IMPLANT DENTISTRY / VOLUME 24, NUMBER 1 2015
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Machine-Driven Versus Manual Insertion Mode: Influence on Primary Stability of Orthodontic Mini-Implants Domen Novsak, DDS,* Magda Trinajstic Zrinski, DDS,† and Stjepan Spalj, DDS, PhD‡
n adequate planning of anchorage is essential for control of the amount, and direction, of teeth movement during orthodontic treatment. Bone anchorage devices are currently the best option for this purpose as they annul reactive forces otherwise present on anchorage teeth, thus expanding the limits of orthodontic treatment possibilities.1 The orthodontic mini-implant, most often used for bone anchorage, is a small cylindrical or conical screw usually measuring from 1.2 to 2.3 mm in diameter and from 4 to 15 mm in length of the body. Orthodontic mini-implants can be self-tapping (having a blunt tip and needing predrilling of a pilot hole), self-drilling (having a drill-like fluted tip, they drill their own path and do not necessarily need a pilot hole), or both self-tapping and self-drilling (designed to allow insertion with or without a pilot hole) and can be inserted either manually or in a machine-driven mode. A systematic review of literature reports adequate stability during treatment of over 80%.2 Some of the factors affecting the long-term stability
A
*Intern, Public Health Center Rijeka, Rijeka, Croatia. †Research Assistant, Department of Orthodontics, School of Medicine, University of Rijeka, Rijeka, Croatia. ‡Assistant Professor, Department of Orthodontics, School of Medicine, University of Rijeka, Rijeka, Croatia.
Reprint requests and correspondence to: Magda Trinajstic Zrinski, DDS, Department of Orthodontics, School of Medicine, University of Rijeka, Kresimirova 40, Rijeka 51000, Croatia, Phone: +38551345638, Fax: +38551345630, E-mail: [email protected] ISSN 1056-6163/15/02401-031 Implant Dentistry Volume 24 Number 1 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000174
Purpose: The study aimed to explore the effect of the insertion method on maximal insertion torque as a measure of primary stability while controlling for the effect of cortical bone thickness, mini-implant length and diameter, and vertical insertion force on insertion torque. Methods: Six types of miniimplants (Dual Top; Jeil Medical, Corp.) with diameters of 1.4, 1.6, and 2.0 mm and lengths of 6 and 8 mm were inserted manually and in a machine-driven mode into pig rib bone samples, and experiments were repeated 10 times, which totaled 120 tested implants in 120 pig rib samples. Cortical bone thickness was measured with a sliding caliper, whereas insertion torque and vertical insertion forces
were recorded with a specially designed device. Results: Significant predictors of better primary stability are thicker cortical bone (explaining 24.2% of variability), wider diameter (20.6%), manual insertion (9.9%), greater length (3.7%), higher maximal vertical insertion force (2.2%), and lower vertical force at maximal insertion torque (1.4%). Conclusions: Manual insertion is associated with higher primary stability of orthodontic mini-implants than mechanical insertion, but thicker cortical bone and larger implant diameter seem to be stronger predictors of primary stability. (Implant Dent 2015;24:31–36) Key Words: insertion mode, insertion torque, mini-implant, vertical force
of mini-implants are low quality and quantity of bone, especially of cortical bone,3,4 excessive force applied and too long extraosseous arm of mini-implant,5 periimplantitis,6 insufficient primary stability,7 and damage to bone during insertion.8 Primary stability is the stability of mini-implants immediately after insertion and it is affected mainly by bone characteristics, insertion technique,4,9 and implant geometrical design characteristics.3,8–10 Mini-implant insertion without predrilling of a guiding hole is expected to result in greater torque.11 In the predrilling method, a smaller hole diameter leads to higher primary stability.9 Concerning
insertion angles, although some previous research recommended angles of 60 to 70 degrees for achieving better primary stability,12 other studies suggest that placement angles lower than 90 degrees decrease anchorage resistance.13,14 The purpose of this study was to examine the effect of the insertion method (manual vs machine-driven) on maximal insertion torque as a measure of primary stability while controlling for the effect of cortical bone thickness, mini-implant length and diameter, and several components of vertical insertion force (vertical force at maximal insertion torque, maximal vertical insertion force at the
32
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NOVSAK
beginning of implantation, and median and deviation of vertical insertion force during implantation). Our research hypothesis was that maximal insertion torque would be increased by machine-driven insertion but significantly mediated by thicker cortical bone, greater mini-implant diameter and length, greater maximal vertical insertion force applied at the beginning of insertion while penetrating cortical bone, and higher median of vertical force during insertion. Uneven vertical insertion forces, that is, larger deviation of forces and greater vertical force at maximal insertion torque were expected to reduce maximal insertion torque.
MATERIALS
AND
METHODS
Six types of mini-implants (Dual Top; Jeil Medical, Corp., Seoul, Korea) made of titanium alloy grade 5 (Ti-6Al4V), with diameters of 1.4, 1.6, and 2.0 mm and lengths of 6 and 8 mm were inserted manually and in a machinedriven mode into pig rib bone samples, and the experiments were repeated 10 times, which totaled 120 tested implants in 120 pig rib samples. All were cylindrical self-drilling bracket-head implants and no predrilling of a guiding hole was performed. To ensure consistency of implantation depth, all implants were inserted up to 1 mm in distance between implant head and cortical surface. Mini-implants were inserted by a single operator perpendicular to the surface of the bone. Insertion angle of 90 degrees instead of 60 to 70 degrees was chosen because it is optimal for obtaining good anchorage resistance, as reported previously.13,14 For manual insertion, a straight hand driver was used with handle and driver shaft length of 111 and 43.5 mm, respectively. The driver was cross-headed. Mechanical insertion was performed with an electric cordless screwdriver (Jeil Medical, Corp.) at 20 rpm.
ET AL
The soft tissue attached including the periosteum was removed with a surgical raspatory. Bone samples of 1.5 cm 3 1.5 to 2 cm were made using a diamond disc with water cooling. Samples were stored in saline solution at room temperature. One mini-implant was inserted in the center of each bone sample. Cortical bone thickness was measured nearest to insertion site at both sides of the sample with an electronic digital sliding caliper (Levior s.r.o., Prerov, Czech Republic) that has an accuracy of 60.03 mm, as reported by the manufacturer. Mean values were used for analysis. Device for Simulating Behavior of Bone in Space
Before the insertion of mini-implants, bone samples were secured with 4 fixation screws into a device designed for the simulation of 3-dimensional biomechanical bone behavior constructed at the Institute of Civil Engineering in Zagreb, Croatia. The samples were placed in an upright position. The device was connected to an oscilloscope PicoScope (Pico Technology, Ltd., St Neots, United Kingdom), which uses electrical potential differences for monitoring force and torque behavior in time as a 2-dimensional graph. Measurements were recorded in PicoLog 5.14.6 software (Pico Technology, Ltd.). The
measured parameters were maximal insertion torque, vertical force at maximal insertion torque, maximal vertical insertion force, and median and deviation of vertical force. Maximal insertion torque was used as a measure of primary stability. The device was calibrated by standardized weights of 100, 200, and 500 g. The research was approved by the Ethical Committee of the School of Medicine, University of Rijeka. Statistical Analysis
For the comparison of differences between groups defined by insertion mode and mini-implant length and diameter, and controlling for the effect of cortical bone thickness, analysis of covariance and Sidak post hoc test were used. Multiple hierarchical linear regression analysis was applied to assess predictive values of all defined biomechanical factors of primary stability. The multicollinearity of predictors was assessed by variance inflation factor and tolerance. All analyses were performed in the statistical software SPSS 10.0 (SPSS, Inc., Chicago, IL).
RESULTS The average cortical bone thickness of samples was 1.42 6 0.34 mm
Bone Models
Pig ribs have previously been used in similar studies as bone models15 due to their anatomical similarity to the human mandible.16 Fresh lumbar pig ribs with diameter of 1 to 1.5 cm and cortical bone thickness of 0.5 to 3.0 mm were used.
Fig. 1. Comparison of maximal insertion torque (N$mm) between tested groups, controlling for the effect of cortical bone thickness (1.42 mm). Error bars represent mean and 95% confidence intervals (CIs). Parentheses connect groups that differ significantly at P , 0.05. There is a discernible trend of increased maximal insertion torque in manual insertion mode and with the increase of implant diameter and length.
IMPLANT DENTISTRY / VOLUME 24, NUMBER 1 2015
Fig. 2. An example of changes in vertical insertion force and torque during time in manual and machine-driven insertion modes of mini-implant 1.6 3 8 mm. The graph shows that in the manually inserted group, torque values rose in waves during insertion. Each wave reflected one rotation of hand. In the machine-driven group, torque values increased in a regular manner.
(range, 0.8–2.4 mm), maximal insertion torque was 106.71 6 46.38 N$mm (range, 39.61–347.49 N$mm), maximal vertical insertion force was 12.74 6 3.18 N (range, 4.82–17.84 N), and vertical force at maximal insertion torque was 8.62 6 3.05 N (range, 2.55–17.7 N). There were no significant differences in cortical bone thickness, maximal vertical insertion force, and vertical force at maximal insertion torque between groups; however, differences in maximal insertion torque between implant design and
insertion method groups were significant (P , 0.001). Although there was a discernible trend of mini-implants inserted in machine-driven mode having lower maximal insertion torque than the manually inserted ones, while controlling for the effect of cortical bone thickness, this was statistically significant only for mini-implants of 1.6 3 8 mm in dimension (Fig. 1; P , 0.001). In the manually inserted miniimplant group, differences were significant between dimensions of 1.4 3 6 mm
33
and 2.0 3 6 mm, whereas in the machine-driven mode, differences were significant between 1.4 3 6 mm and 2.0 3 6 mm, and 1.6 3 8 mm and 2.0 3 8 mm (Fig. 1; P , 0.05). A positive association can be noticed between maximal torque on one side and length and diameter on the other (Fig. 1). In the manually inserted group, torque values rose in waves during insertion. Each wave reflected one rotation of hand during insertion (Fig. 2). In the machine-driven group, torque values increased in a regular manner during insertion. Torque values were higher for the manually inserted group than for the machine-driven insertion group. Vertical insertion force in the manually inserted group rose at the beginning and then remained the same throughout the insertion. In the machine-driven insertion group, the force rose steeply and then slowly decreased until the end of insertion (Fig. 2). In bivariate correlations, maximal torque showed a significant negative linear correlation with machine-driven insertion (r ¼ 0.363; P , 0.001), whereas a positive linear correlation with miniimplant length (r ¼ 0.226; P ¼ 0.013), diameter (r ¼ 0.429; P , 0.001), and cortical bone thickness (r ¼ 0.511; P , 0.001) was demonstrated. Correlations with the defined components of vertical insertion force were not statistically significant. Only predictors that correlated significantly with maximal torque were included in the first regression model. Thicker cortical bone, greater diameter, manual insertion, and greater length of mini-implants were associated with higher values of maximal insertion torque, accounting for 24.5%, 18.4%, 12.3%, and 4.4% of variability, respectively (Table 1). Relating to bivariate correlations, a small decrease of effect size was shown for length, mode of insertion, and cortical bone thickness, whereas it remained the same for diameter. Addition of vertical force at maximal insertion torque (model 2) and maximal vertical insertion force (model 3) did not considerably increase the predictive value of the model (Table 1). Also, addition of median and deviation of vertical insertion force did not substantially increase the predictive value but rather created
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ET AL
Table 1. Hierarchical Regression Analysis for Assessment of Biomechanical Predictors of Mini-Implant Primary Stability
Model/Predictor Model 1* Constant Insertion (1 ¼ manual, 2 ¼ machine) Cortical bone (mm) Implant diameter (mm) Implant length (mm) Model 2† Constant Insertion (1 ¼ manual, 2 ¼ machine) Cortical bone (mm) Implant diameter (mm) Implant length (mm) VFMIT (N) Model 3‡ Constant Insertion (1 ¼ manual, 2 ¼ machine) Compacta (mm) Implant diameter (mm) Implant length (mm) VFMIT (N) MVIF (N)
Correlations
Unstandardized Coefficient B
Standard Error
Standardized Coefficient b
P
Zero order
−141.030 −32.316
29.401 5.368
−0.350
,0.001 ,0.001
−0.363
−0.489
−0.350
67.223 79.523 9.721
7.888 10.757 2.685
0.496 0.429 0.210
,0.001 ,0.001 ,0.001
0.511 0.429 0.226
0.622 0.568 0.320
0.495 0.429 0.210
−139.225 −32.289
30.521 5.392
−0.350
,0.001 ,0.001
−0.363
−0.489
−0.349
67.062 79.616 9.724 −0.208
7.951 10.809 2.696 0.891
0.494 0.430 0.211 −0.014
,0.001 ,0.001 ,0.001 0.816
0.511 0.429 0.226 −0.049
0.620 0.568 0.320 −0.022
0.492 0.430 0.210 −0.014
−170.521 −29.640
32.070 5.354
−0.321
,0.001 ,0.001
−0.363
−0.462
−0.315
67.065 87.177 8.881 −2.562 3.213
7.754 10.928 2.648 1.249 1.226
0.494 0.471 0.192 −0.169 0.220
,0.001 ,0.001 0.001 0.043 0.010
0.511 0.429 0.226 −0.049 0.053
0.631 0.600 0.301 −0.189 0.239
0.492 0.454 0.191 −0.117 0.149
Partial Semipartial
In the first model, relating to bivariate correlations, a small decrease of effect size was shown for length, mode of insertion, and cortical bone thickness, whereas it remained the same for diameter. Addition of VFMIT (model 2) and MVIF (model 3) did not considerably increase the predictive value of the model. *R ¼ 0.782; R2 ¼ 0.612; adjusted R2 ¼ 0.598; F ¼ 45.321; P , 0.001. †R ¼ 0.782; R2 ¼ 0.612; adjusted R2 ¼ 0.595; F ¼ 35.970; P , 0.001. ‡R ¼ 0.796; R2 ¼ 0.634; Adjusted R2 ¼ 0.615; F ¼ 32.663; P , 0.001. MVIF indicates maximal vertical insertion force; VFMIT, vertical force at maximal insertion torque.
multicollinearity problems. Hence, they were dropped out of the regression model. The final regression model demonstrated cortical bone thickness and miniimplant diameter as the most significant predictors of primary stability, accounting for 24.2% and 20.6% of variability, respectively, while controlling for the other predictors. Insertion mode, implant length, maximal vertical insertion force, and vertical force at maximal insertion torque accounted for 9.9%, 3.7%, 2.2%, and 1.4%, respectively. The whole model explained 61.5% of variability of primary stability. Higher primary stability is associated with greater mini-implant diameter, length, manual insertion, thicker cortical bone, lesser vertical force at maximal torque, and greater maximal vertical insertion force (Table 1).
DISCUSSION Since orthodontic mini-implants allow for immediate loading, their
primary stability is very important. Out of all biomechanical factors measured in this research, thickness of cortical bone and mini-implant diameter contribute mostly to higher primary stability, which has been corroborated in previous studies.3,4,8,9,17–19 Implant length contributes less to better initial stability. However, a positive connection to fixation in the bone has been shown.1,3,19 An advantage of this study is multiple regression analysis that allows for quantification of the unique contribution of each assessed parameter to the explanation of variability of insertion torque, that is, controlling for the amount of shared variability among parameters. Although we had hypothesized that machine-driven insertion could be related to higher primary stability, our research showed that manually inserted mini-implants have better initial stability. The advantages of machinedriven mini-implant insertion are ease and comfort in implantation, but this
research shows that it does not contribute to better primary stability of the miniimplant. The tactile sensation seems to be weaker in machine-driven than in hand-driven implantation. This is important in case of contact of mini-implant with the tooth root, when the torque value necessary for insertion suddenly rises.20 Immoderate force can have negative consequencesdoverheating, overcompression, and necrosis of the bone. In view of this, we believe that devices that regulate insertion torque and rotation frequency could be helpful. In our opinion, it may be useful to choose machine-driven over manual insertion in a clinical situation with thick cortical bone to avoid damaging the bone by excessive insertion torque. Several ways of measuring primary stability are used often as follows: histological analysis, percussion test, radiological analysis, measurement of implantation and explantation torque, pull-out test, Periotest and resonance
IMPLANT DENTISTRY / VOLUME 24, NUMBER 1 2015 frequency analysis (RFA),9,18,21,22 and measuring the displacement at lateral loading.23 Apart from experimental methods, there are mathematical ones such as finite element analysis.24,25 Studies have shown that the therapist’s tactile assessment can be a valuable estimate of bone quality.26 Measurement of implantation torque and RFA are considered the most reliable methods of assessment of mini-implant primary stability according to some authors.9,20 However, implantation torque may not be a linear measurement of primary stability. Previous research indicates that there is an optimal interval of torque values in which better mini-implant stability can be achieved, such as a recommend torque from 50 to 100 N$mm for the mini-implant of dimensions 1.6 3 8 mm.27 Too high or too low torque values may result in decreased miniimplant stability.28 Some studies substantiate this only for lower torque, whereas torque values more than 200 N$mm may result in mini-implant fracture.4,29 It is probably necessary to balance the factors influencing mini-implant stability to obtain good results. Vertical force at maximal insertion torque and maximal vertical insertion force barely affect primary stability. The same applies to average values of vertical insertion force and its dispersion. Nevertheless, this study indicates the possibility that higher maximal vertical insertion force applied at the beginning of implantation when the tip of the miniimplant penetrates the compacta, provides for better stability. Presumably, higher maximal vertical insertion force in this critical moment allows for better mini-implant centering during insertion, thus reducing horizontal oscillations that result in mini-implant instability. Our research also suggests that during final insertion, when maximal insertion torque is achieved, the reduction of vertical insertion force contributes to higher primary stability as the cortical bone is probably less displaced and damaged and tighter contact of implant and bone is achieved. The remainder of variability in orthodontic mini-implant primary stability is probably accounted for by the density of compact and spongious
bone, formation of microcracks, and horizontal oscillations of the therapist’s hand during insertion. Additional research should include monitoring of the above-mentioned parameters in the evaluation of primary stability.
CONCLUSIONS Manual insertion is associated with higher primary stability of orthodontic mini-implants than mechanical insertion, but thicker cortical bone and larger implant diameter seem to be stronger predictors of primary stability than manual insertion mode.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ACKNOWLEDGMENTS The authors would like to thank Ivica Anic, Snjezana Glavicic, and Andrej Katalinic for their help in the experimental procedures. A part of the tested mini-implants and the electric cordless screwdriver was donated by the manufacturer Jeil Medical, Corp., Seoul, Korea, to whom authors hereby thank for the donation. This research has been financed by the University of Rijeka (research project 13.06.2.1.53, principal investigator Stjepan Spalj).
REFERENCES 1. Chen Y, Kyung HM, Zhao WT, et al. Critical factors for the success of orthodontic mini-implants: A systematic review. Am J Orthod Dentofacial Orthop. 2009;135:284–291. 2. Reynders R, Ronchi L, Bipat S. Miniimplants in orthodontics: A systematic review of the literature. Am J Orthod Dentofacial Orthop. 2009;135:564.e1–564.e19. 3. Lim SA, Cha JY, Hwang CJ. Insertion torque of orthodontic miniscrews according to changes in shape, diameter and length. Angle Orthod. 2008;78:234–240. 4. Wilmes B, Drescher D. Impact of bone quality, implant type, and implantation site preparation on insertion torques of mini-implants used for orthodontic anchorage. Int J Oral Maxillofac Surg. 2011;40:697–703.
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5. Büchter A, Wiechmann D, Koerdt S, et al. Load-related implant reaction of miniimplants used for orthodontic anchorage. Clin Oral Implants Res. 2005;16:473–479. 6. Cheng SJ, Tseng IY, Lee JJ, et al. A prospective study of the risk factors associated with failure of mini-implants used for orthodontic anchorage. Int J Oral Maxillofac Implants. 2004;19:100–106. 7. Melsen B, Costa A. Immediate loading of implants used for orthodontic anchorage. Clin Orthod Res. 2000;3:23–28. 8. Wilmes B, Ottenstreuer S, Su YY, et al. Impact of implant design on primary stability of orthodontic mini-implants. J Orofac Orthop. 2008;69:42–50. 9. Wilmes B, Rademacher C, Olthoff G, et al. Parameters affecting primary stability of orthodontic mini-implants. J Orofac Orthop. 2006;67:162–174. 10. Katic V, Kamenar E, Blazevic I, et al. Geometrical design characteristics of orthodontic mini-implants predicting maximum insertion torque. Korean J Orthod. 2014;44:177–183. 11. Su YY, Wilmes B, Hönscheid R, et al. Comparison of self-tapping and self-drilling orthodontic mini-implants: An animal study of insertion torque and displacement under lateral loading. Int J Oral Maxillofac Implants. 2009;24:404–411. 12. Wilmes B, Su YY, Drescher D. Insertion angle impact on primary stability of orthodontic mini-implants. Angle Orthod. 2008;78:1065–1070. 13. Petrey JS, Saunders MM, Kluemper GT, et al. Temporary anchorage device insertion variables: Effects on retention. Angle Orthod. 2010;80:446–453. 14. Woodall N, Tadepalli SC, Qian F, et al. Effect of miniscrew angulation on anchorage resistance. Am J Orthod Dentofacial Orthop. 2011;139:e147–e152. 15. Migliorati M, Benedicenti S, Signori A, et al. Miniscrewdesign and bone characteristics: An experimental study of primary stability. Am J Orthod Dentofacial Orthop. 2012;142:228–234. 16. Kim SJ, Yoo J, Kim YS, et al. Temperature change in pig rib bone during implant site preparation by low-speed drilling. J Appl Oral Sci. 2010;18:522–527. 17. Morarend C, Qian F, Marshall SD, et al. Effect of screw diameter on orthodontic skeletal anchorage. Am J Orthod Dentofacial Orthop. 2009;136:224–229. 18. Su YY, Wilmes B, Honscheid R, et al. Application of a wireless resonance frequency transducer to assess primary stability of orthodontic mini-implants: An in vitro study in pig ilia. Int J Oral Maxillofac Implants. 2009;24:647–654. 19. Topouzelis N, Tsaousoglou P. Clinical factors correlated with the success rate of miniscrews in orthodontic treatment. Int J Oral Sci. 2012;4:38–44.
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20. Wilmes B, Su YY, Sadigh L, et al. Pre-drilling force and insertion torques during orthodontic mini-implant insertion in relation to root contact. J Orofac Orthop. 2008;69:51–58. 21. Atsumi M, Park SH, Wang HL. Methods used to assess implant stability: Current status. Int J Oral Maxillofac Implants. 2007;22:743–754. 22. Veltri M, Balleri B, Goracci C, et al. Soft bone primary stability of 3 different miniscrews for orthodontic anchorage: A resonance frequency investigation. Am J Orthod Dentofacial Orthop. 2009;135:642– 648.
ET AL
23. Chatzigianni A, Keilig L, Reimann S, et al. Effect of mini-implant length and diameter on primary stability under loading with two force levels. Eur J Orthod. 2011;33: 381–387. 24. Bardyn T, Gédet P, Hallermann W, et al. Prediction of dental implant torque with a fast and automatic finite element analysis: A pilot study. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2010;109:594–603. 25. Chatzigianni A, Keilig L, Duschner H, et al. Comparative analysis of numerical and experimental data of orthodontic miniimplants. Eur J Orthod. 2011;33:468–475. 26. Alsaadi G, Quirynen M, Michiels K, et al. A biomechanical assessment of the
relation between the oral implant stability at insertion and subjective bone quality assessment. J Clin Periodontol. 2007;34: 359–366. 27. Motoyoshi M, Hirabayashi M, Uemura M, et al. Recommended placement torque when tightening an orthodontic miniimplant. Clin Oral Implants Res. 2006;17: 109–114. 28. Motoyoshi M. Clinical indices for orthodontic mini-implants. J Oral Sci. 2011;53:407–412. 29. McManus MM. Effect of Mini-screw Maximum Insertion Torque on Skeletal Orthodontic Anchorage: Thesis. Iowa City, IA: University of Iowa; 2010.
31
Machine-Driven Versus Manual Insertion Mode: Influence on Primary Stability of Orthodontic Mini-Implants Domen Novsak, DDS,* Magda Trinajstic Zrinski, DDS,† and Stjepan Spalj, DDS, PhD‡
n adequate planning of anchorage is essential for control of the amount, and direction, of teeth movement during orthodontic treatment. Bone anchorage devices are currently the best option for this purpose as they annul reactive forces otherwise present on anchorage teeth, thus expanding the limits of orthodontic treatment possibilities.1 The orthodontic mini-implant, most often used for bone anchorage, is a small cylindrical or conical screw usually measuring from 1.2 to 2.3 mm in diameter and from 4 to 15 mm in length of the body. Orthodontic mini-implants can be self-tapping (having a blunt tip and needing predrilling of a pilot hole), self-drilling (having a drill-like fluted tip, they drill their own path and do not necessarily need a pilot hole), or both self-tapping and self-drilling (designed to allow insertion with or without a pilot hole) and can be inserted either manually or in a machine-driven mode. A systematic review of literature reports adequate stability during treatment of over 80%.2 Some of the factors affecting the long-term stability
A
*Intern, Public Health Center Rijeka, Rijeka, Croatia. †Research Assistant, Department of Orthodontics, School of Medicine, University of Rijeka, Rijeka, Croatia. ‡Assistant Professor, Department of Orthodontics, School of Medicine, University of Rijeka, Rijeka, Croatia.
Reprint requests and correspondence to: Magda Trinajstic Zrinski, DDS, Department of Orthodontics, School of Medicine, University of Rijeka, Kresimirova 40, Rijeka 51000, Croatia, Phone: +38551345638, Fax: +38551345630, E-mail: [email protected] ISSN 1056-6163/15/02401-031 Implant Dentistry Volume 24 Number 1 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000174
Purpose: The study aimed to explore the effect of the insertion method on maximal insertion torque as a measure of primary stability while controlling for the effect of cortical bone thickness, mini-implant length and diameter, and vertical insertion force on insertion torque. Methods: Six types of miniimplants (Dual Top; Jeil Medical, Corp.) with diameters of 1.4, 1.6, and 2.0 mm and lengths of 6 and 8 mm were inserted manually and in a machine-driven mode into pig rib bone samples, and experiments were repeated 10 times, which totaled 120 tested implants in 120 pig rib samples. Cortical bone thickness was measured with a sliding caliper, whereas insertion torque and vertical insertion forces
were recorded with a specially designed device. Results: Significant predictors of better primary stability are thicker cortical bone (explaining 24.2% of variability), wider diameter (20.6%), manual insertion (9.9%), greater length (3.7%), higher maximal vertical insertion force (2.2%), and lower vertical force at maximal insertion torque (1.4%). Conclusions: Manual insertion is associated with higher primary stability of orthodontic mini-implants than mechanical insertion, but thicker cortical bone and larger implant diameter seem to be stronger predictors of primary stability. (Implant Dent 2015;24:31–36) Key Words: insertion mode, insertion torque, mini-implant, vertical force
of mini-implants are low quality and quantity of bone, especially of cortical bone,3,4 excessive force applied and too long extraosseous arm of mini-implant,5 periimplantitis,6 insufficient primary stability,7 and damage to bone during insertion.8 Primary stability is the stability of mini-implants immediately after insertion and it is affected mainly by bone characteristics, insertion technique,4,9 and implant geometrical design characteristics.3,8–10 Mini-implant insertion without predrilling of a guiding hole is expected to result in greater torque.11 In the predrilling method, a smaller hole diameter leads to higher primary stability.9 Concerning
insertion angles, although some previous research recommended angles of 60 to 70 degrees for achieving better primary stability,12 other studies suggest that placement angles lower than 90 degrees decrease anchorage resistance.13,14 The purpose of this study was to examine the effect of the insertion method (manual vs machine-driven) on maximal insertion torque as a measure of primary stability while controlling for the effect of cortical bone thickness, mini-implant length and diameter, and several components of vertical insertion force (vertical force at maximal insertion torque, maximal vertical insertion force at the
32
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NOVSAK
beginning of implantation, and median and deviation of vertical insertion force during implantation). Our research hypothesis was that maximal insertion torque would be increased by machine-driven insertion but significantly mediated by thicker cortical bone, greater mini-implant diameter and length, greater maximal vertical insertion force applied at the beginning of insertion while penetrating cortical bone, and higher median of vertical force during insertion. Uneven vertical insertion forces, that is, larger deviation of forces and greater vertical force at maximal insertion torque were expected to reduce maximal insertion torque.
MATERIALS
AND
METHODS
Six types of mini-implants (Dual Top; Jeil Medical, Corp., Seoul, Korea) made of titanium alloy grade 5 (Ti-6Al4V), with diameters of 1.4, 1.6, and 2.0 mm and lengths of 6 and 8 mm were inserted manually and in a machinedriven mode into pig rib bone samples, and the experiments were repeated 10 times, which totaled 120 tested implants in 120 pig rib samples. All were cylindrical self-drilling bracket-head implants and no predrilling of a guiding hole was performed. To ensure consistency of implantation depth, all implants were inserted up to 1 mm in distance between implant head and cortical surface. Mini-implants were inserted by a single operator perpendicular to the surface of the bone. Insertion angle of 90 degrees instead of 60 to 70 degrees was chosen because it is optimal for obtaining good anchorage resistance, as reported previously.13,14 For manual insertion, a straight hand driver was used with handle and driver shaft length of 111 and 43.5 mm, respectively. The driver was cross-headed. Mechanical insertion was performed with an electric cordless screwdriver (Jeil Medical, Corp.) at 20 rpm.
ET AL
The soft tissue attached including the periosteum was removed with a surgical raspatory. Bone samples of 1.5 cm 3 1.5 to 2 cm were made using a diamond disc with water cooling. Samples were stored in saline solution at room temperature. One mini-implant was inserted in the center of each bone sample. Cortical bone thickness was measured nearest to insertion site at both sides of the sample with an electronic digital sliding caliper (Levior s.r.o., Prerov, Czech Republic) that has an accuracy of 60.03 mm, as reported by the manufacturer. Mean values were used for analysis. Device for Simulating Behavior of Bone in Space
Before the insertion of mini-implants, bone samples were secured with 4 fixation screws into a device designed for the simulation of 3-dimensional biomechanical bone behavior constructed at the Institute of Civil Engineering in Zagreb, Croatia. The samples were placed in an upright position. The device was connected to an oscilloscope PicoScope (Pico Technology, Ltd., St Neots, United Kingdom), which uses electrical potential differences for monitoring force and torque behavior in time as a 2-dimensional graph. Measurements were recorded in PicoLog 5.14.6 software (Pico Technology, Ltd.). The
measured parameters were maximal insertion torque, vertical force at maximal insertion torque, maximal vertical insertion force, and median and deviation of vertical force. Maximal insertion torque was used as a measure of primary stability. The device was calibrated by standardized weights of 100, 200, and 500 g. The research was approved by the Ethical Committee of the School of Medicine, University of Rijeka. Statistical Analysis
For the comparison of differences between groups defined by insertion mode and mini-implant length and diameter, and controlling for the effect of cortical bone thickness, analysis of covariance and Sidak post hoc test were used. Multiple hierarchical linear regression analysis was applied to assess predictive values of all defined biomechanical factors of primary stability. The multicollinearity of predictors was assessed by variance inflation factor and tolerance. All analyses were performed in the statistical software SPSS 10.0 (SPSS, Inc., Chicago, IL).
RESULTS The average cortical bone thickness of samples was 1.42 6 0.34 mm
Bone Models
Pig ribs have previously been used in similar studies as bone models15 due to their anatomical similarity to the human mandible.16 Fresh lumbar pig ribs with diameter of 1 to 1.5 cm and cortical bone thickness of 0.5 to 3.0 mm were used.
Fig. 1. Comparison of maximal insertion torque (N$mm) between tested groups, controlling for the effect of cortical bone thickness (1.42 mm). Error bars represent mean and 95% confidence intervals (CIs). Parentheses connect groups that differ significantly at P , 0.05. There is a discernible trend of increased maximal insertion torque in manual insertion mode and with the increase of implant diameter and length.
IMPLANT DENTISTRY / VOLUME 24, NUMBER 1 2015
Fig. 2. An example of changes in vertical insertion force and torque during time in manual and machine-driven insertion modes of mini-implant 1.6 3 8 mm. The graph shows that in the manually inserted group, torque values rose in waves during insertion. Each wave reflected one rotation of hand. In the machine-driven group, torque values increased in a regular manner.
(range, 0.8–2.4 mm), maximal insertion torque was 106.71 6 46.38 N$mm (range, 39.61–347.49 N$mm), maximal vertical insertion force was 12.74 6 3.18 N (range, 4.82–17.84 N), and vertical force at maximal insertion torque was 8.62 6 3.05 N (range, 2.55–17.7 N). There were no significant differences in cortical bone thickness, maximal vertical insertion force, and vertical force at maximal insertion torque between groups; however, differences in maximal insertion torque between implant design and
insertion method groups were significant (P , 0.001). Although there was a discernible trend of mini-implants inserted in machine-driven mode having lower maximal insertion torque than the manually inserted ones, while controlling for the effect of cortical bone thickness, this was statistically significant only for mini-implants of 1.6 3 8 mm in dimension (Fig. 1; P , 0.001). In the manually inserted miniimplant group, differences were significant between dimensions of 1.4 3 6 mm
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and 2.0 3 6 mm, whereas in the machine-driven mode, differences were significant between 1.4 3 6 mm and 2.0 3 6 mm, and 1.6 3 8 mm and 2.0 3 8 mm (Fig. 1; P , 0.05). A positive association can be noticed between maximal torque on one side and length and diameter on the other (Fig. 1). In the manually inserted group, torque values rose in waves during insertion. Each wave reflected one rotation of hand during insertion (Fig. 2). In the machine-driven group, torque values increased in a regular manner during insertion. Torque values were higher for the manually inserted group than for the machine-driven insertion group. Vertical insertion force in the manually inserted group rose at the beginning and then remained the same throughout the insertion. In the machine-driven insertion group, the force rose steeply and then slowly decreased until the end of insertion (Fig. 2). In bivariate correlations, maximal torque showed a significant negative linear correlation with machine-driven insertion (r ¼ 0.363; P , 0.001), whereas a positive linear correlation with miniimplant length (r ¼ 0.226; P ¼ 0.013), diameter (r ¼ 0.429; P , 0.001), and cortical bone thickness (r ¼ 0.511; P , 0.001) was demonstrated. Correlations with the defined components of vertical insertion force were not statistically significant. Only predictors that correlated significantly with maximal torque were included in the first regression model. Thicker cortical bone, greater diameter, manual insertion, and greater length of mini-implants were associated with higher values of maximal insertion torque, accounting for 24.5%, 18.4%, 12.3%, and 4.4% of variability, respectively (Table 1). Relating to bivariate correlations, a small decrease of effect size was shown for length, mode of insertion, and cortical bone thickness, whereas it remained the same for diameter. Addition of vertical force at maximal insertion torque (model 2) and maximal vertical insertion force (model 3) did not considerably increase the predictive value of the model (Table 1). Also, addition of median and deviation of vertical insertion force did not substantially increase the predictive value but rather created
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ET AL
Table 1. Hierarchical Regression Analysis for Assessment of Biomechanical Predictors of Mini-Implant Primary Stability
Model/Predictor Model 1* Constant Insertion (1 ¼ manual, 2 ¼ machine) Cortical bone (mm) Implant diameter (mm) Implant length (mm) Model 2† Constant Insertion (1 ¼ manual, 2 ¼ machine) Cortical bone (mm) Implant diameter (mm) Implant length (mm) VFMIT (N) Model 3‡ Constant Insertion (1 ¼ manual, 2 ¼ machine) Compacta (mm) Implant diameter (mm) Implant length (mm) VFMIT (N) MVIF (N)
Correlations
Unstandardized Coefficient B
Standard Error
Standardized Coefficient b
P
Zero order
−141.030 −32.316
29.401 5.368
−0.350
,0.001 ,0.001
−0.363
−0.489
−0.350
67.223 79.523 9.721
7.888 10.757 2.685
0.496 0.429 0.210
,0.001 ,0.001 ,0.001
0.511 0.429 0.226
0.622 0.568 0.320
0.495 0.429 0.210
−139.225 −32.289
30.521 5.392
−0.350
,0.001 ,0.001
−0.363
−0.489
−0.349
67.062 79.616 9.724 −0.208
7.951 10.809 2.696 0.891
0.494 0.430 0.211 −0.014
,0.001 ,0.001 ,0.001 0.816
0.511 0.429 0.226 −0.049
0.620 0.568 0.320 −0.022
0.492 0.430 0.210 −0.014
−170.521 −29.640
32.070 5.354
−0.321
,0.001 ,0.001
−0.363
−0.462
−0.315
67.065 87.177 8.881 −2.562 3.213
7.754 10.928 2.648 1.249 1.226
0.494 0.471 0.192 −0.169 0.220
,0.001 ,0.001 0.001 0.043 0.010
0.511 0.429 0.226 −0.049 0.053
0.631 0.600 0.301 −0.189 0.239
0.492 0.454 0.191 −0.117 0.149
Partial Semipartial
In the first model, relating to bivariate correlations, a small decrease of effect size was shown for length, mode of insertion, and cortical bone thickness, whereas it remained the same for diameter. Addition of VFMIT (model 2) and MVIF (model 3) did not considerably increase the predictive value of the model. *R ¼ 0.782; R2 ¼ 0.612; adjusted R2 ¼ 0.598; F ¼ 45.321; P , 0.001. †R ¼ 0.782; R2 ¼ 0.612; adjusted R2 ¼ 0.595; F ¼ 35.970; P , 0.001. ‡R ¼ 0.796; R2 ¼ 0.634; Adjusted R2 ¼ 0.615; F ¼ 32.663; P , 0.001. MVIF indicates maximal vertical insertion force; VFMIT, vertical force at maximal insertion torque.
multicollinearity problems. Hence, they were dropped out of the regression model. The final regression model demonstrated cortical bone thickness and miniimplant diameter as the most significant predictors of primary stability, accounting for 24.2% and 20.6% of variability, respectively, while controlling for the other predictors. Insertion mode, implant length, maximal vertical insertion force, and vertical force at maximal insertion torque accounted for 9.9%, 3.7%, 2.2%, and 1.4%, respectively. The whole model explained 61.5% of variability of primary stability. Higher primary stability is associated with greater mini-implant diameter, length, manual insertion, thicker cortical bone, lesser vertical force at maximal torque, and greater maximal vertical insertion force (Table 1).
DISCUSSION Since orthodontic mini-implants allow for immediate loading, their
primary stability is very important. Out of all biomechanical factors measured in this research, thickness of cortical bone and mini-implant diameter contribute mostly to higher primary stability, which has been corroborated in previous studies.3,4,8,9,17–19 Implant length contributes less to better initial stability. However, a positive connection to fixation in the bone has been shown.1,3,19 An advantage of this study is multiple regression analysis that allows for quantification of the unique contribution of each assessed parameter to the explanation of variability of insertion torque, that is, controlling for the amount of shared variability among parameters. Although we had hypothesized that machine-driven insertion could be related to higher primary stability, our research showed that manually inserted mini-implants have better initial stability. The advantages of machinedriven mini-implant insertion are ease and comfort in implantation, but this
research shows that it does not contribute to better primary stability of the miniimplant. The tactile sensation seems to be weaker in machine-driven than in hand-driven implantation. This is important in case of contact of mini-implant with the tooth root, when the torque value necessary for insertion suddenly rises.20 Immoderate force can have negative consequencesdoverheating, overcompression, and necrosis of the bone. In view of this, we believe that devices that regulate insertion torque and rotation frequency could be helpful. In our opinion, it may be useful to choose machine-driven over manual insertion in a clinical situation with thick cortical bone to avoid damaging the bone by excessive insertion torque. Several ways of measuring primary stability are used often as follows: histological analysis, percussion test, radiological analysis, measurement of implantation and explantation torque, pull-out test, Periotest and resonance
IMPLANT DENTISTRY / VOLUME 24, NUMBER 1 2015 frequency analysis (RFA),9,18,21,22 and measuring the displacement at lateral loading.23 Apart from experimental methods, there are mathematical ones such as finite element analysis.24,25 Studies have shown that the therapist’s tactile assessment can be a valuable estimate of bone quality.26 Measurement of implantation torque and RFA are considered the most reliable methods of assessment of mini-implant primary stability according to some authors.9,20 However, implantation torque may not be a linear measurement of primary stability. Previous research indicates that there is an optimal interval of torque values in which better mini-implant stability can be achieved, such as a recommend torque from 50 to 100 N$mm for the mini-implant of dimensions 1.6 3 8 mm.27 Too high or too low torque values may result in decreased miniimplant stability.28 Some studies substantiate this only for lower torque, whereas torque values more than 200 N$mm may result in mini-implant fracture.4,29 It is probably necessary to balance the factors influencing mini-implant stability to obtain good results. Vertical force at maximal insertion torque and maximal vertical insertion force barely affect primary stability. The same applies to average values of vertical insertion force and its dispersion. Nevertheless, this study indicates the possibility that higher maximal vertical insertion force applied at the beginning of implantation when the tip of the miniimplant penetrates the compacta, provides for better stability. Presumably, higher maximal vertical insertion force in this critical moment allows for better mini-implant centering during insertion, thus reducing horizontal oscillations that result in mini-implant instability. Our research also suggests that during final insertion, when maximal insertion torque is achieved, the reduction of vertical insertion force contributes to higher primary stability as the cortical bone is probably less displaced and damaged and tighter contact of implant and bone is achieved. The remainder of variability in orthodontic mini-implant primary stability is probably accounted for by the density of compact and spongious
bone, formation of microcracks, and horizontal oscillations of the therapist’s hand during insertion. Additional research should include monitoring of the above-mentioned parameters in the evaluation of primary stability.
CONCLUSIONS Manual insertion is associated with higher primary stability of orthodontic mini-implants than mechanical insertion, but thicker cortical bone and larger implant diameter seem to be stronger predictors of primary stability than manual insertion mode.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ACKNOWLEDGMENTS The authors would like to thank Ivica Anic, Snjezana Glavicic, and Andrej Katalinic for their help in the experimental procedures. A part of the tested mini-implants and the electric cordless screwdriver was donated by the manufacturer Jeil Medical, Corp., Seoul, Korea, to whom authors hereby thank for the donation. This research has been financed by the University of Rijeka (research project 13.06.2.1.53, principal investigator Stjepan Spalj).
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