Comparison of Success Rate and Radiological Bone Loss in Patients with Mini-Implant versus Conventional-Sized Implant Removable Prostheses: A Meta-Analysis

Background: The objective of this meta-analysis was to assess whether mini-implants have added benefit in terms of implants success rate and average bone loss over conventional-sized implants after one year of follow-up. Methods: An electronic search of randomized clinical trials was conducted in MEDLINE (via PubMed), Cochrane Central Register of Clinical Trials (CENTRAL) and Web of Science for studies including complete or partial edentulous patients requiring two or four mini-implants or conventional/ standard-sized implants in the maxilla or mandible for implant-supported removable prostheses who completed 12 months of follow-up. Results: The search provided 194 unique articles which were screened for title and abstract. Screening generated 12 articles which went through full-text analysis using eligibility criteria, and 4 articles were included for meta-analysis. Me-ta-analysis of these studies indicated a non-significant difference in the success rate between the two interventions (OR = 1.69 [0.74, 3.85; p = 0.21]). Bone loss estimates resulted in a significant bone reduction (Mean Difference = −0.74 [−0.95, −0.53; p < 0.05]) in favor of two mini-implants when compared with current studies have described the short-term outcomes.


Introduction
In recent years, dental implants have become the predictable treatment option for treatment of missing teeth [1]. They have gained much popularity because of their ability to preserve adjacent tooth structure and bone [2] [3] and due to scientific advances in the field of implant dentistry [4]. For example, in the USA, the prevalence of dental implants among adults with at least one missing tooth increased from 0.7% in 1999 to 5.7% in 2016 [5]. Despite its increasing popularity, there are several clinical aspects such as biologic and technical complications that are still debated [4].
Though not conforming to the proposed classification of dental implant length and diameter, as proposed by [6], recent randomized clinical studies embarked to compare clinical outcomes of using mini-implants over conventional-sized implants. Conventional-sized implants are described as those having a diameter ranging from 3.4 to 5.8 mm [7] while mini-implants as those ranging from 1.8 to 3.3 mm in diameter and 10 to 15 mm in length [8]. Mini-implants are considered to be superior to conventional-size implants as they offer less surgical trauma, cost effectiveness, possibility of immediate loading, possibility of use in atrophy edentulous ridge, and the dramatically smaller percutaneous circumference that may reduce the implant exposure to bacterial invasion [7]. However, the actual clinical benefits of mini implants over conventional-sized implants lack adequate evidence.
In 2017, Mostafa and colleagues performed a systematic review and meta-analysis comparing the implant survival, patients satisfaction, marginal bone loss and incidence of fracture between mini-implants and conventional-sized implants [9].
The study did not find strong evidence to support the use of mini-implant over conventional ones. Based on their analyzed parameters, authors suggested that mini dental implants can be used as an alternative choice to conventional dental implant. However, in their analysis, they did not establish comparisons of findings of resonance frequency analysis (RFA)-measure of implant stability, and implants success rates. In addition, their analysis of marginal bone loss did not consider an implant as a unit of measure. Further, they used individual patients regardless of number of implants placed per patient. Therefore, we conducted this meta-analysis to fill-in gaps of knowledge left from the previous analysis by establishing a statistical comparison of clinical outcomes of mini and conventional-sized implants by pooling together RFA outcomes, success rate and marginal bone loss of the two interventions.

Study Selection
Article screening and eligibility followed PRISMA 2009 Flow Diagram [12]. Each  review authors (K.A and Y.L) retrieved full texts and examined for compliance of all potentially eligible studies. We resolved disagreements by discussion.

Data Collection and Data Items
Reviewers designed a form in excel spreadsheet for data extraction. The two reviewers (K.A and Y.L) conducted the data extraction independently. Discrepancies were resolved in a group and after completing data extraction until reaching consensus. The excel spreadsheet captured data in the form of; Study, Study location (country), Study type, Number of patients in each treatment group, Implants success rate (%), Average radiographic bone level (mm) per study group at one year, Number of recorded prosthetic complications, Patient's satisfaction based on a visual analogue scale (VAS) at one year, resonance frequency analysis (RFA) using the Osstell ® ISQ, and risk of bias domains.

Assessment of Risk of Bias in Included Studies
Two reviewers (K.A and Y.L) independently assessed the included studies for risk of bias (RoB) using the Cochrane Risk of bias assessment tool, which addresses the following domains: selection bias (randomization and allocation concealment); performance bias (blinding of participants and personnel); detection bias (blinding of outcome assessors); attrition bias (incomplete outcome data); reporting bias (selective reporting) and other bias. The RoB summary and graphs are summarized in Figure 2 and Figure 3 respectively.

Summary Measures and Synthesis of Results
Meta-analysis was performed using Review Manager (RevMan) Version 5.3 [13].
For dichotomous data (success rate), we used the number of events in the mini-implants and conventional implant groups to calculate Mantel-Haenszel odds ratios (OR). Analysis of continuous data (radiological bone level) was achieved by using the inverse variance of mean difference. Ninety-five percent confidence interval (95% CI) was used to estimate the precision of the estimated odds ratios and mean differences, and p value of <0.05 was used as a measure of significance. When studies were sufficiently homogeneous (I 2 ≤ 50%), the data was combined by using a fixed-effect model and when substantial heterogeneity was observed (I 2 > 50%), a random-effect model would be used. When substantial heterogeneity was detected, possible explanations would be explored using sensitivity analyses.

Study Selection
The PRISMA flow chart of the study selection process is presented in Figure 1.
The search process brought a total of 194 unique references. After the screening of the titles and abstracts, 12 unique articles were retrieved. Our screening identified no additional studies. After strictly applying the inclusion criteria by reading through the retrieved full texts, 4 studies were eligible for inclusion in the meta-analysis. Excluded studies were 3 review studies, 1 conference abstract, 2 studies were not published in English and 2 studies had findings that do not support meta-analysis. Characteristics of the included studies are summarized in Table 1.  On the other hand, Aunmeungtong et al., 2017 found that after one year-follow up, the lowest mean ISQ score was 66.0 which was recorded in patients with four mini-implants; the highest mean ISQ score was 79.0 in patients with two conventional-sized implants [14]. Data provided by the studies was not sufficient to run a meta-analysis, however statistical analyzes in both studies showed no statistically significant differences in the mean ISQ score between mini and conventional-sized implants.

Success rate of mini-versus conventional dental implants
Three studies [7] [14] [15] with a total of 250 surgically placed implants using either mini or conventional-sized implants were used to compare the success rate of two treatment modalities. A non-significant pooled odds ratio of 1.69 [0.74, 3.85; p = 0.21] was observed suggesting that statistically, none of the treatment modality is superior to the other (Figure 4). Regarding the number of implants per patient, in all studies patients were placed with two conventional-sized implants. For the mini-implants, in two studies [14] [15] patients were placed with four implants and in one study [7] patients were placed with two implants. Comparison of success rate in relation to the number of implants per patient did not provide statistically different findings.

Changes in radiological peri-implant bone level
The radiological peri-implant bone level of mini-implants was compared with conventional-sized implants. Two studies [7] [14] involving a total of 140 surgically placed dental implants compared bone loss to patients who were treated with two mini-implants as compared to two conventional-sized implants. The

Prosthetic Complications
Two studies [14] [15] reported prosthetic complications that occurred within one year after surgical treatment. Aunmeungtong, et al. documented a total of 64 prosthetic complications from 40 patients placed with mini-implants. In patients who received conventional-sized dental implants (n = 20), a total of 52 prosthetic complications were recorded [14]. In the course of one year, Zygogyiannis and colleagues reported a total of 30 complications from 25 patients with mini-implants, and a total of 25 complications from 14 patients with conventional-sized implants [15]. The available information is not enough to establish association between the two types of implants.

Discussion
This meta-analysis merged data to compared implants success rate and radiological peri-implant bone loss between patients placed with mini-implants and those placed with conventional-sized implants. There was no clear evidence of a difference in success rate between the two interventions after one year of follow-up. In the annual radiological bone loss evaluation, we observed a statistically significant mean bone reduction when two mini-implants were compared to two conventional-sized implants but when four mini-implants were compared with two conventional-sized implants the outcomes were non-significant.
Included studies did not offer adequate data to compare RFA outcomes and occurrence of prosthetic complications between the two interventions.
Mini-implants have been reported to be superior to conventional-sized implants in terms of the benefits they offer to patients [7]. Due to their reduced diameter (diameter less than 3), Mini-implants offer high survival rates, immediate stability, less postoperative discomfort, favorable marginal bone loss, and increased satisfaction and quality of life of patients. Overdentures retained with conventional-sized implants offer good long term results, but are also associated with high cost and difficulty with placing especially in situations where there is reduced buccolingual dimensions of bone [17]. However, the current analysis did not find out significant outcome to prove these benefits, maybe, due to short follow-up duration.
Comparing clinical outcomes after one year of follow-up for bone loss seems to be not long enough because it is mainly accepted that crestal bone loss of dental implants during the first year of loading is an inevitable phenomenon and is generally looked upon as an adaptive response to surgical trauma and loading [18]. After the first year, an extensive bone loss might be generally due to an exacerbation of adverse body reactions caused by non-optimal implant components or prosthodontics and/or compromised patient factors [19]. force between the two types of implants [21]. There might be other risk factors which might be linked to implants failure which are not related to implant size.
In addition, even individual genetic disparities do not indicate an increased risk for implant failure [23]. Risk factors documented to play an important role in the success rate of dental implants include cigarette smoking, bruxism, diabetes and bone augmentation [24]. Therefore, if implants are placed by well-trained personnel the success rate is independent on the type or size of the implant.
This review has some limitations. First, the review managed to acquire only four studies that meet the inclusion criteria providing inadequate evidence.
Among the included studies, three reported success rate, two compared radiological bone loss between patients with two mini-implants against two conventional-sized implants. These analyses are not adequate to offer a sound statistical comparison. Therefore, the findings should be analyzed with caution. Second, the review could not find any study that compared radiographic outcomes in patients with four mini and four conventional-sized dental implants. Considering the advantages of mini-implants, more high-quality randomized controlled trials comparing mini with standard diameter implants are required.
In conclusion, available evidence comparing clinical outcomes of mini and conventional-sized dental implants do not provide confidence results due to their short follow-up period. In the current analysis, little amount of evidence is available and do not offer added clinical advantage of mini-over conventional-sized implants. More studies with follow-up times of 10 and more years are needed.