Telescope prosthesis or double crowns
Telescope prosthesis or double crowns are a proven option for the prosthetic treatment of dramatically reduced dentition. However, the production of such a prosthesis places higher demands on the dental practitioner and the technical laboratory involved.
A telescopic crown always comprises two parts:
- The primary crown, or coping, which is permanently fixed in the mouth to anchor teeth, and is preferably made from a suitable gold alloy; and
- The mounted, removable telescopic crown or secondary crown attached to the prosthesis and made of the same alloy material.
History of telescopes
Telescope procedure was first described in 1886 by R Walter Starr and, later, by Goslee and Peeso. Telescope or double crowns thus have their origins in America and not Germany, as the common term ‘German Crown’ might suggest. In Germany, Häupl was one of the pioneers (1929 onwards), as were his pupils Böttger and Rehm, who laid the foundations for the telescope method that remains almost unchanged to this very day. When we talk of telescopic crowns, we generally mean the parallel telescopic crown. With the parallel telescopic crown, the inner telescopic element of each double crown has parallel surfaces, and these are not only parallel to one another but also parallel to the axis of attachment for the remaining telescopic elements. The locking of the inner and outer telescopic crowns provides a connection that can be released in one direction or axis only, by the dentist and the patient themselves.
Double crowns can be used in the following situations:
- where there is a strongly depleted dentition
- uncertain prognosis of individual teeth in a periodontally damaged jaw (existing bone depletion, increased loosening of the anchor teeth)
- with a suboptimal distribution of the remaining teeth
- for the retention of removable bridges
- with implant techniques
When using anchor teeth for this precision mechanical treatment, it is irrelevant whether these are natural teeth or implants, as telescopic attachments do not exclude the use of implants. Rather, the opposite. Precisely through implantology is it possible to indicate a precision mechanical replacement, which could not have been planned in advance due to the absence of natural abutments. The almost universal applicability is characteristic for this anchoring system. Telescopic crowns can be applied as clasp- free connecting elements with purely periodontally and periodontally-gingivally supported partial prostheses.
Advantages of the telescopic system:
- a predominantly axial loading of the anchor teeth leading to a favourable distribution of force
- protection of the anchor teeth from decay
- the option of primary splinting for the securing and fixing of loose teeth
- integrated tilt-avoidance
- a straightforward ability to extend the prosthesis
- the aesthetic advantage of invisible clasps
- the beneficial and straightforward treatment and control of the periodontium and the internal coping.
Disadvantages of the telescopic system:
- requires a high technical effort
- possible overloading of the anchor teeth (possible fracture of the enamel)
- can only be coated/veneered with composites
How double crowns work
The patient expects the prosthesis to be easily inserted and removed. At the same time, the prosthesis has to be sufficiently attached so that it cannot be displaced by motion during speech and eating. In order for these conditions to be met, you need certain physical pre-conditions. These are explained below. In order to achieve a hold between the inner and outer crown, these have to fit in a particular manner.
There are three different types for telescopes fit:
- a clearance fit, in which there is still a small bit of give/play
- the medium fit, in which there is a large tolerance or over-sizing before the joining of the components (which gives totally useless telescopic crowns)
- the pressure fit, where the components are tight and interact such that friction is created during fitting
On the principle that both crown pieces have to join exactly and without obstruction, parallel telescopic crowns are always pressure fittings, which is why telescopic crowns are preferably made from precious alloys because of their high elasticity.
Telescopes – The importance of friction
The inner and outer telescopes are joined together by friction. Stated simply, the friction is due to the interaction between the surface layers of the inner and outer telescope. The binding forces of the telescopic crowns are therefore a consequence of this friction. Friction in telescopic crowns is a value that is difficult to measure. It is principally dependent on the technical construction of the crown, which is influenced by the following factors:
- the number of the planned telescopes
- the length of the friction surfaces of the individual tooth and also the sum of all available telescopes
- the placement of the friction surfaces
- relative to one another. Only opposing facing parallel surfaces can provide the required friction of telescopes
- the elasticity of the materials used, which is why gold alloys are generally used
- the quality of the work