Introduction
A double crown is a pair of crowns that fit one inside the other. The primary (inner) coping is cemented on the abutment; the secondary (outer) crown is built into the removable denture and slides over it. They serve both partially edentulous and nearly edentulous patients.
Telescopic and conical crowns
Retention must be just right: too strong, and removal overloads the periodontium (or implant); too weak, and the denture loosens. A resilient double crown leaves a small space over the occlusal surface of the coping so the denture can sink slightly before the crowns engage.
Indications
- Abutments that are tilted or not parallel: the copings make them parallel.
- Few remaining teeth with reduced periodontal support: the double crowns splint them through the denture.
- Patients who need easy cleaning: the copings are smooth and accessible when the denture is out.
- Strategic planning: if an abutment is lost, the denture is easily extended.
Milling technique
Double crowns, guide planes and attachment shoulders are shaped on a milling machine (a surveyor with a precision handpiece, mechanical or electronic). It cuts surfaces at an exact angle to the path of insertion: 0° for telescopes, the chosen taper for cones.
- Primary copings are waxed and milled in wax to the required parallelism or taper.
- They are cast and milled again in metal with tungsten-carbide cutters, then polished.
- Secondary crowns are made over them, by casting, galvanoforming or CAD/CAM.
- The secondary crowns are joined to the denture framework.
A milling machine is also used to check parallelism of preparations, contour wax patterns for metal crowns and design the partial-denture framework on the cast.