Although the literature has not confirmed that image guidance is superior to fluoroscopic guidance in traditional open procedures for placing pedicle screws, few studies have compared these techniques for placement of percutaneous pedicle screws. Fewer studies have analyzed radiation exposure using the two techniques, and none, to our knowledge, have analyzed recent EMF technology. Accuracy of pedicle screw placement in this study was measured by assessment of cortical breach in both the pedicle and vertebral body as well as by subjective assessment of ideal trajectory. We found that accuracy of placement of lumbar pedicle screws was significantly improved by both of these measures; however, the accuracy was not significantly improved for thoracic and sacral screws. Significant gains in pedicle screw placement accuracy with guidance in the lumbar spine have also been demonstrated in a recent, large meta-analysis [21
Cortical breach has been the most commonly used measure in determining pedicle screw accuracy [21
], but there is significant variability in the definition of these findings in the literature, making comparisons between series difficult. Kosmopoulous and Schizas [21
] reported on the use of 35 different methods of assessment in their meta-analysis and noted that approximately 50% of studies making claims about the accuracy of placement did not clearly define how accuracy was assessed. Even when cortical breach is used as a parameter, considerable variation exists between studies. Some authors have reported only a cortical breach of >2 mm and others reported only on the direction of breach [21
]. The assessment of cortical breach is straightforward on post-procedural CT scans and should represent a minimum reporting requirement to serve as a basis for comparison between studies. The amount and direction of breach are secondary measures that may or may not have significance in relation to the potential of neural injury [32
]; that is, medial cortical breaches have more significance than lateral pedicle cortex breaches.
Although the subgroup analysis indicated that accuracy with navigation was improved in the lumbar spine, increased accuracy of pedicle screw placement in the thoracic spine was not illustrated in this study. In fact, the results suggested a trend toward greater inaccuracy with the EMF navigation system. Similar results have been reported in another study [13
], but this is not a universal finding [2
]. Kosmopoulos and Schizas [21
] noted similar findings in the thoracic group in their meta-analysis. They reported a decreased median accuracy of thoracic pedicle screw placement in vivo with the use of navigation compared with fluoroscopy alone (94.3% accurate vs. 82.2%) and very similar results in cadaver studies (74.9% accurate with navigation and 74.5% without). Possible explanations for this finding are that the learning curve needed to master the navigation technology is steep, and the decreased diameter of the thoracic pedicle makes screw placement inaccuracies more prevalent.
The accuracy of screw placement is of great importance with placement of percutaneous pedicle screws, but operative and radiation exposure times must also be considered. Time for insertion with the EMF technique (and thus operative time) did not vary from that of the standard fluoroscopically guided technique in the current study, which is especially significant because the version of the navigation system in this study was new to all participating surgeons. Thus, shorter operative times would be expected with further familiarity with the navigation system and instrument refinement. Although increased speed has not been shown in other studies, this can be a reasonably expected byproduct of navigation because manipulation of a fluoroscope during surgery can be time consuming, especially in minimally invasive surgeries that require AP and lateral imaging for each level. Elimination of the required movement of the fluoroscope around the operative field should also reduce the potential for contamination of the operative field and concern for infection [4
]. Additionally, simultaneous viewing of both AP and lateral projections facilitates percutaneous screw placement.
An additional outcome measure in this study was total fluoroscopic time and total radiation exposure. The reduction in fluoroscopic time with EMF was highly significant, but the reduction in radiation exposure measured on the dosimeters was not statistically significant. This is likely due to each surgeon’s habit of backing away from the fluoroscope with each exposure, which may provide some protection to the surgeon. Although it was not measured with dosimeters, the highly significant reduction in fluoroscopic time should reduce the exposure to the patient. Total fluoroscopic time and radiation exposure are directly related, and any reduction in exposure time should be of benefit for the patient and the operating room staff. This finding holds special implications for the spine surgeon, as radiation exposure in spinal surgery has been demonstrated to be 10–12 times that of other musculoskeletal surgeries and has the potential to exceed recommended yearly allowances [28
]. Factors unique to spine surgery that may contribute to increased exposure include the increased penetrating beam energy requirements to image the spine adequately [24
], proximity of the surgeon’s hands to the field (which may be exacerbated by the need to maintain alignment of instrumentation), increased Compton scatter at the beam entry site, and the frequent necessity of having either the surgeon or assistant standing next to the beam generator [28
]. Furthermore, poor technique in which the hands are directly irradiated can dramatically increase exposure to as high as 4,000 mrem/min (recommended yearly hand allowance is 50,000 mrem) [26
]. While the consequences of chronic radiation exposure are, as yet, unknown, the increasing exposure beyond recommended limits is certainly cause for concern, especially as minimally invasive techniques requiring fluoroscopy increase in frequency.
Potential disadvantages of the EMF navigation system over traditional fluoroscopy could include system set-up and registration time and the chance of software failure. In addition, there is the need for additional incisions and time to place the spinous process tracking system and the possibility of displacement of the spinous process tracking system. Redo operations in patients in whom the spinous process has been removed previously would present a challenge. Inaccuracies could also be associated with shifting of the attached dynamic reference transmitter position during applications of pedicle screws. Resecuring the bumped transmitter with updated fluoroscopic images allows continued navigation. Minor inaccuracies (i.e., <3 mm) might be of less importance in targeting the L4 to S1 pedicles, as these are the more frequently instrumented pedicles. The 18-in. surgical field may present a potential limitation in the use of EMF navigation when tracking some elongated instruments used in percutaneous approaches.