Stefan Ahren, DDS
Private Practice, Vänersborg, Sweden.

Karl-Erik Kahnberg, DDS, PhD
Department of Oral and Maxillofacial Surgery, Faculty of Odontology, Göteborg University, Göteborg, Sweden.

In all cases, implant treatments are intended to create the most favorable conditions for oral hygiene procedures. Fixture survival has been reported to be very successful, especially in the lower jaw. The importance of oral hygiene procedures to prevent the development of peri-implant inflammations has also been enunciated. However, taking into consideration phonetic problems and food impaction problems with implant superstructures that leave a considerable gap between the bridge and the alveolar crest, we have tried to construct the bridge with close adaptation to the alveolar crest in a series of patients. The risk of poor oral hygiene is obvious, but phonetic, esthetic, and functional aspects can be markedly improved. In the severely resorbed jaws, the design of the superstructure has to compensate for the considerable loss of alveolar bone. It is also important to make the tissue compensation as similar to the surrounding oral mucosa as possible. In the single tooth restoration, both oral surgeons and prosthodontists are eager to make the rehabilitating solution as esthetic as possible by means of deep positioning of the implant and short abutments. If the same concept is applied to the edentulous patients, the clinician could obtain a dentoalveolar substitute, which is a direct continuation of the maxilla or the mandible, with the use of esthetic abutments and close adaptation to the soft tissue. These procedures are done without augmentation surgery and transplant procedures (if there is enough bone for conventional implants). The main risks of implants that are kept below rigid bridge constructions are the difficulties associated with oral hygiene measures and possible marginal bone resorption that result from periodontal disease. However, if the marginal bone conditions, soft tissue conditions, and fixture survival are acceptable, then there are several benefits to improving esthetics, phonetics, and function with these types of prostheses. The objective of this paper is to present the outcomes of 49 consecutive edentulous cases in which a close adaptation of the superstructure has been made. The patients have been followed for up to six years.

MATERIALS AND METHODS.
A consecutive series of 49 patients received a total of 252 fixtures (Nobel Biocare AB, Göteborg, Sweden). There were 17 patients with edentulous mandibles, 20 patients with edentulous maxillas, nine partially edentulous cases, and three single-tooth implant cases. There were 21 women and 28 men in the patient group. The mean age of the men was 65.8 years, and the mean age of the women was 64.0 years. In the lower jaw, self-tapping MARK IIB fixtures (length, 10–18 mm) were used. Standard fixtures (length, 10_20 mm) were used in all cases in the upper jaw. The standard healing time was 4 months in the mandible and 6 months in the maxilla. Both mandibles and maxillas represented the categories 3 and 4 according to the Cawood classification19 of bone volume and bone quality. No surgical complications were noticed, and the healing was uneventful in all patients during both implantation and abutment connection. Estheticon (Nobel Biocare AB, Göteborg, Sweden) abutments (length, 1–2 mm) were used in all cases, enabling a close adaptation of the superstructure to the alveolar crest.
The prosthetic treatment started five to seven weeks after the procedure, enabling the gingival tissue to shrink as much as possible before initiating the prosthodontic procedure. In connection with impression taking and occlusal registration, the dental set-up construction was designed in collaboration between the dentist, the dental technician, and the patient. Photographic documentation was an extremely useful guide for the dental technician’s work. When the metal construction was tested in the patient, the adaptation against the alveolar process was carefully checked. Four weeks after the installation of the implant superstructure, the internal screws were further controlled and tightened if necessary (Figs. 1, 2, and 4).

Fig. 1. Design of the initial type of superstructure with space between the crest and the bridge to allow for cleaning procedures. Tightly adapted superstructure in the upper jaw.
Fig. 2. A closely adapted bridge construction in the upper jaw.
Fig. 3. The bridge has been removed annually to examine the gingival tissue.
Fig. 4. Adaptation of the superstructure to an irregularly formed alveolar crest in the upper jaw.

Radiographic examinations, including panoramic radiograph and intraoral radiographs, were performed before the procedure and after the abutment connection.
The intraoral radiographs were done with a periodic standardized reproducible intraoral technique by using an occlusal impression material. Annual examinations, consisting of the removal of the superstructure and the inspection of the periimplant soft tissues (Fig. 3), were performed. All patients received thorough oral hygiene instruction from a professional dental hygienist who informed them about the use of dental floss, intradental brushes, and wound rinse. The patients were also continually controlled by a dental hygienist every third month for oral hygiene procedures. The surgical procedures were without complications in all of the cases and were done according to a standard procedure with conventional implant surgery. Crestal incisions were used in most of the cases. Abutment connections were made with 1-mm to 2-mm Estheticon abutments.

The follow-up time for the patients varied from two to five years. All patients have been examined annually with standardized intraoral radiographs. In 15 patients, the superstructures were removed annually to allow thorough inspection of the underlying mucosa (Fig. 3). Marginal bone loss was found mainly in the maxillary cases. In the mandibular cases, only one fixture was lost. In the maxillary cases, six fixtures were lost. This translates to a success rate of 98.9% for the individual fixtures in the lower jaw, 96.3% for the upper jaw, and 97.3% for individual implants. The radiologic examination was made by two independent observers and showed results similar to superstructures with a standardized design of a couple of millimeters above the alveolar crest. When measuring the marginal bone height in the periodic identical radiographs, it was found that there was almost no marginal bone reduction in the mandible. In the maxilla, the reduction during the first postoperative year was 0.49 mm and then gradually increased to 0.72 mm the second year, 0.81 mm the third year, and 1.1 mm the fifth year (Figs. 5, 6, and 7 and Table 1). The bone volume and the quality of bone were assessed according to Lekholm and Zarb20 and varied between 1 and 4 in bone volume and 3 and 4 in bone quality. The patient’s satisfaction with the prosthetic constructions and designs was extremely good. For some of the patients, the oral hygiene procedures, including the use dental floss and special techniques to get around the abutments, were difficult or not worth the effort. The marginal conditions, even after a period of six years, appeared healthy with no observable periodontal signs of inflammation as judged by the oral hygienist. These judgments were based on color change, gingival texture, and bleeding on probing.

Fig. 5. Periodic identical intraoral radiographs in the upper jaw one year (A) and five years (B) after the fixture installation and the bridge adaptation.



Fig. 6. Periodic identical intraoral radiographs of implants in the lower jaw after the bridge connection (A) and four years later (B).



Fig. 7. Periodic identical radiographs in the upper jaw of a patient with thin alveolar crest showing initial remodeling and marginal bone loss (A). The situation becomes stable when comparing radiographs four years later (B).



Table 1. Marginal Bone Loss.

DISCUSSION.
The idea of close adaptation of the superstructure to the alveolar crest is aimed at getting an improved esthetic solution and improving the phonetic function of the patient. The patients in our study were able to function well socially and felt that the prostheses were a natural continuation of the alveolar process. Another important functional improvement is that there is no food impaction between the alveolar process and the superstructure. The difficulty with the present design of bridge construction is that it is difficult to make a visual examination of the fixtures without removing the bridge. Logically, it should be much more difficult to clean the area, but the patients did not experience difficulty. The patients performed 90% of their dental oral hygiene with rotating brushes and the rest with superfloss instruments. None of the patients have had pain from the gingival tissue. Instead, they experienced a very comfortable situation.
Visual inspection of the areas performed annually for up to six years has demonstrated gingival mucosal health. The reconstruction of the alveolar crest to a normal configuration to compensate for the loss of bone tissue with a superstructure closely adapted to the soft tissues has had a positive effect on the patients. The very low failure rate, despite difficult cases with deficient bone volume in both the mandible and the maxilla, is favorable for our method.
When measuring the marginal bone height in the radiographs, we found that marginal bone loss was more advanced in the maxilla and that even in five-year follow-up the bone reduction was almost negligible in the mandible. The bone situations in our maxillary cases consisted of very thin narrow ridges with just enough bone to perform conventional implant surgery. In many cases, threads were left exposed on the palatal side. Exposed threads could explain a more rapid marginal bone loss. Remodeling of the bone could account for some of the bone loss in these cases. However, major loss was found in just a couple of patients, indicating that perhaps systemic factors could be influential. The close adaptation of the superstructures does not mean that we create an impossible situation for the patient. However, when comparing the results in our study with longitudinal 15-year studies, the figures are compatible. The clinical and radiographic examinations support this prosthetic design. It also seems that the patients were more able to keep the closely adapted reconstructions hygienic than they were when a space was created between the alveolar crest and the superstructure (Fig. 1). Since the beginning of our study, making these superstructures it has become more routine.

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