iCT Navigation for Transpedicular Screw Fixation in the Thoracolumbar Spine: Experience in 100 Consecutive Cases
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.360699
Source of Support: None, Conflict of Interest: None
Keywords: Intraoperative computed tomography, navigation, pedicle screws, placement, thoracolumbar spine
Due to intraoperative imaging techniques and ongoing implant development, precision and revision rates of pedicle screw placement have significantly improved. Optimal screw placement within pedicles is highly important for achieving maximum biomechanical strength and for avoiding neurovascular complications., Various guidance techniques have therefore been developed, from intraoperative exposure of medial pedicle walls to fluoroscopic guidance, and fluoroscopy-guided,, or computed tomography (CT)-guided navigation.,,,
Intraoperative navigation can be based on a preoperative CT data set, on intraoperative imaging via 3-D C-arm, or on intraoperative computed tomography (iCT). Very recently, robot-assisted pedicle screw placement was introduced as a new additional technique. This study aimed to describe our initial experience with iCT navigation for transpedicular screw fixation in the thoraco-lumbar spine in 100 consecutive cases to confirm the accuracy and reliability of this technique.
After acquisition of iCT (Siemens, Munich) and navigation system (Brainlab, Munich), the authors started to use iCT navigation for all transpedicular screw fixations in the thoracolumbar spine in 2012. Patients from 2012 to 2016 were prospectively documented and retrospectively analyzed. No previous iCT navigation fixation had been performed by the authors, so the presented series reflects the authors' initial experience. All surgeries were performed by the authors at a single institution.
In prone position on a radiolucent carbon table, the spinous processes and facet joints of the concerned levels are exposed through a standard posterior midline approach. The navigation tracker with infrared reflectors is screwed firmly onto a spinous process in the region to be instrumented. The mobile CT scan on rails acquires intraoperative images of the levels to be instrumented, thus matching the bony anatomy to the tracker. The navigation's infrared camera is aimed at the tracker, reading the infrared reflectors of the tracker, the pedicle awl and pedicle probe, and displaying the pedicle screw path and its length on the monitor. The screw driver is the only instrument not navigated. After all pedicle screws are inserted, a verification CT is run to assess the screw placement. If a misplaced screw is seen on CT, it can be removed and re-inserted, to be evaluated by another verification CT [Figure 1]. After the CT confirms correct placement of all screws, rod insertion and the decompression part of the procedure take place.
As the primary endpoint, iCT analysis of pedicle screw placement was performed by the surgeon team, assessing whether screw placement on the verification CT matched the navigation paths. The secondary endpoint for all patients was the one-year follow-up with screw purchase analysis on plain radiographs assessed by inhouse radiologists, as well as clinical assessment by means of patient-rated outcome of back and leg pain on a visual analogue scale (VAS).
One hundred six (106) consecutive patients underwent operation. Five patients were unretrievable for follow-up and one patient had deceased from unrelated heart failure during the follow-up. Therefore, the study population comprised of 100 cases, corresponding to a response rate of 94%. There were 47 women and 53 men with a mean age of 66 years (range 38–86). The follow-up was 12 months in all cases [Table 1].
Surgical indications for stabilization were degenerative instability with stenosis, facet cyst or disc herniation in 82, spondylolytic instability in 9, scoliosis with stenosis in 6, and traumatic fractures in 3 cases. A total of 443 pedicle screws were inserted: 22 in the thoracic spine, 371 in the lumbar spine, and 50 in S1. Thirty pedicles were considered thin with less than 8 mm width on CT, all of them located in the thoracic spine or at L5 in isthmic spondylolisthesis. All pedicle screws were placed relying exclusively on spinal neuronavigation.
Mean surgical time was 151 minutes (range 80–240), and average estimated blood loss was 340 cc (range 100–800). Overall, 435/443 screws (98%) were correctly placed in the first attempt, as shown by correct position on the verification CT. Initial misplacement in 8/443 screws was explained by different factors: Three L5 screws in three different patients and two L4 screws in two different patients were misplaced due to loosened infrared reflectors on the pedicle awl or pedicle probe. Screw removal and re-insertion after exchanging the reflectors resulted in correct position at the second verification CT. Another two L4 screws and one L5 screw were misplaced in a single patient because of the patient's coughing during superficial anesthesia and displacement of the spinous process tracker before the screws were inserted. Tracker re-attachment and re-registration led to correct screw placement at the second attempt. None of the patients with re-positioned screws experienced neurological symptoms at the one-year follow-up. Only one patient with initially medially placed L5 screw suffered from temporary L5 dysesthesia for several weeks. Altogether 100% correct screw placement was achieved intraoperatively at either the first verification CT (98%) or the second verification CT (2%).
One-year follow-up results
Screw loosening was observed in 4/100 patients at the one-year follow-up. An L4 upper endplate fracture both in an 85- and an 81-year-old patients with osteoporosis after L4-5 fixation for degenerative anterolisthesis and stenosis led to severe ongoing back pain; re-operation was not warranted in both cases because of an increasingly fragile medical condition. Subsidence of T11/12 compression fractures led to minor screw displacement with moderate local pain in a 58-year-old male patient with osteoporosis after a T10-L1 fixation. Loosening of L4 screws after L4-S1 fixation for lytic spondylolisthesis in a 60-year-old patient led to moderate local pain.
The average VAS for back pain dropped from 4.9 preoperatively to 2.3 at the one-year follow-up, the VAS for leg pain from 6.4 preoperatively to 0.7 at the one-year follow-up.
Using the Macnab criteria, the patients' self-rating for global outcome at the one-year follow-up was “excellent” in 49% of patients, “good” in 39%, “fair” in 9%, and “poor” in 3%.
Because of a wide range of definition and evaluation methods of misplaced screws, extremely variable misplacement rates are found in the literature.,,, Less than optimal placement of pedicle screws in the thoracolumbar spine reduces biomechanical strength and poses a threat to the neurovascular and visceral structures. Therefore strong incentives exist to improve pedicle screw placement accuracy, promoting the development of navigation systems.
While perfectly functioning navigation systems claim to be precise and reliable, in reality several variables can jeopardize the ideal implant positioning: Bending of instruments, motion of anatomy, and loosening of tracking references can be sources for navigation imprecision., Furthermore, surface matching and fluoro-navigation may create differences between navigation images and reality by image acquisition inaccuracies,,,,, which can be overcome by intraoperative CT guided navigation. As the O-arm (Medtronics, Memphis) acquires a 3D data set of images with a reference frame attached to the spinous process we consider it equivalent to a CT navigation.,
In CT navigation, the precise bony spinal anatomy is acquired over several vertebral levels for controlled implant positioning, and intraoperative CT verifies its own navigation results.,, However, several challenges have to be dealt with:
-Tracker motion and infrared reflector loosening on navigation instruments, as experienced initially in our series, can be minimized with routine and careful observation and is readily corrected during surgery.
-Larger distances between tracker and instrumented levels may increase anatomy motion and thus navigation imprecision, as shown previously. Re-registration of the tracker near the instrumented levels can overcome this problem.
-Mechanical or electrical blockage of the sliding iCT on rails or software failure in transmission of acquired CT images to the navigation system could prevent iCT navigation use. Although this did not happen in the current series, it did happen after the study completion during continuous use of navigation. If such technical failures cannot be overcome within reasonable time, the surgeon has to convert the procedure to traditional anatomic/fluoroscopic screw placement.
-Usage and maintenance of iCT navigation require qualified technicians and radiology assistants. Learning curves of such personnel require patience and training. The estimated average time consumption of iCT navigation use per patient was 20–30 minutes.
-Cost efficiency is difficult to assess: Major investment costs for iCT navigation may be outweighed by overall savings from avoided revisions, avoided law suits, and reputation gains. A cost utility analysis assessing medical and extra-medical variables is difficult to perform. However, Dea et al. found an economical justification for iCT navigation when compared to fluoroscopy guidance in high volume usage, and Costa et al. could not find an economic disadvantage when comparing iCT navigation by means of an O-arm with preoperative CT based navigation.
-Patient habitus could limit iCT use. However, Barsa et al. report 82 surgeries with iCT navigation, documenting “Out of 571 implants inserted in all spinal levels, only five screws (0.87%) did not meet the criteria for correct implant position”” not needing re-placement. They found CT navigation quality sufficient at all segments and independent from the patient habitus.
In our experience, all these challenges were outweighed by the precision and safety of iCT-guided navigation: The overall, initially correct, screw placement in 435/443 screws (98%) is reassuring, as is the correction potential of initially misplaced 8/443 screws, which means that all of our 100 patients left the operating room (OR) with verified, correct thoracolumbar screw placement. In our view, this helps to explain the 96% fixation rate at the one-year follow-up, with screw loosening in 4/100 cases mainly attributed to osteoporosis.
We found the navigation particularly helpful in dysplastic and thin pedicles typically seen in the thoracic spine and in spondylolisthesis, kyphoscoliosis, and revision surgery, where high precision is warranted and anatomical landmarks and orientation are more limited. Because intraoperative verification CT is performed as a final check, postoperative radiographic controls are not necessary. Additionally, as no medical presence near the patient is mandatory during iCT registration or verification and no fluoroscopy is needed, the OR personnel has no radiation exposure, while the patient is subjected to CT radiation on a single event only., Furthermore, Mendelsohn et al. showed that the average total radiation exposure to the patient for iCT-navigated thoracolumbar fixation was 5.69 mSv, less than a single routine lumbar CT scan with 7.5 mSv.
This study naturally has several limitations: There is no control group, screw positions on the intraoperative verification CT were analyzed by the surgeon team itself and not by an independent expert, and the study reflects only a single neurosurgical team's experience.
Altogether, based on our first 100 cases, iCT navigation for transpedicular screw fixation in the thoracolumbar spine seems to be very accurate and reliable.
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments.
Declaration of patient consent
For this type of observational study formal consent is not required. Local ethical committee approval was obtained for this study (BASEC-Nr. 2019-00844).
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Conflicts of interest
There are no conflicts of interest.