Because the maxillary splint is considered a bone- based method because of its attachment to the dental arch, it is the only noninvasive registration method that provides consistently high intraoperative accu- racy (Schramm et al. 2001c; Eggers et al. 2005; Hoff- mann et al. 2005b,c). In surgery of the facial skeleton and skull base region, a registration splint with four markers arranged in an optimum geometrical pat- tern is accurate to less than 1.5 mm, depending on the type of splint used (Naumann 2001; Nilius 2001;
Fig. 32).
Splint-based registration is superior to invasive markers in the facial skeleton because the center of the reference points on the splint is closer to the cen- ter of the facial skeleton than when screw markers are used: The greater the distance of the operative field from the center of the reference markers, the poorer the accuracy of the registration (Fig. 33).
For a distance of up to 10 cm between the reference center and marker point, the accuracy of invasive bone-screw registration and noninvasive splint- based registration is less than 2 mm. When this dis- tance is increased, deviations of up to 8 mm occur when splints are used. This is directly attributable to the increasing distance of the markers from the oper- ative field.
The accuracy (or inaccuracy) of intraoperative navi- gation depends critically on the following five factors:
1. Inaccuracy of the CT data set and its processing and visualization
2. Inaccuracy of the navigation system used 3. Inaccuracy of pointer localization
4. Inaccuracy in registering the patient’s head with the registration system
5. Inaccuracy of the registration method used The first four technical factors in this list can be kept below 0.2–0.3 mm with proper technique (Husstedt et al. 1999). The greatest variation in accuracy results from the type of registration method used. Because the intraoperative accuracy of navigation is the sum of all five factors, the accuracy of the registration sys- tem is considered to be the most important factor.
The mean positional accuracy of 2 mm recom- mended for surgical applications (Sandemann et al.
1994) can be reliably achieved through the use of bone-anchored screw markers (Hassfeld et al. 1997, 2000; Brinker et al. 1998). Their advantages are their reliability and reproducibly high accuracy. They are invasive, however, and this limits their usefulness in emergency and elective procedures (Hassfeld 2000).
Nevertheless, for many years screw markers have been the only feasible method of intraoperative nav- igation in oromaxillofacial patients.
Intraoperative Accuracy
Chapter 9 47
48 9 Intraoperative Accuracy
Fig. 32. Intraoperative accuracy of splint-based and screw- marker-based registration. The intraoperative accuracies of bone marker (BM), PMMA splint (PS), silicone splint (SS), and vestibular splint (VS) determined in laboratory tests can be directly compared based on their bony attachments (a). The
vacuum-formed PMMA splints and screw markers provide a reproducible accuracy of 1 mm in all regions of the skull. The silicone impression splints show markedly higher values in the skull base and cranial vault (b)
49 9 Intraoperative Accuracy
Fig. 33. Intraoperative accuracy as a function of distance from the reference center. The intraoperative accuracies of bone-screw markers (bone marker), vacuum-formed splints (PMMA splint), maxillary silicone impression splints (silicone splint), and vestibular silicone impression splints (vestibular splint) determined in laboratory tests are plotted as a function of distance from the reference center. When this distance is greater than 11 cm, the accuracy values of splint-based regis- tration decline significantly. This is not the case with screw markers distributed symmetrically on the skull
