March 1, 2004
by Dmitri Svirsky, DMD, FACP, FRCD(C)
ABSTRACT: Attachment-retained RPD is not an outdated treatment modality. It is even more contemporary in today’s appearance-oriented society than when it was first introduced. There is significant number of patients who could benefit from this treatment option, both short and long term. However, lack of proper education, overwhelming number of attachments available on the market, multiple adjustments and repairs are making dentist reluctant to offer and provide attachment-retained RPD to their patients. The purpose of this presentation is to outline a simplified approach to this treatment modality.
The advent and success of implant-assisted prosthodontics has changed dentistry tremendously. Treatment planning and decision making is not what it used to be 20 years ago. Perio-prosthetic management of complicated dental conditions was the mainstream of advanced prosthodontics from 1960 to the mid 1980’s. It has got us to where we are now. It has taught us the importance of periodontal, and peri-implant health and the significance of proper maintenance for long-term successful rehabilitation.
Now, site-driven placement of implants, introduced to North America in 1982, is being replaced by prosthetic-driven implant surgeries. Research in bone regeneration has led to the development of numerous techniques allowing the creation of bone where it has been lost or has never existed. While these developments advance the aim of successful prosthodontics to be both functional and esthetic, most advanced procedures are time consuming, psychologically and emotionally exhausting, and very costly.
Semi-precision RPD is the treatment modality that can facilitate both an esthetic and a functional replacement of missing teeth and oral structures. The few retrospective studies available show a survival rate of 83.3% for 5 years, of 67.3% up to 15 years and of 50% when extrapolated to 20 years.1,2
Those numbers don’t equal the 85-97% reported for implant-assisted prosthodontics.3,4 However, there is a significant number of patients who could benefit from this treatment option, both short and long term. RPD provides patients with time to make financial arrangement for future implant-assisted treatment and it then guides site development for the most appropriate implant placement.
Unfortunately dentists rarely utilize semi-precision RPD. In my opinion, general practitioners are discouraged from mastering and utilizing this treatment for a number of reasons:
Lack of proper education in the dental schools. This topic is barely discussed during the lecture cycle, let alone used on the treatment floor. How many educators are actually utilizing this approach in their practices?
The number of the attachments available on the market is overwhelming.5 There are no adequate studies comparing different systems. Doctors are left guessing, which systems work and which don’t. One of the most common resources for information is a laboratory technician.
Multiple initial adjustments and repairs of the fabricated RPD reduce earning potential. What about advise from a laboratory technician at this stage of the treatment?
Not least important, patients generally don’t like RPD. A cross-sectional study of 1303 individuals who had 1603 RPDs found that only one third of dentures were satisfactory.6 Lack of stability was the most common defect. Could it be to some degree our fault? (Fig.1).
Then there are the practical considerations. Even if one knows the procedure, it sometimes doesn’t make sense if you can’t make a reasonable margin on the treatment provided. Dentists are small business operators and a healthy bottom line is important. To recommend the option judiciously, we have to know how much chair time to spend and then let the patient choose between FPD, RPD or implant-assisted restoration with bone regenerative procedures.
After learning, mastering and teaching the technique I use, I have concluded:
You don’t need to know all the attachments
RPD can be comfortable, given it is stable and retentive
RPD can be esthetic
You need to increase your professional fee, because it takes more chair-time to fabricate an attachment-retained RPD
You have to account for a more expensive laboratory bill
FUNDAMENTALS OF THE RPD FABRICATION
The proper diagnosis and achievement of health of the supporting ridges, hard palate tissues, teeth and periodontal structures prior to fabricating RPD cannot be overemphasized. The condition of abutment teeth is important. They must have good periodontal support and average-to-long clinical crowns. It is desirable to splint 2-3 abutments anterior to the edentulous space. It is advantageous to have distal abutment vital.
Understanding the difference in nature and behavior of the tissues supporting RPD is critical.7-9 Those differences multiplied by the function create major stresses in the tooth-soft tissue-prosthesis system. Control of the stresses is the cornerstone to designing and fabricating successful RPD.10
Stress control is achieved through:
– Impression technique
– Broad stable denture base
– Rigid RPD design
– Physiologic shimming
– Occlusal design
– Splinting of abutments
– Intra-coronal attachments
– Clasp assembly design
Another very important aspect is a proper understanding of the limitations imposed on us by the behavior of the materials used in fabrication of RPD. Every clinical and laboratory step has an inherent degree of inaccuracy. The compounding effect of uncontrolled distortions is often poorly fitted attachment components.
Adjustments required to make parts fit and the RPD to sit, lead to the loss of retentiveness of precision attachments and the early breakage of resilient ones, let alone earlier failure of the abutment teeth or RPD, as a result of the increased stresses on them.
Technological distortions are controlled through:
– Selection of the impression material
– Framework casting without attachment
– Denture base processing
– Connection of the attachments to the processed denture bases
– Processing of the artificial teeth to the properly fitting RPD foundation
– Intra-oral adjustments taking into consideration the different nature of the supporting tissues
One of the most important misconceptions is that clasps or attachments retain the RPD. That is wrong and may lead to earlier failures of the supporting tissues or RPD. The retentive devise is supposed to provide resistance to removal of the prostheses along the path of the insertion and to be passive when the appliance is in place.11
One in-vitro study, simulating 1 year’s wear, has shown that all clasp forms demonstrated permanent deformation.12 Sixty percent of retentive arms of circumferential clasp assembly don’t touch teeth at all after 8 years of use.13 There are clinical presentations of patients satisfied with their partials while retentive arms being lost completely. It is other parts of RPD that make it work.
Most commonly attachments are classified as:14
– Precision or semi-precision, depending upon the method of manufacture and tolerance of fit
– Internal or external, according to their intra-coronal or extra-coronal location relative to the abutment tooth
– Rigid or resilient, as determined by the amount of movement allowed between the components parts
Resilient attachments are mostly extra-coronal. They increase unfavorable cantilever forces on abutment teeth and require constant replacement. Non-resilient attachments are subject to wear. That is why a lot of them have some kind of mechanism for an adjustment of lost retentiveness. The precision attachments become semi-precision over time due to wear or to patient requests to make them easier to handle.
Given that, the attachment of choice would combine the best of a conventional clasp assembly, and its ability to adjust the retentiveness, with the esthetics afforded by intra-coronal semi-precision attachments. Cast P.D. (Precision Dowel) attachment with auxiliary retentive arm fits into
Attachments International, Inc., San Mateo, CA sells them under Plastic Dovetail Connector (PDC) name.5 The PDC is an intra-coronal, non-adjustable tapered slide attachment. Both male and female components are made of castable plastic. They come micro (1.8 mm), small (2.2 mm) and medium (2.7 mm) length. The advantages of PDC with the auxiliary arm are:
– Easy to adjust retentiveness
– Easy to maintain
– Easy to replace either with new assembly or to convert to a conventional circumferential clasp
– Female and male parts of the attachments can be cast from gold alloys close in their hardness properties.
Clinical and laboratory protocol
Tooth-supported surveyed retainers are fabricated from the conventional impression in the custom trays.
Appropriate inter-occlusal record is used to mount master cast on an instrument of choice.
Diagnostic tooth arrangement is performed and tried intra-orally.
Master cast is surveyed and RPD design is finalized. Metal copings with guiding planes, occlusal rest seats and female part of a semi-precision attachment on the surfaces adjacent to edentulous areas are fabricated and tried intra-orally.
Subsequently, porcelain is verified to the patient satisfaction. Crowns and/or FPD are finished and custom partial impression tray is made to cover edentulous areas and palate (Fig. 2).
Modified Hindels’s double impression technique16,17 is used to capture mucosal tissue in an undistorted form. Distal extension and palatal areas are impressed with the border-molded tray and zinc oxide egugenol impression paste using selective tissue placement technique.
Accurate impression of the soft tissue will permit impression intra-orally without any help of the dentist as a result of good stability and retention. That is a good indication of the successful impression.
A hole is created in the middle of the palate of the stock metal tray. A pick up impression for both fixed restorations and soft tissue imprint is then made with alginate material while the custom tray is held with minimal finger pressure through the hole in the stock tray.
Excess of the pick up impression material was avoided along the borders of the partial impression (Fig. 3). If you don’t have an in-house laboratory technician, then elastomeric materials must be used for this step.
Crowns and FPD were removed from the pick-up impression. Master cast for fabrication of the RPD framework is poured, surveyed and sent to the laboratory.
Subsequently RPD framework is tried in intra-orally for proper fit and tissue adaptation using pressure indicating paste and disclosing wax respectively.
Denture bases are processed and male part of the attachments completed. The fit, retention and stability of the RPD foundation is verified again and checked with the pressure indicating paste for uniform tissue compression during connecting male attachments.
Troughs for the extensions of the intra-coronal attachments are made to have some space around extensions. The male attachments are secured within female attachments. Heavy consistent finger pressure is applied to the palate while the dental assistant attaches extensions to the denture base with an autopolymerizing pink acrylic resin. This allows achieving the proper relationship between male and female parts of the attachment system irrelevant to casting and acrylic processing inherited distortions.
Upon acrylic setting, another pick-up impression with alginate material is made (Fig. 4). Different at this time is a constant finger pressure that applied through the hole while the material is setting.17,18 The purpose of this step is to record the proper relationship between denture bases to the abutment teeth with the mucosa under the pressure, or to establish so called physiologic shim.
The concept is based on an accurate undistorted impression of the denture-bearing mucosa, establishing the relationship with the abutment teeth under pressure and creating a shim or space at the gingival aspect of the attachment upon the release of the pressure that mimics the movements of the RPD intra-orally while in function.
Shimming is a space between the inferior surfaces of the male and female parts of the attachment. This space can be developed arbitrarily or physiologically.
A new cast is fabricated and a final inter-occlusal record is obtained. Casts are mounted and denture teeth reset.
Final teeth arrangement is verified intra-orally and denture teeth processed to the bases (Fig. 5). Teeth are added to the foundation only when all the components fit individually and then together.
Crowns and FPD are cemented while RPD is in place (Fig. 6 ). The caution to exercise here is to cover all the external surfaces of the crowns, FPD and attachments with Vaseline to allow removal of the RPD.
The most important principles of this technique are that stability of the denture base and subsequent verification of the accuracy of the previous steps, combined with proper impressions and physiologic shimming, will produce predictable results.
Attachment-retained RPD is not an outdated treatment modality. It is even more contemporary in today’s appearance-oriented society than when it was first introduced (Figs. 8-10).19
Dr. Dmitri Svirsky is a Diplomate of the American Board of Prosthodontics and Fellow of the Royal College of Dentists of Canada. He is a Visiting Assistant Clinical Professor at the Boston University School of Dental Medicine, Post-Graduate Program in Prosthodontics. His private practice is limited to conventional and implant prosthodontics.
Oral Health welcomes this original article
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