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

FIGURE 21.1 The surgical planning mode of the StealthStation 2.6.4 (Medtronic Surgical Navigation Technologies, Louisville, CO) includes the x,y,z coordinate system. The x-coordinate corresponds to ‘‘Left/Right,’’ the y-coordinate corresponds to ‘‘Superior/ Inferior,’’ and the z-coordinate corresponds to ‘‘Anterior-Posterior.’’ In this way, each point in the imaging volume has a unique x,y,z value. For cephalometrics, this facilitates the reproducible identification of specific points during the quantitative assessment of images. Other CAS software packages offer similar features. (From Ref. 15.)

thin-cut CT data (Figure 21.2). Users can manipulate the resultant models for review. Such manipulations may include simple rotations or even complex fly-by maneuvers.

Distance measuring: The distance between two points in the preoperative imaging data set volume can be calculated with this tool (Figure 21.3).

Special views: Another optional tool permits the reconstruction of special cut views that present planar data through a specific line in the imaging data set volume (Figure 21.4).

Together these tools provide a means for the quantitative assessment of CT images on a computer workstation. In this way, cephalometric principles may be applied to answer questions about three-dimensional anatomical relationships.

FIGURE 21.3 The LandMarX 2. 6.4 (Medtronic Xomed, Jacksonville, FL) has a distance measuring tool. In this example, distances from the right and left internal auditory canal transverse crests to a fixed point on the malar surface were calculated; in this way, the prereduction and postreduction position of the malar fragment could be monitored during its operative reduction. It should be noted that the trajectories depicted in the axial, coronal, and sagittal images represent only the vector component for that plane, not the true trajectory. As a result, the trajectory may seem inappropriately displaced on each of the planar images. This distance-measuring tool can be used to determine the distance between any two points in the imaging volume. Other CAS software packages include similar software tools.

dardized. Furthermore, the software used for this application has not been optimized for this purpose. As a result, all observations of this area are truly preliminary; it may be anticipated that significant changes will occur.

At first glance, it may seem obvious that software-derived measurements of distance between points on a CT scan correlate well to actual direct measurements in the real world; however, this may not necessarily be true. First, identification of the landmarks on the CT scan images may be imprecise, since difficult user interfaces may confuse even expert users, computer display systems may

FIGURE 21.4 CAS software offers special views that may be manipulated at the computer workstation. This cut-view from the StealthStation 3 (Medtronic Surgical Navigation Technologies, Louisville, CO) is typical. Other systems offer similar capabilities.

poorly project images (due to a lack of sufficient resolution), and finally users may be unfamiliar with the identification of anatomical landmarks via a computer workstation. Furthermore, CT modeling protocols that the software utilizes may introduce errors. These problems reflect the limitations of traditional cephalometrics and the limitations of software for traditional cephalometrics. Fortunately, CAS-based measurements of distance between points on a CT scan correlate well with actual physical measurements. Using a cadaveric skull model, we examined this issue by comparing linear measurements obtained with scientific grade microcalipers with the corresponding linear measurements obtained with the distance-measuring tools of the Stealth Cranial CAS software (Medtronic Surgical Navigation Technology, Louisville, CO); differences between corresponding measurements were not statistically significant.[11]

The choice of parameters for CAS cephalometrics is also problematic. The direct use of parameters from traditional cephalometrics may be considered; however, practical considerations eliminate this option, since one cannot directly extrapolate from hand tracing on a lateral x-ray to CT-based, three-dimensional

modeling on a computer workstation. For this reason, a new set of parameters for CAS cephalometrics is necessary.

It is important to consider the potential clinical ramifications of such parameters. Craniomaxillofacial surgeons are concerned with facial skeleton projection and symmetry (or asymmetry) as well as the resultant soft-tissue contours. The ideal system would provide information about projection. Comparison of relative projection between sides would form a measure of symmetry/asymmetry. To the extent that soft-tissue contours are a function of the underlying bony skeleton, a series of projection measurements from a fixed reference to multiple points along the bony contour would also provide information about the soft-tissue cover’s projection. To the extent that soft-tissue contours reflect the intrinsic properties of the soft tissue, other modeling approaches that include virtual modeling of these mechanical proprieties would be necessary for a complete model of softtissue contours.

Linear projection measurements require two points that define the trajectory of the measurement, which runs from a fixed reference point to a target point. Each point must be easily recognized and reliably identified. Symmetry/asymmetry concerns can be addressed in two ways. In the first alternative, a midline reference point can serve as a frame of reference; measurements to targets on each side from the midline reference point provide quantitative information about projection and symmetry/asymmetry. The second alternative involves the selection of a reference point on each side; measurements to the ipsilateral and contralateral target points provide projection and symmetry/asymmetry data.

We have developed two systems for CAS cephalometrics. The first system focuses upon skull base length and width. In this paradigm, distances between the right and left palatine foramina and the distance between the right and left foramina spinosa serve as measures of skull base width, while the distance from the posterior foramen magnum point (the reference point) and the midpoint of a line between the right and left palatine foramina (the target point) serves as a measure of skull base anteroposterior projection (Figure 21.5) [11]. Admittedly, the clinical impact of this approach is probably limited to specific skull base procedures. The real issue here is the proof of concept; that is, computer-enabled review of CT data provides a means of assessing skull base projection.

Our second system is more directly tailored toward issues of malar contour and projection; as a result, this approach can probably be adapted for other craniomaxillofacial parameters. According to his paradigm, the reference points, which are called skull base reference points (SBR), are the left and right internal auditory canal transverse crests (Figure 21.6). Importantly, the SBRs are not part of the facial or maxillomandibular skeletons; therefore, they are unlikely to be altered by congenital and/or traumatic anomalies that affect the maxillofacial skeleton. Nonetheless, the SBRs provide a suitable reference frame for further measurements. The target points for malar projection (known as the malar points, or MPs)

FIGURE 21.6 The right skull base reference (SBR), shown here on images obtained on the StealthStation 2.6.4 (Medtronic Surgical Navigation Technologies, Louisville, CO), was defined as the transverse crest of the internal auditory canal. The SBR serves as a reference point for the assessment of malar projection.

are the points of greatest curvature on the malar bony contour. In sum, the four points (namely the left and right SBRs and the left and right MPs) define a variety of measurements between each SBR and the ipsilateral and contralateral MP. The potential linear measurements are as follows: the right SBR to the ipsilateral MP (R SBR-iMP), the right SBR to the contralateral MP (R SBR-cMP), the left SBR to the ipsilateral MP (L SBR-iMP), and the left SBR to the contralateral MP (L SBR-cMP). Together these measurements summarize malar projection. Furthermore, R SBR-iMP and L SBR-cMP represent right malar projection, and L SBR-iMP and R SBR-cMP represent left malar projection. Symmetry/asymmetry determinations are reflected in the relative measurements for each side (Figure 21.7).

Our work on the SBR-MP system for CAS cephalometrics has focused on several related projects. In our first project, normative data were developed by performing the SBR-MP measurements on a series of CT scans obtained for

FIGURE 21.7 The StealthStation 2.6.4 (Medtronic Surgical Navigation Technologies, Louisville, CO) distance-measuring tool was used to measure the distance from the left skull base reference point (L SBR) to the ipsilateral malar point (iMP). This distance, known as L SBR-iMP, summarizes information about the position and contour of the left malar bone. It should be noted that the trajectories depicted in the axial, coronal, and sagittal images represent only the vector component for that plane, not the true trajectory. As a result, the trajectory may seem inappropriately displaced on each of the planar images.

surgical navigation during computer-aided transsphenoidal hypophysectomy [12]. In a clinical case report, the SBR-MP measurements were used intraoperatively for computer-aided malar fracture reduction [13]. This initial clinical experience led to a project, in which zygomatic fractures were created in cadaveric skulls and then repaired under CAS guidance [14]. (For a further discussion of computer-aided maxillofacial fracture repair, please see Chapter 24.)

We have also proposed a paradigm for the assessment of the bony nasal pyramid [15]. In this report, the CAS distance-measuring software tools were used to determine the thickness of the nasal bones at the level of the rhinion on thin cut axial CT data. We also proposed a technique for the measurement of bony nasal pyramid projection, which can be operationally defined by the rhinion

FIGURE 21.9 This axial image shows the nasion nasal projection. The StealthStation 2.6.4 (Medtronic Surgical Navigation Technology, Louisville, CO) distance-measuring tool was used to determine this parameter. The nasion nasal projection was defined as the distance from the nasion to a perpendicular reference line through the point defined by the nasomaxillary suture in the axial plane. This reference line runs perpendicular to the axial plane. (From Ref. 15.)

rior vector and nasal bone length, which is an oblique vector. As a result, rhinion projection values are smaller than corresponding nasion projection values.

The rhinion nasal projection and nasion nasal projection also represent asymmetries of the nasal bones. The right and left measurements for each of these parameters would be equal in perfectly symmetrical bony nasal pyramid. To the extent that the corresponding left and right measurements differ, the bony nasal pyramid will be deviated to the side of the lower measurement. This approach may be useful for the characterization of traumatic and congenital bony pyramid deformities. In addition, objective data for the results of rhinoplastic procedures are also possible.

Other applications of computer-aided assessment of the bony nasal pyramid are also feasible. Close monitoring of nasofacial growth and development is desir-