Describe one method that overcomes the problems that may occur when converting wax saddles into acrylic with metal frameworks.

Describe one method that overcomes the problems that may occur when converting wax saddles into acrylic with metal frameworks.

1. If excessive pressure is created in the mould during packing procedures there is the danger of the resin being forced (creeping) between the plate and the cast, disturbing the correct relationship.

2. This may also occur if the investment plaster used to secure the metal framework to the model fractures during boiling out procedures, damaging the relationship of the metalwork to the model, or the vertical dimension, or both and risking possible damage / distortion to the metal framework.

3. In order to avoid the risks of packing the dough too stiff, there is always the danger of contraction porosity if packing too soft, or from under-packing to avoid the above listed dangers.

4. Sodium Alginate may lay against the metalwork forming a 'skin' and become folded into the acrylic dough, appearing as white plaster looking debris in the finished resin; usually around metal-acrylic junctions.

5. Stress fractures around the metal insert areas are another possible danger, and the inclusion of plaster particles has to be guarded against.

6. These, plus all the faults associated with the conversion of wax to acrylic in all acrylic dentures means that there is a high chance of problems occurring during this procedure.

7. In an attempt to prevent these problems, modifications may be made to: (a) the material used as the investment. (b) the flasking procedure

8. An alternative technique which does not use plaster as the investment medium is sometimes referred to as ' The Pour and Cure Technique.

   • With this technique the embedding material may be a reversible hydrocolloid. (Agar-agar) The flask used consists of a square metal base, D shaped body cylinder with two large holes, a metal lid with a hollow circular extension, a box clamp ( U shaped) and two bungs.
   • The denture is sealed to the duplicate model, which is trimmed with a slight outward taper and soaked in water for 10 minutes. The trial denture and model is sealed to the base and the flask assembled around with the posterior body shaped top of the denture facing the posterior flask holes and the holes filled with the bungs.
   • Irreversible hydrocolloid when ready (56°C) is poured through the lid until the flask is filled and allowed to gel. The flask is taken apart and the denture and model is removed from the mould. All traces of wax are then boiled from the model, teeth and metal base. The bungs are removed and two sprue channels are cut to the posterior borders of the denture. These sprue channels should be approximately 10 mm in diameter, tapering to 3-4mm diameter at the denture border. They can be prepared prior to pouring the hydrocolloid by the use of sticks of 'sticky wax' attached to the posterior aspect of the denture
   • The acrylic teeth are re-assembled in the mould, the model coated with sodium alginate and the metal base relocated on the model. These are then re-positioned in the mould and the flask re-assembled. Special 'Pour' type auto-polymerising resins are available for this technique. This is mixed to a fluid consistency and poured into the mould via the sprue channels. If needed air vents may be utilised. The flask is placed in an autoclave under pressure of 15 P.S.1. for 30-40 minutes, depending on manufacturer’s instructions. De-flasking consists of removing the colloidal embedding material from the denture and model.
     • Finishing of the denture base acrylic is usually easier and quicker than the traditional 2-part mould technique, as there is no flash, only two sprue channels which must be removed.