© Borgis - Nowa Stomatologia 2/2005, s. 55-58
Adam Okon1, 2, Anita Okon3
Computer simulation of Xunplates – micro-mechanical devices for moving bones in a cleft palate
Symulacja komputerowa Xunplates – mikromechanicznych urządzeń do przesuwania kości w rozszczepie podniebienia
1Department of Conservative Dentistry, Medical University of Warsaw, Poland
Head: Professor Elżbieta Jodkowska
2Department of Medical Informatics, Medical University of Warsaw, Poland
Head: Professor Robert Rudowski
3Xuntech Medical Systems, Warsaw, Poland
Currently used orthodontic systems, both fixed and removable, have some limitations. Their natural force insertion point is the crown of a tooth or the mucosa, which determines how they work: by changing the inclination of the tooth or by axial movement. In either case, a force configuration is unfavorable, far from the centre of the tooth. These disadvantages are not present in the sub-mucosal orthodontic system.
The micro-plates as an attachment for the forces were introduced in Orthoanchor ® orthodontic system (1, 2). The main disadvantage of this system is the presence of intra-oral wires and direct contact between the bone and the outside world. This problem is to be eliminated in our Xunplates system. All parts of the system: pulling/pushing brackets, wires and micro-plates are planned to be inserted under the surface of the mucosa or periosteum. No part of the system protrudes through the mucosa, which means that it remains sterile. Placed under mucosa, it allows axial tooth movement in favorable, central force insertion point. The most important feature of this system is that it allows bringing the alveolar bones close together in a cleft palate. The application of forces with implanted Xunplates will prevent improper frontal growth and will model bone shape to restrict the size of a future transplant.
The initiation of treatment too early or too late is also a problem we have to face in maxillofacial reconstruction. Early surgical treatment of cleft palate causes inhibition of frontal development of the maxilla after surgical intervention. Late surgical treatment, on the other hand, causes improper bone formation – a wide cleft that makes reconstruction extremely difficult (3, 4, 5).
So what is the ideal solution? Not to inhibit or depress bone growth and to move and/or bend the bone before merger surgery, as in the treatment scenario proposed as a result of our simulations.
Virtual Reality (three dimensional graphics) programmes are widely used in industry for testing non-built (as yet) systems such as cars, airplanes etc. Computer simulation allows for the improvement of such systems without building several expensive prototypes in the process (6, 7, 8, 9). It was obvious that constructing a prototype of a micro device for moving bones in cleft palate would be costly and difficult. That is why we decided to use computer simulation before building the first real prototype.
This paper presents an initial study on the construction of a system of micro devices placed under the mucosa and exerting pressure on the cleft palate to move it into the desired direction of growth.
The aim of the work was to create a system for controlling the rebuilding of the palatal and alveolar bones with an orthodontic system placed sub-mucosally and driven by impulses of an electromagnetic field.
During this study we wanted to test this system using Virtual Reality before building expensive prototypes.
The virtual patient – a newborn with a cleft palate, was created as a deformation of a 3D model of a newborn skull (see Fig. 1). The 3D data were obtained using Computer Tomography (we used internet resources; no child was exposed to radiation for this study). Using computer simulation, based on the literature (7, 8, 9, 10) and radiographs of a cleft palate, bones were removed and a large cleft was created. A smoothing filter was applied locally on the cleft to make it more natural.
Fig. 1. Virtual Patient.
We superimposed the virtual models of our Xunplates system onto a skull model. Xunplates is a system of micro-plates, brackets and titanium wires designed to be inserted during atraumatic surgery under local anaesthesia. Micro-plates tightened with screws are used as bone attachments. The key component of the system is a sub-mucosal micro-mechanical device driven by impulses of an electromagnetic field, which can move the micro-plates in a chosen direction.
The insertion points are chosen earlier using computer tomography. Standard Helical CT with 3D reconstruction and CAD/CAM (Computer Aided Design/Computer Aided Manufacturing) module can be used. Prepared leads will be used for the insertion of elements without visual control. After (2 mm) wounds have healed, the moving process will begin. The micro-mechanical device will be activated using small impulses of an electromagnetic field and will exert a small, constant force on the micro-plate. This process may last several months, because there is no direct contact between the sterile sub-mucosal parts and the outside world. We can choose the direction of movement by turning several micro-devices on and off. Electric power and control data are transmitted through the mucosa using solenoids. An electronic system was planned as a One-wire(r) bus, commonly used in microcontroller´s technology. The level of magnetic impulses used in the device is much lower than that emitted by a cell phone, and should not have biological implications.
During this simulation of Xunplates and a bone displacement, a standard 3D Studio (r) from AutoDesk was used as a software platform. Calculations and rendering were performed using 1,5GB RAM Computer, currently one of the most powerful graphic workstations available.
3D simulation was found to be a useful tool for the virtual testing of new possibilities of treating cleft palate. The 3D model of the cleft palate allowed force planning and simulation of bone movement.
Several types of brackets – components of the Xunplates system were simulated (see Fig. 2).
Fig. 2. Components of the Xunplates system: a.) palate growth stimulator; mechanical components: b.)angular, c.)linear and d.)corner wire retainers; electric parts: e.)solenoid – the power source of the system which works like an antenna transmitting electrical power, f.)electronics controller box, g.)piezoelectric engine for wires. On the figure all the boxes are showed opened; normally they will be closed standard size titanium boxes.
We have tried many arrangements of the system on our virtual patient. One such arrangement, chosen for its best application in achieving the desired force directions, is illustrated in Fig. 3. The whole system can be seen in action: the wire ring for alveolar bone bending, palate growth stimulator and the other brackets.
Fig. 3. Relation between stimulated, desired direction of growth, applied forces and device construction. The whole system can be seen in action: the wire ring for alveolar bone bending, palate growth stimulator and the other brackets.
We have tried to simulate bone changes using the three-dimensional morphing mechanism of our software pack. Three intermediate stages are illustrated (Fig. 4).
a. Start of treatment
b. Intermediate stage
c. End of treatment
Fig. 4. Morphing – the simulation of treatment.
The following scenario of cleft palate treatment was proposed after computer simulation:
I. Planning of modeling cleft palate: Spiral computer tomography; Analysis of the actual bone shape and planning of the target shape; Determination of intermediate stages using three-dimensional morphing. Determination of insertion points for Xunplates using finite element force calculation, based on the skull model from computer tomography. The surgical leads are created using computer engravers or stereo photolithography.
II. Primary Surgery: Atraumatic procedure, sedation and local anesthesia; Small (2×2 mm) mucoperiosteal flaps are raised; Application of brackets with micro screws in pre-determined locations using the prepared leads; Connect of brackets using needle–sized tunnels in the uncut mucosa; wires can be inserted without cutting the mucosa and without visual, thanks to the prepared guide ways.
III. Post surgical period (several months): Stimulation of the system using impulses of an electromagnetic field; Radiological control of bone changes.
IV. Secondary surgery: Removal of the system; a small transplant, if needed; Suturing of the small cleft.
Other possible applications of the system have been noticed, such as post-oncological surgery reconstructions, enhancement of the zeugmatic arch and frontalisation of the maxilla in progeny, plastic surgery.
The determination of insertion points for Xunplates using finite element force calculation is possible, but has not been performed yet; because of the poor skull model applied (lack of internal data). The biological considerations of this system are outside the scope of this study and must be investigated in a future study using an animal model.
Virtual reality helps to answer some questions about a non-built as yet system for moving bones in cleft palate. It allows for several analyses of the actual bone shape and the planning of their target shape based on spiral computer tomography. One of the most valuable applications is the determination of intermediate stages using three-dimensional morphing.
A micro-mechanical device will be built based on virtual reality tests, which have helped to make improvements in the system without building expensive prototypes.
The system demonstrates a new possibility of cleft palate treatment. Other possible applications include post-oncology reconstructions, plastic surgery and maxillofacial surgery. The system needs intensive animal testing and development before it can be used on humans.
The help of Professor Kevin Montgomery from the National Biocomputation Center, Stanford University, USA, for providing the 3D model of the newborn skull, is gratefully acknowledged.
1. Kanomi R.: Mini-implant for orthodontic anchorage. J. Clin. Orthod. 1997, 31(11):763-7. 2.Kanomi R., Tekada K.: Application of titanium mini implant system for orthodontic anchorage. in Biological Mechanisms of Tooth movement and Craniofacial Adaptation pages 253-258 Harvard Society for Advancement of Orthodontics, Boston, Massachusetts, USA. 3.Schultes G., Gaggl A., Karcher H.: A comparison of growth impairment and orthodontic results in adult patients with clefts of palate and unilateral clefts of lip, palate and alveolus. Br. J. of Oral & Maxillofacial Surg. 2000, 38(1):26-32. 4.Prahi., Kuijpers-Jagtman A.M., van´t Hof M.A., Prahl-Andersen B.: A randomised prospective clinical trial into the effect of infant orthopaedics on maxillary arch dimensions in unilateral cleft lip and palate (Dutchcleft). E. J. of Oral S. 2001, 109(5):297-305. 5.Tindlund R.S.: Skeletal response to maxillary protraction in patients with cleft lip and palate before age 10 years. Cleft Palate - Craniofacial J. 1994, 31(4):295-308. 6.Verhey, Janko: Virtual Reality (VR) and Augmented Reality (AR) in medicine. (lecture) http://www.janko-verhey.de/veb-kurse. 7.Mazella F.: The Forcegrid: A Buffer Structure for Haptic Interaction with Virtual Elastic Objects; master thesis NASA Stanford USA. 8.Montgomery K.: User Interface Paradigms for VR-based Surgical Planning: Lessons learned over a Decade of Research. Biocomp. Center Tech. Report., Stanford University, USA. 9.Montgomery K., Stephanides M., Schendel S.: Development and application of a virtual environment for reconstructive surgery. Computer Aided Surgery. 2000, 5(2):90-7. 10.Rivkin C.J., Keith O., Crawford P.J., Hathorn I.S.: Dental care for the patient with a cleft lip and palate. Part 1: From birth to the mixed dentition stage. Br. Dental. J. 2000, 188(2):78-83.