Alternative materials for dental devices: polymers facilitate structurally superior applications.

The dental device industry often has to strike a balance between the requirements for physical strength against the desire for aesthetic appeal. In the limited space environment of the mouth this need for strength has, by tradition, led the industry to use metallic or, to a lesser degree, ceramic materials in most application areas, with only a few exceptions for which more aesthetically tailored polymers (mainly acrylics) play a part.

Metallic alloys such as gold and titanium, and ceramics such as porcelain, zirconia and alumina, are among the most widely used materials in dental applications. Since there are multiple material choices available, dental device designers must thoroughly understand the principles that make these materials behave differently and select the materials accordingly. Gold is usually limited to crowns or bridges, as it is significantly weaker than other materials and has an increased risk of breakage during normal function. Similarly, ceramics are reserved for crowns and healing applications where aesthetics is an important factor. Titanium, because of its high strength, is used in the development of dentures and dental implants, which are inserted into the jaw bone.

Polymer alternative

Recently, there has been a shift towards the use of strong polymers and composites for applications involving some degree of structural performance in load bearing applications such as crowns and bridges, abutments, healing caps and implants. In addition, the use of technologies and production methods, such as molding/ casting developments and the use of CNC/CAD, is further encouraging dental device manufacturers to investigate these material alternatives.

Two such alternative materials are high-performance polyetheretherketones manufactured by Invibio Inc., Greenville, SC. PEEK-OPTIMA polymer is a semi-crystalline thermoplastic that has been used in the development of long term implantable medical devices and pharmaceutical applications having blood or tissue contact for more than 30 days. PEEK-CLASSIX polymer is another thermoplastic, used in the development of medical device applications requiring blood or tissue contact of less than 30 days.

Both are among the most chemically resistant polymers available. They can be repeatedly sterilized using conventional sterilization methods including steam, gamma radiation and ethylene oxide processes without the degradation of their mechanical properties or biocompatibility. Both are characterized by extreme hydrolytic resistance, even at elevated temperatures, and strong ionizing radiation resistance.

They can also be tailored by adding reinforcing fibers that significantly increase their mechanical properties in order to satisfy applications requiring very high strength. Furthermore, their modulus (degree of elasticity or stiffness) may be adapted to closely match that of cortical bone. This is significant as bone requires stimulus from mechanical stress to maintain its structure.

Since their introduction to the medical market, both polymers have gained the confidence and acceptance of the medical community as highly reliable biomaterials. From an engineering viewpoint, their properties have made these biomaterials ideal for the manufacturing of a great variety of medical device applications, and suitable as alternatives to traditional materials in the development of structurally superior dental applications.

Crowns, bridges and dentures

In development of permanent crown and bridge structures, gold alloys are used when maximum strength is desired and aesthetics is not a factor. Similarly, titanium is used when high strength and biocompatibility is needed but appearance is not a factor. Since they are more cost effective, non-precious alloys are often utilized when cost is a concern and when maximum strength is needed, but appearance is not a factor. Ceramic materials, such as porcelain, are often used because they are much more aesthetic than metallic materials. Though ceramics are more natural-looking than other materials, the material is also much more brittle. As a result, it can wear away more easily and can often crack instead of flex from high stress, leading to application failure.

In contrast, a medical-grade polymer such as PEEK-OPTIMA offers a combination of extensive biocompatibility, high strength, stiffness, toughness and good aesthetics. Since this polymer is less brittle than ceramics, it could be used to develop crowns and bridges that can better sustain compressive strength under excessive force brought on by biting or continued grinding of the teeth. Several device manufacturers have begun investigating the polymer as a replacement to traditionally used materials.

Metallic alloys, currently the most widely used material for the development of permanent dentures, present several disadvantages to their use in the mouth. Since alloys are made up of different metals, there is a possibility of biocompatibility problems for sensitive patients. In addition, given that alloys conduct electricity, there is a possibility that dentures made from the materials can produce a "battery effect," a cause for headaches and fatigue problems in some patients. Other patients may have metal allergies that would be exacerbated by the release of metal ions into the body from metal-bearing dental devices. Some experts consider all metals--even non-allergenic or non-toxic metals--to be disruptive and therefore should not be used in the mouth.

A PEEK polymer exhibits almost the same combination of strength and biocompatibility, but without the negative effects attributed to metallics. The modulus of this material can be tailored to the supporting natural tissue or natural teeth in the mouth; therefore, it can be used to develop metal-free dentures that do not restrict biting or chewing sensations, but rather promote a feel similar to natural response. Polymer dentures, which are already being manufactured, have been found not to limit or enhance temperature sensations in the mouth.

Temporary healing devices

Healing applications such as abutments and caps have been made from titanium and ceramics, the latter only occasionally because of its brittleness in a load-bearing application. Titanium healing abutments make up 85% of the market, but have the same disadvantages as metallic dentures, with an added drawback--aesthetics. Because abutments and caps are visible in the mouth while a crown is being made (unlike the supporting structure of permanent dentures), they have to blend in with the surrounding environment of the mouth. For that reason, dental device manufacturers have begun using medical PEEK polymers as alternative materials in the development of healing abutments and caps. Both materials are comparable to titanium in the areas of biocompatibility and strength, and are superior in regards to aesthetics. Using these materials, dental device manufacturers are able to develop non-metallic abutments that closely match the color of natural teeth.

PEEK polymers also enable dental device manufacturers to develop abutments and caps with chair-side adaptability, meaning they can be sculpted by the dentist during surgery. This ability, not possible with titanium abutments, ensures the abutment is flush with the patients' gingiva--the soft tissue that covers the jawbone and surrounds the tooth--prohibiting the soft tissue from receding under the abutment. Such control over the material is critical to the healing process and to ensure the final abutment and crown assembly will fit properly in the gingiva.

Dental implants

Titanium is currently the most widely used material in the development of dental implants. Though implant usage is minimal (only replacing crowns/bridges in about five percent of cases), many anticipate this market will increase significantly, and as this market continues to grow, PEEK polymers will continue to be investigated as an alternative because of the benefits it offers compared to titanium.

Unlike titanium, a medical PEEK polymer can be tailored to match the modulus of bone and therefore allow loading that stimulates bone. Additionally, the polymer easily can be coated and surface modified to enhance bone growth and osseointegration. The polymer will eliminate the electrical conductivity associated with titanium implants and since the polymer has excellent corrosion and acid resistance, it is ideal for long term usage in the demanding environment of the mouth.

Design and manufacturing flexibility

In addition to their mechanical, chemical and biological characteristics, many dental device manufacturers are using medical PEEK polymers because of the broad design and manufacturing flexibility they offer. They may be processed using conventional thermoplastic processing equipment and techniques such as injection molding, extrusion, compression molding and powder coating.

PEEK-OPTIMA polymer is used in injection molding operations to economically mass-produce high performance components without the need for post process annealing or machining. Extrusion may be used to produce film and sheet, monofilament, tubing, rods and compounds with pigments or fillers. The viscosities of these materials are comparable to commodity polymers at their melt temperatures. Three grades are available, including standard viscosity, medium viscosity and low viscosity.

Dental device manufacturers using these polymers also benefit from the reduced development time and costs associated with biocompatibility testing and regulatory approvals. PEEK-CLASSIX polymer is pre-certified to US Pharmacopoeia (USP) Class VI and ISO testing standards and PEEK-OPTIMA polymer has undergone extensive biocompatibility and biostability testing to assure its suitability for implantable devices. Long-term implantation studies have been completed for PEEK-OPTIMA polymer and documentation, including ISO 10993 and USP Class VI, are contained in detailed Master Files with the FDA. These include Device Master Files (MAF) and Drug Master Files (DMF). Armed with this kind of information, dental device manufacturers can reduce the time and costs involved for device clearance with regulatory agencies. The material is also cGMP certified meaning that the company embraces all the principles of "Good Manufacturing Practice" in relation to the manufacturing of unfilled granules, compounds and stock shapes.

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