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| Year : 2014 | Volume
: 25
| Issue : 1 | Page : 128-132 |
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| Modern trends in modeling of extra-oral defects |
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Mukesh Kumar Goyal1, Shelly Goyal1, B Dhanasekar2
1 Department of Prosthodontics and Maxillofacial Prosthetics, Maharaja Gangasingh Dental College and Research Center, Sri Ganganagar, Rajasthan, India
2 Department of Prosthodontics and Maxillofacial Prosthetics, Manipal College of Dental Sciences, Manipal, India
Click here for correspondence address and email
| Date of Submission | 11-Oct-2012 |
| Date of Decision | 21-Nov-2012 |
| Date of Acceptance | 18-Jan-2013 |
| Date of Web Publication | 21-Apr-2014 |
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 Abstract | | |
Maxillofacial prostheses are usually fabricated on the basis of conventional impressions and techniques. The extent to which the prosthesis reproduces normal facial morphology depends on the clinical judgment and skill of the individual fabricating the prosthesis. Recently, as a result of advances in technology, various computer-aided design and manufacturing techniques have been successfully introduced for the automated fabrication of maxillofacial prostheses. These systems are able to provide more consistently accurate reproduction of facial morphology. Keywords: Computer numerically controlled milling, maxillofacial prostheses, rapid prototyping, scanning
How to cite this article:
Goyal MK, Goyal S, Dhanasekar B. Modern trends in modeling of extra-oral defects. Indian J Dent Res 2014;25:128-32 |
Until the recent past, conventional impression materials such as irreversible hydrocolloid or silicones and techniques have been used to fabricate maxillofacial prostheses and extra-oral radiation devices. [1] Potential problems associated with conventional impressions include patient discomfort and distortion of the facial soft tissues. Also, the conventional technique available requires the technician to spend time carving and adapting the prosthesis to a cast of the deficient side of the face. Hence, these techniques rely upon the skill and individual ability of the technician. [2] Recently as a result of advances in digitized imaging technology, it has become possible to obtain non-contact three-dimensional facial measurements and three-dimensional anatomic model.[1]
For convenience, the basic steps involved in the automated fabrication of extra-oral prosthesis can be discussed under the following headings:
- Collection of three-dimensional anatomic data (3-D facial measurements) using scanning techniques:
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- Computerized tomography scanning
- Magnetic resonance imaging scanning
- 3-D optical scanning
- Generation of a 3-D Computer Model - Blue-print
- Manufacturing a physical prototype:
- Computer numerically controlled (CNC) milling
- Rapid prototyping
| Collection of Three-Dimensional Anatomic Data Using Scanning Techniques | |  |
CT scanning
Acquisition of three-dimensional anatomic data became possible in the 1960s with the development of computer-assisted tomography (CT scanning), and expression of that data as 3-D surface images was pioneered by Gobor Herman. Since the initial report of application of that technology for fabricating a prosthetic scalp by Mankovich et al., many basic and clinical investigations regarding computer-assisted designing and manufacturing techniques have been reported in the field of maxillofacial prosthetics. [1]
CT scans are readily available. The use of CT scans allows parts of the body to be serially recorded slice by slice. The three-dimensional computed data obtained can be used to construct an anatomic model or prosthesis of exactly the same dimensions and geometry of the deformed side of the face. Watson et al. used CT data in planning and positioning implants to support an artificial prosthetic ear for patients with hemifacial microsomia. [1] This technique required a large number of contiguous slices (30-40 slices 2-mm apart) for bone depth to be assessed with reasonable accuracy. However, the use of CT scans to obtain 3-D anatomic data cannot be ethically justified because of the high dose of radiation administered. [1]
MRI scanning
MRI scanning is a non-invasive (zero radiation) alternative that projects a three-dimensional image of the soft tissues together with bone. The possible disadvantages remain the length of time the patient is required to remain motionless during the entire length of scanning and the high cost entailed. [2] A further exclusion of this method would arise when many stainless steel wires have been previously used to secure jaw fragments in corrective surgery.
Three-dimensional optical scanning
To avoid the disadvantages of CT scanning or MRI, an optical modeling process for extra-oral defects and body areas was developed. The development was based on experience in the collection of digitized data for tooth-related model dependant representations. The optical three-dimensional scanning unit provides a point cloud or virtual model of the face. [2] The two main types of optical 3-D scanners are discussed below.
3-D scanner based on self-calibrating fringe projection technology (Kolibri mobile)
It is a mobile, multi-view 3-D measuring system developed by the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena, Germany, that facilitates the fully automatic recording of the body part from various directions in one measuring process. The maximum field diameter of the system that is the area that can be recorded at one time is 650 mm. [2] Therefore, the complete human face can be recorded in a single operation. The face is illuminated by two grating sequences rotated 90° from different directions. The object is illuminated from different directions by means of a network of fixed mirrors (M 21 , M 22 , M 23 , M 24 , and M 25 ) and simultaneously observed by cameras from directions. The switching of the projection direction is done by the rotating central mirror M 1 . The observing cameras capture these fringe pictures simultaneously, resulting in at least four phase values for each pixel of the camera. Using these phase values, the 3-D coordinates are calculated. The position and number of the mirrors and cameras can be selected, thereby adapting the system for the relevant body part. Optimum measurement of an extra-oral defect involves four cameras and five projection directions; where more measurements are made from below, two of the cameras as well as three of the projection directions are directed from below to measure the chin. The duration time of recording up to the 3-D point cloud is approximately 20 sec. The measuring accuracy is less than 100 μm. Thus, points with a distance of 100 μm and greater can be recorded separately. [3] The data obtained are adapted for further use by equalizing the point clouds to obtain a computer-aided design (CAD) model based on which a definitive prosthesis is produced. [3] This system is mobile and simple to use. Measurements are made within seconds (approximately 20 sec) and complete human face can be recorded in a single operation. The procedure avoids the stress experienced by patients when conventional modeling methods are used. It avoids exposure to radiation when using a CT or MRI [Figure 1].
3-D laser scanning system
A laser scan of the full face takes 30 sec and is a non-invasive means of collecting digitized data. Coward and Watson emphasized the use of a laser scanner and CAD/CAM (computer assisted manufacturing) systems in the fabrication of auricular prosthesis in the late 90s. [4] Some of the constraints to the use of this technique remain loss of some information of the ear caused by light reflection from the hair and inaccessibility of the internal undercut surfaces of the ear by the vertically projected lines of the laser beam. These problems were overcome by a 3-D laser scanning system (Geodigm Corp., Chanhassen, MN, USA) developed recently to produce a 3-D dental cast known as "e-model." [4] The scanner projects a laser stripe onto the surface of the cast and then processes the images of the laser stripe captured by two digital cameras. The cast is then translated and rotated under computer control to expose all surfaces of the cast to the cameras. This scanning process produces a cloud of over 1 million data points that describe the surface contours of the cast. These 3-D data points are loaded into the scanning systems proprietary e-model software (Geodigm Corp.) and interconnected to form a triangular mesh (tessellate) which is then inverted to produce a 3-D mirror image of the scanned cast. The data obtained are used in conjunction with rapid prototyping machines to obtain the physical prototype. The method also uses a more advanced scanner that requires only one scan, compared to a similar technique described recently by Ciocca et al. that required eight random scans to record all the undercut areas. [5]
| Generation of A 3-D Computer Model (Blue-Print) of the Extra-Oral Defect | | |
Once accurate geometric information of the defect is collected using the scanning techniques, the information is imported into the scanning systems proprietary CAD software package for manipulation and the production of a "blue-print" (CAD model) from which the prototype is manufactured. [6],[7]
| Manufacturing of A Physical Prototype | | |
CNC milling
Data obtained by optical laser scanner have been used in conjunction with computer software which converts 3-D data sets into an instruction sequence for a CNC milling machine to prepare a reverse model of the normal ear. [6]
The data set comprised a depth map of a given view of the scanned object, which was translated into physical movements of a cutter in three axes, enabling an ear pattern to be milled from a block of expanded polyurethane. Plaster of Paris encompassed the polyurethane model to produce a negative mold, into which molten wax was than poured. The wax model ear was adjusted by the maxillofacial technician to create the undercut contours. Disadvantages of this technique are that the milling machine reproduces only the outer surface contours and the details of the internal geometry (undercut contours) are created by freehand carving by the technician. These limitations led to the development of rapid prototyping techniques.
Rapid prototyping
Rapid prototyping is an umbrella term used to describe a variety of technologies that allow quick production of prototype parts from computer files. During the late 1980s, the introduction of rapid prototyping technologies offered new possibilities for modeling extra-oral defects. Developed primarily for the automotive and aerospace industries to shorten the time between design and construction of prototype parts, it operates on the principle of depositing material in layers or slices to build up a model rather than forming a model from a solid block, thus offering a great advantage of creating all the internal geometry as well rather than just the outer surface contours as with a milling machine [8] [Figure 2]. There are currently many variants that are marketed, but the three dominant technologies are most commonly used. Stereolithography uses an ultraviolet laser to solidify a liquid plastic (resin) layer by layer. Laser sintering uses a laser to selectively fuse a thin layer of powered plastic or metal to previously fused layers. Laminated object manufacturing laminates thin sheets of material successively and cuts and destroys material with a laser, leaving behind a solid laminated part. [8]
In each of these techniques, the layers added can be thinner than 0.5 mm with a vertical wall thickness of as low as 0.2 mm. However, the files have become the manufacturing standard for data transmission to rapid prototyping technologies. Stereolithography was the first rapid prototyping technology to be developed, in the 1980s, and is the technique most commonly used to create stereolithographic anatomic (SLA) models for surgery and transferring CAD models to rapid prototyping technologies. [4]
The CAD model of the part to be created is cut into a series of two-dimensional slices. This data is used to control a laser beam that draws each slice of the model in turn on the surface of a tank of resin. The photosensitive resin is instantaneously cured to a solid where the laser beam strikes. At the start of the process, a platform is positioned below the surface of the liquid resin at the thickness of one 0.25 mm slice [Figure 3] and [Figure 4]. Once the first layer has been drawn, it descends to allow resin to cover the top of the model so that the next slice can be constructed at the top of the model. As the platform descends, the model is built from the base up. It has advantages of that the model is created directly from computer data. So, there is no human error and no limit on the complexity of the geometry to be built. [9],[10],[11]
| Discussion | | |
Conventional impression materials have been used for decades in dentistry and maxillofacial prosthetics. However, the challenge for a field that still relies largely on the 16 th century approach of creating prostheses by hand is the vital importance of getting exactly the right fit with existing tissue and bone. [6] This is exactly the same thing that the field of technology has been focusing to address in recent years by coming up with different computer-aided techniques of producing high-quality prototypes for the fabrication of maxillofacial prosthesis. [12] Several techniques have been reported to fabricate a mirror image for maxillofacial prostheses. [7] However, the usefulness of rapid prototyping has been proved beyond a shadow of a doubt. [9] So, recent studies have focused on computer-assisted rapid prototyping machines to sculpt facial prostheses. [6] Although computer-aided design and manufacture techniques have shown some promising applications in the fabrication of facial prosthesis, but it is limited to the production of an intermediate wax pattern, from which the definitive prosthesis is subsequently obtained through conventional procedures. In addition, problems relating to the color compatibility of the facial prosthesis with surrounding tissues remain to be overcome. The development and evaluation of these advances continue till date. [6] It can be anticipated that future developments may include direct fabrication of high-precision computer-designed definitive facial prosthesis without the need to produce an intermediate wax pattern and a color map of the definitive prosthesis by means of a spectrophotometer-assisted color calibration of the surface. [5] So, in this scenario, it would be apt to believe that the profession is destined to go through modernization in the year to come and computer-aided design and techniques may eventually become the next generation of methods of fabrication of extra-oral prosthesis. [10],[11],[12],[13],[14]
| Conclusion | | |
Although advances in both techniques and materials have been remarkable in the past several years, the full potential and utilization of maxillofacial prosthetics is not yet in sight. Developments in materials in industry, particularly the variously plastics and other synthetic products, will have direct application to the complex and exacting requirements of maxillofacial prosthetics. Rapid prototyping can produce physical models without molds or dies using digital methods. [10],[15]
External imaging techniques such as the Breukmann Optotop system offer better accuracy than CT, especially where there is complex anatomy, such as an ear, and they do not require additional interpolation for volumetric modeling between the slices. [3] The use of haptic software such as the Freeform system with the phantom haptic device allows the user to interact with the model through the sense of touch and make adjustments in the size, shape, and scale of the model before tooling the final product. [8] In addition to speed, other advantages include the fact that most 3-D systems are additive methods that allow for production of models that would have been impossible to create by traditional methods. [6] For widespread use, the equipment must become more cost-effective and easier to use, and it must occupy a smaller space. In the meantime, hospitals and clinics in similar geographic areas could consider the establishment of a centralized service. [10] Training programs will continue to emphasize the technical procedures of prosthetics, but the training will be extended to include more sophisticated techniques in the diagnosis to the patient's main problem.
| Acknowledgments | | |
The authors wish to pay their paramount gratitude to Dr. Veena Hegde (Professor and Head) and Dr. Aparna IN (Professor), Department of Prosthodontics and Maxillofacial Prosthetics, Manipal College of Dental Sciences, Manipal, for their constant support and guidance.
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Correspondence Address:
Mukesh Kumar Goyal
Department of Prosthodontics and Maxillofacial Prosthetics, Maharaja Gangasingh Dental College and Research Center, Sri Ganganagar, Rajasthan
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0970-9290.131170
[Figure 1], [Figure 2], [Figure 3], [Figure 4] |
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