INTRODUCTION
Removable prosthodontics has historically been a difficult and frustrating modality. The mastication of food with these appliances assists the edentulous patient in obtaining adequate nutrition; however, complete dentures constructed under even the most ideal conditions will only have a chewing efficiency equal to a fraction of the natural dentition.1 To exacerbate this situation, the absence of natural teeth and continued use of the complete denture causes alveolar bone resorption, resulting in less surface area of foundation for support of the denture base.
The use of endosseous, root-form dental implants to improve comfort and function of removable prostheses has been a major step forward for countless denture wearers. In fact, a panel of relevant experts at the McGill conference in 1992 concluded that the evidence available suggested the restoration of the edentulous mandible with a conventional denture is no longer the most appropriate first choice prosthodontic treatment, and there is overwhelming evidence that a 2-implant overdenture should become the first choice treatment.2
Since clinicians regularly offer this more sophisticated level of prosthodontic treatment, a review of certain principles is appropriate. Removable denture fabrication can either be a highly rewarding experience or a frustrating and demanding encounter. All clinicians want the best results for their patients; however, sometimes our best efforts fall short of patients’ expectations. For this reason, the best way to avoid failures with implant-assisted removable prosthodontics is to have a clear understanding of some basic principles. In any educational discussion of a removable prosthesis, there are always the concepts of support, stability, and retention. In order to understand these concepts as they relate to the implant-assisted prosthesis, one must first understand their relevance in conventional complete denture prosthodontics. When a denture is lacking in one or more of these basic tenets, the prosthesis can be either uncomfortable, dysfunctional, or even totally useless. When presenting for treatment, a correct diagnosis is critical to ascertain the patient’s particular needs in order to ensure a successful result. A lack of support, lack of stability, and lack of retention are each managed in their own unique manner.
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| Figure 1. A preoperative panoramic radiograph. | Figure 2. A mandibular edentulous jaw with 2 dental implants. |
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| Figure 3. A mandibular soft-tissue-supported, implant-retained overdenture prosthesis. | Figure 4. The overdenture in place, with full extension for support and stability. |
Support
Support is defined as the foundation area on which a dental prosthesis rests. It is those areas of the maxillary and mandibular edentulous ridges that are considered best suited to carry the forces of mastication when the dentures are in function.3 The stress-bearing area for support of the mandibular complete denture is from 2 sources:
1. The External Oblique Ridge. This is the primary support area of the complete denture, and it is lined with cortical bone, horizontal to the forces of mastication, and stable after extractions.
2. Alveolar Process/Residual Ridge. Composed of mostly cancellous bone, and often covered with cortical bone, it is susceptible to continued resorption and is not stable after extractions.
The retromolar pad is the pear-shaped tissue located at the posterior end of the alveolar ridge and is stable after extractions. It is composed of non-keratinized, loose alveolar tissue covering glandular tissues, fibers of buccinator muscle, fibers of superior constrictor muscle, fibers of pterygomandibular raphe, and the terminal part of the tendon of temporalis muscle. Although not a primary support structure, it is an important landmark that must be covered with the posterior flange of the prosthesis since it provides for the basal seal of the mandibular denture as well as being an important structure for resisting horizontal displacement.
Stability
Stability is defined as the quality of a prosthesis to be firm, steady, or constant and to resist displacement by functional horizontal or rotational stresses.3 The stability of the denture base is influenced greatly by the size and shape of the residual ridges. Unfortunately, as stated above, the residual ridge is not stable after extractions over time and, as resorption occurs, instability results. It has been shown that ridge shape has a direct relationship specifically to denture stability.4 In fact, this factor is quite significant with respect to the stability of the mandibular denture. Other factors involved in the stability of conventional dentures are denture base adaptation, residual ridge relationship, occlusal harmony, and neuromuscular control.5 Resistance to horizontal displacement, which defines stability, requires maximum adaptation of the denture base to available bone and bony inclines. As an example, adaptation of the denture flanges to the lingual slope of the mandible resists horizontal forces since this incline approaches a 90° angle to the occlusal surface.
Retention
Retention is that quality inherent in the prosthesis acting to resist the forces of dislodgment along the path of placement.3 Factors affecting retention are:
1. anatomic (size and shape of residual ridges)
2. physiologic (quantity and viscosity of saliva)
3. physical (adhesion, cohesion, and surface tension)
4. neuromuscular control
Types of Implant-Assisted Removable Prostheses
Two types of implant-assisted removable prostheses are commonly used in prosthodontics.
The first is a soft-tissue-supported, implant-retained prosthesis. This is an overdenture, which gains primary support from the anatomical structures of the mouth and uses dental implants solely for retention of the prosthesis. It is important to understand how this type of prosthesis gains its support, stability, and retention.
The mandibular prosthesis, which is soft-tissue supported and implant retained, must have full extension into the mandibular external oblique ridge area for its support. It also must have retromolar pad coverage for a basal seal and adequate lingual flange extension for adequate stability. Since implants will provide only retention, anatomic structures must provide support and stability equal to that of a conventional complete denture.
The second type of prosthesis is the implant-supported, implant-retained prosthesis, typically a bar/overdenture. This overdenture does not rely on support, stability, or retention from anatomical structures of the mouth. Instead, the implant-fixed foundation, usually a bar/framework, provides support, stability, and retention for the removable prosthesis. This type of implant-assisted prosthesis need not extend into the support areas of the buccal shelf in the mandible. It also can limit its extension for purposes of stability since this is provided for in the fixed bar/framework component of the prosthesis.
The clinician must carefully ascertain the patient’s needs during the diagnostic evaluation and provide a removable prosthesis that matches the patient’s clinical deficiencies. If the patient has adequate anatomical foundation to satisfy support and stability, an implant-assisted prosthesis that uses implants solely for retention will be adequate. However, if significant deficiencies exist in the anatomic foundation that precludes adequate retention and stability, a fully implant-supported, implant-retained prosthesis is indicated.
CASE REPORT
Diagnosis and Treatment Planning
A 65-year-old male, in good overall health, presented for consultation regarding replacement of his existing complete dentures. Clinical examination revealed an edentulous state with excellent maxillary and mandibular alveolar foundation for a favorable complete denture prognosis. Prosthodontic Classification 1 edentulism was diagnosed. The patient’s existing dentures were poorly fitting, worn, and required replacement.
The treatment plan was for a new maxillary complete denture and a mandibular soft-tissue-supported, implant-retained prosthesis. Since the mandibular prosthesis would utilize 2 implants for retention alone, standard anatomic structures would provide for support and stability. For this reason, the mandibular overdenture was constructed with adequate extension into the external oblique ridge area for support, lingual flanges into the mylohyoid area for stability, and coverage of the retromolar pad for a basal seal.
Two 4 × 13 mm dental implants (Biomet 3i Osseotite Tapered Certain Prevail [Zimmer Biomet]) were placed in the position of teeth Nos. 22 and 27 and were fitted with retentive abutments, and the mating units in metal housings were processed into the denture base (LOCATOR Attachment System [Zest Dental Solutions]). After the laboratory procedures were complete, black processing elements were replaced with standard retentive elements (Figures 1 to 4).
CLOSING COMMENTS
The use of dental implants has substantially changed removable prosthodontic treatment for edentulous patients. Proper diagnosis of the anatomic and physiologic prosthesis foundation with respect to support, stability, and retention is critical for a successful outcome. It is incumbent upon the clinician to determine the type of implant-assisted prosthesis that will satisfy the patient’s needs and then design accordingly. If the treatment plan is for a soft-tissue-supported, implant-retained prosthesis, proper extension and adaptation must follow conventional procedures. If the treatment plan is for an implant-supported, retained prosthesis, support, stability, and retention is provided by the fixed implant framework and, therefore, denture base extension is typically not needed.
Acknowledgment:
The author wishes to acknowledge the assistance of Dr. Mohammad Emadi, prosthodontic resident, Nova Southeastern University College of Dental Medicine, Ft. Lauderdale, Fla.
References
- Winkler S, ed. Essentials of Complete Denture Prosthodontics. 3rd ed. Delhi, India: AITBS Publishers; 2015.
- Thomason JM. The McGill Consensus Statement on Overdentures. Mandibular 2-implant overdentures as first choice standard of care for edentulous patients. Eur J Prosthodont Restor Dent. 2002;10:95-96.
- The glossary of prosthodontic terms: ninth edition. J Prosthet Dent. 2017;117:e1-e105.
- Ribeiro JA, de Resende CM, Lopes AL, et al. The influence of mandibular ridge anatomy on treatment outcome with conventional complete dentures. Acta Odontol Latinoam. 2014;27:53-57.
- Jacobson TE, Krol AJ. A contemporary review of the factors involved in complete dentures. Part II: stability. J Prosthet Dent. 1983;49:165-172.
Dr. Piermatti is a Diplomate of the American Board of Prosthodontics and the American Board of Oral Implantology and serves as a faculty member and director of the Dental Implant Maxicourses at the Rutgers University School of Dental Medicine and the Nova Southeastern University College of Dental Medicine. He also maintains a private practice limited to prosthodontics and dental implantology with offices located in Voorhees, NJ, and Boynton Beach, Fla. He can be reached via email at jpiermatti@yahoo.com.
Disclosure: Dr. Piermatti reports no disclosures.
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