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The telescope procedure was firstdescribed in 1886 by R Walter Starrand, later, by Goslee and Peeso.

Telescope or double crowns thus havetheir origins in America and not Germany,as the common term ‘German Crown’might suggest. In Germany, Häupl was oneof the pioneers (1929 onwards), as werehis pupils Böttger and Rehm, who laid thefoundations for the telescope method thatremains almost unchanged to this very day.

When we talk of telescopic crowns, wegenerally mean the parallel telescopiccrown. With the parallel telescopic crown,the inner telescopic element of eachdouble crown has parallel surfaces, andthese are not only parallel to one anotherbut also parallel to the axis of attachmentfor the remaining telescopic elements. Thelocking of the inner and outer telescopiccrowns provides a connection that can bereleased in one direction or axis only, bythe dentist and the patient themselves.

IndicationsDouble crowns can be used in thefollowing situations:■ where there is a strongly

depleted dentition■ uncertain prognosis of individual teeth

in a periodontally damaged jaw (existing bone depletion, increasedloosening of the anchor teeth)

■ with a suboptimal distribution of theremaining teeth

■ for the retention of removable bridges■ with implant techniques

When using anchor teeth for this precisionmechanical treatment, it is irrelevantwhether these are natural teeth orimplants, as telescopic attachments do notexclude the use of implants. Rather, theopposite. Precisely through implantology isit possible to indicate a precisionmechanical replacement, which could nothave been planned in advance due to theabsence of natural abutments.

■ Precision dental prosthetics with highly

Telescope prosthesis ordouble crowns are a provenoption for the prosthetictreatment of dramaticallyreduced dentition (fewerteeth). However, theproduction of such aprosthesis places higherdemands on the dentalpractitioner and the technicallaboratory involved.

A telescopic crown alwayscomprises two parts: 1. The primary crown, or

coping, which ispermanently fixed in themouth to anchor teeth, andis preferably made from asuitable gold alloy; and

2. The mounted, removabletelescopic crown orsecondary crown attachedto the prosthesis and madeof the same alloy material.

by Ulrich Heker, Master Dental Technician & Chris Thomas, PhD (Molecular Biology)

Single telescope with retention, showingprimary and secondary telescope

Copings in the upper jaw polished andready for modelling the outer crowns

The almost universal applicability ischaracteristic for this anchoring system.Telescopic crowns can be applied as clasp-free connecting elements with purelyperiodontally and periodontally-gingivallysupported partial prostheses.

Telescope during insertion

Complete work for the upper jawseen from below

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The pros and cons of double crownsAdvantages of the telescopic system:■ a predominantly axial loading of the

anchor teeth leading to a favourabledistribution of force

■ protection of the anchor teeth from decay

■ the option of primary splinting for thesecuring 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 theperiodontium and the internal coping

Disadvantages of the telescopic system:■ requires a high technical effort■ correspondingly higher costs■ oversizing of the secondary crowns or

tooth surface loss in preparation■ possible overloading of the anchor teeth

(possible fracture of the enamel)■ can only be coated/veneered with

composites

How double crowns workPhysical principlesThe patient expects the prosthesis to beeasily inserted and removed. At the sametime, the prosthesis has to be sufficientlyattached so that it cannot be displaced bymotion during speech and eating. In orderfor these conditions to be met, you needcertain physical pre-conditions. These areexplained below. In order to achieve a holdbetween the inner and outer crown, thesehave to fit in a particular manner. There are three different types of fit:1. a clearance fit, in which there is still a

small bit of give/play 2. the medium fit, in which there is a large

tolerance or over-sizing before thejoining of the components (which givestotally useless telescopic crowns)

3. the pressure fit, where the componentsare tight and interact such that frictionis created during fitting

On the principle that both crown pieceshave to join exactly and withoutobstruction, parallel telescopic crowns arealways pressure fittings, which is whytelescopic crowns are preferably madefrom precious alloys because of their highelasticity.

The importance of frictionThe inner and outer telescopes are joinedtogether by friction. Stated simply, thefriction is due to the interaction betweenthe surface layers of the inner and outertelescope. The binding forces of thetelescopic crowns are therefore aconsequence of this friction. Friction intelescopic crowns is a value that is difficultto measure. It is principally dependent onthe technical construction of the crown,which is influenced by the followingfactors:■ the number of the planned telescopic

crowns■ the length of the friction surfaces of the

individual tooth and also the sum of allavailable telescopes

■ the placement of the friction surfacesrelative to one another. Only opposingfacing parallel surfaces can provide therequired friction

■ the elasticity of the materials used,which is why gold alloys are generallyused

■ the quality of the work

A prosthesis has to be prepared in such away that the patient can insert it withoutdifficulty. Additionally, it must provide thefeeling of fitting snugly and firmly. Thedenture should also be removable withoutdifficulty whilst not loosening at the wrongmoment or due to sticky foods. Thecriteria must remain valid over a longerperiod of wear.

Note: The force required for removal ofthe prosthesis: 250–300 P is regarded asacceptable to patients. The maximal forcerequired for removal should not exceed650 P, as with higher levels the patientcan often not remove the prosthesis.

Achieving the correct friction of theindividual telescope components is onlypossible with considerable experience andskill as well as a close collaboration withthe dental practitioner.

In the dental surgery, planning and preparation When producing a telescope prosthesis, afew fundamental considerations have to bemade with regard to the arrangement ofthe anchor teeth.

There is a large difference in theproduction of multi-unit bridges, whichrequire a common direction of insertionfrom the start. With crowns, one is lessconcerned with the original alignment ofthe supporting teeth as the primary crownsare applied individually and these aremilled to provide the parallel sides for acommon axis of insertion for thesecondary crown.

As telescopic crowns have multifacetedelements, care must be taken duringpreparation so that there is sufficient roomin the interdental area for effectively fourcrown wall strengths (2 x inner crown plus2 x external crown = circa 0.7–1 mminterdentally and also taking into

engineered connections - Part 2

Prepared teeth upper jaw

consideration the attachment direction). Asimilar situation exists for the occlusallyrequired space (primary component +external crown).

The secondary crowns attached to theprosthesis require a common alignmentaxis. The most important considerationsare therefore: How the preparation mustbe positioned in order to obtain anabutment preparation in which all theprimary components have two opposingparallel surfaces with circa 3.5 to 4 mmaxial length, which in turn must all beparallel to each other.

In addition, you need to consider theimportant aesthetic aspect in yourplanning. The inner crowns have to bemade as robust as possible without makingthe final product too large and chunky aftermilling. If the dental technician can createthe friction surfaces as close as possible tothe tooth base, which can be especiallyproblematic with anterior telescopiccrowns, then it is subsequently easier toalso create a graceful outer telescope.It has been proven beneficial to undertakethe work in two phases.

After the first planning sitting, a plastermodel of the immediate situation isproduced in the laboratory using a

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parallelometer, thus enabling planning tobe carried out at an earlier stage.

The universal application of the telescopiccrown technique, as well as other precisionmechanical attachments such asattachments and bolt or key slides, isfounded on being able to support theprosthesis on demand.

Manufacturing primary crowns After taking impressions using customtrays, the optimal attachmentalignment/direction is established on thecast, for milling using a parallelometer.

The primary crowns are then modelled inwax as with conventional crowns and aresubsequently milled parallel to one anotherin the mill (whilst still in wax!)

Before embedding, spurs are attached tothe wax crowns, which prevent the primaryelements from disengaging from thesubsequent over-impression.

Manufacturing secondary crownsAfter the production (modelling andcasting) of the primary crowns and makingof the master model from the over-impression using a custom tray, thecopings are milled to be parallel to one

another (precision milling). To do this themilling socket with the mounted copingsare attached to a plate that is situated on aball joint. It is then again oriented until anoptimal common ‘telescoping direction/angle’ is found for all the primarycomponents.

The ball joint is then immobilised and allthe exterior surfaces of the primarycomponents are milled parallel to eachother using a special milling machine. It isparticularly at this junction that the dentaltechnician has to be highly skilled.The milling machine has a horizontallymoveable chuck head, similar to aparallelometer. In the subsequent fitting forthe primary component in the mouth ofthe patient, the practitioner tests thecomponent for accuracy and then fixes

Copings in situ

To see more images relating to this and other articles, visit:http://www.german-smile.info/combinedcare/combinedcare_02.htm

Measuring = determining of commondirection of insertion

Wax modelling

Wax milling of the primary crowns

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their relationship via an over-impression inan individual impression tray.

Back in the laboratory, the dentaltechnician prepares the synthetic stumps inthe impression. After casting withpreferably expansion-free plaster, themodel can be trimmed and is ready for thenext step – the milling of the inner parts.The primary elements are removed fromthe model using a so-called ‘spider’ andtransferred to a milling plate.

The parallel surfaces are milled and theremaining surfaces are polished to a mirrorfinish whilst constantly monitoring thestrength. After cleaning, the modelling ofthe external components can begin.

First, the total external surface of theinterior components is coated in a thinlayer of acrylic e.g. GC Pattern Resin®,which provides stability in modelling andalso accurately reproduces the structure ofthe exterior surface, which completelydisappears during burnout. The remainingstructure of the external crown is thencompleted in wax according torequirements (complete or partialveneering).

Once the external parts have beenfurnished with retentions, preferablymesially and distally (as assistance for latersoldering, lasering or gluing), the patternsare prepared and are embedded inreadiness for casting.

Here the accurate measure of specialliquid to investment is of real importance.The casting form is now ready and can beset in the oven. Preheating and casting aredone according to the alloy manufacturer’sinstructions. It is worth taking particularnote of the data sheets (for the product).

After removing the casting, the internalsurfaces of the external componentsshould preferably NOT be shot blasted. Instead, a short soak in Neacid or a similarsolution facilitates the removal of the

About Ulrich Heker Ulrich Heker is the owner-manager ofUlrich Heker Dental Laboratory founded in1996 with the strap line TEETH ‘R’ US. Asa qualified master craftsman (GermanMaster Dental Technician) since 1991, hehas over 26 years’ experience both at thebench and in running a successfulbusiness. Ulrich lives in Mülheim on theriver Ruhr and is an accomplished‘western-style’ rider in his spare time.Ulrich is fluent in English and can easily becontacted at:■ Ulrich Heker

D-45130 Essen Corneliastr. 17. Tel: +49201 797 955Email: Ulrich@Teethrus.deWeb: www.german-smile.info

About Dr Chris ThomasDr Chris Thomas is Director of MiltonContact Ltd, specialising in communicationin pictures, word and person.■ Dr Chris Thomas

Milton Contact Ltd 3 Hall End, Milton, Cambs CB24 6AQ Tel: +44 (0) 1223 440024,Email: chris@miltoncontact.comWeb: www.miltoncontact.co.uk

Ulrich Heker

remnants of the embedding matrix bysteaming. The inner surface should now beclean, smooth and free of micro-bubbles.The surfaces are prepared further usingcareful cleaning with a soft metal brush atthe lowest rotation speed.

Now the secondary crowns can be fittedon the primary crowns.

Note: In order to be able to better handlethe inner crowns, I attach a metal pin toeach internal component. In this way Iavoid damaging the inner crowns with thecrown pliers usually used. (Crowns with awall thickness of up to 0.2 mm do notoffer sufficient resistance for such pliers.)

The precise friction is subsequentlyadjusted using mirror finish fine polishesand (my tip) CONTEX® from Degudent.

Testing the friction strengthFriction can be tested in a fairly simplemanner by tapping the prosthesis firmlythree times, in the correct alignment,against a stable substrate such as the work surface.

Pattern modelling of the copings

Proofing of thickness

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List of literature1. Becker, H: Einflüsse des umgebenden

Mediums auf das Haftverhaltenteleskopierender Kronen. Zahnärztl Welt91, 54, (1982).

2. Becker, H: Wirkungsmechanismus derHaftung teleskopierender Kronen.Zahnärztl. Prax 34, 281 (1983).

3. Böttger, H: Das Teleskopsystem in derzahnärztlichen Prothetik. JA Barth, Leipzig 1961.

4. Diedrichs, G: Ist das Teleskopsystemnoch zeitgemäß? Zahnärztl. Welt 99, 78 (1990).

5. Goslee, H: Principles and Practice ofCrown and Bridge Work Dental Items ofInterest Publishing Co., New York 1923.

6. Hedegard, B: Die Mitarbeit desPatienten – ein Planungsfaktor.Zahnärztl. Welt, Ref.88, 680 (1979).

7. Jacoby, W, Gasser, F: NachträglicheHaltverbesserung von Teleskopkronen.Quintessenz 24, 59 (1973).

8. Jüde, HD, Kühl, W, Roßbach, A:Einführung in die zahnärztliche Prothetik.5.Aufl. Deutscher Ärzte- Verlag, Köln 1996.

9. Kammertöns, H: Haftreibungsprüfung anTeleskop- und Konuskronenarbeiten.Quintessenz Zahntech 14, 11 (1988).

10. Krämer, A, Weber, H:Präzisionselemente in der TeilprothetikTeleskopierende Systeme. Zahnärztl Mitt 80, 2328 (1990).

11. Mack, H: Die teleskopierendeVerankerung in der Teilprothetik.Quintessenz Zahntech 9, 17 (1983).

12. Peeso, FA: Crown and Bridgework.Henry Kimpton, London 1924.

13. Schreiber, S: Die Verankerung vonTeilprothesen mit Teleskopkronen. DtschZahnärztl Z 14, 983-988 (1959).

14. Schwanewede von, H, Anderseck, E:Proteza Teleskopowa – DieTeleskopprothese im stark reduziertenLückengebiß. Prot Stom 35, 166 (1985).

15. Stüttgen, U, Hupfauf, L: Kombiniertfestsitzend – abnehmbarer Zahnersatz. In: Horch, H-H, Hupfauf, L, Ketterl, W,Schmuth,G, (Hrsg): Praxis derZahnheilkunde 6 (Teilprothesen), 2. Aufl.Urban & Schwarzenberg, München –Wien – Baltimore 1988, S.163.

16. Wupper, H: Zur Biomechanikverschiedener Verankerungssysteme –Grundsätze zur Indikation vonGeschieben, Stegen und Teleskopen.Zahnärztl Welt 95, 36 (1986).

■ The friction is inadequate if, after thefirst tap, all the inner crowns protrudevisibly from the external crowns, or evendrop out completely.

■ Friction is optimal if the inner crownshave protruded visibly by the third tapbut have not emerged fully from theexterior crowns.

This straightforward procedure allows eventhe less, well-experienced user to gain arelatively confident insight on the quality offriction prior to the incorporation of atelescopic unit, especially as no furthermeasuring equipment or tools arerequired. Now the crowns are ready to becombined with the casting of the model.The metal pins are removed under heatingfrom the inner components, the primarycrowns are replaced on the model, andeverything is checked again thoroughly.The crowns are now ready to be combinedwith the metal frame. As mentioned above,lasering, gluing and soldering are all validtechniques. With soldering, one shouldstrive for a uniform heating of the solderingblock. If you have already checked theaccuracy of the tooth placement in thepatient with a wax-up, this is transferred tothe metal frame and the prosthesis isassembled for a full test fitting.

CompletionAfter a completed and hopefully successfulfitting, the outer surfaces of the crowns arebonded and coated with composite andthe prosthesis is completed as usual.

Tip: The transition between inner andouter crowns should always be sealedwith wax during this process. Otherwise,possible seepage of the synthetic materialcan bind the two crowns irreversibly.

A final friction test is advisable.

From our experience, friction diminishesover the first days after client use;therefore, ensure friction is not set tooloosely. The probability that a patient willhave kept all anchor teeth 10 years afterinsertion is 80%. The double crownsystem is a versatile and successful way ofachieving the long-term restoration of thepartially edentulous jaw. Insertion andremoval of the appliance and routine oralhygiene are easy to perform, even forpatients with limited manual dexterity. As afull-arch reconstruction, the double crowndenture system enables easy adjustment,modification, and relining with low follow-up costs.

The patient also plays a vital role in thelongevity of their prosthesis. A high qualityprecision dental replacement can only fulfilits task if treated appropriately. The patient,therefore, can also contribute to ensure thesuccess of such quality work by:■ ensuring a commitment towards oral

and dental hygiene■ regular checking of the prosthesis for

intactness, freedom from stresses andadaptation by the gingival surfaces

■ taking great care when removing andinserting the prosthesis

■ ensuring proper care and appropriatehandling of the prosthesis afterinstruction by the practitioner

■ following a short ‘instruction manual’given to the patient by the practitionerfor care of the new custom madeprosthesis.

By following these guidelines the patientshould have many years of a care-free lifewith their telescope double crowndentures.

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Upper metal Cr Co framework

Lower telescopic work from below