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January 2010 | Dental Lab Products
Bench mastery: Kreyer on removables

It’s all in the selection

The needs and expectations placed on today’s removable prosthetic laboratory have changed significantly.

by Robert Kreyer, CDT 

Thirty years ago, removable laboratory services were very basic. Most tasks involved removable repairs, relines, stayplates, acrylic partials, immediate dentures, complete definitive dentures, and cast partial dentures. In the late 1970s, precision attachments became popular for root-retained overdentures and precision partial restorations, expanding product lines further.

Over the next 30 years, removable prosthetic product lines further expanded to include occlusal splints, flexible partials, provisional diagnostic dentures, radiographic stents, surgical implant guides, implant-retained overdentures, and implant-supported fixed-detachable and hybrid prosthetic restorations. And then in the 21st century, dental laboratories introduced tiered product lines from basic to high-end prosthetics based on clinical expectations for meeting a patient’s needs, demands, and pocketbook.

Today’s dental laboratory must be able to provide customers with a wide range of removable prosthetic needs that meet the appropriate esthetic and functional demands of patients.

Refer to slideshow for figures 

Current needs

Removable needs are most associated with patients’ increased esthetic and functional expectations for removable prosthetics and restorative materials in the oral environment. One overlooked need is the selection of appropriate denture teeth to oppose existing restorative dental materials and natural dentition in the mouth.

As we all know, acrylic resin denture teeth wear significantly when opposing porcelain crowns or metal occlusal surfaces (Fig. A). The same wear occurs with implant-retained or -supported prosthetic restorations. When a conventional denture is loaded with implant attachments, a patient’s bite force is increased significantly. This increased bite force results in occlusal wear to conventional acrylic resin denture teeth.

I have seen many implant hybrid restorations where the resin teeth have to be replaced after one year because there is significant wear on the occlusal surface. Replacing denture teeth after only one year does not provide an optimum level of function and esthetics for patients who are in a compromised edentulous state.

Tooth selection factors

Considerations for tooth selection often include price, shade, mold, esthetics, and occlusal schemes. The one factor not usually taken into consideration is functional occlusion in relation to tooth wear. When considering tooth wear due to functional occlusion, the variables to be considered are: opposing occlusion such as natural dentition, porcelain crowns, metal occlusals, complete denture, partial denture, and implant-retained or -supported overdentures (Fig. B).

The question is should tooth selection be a clinical or technical responsibility? All too often, the dental laboratory is given just a shade on the prescription and expected to select anterior or posterior molds from study models.

Study models tell a technician the size and shape of the teeth but not what is opposing, such as natural enamel, amalgam or composite fillings, PMMA acrylic resin, composite, porcelain, precious, or semi-precious or non-precious alloys. Understanding the restorative materials involved determines which type of tooth to select for a conventional denture or implant restoration. If we are designing a mandibular implant overdenture opposing natural dentition, knowledge of opposing restorative materials that will affect on wear is essential to proper tooth selection if our goal is achieving optimum esthetics and function. Only the clinician has this information, so tooth selection or proper communication of these factors must be a clinical responsibility and prescribed accordingly.

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Denture tooth wear characteristics

When natural enamel opposes complete denture prosthetics with acrylic resin (PMMA) teeth, the acrylic teeth will experience excessive wear, altering the prosthetic occlusal surface and causing a significant loss in vertical dimension of occlusion (Fig. C). When porcelain denture teeth oppose natural dentition, the natural teeth will be abraded. When acrylic resin (PMMA) teeth oppose porcelain or ceramic crowns, the acrylic teeth experience extreme wear, usually resulting in a reverse Curve of Wilson (Fig. D).

Metal occlusal surfaces, such as high-noble alloys, that oppose acrylic-resin (PMMA) denture teeth do not affect wear as significantly as non-precious alloys for full cast or metal occlusal surfaces. For cases involving opposing metal surfaces, porcelain denture teeth could be used but are more difficult to grind in and polish and tend to chip or fracture during function (Fig. E). Years ago, denture teeth would have amalgam or gold placed into the occlusal surface to prevent wear and maintain vertical dimension of occlusion. Other techniques included placing porcelain denture teeth in the pre-molar and molar regions.

Highly crossed-linked acrylic resin PMMA teeth such as DCL, IPN or TCR materials, should oppose each other to maintain established vertical dimension of occlusion and occlusal contacts. With complete maxillary and mandibular dentures, never select porcelain teeth to oppose acrylic resin denture teeth. This was common 40 years ago but will result in porcelain wearing away opposing acrylic resin and decreasing vertical dimension of occlusion. For conventional mucosal-supported dentures when esthetics and wear issues are not part of a patient’s needs then crossed- or highly-crossed-linked acrylic resin denture teeth are indicated.

Implant-retained/-supported restorations
Wear with acrylic resin denture teeth has always been an issue when a removable prosthesis is retained by attachments thus increasing a patient’s bite force (Fig. F). An important factor to remember is when loading or retaining a complete denture with implants, the bite force is increased substantially. It is the combination of increased bite force and restorative dental materials that will create excessive wear to new denture teeth in an implant overdenture.

Remember that with implant-retained dentures, stress is not absorbed by periodontal ligaments and surrounding tissue (Fig. G). The actual masticatory forces are considerably higher in edentulous implant patients than in patients with conventional removable restorations.

New tooth form and polymer structure

Removable prosthetics including implant restorations are designed and treatment planned to meet the sophisticated desires and expectations of the modern, mature, and educated patient. Their requirements and expectations go well beyond basic oral functionality; they also expect individualized or personalized esthetics, which is playing an increasingly important role in prosthodontic treatment (Figs. H and I).

In a 2004 Mosby book written by Zarb and Bolender, Prosthodontic Treatment for Edentulous Patients, the authors wrote: “Composite resin teeth have been introduced as a suitable material for fabrication of denture teeth. They contain micro-fine filler particles of silica and have demonstrated wear properties that are clinically acceptable. Continued improvements in the polymer structure may facilitate its widespread use by most clinicians in the future.”

Finally, the future is here in the form of a new, revolutionary polymer structure. This new tooth form is suitable for use with complete conventional dentures, partial dentures, and implant-retained or -supported prosthetic restorations. When esthetics and wear are primary considerations in treatment, there is finally an option in removable prosthetic treatment that will solve these issues. This new denture tooth form (Fig. J) has a very high resistance to wear as well as exceptional esthetics. It is called SR Phonares and is manufactured by Ivoclar Vivadent. The SR Phonares denture tooth line (Fig. K) offers an exceptionally high degree of durability as these teeth are based on a Nano-Hybrid Composite (NHC) material. The tooth molds are shaped according to age-specific characteristics allowing the fabrication of highly personalized implant prosthetic restorations (Fig. L).

Using the SR Phonares NHC prosthetic denture teeth, we can provide removable prosthetics that enable patients who are in a compromised edentulous state to achieve an optimum level of function and esthetics.

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