Учебники / Computer-Aided Otorhinolaryngology-Head and Neck Surgery Citardi 2002

Often access for surgical manipulation can be challenging. Recently, minimal access and endoscopic approaches have been developed to minimize soft tissue scarring [1,2]. These minimally invasive techniques are better suited for fractures that involve little bone loss and minimal comminution, since they rely heavily on normal adjacent facial skeleton for contouring. Traditional incisions and wide access approaches (such as bicoronal and gingivo-buccal incisions) allow visualization of a greater area of the facial skeleton in the immediate operative field; this enhanced access is essential to restoring contour in cases of segmental bone loss and/or severe comminution. Despite open access approaches in these complex cases, normal facial projection still may not be attained. Frequently, in cases of severe bilateral midface trauma, normal facial projection is not available for comparison, and even seasoned surgeons may note suboptimal outcomes in their patients, which may include varying degrees of hypesthesia, dystopia, enophthalmos, malocclusion, and asymmetrical malar projection [3]. Many of these unfavorable results are the result of poorly reduced or misaligned skeletal fragments. Failure to obtain optimal skeletal contour is likely due in part to the inability to confirm three-dimensional skeletal relationships and projection in relation to normal landmarks. In many instances, only a limited number of normal skeletal landmarks are available for reference. Even restoration of normal dental occlusion or normal ophthalmic projection does not necessarily ensure normal midface skeletal contour.

24.1.1Characteristics of the Ideal Guidance System for Craniomaxillofacial Surgery

In patients with complicated fractures, additional guidance systems, which can guide and confirm the reduction, would greatly facilitate the restoration of normal skeletal anatomy. [It should be noted that the term ‘‘guidance system’’ does not refer specifically to computer-aided surgery (CAS); rather ‘‘guidance systems’’ include any technology, technique, and/or methodology that provides additional information about the operative reduction.] These guidances systems must accommodate traditional incisions and approaches as well as the newer minimally invasive techniques in order to be a practical alternative for intraoperative application. Guidance systems that are too cumbersome for efficient use of operative time or that prevent adequate access for reduction or instrumentation will not gain wide acceptance among practicing surgeons. It should be remembered that the reduction of skeletal segments frequently requires the exertion of strong forces on the bony fragments and skeleton. Of course, these forces can disrupt patient positioning. As a result, the guidance system must be able to maintain crucial system accuracy before, during, and after the application of external forces for reduction or osteotomies. Head-fixation devices, which may work well for sinus surgery or neurosurgery, may be rendered useless in craniomaxillofacial

surgery, where stronger forces are frequently necessary to achieve surgical objectives.

This concept of a ‘‘guidance system’’ can reflect a variety of technologies and/or strategies. In this regard, craniomaxillofacial surgeons require additional means for assuring optimal surgical results. Over the past 10–15 years, CAS platforms have gained considerable acceptance for specific applications in sinus surgery, neurosurgery, and spine surgery. Although CAS has not been optimized for craniomaxillofacial applications, it holds tremendous promise in this area. The major limitation of today’s CAS systems is that they provide information about the relative positions of the tips (functions that are useful during sinus surgery, neurosurgery and spine surgery); however, in craniomaxillofacial surgery, the objective of guidance systems is the verification of skeletal projection. The assessment of skeletal projection focuses upon the relative position of a given point, not its absolute position. To the extent that CAS does not provide this type of information, additional development is necessary. Nonetheless, CAS can provide useful information—even in its currently suboptimal form.

24.1.2Computed Tomography Scan Issues in Computer-Aided Craniomaxillofacial Surgery

Recently, computed tomography scans have replaced plain film x-rays as the preoperative imaging modality of choice for midface fractures. Although twodimensional plain x-rays are still obtained in the initial evaluation, CT scans with three-dimensional reconstruction are commonly obtained for preoperative planning. Three-dimensional CT scans offer more information regarding the degree of skeletal displacement and comminution and the volume of bone loss; such information is especially useful in cases of complex midface trauma [4]. Such bony detail is often lost in two-dimensional x-rays, in which the fractures or bone fragments may be superimposed (thereby obscuring skeletal relationships). Recently, helical CT scans have become readily available and are now preferred over earlier generations of scanners. Helical scanners, which generate images more quickly, decrease motion artifact in the resultant CT images. Minimal motion artifact is crucial in CT scans that will be utilized for three-dimensional image reconstruction. CAS systems also require optimal CT images with minimal artifact. In fact, any image artifact whatsoever may lead to erroneous image manipulation and reconstruction. Minor errors in interpretation of positional information could be exponentially multiplied if the CT scan data are artifactual.

Although CT scans provide detailed information about bony anatomy, CT scanners only offer stationary images that the surgeon still must interpret. During presurgical assessment and surgical planning, the surgeon must reconstruct the planar CT images into a mental three-dimensional model from which he or she

extrapolates information to operative field. CAS can dramatically simplify this process. First, CAS permits three-dimensional image manipulation that was not available with traditional plain x-ray and CT films. In addition, the surgical navigation features of CAS permit a direct correlation of the preoperative imaging data with the intraoperative anatomy. The optimal setting for surgical navigation is a rigid structure—like the craniomaxillofacial skeleton. The firmness of skeletal bone permits localization of predetermined points on the bone with accuracy. Unlike soft tissues, which are easily deformed or stretched, the rigid skeleton is an ideal volume for surgical navigation for positional information and even projection information. In theory, specific modeling strategies can be crafted. For instance, virtual computer models can guide surgical planning, and intraoperative surgical navigation can guide the implementation of the resultant strategies. Similarly, computer-based systems can direct the creation of milled models that can be used during surgery. Virtual or milled models can be designed to achieve symmetry with the normal, untraumatized side (if available). In those instances where the injury is bilateral, archived normative data may guide the creation of these guides. Normative data also may be used for computer-aided cephalometric analysis (see Chapter 21).

24.2CAS SYSTEMS

In general, CAS systems consist of a computer workstation, monitor, tracking system, and software tools. The workstation houses the patient’s imaging data and facilitates data manipulation using specific software designed for this purpose. A high-resolution computer monitor displays the image and tracking data for use

24.1 Prior to intraoperative surgical navigation, registration, which functions as a calibration step, must be performed. Registration involves a correlation between corresponding points in the imaging data set and the operating field. In A, external fiducial markers, which have the appearance of small rings, were applied to the patient before her CT scan; during surgery, the surgeon touched each marker sequentially to complete the registration step (StealthStation, Medtronic Surgical Navigation Technologies, Louisville, CO). In this example, a Mayfield head holder secured the patient’s head position, and the large cranial arc DRF (dynamic reference frame), which is attached to the Mayfield, provided a tracking reference for surgical navigation. This DRF, while appropriate for neurosurgical cases, is too cumbersome for craniomaxillofacial surgery. In B, the computer software has calculated a registration based upon the fiducial markers that were built into a special headset (SAVANT, CBYON, Palo Alto, CA). The computer then calculates localizations relative to this reference frame that was defined by the headset. Since the position of the frame relative to the patient is constant (that is, the frame only fits a patient’s head in one way), these localizations show positions relative to the patient.

a similar fashion, prototype CAS systems have also been described for these procedures. As one reviews this area, it becomes apparent that these reports answer specific questions in the development of the ideal CAS system for craniomaxillofacial surgery, but that ideal system has not been developed yet. A brief summary of these efforts follows.

24.3.1CAS-Based Comparison of Prereduction and Postreduction Projection

Only real clinical experiences can determine the true limitations of CAS for facial fracture reduction. As a result, CAS was used to measure prereduction and postreduction position of displaced frontozygomaticomaxillary fractures [5]. Although CAS surgical navigation has acceptable accuracy for sinus surgery, spine surgery, and neurosurgery, its practical accuracy during facial fracture reduction was unknown. Similarly, specific design limitations of CAS were also unknown.

Since maxillary or mandibular dentition is frequently available as a guide for adequate reduction of LeFort fractures and mandibular fractures, the need for CAS for these fractures is relatively small. Therefore, the application of CAS for these fractures is a lower priority. A displaced frontozygomaticomaxillary fracture was selected for this initial trial, since this fracture is representative of a displaced facial fracture in which the reduction relies on adjacent skeletal relationships, rather than dental occlusal relationships.

In this report, LandMarX 2.6.4 (Medtronic Xomed, Jacksonville, FL) was used to measure bilateral malar projection before and after reduction of a unilateral frontozygomaticomaxillary fracture. In this way, quantitative assessment of symmetry was achieved. Projection measurements were performed with the LandMarX distance-measuring tool, a standard software tool that is common in most CAS systems. In order to achieve meaningful measurements, a series of steps were necessary. First, the skull base reference points (SBRs) were defined as the right and left internal auditory canal transverse crest. The SBRs provide a frame of reference for all subsequent measurements. Next, a point on the malar surface was determined in the x,y,z coordinate system. Finally, the distance measuring tools were used to determine the distance between the right and left SBR and each malar surface point before and after fracture reduction. Comparisons before and after reduction provided information about the amount of reduction, and comparisons between sides provided information about symmetry. Transconjunctival, gingivobuccal, and frontozygomatic incisions were used for exposure at these sites during reduction. After initial fracture reduction was performed, the new coordinates for the malar points were recorded. In addition, the new distances from these new coordinates to the SBR points were measured. These measurements were then utilized to assess and adjust the fracture reduction. Then the measurements were repeated (Figure 24.4).

FIGURE 24.4 Intraoperatively CAS can track the projection of the bony midface by localization to specific points on the malar eminence outer surface and the subsequent calculation of distances from predefined skull base reference points. In this example from a revision reduction of a frontozyogmaticomaxillary fracture, the surgical planning tools shows the trajectory and distance between the malar surface and the ipsilateral and contralateral skull base reference points, which were defined as the transverse crest at each internal auditory canal (LandMarX 2.6.4 Medtronic Xomed, Jacksonville, FL).

This project provided useful lessons in several regards. First, the LandMarX system was useful in confirming adequate malar projection in these subjects. In addition, it was observed that smaller dynamic reference frame (DRF) arrays can provide adequate spatial orientation for surgical navigation. In these cases, the smaller ENT or spine DRF was used in the place of the much larger cranial DRF (Figure 24.5). The smaller DRF is less bulky, but it gave a stable reference for optical tracking during the procedure. Since the DRF was screw-fixated to the outer calvarium via a small scalp incision, the surgical navigation was dynamic; that is, the tracking system automatically corrected for patient head movement, since the DRF was directly attached to the cranial skeleton. This DRF arrangement also minimized potential disruption of the DRF due to forces required for fracture reduction.

FIGURE 24.5 For surgical navigation, the CAS system must track the position of the operating field volume. This small dynamic reference frame (DRF) contains small lightemitting diodes (LEDs) that the CAS camera array recognizes (LandMarX 2.6.4 Medtronic Xomed, Jacksonville, FL). This DRF is attached to the skull base post (shown in the inset photo); during the surgery, the post is secured directly to patient’s skull with monocortical bone screws.

24.3.2Sequential Reduction of Frontozygomaticomaxillary Fractures

Since surgical navigation provides precise localization information, similar localization information may also guide fracture reduction. In this approach, preoperative modeling and planning determine the optimal positions of bone fragment(s), and intraoperatively, the bone fragment position is tracked as the fracture is reduced. Obviously, this strategy requires positioning of bone fragments, rather than the usual instruments. As already stated, current CAS systems do not provide information about projection and/or contour; however, relative projection and contour assessments may be derived from the point localizations. Furthermore, the CAS software distance-measuring tools can provide some quantitative data about projection from defined reference points (typically at the untraumatized skull base) through the localization points.

In order to test the applicability of these concepts, the sequential reduction of displaced frontozygomaticomaxillary fractures was performed on cadavers (Figure 24.6) [6]. The goals of this study included the development of techniques