Questions

The review sought to examine the quantitative effects of surface treatment on the mechanical stability of orthodontic MSs. The research question was defined according to the PICO format as follows:

• P (population/patients):

  In vivo studies involving humans or animals.

• I (intervention):

  Surface treatment of MSs.

• C (comparison):

  No surface treatment.

• O (outcome):

  Changes in the mechanical stability of MSs, expressed via MS insertion and removal torque, stability, failure after insertion, and degree of osseointegration.

Study Eligibility

Studies published in the English language that investigated the effects of surface treatment on the mechanical stability of MSs were included. Papers were excluded at this stage if they were in vitro studies, case reports, editorial letters, case series, studies without controls, not investigating the effects of surface treatment on the mechanical stability of MSs, and MSs without surface treatment.

Study Identification

Various research databases were searched, including Cochrane library (Cochrane review, Trails), Medline (PubMed, OVID Medline, and Ebsco), Embase (European studies, pharmacologic literature, conference abstract), Web of Knowledge (social science, conference abstract), SCOPUS (conference abstracts, scientific web pages), CINAHL (nursing and allied health), PsycInfo (psychology and psychiatry), ERIC (education) using key terms focused on the specific search strategy (mini-screws, mini-implants, stability, mechanical, surface, treatment, surface treatment, torque, insertion, removal). For grey literature, the following databases were searched: Google Scholar, Open Grey, National Library of Medicine, Social Science Research. For theses: (EthOS, DART-Europe), Institutional repositories (OpenDOAR, Bielefeld Base, Lenus, RIAN, e-publications@RCSI). No beginning date was used, and the last date of the search was August 2020. Additional studies were sought by searching in the reference lists of all articles included.

Study Selection

All the titles and abstracts were screened independently and in duplicate for inclusion in the study. The interrater agreement for study inclusion, as assessed using an intraclass correlation coefficient, was 0.95. Conflicts were resolved by consensus discussion between the two reviewers.

Risk of Bias Assessment

Risk of bias was assessed using SYRCLE's risk of bias tool for animal studies.²⁵ The selected studies were assessed using the following types of bias: selection bias (domains: sequence generation, baseline characteristics, allocation concealment), performance bias (domains: randomization of animal housing conditions, blinding), detection bias (domains; random outcome assessment, blinding), attrition bias (domain: incomplete outcome data), reporting bias (domain: selective outcome reporting), other (domain: other sources of bias). Each domain was graded as low risk, high risk, or unclear risk depending on yes, no, or unclear judgment, respectively. SYRCLE's risk of bias tool was converted to Agency for Healthcare Research and Quality standards (good, fair, and poor): good quality when all criteria were met (ie, low for each domain), fair quality if one criterion was not met (ie, high risk of bias for one domain) or two criteria unclear, and poor quality if two or more criteria were listed as high or unclear risk of bias or one criterion was not met (ie, high risk of bias for one domain) or two criteria unclear.

Data Extraction and Data Synthesis

The data were extracted independently by the two reviewers using a data extraction sheet, and any differences were resolved by discussion and consensus. The following data were extracted from each included study: first author, publication year, study type, study quality, sample size, inclusion criteria, surface treatment, MSs used, method of analysis, insertion torque values, removal torque values, bone-implant contact ratio, loading information, statistical analysis used, and the authors' conclusion. The meta-analysis was performed using RevMan 5.4, a desktop version of Review Manager software used for Cochrane intervention and flexible reviews. For continuous data, standard mean difference (SMD) was reported with 95% confidence intervals (CIs). In each analysis, I² was used to measure the statistical heterogeneity among studies. According to the values of P and I², the random-effects model (0<P<.1, I² ≥ 50%) was selected.

Study Characteristics

Eight studies were graded as fair, three studies as good, and three studies as poor (Table 1). The data were available from the year 2006 to 2018. Out of the 14 studies included in the review, seven used rabbits, six used dogs, and one used rats. The number of study participants ranged from 6 to 24 (total n = 185), with a mean of 13.2. Eight of the included studies used sandblasting, large grit, acid etching methods of surface treatment (SLA) (Table 2). Loading was applied in nine studies. A total of 949 MSs were used with a mean of 67.8. Removal torque values (RTVs) were assessed in 12 studies, insertion torque values (ITVs) in seven studies, and bone to implant contact ratio was assessed in seven of the included studies. The study period varied from 4 weeks to 16 weeks (Table 3).

Table 1. SYRCLE's Risk of Bias Tool for Included Studies^a

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Table 2. Descriptive Details of Studies Included^a

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Table 3. Details of MSs Used,Loading Force, Area of Placement of MSs, and Outcome Analysis Used^a

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Outcome of Studies

Overall success rate for surface treated MSs ranged from 47.9% to 100%. Mean ITV ranged from 9.6 to 41.8. Mean RTV ranged from 3.4 ± 0.5 to 79.1 ± 11.4 (Table 4). Forest plot of RTV showed significantly higher values for SLA surface treated MSs than control with an SMD of 2.61 (95% CI = 1.49–3.72, I² = 85%, P < .001) (Figure 2). Forest plot of ITV showed an SMD of –6.19 (95% CI = –13.63–1.25, I² = 98%, P = .10), with no statistically significant difference (Figure 3).

Table 4. Outcome of Studies Included^a

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Primary Stability of Surface-treated Miniscrews

The primary stability of MSs is essential because it determines the implant's clinical success rate.²⁷ It is measured in terms of ITVs, and previous authors have recommended values in a range of 5 Ncm to 10 Ncm for better stability.²⁸ The roughness produced by surface treatment may provide space for external discharge of blood and bone particles, thus surface-treated MSs are more conducive to insertion with low ITV compared with conventional machined surface MSs, n which the smooth surface might increase the ITV, resulting in greater damage to surrounding bone structures.²⁹ Surface treatment of MSs facilitates the retention of blood and osteogenic cells through increased surface area and allows migration of these cells at the MS surface. It further enhances fibrin attachment, prevents detachment of fibrin during wound healing, and facilitates bone matrix formation in direct contact with the MSs, thereby improving biocompatibility and stability.^29–32 In the present review, three studies^8,10,18 with SLA surface-treated MSs were used to assess ITV variation, and the forest plot of ITV showed SMD of –6.19 (95% CI = –13.63–1.25, I² = 98%, P = .10), with no statistically significant difference. Wide variation in torque values could be due to variation in length and diameter of MSs, initial pilot hole size, placement method (monocortical vs bicortical placement, wet vs dry placement), and thickness of cortical bone where the MS was placed.^33–37 Overall success rate ranged from 47.9% to 100%, and no association was seen between success rate and increased insertion torque values, disproving the theory that increased placement torque value would induce peri-implant necrosis and subsequent bone-implant interface degeneration.^14,38 When the length, diameter, and geometry were matched, the ITV was reduced.³⁵

Secondary Stability (Retention of MSs)

Retention of MSs depends on their surface roughness and hydrophilicity.³⁹ A rough surface enhances integrin activity at the implant site, thereby inducing cellular responses like cell sticking, migration, proliferation, and differentiation.³² Surface treatment enhances the roughness and hydrophilicity of MSs, thereby increasing retention and RTVs. In the present review, seven studies^{8,10,13,14,18,19,22} were included that assessed the variation in RTV of SLA surface-treated versus conventional machined surface MSs, and the result showed significantly higher RTV for surface-treated MSs with SMD of 2.61 (P < .001). RTV is the measure of bone-implant contact, and increased RTV indicates better osseointegration, with improved secondary stability.^36,37 The new bone formed around inserted MSs is essential for fixation strength in primary stability and for osseointegration in secondary stability, which in turn influence the success rate of MSs.⁴⁰ Three of the included studies^14,23,21 showed significantly higher bone-implant contact values for surface-treated MSs. Roughened surfaces of MSs directly influence the healing mechanism at the bone-implant interface, thereby preventing fibrin detachment during wound healing. In contrast, conventional machined surface MSs cannot retain the fibrin matrix during wound contraction. As a result, osteogenic cells differentiate and synthesize bone without reaching the surface of the implant.^39,41,42

Strengths and Limitations

The present study was the first systematic review with meta-analysis assessing the mechanical stability of surface-treated MSs. The limitation of the present study was significant heterogeneity across the studies included in the meta-analysis. The heterogeneity was caused by variation in the in vivo models, such as placement site of MSs, the animal used in the experiment, follow-up period, and loading or nonloading of MSs. Though for the study we searched for both animal and human studies, all the studies that were included were animal studies due to the lack of clinical studies in humans. The quality assessment of included animal studies rated only three studies as good. The present results point to the need for high-quality randomized controlled trials in future research.