Double (telescopic) crowns are connectors for retaining fixed/removable appliances. Since they were first fabricated and placed in Germany, they are often referred to as "German crowns".
The removable section can be a coverdenture, partial denture or removable/operator-removable telescopic bridge. As with a slide attachment, telescopic crowns can also be used in cases where sectioned bridgework is required to compensate for non-parallel abutments.
The various types of double crownwork for use in the maxilla
The principle of double crowns involves achieving precision of fit between an outer coping cast into, soldered, welded or adhered to the removable section and a coping screw-retained on an abutment or cemented to a natural abutment, which are congruent in form. Electroformed double crowns require a thin gold outer coping to be electroformed and adhered passively into the denture. Inner copings are usually fabricated singly but may also be splinted ("primary splinting"). The removable section splints the abutments to a certain degree ("secondary splinting") yet abutment teeth bearing telescopic units often exhibit increased physiological mobility.
4 inner copings milled parallel for a hybrid prosthesis supported on teeth and implants
Whereas classic double crowns were usually cast with the same metal alloy, nowadays combinations of different materials are possible, e.g. metal-free, zirconia inner copings and high-strength plastic outer copings. This rules out metal corrosion (gap corrosion).
If the circumferential walls of the copings are milled/fabricated cylindrically (only possible in theory) in order to achieve "parallel" fit, we speak of "telescopic crowns" and those with tapering walls are known as "conical crowns". A "ring telescope crown" features an outer crown with open occlusal aspect.
After taking a precision impression (digitally or with impression material), a model is cast from it and the inner copings fabricated on the model. The three-dimensional relationship of the copings to each other and the oral structures is recorded with a second ("pick-up" or "fixation") impression. This impression is used for casting the master model and the removable denture section fabricated on it. To avoid non-parallelism, all copings must be in place during cementation.
Normally double crowns are placed on at least 2 abutments per jaw (one per quadrant). The more double crowns are placed, the better the exerted loads are distributed and, should circumstances dictate, the more abutments can be extracted at a later stage without compromising the splinting effect or denture. To achieve at least linear (such as on right and left canines) or, preferably, triangular or four-cornered support (also on distal posterior teeth or implants placed to increase the number of abutments), the abutments must be spread as widely as possible around the arch of the jaw.
The advantages of double crowns include uniform load distribution across the abutments, straightforward handling for the patient, concealed connectors, firm and lasting retention along with the option of involving as many abutments (teeth and implants) as possible. Their drawbacks are the increased effort required for fabrication and higher costs, increased space requirement (loss of tooth structure due to preparation), unpleasant appearance of the inner copings (after removing the denture) and occasionally odours due to colonisation of bacteria in the gap between the copings ("moist chamber").
Double crowns are retained by means of cone-fit, adhesion, sliding friction and suction-retention (capillary effect of the gap). Whereas conical crowns only achieve form fit and full retention after reaching their end position, telescopic crowns do so as soon as the frictive surfaces first touch. If metallic double crowns are not separated for a longer period, are exposed to very heavy loading (bruxism) or if there are burrs or undercuts in the gap between the copings, the metal surfaces of the inner and outer copings may "cold weld" together and/or jam.
After fitting, double crowns usually have to be tried in repeatedly for a few days before they can be placed/removed smoothly. This involves displacing the teeth and trimming microscopically rough areas to eliminate minimal inaccuracies of fit. If dentures with double crowns are not worn for a longer period of time such as during an illness, they may become impossible to place due to, e.g. the teeth drifting so that the abutments are no longer parallel.
In certain types of double crowns the occlusal space between the two copings compensates for mucosal resilience, the denture may sink in to compensate for the material worn off the contacting surfaces of conical copings. Friction lost by double crowns can be restored by retrofitting or activating existing friction units such as friction-fit pins or press-stud attachments. This is the basic principle of special double crown systems such as the non-frictive ("clearance fit") "Marburg double crown".