3D maxillofacial planning concepts in orthognathic surgery: the evolution
A Scientific essay in Medical Sciences
DOCTORAL THESIS defended in public on 7th July, 2011
The aim of treating abnormalities of the dento-craniofacial complex is to improve the patients’ function and aesthetics. The skeletal remodeling in the course of these treatments implies performing osteotomies, bone grafting, distraction osteogenesis, implant placement and other treatment modalities. The effect of these treatments has a direct influence on the patients’ personal and social wellbeing. Therefore the challenge of maxillofacial surgeons in the 21st century is to achieve excellence in treating more than ever demanding patients. Patients’ demands and expectations have risen dramatically in the past decade, especially when it comes to the changes in facial appearance after maxillofacial procedures. Due to the complexity of the surgical procedures, careful pre-operative planning of the surgical intervention is mandatory to ensure an optimal outcome. Implementation of technical advances in medical three-dimensional (3D) imaging in the field of distraction osteogenesis and orthognathic surgery has been the motivation of this work. This thesis summarizes research work from 1998 through 2010 that basically has been performed in two phases.
In the first phase (1998-2004), the available knowledge in 3D imaging at that time was used to introduce a computer planning system to simulate distraction osteogenesis (DO) in patients with maxillary hypoplasia, in particular cleft patients. This initial research resulted in the development of the Maxilim software (Medicim, Mechelen, Belgium). It was first used to simulate a maxillary DO, using a trans-sinusal maxillary distraction (TSMD) device . As this 3D bone-related planning system proved to be reliable in surgical planning of DO, it was assumed that it might as well be useful in the planning of complex cases in orthognathic surgery. Ideally, the software should be able to virtually predict the effect of orthognathic procedures in exceptionally complex situations, and, if necessary, to adapt it to ensure an excellent outcome.
In the second phase (2004-2010), the Maxilim software was further developed to support planning and simulation of hard and soft tissue changes of orthognathic procedures. In addition an occlusal defining tool was developed and implemented in the software.
The first phase
The initial work focused on the field of distraction osteogenesis (DO) in patients with maxillary hypoplasia, in particular patients with clefts. Shortly after the introduction of extraoral DO, research on the development of intraoral devices started. The external devices were cumbersome, highly visible, and were mostly not well tolerated by patients.
The suggested intraoral devices had two main disadvantages. Firstly, they were still too bulky, and were placed completely under the periosteum. The activation rods were either in continuous contact with lips, causing pain and irritation, or had to be delivered through the skin in the nasolabial region . Secondly, they were all unidirectional, which required a perfect preoperative vector planning.
To overcome the above-mentioned disadvantages, a distractor was designed in a way that the distraction screw could be placed inside the maxillary sinus cavity. The positive results of the initial, experimental work resulted in the development of a trans-sinusal device for distraction of the maxilla in children and adults with mild to severe midfacial hypoplasia (in cooperation with KLS-Martin Medizin-Technik, Tuttlingen, Germany (chapter 2 & 3).
The following challenge that had to be overcome was the prediction of the distraction vector pre-operatively on one hand and transferring this vector planning into the patient per-operatively on the other hand. In cooperation with ESAT KU Leuven, Belgium, software was developed, that could virtually place the distractor device on the maxilla after a virtual Le Fort I type osteotomy was planned and virtually performed. Subsequently different clinical studies were undertaken and were published as preliminary results and later on as a critical evaluation of the clinical applicability and the long-term follow up of the TS-MD (chapter 4a, 4b & 5).
The second phase
Although the above-mentioned 3D bone-related planning system proved to be reliable in surgical planning of DO, it was not possible at that time to predict the changes of the overlying soft tissue envelop during the DO. Therefore, the next step was the development of a biomechanical simulation model that would attempt to simulate the 3D elastic deformation behaviour of facial soft tissues after bony displacement. Four computational strategies were compared to simulate the post-operative facial appearance: a linear finite element model, a non-linear finite element model, a mass spring model and a new mass tensor model. The introduction of this last model showed a significant gain in simulation time compared to the other models. Further the accuracy of the prediction of the new facial outlook after surgery was assessed. The best results were obtained by using the linear finite element model or the mass tensor model (chapter 6).
In order to make this software beneficial in daily practice of orthognathic surgery, two main additional developments had to be implemented. Firstly the occlusal surface of the teeth had to be visualized with high precision, and secondly a virtual planning of dental occlusion was mandatory. Swennen et al. succeeded to implement the first change in the software using their triple scanning technique. It was now necessary to define the virtual occlusion. This was realized by the development of a ‘rigid motion simulation engine’. A rigid motion simulation engine ensured the impenetrability of the dental models. Moreover, this engine enabled the user to identify the desired occlusion semi-automatically. The motion engine allowed almost real-time simulations. This software tool was validated, and the result showed that the presented system allowed a reliable determination of the desired occlusion (chapter 7).
After having integrated the presented occlusal model into an orthognathic planning system, the first complete orthognathic planning system became a reality and was taken into the clinical application. A validation of the soft tissue changes after prediction of the precise intermaxillary relationship became the subject of chapter 8.