In this study, no implants were lost, and two implants had biomechanical problems during the follow-up period. There is a risk for restoring a posterior region using narrow-diameter implants due to the high masticatory force in molar area [14]. Fatigue fracture of the narrow implant body from mechanical weakening has been reported [15]. It has been suggested that a 4-mm-wide implant has a 30 % higher fatigue resistance than a 3.75-mm-wide implant [16]. Therefore, many previous studies reported that wide implants provide better biomechanical characteristics [17], but under certain circumstances, it is difficult to use regular or wide implants. When wide implants are installed in narrow ridges, many clinicians must bone graft around the fenestrated implant surfaces. However, it is postulated that peri-implant grafted bone will be resorbed if the grafted bone does not have an optimal osteogenesis period.
Bone augmentation procedures are often necessary to enlarge the bone width and facilitate regular- or wide-implant positioning. Autogenous bone grafts require complex surgical techniques, and additional risks must be considered. Using narrow implants gives an unskilled clinician surgical freedom and is applicable in patients without the bone width required for regular-diameter implant installation. Although no fenestration has been reported around the implants when narrow implants are used, bone grafts can be necessary to clear a food bolus. In this study, bone grafts over the alveolar bone using slow-resorbing material such as Bio-Oss® (Geistlich, Wolhusen, Switzerland) were used to restore the optimal alveolar bone width for the buccinator mechanism [18].
Using narrow implants reduces the chance of bone dehiscence or fenestration during a flapless surgery. It also prevents lingual dehiscence in the mandibular second molar area during preparation. Given the decreased width of the drills and implants, osteotomy preparation implies less risk of overheating the bone.
Implants positioned too close together can reduce the height of the inter-implant bone crest. It has been suggested that a distance less than 3 mm between two adjacent implants increases bone loss [19]. Narrow implants enable clinicians to easily generate this distance easily. The greatest challenge in replacing missing teeth with implant restoration is for thin gingival biotype cases. Preserving the bone architecture is paramount to a successful final outcome and the peri-implant soft tissue stability. Clinicians want to create an effective barrier to protect the underlying bone from intraoral microorganisms and by-products [20]. Presumably, Ankylos® system provides more space for the soft tissue retention because it has a narrow connection size that produces greater gingival thickness using the platform switch. Tight and stable soft tissue integration during implant restoration facilitates long-term success (Fig. 2).
The fixture-abutment connection type is also important for implant longevity. Quek et al. reported that narrow-diameter implants are more easily broken than wider implants because they have a narrow platform diameter [21]. However, different results are expected from implants with an internal connection. Certain studies have shown that the biomechanical stability of internal conical connection implants is better than in butt-joint implants [22, 23]. Herein, we used implants with a conical connection, and the force on implants with a conical connection was not focused on a screw but a connection.
In the previous study, the effect of the joint design on the fatigue strength and failure mode in the conical connection system was significantly better for the butt-joint system [22]. Therefore, it would be difficult to apply Quek’s result [21] to the system used herein. In an article published by Zipprich et al., 10 implant systems that used either conical or flat-to-flat connections were compared relative to their dynamic lateral load responses under simulated clinical conditions. The clearance-fit systems produced micromovements, whereas the systems with a conical fit (Astratech® and Ankylos®) generated no movement at all [24]. There will be no micromovement during functional load, and fewer loads on the abutment screws produce few if any screw loosening problems.
One of the pitfalls in using narrow implants is the risk of fracture to the fixture or abutment. The thickness of the fixture titanium wall is important. Where the fixture titanium is too thin around the abutment, the tendency is to lose bone upon loading. Therefore, it is important to secure sufficient fixture titanium around the abutment. A previous study showed that reinforcing the neck region is necessary in reduced-diameter Straumann® tissue level implants [25]. Quaresma et al. reported that a conical-connection implant produces lower stress on the alveolar bone and prosthesis and greater stress on the neck portion of the abutment-prosthesis complex [26]. It has a weak point at the neck portion of the abutment, especially when it is used in the posterior region. Herein, we observed no abutment fracture cases. We presumed that narrow fixtures were generally used in the narrow width of the bone because patients with a narrow bone width may have a weaker occlusal force than patients with normal- or wide-width bones [6]. Interestingly, the Ankylos® system design generates the same abutment for 3.5-mm-, 4.5-mm-, 5.5-mm-, and 7-mm-diameter implant fixtures. Thus, the crown margin position is only determined by the abutment, and after healing, several millimeters of play are available to define the final emergence profile; further, the same connection size between fixture and abutment can be used for all regions in the mouth. If the bone width is wide enough for installation of a wide implant, there is no reason not to use wider implants. However, it should also be noted that sufficient bone housing around the implants may be more important than implant diameter. The premise should be biomechanical stability. In Asian patients, who usually have narrow ridges and thin gingival biotype, using narrow implants enables bicortical installation in the posterior region. However, when treating patients with severe bruxism, heavy masticatory forces and oral habits, a narrow implant is not advised. We observed one case of cement loss in a bruxing patient. The loading history of the implant and the time required for the functional adaptation of the bone to implants may be more important than the implant itself. Development of better biomechanical properties in implants will facilitate narrow implant use in the posterior region. No statistically significant association between the variables and bone loss were detected herein given the combination of the factors described above, such as platform switching and conical connection. A single factor does not produce implant treatment success. Streckbein et al. also reported that low levels of bone strain are observed where a platform switch compensates for a small cone angle in the Ankylos® system [27]. For the PTV value, the groups with a loading period over 3 years is smaller than the group with a loading period of over 1 or 2 years, which is likely related to bone remodeling completion.