Rehabilitating total or partial edentulism with dental implants has been a frequently utilized and predictable treatment method for over 20 years (24). With advancing applications and increased research efforts, the focus is on achieving the long-term survival of dental implants (25). Mandibular cortical thickness (MCT), detectable through radiographic examinations such as OPG or periapical radiographs, serves as an early indicator in implant survival (26). Adell et al. were the first to suggest that uncontrolled MCT could lead to long-term implant failure (27). Although the developmental mechanism of MCT is not fully elucidated, it is known to be influenced by various factors such as oral hygiene, occlusal trauma, the presence of microgaps between implant and abutment, implant neck design, implant diameter, the jaw where the procedure is performed, the type of prosthesis applied, and the surface characteristics of the implant (28).
Following the acknowledgment of the concept of MCT as an early indicator for failed implants, numerous studies have explored factors affecting MCT (28–33). However, existing literature often focuses on changes in MCT based on one or a few parameters. In this study, considering the multifactorial etiology of MCT, we aimed to evaluate changes in MCT and fractal analysis (FA) based on factors such as height, diameter, surface preparation method, applied jaw, and neck design.
In a study encompassing 4591 implants, French et al. evaluated MCT between jaws (maxilla/mandible) and reported a significantly higher MCT in the upper jaw compared to the lower jaw (34). Similarly, Nitzan et al. found a significant excess of MCT in the upper jaw compared to the lower jaw in their study (35). In our study, MCT in implants applied to the upper jaw (DMCT) was found to be significantly higher than those applied to the lower jaw. Our study results align with the literature, particularly in terms of DMCT. The higher presence of MCT in the upper jaw may be explained by the spongy structure of the maxilla, leading to greater compression under functional forces and a decrease in micro-level intraosseous vascularization (36–38).
Zimmermann et al. conducted a systematic review, examining 22 articles on MCT in implants restored with fixed and removable prosthetics. They reported no statistically significant relationship between prosthetic superstructure and MCT (39). Saravi et al. conducted a systematic review focusing on studies of MCT in fixed and removable prosthetics and reported no statistically significant difference in MCT between groups using fixed and removable prosthetics (40). Similarly, Pauletto et al., in their systematic review and meta-analysis, found no statistically significant difference in MCT between patients using fixed or removable prosthetics on implants (41). In contrast, Toti et al. compared patients with a full fixed prosthesis and those with a partial fixed bridge in their study of 209 implants. According to their findings, patients with a full fixed prosthesis had significantly higher MCT (42). These findings are consistent with our study results. In our study, MMCT was significantly higher in patients using implant-supported removable prosthetics (with locator attachments). Comparable MMCT values were observed in patients with full fixed restorations and fixed partial bridges. Shahin et al., in their review, reported that factors such as prosthesis type, implant-abutment connection, and implant position affect the stress distribution on implants (43). In our study, MMCT was significantly lower in patients using single crowns on implants than in other groups. When examining the relationship between DMCT values and prosthetic superstructure type, patients with single crowns showed significantly lower DMCT values.
Maminskas et al., in their study examining stress distribution on bone around implants, reported that forces on implants varied in different axes and intensities (44). Geng et al., in their review, reported that stress distribution in implant-supported bridge restorations was higher and more variable than in single crown restorations (45). Dorj et al. attributed the significantly higher MMCT in implant-supported removable prosthetics to the multifaceted forces exerted on the bone due to removable prosthetics (46). Studies have shown that during function, the stress on dental implants concentrates in the neck region of the implant (47, 48). In line with these findings, the significantly lower MMCT in implant-supported single crowns in our study can be explained by the lower and unidirectional stress on implants in single crowns.
Abrahamsson et al., in their review covering publications examining the relationship between MCT and implant surface properties, reported that the implant surface preparation method did not significantly affect MCT (49). Similarly, Kim et al., in their study of 236 implants with two different surface preparation methods, found no statistically significant difference in MCT between the two groups (50). Donati et al., examining MCT on 149 implants with different surface features, reported no statistically significant difference in MCT between groups during a 20-year follow-up (51). Our study also yielded similar results to the literature. The 378 implants examined were divided into two groups (SLA Group and AND Group) based on surface preparation methods, and there was no significant difference in MMCT/DMCT values between the groups.
Benedikt et al. conducted a randomized controlled study comparing marginal bone levels in implants with and without microgrooves in the neck portion. They reported no statistically significant difference in MCT between the groups (52). In contrast, Bratu et al. examined MCT in two different implants with microgrooves in the neck region and reported significantly lower MCT in implants with microgrooves (53). Bratu et al. also reported the presence of premature soft tissue around the cover screw and implant neck in polished neck implants after a 4-month osseointegration period. The researchers attributed this to better osteoblast proliferation on rough surfaces (implants with microgrooves). Niu et al. reported significantly lower MCT in implants with microgrooves compared to those without microgrooves, attributing this to the decrease in cutting stress transferred to the bone in implants with microgrooves (54). Most studies have compared situations with and without microgrooves on dental implants. All implants included in our study had microgrooves. Therefore, a more specific and dimensional grouping was made based on the coronal length of microgrooves. According to our results, both MMCT and DMCT were significantly lower in implants with coronal microgrooves of 1 mm.
Studies have shown that microgrooves reduce cutting stress transferred from the neck region of the implant to the bone during function (55, 56). Kinni et al. reported in their study that dental implant design could alter the transmission of force to the bone (57). Chowdhary et al., in their study, suggested that microgrooves convert cutting stress, which is less resistant in bone after implant placement, into compressive stress, which is more resistant in bone, potentially reducing MCT (58). Hermann et al. evaluated MCT in implants with a rough neck region and a polished surface and reported lower MCT in implants with a polished neck (59). They attributed this to the lower plaque accumulation seen on polished surfaces. Kerr et al. reported in their study that microgrooves on rough surfaces could provide a substrate for bacterial adhesion in the presence of possible contamination in the implant area (60). In light of this information, although microgrooves in the neck region may provide mechanical benefits by converting cutting stress into compressive stress, the lower MMCT in Group MT 1 may be associated with increased surface area susceptible to bacterial adhesion in implants included in Group MT 2.
Mu et al. took OPGs of dental implants in 48 patients immediately before prosthetic loading and at the 12th month after loading and compared them in terms of FD (61). They reported a significant increase in FD after functional loading. Similarly, Wilding et al. conducted a similar study using follow-up OPGs at the 24th month and reported a significant increase in FD (21). The researchers attributed the increase in FD after functional loading to microtrauma in cranial bones during function, leading to an increase in cortical bone and trabecular number (62). Julius Wolff reported in his study in 1892 that bone tissue responds differently (building or resorption) to different intensities of stimuli (63). This phenomenon became known as Wolff's Law. Subsequent studies on the subject have shown that osteoblasts respond adaptively to minimal stresses on bone (64). Stanford et al. reported in their study that positive remodeling leading to increased trabeculation could develop in the bone tissue surrounding dental implants under occlusal loading (65). In our study, similarly, FD significantly increased in both mesial and distal surfaces between T0 and T1, i.e., after prosthetic loading. Considering the increased microtrauma during the osteointegration process and prosthetic loading, the increase in bone remodeling is expected. In this regard, the increase in FD can be explained by the increase in trabeculation.
Papantonopoulos et al., in a multi-group study involving 94 implants, examined the relationship between peri-implantitis and various factors such as smoking, systemic diseases, radiographic data, bleeding on probing, and FD (66). Although they found a lower incidence of peri-implantitis in patients with higher preoperative FD, they reported no significant relationship between peri-implantitis and bone loss around the implant. Lang et al. examined the changes in parameters such as pocket depth, bleeding on probing, and FD between two groups of patients with and without peri-implantitis, consisting of 104 patients. While they found a statistically significant difference in pocket depth and bleeding on probing between the two groups, there was no statistically significant difference in FD (67). There is no study in the literature evaluating the effects of dental implant design on MCT and bone quality through fractal analysis. In this aspect, our study is an original contribution that could provide new insights. Our fractal analysis revealed no significant relationship between FD and factors such as the jaw where the implant was applied, implant diameter, implant length, prosthetic type, and surface characteristics. However, when looking at the relationship between FD and implant neck design, an increase in FD in implants with coronal microgrooves of 1 mm was significantly higher than in implants with coronal microgrooves of 3 mm. This indicates that the increase in trabeculation after 3 months of loading is greater in implants with coronal microgrooves of 1 mm. However, when looking at the correlation of the mentioned data with MCT, the MCT in implants with coronal microgrooves of 1 mm is significantly lower than in implants with coronal microgrooves of 3 mm. It is known that microgrooves regulate stress distribution. However, while the increase in FD in our study group with more microgrooves in the coronal direction is not statistically significant, on the contrary, FD has significantly increased in the group with fewer microgrooves. Implants with rough surfaces in the neck region are known to be more prone to plaque accumulation and bacterial adhesion. Therefore, implants with coronal microgrooves of 3 mm provide a larger surface area that allows bacterial colonization in case of attachment loss due to peri-implantitis. This may explain the increased MCT in implants with coronal microgrooves of 3 mm. Our data suggest that the coverage of microgrooves in dental implants with a coronal 1 mm is sufficient to regulate stress transmission and minimize MCT.
The limitations of our study are as follows:
• Since our study was conducted retrospectively, specific data related to the surgeries where dental implants were placed (insertion torque, complications, etc.) were not included in the study.
• As our study was conducted retrospectively, clinical data related to patients at T1 time (pocket depth, bleeding on probing, hyperemia, etc.) were not included in the study.
• Our study aimed to maintain a high number of participants in each group since it was a multi-group study. Therefore, in grouping the data according to implant localization, only upper and lower jaw categorization was performed based on the available data, and the anterior-posterior region differentiation could not be made due to insufficient archives.