Cephalometric Tracing

A Cephalometric tracing is an overlay drawing produced from a cephalometric radiograph by digital means and a computer program or by copying specific outlines from it with a lead pencil onto acetate paper, using an illuminated view-box.

Tracings are used to facilitate cephalometric analysis, as well as in superimpositions, to evaluate treatment and growth changes.

Computerized Cephalometrics

The process of entering cephalometric data in digital format into a computer for cephalometric analysis.  Digitization (of radiographs) is the conversion of landmarks on a radiograph or tracing to numerical values on a two- (or three-) dimensional coordinate system, usually for the purpose of computerized cephalometric analysis. The process allows for automatic measurement of landmark relationships. Depending on the software and hardware available, the incorporation of data can be performed by digitizing points on a tracing, by scanning a tracing or a conventional radiograph, or by originally obtaining computerized radiographic images that are already in digital format, instead of conventional radiographs. Computerized cephalometrics offers the advantages of instant analysis; readily available race-, sex- and age-related norms for comparison; as well as ease of soft tissue change and surgical predictions.

Finite Element Analysis

Finite elemental analysis is used in digital imaging and is an engineering technique of stress analysis, the basic concept of which is the visualization of a structure as an assemblage of a finite number of discrete structural elements connected at a finite number of points. The finite elements are formed by figuratively “cutting the original structure into segments. ” For two-dimensional applications, triangles of various sizes and shapes usually are the finite elements of choice. Each element retains the mechanical characteristics of the original structure. Some characteristics of the material have to be specified (depending on whether it is isotropic or not).

Additionally, a numbering system is required to identify the elements and their connecting points, called “nodes. ” A coordinate system also must be established to identify uniquely the location of the nodal points. A large number of simultaneous linear equations are computer-generated, which establishes compatibility within each element.

The technique has some very distinct advantages as a research tool, among which is the ability to obtain an estimate of the stresses throughout the structure under consideration. Further, the inclusion of any type of anisotropy and inhomogeneity conceptually is possible by inserting the appropriate distribution of material properties at the nodes of the elements. However, when it is applied to structures such as a tooth, there are some practical limitations, as relatively little is known about the mechanical properties of dental and especially periodontal tissues.

Moire Diffraction

This is a technique with many applications in medicine, is used mainly to map three-dimensional contour. In orthodontics it has been used for evaluation of facial asymmetry. The technique uses a series of lines produced by a transparent grid. The grid is placed in front of the object that is to be contour-mapped. A light source is offset from the viewing angle. The light passes through the diffraction grading twice: firstly on its way from the source to the object and secondly after it has been reflected off the object. It then is recorded by a film or viewed by an investigator. An interference pattern of light and dark lines or fringes is created; each fringe represents a set of equidistant points from the grid. The fringes appear superimposed on the object as a series of contour plots of similar elevation. The method is limited by the viewing angle of the system to the object. Areas of rapid elevation change on the object are difficult to characterize because of inability to distinguish the line separation. This requires evaluation of the object from different viewing angles.

Visual treatment objective (VTO)

A treatment planning and communication aid that may be used to define the tooth movements and/or surgical changes required to achieve the desired facial goals. Essentially it consists of the patient’s pretreatment lateral cephalometric tracing, modified to demonstrate the changes that are anticipated in the course of treatment. This can be accomplished with the help of a computer program. A VTO of a growing child is an estimate of the growth of a growing patient with the orthodontic treatment outcome, and is a very helpful tool in arriving at a final treatment plan. Visual treatment objectives performed for orthognathic surgical treatment planning are sometimes referred to as STOs (Surgical Treatment Objectives).

The construction of a VTO can be very helpful in exploring various treatment options, but it is important, once a plan is determined, that the clinician goes back and makes sure that it is the product of a logical and practical approach to the problem. The VTO can be linked directly to, and evaluated in conjunction with, the occlusogram.

Occlusogram

An occlusolgram is a graphic representation of the arches from the occlusal view. Occlusograms are mainly used as treatment planning aids to assist in defining the specific tooth movements required within and between arches (in the sagittal and transverse planes) to achieve treatment goals. An occlusogram is essentially a two-dimensional diagnostic setup and is directly correlated with the Visual Treatment Objective (VTO). It can be constructed from tracings of radiographs or photographic or photostatic copies of the occlusal aspects of the maxillary and mandibular study casts. The tracings of the teeth of both arches are superimposed on each other to reproduce the existing occlusal relationship, using index points that are marked on the images or models and subsequently transferred to the tracings. Anticipated movements of individual teeth as well as the need for extractions then can be determined, to simulate the desired treatment goal.

Superimposition

Superimposition is the process of placing two images upon each other, registering on structures that remain relatively stable during the time period separating the two images, to evaluate the changes brought about by growth and/or treatment. In orthodontics, most commonly applies to cephalometric tracings or occlusograms.