Effect of Operator-Related Factors on Failure Rate of Orthodontic Mini-Implants (OMIS) used as Temporary Anchorage Devices (TAD); Systematic Review

Aim: This review aimed to determine the operator-related variables that may influence the clinical performance and failure rate of orthodontic mini-implants (OMIs) used as anchorage devices.

Materials and Methods: A search was performed through electronic databases; PubMed, EMBASE searched via ScienceDirect and Cochrane Library. Reference lists were limited to English papers ranging from 2012 to 2018. Eligibility criteria were defined by considering the (PICOS) question patients who received OMIs for orthodontic anchorage. Inclusion and exclusion criteria were performed independently by two authors.

Results: A lot of factors have been proven to affect the success rate of the OMIs, whereas root-proximity and secondary insertion of the mini-implant revealed to be the most significant factors for OMIs failure.

Conclusions and Recommendations: The OMIs should be placed as far as possible from the root, and secondary insertions of failed primary implants should also be avoided.

Keywords: Mini-Implants; Temporary Anchorage Device; OMIs; TAD; Orthodontic Anchorage

To achieve the best successful results in orthodontic treatment, anchorage control should be thoroughly managed. The most recent way to gaining this goal is by using mini-implants which have been accepted all over the world [1-5].

Mini-implants are the smallest temporary anchorage devices (TAD) that can be used in different sites of the oral cavity, and in areas that are not reachable by any other types of orthodontic anchorage appliances [6, 7]. Such devices are also accepted by most of the patients [8,9].

A lot of research has been conducted to test the success rate of orthodontic mini-implants (OMIs), showing an average success rate of approximately 84% [10,11]. Further research (meta-analysis) reported an overall failure rate of 13.5% for orthodontic mini-implants [12].

The failure rate of orthodontic mini-implants proved to be affected by lots of variables which including: Patient-related factors comprising: oral hygiene measures, smoking, cortical bone thickness, as well as age of the patient [13-16].

Operator-related factors (technical factors) comprising : root proximity, insertion torque, insertion angle, besides amount of orthodontic load [OMIs are stable within forces of 50 g (0.5 N) to 450 g (4.5 N)], direction of load, time of loading (Immediate vs delayed), primary or secondary (re) insertion as well as placement site [12,13,16-22,26-34].

Mini-implant - related factors comprising: screw-diameter, screw length, implant material and insertion method [13,15,35-40]. In general, a success rate of OMIs greater than 80% should encourage the operator to use it. Scanning of the latest systematic literature reviews and meta-analyses, the technical operator-related factors revealed to have the main impact on OMIs success [10,12,41,42].

This review will try to extend and focus on the parameters related to operator related variables, that could influence the failure rate of orthodontic mini-implants (OMIs).

The selection criteria for this review were defined by considering the PICOS question as following:

1- Population (P): Patients of both sexes, without restriction on age, ethnic, or socioeconomic groups were included. Their orthodontic treatment with fixed appliances required skeletal anchorage.
2- Intervention (I): Intervention comprised the placement of orthodontic MIs for skeletal anchorage.
3- Comparison (C): OMIs insertion angle, amount of orthodontic load,direction of load and placement site were compared.
4- Outcome (O): Mini-implant fracture, patient pain or discomfort and loss of mini-implant stability considered as failure. These outcomes are evaluated twice, primary and secondary: -

• Primary outcome: evaluating all described signs before OMIs functions finishing. Measured immediately after implant insertion.
• Secondary outcome: evaluating all described signs after OMIs functions finishing Measured after the healing phase. 5- Study design (S): (Table 1).

Databases: With filtering of the last 5 years researches, only English papers were selected, because studies of languages other than English (LOE) mainly tend to be of lower quality than studies written in English. Moreover, few of these studies could have the criteria for inclusion into the review, but are still not representative of all the LOE studies [43,44]. Hence, the studies were limited to English language only.

Our search was started at 2018-1-14. The Electronic databases and search strategies are shown in Appendix 1.

All papers were collected in Reference manager (EndNote X7), and managed as following:

All titles and summaries of collected publications were reviewed in order to exclude inadequate articles. Full versions of remaining, possibly appropriate articles were reviewed. Full texts of articles’, which eligibility could not be evaluated by reviewing their summaries, were read in order to avoid incorrect exclusions. The process of articles’ selection is presented in the PRISMA flow diagram (Figure 1).

Two authors independently extracted study characteristics and outcomes from the included studies. Miniscrew implant failure counts were extracted as a binary outcome and converted to failure event rates. The primary outcome was the overall miniscrew implant failure rate, and associated factors were the secondary outcomes. Risk factors were assessed by comparing two or more event rates provided by a study.

Two authors assessed independently the risk of bias of the included studies using the Cochrane Collaboration’s tool for assessing risk of bias by means of RevMan (version 5.2) as guided by the Cochrane Handbook for Systematic Reviews of Interventions [45]. The following domains were considered: (1) adequate sequence generation, (2) allocation concealment, (3) blinding of participants and personnel, (4) incomplete outcome data, (5) selective outcome reporting, and (6) other sources of bias. For all included studies, the risk of bias for each domain was judged as low risk, high risk, or unclear risk. Each randomized controlled trial was assigned an overall risk of bias in terms of low risk (low for all key domains), high risk (high for ≥1 key domain), and unclear risk (unclear for ≥1 key domain).

357 articles were collected after primary electronic database search. The search results are shown in the PRISMA flow diagram. 32 duplicated items were found, and the remaining 323 articles analyzed their titles and abstracts in detail. The articles which had not confirmed the inclusion requirements were rejected and 56 articles full texts were downloaded and read. After applying the inclusion and exclusion criteria, 16 articles were kept, complete list of included studies shown at Table 2. The excluded 39 papers after full text screening were mentioned in Appendix 2

• All included studies were evaluated for the quality based on modified Feldmann and Bondmark suggested method under five criteria: 1) sample size, 2) research method, 3) research object description, 4) research technique and 5) study design. After qualitatively evaluating all articles, they were divided into two categories: of high (8-10 points) (3-9,11, 34-36) and medium (6-7 points) (10, 12) quality (Table 3) [46].
• 4418 OMIs of 12 different manufacturers (Chopra et al. 2015) and 4 different types of materials (Titanium, titanium alloy, Titanium-vanadium alloy and stainless steel) which had been threaded in 1709 patients’ upper and lower jaws at different areas, were analyzed.
• The samples of analyzed OMIs were not less than 28 OMIs (Albogha et al. 2016) and not exceeding 1375 OMIs (Melo et al. 2016). The number of 10-570 patients were included in the search. The analyzed OMIs were used for anchorage of the dentition for at least 3.5 months. The success rate of MI was assessed in the analyzed articles.
• Diameter of OMIs ranged from 1.2-2.3 mm and their length ranged from 6-12 mm (Table 2).
• The technical operator-related factors affecting the success rate of OMIs included; selected placement site (including root proximity), insertion torque, insertion angle, amount of orthodontic load, direction of load, time of loading (Immediate vs delayed) and primary or secondary (re) insertion. The included studies focused on: insertion site (including root proximity), insertion angle (most of included studies focused on vertical angle), amount, direction, as well as onset of loading.
• Uesugi et al. 2017 described the effect of secondary insertion of OMIs on the success rate of OMIs, being about 44.2% for all re-inserted types.
• OMIs were inserted in different areas, but most of the studies placed them between the 2nd premolar and 1st molar (especially in the Maxilla). These inserts were used for different purposes but most of authors used it for retraction of the anterior segment.
• The applied load used in all included studies, and it ranged from 50-300 gm, while a few papers did not even describe the amount of load applied (Table 2).
• The OMI stability/success/failure affecting factors were analyzed in all articles, however, authors had given different definitions of a “successful” MI (Table 2). A successful MI is that implant which performs its’ function as a skeletal anchorage device for a certain period of time (6-12 months), or during the entire orthodontic treatment period without any notable mobility, surrounding soft tissue inflammation or any other pathologies.
• Root proximity has been found to be the most significant factor for OMIs failure, and therefore at least 1mm clearance should exist between root and OMIs. Janson et al. 2013 declared that: OMIs root proximity didn’t influence the success rate as long as there was no periodontal ligament invasion. Albogha et al. 2016 stated that if OMIs is slightly apically inclined, reducing the vertical angulation, the OMI will be away from the roots. He also declared that with a small interradicular width, the OMI

should be placed closer to the root opposing the force direction that will be applied later. Garg et al. 2015 supported the evidence of Albogha et al. 2016 by proving that the OMIs do not remain absolutely stationary like the end-osseous implant throughout orthodontic loading. Therefore, it is mandatory that in case of small interradicular, the OMI should be placed closer to the root opposite to the future force direction.
• Almost all studies found that the onset of OMIs loading, either immediate or delayed, affects the success rate of OMIs insignificantly, or even having no effect at all. In 2015, Jeong et al. recommended the delay of load application, as he found that the immediate loading increased the risk of failure.
• OMIs vertical angulation was measured by different ways in different articles, but not all articles measured the angulation of the mini-implant (Table 2). Some authors measured the angulation of mini-implants to root and others measured it from mini-implant surface to alveolar bone and finally, others measured it to the occlusal plane. The mini-implant angulation ranged from 40-90° with an exception of Jing et al. 2016, who started his measurement from 10°-90°. In 2013, Jung et al., and Park et al. 2018 declared that cortical bone thickness increased with decreased vertical placement angle, and the success rate increased as the cortical bone thickness increased. Although this association was not statistically significant. All authors consider the OMIs angulation change not a statistically significant.
• The success rate of OMIs used during orthodontic treatment in all included studies ranged from 79.2% to 97%, though the success rate was not presented in some articles.
• The authors in several included studies described many operator-related factors affecting success rate of OMIs. However, the statistically significant factors that affect OMIs success rate were: root proximity as well as secondary insertion of pre-failed OMI.

• Many operator-related factors can affect the success rate of orthodontic mini-implants OMIS, and it should be taken into consideration before placement of the implant.
• The operator should give extra welling to the root proximity and should prevent any secondary insertion of pre-failed OMIs.

• Place the OMIs as far away as possible from the root, and if the space between roots are thin, make the OMI away from the root of force application.
• Avoid secondary insertions of pre-failed OMI.