We were in the wet lab today, to take impressions and pour up stone models of our typodont. To take an impression, you first find a suitable size tray that will fit to the posterior teeth, without allowing for too much facial-to-lingual movement. Trays can be manipulated in a number of ways in order to better fit the patient, including heating up of the plastic in order to bend it, or adding Triad composite material to the end of the tray if it is too short.
Once you have the correct size tray, you can start by mixing the alginate. Based on the product we used, for each pouch of pre-packaged alginate, you add 55 mL of cold water (recall: cold water = retards setting). The water is added to a mixing bowl first, and then the alginate powder. Initially, you want to mix the water and powder in order to dampen it; this keeps the material in the bowl when you begin to mix it vigourously. You then begin to systematically spread it along the side of the bowl using your spatula. When it begins to have a uniform paste-like feel, you begin to transfer it to the tray. This should be done in two scoops, with the top smoothed. It is also important to make sure it is pushed down into the holes on the bottom of the tray so that it is mechanically locked into the tray. This ensures that the alginate impression does not get separated from the tray upon removal. Finally, place the tray from posterior to anterior, ensuring that you don't 'burn' the occlusal surface by pushing the arch in too deep. As well, make sure that alginate covers the entire vestible up to the land area in order to get a good gingival impression.
Remember, this all has to be done as quick as possible, as the alginate sets quickly. And no bubbles.
Once the impression sets, then you remove from mouth (should take 3-5 minutes). The stone powder also comes pre-packaged, and ours needs 40 mL of water. Same mixing procedures, all though time is not as imperative. Once it's mixed, you need to place the mixing bowl on the vibrator to work out any air bubbles in the stone. You follow this up with a double pour procedure. You take small amounts of stone, and run it through the occlusal surface, seeing every tooth get filled. Once you get to the end, you vibrate off any extra. This helps limit any bubbles from occuring at the occlusal surface. You then proceed to fill it up at a quicker pace, giving it a nice large base for further processing. You then let it harden for about 45 minutes (recall: the two setting phases of stone, dental materials lecture!). Carefully pry off the impression tray. It may be necessary to use a buffalo knife to remove any stone mixture that may have slopped over the edge and mechanically locked the model in place.
How to make a base comes next lecture.
General and Oral Histology
Histo lecture today focused on the histology of the oral cavity. We first looked at salivary glands, which classified based on type (whether they are major or minor) and classified by if they secrete serous, mucous, or a mixture of both.
There are three major salivary glands in the oral cavity, those being:
- parotid gland: drained by Stenson's duct, secretes only serous fluid
- submandibular gland: drained by Wharton's duct, secretes both serous and mucous, but mainly serous
- sublingual gland: drained by ducts of Rivinus (many), secretes both serous and mucous, but mainly mucous
Saliva has a number of functions, including:
- moistening oral mucosa and dry food, cooling hot food
- buffers acids through bicarbonate ions
- breaks down carbohydrates by alpha-amylase and fats by lingual lipases
- controls bacterial flora
- helps in remineralization through calcium and phosphate concentrations
- protects teeth by forming enamel pellicle (micrometer of organic film that adsorbs onto enamel surface
You tend to see increased salivary flow rates when chewing. It can also be increased through pregnancy-related hormonal changes, olfactory stimuli, and certain medicines and drugs. Flow rate can be decreased by menopause-related hormone changes, stress, and anti-adrenergic and anti-cholinergic drugs. Dry mouth is clinically referred to as xerostomia.
Typically find four types of cells in oral mucosa, including the keratinocytes (protection), melanocytes (secretes melatonin), Merkel Cells (touch receptors), and Langerhan's cells (antigen-presenting cells).
You see two types of epithelial layers, a non-keratinized stratified squamous layer called the lining mucosa, and a parakeratinized stratified squamous layer called the masticatory mucosa. The soft tissues (lips, gingiva, soft palate) are lined with lining mucosa, and you find many minor salivary glands that secrete mucosa and submucosa. The epithelium covering hard tissues (ie. the hard palate) have masticatory mucosa to protect the tissue from mastication forces. The masticatory mucosa typically doesn't have submucosa, and are attached directly to the hard tissue.
We finished up by talking about the specialized salivary glands, or the papillae. Four types: filiform papillae (no real taste buds), fungiform papillae, foliate papillae, and circumvallate papillae.
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