The biomechanical classification of a removable partial denture (RPD) based on the nature of the supporting tissue can be divided into 3 parts: tooth borne, tooth-mucosa borne, or entirely mucosa borne. The tooth-supported RPD usually has the edentulous space bounded by abutment teeth. The tooth-mucosa borne partial denture, in the majority of cases, are distal extensions bilateral or unilateral (Kennedy Class I and II). Finally, the last category (entirely mucosa borne) is usually not considered for definitive treatment, functioning in the cases when interim or transitional prostheses are required.
VERTICAL SEATING FORCES
The origin of the vertical seating forces on an RPD can be caused by mastication, swallowing, and parafunctional movements; resulting in tissue-ward movement of the extension base, as well as rotation around the fulcrum line. Different ways to counteract these movements include: use of the denture base, nonstress releasing clasps assemblies, and rests. In the case of a maxillary RPD, the major connector also counteracts these movements. On the other hand, for the tooth-borne removable partial denture, the vertical forces are counteracted by the teeth adjacent to the edentulous space, transmitting them to the teeth and from there to the underlying bone.
From the descriptions above, it is logical to assume that a tooth-borne partial denture would be more comfortable and less traumatic to the edentulous tissue and abutment teeth. In addition, it would provide the patient with enhanced retention. Since there would be no edentulous distal extension, minimal vertical seating movement would be counteracted by the soft tissues.
IMPLANTS AS AN ALTERNATIVE
Several years ago when a distal extension case presented to our practices, no alternative in the design of the removable partial denture was available. Common complaints associated with the Kennedy Class I and Class II removable partial denture are lack of stability, minimal retention, and unaesthetic retentive clasping. With the introduction of dental implants, the possibility of their placement in strategic locations within the edentulous space can transform a distal extension tooth-tissue borne partial denture into a tooth-implant borne partial denture, mimicking the tooth-borne partial denture.
OPTIONS HAVE INCREASED OVER TIME
The options for restoring edentulous areas have changed dramatically in the last 20 years, especially with the introduction of endosseous dental implants. Since the pioneering work by Bränemark, the use of osseointegrated implants to aid in restoring missing teeth has become the treatment of choice for all patients. Endosseous implants may vary in shape, length, width, surface coating; yet major brands all report a success rate of 95% in a period of 5 to 10 years.
The conventional endosseous implant placement protocol, according to Bränemark, et al involves a healing period of 3 to 6 months with a submerged placement. For implants placed immediately into extraction sites, the protocol involves a healing period of just 8 to 12 months. The most important requirement for implant-supported restorations is osseointegration of the dental implant. This process can be affected by the bone quality, surgical technique, patient habits, and other factors. Nonaxial loading during function can contribute to loss of osseointegration; this loading can be the product of different causes, primarily by the incorrect implant localization and angulations. Fabrication of surgical guides for the correct placement of an endosseous implant is imperative to avoid aesthetic, functional, and phonetic complications.
This article describes a simple technique to transform a bilateral distal extension (Kennedy Class I) tooth-mucosa borne RPD into a tooth-implant borne (Kennedy Class III) RPD.
DISCUSSION
To date, the number of RPDs placed with implants and natural teeth has been very limited. Mitrani, et al reported that the bone loss on posterior implants used for the distal extension on an RPD is less than 1.0 mm after its functional loading, and less than that when the implants are used only as a vertical stop and using resilient attachments.
Brudvik reported that when the implants are connected with other attachments, including conventional clasping on the other teeth (or precision crowns), lateral stability and retention could be enhanced. Extracoronal attachments such as an O-ring should be utilized when a single implant abutment is used on an RPD. On the contrary, stress relief such as a resilient wire should be used when intracoronal attachments are used.
Due to the difference in structure of implants and natural teeth, their mobility also differs. If a combination of implants and natural teeth is necessary, possible overloading of the implant(s) and the potential for intrusion of the natural teeth should be monitored carefully. The natural teeth, which were used as abutments in this case showed no mobility and had adequate crown-to-root rations.
CONCLUSION
The author presented this case report article because discussion regarding the application of implants with RPDs is limited in the literature. It is the author’s opinion that patients can benefit greatly with this treatment approach when performed with proper diagnosis, treatment planning, and technique.
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