Immediate Functional Load of Mandibular Implant Overdentures: A Surgical and Prosthodontic Rationale of 2 Implant Modalities

The viability and predictability of osseointegrated endosseous implants in treating patients who are completely and partially edentulous have been supported with the high success rates reported in numerous studies.1–4 Classic guidelines for osseointegration include a long healing period, during which functional load should be avoided. Periods of 3 to 4 months and 4 to 6 months have been recommended as healing times for osseointegrated implants placed in the mandible and maxilla, respectively.5,6 The long healing periods might impose an intolerable situation esthetically, functionally, psychologically, and socially on some patients, especially in a completely edentulous situation.

In 1986, Babbush et al7 described a technique of immediately loading 4 titanium plasma-sprayed implants placed in the mandibular symphysis with an overdenture. The implants were rigidly splinted with a metal bar, and the denture was relined with soft liner within 2 to 3 days after the surgery. The final prosthesis with the clips was placed 2 weeks later. The authors reported a cumulative failure rate of 12% after 8 years of follow-up. Since then, several studies have focused on both fixed and removable immediately loaded implant-supported prostheses in patients who are completely edentulous, and high success rates comparable with conventionally loaded implants have been reported.

The success rates of subperiosteal dental implants have been disputed.8,9 Minimal documented evidence of the predictability of these devices has lessened the benefits that the implant offers for patients with severely atrophic mandibles.10,11 One of the guidelines of these particular implant modality dictates that the implant upon insertion be loaded with a provisional overdenture. In most cases, the patient's existing denture is modified so it can be used as a provisional denture for 3 months before the final prosthesis is inserted.

The purpose of this article is to describe the surgical and prosthodontic procedure for the immediately loaded mandibular implant overdenture of 2 different implant modalities: root-form implant overdenture and subperiosteal implant overdenture. The clinical rationale and outcomes of each modality are also discussed.

The patients selected for immediately loaded mandibular implant bar overdentures fulfilled the following criteria: (1) had completely edentulous maxilla and mandible (Figure 1), (2) had enough bone between the mental foramina to allow for placement of 4 endosseous root-form implants with a minimum length of 12 mm, (3) had bone quality better than type IV according to Lekholm and Zarb,12 and (4) had a medical history that did not contraindicate implant treatment.

The patients selected for immediately loaded mandibular subperiosteal implant bar overdentures fulfilled the following criteria: (1) had completely edentulous maxilla and mandible (Figure 1), (2) had enough bone to allow for placement of a subperiosteal implant framework without having to augment the mandible with simultaneous bone graft (division C according to Misch13), and (3) had a medical history that did not contraindicate implant treatment.

After the patient had been selected as a candidate for immediately loaded mandibular implant overdenture, new maxillary and mandibular complete dentures with proper extensions as well as proper occlusal schemes were fabricated. Adjustments were made so that the patient was as comfortable as possible with the new dentures. A radiographic template was then fabricated with clear acrylic resin (Splint Resin Polymer, Great Lakes Orthodontics, Towanda, NY) by duplicating the mandibular complete denture. Radiopaque markers, such as gutta percha (Dental stopping, Coltene-Whaledent, Cuyahoga Falls, OH) or barium sulfate (E-Z-HD, E-Z-EM), were used during computerized tomography (CT) scanning to determine the implant position and data to produce for the subperiostal implant protocol a 3-dimensional life-size cast or model of the mandible by stereolithography methods.14–16 (Figure 2). The radiographic template was later modified into a surgical template.

Implant placement surgery was performed under local anesthesia by infiltration in the mandible. A midcrestal incision was made, a full-thickness flap was reflected, and the mental foramina were identified bilaterally. The surgical template was then placed in position for evaluation of the clearance for the attachment system. In the patient shown in Figure 3, alveoloplasty was necessary because of the space inadequacy.

The maxillary complete denture was used to locate the midline, and a slight mark was made on the mandible. The 2 posterior implants were located approximately 5 mm anterior to the mental foramina. The 2 anterior implants were about 7 to 10 mm anterior to the posterior implants; the anterior implants should be equidistant from the midline. All implant positions should be within the confines of the surgical template (Figure 4).

A complete series of osteotomies were performed according to the manufacturer's protocol, and the implants were placed into the osteotomy sites. The surgical template and maxillary complete denture were used to verify buccolingual and mesiodistal angulation of the implants, respectively.

Transmucosal abutments of appropriate heights to create a proper plane for the framework fabrication were placed with the appropriate abutment wrench. The tops of the transmucosal abutments should follow the anterior occlusal plane of the maxillary complete denture. The recommended torque for the abutments should not be attempted, because this might cause the implant to thread into a deeper level.

Plastic bridge copings were roughened to create a retentive surface and were placed on top of the transmucosal abutments. The flap was then preliminarily closed with Gore-Tex sutures (3i/WL Gore, Newark, Del). English-Donnelle-Staubi (EDS) plastic bars (Attachments International, San Mateo, Calif) were prepared to the proper lengths and placed between the plastic bridge copings. The bar and the bridge copings were joined with autopolymerizing acrylic resin (GC Pattern Resin, Alsip, Ill). The anterior bar should be parallel to the occlusal plane of the maxillary complete denture and perpendicular to the line bisecting the mandible (Figure 5).

The resin pattern was left to set intraorally for approximately 20 minutes. It was then removed and transferred to the laboratory, where the metal framework was fabricated in type IV gold alloy (Monogram IV, Leach & Dillon, Cranston, RI). Meanwhile, the protective titanium healing caps were placed on the transmucosal abutments and the final flap closure was performed, leaving the healing caps exposed.

Amoxicillin 500 mg and ibuprofen (Motrin, Upjohn, Kalamazoo, Mich) 800 mg were prescribed for antibiotic coverage and pain control. The patient was instructed to use 0.12% chlorhexidine gluconate (Peridex, Procter & Gamble, Cincinatti, Ohio) twice a day for 2 weeks and to start brushing the bar regularly with an end-tufted brush (John O. Butler, Chicago, Ill) 1 week after the surgery. The patient was placed on a liquid diet for the next 2 weeks.

The try-in of the metal framework was completed within 24 hours after the surgery. After satisfactory fit and stability of the framework were verified both clinically and radiographically, the bar was placed in the patient's mouth (Figures 6 and 7). The patient did not wear the mandibular denture during the soft-tissue healing.

The sutures were removed 2 weeks after the surgery (Figure 8). The mandibular denture was relieved so that it was not touching the bar when placed in position. The pickup impression of the bar was made with the mandibular denture with polyvinyl siloxane impression material (Reprosil, Dentsply, York, Pa). The metal housing for the EDS clip (Attachments International) was incorporated during the relining of the mandibular overdenture. After tissue surface adjustment, the EDS clip was placed in the metal housing. Clinical remount was performed, and occlusion was adjusted for bilaterally balanced occlusion. Afterward, the final prostheses were placed in the patient's mouth (Figure 9). Oral hygiene instructions were reinforced. Although no food restriction was imposed, the patient was recommended to follow a soft diet for the first 2 to 3 weeks after prosthesis placement.

The patient was asked to return the next day for a postinsertion check and any necessary adjustments. Follow-ups were scheduled at 1, 3, and 6 months after prosthesis placement and every 6 months thereafter.

After the protocol outlined by Truitt et al17 for the morphological replication of mandibles by using 3-dimensional (3-D) modeling and sterelithoradiography, subperiosteal implants were designed and fabricated from surgical-grade Vitallium (Howmedica, Kalamazoo, Mich) (27% chrome, 66% cobalt, 7% molybdenum) and coated with hydroxyapatite (HA). They were also fabricated in 1 piece with the continuous intraoral bar in place as part of the implant at the time of insertion (Figure 10). The intraoral incisions were made from the right retromolar pad and extended along the crest of the ridge to the contralateral retromolar pad. By using periosteal elevators, the mucoperiosteum was reflected from the bone, both buccally and lingually; the mental foramina were identified; and the dissection was completed with minimal trauma to the neurovascular bundles. During implantation, the tissues were carefully retracted with nylon-covered instruments to prevent transfer of metal to the passivated or HA-coated implant. When the implants were fully seated, the approximation of bone to metal was noted and recorded, and any discrepancies were then filled with an appropriate graft material. The tissues were closed with 4-0 vicryl sutures (Figures 11 and 12).

Immediately after implant placement and complete closure, the patient's existing denture was modified and adapted for use as a transitional denture. The denture modification consisted of removal of a portion of the buccal and lingual flanges of the denture and the relining of the same denture at the correct vertical dimension of occlusion (Figures 13 and 14).

The modified denture was subsequently adapted and relined, thereby increasing its retention and function for 2 months before the final prosthesis was fabricated.

For the final mandibular overdenture, record bases were made at the time of implant fabrication, which accurately fit on the intraoral bar. These record bases facilitated the intraoral bar and border molding impressions and were also used to obtain the interocclusal records. The record bases had Dolder clips (Attachments International) added to them for retention purposes. Once the interocclusal records were obtained, the subsequent steps involved conventional denture fabrication procedures.

Recently, clinical studies have reported comparably high implant success rates (85%–100%) in immediate loading in patients who were completely edentulous.7,18–24 Peri-implant bone response of the immediately loaded implants is also favorable and comparable with that of conventional delayed-loading implants.24 It is difficult to compare subperiosteal implant overdentures with root-form implant overdentures; however, some elements are common and can be the bases for evaluation of these modalities.

Dense trabecular bone provides better intimacy of initial fit and implant-bone contact area, which translates to better primary stability.25–27 It has also been documented that implant failure rates in type IV bone are significantly higher than in type I to III bone.28–30 In addition, Schnitman et al21 reported that 3 of 4 failed immediately loaded implants were placed distal to the mental foramen, where the bone quality is inferior to the symphysis area.

No comparative studies have reported the success rate of subperiosteal implants among the different bone types; however, some anecdotal reports describe significantly higher failure rates in the maxilla than in the mandible.

Bone quantity dictates the implant diameter and length. Wider-diameter or longer implants provide greater surface area for initial bone-implant contact as well as for osseointegration. Positive correlation has been established between implant diameter or length and the removal torque or push-out force values.31–33 Higher implant failures for short implants (≤10 mm) were also reported in both immediate21,22 (??) and delayed loadings.30,34–36 Although immediate loading of wide-diameter implants has not been reported, it should be considered when available bone permits the placement of such implants.

The authors of this article have limited the application of the subperiosteal implant to the severely atrophic mandible. Considering the arguments against the use of short endosseous implants, class D according Misch,13 mandibles can successfully be treated with subperiosteal implants without bone grafting. The authors have achieved positive results in the treatment of severely atrophic mandibles.

Threaded implants should be used in immediate loading situations because they provide the strongest immediate mechanical retention after placement.37 

The design according to James has the only documented evidence about the benefits of the lateral support vs the designs with support from only the crest of the ridge. Since 1983, the authors of the current paper have exclusively used the avowed design. The implant, in general, uses maximum support for the body of the mandible and is HA coated.

Although no significant differences have been in the implant success rates between smooth and rough implant surfaces, recent literature seems to favor a rough surface in achieving a greater magnitude and faster rate of osseointegration.38–41 Therefore, in immediate loading situations where maximum magnitude and rate of osseointegration are desired, implants with surface treatments should be considered.

Human histology has demonstrated the benefits of HA-coated subperiosteal implants. Positive correlation has been found in the quality of survival between the coated and the noncoated subperiosteal implants.42 

Engaging both the superior and the inferior cortical bone significantly increases the implant stability and the removal torque.43 In addition, Chiapasco et al23 found no correlation between the implant length and the success rates as long as there was bicortical stabilization. Bicortical initial stabilization could also be enhanced by minimal countersinking.20 

It has been frequently stated that implant surgery is prosthetically driven; this is especially true in an immediate loading situation. The final prostheses dictate implant position, angulation, number, and distribution as well as bar length and orientation. The interarch distance and the attachment systems dictate the depth at which the implants are placed. Therefore, fabrication of the final prostheses before implant surgery is very important. The use of the maxillary complete denture and surgical template is very essential during implant surgery. The maxillary complete denture is used to locate the midline, as a guide for mesiodistal implant angulation and implant level, and as a guide for horizontal bar orientation. The surgical template is used to evaluate the clearance for the attachment system, confine the implant positions, guide buccolingual implant orientation, and guide sagittaly bar orientation.

The attachment system used in immediate loading situations should involve prompt, rigid splinting of the implants. Lum et al42 showed that direct bone-implant interface was achieved when immediately loading HA-coated blade-form implants with rigid fixation to a firm natural tooth. Babbush et al7 also showed, in the immediate loading study, that apart from infection, failure was related to delayed application of a rigid splinting bar. The authors of the current paper have also previously reported on the recommended guidelines for immediate functional load, which include the immediate rigid splinting bar placed in the patient's mouth within 24 hours after surgery.44 In this report, and in the case of the root-form implant modality, the EDS bar-and-clip system was used and the rigid splinting bar was placed in the patient's mouth within the same period after the implant surgery. The Dolder bar-and-clip system has been used since 1984 for the subperiosteal implant modality. During the fabrication of the subperiosteal implant, the intraoral bar is incorporated to the wax pattern and corresponds to the same bar dimensions of the Dolder system.

The bar orientation should be parallel to the arbitrary transverse horizontal axis horizontally and sagittaly.45,46 Because the plastic bar pattern is fabricated at the time of surgery, the bar cannot be aligned parallel to the condylar components in the articulator. However, the anterior occlusal plane of the maxillary complete denture and the surgical template can be used as the guides for horizontal and sagittal bar orientations, respectively. The bar should be parallel to the anterior occlusal plane of the maxillary complete denture and perpendicular to the line connecting the lingual frenum and the midline of the surgical template.

Improper bar length can cause unfavorable force distribution, which may lead to peri-implant bone resorption.47 The optimal bar length has been postulated as 18 to 23 mm, which corresponds to a distance of 22 to 27 mm between the centers of the implants.47 Nevertheless, this bar length may not be routinely achieved because of anatomic limitations such as the arch form and the location of the mental foramen.

The location of the intraoral bar for subperiosteal implants is based on the evaluation of the CT scan data in relation to the radiographic templates. The intraoral bar is traced on the 3-D generated model of the mandible following specific guidelines. The radiopaque material present in the radiographic template aids in the location of the transmucosal posts that interconnect the intraoral bar. The main advantage of this modality is that the bar is an integral part of the implant; it is attached to it from the time of insertion and, in essence, is ready to receive a transitional overdenture immediately after surgery.

Although the implant success rates of mandibular overdentures supported by 2 and 4 conventionally loaded implants were not different,4,48–50 Rangert et al51suggested that load transferred to individual implants could vary because of the number of the implants and their positions. They have shown that when implants are in line (usually in situations with 2 implants), the bending moment encountered is higher than when the implants are in configurations (eg, tripod or quadrilateral) that create offsets. In immediate loading situations, excessive load in any direction can be detrimental. To provide the offset configuration as well as to allow anterior bar orientation to be parallel to the transverse horizontal axis, 4 implants may be the ideal minimum number.

The implants should be placed within the confines of the surgical template. To avoid any neurovascular injury, the posterior implants are placed approximately 5 mm anterior to the mental foramina. The anterior implants are positioned so that the optimal bar length is achieved without encroaching into the tongue space, and the distance between the centers of the anterior and posterior implants is at least 7 mm for hygiene purposes. During implant placement, the implant angulation should be perpendicular to the occlusal plane to avoid any undue overloading.52 The maxillary complete denture and surgical template can be used as guides for mesodistal and buccolingual implant angulation, respectively.

The location of the implant posts as well as the height of the bar in relation of to the crest of the ridge is “prosthetically driven,” and, specifically, the implant designs used in this report were done according to the concepts outlined by James et al.10 

Following the described protocol, which was approved by the Institutional Review Board of Loma Linda University, a prospective study was conducted on 5 patients.24 Twenty implants were placed, and all remained successfully osseointegrated after 1 year of loading. The mean marginal bone loss was 1.16 mm at 12 months.24 

Although no major complications or loss of osseointegration were encountered, a precautionary measure should be taken. In the case of early failures (less than 3 months after the implant placement surgery) of 1 or more implants, it is recommended that the unaffected implants be left unloaded until they are fully integrated for subsequent restoration in the conventional manner. If the remaining implants are not adequate for supporting a mandibular overdenture, more implants should be implemented accordingly. The patient must be informed of such a possibility before the treatment.

The advantages of immediately functional loaded implant prostheses include the shorter treatment time and the elimination of the provisional stage as well as the second-stage surgery. This article describes the surgical and prosthodontic procedure of the immediately loaded mandibular implant bar overdenture along with its clinical rationale. Although the high implant success rates reported with this treatment modality might suggest that it is a viable and predictable treatment option, careful patient selection and treatment planning are still as important as or even more important than the treatment itself.

Achieving primary stability and maximizing the magnitude and the rate of osseointegration seem to be the key elements in achieving and maintaining osseointegration in such situations.

Following the described protocol, which was approved by the Institutional Review Board of Loma Linda University, a retrospective study (unpublished thesis) was conducted on the last 5 patients treated with superiosteal implants from June 1995 to June 2000 who had immediate ovedentures in function for at least 3 years. After 1 year of loading, all 5 implants that were placed have successfully provided immediate functional load since the day of the surgery. Bone loss around subperiosteal dental implants is often difficult to measure because of the metal superimposition from the implant struts. Radiographs were taken of each implant post at implant placement and at 3-, 6-, 9-, and 12-month intervals. The detectable mean marginal bone loss was 1.2 mm at 1 year.

No major complications or loss of implants occurred; however, some observations are in order. It must be emphasized as reported previously that many of these implants required “minor corrective” soft-tissue surgery. In this particular report, 3 of the 5 implants placed required corrective surgery.

The surgical and prosthodontic procedures for the immediately functional loaded mandibular implant overdentures described in this article demonstrate the differences between the 2 modalities. The subperiosteal implant modality provides an overdenture that is supported by the implant in its entirety. The technique appears to be highly sensitive and, apart from the information presented here, lacks documented reproducible data. The root-form implant overdenture modality uses the retention given by the implant bar-and-clip system and partial support given by the edentulous ridge. The technique is simpler and has been reproduced and reported by other clinicians. Within the limitations of this article, both techniques achieve the goal of providing predictable immediate function after surgery.