(When you were in preclinic the primary model and the master cast were exactly the same. The borders could not be overextended. This is not so for the clinic.)
There should be minimal or no blockout of the model to make the tray. Check with your instructor if you think there will be a problem.
If you consider that the primary impression was overextended, then using the primary model to determine the tray borders will be inaccurate. You must use the mouth to determine how to shorten the tray borders.
You can use a perio probe on the patient to measure the distance from the crest of the ridge to the fold. Then measure your tray in that area to see if you are 2-3 mm shorter.
The patient's old denture probably has correct borders. Your tray borders should be shorter than the denture borders.
When checking to see if there is room for compound, students pull the lip too vigorously, moving the fold area so it looks like there is room between the tray border and the fold. Most students invariably make the tray borders too long. The lips should be relaxed as possible when determining border length.
Be especially sensitive about the frena. Sometimes they almost reach the crest of the ridge. Check all three (or four) in each arch. They should not touch the tray. (Don't forget the lingual frenum.)
If your tray borders are too long then the denture will be overextended. If your tray borders are too short then you can add more compound. My mottos are: "When in doubt, cut it out." or "When in doubt ask an instructor."
Make the tray borders 2mm thick. This will reduce the flexibility and make the border molding easier. The compound can now sit on the thickened tray border. The result will be an impression that has an optimum denture border width.
If you have a torus then relieve the tray in that area. Otherwise you may have a pressure spot. It is also important to relieve where there is mobile tissue. There will be less distortion. A lot of mobile tissue may need surgery.
The tray may (meaning, it is permissible to) be overextended posteriorly. On the maxilla, this will ensure that you will have the hamular notch on the master cast. A long cast does not mean the denture will be too long. When you do the posterior palatal seal you will determine the proper length of the denture.
The tray borders should be smooth and rounded. Don't aggravate the patient by causing them pain when the tray is inserted.
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©1999 by Julius Rosen, D.D.S.