• For the retention ofremovable bridges.

The almost universalapplicability is characteristicof this anchoring system.Telescopic crowns can beapplied as clasp-freeconnecting elements withpurely periodontally andperiodontally-gingivallysupported partial prostheses.

The pros and cons of double crownsThe advantages of thetelescopic system include:• A predominantly axialloading of the pillars, leadingto a favourable distribution offorce and thus helping toprotect the anchor teeth fromdecay • The option of primarysplinting for securing andfixing loose teeth• Integrated tilt-avoidance • A straightforward ability toextend the prosthesis even upto a full denture, with adistinct aesthetic advantagebecause no clasps are used.The treatment allows forcontrol of the periodontium

CLINICAL

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Telescopic prostheses (alsoknown as double crowns) area proven option for prosthetictreatment where there isdramatically reduceddentition (and in some casesmight even serve as analternative to implants).However, the production ofsuch a prosthesis demandsconsiderable technical skillsfrom both the dentist and thedental technician; of equalimportance is goodcommunication between thedental team.

The telescopic attachmentconsists of two parts: 1. The primary (inner) crown,or coping, which ispermanently fixed to theanchor teeth and is preferablymade out of a preciousmetal – a high-gold alloy2. The secondary (outer)crown incorporated into theprosthesis, made out of thesame alloy.

The usual type of doublecrown system, next to theconus type, is the paralleltelescopic crown.

They are so-named because all of the surfaces ofthe primary (inner) and thesecondary (outer) telescopeare not only parallel to oneanother but also parallel tothe axis of each incorporatedtooth.

However, in the caseof a conus telescopic systema 4º angle of both telescopesto the axis of the tooth isaimed for, based upon by theexact preparation carried outby the dentist.

IndicationsDouble crowns can be used inthe following situations: • Where there is a stronglydepleted dentition withuncertain prognosis ofindividual teeth in aperiodontally damaged jaw(existing bone depletion,increased loosening of theanchor teeth) with a sub-optimal distribution of theremaining teeth

Ulrich Heker is a MasterDental Technician and theowner and manager ofTEETH‘R’US in Essen,Germany.

Verena Tunn-Salihoglu is apractising dentist inAberdeen, Scotland.

In the first of twoarticles, Ulrich Hekerand Verena Tunn-Salihoglu discuss thetechnical side oftelescopic dentures,focusing on theclinical aspects of thetreatment in nextmonth’s instalment

| 21 October 2010 | Irish Dentist

and the internal coping• A cost-effective alternativeto implants.

Disadvantages of thetelescopic system include:• It requires a high level oftechnical expertise• There are correspondinglyhigher costs• Over-sizing of thesecondary crowns can occurif the pile has not beenefficiently reduced • Compared to a porcelain-fused-to-metal (PFM)crown, higher toothsubstance loss duringpreparation can only becovered/veneered withcomposites

How double crowns workPhysical principlesThe patient expects theprosthesis to be inserted andremoved easily. At the sametime, the prosthesis has tobe sufficiently attached sothat it cannot be leveragedoff by motion during speechand eating. For this to beachieved, some existingphysical needs must be met.

Telescopic crowns in practice

CLINICAL

22 | October 2010Irish Dentist | www.IrishDentist.ie

In order to obtain a holdbetween the primary andsecondary telescopic crownelements, they have to fit in aparticular manner. There arethree different types of fit:1. A clearance fit, in whichthere is still a small bit ofgive/play 2. A medium fit, in whichthere is a large tolerance orover-sizing before the joiningof the components (whichgives totally useless telescopiccrowns)3. The pressure fit, where thecomponents are tight andinteract such that friction iscreated during fitting.

On the basis that bothcrown pieces have to joinexactly and withoutobstruction, paralleltelescopic crowns are alwaysof the pressure-fit type,which is why there is thepreference that telescopiccrowns are made fromprecious alloys, given theirhigh elasticity.

The importance of frictionThe inner and outertelescopes stay togetherbecause of friction. Statedsimply, the friction is due tothe interaction between thesurface layers of the innerand outer telescope and thebinding forces of thetelescopic crowns are aconsequence of this friction.

Friction in telescopiccrowns is a value that isdifficult to measure. It isprincipally dependent uponthe technical construction ofthe crown, which isinfluenced by the followingfactors:• The number of plannedtelescopic crowns• The length of the frictionsurfaces of the individualtooth and the sum of allavailable telescopes• The placement of thefriction surfaces relative toone another. Only opposite-facing parallel surfaces canprovide the required frictionwith the elasticity of thematerials• The quality of the work.

The prosthesis has to beprepared in such a way that

the patient can insert itwithout difficulty.Additionally, it must providethe feeling of fitting firmly.The denture should also beremovable without difficulty,while not loosening at thewrong moment. All of thesecriteria must remain validover a long wear period.(NB: The force required forremoval of the prosthesis –250-300 P – is regarded asacceptable to patients. Themaximum force required forremoval should not exceed650 P, because if a higherlevel of force is needed, oftenthe patient cannot manage.)

Achieving the correct levelof friction for the individualtelescopic components isonly possible as a result ofconsiderable experience andskill by both partiestechnically involved – thedentist and the dentaltechnician – and theirinterdisciplinarycommunication.

The success also dependson the precision of each stepand a keen eye for detail.

Principles and next steps This article provides anoverview of the technicalaspects of telescopicprostheses or double crownsand their almost universalapplicability. The basicprinciples of how they workand the importance ofachieving the right level offriction are described.Success depends upon goodcommunication andtechnical skill by both thedentists and the technician.

Look out for the secondarticle on our precisiondental prosthetics withhighly engineeredconnections, which will bepublished in the next issueof Irish Dentist and willillustrate the clinical aspectsof this kind of treatmentmodality.

For a full list of thereferences used in thewriting of this article,please emailrefs@irishdentist.ie.

A telescopic crown during insertion

A single telescopic attachment with retention, showing the primaryand secondary crowns

This upper jaw prosthesis is now ready for modelling of the outercrowns