Flexible Partial Denture Variations: The Use of Circumferential, Combination, and Continuous Clasp Designs

INTRODUCTION
The development of flexible partial denture materials has allowed dentists to rethink the possibilities of long-term treatment of partial edentulism. These materials can be used without the concurrent weaknesses of methylmethacrylate (ie, frequent fracture, poor retention, lack of stability) or the technical difficulties and expense of metal castings. It is the purpose of this article to review the previously introduced clasps that have been found to be exceptionally functional with flexible partial design and to introduce new variations in clasping and design expectations.

Types of Clasps
(In conjunction with reading this article, you may wish to review the author’s previous article entitled “Flexible Removable Partial Dentures: Design and Clasp Concepts,” published in December 2008, available in our article archive on our Web site dentistrytoday.com.)
The circumferential/ring clasp (Figure 1) may be used on any freestanding tooth. It is particularly useful on mesially tipped mandibular molars (Figure 2). When seen from the posterior aspect, its use on a mesially tipped second or third molar can be easily imagined (Figure 3). The circumferential clasp may be used as a single clasp, in combination with multiple clasps (Figure 4), or in a unique combination called the combination clasp.

Figure 1. A circumferential/ring clasp.	Figure 2. Circumferential/ring clasp, as used for a mesially tipped molar.
Figure 3. Circumferential clasp seen from distal.	Figure 4. Circumferential clasp used with a conventional clasp.
Figure 5. Combination clasp using a circumferential combined with a conventional clasp.	Figure 6. Continuous clasp involving multiple teeth.

The combination clasp (Figure 5) is a circumferential clasp in combination with a conventional clasp. It provides the strength and retention of the circumferential clasp with the engagement of a conventional clasp. Again, the strength, retention, and stability of the combination clasp contribute to a higher degree of satisfaction for the patient and the dentist by allowing the clinical goals of partial denture treatment to be met.
The continuous clasp is a clasp that encircles several teeth, gaining a vertical component to resist “folding” by the axial wall as it goes around the distal of the last tooth and is connected at some point of its mesial run (Figure 6).

Clasp Design and Material Considerations
The strength of any clasp is in the design of the clasp, and the material from which it is made. The majority of clasp designs, regardless of the material, are cantilevers, gaining strength by a connection of only one end. A cantilever can be strong if it is short, massive, externally braced, or a combination of these factors. The original designs of flexible clasps sought to achieve strength by use of a broad and relatively flat clasp, gaining strength through size; strength was limited due to its single-plane design and flexible nature. However, the flexible partial clasp may gain much more effective strength by a combination of external bracing, which effectively shortens the flexible component and introduces a second plane of resistance (the combination clasp) or by external bracing (circumferential clasp; continuous clasp).
This concept is grasped by imagining a flat piece of metal, whether broad or narrow of any length. The longer it is, the more easily it can be bent. However, connect another piece of metal, at right angles (ie, the I beam concept/channel concept), and rigidity and strength will exponentially increase. The connection of another component in a different plane provides the strength for all clasps.
The 3 main clasps of a flexible partial are the circumferential clasp, the combination clasp, and the combination clasp. Elimination of a single weak component (a broad flat cantilever) by completing the circle and attaching it to itself forms the strong “circumferential” clasp. A circumferential clasp with a short contilevered conventional clasp forms the combination clasp.  And when a circumferential clasp encircles multiple teeth, it becomes the combination clasp.
Close attention to the design, allowing for the incorporation of 2 right angle planes to resist flex, is essential for retention and stability.

PARTIAL DENTURE DESIGN CONSIDERATIONS
The realities of dental practice often preclude fabrication of diagnostic casts and survey/design as taught by our dental schools. While it is true that experience can often take the place of careful consideration and planning, it is also true that nothing can guarantee long-term success as much as careful consideration of good diagnostic casts surveyed with even the most simple devices, a careful application of the clasp design considerations by modest tooth preparation, the fabrication of an accurate master cast, and the generation of clear instructions for the dental laboratory team.
A well-executed flexible partial can predictably provide years of service, providing a stark contrast to taking a “quick impression” and accompanying it with a generic “make a partial” laboratory prescription. Having done it both ways in my career, I can unequivocally state that planning and careful execution pay off with much greater dividends in the long run, far exceeding the short-term benefits of the “quick” methodology. The flexible partial is simply too powerful a “tool” to be used, or thought of, casually.
The survey and design are simple, following the basic concepts taught by the schools. Look at the survey lines on the tooth and carry in the mind the basic pattern of a circumferential clasp while thinking, “What must I do to this tooth (or any other tooth) to make it possible to use a circumferential clasp or combination clasp?” Keeping this matrix in mind will simplify the design dilemma.
The successful use of the clasps depends on a relatively consistent survey line around the tooth. Minor enameloplasty will provide a one to 2 mm band around the tooth which the flexible circumferential clasp may accurately and passively fit (Figure 7).
I recommend the use of tapered diamonds (such as an 878-2861 [Henry Schein Dental]) to lightly create the guide plane and zone-of-fit for the circumferential clasp (Figure 8). Finish by using a fine finishing diamond (such as an 878-2896 [Henry Schein Dental]) with copious amounts of water.

Figure 7. Establish zone of guide plane/fit following survey and design.	Figure 8. Wide space with only tooth No. 8 missing. Patient had experienced repeated fractures of previous partial dentures.

As in all removable partial design, a certain amount of creativity is a necessity, which is individually expressed. For that reason, it may be expected that, given any particular situation, several designs may be possible; however, certain basic elements of design may be neglected only at the peril of the long-term restoration.

CASE REPORTS
Case 1
A 23-year-old male presents with a wide space, missing only tooth No. 8 (Figure 8). An interim acrylic partial denture (the ubiquitous “flipper”) had been made, but it had proved unsatisfactory with the replacement tooth easily having broken off several times since it was delivered. The decision was made to try a flexible partial denture utilizing a limited continuous clasp.
Very little tooth modification was necessary. Impressions were made with sterilized stock metal trays (Rimlock Trays [DENTSPLY Caulk]) and a fast set alginate (Jelptrate Plus [DENTSPLY Caulk]). The impressions were then poured within 2 minutes following removal from the mouth in improved dental stone (Die Kenn [Modern Materials/Heraeus Kulzer]). The prosthesis provided is shown (Figure 9) on the cast, and from both its superior aspect (Figure 10) and the intaglio surface (Figure 11).
After several months of use, patient satisfaction remained high (Figure 12) and there was no incidence of breakage.

Figure 9. Flexible partial on cast.	Figure 10. Superior view.
Figure 11. Tissue surface view, showing crossover points between teeth Nos. 5 and 6; 10 and 11.	Figure 12. Clinical view of flexible prosthesis.

Case 2
A 38-year-old male presented with missing anterior and posterior teeth, retaining only teeth Nos. 21, 22, 27, 28, and 29. He stated that he was satisfied with his existing complete upper denture, but he expressed extreme unhappiness about his previous “plastic” partials and his one experience with a “combined metal/plastic” partial. He couldn’t decide which he disliked more; the metal, or the all plastic. Following demonstration of the flexible partial, he agreed to treatment with “this new kind of partial plate.”
As previously described in case 1, accurate impressions were taken and dental stone casts were made.
A design utilizing 2 continuous clasps was developed and fabricated (Figure 13). The continuous clasp on the left side shows how 2 teeth were incorporated (Figure 14). Clinically, this flexible partial was retained very well and actually somewhat difficult to take out. The aesthetic possibilities are clearly shown (Figures 15 and 16).

Figure 13. Bilateral continuous clasps.	Figure 14. Laboratory fabrication of continuous clasp.
Figure 15. Aesthetic result; left side.	Figure 16. Aesthetic result; lower right.

Case 3
A 36-year-old male presented with multiple missing teeth, several heavily restored teeth, and a history of failed plastic partial dentures. His dental history was one of failed fillings, and he expressed unhappiness at his initial visit with his smile and the frequently breaking partial dentures. Following demonstration of the flexible partials, he became enthusiastic about trying “something new.”
Following a prophylaxis, light tooth preparation was performed with only the finishing bur (Schein 878-2861) to create circumferential guide planes on multiple teeth suitable for a long continuous clasp (Figure 17) and 3 circumferential clasps (Figure 18).
Impressions and working models were made in the usual fashion. A partial design was employed that incorporated the 3 remaining maxillary right teeth, using circumferential clasps and a long run continuous clasp extending around the distal tooth on the contralateral side; this was required due to the lack of interocclusal options on the maxillary left. The aesthetic options pleased the patient (Figure 19).
The stability and retention were excellent. Breakage is not expected at any time.

Figure 17. Continuous clasp involving multiple teeth.	Figure 18. Three circumferential clasps.

Figure 19. Aesthetics of the flexible partial denture.

Case 4
Flexible partial design holds promise in the post-treatment restoration of trauma patients.
A 19-year-old male presented with an avulsed maxilla due to a gunshot. The bullet had entered the right orbit and exited through the anterior maxilla of the mouth. Although the anterior damage was devastating, the posterior teeth remained intact and stable (Figure 20).
Continuous clasps on both the right and left maxillary teeth provided the necessary stability and retention for an aesthetic solution (Figure 21). A useful element of this treatment was the light weight of the flexible prosthesis by the elimination of a cast metal component or a heavy acrylic section.

Figure 20. Trauma to the patient presented a difficult restorative dilemma.	Figure 21. Bilateral stabilization and light weight resulted in a good solution.

Case 5
Flexible partial design holds promise in the post-treatment restoration of cancer patients.
A 23-year-old woman presented with large palatal defect following a hemimaxillectomy and radiation therapy for squamous cell carcinoma. Although healing had progressed in a fairly normal fashion, an obturator that had been previously delivered to the patient caused a focal penetration of the oral mucosa on the remaining left posterior palatal surface. This was presumably due to the weight and friction involved with a definitive hollow bulb obturator in terms of weight and stability.
A flexible partial was fabricated, following impression-taking and block-out of the most superior aspects of the surgical wound (Figure 22). A conventional flexible clasp was utilized for the maxillary right, and a continuous clasp was developed for multiple points of retention and stability for the maxillary left (Figure 23).
The light weight of the flexible partial was kind to the mucosa and the remaining teeth. It was noted that, although the goal had been to increase retention and stability, the accidental finding of the great weight reduction (using new clasp designs and thinner flexible base/clasps) was of extreme significance.

Figure 22. Large palatal defect that required careful planning and design.	Figure 23. Continuous clasp on maxillary left distributed the forces of this light weight flexible partial.

SUMMARY
The new design potential of the flexible partial and its clasp allows for a new treatment approach to the well-established problems of retention, stability, and strength. The 4 main clasp designs include the conventional, the circumferential, the combination, and the continuous clasp. The proper use of these various designs can be a strong foundation upon which to develop the clinical strengths of the flexible partial denture.

Dr. Kaplan is a graduate of Indiana University School of Dentistry. He received his postgraduate training in prosthodontics through the Air Force at Wilford Hall Medical Center and completed his maxillofacial prosthodontic Fellowship at the University of Chicago. He is currently working with the US Army DENTAC unit at Fort Hood, Texas. He can be reached at pgk.dent@gmail.com.

Dr. Kaplan reports no disclosures. The views expressed in this article are those of the author and do not reflect official policy or position of the Department of the Army, department of Defense or the US Government.