Sunday, 11 March 2012

How to make a special tray

Upper spaced tray
1.      Make sure the cast is trimmed close to the sulcus
2.      Design the dimension and form of the tray drawing a margin liner around the model approximately one to two mm short of the deepest part of the sulcus
3.      Soak the cast in the water for some minutes so that the wax will not stick to the cast
4.      Heat, adapt and cut the wax onto the model following the prearranged guide marks
5.      A second layer of wax has to be applied to obtain a 2 to 4mm of thickness
6Using a blade occlusal stops has to be made  to provide stability
7.      Adapt the light-cured over the wax space
8.       Press the palate down first after the ridges and   sulcus and carefully around the remaining teeth
9.       Remove the light-cured acrylic excess with a   knife
10.       Using the remaining light-cure produce a straight handle due existence of anterior teeth
11.       Fixed the cable on the anterior part of the tray
12.       Make holes around the tray with carbide bur to allow impression material’s retention
13.       Leave the tray 5 minutes on the light curing box
14.       Remove the tray from the model
15.       Remove all the wax remaining after curing
16.   Trim the sharp edges and irregularities
 Apply varnish on the tray and until to set.

Lower close fitting tray
1.      Follow the same procedures describe above sequence 1-2-3 and 4
2.      For the close fitting apply only one layer of wax
3.      For the close fitting tray is not necessary to produce occlusal stops
4.      Adapt the light-cured over the wax space pressing around the ridges, buccal and lingual sulcus
Remove the light-cured acrylic excess
5.      Remove the light-cured acrylic excess with a  knife
6.      Using the remaining light-cure to produce stub style handle design for edentulous patients
7.      Leave the tray 5 minutes on the light curing box
8.      Remove the tray from the model
9.      Remove all the wax remaining after curing the tray
10.      Trim the sharp edges and any irregularities
Apply varnish on the tray and wait until sets with a  knife around the lingual and buccal sulcus

How a poorly designed and/or constructed tray can have a negative effect on the outcome of the master cast.

The custom made impression trays are designed by the dental technician for an individual patient, they are constructed based on the preliminary cast (first impression) and is used no more than one time to take the secondary impression from the patient being discarded.

The material most common used to produce the special trays is the light-cured acrylic due to its price, ease manipulation to be adapted onto the model and working time. There are many advantages to use custom trays:
·         The distortion of tissues are controlled
·         Control over the flow of the impression material
·         Great support of the impression material
·         Minimize patient’s discomfort fitting
·         Save impression material
·         Reduced distortion during pouring of model
·         Used for muco-static and muco-compressive impressions

Basic principles are very important to be followed when custom trays are constructed. Custom trays need to be rigid enough to support the impression material, some times it could be too flexible, then the secondary impression will not be accurate due the pressure applied on the tray forcing it to bend causing distortion on the impression to be taken.
The stability of the tray can have a negative effect on the impression taken due to the thickness of the light cure used when constructed. The exact thickness must beapproximately 2mm, and 1mm short to the mucobuccal fold to allow for border moulding. Sharp and rough edges have to be trimmed appropriately for the reason that
may irritate the patient.

The impression tray made with light-cured have to become rigid when goes into the light-cured box. The dental technician as well has to make the tray no too thin, especially where the handle is positioned because it probably will break due the pressure applied on this region.

The custom tray cannot be too large, the tissues located around the edges of the impression will be affected being distorted on the impression, or if the tray is too small the tissues will not receive support, the full depth and width of the anatomical sulcus will certainly not be obtained.

Some special trays are required to be made with holes; it will be requested by the dentist if necessary depending the type of impression taken and the material to be used on. These holes have to be made distributed all over the tray being not too big and not to small, the impression material before set will undergo pressure so parts of it will flow out side these holes. These holes will stop the impression to be distorted then the secondary cast will be more accurate when poured.

The design from the handles is very important when a custom tray is made. The dental technician has to be aware about these 3 basic designs:
Stepped: The handle can protrude from the mouth
Without distorting the lips
Straight: Used for fully dentate patients or where
anterior teeth are standing
Stub: Used for edentulous patients, who require some
lip support                                                                                                                                               
There are many reasons that a handle’s position on the custom tray could affect directly the impression taken. The impression taken from mandible or maxilla requires different support made by dentist’s hands also the condition of the patient’s dentition, oral tissues and ridges may affect the type of the handle to be constructed. Using the right handle’s shape will be possible to achieve:

·         Better support of the impression material
·         Controlled distortion of tissues
·         Reduce the discomfort for the patient
·         Obtain accurate impression from denture related surfaces, alveolar ridges and mucosal tissues
·         Reduce the impression material used.

The handle should be positioned in the anterior so that it does not interfere with placement of tray or border moulding procedures.
The final outcome cast when the handles are not in the right position will result in a poor representation from the tooth surface, ridges and soft tissues

When a custom made tray is constructed is essential to make stops, however this design does not apply for close fitting trays. The stops are used to hold the inner surface of the tray out of contact with the patient’s tissue when the impression is taken.

The second impression taken using a custom made tray without stops possibly will display problems on the oral impression. Probably, the final outcome for the second cast when poured will be an inadequate representation of the patient’s dentition, as a result the dental technician will not be able to construct indirect dental restorations or removal dental appliances with acceptable accuracy.

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