Dental Tribune Asia Pacific Edition | 1+2/201614 TRENDS & APPLICATIONS

Thermoplastic materials have beenused in aviation and space engineer-ing for a long time. Owing to theirhigh mechanical strength and lowmodulus of elasticity, they have be-gun to increasingly replace metal inmany manufacturing industries too,particularly in those where metalhas been the dominant choice untilnow. Implants for intervertebraldiscs, as well as hip and knee joints,are made of PEEK, a thermoplasticpolymer. Four million implants havebeen fitted during the last 15 yearswith outstanding success. In recentyears, thermoplastic materials havealso been used in dental technology.This article discusses a number ofcommon plastic materials that havebecome alternatives for use in themanufacture of non-metal tele-scopic dentures.

About 15 years ago, the first at-tempts were made, not without initial problems, to produce non-metal telescopic dentures. Thesedentures were made by injectionmoulding using a polyamide (PA)in the dental laboratory. A wax

mould of the framework, bar andsecondary crowns is made as an integral part, embedded in plasterin a flask and the wax boiled out.The plastic material, which is avail-able in the laboratory as granularmaterial, is heated in the injectionmoulding device and injected intothe mould. After a period of cool-ing, which should not be shorterthan specified, the prosthesis is re-moved from the mould and fin-ished. Special milling cutters areneeded because the material tendsto become viscid when cut. Very

importantly, absolutely no metalmust be entrained. If the denturewere to cut by a tool previouslyused for cutting metal, minutemetal particles would be incorpo-rated into the thermoplastic mate-rial by the milling cutter. Frictionwould easily be controlled by ex-pansion plaster.

The good sliding properties andthe high friction of the secondarycrown particularly surprised us.When inserted, the secondary crownslides along the primary crown and

is retained partly by clamping andpartly by suction. Our patientsfound the good sliding propertiesand the light weight comfortable.The modulus of elasticity of PA isvery low, which lends flexibility tothe material. This gives the patient asensation of a readily adapting den-ture, rather than a foreign body, inhis or her mouth (Figs. 1–3).

The low modulus of elasticity,however, turned out to be the

greatest drawback of the material.The moduli of elasticity of all plas-tic materials used for bonding arevery high and two moduli as wideapart as these cannot be bonded reliably for a long time by anymeans available to dental labora-tory technicians. As a consequence,many dentures develop cracks andspalls in the bonds after severalmonths. In addition, the largepores on the surface of the dentureled to discoloration, particularly inpatients with an altered acid–basebalance.

FPMA short while after PA, the indus-

try launched a successor materialwith FPM. This thermoplastic fluo-ropolymer offers some flexibility,but less than that of PA, however.The modulus of elasticity is mar-ginally higher than that of PA, butdistinctly lower than that of metal.Consequently, similar problems asthose encountered with telescopicdentures of PA occurred.

PMMAWe have obtained good results

with PMMA (polymethylmetha -crylate). This plastic material isvery hard and inflexible. Finishablein different colours, it is used forcomplete dentures and occlusalsplints, as well as for long-termtemporary dentures, crowns andbridges. The material is not suscep-tible to plaque, and discoloration isvery inconspicuous.

The moduli of elasticity of bond-ing materials and PMMA are simi-lar; thus, cracks and spalls of bondsdid not occur. Patients who had previously worn a telescopic pros-thesis of PA or fluoropolymer, how-ever, complained that the dentureof the new material was uncom-fortable to wear. PMMA’s lack offlexibility gave patients the sensa-tion of having a foreign body intheir mouth (Figs. 4–6).

Unfortunately, denture breakswere reported after some time, particularly in free-end situations.Also, dentures not lined regularlyand exposed to high force tendedto break. We believe one reason forthat is the fairly high modulus ofelasticity, which makes the mate-rial somewhat brittle. The greatestproblem, however, is that thermo-plastic materials cannot be re-paired. There is no way of repairingcracks or fractures. The only solu-tion is to make a new denture.

PEEKPEEK (polyetheretherketone)

was first used for telescopic den-tures about six years ago. In generalmedicine, it has been used for hip,knee and intervertebral disc im-plants for almost 15 years. Accord-ing to German company Evonik Industries, as many as four millionimplants have been fitted and not a single case of proven allergy tothat material has been reported.The modulus of elasticity of PEEK issimilar to that of bone, with posi-tive consequences for integration.This is one of the reasons that PEEKmerits the attention of dental lab -oratory technicians. Finally, thereis a material with a hardness simi-lar to that of bone, not as soft as PA or FPM plastics and not as hardas PMMA. These very rigid materi-als often cause dental techniciansproblems, for example with all- ceramic solutions for the upperjaw, where craniomandibular prob-lems frequently arise.

PEEK is a very light-weight ma -terial with a long history of use inspace flight. Non-conductive, it has

Thermoplastic materialsin dental technologyAn overview in the case of non-metal telescopic dentures

By Claudia Herrmann, Germany

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been used in semiconductor tech-nology for a long time. This prop-erty also offers benefits for use inthe oral cavity.

The pharmaceutical industryuses PEEK in production. Parts incontact with the product are madeof PEEK owing to its low discol-oration and high resistance to wearand corrosion. Both properties arealso very useful for dental tech -nology.

PEEK is indicated for removable,as well as conditionally removable,prostheses. Therefore, bridges,crowns, telescopic dentures and attachments, as well as screw- retained superstructures, can befabricated. The material has verygood sliding properties and pa-tients report that it is extremelycomfortable to wear.

There are two different methodsof manufacture. One is injectionmoulding and the other isCAD/CAM milling. The minimumthickness of telescopes is 0.6 mm.The minimum thickness of frame-works and bars is distinctly higher,but varies depending on the de-sign and the size of the telescopicprosthesis, as well as the number of available telescopes. Generally, a PEEK telescopic prosthesis will bea little thicker than a metal tele-scopic prosthesis. It is an absolutenecessity that the primary crownbe made of zirconia, as abradedmetal particles would otherwisecollect under the secondary crown.

The veneer bond strength wastested in a study at the University ofRegensburg in 2012. In order to passthe test, a value of 5 MPa had to beachieved. Of all the veneering sys-tems tested, PEEK scored 10 MPaand above and passed all of thebond strength tests. In other tests,such as discoloration and shearstrength, it also achieved very posi-tive results, confirming the suit-ability of PEEK for use in the oralcavity. When subjected to load at fracture tests, a PEEK bridgeachieved 2,354 N and was far su-perior to a ceramic bridge, with1,702 N. Hence, PEEK can withstandhigher loads in the oral cavity thancan ceramic material, and so wide-span telescopic dentures can bemade of PEEK.

It is necessary when handling telescopic dentures of PEEK to apply ceramic guidelines becausethe material could otherwise beweakened owing to crack propa-gation. In addition, the prosthetic design must follow certain criteria.For example, a prosthesis without atransverse bar must always includea backing plate in the secondarypart to provide sufficient stability.Dental technicians required tomake non-metal telescopic pros-theses should therefore receivesufficient training and instructionso that the required high-qualitylevel can be maintained. Those whowork with PEEK only rarely and whotherefore lack experience are ad-vised to have telescopic prosthesesof PEEK designed and cut in a spe-cialised laboratory. Even in our

laboratory, we have come acrossPEEK prostheses with cracks, butthese have invariably been due tomanufacturing mistakes. Prosthe-ses made correctly exhibit nocracks. Cracks and spalls of the veneering of PEEK dentures can befound about as often as in tele-scopic prostheses of metal—that is,rather seldom.

PEEK is extremely resistant toplaque and inert to acids and chem-icals; therefore, the denture can be

cleaned with a chemical dentalcleaner.

Friction is one of the most criticalcharacteristics of telescopic pros-theses. The friction of PEEK is verygood and can be controlled ex -cellently with expansion plaster.However, most important is thatfriction is permanent. We made ourfirst telescopic prostheses of PEEKabout five years ago and we havenot observed any loss of friction inthat time (Figs. 7–13).

ConclusionOur laboratory has the experi-

ence of having made over 300 non-metal telescopic prostheses overthe course of 11 years. After initialproblems and several tests, PEEKhas finally proven a suitable mate-rial for telescopic dentures in thelong term. Non-metal telescopicprostheses are in no way inferior to metal telescopic dentures, pro-vided they are made profession-ally. On the contrary, the light

weight, the high wear comfort andthe absence of metal, in particular,are compelling arguments for den-tal technicians and patients alike.

15Dental Tribune Asia Pacific Edition | 1+2/2016TRENDS & APPLICATIONS

Claudia Herr -mann is ownerand CEO of Dental -LaborHerrmann inBad Tölz in Ger-many. She canbe contacted atabt@dl-herrmann.de.

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