Prosthetic Replacement Options for Restoring Kennedy Class I Bilateral Distal Extension Cases
George E. Bambara, DMD, MS
The Kennedy classification is a system of labeling dental arches based on relationships of edentulous spaces to abutment teeth. Four basic classifications exist with various modifications allowing for missing teeth (Table 1).1 The exception is the Class IV, which has no modifications. Essentially, the Kennedy system allows practitioners to plan treatment around the remaining soft and hard tissue.
Successfully restoring bilateral posterior segments in either arch can be very rewarding and requires careful and meticulous attention to detail. It is important to recognize that the distal extension areas and the abutment teeth share occlusal loading forces. Successful bilateral distal-extension area restorations will depend on the skill of the dentist and his or her ability to recognize the potential for destructive forces being introduced as a result of poor case design and the improper handling of soft tissue.
After a full dental examination, various treatment options and the desires of the patient need to be addressed. This article considers three different treatment-planning modalities for restorations in patients who have lost either upper or lower posterior teeth bilaterally and have no distal abutments (ie, Kennedy Class I bilateral distal extension) (Figure 1).
OPTION ONE: THE REMOVABLE PARTIAL DENTURE
Removable partial dentures (RPDs) can be made with or without a cast framework. If the framework is not cast, a full acrylic, rubberized, flexible partial is made with a C-shaped clasp made of the same material. Valplast (Valplast International Corporation, Long Island City, NY), SunFlex (Sun Dental Labs, Clearwater, FL), and Flexite (Flexite Company, Mineola, NY) (Figure 2), are examples of this type of RPD. Partial dentures without cast frameworks lack support and stability. They depend solely on soft tissue support unless provisions are made for rest seats.2,3 Variations of this flexible design exist and some may incorporate a metal framework into the material or may add metal clasps or metal rest seats. Generally, a softer, less rigid partial is considered to be more comfortable for the patient as well as being a very cosmetic alternative. Flexible partials provide comfort and cosmetics. Various degrees of support and stability must be part of the treatment-planning process.
A cast RPD has a full cast-metal framework complete with full direct retentive and reciprocating clasping, indirect retainers, and major and minor connectors (Figure 3 ). The cast partial has historically been the option of choice because the lack of posterior abutments in Kennedy Class I cases obviated the possibility of fixed bridgework.3 Cast partials are rigid and retentive, and thus add stability to the denture. They do not allow for movement and need to be well supported by the distal extensions of the soft tissue and the dense osseous retromolar pads.1-3 Clasp design is extremely important, as is impression taking, to secure a proper fit of the framework in relation to the teeth and supporting tissues. Clasps designs vary and must be chosen so as to avoid destructive Class 1 lever systems (Figure 4). These levers act as crowbars and can, over time, generate destructive forces on the abutment teeth. To avoid destructive forces in bilateral distal-extension treatment planning, the rest seats should be placed in the mesial fossa of the posterior abutments and the retentive tip should be placed distally below the disto-facial infra-bulge so as to disengage under masticatory force.3,4 This type of preferred lever is considered a Class 2 and resembles a wheelbarrow (Figure 5). Two clasp designs that fulfill Class 2 lever requirements in bilateral distal-extension cases are the Reverse Akers clasp and the RPI. Both clasps are designed to divert occlusal loading forces away from the abutment teeth. Rest seats as well as the soft tissue provide support for the partial denture and aid to stabilize the occlusion. The rest seats also direct the occlusal forces favorably down the long axis of the abutment teeth. Depending on whether the rest is placed in the mesial or distal fossa will dictate the character, magnitude, and direction of the resulting forces.
OPTION TWO: THE PRECISION-ATTACHMENT RPD
Precision attachments have been used in removable and fixed prosthetics for over a hundred years and have contributed to the success of RPDs, overdentures, segmented fixed prosthetics, and implants. Studies have demonstrated that precision-attachment partials last longer, wear less, need less adjustments, look better, work better, are less destructive, protect abutment teeth, are easier to clean, and are worn most of the time by patients who have them.5,6 This is also evidenced by dentists who treatment plan attachments and recall their patients on a regular basis.
The precision-attachment partial is similar to the cast partial in that its framework is cast in metal. The precision attachment itself is formed around a template or machined to exact specifications in a high noble metal. The attachment is placed in the distal portion of the crown wax-up and the crown is cast. It can also be soldered into place. It should not over-bulk the crown but fit within its confines and contours. At least two crowns need to be splinted together or double-abutted and placed on both sides of the distal extension areas. Precision attachments like the Stern’s G/L or the Stern’s Latch, (Sterngold Dental, LLC, Attleboro, MA) are two examples of precision attachments that can be used to treatment plan distal-extension cases (Figure 6). Precision attachments themselves functions as clasps. They provide adjustable retention and are rigid. They are extremely esthetic and do not allow any metal to show as minor cast connectors do (Figure 7). They are generally box-like or rectangular and redirect the forces of occlusion toward the teeth and away from the soft tissue. Many times dentists will splint the anterior segment of teeth if there is a questionable periodontal prognosis or if complete cosmetic rehabilitation is desired. Precision-attachment partial dentures are a superior alternative to the clasped partial denture, especially in Kennedy Class I bilateral distal-extension cases.5,6
OPTION THREE: IMPLANTS
Compared to the clasped partial denture, implants are a relatively new phenomenon in modern dentistry. Implants have solved the problem of having to use other teeth as abutments to fill the space of a missing tooth. Implants, as free-standing units, are easier to clean and are the closest replacements to natural teeth dentistry has to offer.
Treatment planning Kennedy Class I bilateral distal extensions areas using implants totally eliminates the need for major or minor connectors, as fixed prosthetics are the restorations of choice (Figure 8). Class 1 levers do not exist unless cantilevers are used. The patient’s posterior teeth can now function without any connection to their anterior teeth. Depending on the patient’s quantity and quality of remaining bone, the abutment teeth may be connected together as a splint or may be inserted as individual free-standing units. The implant units are kept separate from any crowns or bridgework planned, and also are not connected to natural teeth. Fully integrated implants are completely surrounded and attached to bone. There is no physiologic movement of the implant body in the surrounding bone. Natural teeth are surrounded by fibrous, ligamentous tissue with a neurovascular bundle apparatus that provides a feedback mechanism for spatial relationships and pain. This apparatus allows for limited physiologic tooth movement in all directions.7 Based on these differences, implants and natural-teeth abutments are treated as separate units. The implant crowns or bridges may be screwed down to the implant using a screw attachment, such as a threaded tube and screw or a Combi-Snap (Sterngold Dental, LLC) (Figure 9). Screws allows for future retrievability of the abutments. They are used when abutments are short, the patient is a heavy bruxer, or in extreme Class II or III occlusions. They may also be cemented down to the implant abutments and treated as a traditional crown-and-bridge.
Treatment planning Kennedy Class I bilateral distal-extension cases provides a challenge to the restoring dentist. After carefully listening to the patient’s desires and fully analyzing the patient’s study models, radiographs, periodontal charting, dexterity, and medical history, a successful prosthesis can be fabricated that will meet and exceed the patient’s desires and will play a significant role in preserving the patient’s remaining dentition.
1. Grasso J, Miller E. Removable Partial Prosthodontics. 2nd ed. St. Louis, Mo: C.V. Mosby Co; 1980.
2. Applegate OC. Essentials of Removable Partial Denture Prostheses. 3rd ed. Philadelphia, Penn: W.B. Saunders Co; 1966.
3. Cinotti WR, Grieder A. Periodontal Prosthesis, Volumes 1 and 2. St. Louis, Mo: C.V. Mosby Co; 1968.
4. Shillingburg HT. Fundamentals of Fixed Prosthodontics. Chicago, Il: Quintessence Publishing Co, Ltd; 1978.
5. Preiskel HW. Precision Attachments in Prosthodontics: Overdentures And Telescopic Prostheses. Volume 2. Chicago, Il: Quintessence Publishing Co, Ltd; 1985.
6. Feinberg E. Diagnosing and prescribing therapeutic attachment-retained partial dentures. NYS Dent J. 1982;48(1):27-29.
7. Clepper D. Syllabus of Prosthetics for Osseointegrated Implants. Augusta, Ga: Omega Publications; 1997.
About the Author
George E. Bambara, DMD, MS
Staten Island, New York