Special Tray And Model

Question 1

Spacer

Answer 1

spacer adaptation is important in the manufacture of special tray- dental impression materials have an optimum thickness to use correctly and use of spacer will provide this,. In addition to optimum impression material the amount of spacer dictates whether the impression is to be a mucostatic or mucocompressive impression.

Question 2

Tissue stops

Answer 2

allow the tray to locate and maintain the desired thickness of impression material- whilst not necessary used in all cases these can
a)Stop the tray from condensing and touching tissues altering the impression surface
b)Allow the clinician to adequately assess the tray for under and over extension

Question 3

Handles

Answer 3

help for loading/placement and orientation of the tray in the mouth- however can then be a source of inaccuracies, especially for complete dentures. Distortion of the lip would then distort the functional sulcus and additionally encourage the clinician to adapt a poor impression taking technique leading to an over extended impression

Question 4

Secondary cast

Answer 4

The clinicians goal when taking a master impression is to record the oral structure as accurately as possible using the correct technique
•Over extended impression the denture will contact tissues that are possibly moveable this will result in damage/trauma and an unstable denture.
•Under extended impression- the impression material is not taken to the full depth of the sulcus. However, if the impression material extends to the full extent of the sulcus but is not supported by the tray during casting it is possible for the weight of the stone or the turning over of the impression to the base of the model can distort the model (bottom left demonstrates possible distortion through lack of tray support).

Question 5

Retention in a model

Answer 5

However it is imperative to construct the model correctly, removing the border moulding of the denture (bottom right figure) removes the functional width of the impression and therefore can ultimately result in a lack of retention in the denture

Question 6

Mucostatic/Nonpressure minimal pressure technique all denture bearing areas are recorded in a none displaced state.

Answer 6

Mucostatic/Nonpressure minimal pressure technique all denture bearing areas are recorded in a none displaced state. This is undertaken by provided constant relief or using a fluid impression material, as none moveable tissues only are recorded the denture relies on interfacial surface tension for retention- varying levels of residual ridge resorption have been recorded with this technique (Smith et al,. 1999). This type of impression technique is commonly used for the primary impression however the issue with secondary models in this technique has been included.

Question 7

Selective pressure

Answer 7

Selective pressure- often used with ‘problematic’ tissue areas such as a
1.Displaceable flabby anterior maxillary ridge
Border moulding is undertaken then a selected material such as ZOE or PVS is used, the area where there is an issue is marked and then removed from the impression (at this point the tray should be no longer retentive and a differing material is then syringed into the area and the tray reinserted- on setting it is important to see whether the peripheral seal has been re-established

Question 8

Which stone should be used and why

Answer 8

removable partial dentures type IV stone should be used, this is due to the abrasion that the models may undergo. Even with careful use of the surveyor it is possible for the model to become damaged or chipped. Accurate use of the materials should be observed, whilst adding of more water is unlikely to cause volumetric changes it will mean the model is ‘weaker’

Question 9

CAD benefits

Answer 9

The benefits of scanning impressions include speed, efficiency, the ability to store the resulting information and ease of transferring the information between clinic and laboratory. Several studies have examined the accuracy of the impressions from single unit to full arch scans. However there are few from looking at the patients/clinician acceptance.

Question 10

Patients preferences on CAD

Answer 10

The above author found that
•digital was more efficient from a clinical standpoint
•Patients prefer digital impressions for a multitude of reasons including but not limited to discomfort, time and taste