pediatric scoliosis surgeons. One of the drawbacks of previous reports of the freehand technique is that the sagittal trajectory is not clearly defined. Our technique fills this gap, and this series demonstrates that the technique produces a reliable and consistent result.
Kim et al. have shown in their study a constant angular relationship with the SAP and the pedicle axis; the line perpendicular to the SAP can act as a trajectory. Therefore, they suggested that the SAP might be the only accurate and safe reference for pedicle screw insertion in the thoracic spine perpendicular to the SAP using freehand technique. Our study corroborates this finding and gives a practical and reproducible solution to determine a perpendicular trajectory to the SAP by using an O’Connell dissector.
Baaj et al. have done extensive review of existing strategy for freehand pedicle insertion in thoracic spine and reported a paucity of evidence to guide sagittal trajectory in thoracic pedicle; they recommended a sagittal trajectory orthogonal to dorsal curvature of spine. We have shown in this study that using 90° metal marker (O’Connell dissector) on the SAP was more easily reproducible particularly in scoliosis where there is a three-dimensional deformity.
One of the strengths of our study is that the technique has been developed and validated in a cohort of scoliosis patients, where due to rotation of vertebrae, inserting pedicle screws is most challenging. This technique can be easily extrapolated to trauma and tumor cases.
One of the important drawbacks of our study is that we did not get postoperative CT to more reliably document the rate of clinically insignificant breach. We therefore cannot conclude our accuracy; however, given only one screw was revised 6 months after surgery and no other breach was identified on review of plain radiograph, we can state that apart from one screw other 389 screws placed using this technique were acceptable. It is also to be stated that we have low threshold to recheck position of our pedicle screw intra-operatively if there is any doubt on final fluoroscopic image which also contributes to our high accuracy.
Our results are consistent with other studies that reported that the freehand technique when performed by experience and suitably trained surgeons produces good outcomes, with no intra-operative neurological compromise, or return to theater for misplacement of pedicle screw.,
| Conclusion|| |
Our technique is a useful addition and improvement to freehand pedicle screw insertion. Sagittal trajectory of thoracic pedicle is clearly defined on a fixed bony landmark, orthogonal to SAP and O’Connell dissector used as a simple guide to sagittal trajectory of thoracic pedicle. The technique is easily reproducible and will aid in making the freehand technique both easier to perform and to teach.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]