Making Tomographic Slices And Superimposing Contour Lines On Photographs

Tomographic Slices

Slice each of the resulting quadrants horizontally, 5 mm above the plane of the teeth (i.e. with the blade of the knife parallel to the surface of the impression material corresponding to the plane of the teeth).  It is then possible to measure the distance of this plane from the roof of the mouth, and to draw a line round it.

In order to assist with the scaling it is also advisable to draw round the base of the teeth.   Check with your local biology or physical anthropology departments for a tomograph with which to make the 5 mm horizontal slices.  Otherwise, it is relatively easy to make a tool to cut the alginate mold parallel to the plane of the teeth.  On a flat, smooth, 190 x 125 mm wooden board paste heavy cardboard strips parallel to each other, 70 mm apart, resulting in a two “walls” of height 5mm.  On top of, and perpendicular to the cardboard walls place an 80 mm long razor blade, creating a wide slicing area.  Push each quadrant of the dental impression through this tool so as to cut off the bottom 5 mm of impression material in the occlusal plane.  Place the quadrants back together at the origin on the graph paper, trace them, and repeat the process until all of the impression material is drawn in this way. 

Alternatively, you can use a cast to get the contour lines: fill the cast with liquid to depths of 5, 10, and 15 mm, photographing each depth.

Superimposing on Photographs

To superimpose the contour lines and the marks of the teeth accurately on the photograph, it is first necessary to be certain that the photographs are not distorted. They will be foreshortened, if the mirror was not at a 45 angle to the plane of the teeth.  The scaling can be done quite easily if the photograph has been entered into a computer, either by scanning it, or by conversion from the video.  The contour lines, location of the teeth and the tracing of the sagittal section can also be scanned in, and then the image of the roof of the mouth scaled independently in each direction so that obvious landmarks, such as the distance between certain teeth are adjusted appropriately. 

For a detailed illustration of this process please refer to Ladefoged’s chapter on static palatography in his 2003 book, Phonetic Data Analysis.