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Basic and Clinical Research

A Comparison of the Mechanical Measures Used for Assessing Orthodontic Mini-Implant Stability

Hosein, Yara K. PhD; Dixon, S. Jeffrey DDS, PhD; Rizkalla, Amin S. PhD; Tassi, Ali DDS, MClD

Author Information
doi: 10.1097/ID.0000000000000514
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Abstract

The use of mini-implants as skeletal anchorage devices in orthodontic tooth movement has expanded over the past decade.1–3 Their clinical success is largely influenced by the mini-implants' stability postimplantation. Once inserted, they maintain their function as an anchor by mechanical retention achieved at the implant-bone interface (termed primary stability). As such, any measures used to assess mini-implant stability would be expected to quantify the extent of retention or primary stability achieved.

Within the orthodontic research community, a variety of mechanical measures have been used to assess mini-implant stability.4 These include insertion torque (IT),5–8 Periotest value (PV),9–11 and pull-out load (POL).7,12–14 Each of these stability measures assesses the mini-implant's resistance to an applied force. IT is a measure of the rotational resistive force experienced by the mini-implant during advancement into the bone and can be indicative of the strength of the implant-bone connection.6 Higher torques indicate greater frictional resistance between the mini-implant and bone during insertion, suggesting a stronger implant-bone connection. In comparison, Periotest is a percussion device that measures implant mobility based on the embedded mini-implant's resistance to the application of a lateral tapping force. Depending on the duration of contact between the pressure-sensitive tapping head of the Periotest and the implant, a stability value is output, where shorter duration of contact is indicative of a less mobile mini-implant.15 Similarly, pull-out tests assess the embedded mini-implant's resistance to axial forces by measuring the magnitude of force required to cause mini-implant removal from the bone7,13,14,16,17 and the displacement of the mini-implant in response to the applied force. High axial loads accompanied by small screw displacement (ScrD) upon pull-out are indicative of a stable mini-implant. Although all of the aforementioned measures have been used to quantitatively define mini-implant stability, their assessments are specific to the application of a unidirectional force, which is unlike the multidirectional loading experienced by mini-implants in clinical situations in vivo.

Clinically, mini-implants are exposed to a variety of forces, depending on their location of placement and attachment to the orthodontic appliance. Common mini-implant placement sites include the buccal cortical plate in the maxilla and mandible, maxillary palate, palatal aspect of the maxillary alveolar process, and the retromolar area in the mandible.18 Depending on its placement position relative to the applied orthodontic force (from the attached appliance), the mini-implant may be exposed to multidirectional loading. For example, a mini-implant placed in the buccal cortical plate will primarily experience lateral loading perpendicular to the long axis of the implant, with some degree of torsional load dependent on the location of the appliance attachment at the mini-implant head. In comparison, a mini-implant placed in the palate or in the palatal aspect of the maxillary alveolar process may also experience force components along the long axis of the implant (ie, axial forces). Additionally, forces from occlusal function or parafunction may be transmitted to the mini-implant through the attached orthodontic appliance and can contribute to mini-implant loading in a variety of directions. However, few studies have considered multidirectional loading in their in vitro assessment of mini-implant stability.7,12–14,19,20

In addition to the singular force directionality of the mechanical measures currently used to assess mini-implant stability, some are restricted in use due to their invasive nature (eg, pull-out force and displacement) or limited availability in clinical practice (eg, IT). Although the Periotest measurement system is noninvasive and permits clinical assessment of implant stability, it has not been extensively adopted in orthodontics research.10,11,15,21 Considering these limitations, the described mechanical measures have frequently been used independently or interchangeably when quantifying mini-implant stability in the dental literature.22–31 However, there are no known studies that have specifically compared all 3 mechanical measures to determine their agreement in quantifying mini-implant stability. An understanding of the relationship among these measures will validate their interchangeable use as predictors of mini-implant stability.

The present study investigated different mechanical measures used to assess mini-implant stability. We compared an array of mini-implant designs using stability measures that are extensively reported in the dental literature to determine the relationship between and reliability of these various measures. It was hypothesized that mini-implants would show differences in their stability based on measures of IT, PV, POL, and ScrD and, when relationships were compared between the various stability measures, some measures would show stronger correlations and reliability than others.

Materials and Methods

Ninety self-drilling (n = 15 per mini-implant) orthodontic mini-implants from 6 different manufacturers (Aarhus, Medicon, Tuttlingen, Germany; Dual-Top, Jeil Medical Corporation, Seoul, Korea; OrthoEasy, Forestadent, Pforzheim, Germany; Tomas-pin, Dentaurum, Turnstrasse, Ispringen, Germany; Unitek, 3M, Monrovia, CA; and VectorTAS, Ormco, Glendora, CA) were used in this study. All mini-implants were 8 mm in length, with diameters ranging between 1.4 and 1.8 mm. Six different mini-implant designs were included in the study to obtain a range of stability responses for comparing the various stability measures.

A custom-made device was used for manual insertion of mini-implants into artificial bone blocks with uniform material properties (cortical layer: 40 pcf; cancellous layer: 20 pcf) (Sawbones; Pacific Research Laboratories, Vashon, WA) (Fig. 1A). During insertion, a torque sensor (AMTI 6-DOF; Advanced Mechanical Technology Inc., Watertown, MA) secured at the base of the bone block measured torques experienced during mini-implant advancement. The insertion device was designed with a universal driver supported by a stabilizer, which allowed insertion of mini-implants in a vertical orientation, without contribution of off-axis loading along the length of the mini-implants. Torques experienced throughout insertion were measured by the torque sensor, and the maximum IT for each mini-implant was identified (Instron WaveMatrix Software; Instron, Norwood, MA). This stability measure was used to define the frictional force experienced at the implant-bone interface during mini-implant insertion.

Fig. 1
Fig. 1:
Mini-implant stability measurement methods.A, IT was measured using a custom-designed device, which incorporated a torque sensor secured at the base of the bone block sample and a stabilized universal driver used for mini-implant insertion. B, Periotest tool was used to assess mini-implant mobility based on percussion tests. C, Pull-out tests were used to measure the strength of the mini-implant and bone interface connection.

Immediately after insertion, the mechanical stability of the implanted systems was assessed using the Periotest tool (Periotest, Medizintechnik Gulden, Germany) (Fig. 1B). The Periotest was used to measure mobility of the embedded mini-implants and it output resultant stability values. The lower the PV observed, the greater the stability of the mini-implant, with negative PVs indicating greatest stability.

Subsequently, the mini-implants were placed in a materials testing machine (Instron 8874) for pull-out testing (Fig. 1C) to further assess mini-implant stability. A custom-built attachment clamp was used to connect the head of all mini-implants to the materials testing machine, allowing pull-out of the mini-implant screws from the bone blocks. The machine's displacement control setting was used to extract the mini-implants at a rate of 2 mm/min, to a maximum displacement of 3 mm. Load-displacement curves were generated for each of the mini-implants throughout testing, and measures of force and displacement at mini-implant pull-out were recorded. The POL offered information on the strength of mechanical retention achieved by the mini-implant, and the ScrD at pull-out measured the mini-implant's resistance to motion during application of the axial load.

For statistical analysis of the stability results, Shapiro-Wilk (α = 0.05) tests were first used to test for normality among the stability measures for each mini-implant. For normally distributed data, one-way ANOVA with post hoc Tukey tests (α = 0.05) were used to compare the stability among mini-implants for each measure. If the stability data failed normality tests, a Kruskall-Wallis test with Dunn's multiple comparison test (α = 0.05) was used to detect differences in the measurements between mini-implants. To determine the association between stability measures for all mini-implants, Spearman rank correlation analyses were performed on the grouped data. In addition, to assess the reliability of the various measures of mini-implant stability, the coefficients of variation (COVs, %) were compared for all measures.

Results

Overall, this study found differences in mini-implant stability, specific to the mechanical measure used for assessment (Fig. 2). Unitek, Tomas-pin and Dual-Top showed the highest ITs (P < 0.05), with no differences found between them (P > 0.05) (Fig. 2A). When comparing POLs, Unitek displayed the highest force at mini-implant pull-out (P < 0.05, Fig. 2D). Similarly, Unitek showed the lowest mobility values (P < 0.0001) during Periotest assessments (Fig. 2B). However, comparisons of axial displacement during pull-out revealed that Aarhus and Dual-Top mini-implants experienced the smallest displacements among the group (P < 0.05, Fig. 2C).

Fig. 2
Fig. 2:
Stability results for all mini-implant designs tested based on measures of (A) IT, (B) PV, (C) ScrD, and (D) POL. Stable mini-implants were identified by high IT, low PV, low ScrD, and high POL. Overall, mini-implants showed variations in stability results, which were not consistent across the 4 individual stability measures. Data presented are mean ± standard errors, n = 15. Bars labeled with the same lower case letters are not significantly different (P > 0.05).

When the degree of association among stability measures was assessed, correlation analyses found moderate relationships between ITs and PVs (r = −0.68; P < 0.0001, Fig. 3A) and between ITs and POL (r = 0.66; P < 0.0001, Fig. 3C), where mini-implants with high ITs also displayed high POLs and low PVs. Similarly, POL moderately correlated with PVs (r = −0.55; P < 0.0001, Fig. 3E). By contrast, axial ScrDs showed only weak correlation with PVs (r = −0.41; P < 0.0001, Fig. 3D) or no correlation with other stability measures (P > 0.05, Fig. 3, B and F).

Fig. 3
Fig. 3:
AF, Scatter plots showing the association between each of the stability measures. A, PV and IT. B, ScrD and IT. C, POL and IT. D, ScrD and PV. E, POL and PV. F, ScrD and POL. The line of the best fit (linear regression) for each scatter plot is depicted in solid black, with its corresponding 95% confidence interval represented by the dotted lines. Overall, the greatest association was found between IT and PV, IT and POL, and POL and PV.

COV analysis showed that Periotest results had the greatest variability (% COV: 11%–100%) compared with IT (≤11%), POL (≤4%), and ScrD (≤19%) for all mini-implant designs. In addition, it was found that the highest variability in Periotest measurements was for those mini-implants that appeared to be more stable based on low PVs (Table 1).

Table 1
Table 1:
Summary of Stability Data From Measures of IT, PV, POL, and ScrD

Discussion

Implant stability is an important measure for the development and testing of new mini-implant designs and for assessing the clinical success of orthodontic mini-implants. Ideally, a stability measure should quantify the extent of anchorage achieved on insertion and throughout the course of treatment. Within the dental literature, there are a variety of measures used to define mini-implant stability,4 dependent on the resistance of the mini-implant to a unidirectional load. However, clinical loading of mini-implants is not singular, and the direction of the force vector that acts to destabilize the mini-implant is dependent on the location of implant placement, the force from the attached orthodontic appliance, as well as other forces in the oral cavity (eg, occlusal function) that can be transmitted to the mini-implant.

Mini-implants are placed at various locations in the mandible and maxilla—the buccal cortical plates, palate, palatal aspect of the maxillary alveolar process, and the retromolar area in the mandible.18 Depending on the mini-implant position relative to the applied force, the resultant load applied at the mini-implant head may be multi-modal, with a combination of axial, torsional, and lateral components. As such, any assessment of mini-implant stability used to predict clinical success should ideally consider the implant's resistance to all of these forces. Despite this, mechanical measures used to quantify mini-implant stability have focused on singular modes of loading and have been used interchangeably within the literature to predict the success of mini-implants. To address this need, we investigated each mechanical measure and determined their associations and reliability in quantifying mini-implant stability.

Differences were found in the stability of 6 different mini-implants based on the measures of IT, load and ScrD at pull-out, and PV. Analysis of the results showed that differences in stability among the mini-implants were specific to the mechanical measure used for assessment. For example, VectorTAS, Ortho Easy, and Aarhus mini-implants showed the least stability when assessed by IT but stability comparable to other mini-implants when assessed by POL, ScrD, or PV. These differences in mini-implant stability results show the specificity of an implant's resistance to the type of load being applied. When these mechanical measures are used independently in the dental literature, this specificity may lead to discrepancies in predicting the clinical success of mini-implants.

Similarly, the different stability measures showed variations in the strengths of correlation for all mini-implant designs tested. Moderate correlations were found between ITs and PVs and between ITs and POLs. On the other hand, weak or no correlations were observed between the other stability measures. In addition, no correlation coefficients greater than 0.68 (P < 0.05) were found between stability measures. Based on these findings, we propose that the associations are not strong enough to confidently use these mechanical measures interchangeably in defining mini-implant stability.

In addition to the relationships between stability measures, this study compared the reliability of each measure based on % COVs. Periotest showed the highest variability in its measurement of mini-implant stability, with a relatively constant error (SD ≈ 1.0) found during the assessment of each mini-implant. As such, when the PV increased for less stable mini-implants, the percentage variation decreased, indicating better reliability of the Periotest in detecting unstable mini-implants. These results were similar to those reported in the literature,32,33 when PVs were compared with Osstell ISQ values for dental implants. Considering this, it is suggested that future studies investigate the sensitivity of the Periotest in detecting varying degrees of mini-implant instability, to determine its threshold and validate its use in assessing the potential for mini-implant loosening.

To simplify interpretation of the various stability responses of mini-implants, the authors suggest that mechanical measures be classified into 2 main stability categories—interface strength and interface motion.34–36 Interface strength measures the maximum load required to cause instability at the implant-bone interface, whereas interface motion quantifies mobility at the interface as a result of the applied load. Torque and POL are examples of interface strength measures, whereas PV and ScrD at pull-out measure interface motion. Although both categories quantify the stability of mini-implants, it is important to understand the necessity of each in defining mini-implant success. A mini-implant with optimal stability would demonstrate high interface strength and low interface motion. A comprehensive assessment of mini-implant stability should incorporate measures that quantify both interface strength and motion when exposed to relevant orthodontic loads.

Application of relevant orthodontic loads is required when assessing mini-implant systems. As mentioned previously, orthodontic loading of mini-implants is dependent on the location of mini-implant placement and the direction of the force from the orthodontic appliance. In addition, forces resulting from occlusal function or parafunction may be transmitted to the mini-implant. The resultant force vector will be a combination of individual force components—axial (z direction), lateral (x or y direction), and torsional (moment about the z axis)—where the contribution of each force component can lead to instability and resultant loosening of mini-implants. Each of these forces was assessed via the measures used in this study: POL/displacement, PV, and IT, respectively. The authors suggest that assessment of mini-implant stability should involve testing the mini-implant's response to each of these loading modes and combining the results in the interpretation of overall stability. This will allow for a comprehensive understanding of the mini-implant's stability response, specific to placement location and function.

This study compared stability measures obtained using 6 different mini-implant designs inserted into a standardized bone model, with the expectation that various mini-implant designs would show varying degrees of stability. The authors intentionally chose different mini-implant models to determine whether stability results for each mini-implant were affected by the mechanical measure used in its stability assessment (eg, IT, Periotest, and pull-out). Based on the findings showing variations in mini-implant stability specific to the measure used for assessment, caution should be taken when predicting the success of mini-implants based on in vitro data using just one of these stability measures.

Conclusion

Results from this study comparing commonly used mini-implant stability measures found that differences in mini-implant stability response were specific to the mechanical measure used for assessment. At best, only weak to moderate correlations were found between stability measures, and Periotest showed the lowest reliability in the reproducibility of its results. Overall, the findings of this study highlight the need for more robust approaches to assess stability to better predict the clinical success of mini-implants.

Disclosure

This research was funded by an internal research grant from Schulich Dentistry, Western University. In addition, Y. K. Hosein was supported in part by the Joint Motion Program, a CIHR Strategic Training Program in Musculoskeletal Health Research and Leadership. The authors certify that the research is original, not under publication consideration elsewhere, and free of conflict of interest.

Acknowledgments

The authors acknowledge the manufacturers for donating the mini-implants used in this study: Aarhus, Medicon, Tuttlingen, Germany; Dual-Top, Jeil Medical Corporation, Seoul, Korea; OrthoEasy, Forestadent, Pforzheim, Germany; Tomas-pin, Dentaurum, Turnstrasse, Ispringen, Germany; Unitek, 3M, Monrovia, CA; and VectorTAS, Ormco, Glendora, CA. Special thanks to Dr. Cynthia E. Dunning and the Jack McBain Biomechanical Testing Laboratory at Western University for use of their mechanical testing facilities.

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Keywords:

torque; pull-out load; implant loosening; Periotest; mechanical retention

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