Computer Assisted Orthognatic Surgery based on 3D Cephalometry



Computer Assisted Orthognatic Surgery based on 3D Cephalometry.

A New Approach with 3D Surgical Wavers.

Max J. Zinser MD, DDS; Robert A. Mischkowski MD, DDS; Marian Durond MPH and Joachim E. Zöller MD, DDS, PhD

Introduction: Cephalometric analysis is conventionally performed on a standard set of lateral and frontal X-rays. They hence only offer a limited means to evaluate and characterize complex pathologies, e.g. deformities involving asymmetry. Computerization and sophisticated radiographic technology in orthodontics and orthognathic surgery have contributed immensely to the stockpile of knowledge. Yet in spite of the tremendous technological and scientific advances, the question of whether treatment planning for orthognathic surgery is an art or a science continues to loom in the minds of clinicians. Another significant improvement can be attributed to the development of highly sophisticated software for 3D visualisation of radiological data and segmentation of anatomical defined objects (Burgielski et al. 2002). As these objects can be moved in all spatial planes, the software becomes a valuable tool for pre-surgical planning of procedures associated with translocation of bony segments of the skull of facial skeleton. Although excellent navigation hardware and 3D planning software are already available, there is still a need for tools featuring simple and rapid synthesis of both, the realization of a pre-surgically planned situation and the valid intraoperatively transformation. Several approaches to solve the problem of bony segment translocation with a navigation system have been already attempted (Wagner et al. 1996a, Marmulla and Niederdellmann 1998 a, b, Troulis et al. 2002). Different approaches have to be made using the principles of augmented or virtual reality (Mischkowski et al. 2006).

The purpose of the present study is to introduce a new surgical waver concept based on computer assisted surgery. The transformation of the presurgical information (fig. 1) does not occur as above described by means of navigation, in contrast to that, new designed surgical wavers were used and introduced to determine and transform the segmented jaws (fig. 2).

The novel surgical wavers were individually made in cooperation with Materialise Belgium, by means of stereolithography based on presurgical CT or cone beam data. A further goal of this study was the evaluation of the precision, reliability and feasibility of this new technique including the capability of soft-tissue prediction. Finally the pre- and postoperative accuracy was evaluated and compared to the conventionally planning based on x-rays and plaster models.

Methods: This study contains 15 patients with orthognatic deformities (open bite, class II and class III). A virtual reality workbench is used for surgical planning based on spiral computed tomography and cone beam database. The orthognatic surgery planning was done preoperatively and virtually, referring to anatomical landmarks (basal skull) as a reference plane. Perpendicular to that plane through the nasion the facial median-plane was created. These symmetrical planes represent the ideal virtual facial situation (Fig. 2). In order to determine the real facial assymetrie the coordinates were drawn combining the occlusal plane perpendicular to the first incisors (Fig. 2). Based on these symmetrical planes the surgical techniques including the bone segmentation, i.e. high, low Le Fort I, II osteotomies or mandibular sagital splits were determined in order to get the best symmetrical result. Based on this virtual augmented surgeries custom dental or surgical wavers have been developed to transfer the virtual planning to the intraoperative situation. We used the new software (CMF / Simplant pro 9.21) from Materialise (Leuven, Belgium). The major objective of this “new technique“ contains the surgical transfer of the presurgical virutual planning as exact as possible as well as the repositioning of the centrical position of the temporo-mandibular-joint. For that our surgical concept is based on 3 splints. The first waver is used to define reference holes in the segmented bone before the transposition was performed. After the osteotomies the second surgical waver fix the segmented bone in the new “desired virtual” position. Therefore the wavers were built in a three-dimensional way connecting the reference holes (Fig. 3). Thereafter the bony fixation can be performed. In concern of bimaxillary osteotomies the third waver fix the definitive occlusion and reposition the mandibular joint in the centrical position. To compare the achieved bony translocation with the pre-surgical planning, 3D-cephalometric analysis of pre- and post-operative CT or DVT (cone beam) data were performed using Amira® software. Therefore the pre- and postoperativ CT or DVT dataset were fused.

Results: The new 3D computer assisted approach was applied for orthognatic surgeries in 15 patients, 10 underwent bimaxillary osteotomies and 5 monomaxillary osteotomies. 10 patients were females 5 were males. Maxillary retrognathia and mandibular prognathia was the diagnosis in 10 cases. One patient had in addition an circular open bite and 2 patients each had bimaxillary laterognathia and mandibular laterognathia to be corrected; the occlusal plane angle was adjusted in two more patients and vertical maxillary excess was treated by maxillary impaction in another patient. 3 cases had mandibular prognathia and 2 cases mandibular retrognathia. The surgery time was prolonged by approximately 30 min. due to the new surgical handling when using the new 3D-wavers. With increased experience, this extra time related to the technical handling will be diminished. In beginning of the development phase problems with the stability of the wavers occurred. These could be solved, however, through another design and another more stable material.

The great advantage of these technique consists in the rigid fixation of the bone parts (mandibula, maxilla) through the wavers. This facilitates the fixation of the osteotomy plates.

The control or comparison group contained also 15 patients with similar clinical picture. The planning was carried out, however, classically with X-rays and the conventional surgical wavers.

The 3D-cephalometric analysis based on pre- and postoperatively CT or DVT (Galileos® , Sirona, Germany) data showed that the accuracy when using the “new surgical 3D splints” including the computer assisted approach was within a range of 1 mm of the surgical plan. In the control group (classical planning) the accuracy was within a range of 1,5 mm including a tendency to advance the maxilla less than the planned distance was noticed.

Conclusion: Computer generated 3D-image prediction is suitable for patient education and communication. This simulation system will be particularly useful for the selection of an optimal operative method in cases in which change in facial soft tissue shape should be carefully deliberated. In the authors hand, the use of this computer-assisted-orthognatic surgical technique using surgical templates has resulted in outcomes close to those predicted by the planning process. The “new computer assisted 3D-splints” have the potential to become a valuable tool in orthognathic surgery. Up to now monomaxillary and bimaxillary osteotomies were evaluated. The application to other types of more complex osteotomies, including segment osteotomies are currently under investigation.

Figure 1: Figure 2:

Preoperative virtual planning of a Preoperative virtual planning. The

bimaxillary osteotomy (CMF,Materialise symmetrical planes were indicated

Belgium)

Figure 3:

Surgical application of the “new 3D wavers”. The maxilla is fixed with the 3D wavers in the new desired position.

References:

1. Burgielski Z, Jansen T, Rymon-Lipinski B, Hanssen N, Keeve E: Julius- a software framework for computer-aided-surgery. Biomed Tech (Berl) 47 (Suppl 1 Part 1): 101-103, 2002

2. Marmulla R, Niederdellmann H: Computer-assisted bone segment navigation. J Cranio-Maxillofac Surg 26:347-359, 1998a

3. Mischkowski R, Zinser MJ, Kübler A, Ulbricht H, Seifert U, Zöller J: Positionierung der Maxilla mittels “Augmented Reallity” Technik bei Le Fort I Osteotomien. Dtsch Zahnärztl Z Supplement: 30-32, 2005

4. Mischkowski, RA, Zinser MJ, Kübler A, Krug B, Seifert U, Zöller JE: Application of an augmented reality tool for maxillary positioning in orthognatic surgery – A feasibility study. J Cranio-Maxillofac Surg 34:478-483, 2006

5. Troulis MJ, Everett P, Seldin EB, Kikinis R, Kaban LB: Development of a three-dimeneional treatment planning system based on computed tomographic data. Int J Oral Maxillfac Surg 31:349-357, 2002

6. Wagner A, Ploder O, Ensilidis G, Truppe M, Ewers R: Image-guided surgery. Int J Oral Maxillofax Surg 25:147-151, 1996a

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