Chapter 14 The Edentulous Mandible: An Organized Approach to Implant-Supported Overdentures

The dental profession and the public are more aware of the problems associated with a complete mandibular denture than any other dental prosthesis. The placement of implants enhances the support, retention, and stability of an overdenture. As a result, patients are very willing to accept a treatment plan for a mandibular implant overdenture. There is greater flexibility in implant position or prosthesis fabrication with a mandibular implant overdenture, and, as a result, it is also an ideal treatment modality to begin a learning curve in implant dentistry. Therefore one of the most beneficial treatments rendered to patients is also one of the best introductions for a dentist into the discipline of implant dentistry. In comparison, maxillary implants for overdentures are not used as often and are also more difficult to place and restore.

The concept of mandibular implant–supported overdentures has been used for many years. Successful reports were published originally with mandibular subperiostal implants or with immediately loaded and stabilized root form implants in the anterior mandible.^1,² An increased awareness from the profession and patients allows a variety of clinical situations, bone densities, biomechanics, and patients’ desires to restore an ever-growing number of patients with implant-supported overdentures.³^–³² As the data gathered on this prosthetic modality grow, overdentures are gaining even greater acceptance and justifying their place in the armamentarium of the implant dentist.

From a bone volume conservation standpoint, complete edentulous patients should be treated with enough implants to support overdentures in the maxilla and mandible. The continued bone loss after tooth loss and associated compromises in esthetics, function, and health make all edentulous patients implant candidates. The average denture patient does not see a dentist regularly. In fact, 10 or more years usually separate dental appointments of edentulous patients. The more often a patient wears a denture, the greater the bone loss. Yet 80% of denture patients wear their dentures day and night, which accelerates the bone loss.

Because of the long hiatus between dental visits, the amount of resorption from initial denture delivery to the next professional interaction already has caused the destruction of the original alveolar process. The bone loss that occurs during the first year after tooth loss is 10 times greater than in following years. In the case of multiple extractions, this often means a 4-mm vertical bone loss within the first 6 months. As the bony ridge resorbs, the muscle attachments become level with the edentulous ridge.³³^–³⁵

Rather than waiting until the patient has lost most of the residual bone, the dentist should inform and emphasize to the patient the benefits of implants and why they should be inserted before the bone is lost. Therefore the profession should treat bone loss from extraction in a similar fashion as bone loss from periodontal disease. Rather than waiting until the bone is resorbed or the patient complains, the dental professional should educate the patient about the bone loss process caused not only by periodontal disease, but also by the lack of stimulation and its consequences of bone resorption, and explain how implants are available to treat the condition. Therefore most completely edentulous patients should be informed of the necessity of dental implants to maintain bone volume, function, masticatory muscle activity, esthetics, and psychologic health. Ideally, patients who have unsalvageable teeth should be given the option to include implants to support the future prosthesis. The traditional complete denture may be presented as a temporary measure to provide cosmetic and oral function during implant treatment.

The next progression in the implant philosophy is to convert all mandibular implant and soft tissue–supported restorations to a completely implant-supported prosthesis. The majority of mandibular overdentures are supported by two implants anterior to the foraminae and soft tissue support in the posterior regions (Figure 14-1). Yet posterior bone loss occurs four times faster than anterior bone loss. In the completely edentulous patient, the eventual paresthesia and mandibular body fractures are primarily from posterior bone loss. The anterior implants allow improved anterior bone maintenance, and the prosthesis benefits from improved function, retention, and stability. However, the lack of posterior support in two- and three-implant overdentures may cause accelerated posterior bone loss. To the contrary, studies by Wright et al.³⁰ and Reddy et al.³⁶ found prostheses completely supported by implants in the edentulous mandible actually may increase the posterior bone volume (even though posterior implants are not inserted). As a result, complete implant-supported restorations should be the restoration of choice.

Figure 14-1 The majority of mandibular overdentures performed by dentists are with two implants, regardless of factors such as the remaining bone, the opposing arch, and patient complaints. Although this treatment is an improvement compared with a denture, the posterior bone loss continues and the anterior implants may experience greater problems than when additional implants are used.

Financial considerations have been identified as the reason for the selection of a limited treatment, which may consist of two or three implants to support the overdenture.³⁷^–³⁹ These restorations may be used as transitional devices until the patient can afford to upgrade the restoration. When a partially edentulous patient cannot afford to replace four missing first molars, the dentist often will replace one at a time over many years. Likewise, the dental implant team can insert one or two additional implants every few years until finally a complete implant-supported prosthesis is delivered.

The ultimate goal may be designed in the beginning of treatment and may take many years to complete. However, the advantage of developing a treatment plan for long-term health, rather than short-term gain, is beneficial to the patient. As such, if finances are not an issue, the dentist should design a prosthesis that is completely supported, retained, and stabilized by implants. If cost is a factor, a transitional implant-retained restoration greatly improves the performance of a mandibular denture. Then the dentist may establish a strategy for the next one or two steps for the final restoration.

REVIEW OF THE LITERATURE

In 1986 a multicenter study reported on 1739 implants placed in the mandibular symphysis of 484 patients. The implants were loaded immediately and restored with bars and overdentures with clips as retention. The overall success rate was 94.² Engquist et al.³ reported a 6% to 7% implant failure for mandibular implant–supported overdentures and a 19% to 35% failure for maxillary implant overdentures. Hyperplasia below the bar occurred in 25% of the patients. Prosthetic complications were not reported. Jemt et al.⁴ reported on a 5-year prospective, multicenter study on 30 maxillae (117 Brånemark implants) and 103 mandibles with 393 implants. Survival rates in the mandible were 94.5% for implants and 100% for prostheses; in the maxilla, the survival rates were 72.4% for implants and 77.9% for prostheses. Higher failure rates in the maxilla were related directly to poor density and quantity of bone with a characteristic cluster failure pattern.

Wismeijer et al.⁵ reported on 64 patients with 218 titanium plasma-sprayed implants, with a 97% survival with overdentures in a 6.5-year evaluation. Naert et al.¹⁴ found 100% implant success at 5 years for overdentures with different anchorage systems. In Belgium, Naert at al. reported on 207 consecutively treated patients with 449 Brånemark implants and Dolder-bar overdentures. In this report, the cumulative implant failure rate was 3% at the 10-year benchmark.^9,¹⁰ In this long-term report, prosthetic complications related to overdenture relining and loose retention occurred 10% of the time. More care was needed if ball attachments were used rather than a Dolder bar.⁹ In a 10-year study reporting on attachment methods, balls provided the greatest retention and magnets always scored the lowest.¹⁰ Similarly, Hutton et al.¹¹ reported 97% survival rates for mandibular overdentures.

Misch ¹² reported less than 1% implant failure and no prosthesis failure over a 7-year period with 147 patients when using the organized treatment options and prosthetic guidelines presented in this chapter. Kline et al.¹³ reported on 266 implant-supported overdentures for 51 patients, with an implant survival rate of 99.6% and a prosthesis survival rate of 100%. Mericke-Stern et al.¹⁶ reported 95% implant survival with two implant overdentures. In a randomized clinical report, Awad et al.⁶ compared satisfaction and function in complete dentures (48 patients) versus two implant-supported overdentures (IODs) in 56 patients. There was significantly higher satisfaction, comfort, and chewing ability in the IOD group. A similar study in a senior population yielded similar results.⁷ Thomason et al.,⁸ in the United Kingdom, reported a 36% higher satisfaction for the implant IOD patients than the complete denture wearers in the criteria of comfort, stability, and chewing. In Canada, Attard and Zarb ⁴⁰ followed IOD wearers for 20 years with a success rate of 84% and 87% for prosthesis and implants, respectively. In a 10-year study of IODs in Israel, with 285 implants and 69 implant over dentures, Schwartz-Arad et al. reported implant survival was 96.1% with higher success rates in the mandible.⁴¹ Many reports have been published over the last two decades that conclude that implant-supported overdentures represent a valid beneficial option for denture wearers.³^–⁴¹ It should be noted that the majority of reports are for implant overdentures supported by only two implants.

ADVANTAGES OF IMPLANT OVERDENTURES

Traditional overdentures must rely on the remaining teeth to support the prosthesis. The location of these natural abutments is highly variable, and they often comprise past bone loss associated with periodontal disease. For a mandibular implant–supported overdenture, the implants may be placed in planned, specific sites, and their number may be determined by the restoring doctor and patient. In addition, the overdenture implant abutments are healthy and rigid and provide an excellent support system. As a result, the related benefits and risks of each treatment option may be predetermined.

The patient gains several advantages with an implant-supported prosthesis (Box 14-1). Minimal bone resorption of the anterior residual ridge occurs with the placement of implants. After the extraction of mandibular teeth, an average of 4-mm vertical bone loss occurs during the first year after treatment. This bone loss continues over the next 25 years, with the mandible experiencing a fourfold greater vertical bone loss than the maxilla.³⁵ The bone under an overdenture may resorb as little as 0.6 mm vertically over 5 years, and long-term resorption may remain at less than 0.05 mm per year.^4,^40,^42,⁴³

Bone loss dictates the appearance of the inferior third of the face. A maxillary overdenture often provides improved support for the lips and soft tissues of the face compared with a fixed prosthesis because the prosthesis contour does not have to accommodate daily hygiene requirements. Denture teeth also provide an esthetic replacement for the natural dentition, which is more challenging for the technician to recreate with porcelain-fused-to-metal restorations. For the laboratory to create pink interdental papilla, as well as replace the soft tissue drape, is easier with an overdenture compared with porcelain-metal fixed restorations. In addition, the teeth can be positioned in the most esthetic position, without any restriction as to the relationship to the atrophied crest, because stability now is provided by the implant and does not depend on tooth position on the crest of the ridge.

Soft tissue abrasions and accelerated bone loss are more symptomatic of horizontal movement of the prosthesis under lateral forces. An implant-supported overdenture may limit lateral movements and direct more longitudinal forces. A mandibular denture may move 10 mm during function. Under these conditions, specific occlusal contacts and the control of masticatory forces are nearly impossible. An implant overdenture provides stability of the prosthesis, and the patient is able consistently to reproduce a determined centric occlusion.⁴⁴

A study of chewing efficiency compared wearers of complete dentures with wearers of implant-supported overdentures. The complete denture group needed 1.5 to 3.6 times the number of chewing strokes compared with the overdenture group.⁴⁵ The chewing efficiency with an implant overdenture is improved by 20% compared with a traditional complete denture.^6,^7,^46,⁴⁷

Higher bite forces have been documented for mandibular overdentures on implants. The maximum occlusal force of a patient with dentures may improve 300% with an implant-supported prosthesis.⁴⁸ Mericke-Stern⁴⁹ and Mericke-Stern et al.⁵⁰ compared mastication between root overdentures and implant overdentures. The former was more discriminative, whereas the latter developed slightly harder chewing strokes and tended to masticate more vertically. Jemt et al.⁵¹ showed a decrease in occlusal force when the bar connecting implants was removed, which they attributed to the loss of support, stability, and retention. If enough implant support is provided, the resulting prosthesis may be completely supported, retained, and stabilized by the implant (removable prosthesis type 4 [RP-4]).

The complete mandibular denture often moves during mandibular jaw movements during function and speech. The contraction of the mentalis, buccinator, or mylohyoid muscles may lift the denture off the soft tissue. As a consequence, the teeth may touch during speech and elicit clicking noises. The retentive implant overdenture remains in place during mandibular movement. The tongue and perioral musculature may resume a more normal position because they are not required to limit mandibular denture movement.

The implant overdenture may reduce the amount of soft tissue coverage and extension of the prosthesis. This is especially important for new denture wearers, patients with tori or exostoses, or patients with low gagging thresholds. Also, the existence of a labial flange in a conventional denture may result in exaggerated facial contours for the patient with recent extractions. Implant-supported prostheses do not require labial extensions or extended soft tissue coverage.

Soft and hard tissue defects from tumor excision or trauma do not permit the successful rehabilitation of the patient with traditional denture support. Hemimandibulectomy patients and other maxillofacial patients also may be restored successfully with an implant overdenture.^52,⁵³

The implant overdenture also provides many practical advantages over the implant-supported complete fixed partial denture (Box 14-2). Fewer implants are required, because soft tissue areas may provide additional support. Regions of inadequate bone for implant placement therefore may be eliminated from the treatment plan, rather than necessitating bone grafts or placing implants with poorer prognosis. Abutments do not require a specific mesiodistal placement because the prosthesis completely covers the implant abutments.

The esthetics for many edentulous patients with moderate to advanced bone loss are improved with an overdenture compared with a fixed restoration. Soft tissue support for facial appearance often is required for an implant patient because of advanced bone loss, especially in the maxilla. Interdental papilla and tooth size are easier to reproduce or control with an overdenture. Denture teeth easily reproduce contours and esthetics compared with time-consuming and technician-sensitive porcelain-metal fixed restorations. The labial flange may be designed for optimal appearance, not daily hygiene.

Hygiene conditions and home and professional care are improved with an overdenture compared with a fixed prosthesis. Peri-implant probing is diagnostic and easier around a bar than a fixed prosthesis because the crown often prevents straight line access along the abutment to the crest of the bone. The overdenture may be extended over the abutments to prevent food entrapment during function. Speech is not compromised because the denture may extend onto the soft tissues in the maxilla and prevent air and saliva from escaping.

An overdenture may be removed at bedtime to reduce the noxious effect of nocturnal parafunction, which increases stresses on the implant support system. The overdenture may provide stress relief between the superstructure and prosthesis, and the soft tissue may share a portion of the occlusal load. The prosthesis is usually easier to repair than a fixed restoration. Reduced laboratory fees and fewer implants allow the restoration of patients at reduced costs compared with a fixed prosthesis. In addition, long-term denture patients do not appear to have a psychologic problem associated with the ability to remove their implant prostheses.⁵⁴^–⁶²

When cost is a factor, two implant-retained IODs may improve the patient’s condition at a lower overall treatment cost than a fixed implant–supported prosthesis.^42,^44,⁶³ A survey by Carlsson et al.⁶⁴ in 10 countries indicated a wide range of treatment options. The proportion of implant overdentures selection versus fixed implant dentures was highest in the Netherlands (93%) and lowest in Sweden and Greece (12%). Cost was cited as the number one determining factor in the choice.

The primary indications for a mandibular implant overdenture are problems often found with lower dentures, such as lack of retention or stability, decrease in function, difficulties in speech, tissue sensitivity, and soft tissue abrasions. If an edentulous patient is willing to remain with a removable prosthesis, an overdenture is often the treatment of choice. In addition, if cost is a problem for the patient, the overdenture may serve as a transitional device until additional implants may be inserted and restored.

DISADVANTAGES OF IMPLANT OVERDENTURES

The primary disadvantage of a mandibular overdenture is related to the patient’s desire, primarily because they do not want to be able to remove the prosthesis. A fixed prosthesis often is perceived as an actual body part of the patient, and if a patient’s primary request is not to remove the prosthesis, an implant-supported overdenture would not satisfy the psychological need of this patient.

The mandibular overdenture treatment plan requires more than 12 mm of space between crestal bone and the occlusal plane (Figure 14-2). When sufficient crown height space is lacking and the prosthesis is more prone to component fatigue and fracture, an overdenture is more difficult to fabricate than a porcelain-to-metal fixed prosthesis. The 12-mm minimum crown height space provides adequate bulk of acrylic to resist fracture, space to set denture teeth without modification, and room for attachments, bars, soft tissue, and hygiene. In the mandible, the soft tissue is often 1 to 3 mm thick above the bone, so the occlusal plane to soft tissue should be at least 9 to 11 mm in height. An osteoplasty to increase crown height space before implant placement or a fixed restoration is often indicated when abundant bone height and width are present.

Figure 14-2 The mandibular overdenture requires at least 12 mm between the soft tissue and the occlusal plane to provide sufficient space (15 mm from bone level to occlusal plane) for the bar, attachments, and teeth.

Mandibular overdenture wearers often incur greater long-term expenses than those with fixed restorations. Attachments such as O-rings or clips wear and must be replaced regularly. Replacements appear more frequent during the first year, but remain a necessary maintenance step.^17,^20,^34,⁶⁵^–⁷² Denture teeth wear faster on an implant overdenture than with a traditional denture because bite force and masticatory dynamics are improved. A new overdenture often is required at 5- to 7-year increments because of denture tooth wear and soft tissue support changes. Therefore patient education of the long-term maintenance requirement should be outlined at the onset of implant therapy.⁷³

A primary concern for RP-5 overdentures compared with RP-4 or fixed restorations is the continued bone loss in the posterior regions. The posterior bone resorbs faster than the anterior bone, and implant prostheses with posterior soft tissue support may accelerate posterior bone resorption two to three times faster than in a complete denture wearer.⁷³^–⁷⁵ By contrast, patients wearing fixed implant–supported prostheses showed no bone loss and usual occurrences of bone apposition.^76,⁷⁷ Therefore the short-term benefit of decreased cost may be offset by the accelerated bone loss that is a primary consideration, especially in the younger edentulous patient. As previously discussed, all implant overdentures would benefit if they were completely implant supported, and the recommendation is to consider a RP-5 prosthesis as an interim device designed to enhance the function of the patient. These prostheses should not be considered as an end result for all patients. Instead, a regular evaluation of patients’ performance paired with patient education should enable the transformation to a RP-4 restoration. In addition, reports indicate that mandibular implant overdentures may cause a combination-like syndrome, with increased looseness, subjective loss of fit, and midline fracture of the upper denture.⁷⁸^–⁸² Although not yet established as a cause-and-effect situation, the condition also appears to be controlled by the choice of proper occlusal scheme.⁸⁰

A side effect of a mandibular overdenture is food impaction. The flanges of the prosthesis do not extend to the floor of the mouth in the rest position (to eliminate sore spots caused by elevation of the floor of the mouth during swallowing). However, during eating, food particles migrate and become impacted under the prosthesis during swallowing. A similar condition is found with a traditional denture. However, because a lower denture “floats” during function, the food more readily goes under and out, whereas the implant overdenture traps the food debris against the implants, bars, and attachments (Box 14-3).