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Loss of anchorage of miniscrews: A 3-dimensional assessment
ORIGINAL ARTICLE
Loss of anchorage of miniscrews: A 3-dimensional assessment Amr Ragab El-Beialy,a Amr Mohamed Abou-El-Ezz,b Khaled Hazem Attia,c Ahmad Mohamed El-Bialy,d and Yehya Ahmed Mostafae Cairo, Egypt Introduction: Miniscrews have exponentially empowered the orthodontic anchorage armamentarium. However, the behavior of miniscrews under loading has not yet been fully investigated. Methods: Forty miniscrews were used for retraction of the maxillary and mandibular canines into the spaces of extracted first premolars. After a healing period of 2 weeks, a loading force was applied between 150 and 250 g for 6 months. The displacement of the miniscrews, their placement angles, and their implanted percentages into the bone were assessed by using superimposed 3-dimensional volumes of the maxilla and the mandible before and after 6 months of loading of the miniscrews. Results: Miniscrews were displaced in the direction of orthodontic loading. The displacement was experienced in the movement of the head and tail, and extrusion of the miniscrews. These movements were not correlated with the placement angle or the length of miniscrews in the bone. Conclusions: Movement of miniscrews is to be expected during orthodontic loading. This phenomenon should be considered when determining their dimensions and placement angles to provide the maximum range of action for orthodontic mechanics without midtreatment replacement of miniscrews. (Am J Orthod Dentofacial Orthop 2009;136:700-7)
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ince the introduction of temporary anchorage devices in orthodontics, they have been used to correct various problems: open bites,1-4 deep5 bites, Class II6,7 and Class III8 malocclusions, and canine retraction.9,10 These miniature tools have been advocated for dental intrusion,11-14 en-masse retraction of anterior teeth,15-17 distalization,18,19 and correction of skeletal deformities.20 However, the behavior of miniscrews during treatment has been rarely studied.21 Various protocols have been reported for placing miniscrews. Some authors reported making an incision and reflecting a flap before placing the miniscrew.21-23 Others made a simple incision of the mucoperiosteum
From the Cairo University, Cairo, Egypt. a Assistant lecturer, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine. b Professor, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine. c Assistant professor, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine. d Assistant professor, Systems and Biomedical Engineering Department, Faulty of Engineering. e Professor and head, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Yehya Ahmed Mostafa, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine, Cairo University, 52 Arab League St, Mohandesseen, Giza, Egypt; e-mail, yehya. [email protected]. Submitted, July 2007; revised and accepted, October 2007. 0889-5406/$36.00 Copyright Ó 2009 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2007.10.059
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without flap reflection.4,8,24 Another protocol used a transmucosal method for intrabony placement of miniscrews.2,11,25-27 Herman et al10 used the transmucosal protocol for placing mini-implants, and 19 of 39 implants failed. Thus, they adjusted the placement procedure to include a vertical incision and elevation of a soft-tissue flap before placing the miniscrews to visualize the contour of the alveolar bone. Miyawaki et al28 conducted a study comparing flap and flapless surgical procedures for placing miniscrews. Their results showed no significant association between the placement technique and the success rates of the miniscrews. However, patients who had flap surgery complained more frequently of swelling and pain after the procedure. In a systematic review of the optimum healing period for miniscrews, Ohashi et al29 concluded that a 2-week healing period was ample before orthodontic loading. Piattelli et al30 stated that overheating of bone is a risk factor for implant failure. Ohmae et al31 used mini-implants for orthodontic intrusion. They reported stability without mobility or displacement, which was tested with superimposed periapical radiographs. For many years, the lateral cephalometric radiograph, although a 2-dimensional representation of a 3-dimensional (3D) clinical situation, has been the main tool for orthodontic analysis. However, computed tomographic (CT) imaging has the versatility to compile
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Fig 1. A, Loaded miniscrews; B, miniscrew at the end of the experiment.
and manipulate data to acquire a 3D volume of the patient and has exponentially empowered the orthodontic analysis armamentarium.32-37 The purpose of this study was to investigate the stability and behavior of miniscrews under orthodontic loading with a 3D volumetric analysis. MATERIAL AND METHODS
The study design was reviewed and approved by the Institutional Review Board of Cairo University. Informed consent was obtained from the patients before clinical procedures. Forty miniscrews (Absoanchor, Dentos, Daegu, Korea) (22 maxillary, 18 mandibular) were used for canine retraction. The tapered miniscrews (diameter, 1.2 mm; length, 8 mm) were transmucosally placed through the buccal attached gingiva into the interradicular space between the second premolar and the first molar in the maxillary and mandibular arches of 12 patients. One orthodontist placed all miniscrews. The surgical site locator previously illustrated was used to identify the point of entry of the miniscrew into the buccal alveolar plate.25 After administration of local anesthesia, the miniscrews were placed followed by extraction of the first premolars. Under copious irrigation, a pilot hole was made at the placement point to traverse the dense buccal cortical plate. The miniscrews were selfdrilled through the pilot hole into the alveolar bone. The head of the miniscrew was left exposed in the oral cavity (open technique) for orthodontic attachment. Two weeks later, retraction of the canine began by using a nickel-titanium coil spring attached between the head of the miniscrew and the hook of the canine bracket, delivering a force of 150 to 250 g (Fig 1, A). The force level was monitored and reactivated when necessary every 2 weeks. The experiment ended 6 months after loading the miniscrew (Fig 1, B). Two sets of records were made. The first data set was taken after placement of the miniscrews but before loading, and the second was taken after 6 months of loading of the miniscrews. These data sets comprised high-speed dental CT scans of the skull and the mandi-
Fig 2. A, 3D volume of the maxilla; B, 3D volume of the mandible.
ble (spiral CT, Multislice 16, Somatom, Siemens, Germany; effective milliampere amount was based on a preliminary scanogram, 80 mA; spiral scanning was done at 120 Kv, high resolution mode, 0.75-mm scanning slice thickness and 0.5-mm table increment). For each patient, the following computer procedure was adopted. The patient’s CT data were saved in DICOM format (Digital Imaging and Communications in Medicine) and managed by using computer software (3D-Biodent, Systems and Biomedical Engineering Department, Faculty of Engineering, Cairo University, Cairo, Egypt) to tile up the images and rebuild the 3D volume of the patient’s head. The patient’s 3D volume
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Fig 4. A, Maxillary volume with the points recorded (arrows); B, registered mandibular volumes with the points recorded (arrows).
Fig 3. A, Maxilla before loading registered on maxilla after loading; B, mandible before loading registered on mandible after loading.
before loading the miniscrews was manipulated to separate the maxilla from the mandible (Fig 2) automatically with a software option specifically written to perform this task.36 The same procedure of separation was followed for the patient’s volume after the experiment. Maxillary volumes (before and after loading) were registered (superimposed) together (Fig 3, A). The registration is based on a technique of applying a modified iterative closest point.37 The same procedure was used for the registration of mandibular volumes (before and after loading, Fig 3, B).The body of the maxilla, and the body and ramus of the mandible, were used in the registration process, disregarding the dental misfit. The window level (voxel level of gray scale) of the registered volumes was then adjusted to subtract the bone for clear visualization of the dentition and the miniscrews. Coordinates of points on the miniscrews were recorded, representing the head, tail, and point of penetration of the miniscrew into the bone. Also, the coordinates of a point on the external surface of the bone 1 cm
apical to the point of penetration of the miniscrew into the alveolar bone was recorded. These points were recorded before and 6 months after loading of miniscrews for maxillary (Fig 4, A) and mandibular (Fig 4, B) volumes. By subtracting the coordinates of the same defined points on the before and after volumes, the measurements of the miniscrews’ displacement were made directly without the need to use reference points or planes. The recorded coordinates of these points were used to measure the following: movement of the miniscrew head, movement of the miniscrew tail, extrusion of the miniscrew, original placement angle of the miniscrew into the external cortical plate of bone, and implanted percentage of miniscrew length into the bone, calculated as a percentage of the threaded portion of the miniscrew. Miniscrews were recorded as successful if they remained stable during loading until the end of the clinical experiment. The measurements were repeated by the same observer 2 weeks later. The data were statistically analyzed. The means and standard deviations were tabulated. The Pearson correlation coefficient to correlate the variables under investigation was used. The paired-samples t test, concordance correlation coefficient, and the Pearson correlation coefficient were used to determine the level of significance and the correlation of intraobserver measurements. RESULTS
The overall success rate in this study was 82.5%, with 7 failed miniscrews (4 maxillary, 3 mandibular) of the 40 used (Table I). All failed miniscrews
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Outcome and location of miniscrews in sample (n 5 40)
Table I.
Miniscrews
Outcome Success Failure
Location
N
%
Maxilla
Mandible
33 7
82.5 17.5
18 4
15 3
contacted or even grooved the roots of the adjacent teeth (Fig 5). The failed miniscrews had severe mobility or became dislodged after the 2-week healing period after placement and before loading. These failed miniscrews were not replaced during the experiment and were excluded from the study. The remaining 33 successful miniscrews were stable throughout force application, with a success rate of 100% for the loaded miniscrews. Statistical analysis of the miniscrews showed nonstatistically significant differences between the 33 successful and the 7 failed miniscrews, with respect to the placement angle and the implanted percentage of miniscrew length (Table II). On average, the heads of the miniscrews were displaced in the direction of force application; their tails moved in the opposite direction (Table III). For the successful miniscrews, the Pearson correlation coefficient test was used to correlate the variables (Table IV). A significant correlation among movement of the head, movement of the tail, and extrusion of the miniscrews was seen. A nonsignificant correlation was noticed for the implanted percentage of miniscrew length and the placement angle with the movements of the head and tail, and extrusion of the miniscrews. Five of the successful miniscrews had contacted the roots of the adjacent teeth during placement. Although they had a statistically significant higher amount of displacement during force application compared with the successful miniscrews that did not contact the adjacent roots (Table V), they remained clinically stable during force application. Error analysis with paired-samples t tests showed a nonstatistically significant difference between the intraobserver measurements (Table VI). These measurements showed reasonable results for conformity and equivalency with the concordance correlation coefficient and the Pearson correlation coefficient tests (Table VII), respectively. DISCUSSION
Since the introduction of miniscrews, many studies in the orthodontic literature have described their clinical
Fig 5. A failed miniscrew contacting the root of the adjacent tooth.
applicability for absolute anchorage. Relatively few articles were critical of the effect of loading and the factors affecting their success rates.23,28,38 Our study focused on evaluating the behavior of miniscrews on loading from a 3D perspective. The overall success rate of the miniscrews in this study was 87.5% (Table I). This rate was relatively lower than reported rates.9,28 This might be because all failed miniscrews in this study had inadvertently contacted the roots of adjacent teeth; this is not an uncommon clinical situation.38,39 The unwanted miniscrew-root contact might have resulted from placing the miniscrews at the same visit as extracting the first premolars. Extraction of teeth necessitated the administration of local infiltration anesthesia, which obviated the sensation and pain perception of the teeth and the associated periodontium. This level of anesthesia obscures the discomfort and pain that usually accompany the miniscrew-root contact with placement under topical anesthesia. Hence, a possible explanation is that encroachment of a miniscrew on the periodontal ligament triggers a severe inflammatory reaction that extended along the alveolar bone and resulted in peri-implantitis. This explanation supports the results of Kuroda et al,40 who concluded that the proximity of miniscrews to the adjacent tooth root is a major risk factor for failure of miniscrew anchorage. Under topical anesthesia, the pain and discomfort from the miniscrew-root contact allow the orthodontist the flexibility necessary to adjust the direction and angulation of the miniscrew. Hence, it can be suggested that the application of only topical anesthesia during placement should not be optional. However, the success rate achieved in this study of the actively loaded miniscrews was 100%. Miniscrews
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Table II.
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Descriptive statistics of successful and failed miniscrews
Variable Placement angle Implanted percentage of miniscrew length
Group
Minimum
Maximum
Mean
SD
t
P
Successful Failed Successful Failed
14 18.4 51.1% 55%
49.9 52 100% 100%
34.336 35.629 79.1% 71.2%
9.043 13.951 13.2 14.7
0.311
0.757*
–1.410
0.167*
*Nonsignificant at P .0.05.
Average displacement of successful miniscrews (n 5 33)
Table III.
Variable Movement of head (mm) Movement of tail (mm) Extrusion (mm)
Minimum
Maximum
Mean
SD
0.174 0.341 0.014
4.121 1.796 2.557
1.080 0.828 0.548
0.787 0.401 0.586
that were stable after the healing period remained anchored to the bone throughout the 6 months of loading. The same success rate was reported by Thiruvenkatachari et al.9 The amalgamation of the CT and the 3D volumetric analyses into orthodontics disclosed new avenues for orthodontic treatment planning and evaluation via a 3D tool simulating the true clinical situation.32-35 This tool provided accurate and reliable answers to the question under study. Various values for the vertical angles of miniscrew placement have been reported. It seems that these angles were assessed visually because the measuring tools were not mentioned.6,10,15,18,28 In this study, the vertical angle of placement of the miniscrews with the buccal alveolar plate was measured on the 3D volume, ranging from 14 to 49.9 , with a mean of 34.336 6 9.043 for the successful miniscrews (Table II). The precision of the measurements of the 3D-Biodent software proved to be highly accurate.35 However, the results indicate that the vertical placement angle and the implanted percentage of miniscrew length did not determine the success of the miniscrews (Table II). The horizontal angle between the miniscrew and the alveolar buccal plate that indicates the distal or mesial position of the head of the miniscrew was not measured. The placement of the head of the miniscrew in a favorable supportive position is dictated by the topography of the area. The consensus of opinion about the placement spot is midway between the roots of the adjacent teeth. Hence, the vertical placement angle is controllable by the clinician, whereas the horizontal angulation is clinically dictated. The 3D registration (superimposition) of the before and after loading maxillary and mandibular volumes showed that the miniscrews were displaced under ortho-
dontic loading, although they were clinically stable (Table III). This finding is consistent with that of Liou et al.21 However, their assessment method was questionable because they used lateral cephalograms to assess the movement of potentially overlapping bilateral screws. Moreover, the evaluation of such fine movements was accomplished on lateral cephalometric radiographs regardless of their inherent limitations.41-46 The amount of displacement of miniscrews in this study was not correlated to the placement angle or the percentage of implanted miniscrew length (Table IV). Liou et al21 reported extrusion of miniscrews under loading. This finding was apparent clinically in our study, as evidenced by an increase in the amount of the miniscrew head projecting outside the alveolar bone. This finding might necessitate tightening of the miniscrews. In contrast, 5 of the 33 successful screws contacted the periodontal ligaments of adjacent teeth but without detrimental peri-implantitis. The mean displacement of these 5 miniscrews was higher than the rest of the successful miniscrews. A statistically significant difference was recorded regarding head movement and extrusion of the miniscrews (Table V). Two of the 5 miniscrews were placed in the mandible and had the greatest movements of the screw heads on loading (4.121 and 3.9 mm, Table V). These movements might have stemmed from a mild form of peri-implantitis; nevertheless, the anchorage potential of the miniscrews was not threatened. This finding was also reported by Kuroda et al.40 Canine retraction in the quadrants with failed miniscrews was accomplished by replacement and subsequent loading. These quadrants were not included in our statistical analyses. The 3D volumetric analysis might still be considered a new tool in orthodontics. Hence, several intraobserver tests were used to verify the equivalence, association, and agreement of measurements at different times. Assessment of intraobserver error was carried out by using the paired-samples test, the concordance correlation coefficient test for equivalence of the intraobserver measurements, and the Pearson correlation coefficient to test the association between the observations
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Table IV.
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Pearson correlation coefficient of the variables
Variable Movement of head Movement of tail Extrusion Implanted percentage of miniscrew length Placement angle
Movement of head
Movement of tail
Extrusion
1.00
0.462† 1.00
0.832† 0.396* 1.00
Implanted percentage of miniscrew length
Placement angle
–0.324 –0.165 –0.300 1.00
–0.080 0.261 –0.081 –0.166 1.00
†
*Significant at P \0.05; Significant at P \0.01.
Table V.
Average displacements of successful miniscrews with and without root contact
Variable Movement of head (mm) Movement of tail (mm) Extrusion (mm)
Successful miniscrews
Sample (screw)
Minimum
Maximum
Mean
SD
t test
Root contact No root contact Root contact No root contact Root contact No root contact
5 28 5 28 5 28
0.5418 0.1739 0.3405 0.3606 0.1067 0.0145
4.1205 3.019 1.078 1.7958 2.5567 1.5452
1.7589 0.9588 0.6357 0.8623 1.13534 0.44303
1.403 0.5848 0.331 0.408 0.972 0.438
* NS *
NS, Not significant. *Significant at P \0.05
Table VI. Paired samples test for intraobserver error assessment Variable Movement of head Movement of tail Extrusion Implanted percentage of miniscrew length
t
P
–0.77 –0.29 0.26 –0.18
0.4447 0.7761 0.7941 0.8546
Not significant at P .0.05.
(Tables VI and VII). Evaluation of the intraobserver measurements varied insignificantly in the interpretation from 1 occasion to another; this supports the reliability of the 3D volume as a tool for measurements. Our results indicate that miniscrews, as stable anchorage units that have greatly enriched the orthodontic anchorage armamentarium, experience some displacement under loading. However, we support the hypothesis that the displacement figures for the miniscrews in this study have limited clinical significance because they apply only to the miniscrews used in this study, with the given dimensions, in the given quality and quantity of bone, with the reported percentage of implanted length of the miniscrews, for the healing time provided, the initial loading forces reported, and the duration of force application mentioned. These fig-
ures might also be influenced by more variables, such as the rate of increase of loading on the miniscrews. In short, these factors are variables that are subject to clinical alterations in different situations. Movement of miniscrews is highly possible; therefore, careful attention to the technique of placement and continued monitoring of the temporary anchorage device is recommended. This phenomenon of displacement of miniscrews should be considered when determining the dimensions of the miniscrew and the placement angle. The objective is to provide the maximum range of action for the attachment of orthodontic mechanics without midtreatment replacement of the miniscrews, especially when clinical situations necessitate long loading times. Of prime importance is the patient’s communication because a predetermined outcome is the power of knowledge, whereas a postoutcome defense is an excuse for amateurism. CONCLUSIONS
1. 2.
3.
Miniscrews are successful anchorage units for canine retraction. Even though miniscrews are considered clinically stable anchorage units, they experience some displacement under loading, seen with 3D evaluation. Miniscrews should be placed with no more than topical anesthesia.
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Table VII.
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Concordance correlation coefficient and Pearson correlation coefficient of intraobserver readings Concordance correlation coefficient Confidence limits
Variable
CCC
Lower
Upper
Pearson correlation coefficient
Movement of head Movement of tail Extrusion Implanted percentage of miniscrew length
0.908 0.657 0.863 0.830
0.840 0.458 0.767 0.717
0.948 0.793 0.921 0.900
0.910 0.660 0.867 0.839
CCC, Concordance correlation coefficient. High association and equivalence with values approaching 1.
4.
The patient should be informed before placement of miniscrews that their position and angulation might need to be redirected during treatment in case of their displacement.
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29. Ohashi E, Pecho O, Moron M, Lagravere M. Implants vs screw loading protocols in orthodontics. A systematic review. Angle Orthod 2006;76:721-7. 30. Piattelli A, Piattelli M, Mangano C, Scarano A. A histologic evaluation of eight cases of failed dental implants. Is bone overheating the most probable cause? Biomaterials 1998;19:683-90. 31. Ohmae M, Saito S, Morohashi T, Seki K, Qu H, Kanomi R, et al. A clinical and histological evaluation of titanium mini-implants as anchors for orthodontic intrusion in the beagle dog. Am J Orthod Dentofacial Orthop 2001;119:489-97. 32. Lagravere MO, Hansen L, Harzer W, Major PW. Plane orientation for standardization in 3-dimensional cephalometric analysis with computerized tomography imaging. Am J Orthod Dentofacial Orthop 2006;129:601-4. 33. Cevidanes LHS, Styner MA, Proffit WR. Image analysis and superimposition of 3-dimensional cone-beam computed tomography models. Am J Orthod Dentofacial Orthop 2006;129:611-8. 34. Chen Y, Duan P, Meng Y, Chen Y. Three-dimensional spiral computed tomographic imaging: a new approach to the diagnosis and treatment planning of impacted teeth. Am J Orthod Dentofacial Orthop 2006;130:112-6. 35. Al-Zanati H. Accuracy of dental arch dimension using conventional and three dimensional orthodontic models [thesis]. Cairo, Egypt: Cairo University; 2007. 36. Elzalabani S. 3D segmentation and reconstruction system for the mandible and teeth [thesis]. Cairo, Egypt: Cairo University; 2007. 37. El Bakry OM. Surface registration for the analysis of 3D orthodontic treatment [thesis]. Cairo, Egypt: Cairo University; 2007.
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38. Kuroda S, Sugawara Y, Deguchi T, Kyung HM, Yamamoto TT. Clinical use of miniscrew implants as orthodontic anchorage: success rates and postoperative discomfort. Am J Orthod Dentofacial Orthop 2007;131:9-15. 39. Kravitz ND, Kusnoto B. Risks and complications of orthodontic miniscrews. Am J Orthod Dentofacial Orthop 2007;131(Suppl): S43-51. 40. Kuroda S, Yamada K, Deguchi T, Hashimoto T, Kyung HM, Yamamoto TT. Root proximity is a major factor for screw failure in orthodontic anchorage. Am J Orthod Dentofacial Orthop 2007; 131(Suppl):S68-73. 41. Ahlqvist J, Eliasson J, Welander U. The effect of projection error on cephalometric length measurements. Eur J Orthod 1986;8:141-8. 42. Ahlqvist J, Eliasson J, Welander U. The effect of projection error on angular measurements in cephalometry. Eur J Orthod 1988;10: 353-61. 43. Chen Y, Chen S, Chang H, Chen K. Comparison of landmark identification in traditional versus computer-aided digital cephalometry. Angle Orthod 2000;70:387-92. 44. Forsyth DB, Shaw WC, Richmond S. Digital imaging of cephalometric radiography, part 1: advantages and limitations of digital imaging. Angle Orthod 1996;66:37-42. 45. Forsyth DB, Shaw WC, Richmond S, Roberts CJ. Digital imaging of cephalometric radiography, part 2: image quality. Angle Orthod 1996;66:43-50. 46. Rudolph D, Sinclair P, Coggins J. Automatic computerized radiographic identification of cephalometric landmarks. Am J Orthod Dentofacial Orthop 1998;113:173-9.
Loss of anchorage of miniscrews: A 3-dimensional assessment Amr Ragab El-Beialy,a Amr Mohamed Abou-El-Ezz,b Khaled Hazem Attia,c Ahmad Mohamed El-Bialy,d and Yehya Ahmed Mostafae Cairo, Egypt Introduction: Miniscrews have exponentially empowered the orthodontic anchorage armamentarium. However, the behavior of miniscrews under loading has not yet been fully investigated. Methods: Forty miniscrews were used for retraction of the maxillary and mandibular canines into the spaces of extracted first premolars. After a healing period of 2 weeks, a loading force was applied between 150 and 250 g for 6 months. The displacement of the miniscrews, their placement angles, and their implanted percentages into the bone were assessed by using superimposed 3-dimensional volumes of the maxilla and the mandible before and after 6 months of loading of the miniscrews. Results: Miniscrews were displaced in the direction of orthodontic loading. The displacement was experienced in the movement of the head and tail, and extrusion of the miniscrews. These movements were not correlated with the placement angle or the length of miniscrews in the bone. Conclusions: Movement of miniscrews is to be expected during orthodontic loading. This phenomenon should be considered when determining their dimensions and placement angles to provide the maximum range of action for orthodontic mechanics without midtreatment replacement of miniscrews. (Am J Orthod Dentofacial Orthop 2009;136:700-7)
S
ince the introduction of temporary anchorage devices in orthodontics, they have been used to correct various problems: open bites,1-4 deep5 bites, Class II6,7 and Class III8 malocclusions, and canine retraction.9,10 These miniature tools have been advocated for dental intrusion,11-14 en-masse retraction of anterior teeth,15-17 distalization,18,19 and correction of skeletal deformities.20 However, the behavior of miniscrews during treatment has been rarely studied.21 Various protocols have been reported for placing miniscrews. Some authors reported making an incision and reflecting a flap before placing the miniscrew.21-23 Others made a simple incision of the mucoperiosteum
From the Cairo University, Cairo, Egypt. a Assistant lecturer, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine. b Professor, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine. c Assistant professor, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine. d Assistant professor, Systems and Biomedical Engineering Department, Faulty of Engineering. e Professor and head, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Yehya Ahmed Mostafa, Department of Orthodontics and Dentofacial Orthopedics, Faculty of Oral and Dental Medicine, Cairo University, 52 Arab League St, Mohandesseen, Giza, Egypt; e-mail, yehya. [email protected]. Submitted, July 2007; revised and accepted, October 2007. 0889-5406/$36.00 Copyright Ó 2009 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2007.10.059
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without flap reflection.4,8,24 Another protocol used a transmucosal method for intrabony placement of miniscrews.2,11,25-27 Herman et al10 used the transmucosal protocol for placing mini-implants, and 19 of 39 implants failed. Thus, they adjusted the placement procedure to include a vertical incision and elevation of a soft-tissue flap before placing the miniscrews to visualize the contour of the alveolar bone. Miyawaki et al28 conducted a study comparing flap and flapless surgical procedures for placing miniscrews. Their results showed no significant association between the placement technique and the success rates of the miniscrews. However, patients who had flap surgery complained more frequently of swelling and pain after the procedure. In a systematic review of the optimum healing period for miniscrews, Ohashi et al29 concluded that a 2-week healing period was ample before orthodontic loading. Piattelli et al30 stated that overheating of bone is a risk factor for implant failure. Ohmae et al31 used mini-implants for orthodontic intrusion. They reported stability without mobility or displacement, which was tested with superimposed periapical radiographs. For many years, the lateral cephalometric radiograph, although a 2-dimensional representation of a 3-dimensional (3D) clinical situation, has been the main tool for orthodontic analysis. However, computed tomographic (CT) imaging has the versatility to compile
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Fig 1. A, Loaded miniscrews; B, miniscrew at the end of the experiment.
and manipulate data to acquire a 3D volume of the patient and has exponentially empowered the orthodontic analysis armamentarium.32-37 The purpose of this study was to investigate the stability and behavior of miniscrews under orthodontic loading with a 3D volumetric analysis. MATERIAL AND METHODS
The study design was reviewed and approved by the Institutional Review Board of Cairo University. Informed consent was obtained from the patients before clinical procedures. Forty miniscrews (Absoanchor, Dentos, Daegu, Korea) (22 maxillary, 18 mandibular) were used for canine retraction. The tapered miniscrews (diameter, 1.2 mm; length, 8 mm) were transmucosally placed through the buccal attached gingiva into the interradicular space between the second premolar and the first molar in the maxillary and mandibular arches of 12 patients. One orthodontist placed all miniscrews. The surgical site locator previously illustrated was used to identify the point of entry of the miniscrew into the buccal alveolar plate.25 After administration of local anesthesia, the miniscrews were placed followed by extraction of the first premolars. Under copious irrigation, a pilot hole was made at the placement point to traverse the dense buccal cortical plate. The miniscrews were selfdrilled through the pilot hole into the alveolar bone. The head of the miniscrew was left exposed in the oral cavity (open technique) for orthodontic attachment. Two weeks later, retraction of the canine began by using a nickel-titanium coil spring attached between the head of the miniscrew and the hook of the canine bracket, delivering a force of 150 to 250 g (Fig 1, A). The force level was monitored and reactivated when necessary every 2 weeks. The experiment ended 6 months after loading the miniscrew (Fig 1, B). Two sets of records were made. The first data set was taken after placement of the miniscrews but before loading, and the second was taken after 6 months of loading of the miniscrews. These data sets comprised high-speed dental CT scans of the skull and the mandi-
Fig 2. A, 3D volume of the maxilla; B, 3D volume of the mandible.
ble (spiral CT, Multislice 16, Somatom, Siemens, Germany; effective milliampere amount was based on a preliminary scanogram, 80 mA; spiral scanning was done at 120 Kv, high resolution mode, 0.75-mm scanning slice thickness and 0.5-mm table increment). For each patient, the following computer procedure was adopted. The patient’s CT data were saved in DICOM format (Digital Imaging and Communications in Medicine) and managed by using computer software (3D-Biodent, Systems and Biomedical Engineering Department, Faculty of Engineering, Cairo University, Cairo, Egypt) to tile up the images and rebuild the 3D volume of the patient’s head. The patient’s 3D volume
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Fig 4. A, Maxillary volume with the points recorded (arrows); B, registered mandibular volumes with the points recorded (arrows).
Fig 3. A, Maxilla before loading registered on maxilla after loading; B, mandible before loading registered on mandible after loading.
before loading the miniscrews was manipulated to separate the maxilla from the mandible (Fig 2) automatically with a software option specifically written to perform this task.36 The same procedure of separation was followed for the patient’s volume after the experiment. Maxillary volumes (before and after loading) were registered (superimposed) together (Fig 3, A). The registration is based on a technique of applying a modified iterative closest point.37 The same procedure was used for the registration of mandibular volumes (before and after loading, Fig 3, B).The body of the maxilla, and the body and ramus of the mandible, were used in the registration process, disregarding the dental misfit. The window level (voxel level of gray scale) of the registered volumes was then adjusted to subtract the bone for clear visualization of the dentition and the miniscrews. Coordinates of points on the miniscrews were recorded, representing the head, tail, and point of penetration of the miniscrew into the bone. Also, the coordinates of a point on the external surface of the bone 1 cm
apical to the point of penetration of the miniscrew into the alveolar bone was recorded. These points were recorded before and 6 months after loading of miniscrews for maxillary (Fig 4, A) and mandibular (Fig 4, B) volumes. By subtracting the coordinates of the same defined points on the before and after volumes, the measurements of the miniscrews’ displacement were made directly without the need to use reference points or planes. The recorded coordinates of these points were used to measure the following: movement of the miniscrew head, movement of the miniscrew tail, extrusion of the miniscrew, original placement angle of the miniscrew into the external cortical plate of bone, and implanted percentage of miniscrew length into the bone, calculated as a percentage of the threaded portion of the miniscrew. Miniscrews were recorded as successful if they remained stable during loading until the end of the clinical experiment. The measurements were repeated by the same observer 2 weeks later. The data were statistically analyzed. The means and standard deviations were tabulated. The Pearson correlation coefficient to correlate the variables under investigation was used. The paired-samples t test, concordance correlation coefficient, and the Pearson correlation coefficient were used to determine the level of significance and the correlation of intraobserver measurements. RESULTS
The overall success rate in this study was 82.5%, with 7 failed miniscrews (4 maxillary, 3 mandibular) of the 40 used (Table I). All failed miniscrews
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Outcome and location of miniscrews in sample (n 5 40)
Table I.
Miniscrews
Outcome Success Failure
Location
N
%
Maxilla
Mandible
33 7
82.5 17.5
18 4
15 3
contacted or even grooved the roots of the adjacent teeth (Fig 5). The failed miniscrews had severe mobility or became dislodged after the 2-week healing period after placement and before loading. These failed miniscrews were not replaced during the experiment and were excluded from the study. The remaining 33 successful miniscrews were stable throughout force application, with a success rate of 100% for the loaded miniscrews. Statistical analysis of the miniscrews showed nonstatistically significant differences between the 33 successful and the 7 failed miniscrews, with respect to the placement angle and the implanted percentage of miniscrew length (Table II). On average, the heads of the miniscrews were displaced in the direction of force application; their tails moved in the opposite direction (Table III). For the successful miniscrews, the Pearson correlation coefficient test was used to correlate the variables (Table IV). A significant correlation among movement of the head, movement of the tail, and extrusion of the miniscrews was seen. A nonsignificant correlation was noticed for the implanted percentage of miniscrew length and the placement angle with the movements of the head and tail, and extrusion of the miniscrews. Five of the successful miniscrews had contacted the roots of the adjacent teeth during placement. Although they had a statistically significant higher amount of displacement during force application compared with the successful miniscrews that did not contact the adjacent roots (Table V), they remained clinically stable during force application. Error analysis with paired-samples t tests showed a nonstatistically significant difference between the intraobserver measurements (Table VI). These measurements showed reasonable results for conformity and equivalency with the concordance correlation coefficient and the Pearson correlation coefficient tests (Table VII), respectively. DISCUSSION
Since the introduction of miniscrews, many studies in the orthodontic literature have described their clinical
Fig 5. A failed miniscrew contacting the root of the adjacent tooth.
applicability for absolute anchorage. Relatively few articles were critical of the effect of loading and the factors affecting their success rates.23,28,38 Our study focused on evaluating the behavior of miniscrews on loading from a 3D perspective. The overall success rate of the miniscrews in this study was 87.5% (Table I). This rate was relatively lower than reported rates.9,28 This might be because all failed miniscrews in this study had inadvertently contacted the roots of adjacent teeth; this is not an uncommon clinical situation.38,39 The unwanted miniscrew-root contact might have resulted from placing the miniscrews at the same visit as extracting the first premolars. Extraction of teeth necessitated the administration of local infiltration anesthesia, which obviated the sensation and pain perception of the teeth and the associated periodontium. This level of anesthesia obscures the discomfort and pain that usually accompany the miniscrew-root contact with placement under topical anesthesia. Hence, a possible explanation is that encroachment of a miniscrew on the periodontal ligament triggers a severe inflammatory reaction that extended along the alveolar bone and resulted in peri-implantitis. This explanation supports the results of Kuroda et al,40 who concluded that the proximity of miniscrews to the adjacent tooth root is a major risk factor for failure of miniscrew anchorage. Under topical anesthesia, the pain and discomfort from the miniscrew-root contact allow the orthodontist the flexibility necessary to adjust the direction and angulation of the miniscrew. Hence, it can be suggested that the application of only topical anesthesia during placement should not be optional. However, the success rate achieved in this study of the actively loaded miniscrews was 100%. Miniscrews
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Table II.
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Descriptive statistics of successful and failed miniscrews
Variable Placement angle Implanted percentage of miniscrew length
Group
Minimum
Maximum
Mean
SD
t
P
Successful Failed Successful Failed
14 18.4 51.1% 55%
49.9 52 100% 100%
34.336 35.629 79.1% 71.2%
9.043 13.951 13.2 14.7
0.311
0.757*
–1.410
0.167*
*Nonsignificant at P .0.05.
Average displacement of successful miniscrews (n 5 33)
Table III.
Variable Movement of head (mm) Movement of tail (mm) Extrusion (mm)
Minimum
Maximum
Mean
SD
0.174 0.341 0.014
4.121 1.796 2.557
1.080 0.828 0.548
0.787 0.401 0.586
that were stable after the healing period remained anchored to the bone throughout the 6 months of loading. The same success rate was reported by Thiruvenkatachari et al.9 The amalgamation of the CT and the 3D volumetric analyses into orthodontics disclosed new avenues for orthodontic treatment planning and evaluation via a 3D tool simulating the true clinical situation.32-35 This tool provided accurate and reliable answers to the question under study. Various values for the vertical angles of miniscrew placement have been reported. It seems that these angles were assessed visually because the measuring tools were not mentioned.6,10,15,18,28 In this study, the vertical angle of placement of the miniscrews with the buccal alveolar plate was measured on the 3D volume, ranging from 14 to 49.9 , with a mean of 34.336 6 9.043 for the successful miniscrews (Table II). The precision of the measurements of the 3D-Biodent software proved to be highly accurate.35 However, the results indicate that the vertical placement angle and the implanted percentage of miniscrew length did not determine the success of the miniscrews (Table II). The horizontal angle between the miniscrew and the alveolar buccal plate that indicates the distal or mesial position of the head of the miniscrew was not measured. The placement of the head of the miniscrew in a favorable supportive position is dictated by the topography of the area. The consensus of opinion about the placement spot is midway between the roots of the adjacent teeth. Hence, the vertical placement angle is controllable by the clinician, whereas the horizontal angulation is clinically dictated. The 3D registration (superimposition) of the before and after loading maxillary and mandibular volumes showed that the miniscrews were displaced under ortho-
dontic loading, although they were clinically stable (Table III). This finding is consistent with that of Liou et al.21 However, their assessment method was questionable because they used lateral cephalograms to assess the movement of potentially overlapping bilateral screws. Moreover, the evaluation of such fine movements was accomplished on lateral cephalometric radiographs regardless of their inherent limitations.41-46 The amount of displacement of miniscrews in this study was not correlated to the placement angle or the percentage of implanted miniscrew length (Table IV). Liou et al21 reported extrusion of miniscrews under loading. This finding was apparent clinically in our study, as evidenced by an increase in the amount of the miniscrew head projecting outside the alveolar bone. This finding might necessitate tightening of the miniscrews. In contrast, 5 of the 33 successful screws contacted the periodontal ligaments of adjacent teeth but without detrimental peri-implantitis. The mean displacement of these 5 miniscrews was higher than the rest of the successful miniscrews. A statistically significant difference was recorded regarding head movement and extrusion of the miniscrews (Table V). Two of the 5 miniscrews were placed in the mandible and had the greatest movements of the screw heads on loading (4.121 and 3.9 mm, Table V). These movements might have stemmed from a mild form of peri-implantitis; nevertheless, the anchorage potential of the miniscrews was not threatened. This finding was also reported by Kuroda et al.40 Canine retraction in the quadrants with failed miniscrews was accomplished by replacement and subsequent loading. These quadrants were not included in our statistical analyses. The 3D volumetric analysis might still be considered a new tool in orthodontics. Hence, several intraobserver tests were used to verify the equivalence, association, and agreement of measurements at different times. Assessment of intraobserver error was carried out by using the paired-samples test, the concordance correlation coefficient test for equivalence of the intraobserver measurements, and the Pearson correlation coefficient to test the association between the observations
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Table IV.
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Pearson correlation coefficient of the variables
Variable Movement of head Movement of tail Extrusion Implanted percentage of miniscrew length Placement angle
Movement of head
Movement of tail
Extrusion
1.00
0.462† 1.00
0.832† 0.396* 1.00
Implanted percentage of miniscrew length
Placement angle
–0.324 –0.165 –0.300 1.00
–0.080 0.261 –0.081 –0.166 1.00
†
*Significant at P \0.05; Significant at P \0.01.
Table V.
Average displacements of successful miniscrews with and without root contact
Variable Movement of head (mm) Movement of tail (mm) Extrusion (mm)
Successful miniscrews
Sample (screw)
Minimum
Maximum
Mean
SD
t test
Root contact No root contact Root contact No root contact Root contact No root contact
5 28 5 28 5 28
0.5418 0.1739 0.3405 0.3606 0.1067 0.0145
4.1205 3.019 1.078 1.7958 2.5567 1.5452
1.7589 0.9588 0.6357 0.8623 1.13534 0.44303
1.403 0.5848 0.331 0.408 0.972 0.438
* NS *
NS, Not significant. *Significant at P \0.05
Table VI. Paired samples test for intraobserver error assessment Variable Movement of head Movement of tail Extrusion Implanted percentage of miniscrew length
t
P
–0.77 –0.29 0.26 –0.18
0.4447 0.7761 0.7941 0.8546
Not significant at P .0.05.
(Tables VI and VII). Evaluation of the intraobserver measurements varied insignificantly in the interpretation from 1 occasion to another; this supports the reliability of the 3D volume as a tool for measurements. Our results indicate that miniscrews, as stable anchorage units that have greatly enriched the orthodontic anchorage armamentarium, experience some displacement under loading. However, we support the hypothesis that the displacement figures for the miniscrews in this study have limited clinical significance because they apply only to the miniscrews used in this study, with the given dimensions, in the given quality and quantity of bone, with the reported percentage of implanted length of the miniscrews, for the healing time provided, the initial loading forces reported, and the duration of force application mentioned. These fig-
ures might also be influenced by more variables, such as the rate of increase of loading on the miniscrews. In short, these factors are variables that are subject to clinical alterations in different situations. Movement of miniscrews is highly possible; therefore, careful attention to the technique of placement and continued monitoring of the temporary anchorage device is recommended. This phenomenon of displacement of miniscrews should be considered when determining the dimensions of the miniscrew and the placement angle. The objective is to provide the maximum range of action for the attachment of orthodontic mechanics without midtreatment replacement of the miniscrews, especially when clinical situations necessitate long loading times. Of prime importance is the patient’s communication because a predetermined outcome is the power of knowledge, whereas a postoutcome defense is an excuse for amateurism. CONCLUSIONS
1. 2.
3.
Miniscrews are successful anchorage units for canine retraction. Even though miniscrews are considered clinically stable anchorage units, they experience some displacement under loading, seen with 3D evaluation. Miniscrews should be placed with no more than topical anesthesia.
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Table VII.
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Concordance correlation coefficient and Pearson correlation coefficient of intraobserver readings Concordance correlation coefficient Confidence limits
Variable
CCC
Lower
Upper
Pearson correlation coefficient
Movement of head Movement of tail Extrusion Implanted percentage of miniscrew length
0.908 0.657 0.863 0.830
0.840 0.458 0.767 0.717
0.948 0.793 0.921 0.900
0.910 0.660 0.867 0.839
CCC, Concordance correlation coefficient. High association and equivalence with values approaching 1.
4.
The patient should be informed before placement of miniscrews that their position and angulation might need to be redirected during treatment in case of their displacement.
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