Evidence of benefit of surgical navigation in ENT 20
General rules for using navigation systems 20
Imaging protocols 21
Navigation for sinus and anterior skull base surgery 21
Navigation for sellar and parasellar approaches 25
Computer-assisted (image-guided) surgery in pediatric ENT 27
Image-guided neurotologic surgery 33
The craniofacial region is a complex association of bone structures and soft tissue through which numerous essential neurovascular and sensory structures pass. Remodeling by pathologic processes and previous treatments often hinders the surgeon’s spatial orientation, which is further aggravated by the natural narrowness of the approaches and by frequently hemorrhagic conditions. The endoscopes generally used for surgery in this region seriously deform imaging geometry. Thus, even highly experienced head and neck surgeons often encounter considerable problems of anatomic identification, as human beings lack the sensory apparatus needed to ensure exact positioning.
Neuro-otologic and later rhinologic approaches, greatly developed during the last 30 years, entail an even more pressing need for three-dimensional localization and orientation, which computer-assisted technologies have very largely met. The present chapter reviews the clinical application of navigation systems, the technological aspects of which were dealt with in the previous chapter.
Evidence of benefit of surgical navigation in ENT
In endoscopic endonasal surgery, navigation is intended to enhance efficacy and safety [ ]. The complications rate is already fairly low, at 0–4.3% depending on the report, but the associated morbidity is severe and may be life-threatening [ ]. Meta-analyses assessing the benefit of navigation systems are severely biaised by comparing procedures in heterogeneous pathologies, performed by surgeons of varying experience, and whose experience, moreover, varies over time [ , ]. Morbidity and mortality in endonasal surgery is poorly predictable, greatly depending on intraoperative conditions, and notably hemorrhage. The ideal design would be a large-scale prospective multicenter trial randomizing operated sides between navigation and no navigation, and classifying the surgeons, but this would not be ethically acceptable and has in fact never been attempted. Published studies mainly report, without precise quantification, shorter operative times [ ]; some also report reduced operator stress, but again without quantification.
In our opinion, the question cannot be answered if formulated in this way, as it fails to take account of the ultimate objective of surgery: complete lesion resection, sparing surrounding structures. However, the wider the resection, the greater the risk of lesion, and this is true for a simple sinonasal polyposis as for a pituitary adenoma or vestibular schwannoma. In our view, it is this general rule that navigation should seek to overturn, improving both safety and resection, in a non-zero-sum game. Unfortunately, there are almost no relevant studies [ ], perhaps partly because navigation has become so well-established that its interest no longer seems to need demonstrating.
General rules for using navigation systems
The basic purpose of navigation systems is not to replace the surgeon but to enhance his or her performance by adding an artificial sense to the visual and tactile informations. Like any machine, the navigator may provide faulty information, and the right way of using it is based on constant critical analysis of the information displayed.
System accuracy is liable to drift during the procedure and needs to be regularly checked against straightforward landmarks, especially at start of surgery, and further periodic checks should be made throughout ( Figure 3.1 ), as accuracy can vary within a given working volume according to a complex distribution function [ ]. When the displayed position differs markedly from that estimated by the operator, the discrepancy should be resolved by checking, for example, bordering landmarks. These checks enable not only safe operating, but also an estimation of the value and limitations of the equipment. They also encourage a certain humility and willingness to keep on learning anatomy, no matter how experienced you may be.
Long flexible instruments should not be used, as any excess pressure deforms their geometry and hence the calibration, with consequent loss of precision in proportion to instrument deformation. This is especially important in frontal sinus surgery, where bone dissection requires pressure on relatively long and fine instruments.
We do not feel that navigation should be reserved only for the most difficult cases. This would mean encountering inherently complex equipment for the first time just when the surgeon is most in need of it and single-mindedly concentrated on the surgical procedure. Despite what is regularly claimed, navigation systems do not, in our opinion, have specific indications: they should be deployed according to common sense. The only limitations are the set-up time and, for certain systems, the cost of consumables.
Navigation is also a valuable educational tool. Because it continuously correlates the location of the surgical instrument simultaneously in the operating field and on patient imaging, it is obviously an accelerator of learning curves, although few publications have focused on this. Large display screens, now common in operating theaters, disclose where the operator is working, and thus help all those involved in the surgical procedure (anesthetists, nurses, etc.) to enhance their participation.
The essential component of navigation is patient imaging, and certain principles need to be known to optimize image quality ( Table 3.1 ). CT scanners are recalibrated regularly and provide images with far more precise voxel coordinates than MRI. Multimodal MRI-CT is very useful in tumor resection, and for this the MRI scanner needs to be registered on the CT scanner and not vice-versa. Native axial slices should be used in DICOM format 1
1. DICOM: Digital Imaging and Communication in Medicine, a standard defining the way medical images are encoded. Basically, a header contains the patient’s identification and image parameters such as voxel size., rather than derived axial reconstructions, the accuracy of which is dependent on the manufacturer’s particular algorithm.
|CT-scan parameter||Recommended value||Explanation|
|Field of view (FOV)||190 ~ 210 mm||Wider FOV decreases accuracy |
Smaller FOV could preclude useful anatomical data
|Slice thickness||0.4 ~ 1 mm||Thicker slices decrease accuracy |
Thinner slices increase radiation dose without real benefit
|Slice overlapping||Not overlapped||Overlapped slicing increases density resolution at the expense of spatial resolution|
|Gantry tilt||Zero degrees||Tilted slices create non-Cartesian volume|
|Lowest slice level||Hard palate||Lower acquisition would include dental artifacts|
|Upmost slice level||35 mm above nasion||Higher acquisition would increase radiation dose without added benefit|
Cone-beam CT generally shows excellent spatial resolution, but poorer density resolution than conventional CT (around 1,024 gray levels, compared to 4,096), although this is usually innocuous; however, the field of view is often restricted, which may hinder registration.
Navigation for sinus and anterior skull base surgery
Image processing by virtual navigation, MPR 2
2. MPR: multiplanar reconstruction (sagittal and coronal slices are reconstructed from the axial series).or surface/volume-rendering reconstruction, often built into navigation systems, enable much finer analysis than each sequence alone. The possibility of locating an anatomic point simultaneously in all three dimensions improves not only diagnosis of certain lesions but also surgical strategy. The slope of the olfactory fossa with respect to the ethmoid roof and its thickness, the position of the anterior ethmoidal artery and its intra- or extra-osseous course can thus be known before beginning surgery. Likewise, a more or less voluminous agger nasi cell and a lateral, medial or, on the contrary, posterior uncinate process upper insertion can be analyzed very rapidly. If a sphenoidal sinus approach is planned, the size of the Onodi cell, the presence, volume, symmetry and extent of any lateral sphenoid sinus recess and possible protrusion of the optic nerve or carotid canal can be known in advance. These structures can also be annotated on the image, to facilitate access or avoidance during surgery.
Navigation has been reported as a precious aid in primary surgery [ ], but is more commonly used in revision procedures [ ]. Loss of landmarks and local alterations such as synechia or mucocele, problematic neighboring structures such as protrusion of the papyraceous plate or ethmoid roof, aggressive pathology such as invasive fungal sinusitis, inverted papilloma or tumor, neo-osteogenesis and finally hemorrhage may all be associated, complicating 3D localization by identification of anatomic structure. Clinically, functional failure of revision ethmoidectomy seems less frequent with navigation [ ]. Some studies also reported better resection quality, for comparable pathologies [ ].
Sphenoid sinus surgery
Diagnostic navigation now provides histologic data on sphenoid sinus lesions, where biopsy would not have been readily performed 20 years ago.
Navigation for therapeutic purposes also facilitates direct sphenoidotomy via the sphenoethmoidal recess, without posterior ethmoidectomy, thus enabling precisely targeted surgery. Navigation is also helpful in case of very large sphenoid sinus fungus balls, especially extending into the lateral sphenoidal recess, or to explore bone destruction to screen for active meningeal breach ( Figure 3.2 ).
Arterial pedicle hemostasis
Difficult hemorrhagic conditions, narrow nasal cavities, mucosal trauma and inflammation following nasal packing can make epitaxis haemostasis difficult to achieve for non-expert operators. Navigation provides very straightforward location of the sphenopalatine foramen, even under hemorrhage. Some navigation systems include a stereotactic location module to create an instrument guidance tunnel, greatly simplifying the procedure ( Figure 3.3 ). Likewise, the anterior ethmoidal artery can be easily identified ( Figure 3.4 ).
Location and closure of meningeal breaches
Certain osteo-meningeal breaches with chronic cerebrospinal fluid leakage can be identified on imaging, but exact location for closure is hard to determine. Navigation can specify their exact coordinates, enabling direct intraoperative approach ( Figure 3.5 ).
Frontal sinus surgery
Frontal sinus inflammation and infection raise difficult issues for the rhinologist. Surgery is hindered by the anatomic complexity of the frontal recess, the curvature and sometimes considerable lateral extension of the sinus itself, its degree of anteversion, close relations with the anterior cranial fossa and, more generally, its high location next to the nasal tract. Likewise, it is confined by many structures often implicated in the pathology, laying pitfalls for even experienced surgeons. Finally, fibrosis caused by previous procedures of varying success often further complicate matters.
Whether surgery seeks to restore frontonasal drainage (Draf I and II sinusotomy) or gravitational derivation (Draf III), navigation provides undeniable aid, and this may be one of its foremost indications in ENT. Notably, it enables:
the likely position of the anterior ethmoidal artery to be determined, making frontal recess dissection safer and faster;
quicker frontal ostium access, and forewarning of the classic pitfalls of suprabullar and supraorbital cells, which, if confused with frontal sinus itself, may lead to cerebral penetration;
identification of elements liable to confine the frontal sinus, so as to release it (Draf I): high uncinate process insertion or agger nasi cell ( Figure 3.6 );
assessment of the immediate relations between the frontal recess, posterior sinus wall and skull base;
location within a multicompartmental sinus, notably comprising a more or less convoluted frontoethmoidal cell, inducing frontal confinement ( Figure 3.7 );
location of the frontal nasal spine and estimation of the space that could be freed by drilling it (Draf II, Figure 3.8 );
exact estimation of the lateral and medioposterior limits formed by the orbital plate and olfactory fossa, to optimize bone drilling in Draf III ( Figure 3.9 );