The primary crowns are then modelled in wax as with conventional crowns and are subsequently milled parallel to one another in the mill (whilst still in wax!) Before embedding, spurs are attached to the wax crowns, which prevent the primary elements from disengaging from the subsequent over-impression.
Telescopes manufacturing secondary crowns
After the production (modelling and casting) of the primary crowns and making of the master model from the over- impression using a custom tray, the copings are milled to be parallel to one another (precision milling). To do this the milling socket with the mounted copings are attached to a plate that is situated on a ball joint. It is then again oriented until an optimal common ‘telescoping direction/ angle’ is found for all the primary components.
The ball joint is then immobilized and all the exterior surfaces of the primary components are milled parallel to each other using a special milling machine. It is particularly at this junction that the dental technician has to be highly skilled. The milling machine has a horizontally moveable chuck head, similar to a parallelometer. In the subsequent fitting for the primary component in the mouth of the patient, the practitioner tests the component for accuracy and then fixes their relationship via an over-impression in an individual impression tray. Back in the laboratory, the dental technician prepares the synthetic stumps in the impression. After casting with preferably expansion-free plaster, the model can be trimmed and is ready for the next step:
The primary elements are removed from the model using a so-called ‘spider’ and transferred to a milling plate. The parallel surfaces are milled and the remaining surfaces are polished to a mirror finish whilst constantly monitoring the strength. After cleaning, the modelling of the external components can begin. First, the total external surface of the interior components is coated in a thin layer of acrylic e. g. GC Pattern Resin®, which provides stability in modelling and also accurately reproduces the structure of the exterior surface, which completely disappears during burnout.
The remaining structure of the external crown is then completed in wax according to requirements (complete or partial shading). Once the external parts have been furnished with retentions, preferably mesially and distally (as assistance for later soldering, lasering or gluing), the patterns are prepared and are embedded in readiness for casting.
Note: In order to be able to better handle the inner crowns, I attach a metal pin to each inner crown with a little Pattern Resin. In this way I avoid damaging the inner crowns with the crown pliers usually used. (Crowns with a wall thickness of up to 0. 2 mm do not offer sufficient resistance for such pliers.) The precise friction is subsequently adjusted using mirror finish fine polishes and (my tip) CONTEX® from Degudent.
Testing the friction strength of telescopes
Friction can be tested in a fairly simple manner by tapping the prosthesis firmly three times, in the correct alignment, against a stable substrate such as the work surface.
The friction is inadequate if, after the first tap, all the inner crowns protrude visibly from the external crowns, or even drop out completely. Friction is optimal if the inner crowns have protruded visibly by the third tap but have not emerged fully from the exterior crowns. This straightforward procedure allows even the less, well-experienced user to gain a relatively confident insight on the quality of friction prior to the incorporation of a telescopic unit, especially as no further measuring equipment or tools are required. Now the crowns are ready to be combined with the casting of the model. The metal pins are removed under heating from the inner components, the primary crowns are replaced on the model, and everything is checked again thoroughly. The crowns are now ready to be combined with the metal frame. As mentioned above, lasering, gluing and soldering are all valid techniques. With soldering, one should strive for a uniform heating of the soldering block. If you have already checked the accuracy of the tooth placement in the patient with a wax-up, this is transferred to the metal frame and the prosthesis is assembled for a full test fitting.
A final friction test of telescopes is advisable.
From our experience, friction diminishes over the first days after client use; therefore, ensure friction is not set too loosely. The probability that a patient will have kept all anchor teeth 10 years after insertion is 80%.
The double crown system is a versatile and successful way of achieving the long-term restoration of the partially edentulous jaw. Insertion and removal of the appliance and routine oral hygiene are easy to perform, even for patients with limited manual dexterity. As a full-arch reconstruction, the double crown denture system enables easy adjustment, modification, and relining with low follow- up costs. The patient also plays a vital role in the longevity of their prosthesis.