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Intraoperative Image-Guided Navigation for Lumbar Interbody Fusion
Introduction
Minimally invasive spine surgery has gained much popularity in recent years owing to the reductions in patient morbidity, length of hospital stay, and costs. Although these short-term outcomes have seen marked improvements, there has been little improvement in the long-term outcomes when comparing minimally invasive lumbar interbody fusion (MIS-LIF) to open techniques.
When first developed, MIS-LIF required extensive fluoroscopy to ensure accurate interbody cage placement and extensive imaging for the percutaneous placement of pedicle screws. The consequent accumulation of radiation from many of these minimally invasive procedures may result in dangerous radiation dosages to the surgeons who perform these procedures. As such, there has been an increased development and use of navigation-based techniques that rely on the use of intraoperatively acquired images with subsequent image registration allowing for navigation of interbody cage and percutaneous screw placement. This approach exposes surgeons to much less radiation while maintaining comparable accuracy.
In this chapter, we review various advanced imaging modalities related to both accurate interbody cage and associated pedicle screw placement in the lumbar spine.
Fluoroscopy
Prior to the advent of advanced imaging modalities relying on computer-aided image processing and registration, fluoroscopy was utilized to ensure proper cage placement. This method requires successive anterior-posterior and lateral C-arm images to ensure that the cage is inserted orthogonal to the disk space. In patients with deformity or multilevel degenerative disease, ensuring a perfect orthogonal position can require tilting the table to acquire appropriate images; consequently, with the repetitive imaging, there can be significant radiation exposure. Additionally, the accuracy of cage and pedicle screw placement has been a concern, particularly in comparison to more open techniques where visualization is much easier.
Given the increased exposure to large amounts of low-level radiation that can result from high case volumes, several studies have aimed to quantify the average surgeon radiation exposure during MIS-LIF cases. Regarding lateral lumbar interbody fusion (LLIF) cases, Taher et al. found that during eighteen cases fusing a mean 2.4 levels, average total fluoroscopy time was 88.7 seconds, including fluoroscopy at the beginning of the case to ensure accurate positioning. Of note, significant increases in radiation exposure were noted in unprotected areas when compared to the dosimeter located under the lead apron of the primary surgeon. Bindal et al. observed an average fluoroscopy time of 101 seconds during minimally invasive transforaminal interbody fusion (MIS-TLIF), with radiation exposures that were generally improved in a later study by Funao et al., who used a one-shot fluoroscopy technique in an attempt to lessen or reduce surgeon radiation exposure. Other similar low-dose fluoroscopy protocols have been developed to decrease radiation exposure during MIS-TLIF cases.
The significance of such radiation exposures to the surgeon is unclear, although various authors have suggested that exposures may have a more critical impact on younger surgeons who are beginning their practice and have a lifetime of fluoroscopy-dependent spine procedures ahead of them. With this in mind, Taher et al. calculated that 2700 LLIF procedures theoretically could be performed each year without exceeding standards for “safe” occupational radiation exposure. Although this may be true, an interest in reducing surgeon radiation exposure persists.
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