Influence of risk factors on the long-term survival of oral rehabilitation with extra-narrow implants: a retrospective study

Abstract Objective This study aimed to retrospectively collect clinical data to evaluate the influence of possible risk factors on the long-term success of implant treatment with extra-narrow (2.9 mm diameter) implants in a daily dental practice setting. Methodology Data were collected from records of patients who received at least one extra-narrow implant from 2012 to 2017, regarding implant survival, prosthesis survival, patient characteristics, and implant characteristics. The association between the dependent variables “implant survival”, “prosthesis survival,” and “adverse events” related to patient and implant characteristics was statistically evaluated by chi-square tests. Moreover, implant and prosthesis survival were analyzed by Kaplan-Meier survival curves. Results The sample was constituted of 58 patients (37 women and 21 men) with a mean age of 54.8 years old (SD: 12.5), followed up for up to eight years. In total, 86 extra-narrow implants were placed within this sample. Four implants were lost, resulting in an implant survival rate of 95.3%. A total of 55 prostheses were inserted and only one (1.8%) was lost, resulting in a prosthesis survival rate of 98.2%. The mean implant and prosthesis survival time was, respectively, 7.1 years and 6.3 years, according to the Kaplan-Meier survival analysis. A correlation was found between smoking and implant loss, which makes implant loss eight times more likely to occur in smokers than non-smokers. A significant association was also found between prosthesis loss and previous need of prosthesis repair. However, it was not considered clinically relevant. No association was found between the occurrence of adverse events and later implant or prosthesis loss. Conclusion High implant and prosthesis survival rates were found in the long term for treatment with extra-narrow implants. Moreover, a significant correlation between smoking and implant loss was observed.


Introduction
Dental implants are widely used with great success and long-term survival rates in completely and partially edentulous patients. 1,2 The implant diameter choice is based on several factors and, since an adequate bone volume and interdental space are required to produce good results, single-tooth rehabilitation in the anterior region can be challenging. 3 Moreover, when placed in the atrophic alveolar bone, standard-diameter implants can expose their threads and lead to failure. 4 This is common in cases of agenesis, present in 2.2% to 7.6% of the population, 5 and after tooth extraction, in which the alveolar bone resorption is progressive. 4 Other conditions, such as trauma, neoplasia, and denture wearing, are related to reduced space. 6 Some treatment approaches are suggested to successfully manage patients with limited space for standard dental implants, such as bone augmentation techniques. However, these approaches are more invasive, presenting higher risks of complications, besides a longer time and additional costs. 7 In cases of limited mesiodistal space, orthodontic treatment and adhesive partial denture are suggested, but it might not meet all patients' expectations. Thus, narrowdiameter implants emerge as a reliable alternative.
Although there is no consensus in the literature on the definition of narrow implants, in general, implants with a diameter narrower than 3.5 mm are considered narrow whereas implants with diameters narrower than 3.0 mm are described as extra-narrow or mini implants. 8 Reduced bleeding, postoperative discomfort, and healing time are some of the reported advantages of these implants when compared with grafting procedures. 4 Moreover, narrow and extranarrow implant survival rates from 80% to 100% were reported in a follow-up period of up to seven years. 5,6,9,10 Regarding aesthetic aspects, which are especially important in the rehabilitation in the anterior region, good results seem to be produced by narrow implants. 11 The reduced diameter makes it possible to achieve an adequate 3-dimensional position, respecting the necessary distance between implant and adjacent teeth, as well as surrounding bone, to facilitate papillae formation and its maintenance in the long term. 9 This is especially important to achieve good aesthetic outcomes in upper and lower lateral incisors and central incisors, which present the smallest mesiodistal dimensions. 12  The association between the dependent variables "implant survival," "prosthesis survival," and "adverse events" and patient and implant characteristics was evaluated by chi-square tests and by estimating the relative frequencies, odds ratios (OR), and 95% Their length ranged from 10 to 14 mm, to support single or multi-unit fixed and removable prostheses in maxilla and mandible. Patients were followed up for a mean period of 2.8 years (SD: 1.9; up to 8.0). Four implants were lost due to lack of osseointegration, resulting in an implant survival rate of 95.3%. Three of these losses occurred before loading. from 36 to 60 N.cm were reached in most implants (39;45.3%). A peri-implant bone loss greater than 1.5 mm was observed in six (7.0%) implants. No significant association was found between implant characteristics and implant loss ( Table 2).
In total, 55 prostheses were inserted. Only one For the other six implants, the prothesis type was not reported.
No correlation was found between patients' medical conditions and prosthesis loss (Table 3).
However, a significant association was found between prosthesis loss and previous need of prosthesis repair (p=0.040; Table 4).
Regarding adverse events, four (4.7%) occurrences were reported: two (2.3%) chronic pain episodes and two (2.3%) local inflammatory reactions. No correlation was found between patient characteristics or implant-related variables and adverse events (Tables 5 and 6).   From variables that showed significance by the chisquare test, factors that could influence survival were evaluated. Thus, it was found that the mean implant survival time for non-smokers was greater than for smokers (3.7 years for smokers and 7.2 for nonsmokers; p=0.002) ( Figure 3 and Table 7). The mean implant and prosthesis loss time was, respectively, 1.7 years (SD: 1.9) and 0.9 years. On the other hand, smoking habits increased by    Since retrospective observational studies use data that were originally collected for other purposes, not all relevant information might have been available for analysis, and this is a limitation of this study. There could be missing data due to poor registration quality or variables that were not considered to be registered in advance. In both cases, the origin of missing information can lead to information bias. Moreover, due to this study design, it may be difficult to assess the temporal relationship between data found, leading to a potential confounding bias. Information related to date of implant and prosthesis placement were collected, as well as concerning risk factors. This