Twenty million adults in the United States have completely edentulous mandibular arches.1 Over the past 15 years, many of these patients have been treated with an implant overdenture (IOD), which is a vast improvement compared with a traditional complete denture. The most common treatment option for an IOD is two anterior independent implants (option OD-1) (Figure 24-1). Most often, a fixed prosthetic option is not even presented to the patient. Although these restorations have more retention than a denture, they do not prevent the continued bone loss in the posterior regions of the mouth. This continued bone loss affects esthetics, function, and long-term health. It may eventually cause paresthesia and fracture of the mandible (Figure 24-2). The retention, stability, and support for a complete-arch overdenture are so superior to the other treatment options that almost all implant candidates should eventually have an implant-supported full-arch prosthesis (Figure 24-3). There are many reasons full-arch implant options (including a fixed restoration) should be part of the treatment options presented to an edentulous patient.
Comparing Fixed versus Removable Implant Prostheses
The number and location of implants for an overdenture which is completely implant supported, retained and stabilized by implants and a connecting bar and fixed prosthesis, is most often the same. The laboratory and component costs for a hybrid fixed restoration (denture teeth, acrylic and metal substructure) are often similar to those for a fully implant-supported overdenture (a type 4 removable prosthesis [RP-4]) with a connecting bar and attachments. The chair time required to fabricate an overdenture and bar is similar to that for an implant-supported fixed prosthesis. Therefore, these two restoration types should have a similar fee. Yet because dentures and partial dentures typically cost several times less than fixed restorations on teeth, the doctor often charges half the fee for an IOD and bar compared with a fixed restoration. As a result, many patients have preferred a fixed prosthesis but often for financial considerations chose a removable prosthesis. However, implant number, laboratory fees, component fees, and chair time are similar for these two types of prostheses. If the patient fees for these two restorations were similar, most patients would opt for a fixed prosthesis (Figure 24-4).
A fixed restoration provides the psychological advantage over an IOD of acting and feeling similar to natural teeth. An overdenture, even if fully implant supported, remains a removable prosthesis. A common remark heard from patients with implant fixed restorations is, “These implant teeth are better than my own teeth.” Patient comments related to IODs most often state, “These overdentures are better than my denture.”
Removable IODs require greater maintenance and exhibit more frequent prosthetic-related complications than fixed restorations. For example, Walton and McEntee noted that there were three times more maintenance and adjustments for removable prostheses compared with fixed restorations.2 IODs often require attachments to be changed or modified every 6 months to 2 years, and denture teeth often wear, requiring a new prosthesis to be fabricated every 5 to 7 years.2–7 In a review of literature by Goodacre et al., IODs have retention and adjustment problems 30% of the time, relines 19% of the time, clip or attachment fracture 17% of the time, and fracture of the prosthesis 12% of the time.6 Fixed prostheses need less repair and less maintenance and often last the life of the implant support. Although porcelain fractures with a fixed restoration may occur and be costly to repair, over a lifetime, the implant-supported removable prosthesis is often more expensive.
Unlike the labial flange of a maxillary denture, the labial flange of a mandibular overdenture rarely is required for esthetics. A mandibular overdenture often traps food below its flanges, similar to a denture. Dentures and IODs are border molded to the muscle attachment level to allow the floor of the mouth to raise during swallowing. As a consequence, food accumulates below the denture flange while the muscles are at rest and then is compressed under the restoration during deglutition. The contour of a fixed restoration is less prone to food entrapment. The daily care for a bar IOD (RP-4) may be similar to that for a fixed mandibular restoration because ridge lap pontics are not required for esthetics or speech, as with some maxillary fixed prostheses.
A more recent clinical study by Wright et al. has evaluated posterior mandibular bone loss in IODs (type 5 removable prosthesis [RP-5]) compared with cantilevered fixed prostheses from anterior implants.8 The annual bone loss index observed in the RP-5 overdentures ranged from +0.02 to −0.05 with 14 of 20 patients losing bone in the posterior regions. On the other hand, the fixed prostheses group had a range from +0.07 to −0.015 with 18 of 22 patients gaining posterior bone area (see Figure 24-3). Reddy et al. also found a similar clinical observation in 60 consecutively treated cantilevered fixed prostheses supported by five to six implants placed between the foarmina.9 The mandibular body height was measured 5, 10, 15, and 20 mm distal to the last implant. The baseline measurements up to 4 years after function increased from 7.25 ± 0.25 mm to 8.18 ± 0.18 mm. Nearly all of the bone growth occurred during the first year of function. Therefore, an important role for the complete implant-supported restoration is the maintenance and even regeneration of posterior bone in the mandible. This is especially important because continued posterior bone loss in this region may lead to paresthesia and even mandibular body fracture (Figure 24-5; Box 24-1).
Too often, the dentist offers overdentures as the only option for edentulous patients rather than including fixed treatment options. The advantages of a fixed restoration over an overdenture warrant almost all edentulous patients to be given an option for a fixed prosthesis. This chapter discusses fixed treatment planning options for the completely edentulous mandibular arches.10 These same implant location options may also be used for RP-4 restorations in the mandible.
The amount of force transmitted to an implant-fixed prosthesis is similar to that of a completely implant-supported overdenture (RP-4). In one aspect then, the number of implants to support either prosthesis type should be similar. Mandibular overdentures may be removed at night to decrease the risk of nocturnal parafunctional overload. However, most mandibular edentulous patients also have an edentulous maxilla. Therefore, if the patient is willing to remove the maxillary dentureat night, the risk of nocturnal parafunction also may be eliminated for patients with mandibular fixed prostheses. As a consequence, the number of implants required to restore a fixed prosthesis may be similar to that for a fully implant-supported overdenture.
When the patient has natural teeth or implants in the maxilla, more implants usually are indicated for the mandibular fixed prosthesis (compared with patients with an upper denture) to reduce the risk of occlusal overload, or a reduction in cantilever length is necessary. Force factors such as parafunction, crown height, masticatory dynamics, and the bone density of the implanted regions also may modify the implant position, implant number, size, and design for a fixed prosthesis. Increased force factors contribute to uncemented restorations, screw loosening, component fracture, crestal bone loss, and even implant failure. As a result, the fixed prosthesis often may require an improved biomechanical position or greater implant number compared with an IOD support system.
Many reports have addressed the dimensional changes of the mandible during jaw activity as a result of masticatory muscle action.11–19 Five different movements have been postulated.18 Medial convergence is the one most commonly addressed.19 The mandible between the mental foramina is stable relative to flexure and torsion. However, distal to the foramina, the mandible exhibits considerable movement toward the midline on opening.15,16 This movement is caused primarily by the attachment of internal pterygoid muscles on the medial ramus of the mandible.
The distortion of the mandible occurs early in the opening cycle, and the maximum changes may occur with as little as 28% opening (or about 12 mm).11 This flexure has also been observed during protrusive jaw movements.20,21 The greater the active opening and protrusive movements, the greater the amplitude of mandibular flexion. The amount of movement varies among individuals and depends on the density and volume of bone and the location of the site in question. In general, the more distal the sites, the more medial flexure. The amplitude of the mandibular body flexure toward the midline has been measured to be as much as 800 microns in the first molar–to–first molar region to as much as 1500 microns in the ramus-to-ramus sites (Figure 24-6). In a study by Hobkirk and Havthoulas on deformation of the mandible in subjects with fixed dental implant prostheses, medial convergence up to 41 mm was observed.22
Torsion of the mandibular body distal to the foramina has also been documented in both animal and human studies.23–26 Hylander evaluated larger members of the rhesus monkey family (macaque) and found the mandible twisted on the working side and bent in the parasagittal plane on the balancing side during the power stroke of mastication and unilateral molar biting26 (Figure 24-7). Parasagittal bending of the human jaw during unilateral biting was confirmed by Marx,27 who measured localized mandibular distortion in vivo in humans by using strain gauges on screws attached to cortical bone in the symphyseal and gonial regions. Abdel-Latif et al. confirmed that the mandibles of patients with implant prostheses measured up to 19 degrees of dorsoventral shear.23 The torsion during parafunction is caused primarily by forceful contraction of the masseter muscle attachments (Figure 24-8). Therefore, parafunctional bruxism and clenching may cause torsion-related problems in the implant support system and prosthesis when the mandibular teeth are splinted from the molar to molar regions.
The posterior bone gain in edentulous patients restored with cantilevered prostheses from anterior implants may be a consequence of the mandibular flexure and torsion, which stimulate the bone cells in the region. Because the bite force may increase 300% with an implant prosthesis compared with a denture, the increased torsion may stimulate the posterior mandibular body to increase in size, as reported by Reddy et al.9 and Wright et al.8
Misch has observed the increase in flexure in the posterior mandible is a result of the mental foramen weakening of the facial cortical plate.10 As such, the mandible flexes and has torsion distal to the foramen. The most common position of the mental foramen is between the first and second premolar teeth. Therefore, when bilaterally splinting teeth distal to the premolar positions, mandibular dynamics should be considered.22 Posterior rigid, fixated implants splinted to each other in a full-arch restoration are subject to a considerable buccolingual force on opening and during parafunction.22,28,29 A study by Miyamoto et al. identified jaw flexure as the primary cause of posterior implant loss in full-arch splinted mandibular prostheses.28 The more distal the rigid splint from one side to the other, the greater the risk that mandibular dynamics may influence the implants or prosthesis prognosis. In addition, the body of the mandible flexes more when the size of the bone decreases.10 As a result, the division C minus height (C–h) or division D mandible flexes or exhibits torsion more than the division A mandible, all other factors being similar.
The difference in movement between an implant and a tooth has been addressed as a concern for dentists when splinting these objects together. The natural tooth movement ranges from 28 microns apically and 56 to 108 microns laterally. In contrast, the rigid implant has movement up to 5 microns apically and 10 to 60 microns laterally. Yet the mandibular flexure and torsion may be more than 10 to 20 times the movement of a healthy tooth. Therefore, the flexure and torsion of the mandibular body are more critical in the patient evaluation compared with whether an implant should be joined to the natural dentition.
Past authors have suggested four implants in the mandible with a full-arch splinted fixed restoration—two in the first molars and two in the canine regions30 (Figure 24-9). Additional implants have been used with this full-arch splinted restorative option, with up to four other implants in the premolar and the incisor regions.31 However, complete cross-arch splinting of posterior molar implants with a rigid, fixated prosthesis should be reconsidered in the mandible. The flexure of the mandible is thwarted by the prosthesis,22,23,29,32–34 but this introduces lateral stresses to the implant system (cement, screw, crestal bone, and implant–bone interface). These lateral stresses place the molar implants, screws, and bone at increased risk because of the mandibular flexure and torsion previously addressed.
In complete mandibular subperiosteal implants, pain upon opening was noted in 25% of the patients at the suture removal appointment when a rigid bar connected molar-to-molar regions. When the connecting bar was cut into two sections between the foramina, the pain upon opening was immediately eliminated. This clinical observation does not mean that the other 75% of patients did not have flexure of the mandibular arch upon opening. The observation does demonstrate, however, that flexure may be relevant to postoperative complications. Consequences of a treatment plan with cross-arch connection of posterior mandibular implants may include bone loss around the implants, loss of implant fixation, material fracture (implant or prosthesis components), unretained restorations, and discomfort upon opening. Until clinical studies become available and state otherwise, full-arch splinted restorations joining bilateral molar implants in the mandible should not be a treatment of choice.