For edentulous patients, successful denture therapy is influenced by the biomechanical phenomena of support, stability, and retention.42-44 Retention, or the resistance to movement of the denture away from the supporting tissues, is critical. Unfortunately, the physical, physiologic, and mechanical factors associated with denture retention are not completely understood. Physical forces influencing denture retention are believed to include adhesion, cohesion, capillary attraction, surface tension, fluid viscosity, atmospheric pressure, and external forces imparted to the prostheses by oral-facial musculature.45-51 Of these, interfacial surface tension associated with the saliva layer between the denture base and supporting soft tissues is quite important. This is particularly true for maxillary prostheses. Retention is realized as this saliva layer maximizes contact with approximating prosthetic and mucosal surfaces. Therefore, xerostomic patients who experience a quantitative or qualitative reduction in saliva may have reduced complete denture retention due to decreased interfacial surface tension.52-54
In the maxilla, alveolar resorption may obscure anatomic landmarks required to identify an effective postpalatal seal area. An ineffective or improperly located postpalatal seal may compromise denture retention.55 Therefore, reduced vertical alveolar height in a severely atrophic edentulous maxilla may result in poor denture stability and inadequate denture retention.56,57
The typical pattern of residual ridge resorption results in the medial-lateral and anterior-posterior narrowing the maxillary denture foundation and a perceived widening of the mandibular denture foundation.58-62 Resultant changes in horizontal maxillomandibular ridge crest relationships may necessitate setting posterior denture teeth in cross-bite. This arrangement may complicate force distribution to the denture bearing tissues. If cross-bite posterior denture occlusion is not carefully developed and managed in patients with severe residual ridge resorption, denture instability may result.63
The objective of complete denture therapy for patients with severe reduction of residual ridges is not solely the replacement of missing teeth. Rather, complete dentures must be designed to replace both the missing dentition and associated supporting tissues. In doing so, the denture base may occupy a substantial volume. Since denture base coverage of the hard palate is necessary to satisfy mechanical requirements of the prosthesis, and not to replace missing anatomic structures, care must be taken to limit denture base thickness in this area. In addition to replacing missing oral tissues, complete dentures structurally redefine potential spaces within the oral cavity. Inappropriate denture tooth positioning and physiologically unacceptable denture base contour or volume may result in compromised phonetics,64 inefficient tongue posture and function,59,65 and hyperactive gagging.66-69 Carefully designed external denture contours (i.e., cameo or polished denture surfaces) may contribute substantially to prosthesis stability and retention.70 Successful denture wearers master patterns of oral-facial muscular activity serve to retain, rather than displace, their prostheses. When optimally contoured, complete dentures occupy space in the oral cavity defined by the physiologic limits of acceptable muscular function, thus acquiring stability and retention during mastication, deglutition, and phonation.71,72 Conversely, poorly designed prostheses that do not accommodate anticipated muscular function may yield compromised denture stability and reduced retention.
Complete denture retention is, in part, influenced by denture occlusion. Most denture wearers consciously or subconsciously perform random, empty-mouth occlusal contacts throughout the day.73 These contacts may result from functional activity (e.g., swallowing) or parafunction (e.g., bruxism or clenching). A bilaterally balanced denture occlusion is intended to minimize the adverse consequences of functional and parafunctional empty-mouth loading by widely distributing these forces to the denture bearing structures.74 Therefore, a properly balanced denture occlusion may serve to dampen potentially detrimental occlusal forces acting to disrupt denture stability. A balanced occlusion is dependent on effective clinical and laboratory procedures. Accurate and precise registration of maxillomandibular relationships, meticulous articulation of master casts, careful positioning of denture teeth, and correct processing of denture bases must be accomplished. Both laboratory and clinical remount procedures are essential if optimal occlusal balance is to be achieved prior to delivery of the prostheses. Finally, periodic recall of all edentulous patients allows reevaluation of the denture occlusion; a clinical remount can be performed when correction is indicated.
Complete maxillary and mandibular dentures have long been considered the standard of care for treating edentulous patients. While most edentulous patients express relative satisfaction with their maxillary complete dentures, many do not enjoy equally successful mandibular denture comfort and function.75,76 The use of endosseous dental implants to assist in the support, stability, and retention of removable prostheses is now considered an effective treatment modality for the edentulous patient. Individuals wearing implant-assisted overdentures typically report improved oral comfort and function when compared to conventional, mucosa-supported prostheses.77-82 Except when contraindicated due to financial or surgical considerations, implant-assisted overdentures are usually the treatment of choice. A symposium held at McGill University addressed the efficacy of implant-assisted overdentures for treatment of edentulism. After thorough review of existing information, the following consensus statement was formulated:
“The evidence currently available suggests that the restoration of the edentulous mandible with a conventional denture is no longer the most appropriate first choice prosthodontic treatment. There is now overwhelming evidence that a two-implant overdenture should become the first choice of treatment for the edentulous mandible.”83