Carlos Flores-Mir DDS, DSc, FRCD(C); Analia Veitz-Keenan DDS
Mini-implants are effective as anchorage, but their success depends on proper initial mechanical stability and loading quality and quantity.
The authors followed accepted guidelines for conducting a systematic review, however the included studies were not methodologically sound and had inherent differences in design and quality.
In patients undergoing orthodontic treatment with adjunctive mini-implants what factors determine the success of these additional anchorage devices?
The authors searched three databases for English-language articles published up to 2007. They also performed hand-searches. Their criteria included human trials that studied the use of mini-implants for orthodontic anchorage. Two independent reviewers evaluated the selected studies. They both assessed study design, sample size, measurement methods, method-error analysis, blinding, statistics and confounding variables.
The authors identified 324 articles, 16 of which they selected for inclusion. Of those selected studies, 14 involved a self-tapping placement method that required a pilot drill to prepare a hole in the bone; this method yielded success rates ranging between 85 and 100 percent. The other two studies involved a self-drilling placement method; this method produced a success rate of 70 percent in one study while the other study did not evaluate the survival rate of the mini-implants. Of the 16 selected studies, six evaluated mini-implants with immediate loading and the success rates ranged from 75 percent to 100 percent (all self-tapping method). In the remaining 10 studies, the mini-implants were loaded after healing times of two to four weeks, with success rates ranging from 0 percent to 100 percent. In most of the studies, the orthodontic load on the mini-implants was 100 to 200 grams of force as direct anchorage. In some studies, the diameter of the screws (from 1.5 to 2.3 mm) was associated with their stability with a success rate ranging from 80 and 100 percent.
Based on the available data for self-tapping mini-implants, the diameter and the length of the implant should be 0.2 to 0.5 mm larger than the width and the depth of the bone hole for optimal placement torque. For mini-implants, healing time is unnecessary. The selection of the mini-implant size depends on the available bone. Published success rate lies between 70 and 100 percent for the included studies.
Source of Funding:
Importance and Context:
Practitioners are increasingly using mini-implants as an alternative orthodontic anchorage method. They appear to overcome previous anchorage limitations in complex cases and are conducive to better patient acceptability and compliance in comparison with external anchorage devices like an orthodontic headgear. The success rate for the mini-implants appears relatively high but failures are common. A better understanding of placement methods, loading protocol and optimum sizes can help minimize failures.
Strengths and Weaknesses of the Systematic Review:
The authors followed accepted guidelines for conducting a systematic review. They limited their broad search strategy to include English-only publications, which weakened the review. The review included a flow diagram of their literature search. The authors retrieved all the possible data and evaluated several factors related to success such as implant shape, material, diameter, length, implant number, loading quantity, healing period, duration and success rate.
Strengths and Weaknesses of the Evidence:
The available evidence from the review comes from 16 prospective and retrospective studies. The included studies have a high risk of bias, no comparison groups, unclear treated population and poorly defined criteria for measuring success. Due to these differences, the authors could not combine the data from the studies and provided only a narrative assessment.
Implications for Dental Practice:
Mini implants seem to be an interesting alternative for orthodontic treatment when other options for anchorage are not acceptable. The current review sheds some light into some concerns, but the results should still be considered with caution. Self tapping of mini-screws shows a higher success rate than self-drilling. Allowing healing time seems unnecessary. The diameter of the mini-screws should be considered as it is associated with its success. New related study results need to be consider as this systematic review could get quickly outdated with the pace of research in this area.