Stability comparison between commercially available mini-implants and a novel design: Part 1
To compare mechanical stability among five mini-implant designs—a newly invented design and four commercially available designs that vary by shape and threading; to calculate external surface area of each design using high-resolution micro-computed tomography; and to evaluate the relationship between surface area and stability results. The four commercially available mini-implants—single-threaded and cylindrical (SC), single-threaded and tapered (ST), double-threaded and cylindrical (DC), double-threaded and tapered (DT)—and a new implant that is designed to engage mostly in cortical bone with shorter and wider dimensions (N1) were inserted in simulated bone with cortical and trabecular bone layers. The mechanical study consisted of torque measurements and lateral displacement tests. External surface area was computed using a 25-µm micro-CT. Maximum insertion torque, maximum removal torque, and force levels for displacements were the highest in N1, followed by DT, ST, DC, and SC (α = .05). The surface area was largest in DT, followed by N1, ST, DC, and SC. Surface area engaged in cortical bone, however, was the greatest in N1. The surface area of mini-implants had positive correlation with stability. Among commercial designs, both added tapering and double threading improved stability. N1 was the most stable design within this research design. The new design has the potential to be clinically superior; it has enhanced stability and there is diminished risk of endangering nearby anatomic structures during placement and orthodontic treatment, but the design requires refinements to reduce insertion torque to avoid clinical difficulty and patient discomfort.Abstract
Objective:
Materials and Methods:
Results:
Conclusion:
INTRODUCTION
Over the past decade, orthodontic mini-implants have been proven to be useful in establishing absolute anchorage and introducing compliance-free, more efficient, and more precise biomechanics to orthodontic treatment.1 Despite the tremendous success of mini-implants in facilitating treatment outcomes, however, the stability of mini-implants remains an important issue that needs to be resolved because failure rates are widely variable and could be as high as 25%.2,3 As the potential uses of mini-implants have broadened, recent studies have focused on examining factors contributing to failure and evaluating stability.4,5 The stability of mini-implants is generally defined with two main components.6 Primary stability is established from the mechanical retention between the mini-implant surface and bone.7 It is dependent on the thickness and integrity of the cortical bone, the mini-implant design, and loading protocol.7,8 Secondary stability is achieved through continuous bone remodeling around the mini-implant, leading to osseointegration.9,10
To achieve optimal stability, previous studies have examined various mini-implant parameters, such as loading protocol, surface treatment, shape, diameter, length, tapering, and thread count.7,9,11–13 Researchers concluded that stability can be enhanced by maximizing the interlocking surface area (SA) between the bone and implant, which can be achieved by increasing diameter, adding threading, and tapering mini-implants.4,11,13 However, these design modifications may cause patient discomfort and bony microfractures during placement.4,14 In recent years, mini-implants with various thread designs have been developed with the goal of improving stability, but few studies have tested the stability of these mini-implants with a new thread design.
Among many known methods for assessing implant stability,8,15 insertion torque is often measured in determining initial stability.8,14 Removal torque should be comparatively high to prevent unscrewing and is considered to be a better measurement of stability.7 In addition to torque testing, the lateral displacement test to measure the force required for displacement of 0.01, 0.02, and 0.03 mm was performed.
The potential bone–implant contact area is important for mechanical interlocking in establishing primary stability and osseointegration in secondary stability.4,16 Although information on SA would have been valuable, this parameter is very difficult to calculate using mathematical methods alone because of the complexity of mini-implant design features such as threading and taper. In this study, we used micro-computed tomography (μ-CT) to precisely compute the mini-implant surface area engaged in cortical and trabecular bone layers for each design.16 The use of μ-CT has been proven to be an accurate and reproducible method of quantifying microstructures in the field of biomedical and material research.17
The aim of this study was to compare the stability among five mini-implant designs using torque and lateral displacement tests. As root proximity5 and root contact are major factors in mini-implant failure,18,19 a new design (N1) was invented. The N1 design is short to engage mostly in cortical bone but wide to optimize stability. Its exterior is single threaded and cylindrical and its interior has a hollow center to facilitate insertion.
MATERIALS AND METHODS
According to the shape and thread design, Ti-6Al-4V alloy mini-implants (Biomaterials Korea, Seoul, Korea) (Table 1) were divided into five groups: single-threaded and cylindrical (SC), single-threaded and tapered (ST), double-threaded and cylindrical (DC), double-threaded and tapered (DT), and the single-threaded and cylindrical new design with shorter and wider dimensions (N1) (Figure 1).
Citation: The Angle Orthodontist 81, 4; 10.2319/092410-556.1
For each test, 20 mini-implants of each design were inserted by one technician into biosynthetic bone (170 × 120 × 40 mm [length × width × height] Sawbones, Vashon, Wash). The bone block was composed of two layers: 2-mm-thick20 30 pounds per cubic foot (PCF) cortical bone simulation11 and 10 pcf trabecular bone (Table 2). A 1-mm-thick plastic sheet was used to simulate soft-tissue thickness. All mini-implants were inserted perpendicular to the bone surface until their necks contacted the soft-tissue simulation.
Torque Test
As in the study by Kim and colleagues,11 each design of mini-implants (n = 20) was inserted and removed using a surgical engine and contra-angle handpiece (ElcomedSA200C, W&H, Bürmoos, Austria). The unit was calibrated each time and rotational speed was set for 30 rpm.11 Maximum insertion torque (MIT) and maximum removal torque (MRT) were measured.
Lateral Displacement Test
After a new set of 100 mini-implants was properly inserted using a manual driver, mechanical testing was performed using the compression mode of Instron 5560 (Instron Corp, Norwood, Mass) (Figure 2). With 1 kN load cell, applied force value and lateral displacement data were computed with Bluehill®2 Version 2.2, integrated software (Instron Corp, Norwood, Mass). The biosynthetic bone with mini-implants inserted was secured with the vice and positioned to allow the indenter to deliver a force perpendicular to the neck of the mini-implants in a downward direction. The software was programmed to record the perpendicular force as implants were displaced at 0.01 mm, 0.02 mm, and 0.03 mm, respectively, by the indenter from their original position. After a row of mini-implants was tested, it was removed to allow the machine to access the next row.
Citation: The Angle Orthodontist 81, 4; 10.2319/092410-556.1
Surface Area Measurement
Mini-implants were scanned using a high-resolution μ-CT (Skyscan1172F, Virginia Beach, Va) at an image resolution of 25 µm, using 90 kV and 110 µA with an aluminum-copper filter. Three-dimensional (3D) data sets were reconstructed from two-dimensional (2D) images, corrected of ring artifacts and beam hardening and fine-tuned for alignment. The orientations of the data sets were standardized three-dimensionally. As shown in Figure 3a, the implants were viewed in 3D μ-CT image-viewing software to locate the base of tissue collar (TC), soft tissue, cortical bone (CB), and trabecular bone (TB) sections (the area from CB to the tip). External surface was divided according to the thickness of soft-tissue template and cortical bone used in the mechanical study. The optimal threshold of 65 was determined for 2D binarized and 3D images to depict the true morphology.21 The cylindrical volume of interests was drawn to enclose CB and CB + TB areas. To calculate the surface area of the CB region, the 3D SA parameter, bone surface (BS), was measured and subtracted by the 2D SA parameter, bone area (B.Ar), of the top and bottom cross-sections that do not contribute to SA engaged in the bone. Similarly, the SA of CB + TB was calculated as B.Ar of the top cross-section of the CB + TB subtracted from BS.21 Finally, 3D images were rendered for visualization (Figure 3b).
Citation: The Angle Orthodontist 81, 4; 10.2319/092410-556.1
Statistical Method
Mean MIT, MRT, and force levels for each lateral displacement distance were compared across designs using one-way analysis of variance. Statistical significance was defined as P < .05 using the Fisher least significant difference criterion. Means and standard deviations were reported. Scatter plots were examined to assess the relationships between SA and torque values and between SA and force levels for each displacement. Correlation coefficients (r) were obtained using Pearson's correlation.
RESULTS
Torque Test
Both mean MIT and MRT were highest in N1, followed by DT, ST, DC, and SC (Table 3, Figure 4). The MRT was lower than the MIT in all groups. The means of MIT in all groups were statistically different at P < .05, and the means of MRT in all groups were significantly different except between SC and DC and between ST and DC (Table 4). DT showed higher MIT and MRT than DC or ST. Adding tapering or double threading separately increased MIT and MRT.
Citation: The Angle Orthodontist 81, 4; 10.2319/092410-556.1
Lateral Displacement Test
As in the torque tests, the mean force required for displacement of all three distances was greatest in N1, followed by DT, ST, DC, and SC (Figure 5a, Table 5). The means in all groups were significantly different (P < .05), except for SC and DC at the 0.01-mm displacement (Table 6). Besides registering the highest MIT and MRT, N1 also required the greatest force for all displacement distances. Adding tapering and double threading increased the force required as in the torque tests. Figure 5b shows that the initial displacement of 0.01 mm from the original position required the highest force compared to the additional force required for subsequent 0.01-mm displacements.
Citation: The Angle Orthodontist 81, 4; 10.2319/092410-556.1
Surface Area Calculation
Figure 6a represents the differential SA embedded in cortical bone and trabecular bone layers for each design. N1 had the greatest SA in cortical bone. The total external SA was largest in DT, followed by N1, ST, DC, and SC. Figure 6b also shows that as the SA increases, mean MIT and MRT also increase with a positive correlation in a linear relationship except for N1. There is a stronger positive correlation (r > .9) with torque among the four commercial designs than among all designs including N1 (r > .75). Figure 6c shows that as external SA increases, average force required for displacement increases in a linear relationship. There is a strong positive correlation (r > .95) between SA and force. N1 had less total SA than DT, but it required greater MIT, MRT, and force.
Citation: The Angle Orthodontist 81, 4; 10.2319/092410-556.1
DISCUSSION
The order of stability was established as N1 followed by DT, ST, DC, and SC. Possible explanations for this order may stem from differences in diameter, tapering, and threading. Because of its large diameter (4.1 mm), N1 required considerably high torque and force for lateral displacement consistent with previous studies.22 In addition, both tests proved that tapered mini-implants were more stable than cylindrical ones with either single thread or double thread.7,11 Because tapered mini-implants are wider in the neck, tighter engagement and greater contact with cortical bone is established, lending increased stability. Similarly, double threading increases contact area with cortical bone.
In this study, SA values were successfully computed using μ-CT scans despite the complexity of the design of implants studied. The positive proportional relationship and the correlation between external SA and stability values (MIT, MRT, force for displacement) reaffirmed the importance of the contact area between implant and bone for initial mechanical interlocking.4 At the same time, increased implant–bone contact area allows for greater osseointegration, thereby enhancing secondary stability as well.16 Direct bone–implant contact is reported to be 10% to 58%.23 With growing versatility in clinical uses of mini-implants, they are remaining inserted for longer periods, including the retention phase, making osseointegration an increasingly important factor in mini-implant stability.24
Interestingly, N1 required significantly higher force for all displacement distances than DT, although N1 had a smaller surface area than DT. Similarly, N1 generated greater MIT and MRT than DT. This trend shows that surface area in cortical bone is a stronger contributor to mini-implant stability than in trabecular bone.12 Because we used commercially available designs, the length of cylindrical designs (5.5 mm) was shorter than that of tapered designs (6.1 mm). Numerous studies25–29 have established a consensus that screw length is not associated with failure or stability, and our study also highlighted the importance of engagement in cortical bone. Thus, differences in length can be deemed to have little impact in comparisons of stability in this study.
The study by Kim and colleagues7 reported the decrease in MIT for “dual-thread” implants compared to single-threaded implants. This disparity in findings is attributable to the essential difference in design between Kim and colleague's dual-thread and this study's double-thread. Dual-thread design has a thread count that is doubled per unit length in the neck region, possibly causing insertion rate to be decreased by half. However, because double-thread design has additional overlapping set of threads with the same thread count per unit length as single threading, insertion rate is not altered (Figure 7).
Citation: The Angle Orthodontist 81, 4; 10.2319/092410-556.1
The lateral displacement test was first introduced in this study and showed results consistent with the torque tests. Unlike the study of Florvaag and colleagues,30 which investigated mini-implant deformation up to 3.5 mm using the pull-out strength test, which applies vertical force, this study used a test with perpendicular force, which is consistent with how orthodontic forces are applied on mini-implants. Also, this study focused on initial loosening at microlevels of 0.01, 0.02, and 0.03 mm, as initial mobility is the key determinant for success of mini-implants.31 After initial enlargement of bony socket or possible microfractures created with initial movement, mechanical retention between bone and implant diminishes and less additional force was needed to displace further distances (Figure 6).
Considering that orthodontists use light force of less than 200 g for tooth movements, all designs used in this study could withstand orthodontic force as the force required for 0.01 mm displacement exceeded 200 g. The force needed to displace 0.02 mm and 0.03 mm for more stable designs (ST, DT, and N1) approximated or exceeded the 500 g needed for orthopedic movements. With the development of designs that provide better stability, mini-implants can aid orthopedic movements and minimize adverse effects on anchor teeth. Thus, mini-implants with headgear tubes or hooks for face masks can become reality.
Overall, N1 is a very stable design, as proven by both tests. The N1 design, engaged mostly in cortical bone, has the potential to be clinically superior as it can avoid root proximity in areas with adequate cortical bone thickness, which is a major factor in mini-implant loosening and risk of root resorption and ankylosis.18,19,27 Furthermore, in carefully selected cases, N1 can not only reduce the risk of damage to neurovascular bundle and sinuses but can also perform en masse retraction more efficiently without the limitation of inter radicular space. The MIT of N1 (15.6 Ncm), however, exceeds recommended insertion torque range of 5 to 10 Ncm,25 and N1 has a low torque ratio of MRT to MIT because of this high MIT. Excessive insertion torque can introduce clinical difficulty of insertion and bone strain, which in turn can adversely affect the stability of mini-implants.14,25 The N1 design requires improvements to reduce MIT to facilitate placement and to lower the risk of bone strain while maintaining high MRT. In addition, clinical trials, in vivo studies, and histologic analysis to evaluate bone remodeling with N1 are required to further investigate its use.
CONCLUSIONS
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Mini-implants with increased diameter, tapering, and double threading can maximize interlocking SA between bone and implant, thereby increasing primary stability.
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As the implant surface area embedded in bone increases, stability increases. The large SA of N1 in cortical bone yielded the highest stability despite its having a smaller total SA than DT, suggesting that SA in the cortical bone plays a more vital role in mechanical retention. Increasing cortical bone–implant contact should be considered for future designs.
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The new design (N1) is promising as it showed superior stability with less risk of endangering nearby anatomic structures during placement and orthodontic treatment. However, modifications of N1 to lower insertion torque are indicated to avoid clinical difficulty and patient discomfort.
Characteristics of five mini-implant designs. Ta indicates tapered; H, hollow; TD, thread design.
Instron 5564 Universal Testing Apparatus for lateral displacement test.
(a) Different sections of surface-area measurements. (b) μ-CT scanned three-dimensional images of five designs.
Means of maximum insertion and removal torque for all groups.
(a) Lateral displacement test summary. (b) Force required for subsequent 0.01-mm displacement.
(a) Surface area (SA) divided into cortical and trabecular bone. (b) The relationship between SA and torque values. (c) The relationship between SA and force required for lateral displacement.
Thread-pattern comparison between dual-threaded and double-threaded implants.
Contributor Notes