Pedicle screw placement in the thoracic spine can be more challenging because of the smaller pedicles, potential deformity, the spinal cord’s inability to tolerate significant deformation, and the difficulty of obtaining good intraoperative fluoroscopy at many levels [3
]. Vaccaro et al. [3
] had shown in a cadaver study that without the use of fluoroscopy, as many as 41% of thoracic pedicle screws violated the cortex and 23% of the screws had medial violation and entered the spinal canal. Nevertheless, pedicle screws have superior biomechanical properties as compared to hooks and have the ability to control the spine in the axial, coronal and sagittal planes [1
]. Moreover, unlike hooks, a properly inserted pedicle screw does not intentionally enter the spinal canal and can be just as strong in the presence of a laminectomy.
Fluoroscopy has been the most commonly used method to guide the insertion of pedicle screws into thoracic spine. In most surgical series, a 10–20% rate of cortical violation is reported, although rates as high as 40% have been noted in some series [4
]. Most of the violations were either contained within the rib head or protruded <2 mm medially. The incidence of neurovascular injury or need for screw revision has been <2% of patients [4
]. In cases of scoliosis correction, fluoroscopy offers less anatomic information about the orientation and position of the pedicles due to the severe deformity, rendering it less useful [15
Computer-assisted image guidance has also been used by spine surgeons. Many studies have shown that image guidance significantly decreases the cortical violation rate to less than 10% [16
]. The biggest drawback, however, is the increased operative time; some have calculated that image guidance increases the screw insertion time by as much 50% at each level [6
]. Moreover, the intervertebral anatomical relationship may change during surgery, increasing the potential risk of registration error with navigation [15
]. Some authors have used a mixed strategy such as using image guidance for the upper and mid-thoracic spine—which tends to have smaller pedicles—and using the free-hand or fluoroscopic guidance techniques for the lower thoracic spine [17
]. Recently, the free-hand technique has gained popularity. Kim et al. [7
] reported a series in which 8,000 screws had been placed by the free-hand technique without neurovascular complications, and only 8% had significant breaches. The free-hand technique relies on anatomy and the tactile feedback for pedicle screw insertion. The anatomy can be learned, but the tactile feedback must be experienced first-hand. Thus, we feel it is important that the residents actually cannulate the pedicles themselves on both sides of the spine to maximize their experience. Moreover, detecting a breach of the pedicle wall by probing the pedicle is a skill with significant learning curve [22
]. There are few substitutions for this experience other than personally feeling what a breach is like.
Our results have shown that under supervision, free-hand thoracic pedicle screws placed by neurosurgery residents have a 15% cortical violation rate, which is comparable to most surgical series. The majority of the violations ranged between 2 and 4 mm. It has been shown that medial cortical violations of <4 mm are unlikely to cause any neurological complications [7
It is interesting that the PGY-6 residents had a 19% cortical breach rate, slightly higher than the 13% breaching rate of the PGY 2 and 3 residents. Many of the breaches originate from two patients, who account for 10 out of 21 breaches for the PGY-6 residents. One possible explanation is that the junior residents most likely had more strict supervision for placement of screws whereas the chief residents were given more independence (though still supervised) during placement of the screws. We had more lateral cortical breaches than medial breaches. The most likely explanation is the tendency to err laterally instead of medially. No vascular or pleural injuries were observed in these cortical violations.
This study is based upon postoperative thoracic spine CT scans and chest CT scans because CT scans have been shown to be more sensitive than radiographs in detecting cortical breaches [24
]. We do not routinely obtain postoperative CT scans on every patient undergoing thoracic fusion because of cost and added radiation exposure, but we do perform CT scans if they are clinically indicated. Indications for CT in this patient cohort included: planning for further surgery, evaluation of bony resection, baseline to follow tumor recurrence or progression, radiosurgery treatment planning, or evaluation of possible chest pathology (e.g. pulmonary embolism, pleural effusion, etc.). One could argue that this created a selection bias: how does one know whether patients with “well-placed” screws seen on X-ray were selected for CT scanning and patients with “poorly placed” screws were excluded? This would be the case if we had selectively chosen to obtain CT scans in patients to evaluate pedicle screw placement; however, the indication to obtain a CT scan was not related to the placement of pedicle screws. That is to say, CT scans were not obtained because we wished to evaluate “well-placed” or “poorly placed” pedicle screws; rather, they were obtained for clinical reasons without regards to intraoperative placement of the screws. Thus, because the CT scans were not obtained for evaluation of pedicle screw placement per se, we felt that this essentially became a reasonable random sampling of thoracic pedicle screw cases in this time period.
Complex techniques in neurosurgery such as aneurysm clipping or thoracic screw placement require significant training and experience before the surgeon can master the technique. Several studies have analyzed the outcome of aneurysm clipping by neurosurgeons in training and noted no significant difference in outcome as compared to staff neurosurgeon-performed cases [26
]. However, it is important to ensure that quality care is nonetheless delivered, despite the role of training. For spinal instrumentation techniques, the anatomical relationship of bony landmarks can be learned by practicing on saw-bone models. Practicing on cadavers is another way to gain additional experience. However, neither saw-bones nor cadavers can replace the actual experience, especially the tactile feedback experience. Moreover, availability of cadavers is often limited. Therefore, our program does not have routine saw-bone or cadaver training sessions for residents. From our experience, training takes place best in the operating room. The typical training route consists of observation of the technique first, followed by execution of the technique under strict supervision. Thus, it is important to study the outcome of surgery in an advanced technique such as free-hand thoracic screw placement performed by trainees.