Accuracy of the Freehand (Fennell) Technique Using a Uniform Entry Point and Sagittal Trajectory for Insertion of Thoracic Pedicle Screws: A Computed Tomography-based Virtual Simulation Study
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.284379
Source of Support: None, Conflict of Interest: None
Keywords: Freehand technique, pedicle screw simulator, thoracic pedicle screwsKey Message: The free hand technique ( Fennel et al) serves as an effective guidance for placement of thoracic pedicle screws. At T11 and T12 levels, an axial angle less than 10° is necessary to prevent pedicle breach.
Pedicle screw fixation is the most widely used technique for internal stabilization of the spine as it offersgreater pull-out strength. Pedicle screws can be inserted safely using intraoperative aids such as fluoroscopy and computed tomography-guided (CT) navigation., Concerns regarding radiation exposure while using fluoroscopy and the lack of widespread availability of navigation systems have led to the use of freehand techniques for insertion of pedicle screws. Studies have demonstrated that these techniques are safe, reliable, have a low incidence of screw malposition, and neurovascular complications.,
The accuracy and reliability of various freehand techniques can be studied using preoperative simulation software, which also serves as effective teaching aids for surgical trainees. Pedicle screw simulator (PSS) is a versatile module which can be used for presurgical planning in 3D Slicer, an open-source platform for medical image informatics, processing, and analysis.,
Fennell et al. have described a simple and effective freehand technique of thoracic pedicle screw placement using a uniform entry point and sagittal trajectory for all levels. In this study, we have tested the accuracy of freehand technique (Fennell et al.) by virtually simulating it in PSS.
Normal CT thoracic spine obtained from CT thorax data of five patients who had been screened for suspected lung pathology was used in this study. The CT scans were acquired in the supine position in a Siemens Emotion 6 CT scanner with 1 mm collimation and 0.75 table pitch. The X, Y, and Z spacing (slice thickness) of CT data were 0.586, 0.586, and 1 mm, respectively. The interslice gap was 1mm, which provided consecutive axial slices without any gap. The matrix size of each slice was 512 × 512 and the field of view (FOV) was 300 mm. The ideal screw trajectory and freehand (Fennell) technique were simulated in all the 240 pedicles using a 4mm screw cylinder in PSS.
Free hand (Fennell) technique
Fennell et al. describe a uniform entry point 3 mm caudal to the junction of the lateral margin of the superior articulating process and the transverse process. A sagittal trajectory, orthogonal to the dorsal curvature of the spine at that level and an approximate axial (medial-lateral) angulation of 30°at T1-T2 and 20°from T3-T12was described by them for freehand insertion of thoracic pedicle screws.
Ideal trajectory technique
The entry point described by Fennell et al. which was 3 mm caudal to the junction of the lateral margin of the superior articulating process and the transverse process was adopted for simulating this technique. We chose a sagittal trajectory parallel to the superior endplate of the corresponding vertebra. An ideal axial trajectory through the midsection of the pedicle was identified. Screw insertion was simulated using the entry target (ET) mode in the PSS. This mode in the software also calculated the axial angle.
PSS module in the 3D slicer environment was used to simulate the Fennell technique. Vertebral endplate (pitch angle) was corrected to ensure the endplates are parallel at the corresponding spinal level and the vertebral rotation in the axial plane (yaw angle) was corrected to align the spinous process.
Entry point and trajectory
Entry point described by Fennell et al.and a sagittal trajectory parallels to the superior endplate of the corresponding vertebra, was chosen to simulate both the freehand technique and the ideal trajectory. We used the entry angle (EA) mode inPSS to simulate the pedicle screw insertion by the freehand (Fennell) technique. Insertion angle (axial) of 30° was used for T1 and T2. At T3-T12 levels, axial angle of 20°was used. [Figure 1] and [Figure 2] show the use of EA mode.
To identify the ideal axial trajectory, entry target (ET) mode of PSS was used. Target fiducial was placed in the vertebral body such that the screw passes through the midsection of the pedicle. The software then calculated the axial angles for this ideal trajectory.
Presence of pedicle breach in the mediolateral direction in the Fennell technique and the ideal trajectory method was noted. The ideal axial angle and angle difference between ideal (axial) trajectory and Fennell technique were analyzed.
Total of 240 pedicle screw insertions was simulated. 120 by the Fennell technique and 120 using the ideal trajectory method.
A sagittal trajectory parallel to the superior endplate caused no pedicle breach in the cranial-caudal direction at any level in both groups. In all the subjects where the screw was placed using the Fennell technique, no medial or lateral breach of the pedicle cortex was noted while using the axial trajectory of 30° at T1-T2 and 20° from T3-T10 but there was a medial cortical breach at T11 and T12in all subjects as shown in [Figure 3]. In all the pedicle breaches at T11 and T12, less than 25% of the screw diameter was outside the medial cortex.
There was no medial or lateral breach of the pedicle cortex was seen in any subject when the ideal trajectory technique was used to simulate screw placement.
Ideal axial trajectory and angles
Mean axial angles at T1, T2, T3, T4, T5, T6, T7, T8, T9, and T10 were 29.9, 30.4, 21.5, 19.5, 20.3, 21.7, 20.4, 21, 20.9, and 21.4, respectively. Mean axial angles at T11 and T12 were 2.8° and 6.5°, respectively. The difference in axial angles between Fennell technique and ideal (axial) trajectory is tabulated in [Table 1]. [Figure 4] shows an ideal trajectory at T11 and T12.
Thoracic pedicle screw insertion is challenging and involves a steep learning curve, as the pedicles are smaller, with complex morphology, and malposition can be associated with neurovascular complications., CT-based simulation is an effective method to study the effectiveness of various freehand techniques, and also aids in surgical training. In this CT-based simulation study with PSS, we found that a sagittal trajectory parallel to the superior endplate did not produce cranial-caudal pedicle breach at any of the levels in the thoracic spine. At T11 and T12 levels, a medial cortical breach in the pedicles was noted on simulating the Fennell technique and the in ideal axial angles at T11 and T12 levels were less than 10°.
Pedicle screw fixation is the most widely used technique for internal stabilization of the spine as it offers a greater pull-out strength. Pedicle screws can be inserted safely using intraoperative aids such as fluoroscopy and computed tomography-guided (CT) navigation., Intraoperative fluoroscopy in long level fusion surgeries exposes the surgeon and the patient to radiation for a longer duration. CT-guided navigation is limited by its cost and increases operating time. Free hand techniques to insert pedicle screw are gaining popularity in high volume centers especially in cases requiring long level fusion.
Fennell et al. described a uniform entry point and sagittal trajectory, for all levels of the thoracic spine. A sagittal trajectory orthogonal to the dorsal curvature of the spine at the corresponding spinal level, and an axial angle of 30° for T1 and T2, and 20° from T3 to T12, from a point 3 mm caudal to the junction of the lateral margin of the superior articulating process and the transverse process, was described as their conclusion. In this study, we virtually simulated thoracic pedicle screw insertion, using the entry point and axial trajectory as described by Fennell et al., using PSS. PSS, a preoperative planning software module enabled us to orient the vertebra to identify the entry point and choose the trajectory for Fennell technique using EA mode. We chose a 4mm screw cylinder taking into account the probability of pedicle expansion which cannot be studied in a virtual environment.
We noted that the entry point described by Fennell et al. was an effective starting point. However the description of the dorsal curvature of the spine is vague and hence we chose to use a sagittal trajectory parallel to the superior endplate at all levels, as this was easier to understand and this trajectory is well-established across many studies., This corresponded to a trajectory orthogonal to the curvature of the thoracic spine.
This simplifies the understanding of cranial-caudal angulation required for thoracic pedicle screw insertion, and we had no pedicle breaches in cranial-caudal direction at any level. We also found that the axial trajectory of 30°at T1-T2 and 20°from T3 to T10 is very effective for screw placement as described by Fennell et al. However, we noticed there was a significant difference in the axial angles at T11 and T12 compared with T3-T10 levels. In our study, the mean axial angle at T11 and T12 was less than 10° emphasizing the fact that lower thoracic pedicles are relatively straight in the axial plane. The transition in the pedicle morphology at T11-12 is probably due to the fact that the rib head articulates more posteriorly towards the base of the pedicle at T11 and T12 and this finding of straightening of the pedicle angulation at T11-12 has been well established across studies.,
Fennell technique was effectively simulated using PSS. A uniform entry point for all thoracic spine vertebrae, to insert pedicle screw is effective guidance for freehand placement of thoracic pedicle screws. A sagittal trajectory parallel to the superior endplate is orthogonal to the dorsal curvature of the corresponding spinal level and did not produce a cranial or caudal breach of the pedicle. At T11 and T12 levels, an axial angle less than 10° is necessary to prevent pedicle breach.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]