Case presentation of two patients using diagonal platform-switched double implants for maxillary single-first-molar replacement as the alternative of a single-tooth implant

A single-tooth implant restoration is generally performed for maxillary single-first-molar replacement. If the interdental space between the second premolar and the second molar is large enough, a double-implant placement can be performed to avoid creating mesiodistal cantilever and to distribute occlusal loading forces. If there is not adequate space for a double-implant placement to be performed mesiodistally along the crest of the alveolar ridge line, they should be placed along a diagonal line offset lingually to increase the space. This procedure has two primary advantages. First, greater stability is provided by a double-implant placement. Resistance to lateral forces (palatal-buccal) is much stronger than two implants placed along the alveolar crest ridge line. Residual palatal and buccal bone can provide support against occlusal forces, provided that there is adequate residual bone in these regions. If anatomical conditions are favorable, the placement of two diagonal implants in the palatal and buccal residual bones can be a rational procedure. We report on two typical patients. The progress of these patients was followed using computed tomography for 7 and 6 years, respectively.


Background
We report on two patients who were missing a maxillary first molar. In general clinical practice, only a single-tooth implant seems to be used as a prosthesis in patients with a missing maxillary molar tooth [1]. In addition, if there is insufficient residual bone between the bottom of the sinus and the crest of the alveolar ridge, GBR (guided bone regeneration) technique [2] or sinus elevation [3] surgery is required. Even though these procedures can promote osteogenesis, whether the quality of newly formed bone is similar to that of the existing residual bone remains unclear [4][5][6][7][8]. If there is enough residual D2 or D3 bone (Misch Bone Density Classification) [9] for implant placement, the use of such bone is recommended whenever possible. If we accept the above, a double-implant placement [10,11] inserted in the buccal and palatal sides, not the center of the alveolar ridge, may be a reasonable option for maxillary-first-molar replacement because these regions sometimes retain more bone volume than the center of defects.

Patient 1
The patient was a 51-year-old man who was a nonsmoker. He was referred to my clinic in April 2004. The #26 tooth had already been extracted because of severe periodontitis. He returned to my clinic in March 2006 to receive implant treatment (Fig. 1). The medical history was unremarkable. A hematological analysis was performed to assess his overall medical status. The #26 tooth was confirmed to be missing, and there was severe bone resorption around the buccal roots of #27, associated with an apical lesion (Fig. 2).

Patient 2
Similar to the first patient, our second patient was a nonsmoking man, 43 years of age. He was referred to my clinic by another dentist in January 2008 and was first examined at that time. He complained of pain due to serious periodontitis in #26. The medical history was unremarkable. He was examined in a similar manner as patient 1.
In the same month, #26 was extracted. After extraction, bone resorption including the buccal cortical plate occurred more severely than in most patients (Figs. 5 and 6). Two months after extraction, only GBR was performed to repair the bone defect. Double implants (Ankylos implants; diameter, 3.5 mm; length, 9.5 and 11 mm) were placed diagonally in the missing region 3 months after GBR (Fig. 7). In November of the same year, a screwretained superstructure was seated (Fig. 8).

Treatment results
The patients' clinical progress has been good as of 7 years and 5 months in patient 1 (Figs. 9 and 10) and 6 years and 7 months in patient 2 (Fig. 11). Computed tomographic (CT) scans showed that both patients' implants were inserted at appropriate positions, as planned before surgery (Figs. 12 and 13).

Discussion
Insertion of a single implant for single molar replacement is established practice. Levin et al. reported that a single implant can serve as a good long-term and predictable treatment modality to replace a single molar with low complication and failure rates [16].
However, weak points of single-first-molar replacement were reported.
Mazor et al. reported that 33 patients received 66 narrow double implants of 3 mm replacing 33 missing first molars. All implants survived the follow-up time of 10 to 18 months. Wide-diameter implants are a suitable alternative for replacing a missing molar in some cases, but there has been a greater incidence of screw loosening, resulting in a higher failure rate. However, the use of a double-implant placement has been successfully demonstrated to be a functional and more biomechanically sound method of molar replacement. He concluded that replacing a single missing molar with two narrow dental implants might serve as a viable treatment option providing good and predictable long-term results [17].
Sullivan reported that when using standard-diameter 3.75-mm titanium screw implants in molar regions, up to 14 % of single-tooth, molar implants could fracture, so there remains an unacceptable risk of implant fracture [18].
In addition, a double-implant placement has been reported to be superior to a single stand implant. The advantages and disadvantages of a single standard implant, a wide-diameter implant, and double implants for single-molar replacement have been debated with biomechanics [10,11,[19][20][21].    If there is 13 mm between the maxillary second premolar and the second molar, a double-implant placement can be performed in the crest of the alveolar ridge line. When 3.5-mm diameter implants are used, there should be a minimum space of 3 mm between implants [22] and 1.5 mm between the implants and adjacent teeth [23] (1.5 + 3.5 + 3 + 3.5 + 1.5 = 13 mm). However, the average Japanese mesiodistal dimension of the maxillary first molar is between 10 and 11 mm [24]. Therefore, if a double-implant placement is used, it has to be placed diagonally to create a larger inter-implant space and accommodate international implant sizing [1].
As for the vertical bone dimension, if there is insufficient residual bone volume between the bottom of the sinus and the crest of the alveolar ridge, GBR or sinus elevation surgery is required for all implants. If there is sufficient existing residual D2 or D3 bone near the buccal and palatal cortical plates, the use of such bone is recommended whenever possible. If double implants can engage these cortical plates, greater support can be obtained with diagonal double implants than with a single stand implant.
In both patients, we used Ankylos implants, which have a medialized step between the implant body and abutment. Therefore, Ankylos is a platform-switched implant. The benefits of this shape have been reported. With platform-switched implants, the shape affects the cortical bone more than the trabecular bone [25], which contributes to the maintenance of inter-implant bone height and soft tissue level [26] and reduces bone loss [27]. The horizontal discrepancy between the outer edge of the implant platform and the implant-abutment interface influences post-restorative biologic processes [28]. This horizontal discrepancy may reduce bone resorption around the implant neck because it horizontally offsets the bone away from the microgap that exists between the implant and abutment. This increased distance is thought to attenuate the loss of crestal bone height [29].
Effects of the inter-implant distance between platformswitched implants have been reported. Even when the inter-implant distance was less than 3 mm, crestal bone resorption was prevented [30][31][32][33][34]. Previous studies thus support the advantages of platform-switched implants as compared with platform-matched implants when double implants are placed adjacently.
In patient 1, the platform-switched double-implant placement was performed very close to each other (Fig. 3). If platform-matched implants had been used, inter-implant bone loss might have occurred. Fortunately, there was no evidence of bone resorption on the periapical X-ray obtained 7 years and 5 months after superstructure placement (Fig. 10). In addition, there has been no problem with the emergence profile because platform-switched implants provide a larger volume of soft tissue around the connecting area between the implant and abutment.
In patient 2, severe bone resorption including the buccal cortical plate occurred after #26 extraction and was greater than we had anticipated. Therefore, only GBR was performed without implant insertion to repair the severe bone resorption (Fig. 6). CT was performed to evaluate the GBR site after 3 months. Then, a double-implant placement was performed diagonally with another session of GBR to increase bone volume (Fig. 7).
The use of platform-switched implants has been associated with coronal extension of the interimplant bone peak beyond an imaginary line connecting the two implant-abutment interfaces [22]. In patient 2, even  though the double-implant placement was performed adjacently, new bone formation over the implant shoulder was fortunately confirmed on a periapical X-ray (Fig. 14).
In diagonal double-implant placement, the mesial implant should be placed more buccally and the distal implant more palatally to improve esthetics [1]. In addition, oral hygiene is a very important factor related to inter-implant space. It might be easier for a patient to use an interdental brush from the palatal side rather than from the buccal side, especially in the maxillary molar region.
On the other hand, Bhat and Handelsman reported that there is a surgical risk due to the precision needed in a double-implant placement, because the mesiodistal space is often very limited. Therefore, any misplacement of double implants will ruin the mesial or the distal site, and misplacement also may damage the adjacent roots or compromise the embrasure space, so that only one implant can be placed [35]. This kind of careful attention should be paid when using a double-implant placement in single molar missing.

Conclusions
We described two patients in whom diagonal platformswitched double-implant placement was performed for maxillary single-first-molar replacement. In such patients, diagonal double implants have some advantages over wide-diameter implants. There was no bone resorption around double implants' necks. We therefore consider this method as one option for maxillary single-first-molar replacement.