Clinical and radiographic assessment of narrow-diameter and regular-diameter implants in the anterior and posterior jaw: 2 to 6 years of follow-up

Dental implants are devices used to secure artificial teeth and are widely used to restore missing teeth, a practice that has existed worldwide for several decades. In some clinical scenarios, such as cases of inadequate bone volume, treatment with regular-diameter implants (RDIs) is often hampered, leading to relatively unfavorable clinical outcomes [1]. For this purpose, advanced and complex surgical techniques including bone splitting, guided bone regeneration, and distraction osteogenesis are often required to increase the horizontal bone dimension [2, 3, 4]. However, these surgical interventions are unrealistic for some patients, because they may involve extended treatment time, can have unforeseen complications, and can be costly [5].

Extensive research has led to the development of multiple types of dental implants that vary in shape and size according to the patient's needs [6]. Among these dental implants, the short- and long-term survival of artificial narrow-diameter implants (NDIs) has been extensively researched in patients with reduced horizontal bone height in edentulous jaws [7, 8, 9, 10]. According to Al-Johany et al. [11], NDIs are defined as dental implants with diameters ranging from 3.0 mm to 3.75 mm. NDIs are designed to work well in edentulous areas where there is limited mesiodistal space, such as the maxillary lateral and mandibular lower incisors [12, 13]. Using a RDI in such limited spaces may damage the adjacent natural dentition. Furthermore, several reports on the survival rates, aesthetic outcomes, and technical complications of NDIs have been published. Although results regarding clinical and radiographic peri-implant parameters of NDIs in the esthetic zone are widely documented, few clinicians have studied the clinical success rate of NDIs in the posterior jaws [14, 15]. In their short-term clinical study, Tolentino et al. [16] observed 100% implant survival and success rates for NDIs in the molar region of the mandible. A long-term retrospective study with up to 11 years of follow-up reported an overall survival rate of 95.1% for NDIs in the posterior region [17]. A recent meta-analysis reported a mean implant survival rate of 97.7% for NDIs that was reflective of clinical success in all regions, including posterior single-tooth restorations [18].

Nevertheless, there are still several limitations that should be taken into account when considering the introduction of NDIs in the posterior jaws. These include the risk of fracture of the screw and implant fixture due to the thin fixture wall of the NDI, as well as the risk of overload stemming from the reduced ratio of the diameter of the implant to the occlusal surface area, which can produce cantilever effects [19]. These factors may contribute to the complication rate and thereby reduce the overall clinical success of NDIs placed in the posterior region. Therefore, the present retrospective clinical study aimed to evaluate and compare the clinical and radiographic parameters, complication rates, and patient satisfaction levels of fixed prostheses supported by NDIs placed in the anterior and posterior regions.

This study focused on the comparative analysis of NDIs placed in the anterior and posterior regions. Their clinical peri-implant parameters, including plaque levels, bleeding scores, PD, and radiographic evidence of bone loss, were recorded. In addition, technical complication rates and patient satisfaction were evaluated. The results of the present retrospective study suggest that clinical and radiographic parameters showed statistically similar outcomes during the follow-up period. In addition, NDIs placed in both the anterior and posterior regions offered acceptable patient satisfaction and reasonable complication rates.

Clinical peri-implant parameters reflective of health were observed at various follow-up intervals. These were indicative of the meticulous oral hygiene maintenance practiced by all of the patients during follow-up. This relates to the importance of oral hygiene care and implies that dental implants could survive longer if plaque levels were kept low [26, 27]. It should be noted that PSs around NDIs placed in the posterior region (26.5%) were higher than those associated with anterior NDIs (18.2%). However, this difference was not statistically significant. Nevertheless, a possible reason for this difference could be that the posterior region is considered to be a difficult-to-maintain area and is a region where effective oral hygiene may not be practiced optimally. In addition, BOP was the only parameter that showed statistical significance—in particular, a statistically significant difference between the posterior and anterior NDIs. This may be related to known inflammatory signs in the posterior region, which in turn may be due to the plaque levels around NDIs placed in the posterior jaw.

None of the NDIs placed in the anterior or posterior region were associated with serious complications, including implant fracture. A previous meta-analysis found 5-year dental implant fracture rates of only 0.08% and 0.5% for single crown restorations and splinted restorations, respectively [28]. This may demonstrate that these implants with a 3.3-mm diameter yield predictable outcomes in the posterior jaw. The technical complications associated with single crowns were also higher than those associated with splinted crowns (6.5% vs. 13.2%, respectively). All 12 fixed restorations either became loosened or had some level of chipping observed in the splinted group. This might explain the significantly higher supra-structure complication rates in splinted restorations compared to single crown restorations.

Statistically significant differences with regard to CBL were observed between splinted and single restorations (P=0.22). These outcomes contradict the results presented in the long- and short-term studies conducted by Shi et al. [28] and Al-Aali et al. [29], respectively. The study by Shi et al. [28] showed that the CBL was 1.2 mm in splinted restorations and 1.3 mm in single restorations at an 8-year follow-up assessment and that the difference between splinted and single restorations was not considered clinically meaningful. This lack of a clinically meaningful difference may indicate that both NDI-supported single and splinted crowns could maintain the CBL.

It is noteworthy that the average CBL around splinted crowns was statistically significantly higher than the average CBL around single crowns (1.51 mm vs. 1.03 mm, respectively; P=0.022). This difference might be explained by the higher occlusal force and more persistent inflammation around splinted restorations compared to single restorations [29, 30]. The impact on oral cleanliness around splinted restorations is another important factor, as more debris can be retained around splinted crowns, requiring more care.

Some noticeable limitations are present in this study. First, survival analysis using advanced statistical methods such as Kaplan-Meier analysis was not performed. Additionally, with the strict eligibility criteria imposed, the outcomes may not translate to other cohorts, including tobacco smokers and individuals with systemic disorders such as uncontrolled diabetes mellitus. Finally, a limited number of NDIs were studied in the maxillary jaw, even though the maxilla and mandible differ in their bone mineral density, suggesting a potentially different outcome in mandibular NDIs. Therefore, future studies should be undertaken to confirm the clinical efficacy of NDIs in maxillary jaws compared to mandibular jaws.

NDIs placed in the anterior and posterior regions of the jaws function equally well in terms of peri-implant soft and hard tissue health, although NDIs demonstrated an increased risk of prosthetic complications. NDIs placed in both regions offered acceptable patient satisfaction and reasonable complication rates. NDI-supported prostheses in either the anterior or the posterior region could be a promising treatment option, especially in areas where advanced surgical interventions such as bone augmentation should be avoided.