Easy placement is an advantage of orthodontic

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1 ORIGINAL ARTICLE Miniscrew stability evaluated with computerized tomography scanning Jung-Yul Cha, a Jae-Kyoung Kil, b Tae-Min Yoon, c and Chung-Ju Hwang d Seoul, Korea Introduction: In this study, we aimed to determine the effect of bone mineral density (BMD), cortical bone thickness (CBT), screw position, and screw design on the stability of miniscrews. Methods: Ninety-six miniscrews of both cylindrical and tapered types were placed in 6 beagle dogs. The BMD and CBT were measured by computerized tomography and correlated with the placement and removal torque and mobility. A regression equation to predict the placement torque was calculated based on BMD, CBT, screw type, and screw position. Results: The placement torque showed a positive correlation in the order of removal torque (0.66), BMD of the cortical bone (0.58), and CBT (0.48). Placement and removal torque values were significantly higher in the mandible compared with the maxilla. Tapered miniscrews had higher placement torque than did the cylindrical type (P \0.001). However, the removal torque was similar in both groups. Placement torque was affected by screw position, screw type, and BMD of cortical bone, in that order. Conclusions: BMD of cortical bone, screw type, and screw position significantly influence the primary stability of miniscrews. (Am J Orthod Dentofacial Orthop 2010;137:73-9) Easy placement is an advantage of orthodontic miniscrews. In addition, the tooth can be moved without the patient s cooperation; this enhances treatment efficiency. Some success has been reported regarding the use of miniscrews as orthodontic anchorages. However, the clinical application of a miniscrew does not guarantee treatment success, and its stability is essential before it can be used as orthodontic anchorage. 1-3 It is important to ensure the initial stability of an orthodontic miniscrew because most failures occur during the initial stage. Furthermore, stability of the miniscrew in the initial stage reduces its micromovement, thus allowing for an appropriate environment that supports the healing process surrounding the bone. The success of an implant or miniscrew is determined by the patient s general condition, the biocompatibility of the materials, the placement procedure, and bone a Assistant professor, Department of Orthodontics, College of Dentistry, Yonsei University, Seoul, Korea. b Private practice, Seoul, Korea. c Postgraduate student, Department of Orthodontics, College of Dentistry, Yonsei University, Seoul, Korea. d Professor, Department of Orthodontics, Dental Science Research Institute, The Institute of Craniofacial Deformities, College of Dentistry, Yonsei University, Seoul, Korea. The authors report no commercial, financial, or proprietary interest in the products or companies described in this article. Reprint requests to: Chung-Ju Hwang, Department of Orthodontics, College of Dentistry, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul , Korea:; , hwang@yuhs.ac. Submitted, August 2007; revised and accepted, March /$36.00 Copyright Ó 2010 by the American Association of Orthodontists. doi: /j.ajodo quantity and quality. 4,5 The initial stability of miniscrews is considered essential in clinical use because of immediate or early loading in many patients. 6 Two factors affect the initial stability of a screw: the screw factor and the host factor. The screw factor is related to the characteristics of the screw design, including diameter and length. 7 Various screw designs have been introduced to enhance initial stability. 8 The goal is to increase initial fixation by inducing controlled compressive forces in the cortical bone layer. In the prosthodontic field, it was reported that a tapered shape can enhance stability for immediate loading by increasing the mechanical contact between the dental implant and the surrounding bone. 9 The host factor is related to the quantity and quality of the bone where the screw is placed. Cortical bone thickness (CBT) (quantity) can affect the initial stability of a screw. 10 Finite element analysis showed that, when a lateral force is applied to the screw, most of the force is concentrated on the cortical bone. 11,12 For this reason, the previous studies focused on the anatomic background related to CBT rather than bone quality, to improve the stability of the miniscrew. On the other hand, several studies have used computed tomography (CT) in prosthodontics and orthopedics to study factors related to bone density It was reported that bone density differs according to sex, age, and physical condition. CT was recently suggested to successfully measure bone mineral density (BMD) in orthopedics. BMD has been used as a parameter to establish a treatment plan to ensure the stability of implants in dentistry

2 74 Cha et al American Journal of Orthodontics and Dentofacial Orthopedics January 2010 There are 2 methods for measuring BMD. One is with Hounsfield units (HU), and the other is to measure BMD indirectly by using a hydroxyapatite block. Since Hounsfield units can be affected by the loaded voltage of x-rays and the protocol used, bone density analysis with a BMD calibration standard (hydroxyapatite block) has greater advantages for evaluating bone quality more accurately. 18 One assessment method of initial stability is to measure the torque during placement. 9,13,19 Cheng et al 19 proposed that placement torque could be used as an indicator for initial stability, and, to obtain initial stability, a certain amount of torque is necessary. Placement torque is the measurement of the resistance at the screw-bone interface; it reflects the level of bone deformational strain caused by the miniscrew. In this study, we hypothesized that BMD values can be used to predict the mechanical stability of miniscrews by evaluating placement torque. Our aim was to determine the effect of BMD, CBT, screw position, and screw design on the stability of miniscrews. MATERIAL AND METHODS For this study, miniscrews were placed in 6 beagle dogs (age, 1 year; weight, 12 kg). Their purchase, selection, and management, and the experimental procedures were carried out according to prescribed conditions of the institutional review board, the Animal Experiment Committee of Yonsei Hospital, Seoul, Korea. A nondrilling type of miniscrew, 1.4 mm in diameter and 7 mm in length, was used. Both the cylindrical type (OAS-1507C) and tapered type (OAS-1507 T, Biomaterials Korea, Seoul, Korea) were selected; 96 screws were used (Fig 1). The location of the orthodontic miniscrew placement was determined after evaluating the reconstructed CT image, taken before placement, to determine whether there was sufficient interdental space. Sites selected were between the roots of the second, third, and fourth premolars, and the first molar in the mandible, between the roots of the second and third premolars, the first molar, and between the second and third premolars in the maxilla (Fig 2). The animals were injected subcutaneously with 0.05 mg per kilogram of atropine followed by an intravenous injection of rompun, 2 mg per kilogram, and ketamine, 10 mg per kilogram, to induce general anesthesia. The anesthesia was maintained with 2% enflurane, and each animal s temperature was maintained with a heating pad and an electrocardiogram, and monitored. When placing the mini-implant, 2% hydrochloric acid lidocaine containing 1:100,000 epinephrine was infiltrated into the placement area. Before placement, a 5-to-10 mm gingival incision was made under salinesolution irrigation, and complete placement of the screw into the alveolar bone was confirmed. Screw placement was performed manually with 70 to 90 angulations to gingival surface in consideration of the buccolingual width of alveolar bone for each experiment. In all miniscrews, a force of 250 to 300 g was applied with an elastomeric chain engaged reciprocally from a cylindrical miniscrew to a tapered miniscrew immediately after the placement of miniscrews, and the elastic chain was exchanged every 3 weeks. Screw mobility was measured twice on each miniscrew by using a periotest (Simens AG, Bensheim, Germany) before removing the screw. The sleeve of the handpiece of the periotest was positioned with 1 to 2 mm from the screw head perpendicularly after an elastomeric chain was removed. The average of 2 measurements for a miniscrew was recorded as the mobility value. During the test period, a chlorhexidine solution was applied daily to maintain the animals oral hygiene (Fig 3). The placement torque was the highest (in newtons per square centimeter) when the miniscrew was placed completely into the bone. The highest removal torque was measured during a quarter initial turn by using a torque sensor (MGT50, Mark-10 Co, New York, NY). To measure the BMD of the implanted area, CT scanning was done 3 days before placing and removing the miniscrews. Before scanning, each dog was placed under intramuscular sedation. The gantry of the CT device was placed parallel to the occlusal plane, and the dog s head was fixed with a strap. The CT scan was performed with a high-speed advantage CT scanner (GE Medical System, Milwaukee, Wis) by using standard CT protocol (high-resolution algorithm, matrix, 120 Kv, 200 ma) at a table feed of 6 mm per second. The CT data were reconstructed to 1-mm thick transaxial images. A calibration standard (Dental Phantom, Image Analysis, Columbia, Ky) was applied to calibrate the level in Hounsfield units in the dogs during the CT scanning. The calibration standard containing 3 compartments with 0, 75, and 150 mg of hydroxyapatite per cubic centimeter was attached to the phantom for BMD calibration. Calibration was performed by measuring the Hounsfield unit values in the 3 compartments of the dental phantom and relating these values to determine the BMD with a linear equation. The calibrations were performed for each dog. The scanned images were analyzed by V-implant (CyberMed, Seoul, Korea). The CT data taken before removing the screws were reconstructed to a transaxial

3 American Journal of Orthodontics and Dentofacial Orthopedics Cha et al 75 Volume 137, Number 1 Fig 1. Features and measurements of the miniscrews tested (mm). was based on a linear relationship: BMD 5 ahu1 b, where a and b are calibration coefficients. Pearson correlation analysis was used to determine the association between placement torque, removal torque, mobility, BMD, and CBT. The categorical variables (screw type and position) were statistically evaluated by using independent t tests to determine any differences in placement torque. The results suggest that the variables were linearly correlated. Thus, a multiple regression model was used with placement torque as the dependent variable, and CBT, BMD, screw type, and screw position as the independent variables. Fig 2. Schematic diagram indicating the approximate locations of screws. Circles indicate screw heads, and orthodontic forces were loaded reciprocally between a cylindrical miniscrew (CS) and a tapered miniscrew (TS). image at 1-mm intervals for each occlusal plane of the maxilla and mandible (Fig 4, A). The vertical location and inclination of each screw was localized by using a reconstructed image at the coronal plane from the postoperative scans. This information was transferred to the preoperative images. From the preoperative scanning data, the average BMD of a cylindrical area (diameter, 2 mm; depth, 5 mm) for each screw site was measured as the BMD total by Hounsfield unit calibration. The CBT and average bone mineral density in the cortical area (BMD cortical) were also calculated (Fig 4, B). Statistical analysis To determine the relationship between Hounsfield units and BMD values, a calibration regression equation was derived individually for all experimental dogs and RESULTS A regression equation was obtained by using the average Hounsfield value (Table I, Fig 5) for the 3 parts in the standard calibration of each beagle. The parameter a (inclination) was calculated within the range of to 0.935, and the intercept b ranged from 3.80 to The correlation coefficient of all regression equations was A correlation was observed between placement and removal torque, mobility, screw type, and BMD cortical. The mobility showed a negative correlation with placement torque ( 0.577) and a positive correlation with BMD cortical (0.575) (Table II). The values of placement and removal torque, CBT, and BMD were significantly higher in the mandible compared with the maxilla (P \0.05). The tapered screw (14.57 N per square centimeter) had higher placement torque than the cylindrical screw (9.69 N per square centimeter); this was statistically significant in the mandible (P \0.001) (Table II). There were no significant differences in the removal torque between the cylindrical and tapered screws. The values of CBT and BMD were similar in the tapered and cylindrical screw groups (Table III).

4 76 Cha et al American Journal of Orthodontics and Dentofacial Orthopedics January 2010 Fig 3. Experimental protocol used in this study. BMD cortical had significant influences on placement torque (R ) (Table IV). Fig 4. Procedure of BMD measurement in the region of interest: A, position of calibration phantom and reconstructed image according to the occlusal plane; B, measurement of Hounsfield units for the region of interest (red box). Multiple regression analysis showed that, among all the variables (CBT, BMD cortical, BMD total, screw type, screw position), screw position, screw type, and DISCUSSION Recently in orthodontics, the use of miniscrews has been generalized for the preparation of orthodontic anchorage in clinical patients. However, studies on the stability of orthodontic miniscrews are still in their early stages compared with studies of prosthodontic implants. Factors that can affect a miniscrew s stability are design, 20 placement procedures, 21,22 and quantity and quality of the bone related to the host factor. 5,23 This study focused on the assumptions that the quantity (thickness) and quality (density) of the bone have significant effects on a miniscrew s stability. Information on bone density can be obtained from the measurement of the Hounsfield units, but there are issues regarding the reliability of many CT scanners, since CT values are affected by changes in the effective energy of the x-ray source, the so-called beam-hardening effect. 18 To calibrate Hounsfield unit values, reference calibration standards for these CT values are needed for each subject. Calibration coefficients had a range of to in this study. The same 1000-HU value for 2 subjects can have a significant discrepancy of about 120 BMD units. Even though 2 subjects might have the same Hounsfield unit value, they can have different BMD values, and calibration coefficients must be used to calculate the BMD values. We therefore used a hydroxyapatite phantom to provide accurate calibration. Regarding the relationship between BMD and placement torque, there was a positive correlation in the cortical part of the alveolar bone but no similar correlation in the total bone. Previous studies 15,17 of prosthodontic implants reported high correlation

5 American Journal of Orthodontics and Dentofacial Orthopedics Cha et al 77 Volume 137, Number 1 Table I. Calibration of BMD by using Hounsfield units (HU) in the 3 compartments of the dental phantom Calcium hydroxyapatite 0 mg/cm 3 75 mg/cm mg/cm 3 HU HU HU Calibration coefficients Dog Mean SD Mean SD Mean SD Parameter a Parameter b Correlation coefficient Table II. Pearson correlation test and independent t test for placement torque Variable Correlation coefficient P value Removal torque Mobility CBT BMD cortical BMD total Mean 6 SD Screw type Cylindrical Tapered Screw Maxilla position Mandible Fig 5. Graph depicting calibration of BMD values from Hounsfield units for dog 1. The linear regression equation is shown on the graph, and the values are listed in Table I. mg HA/cm 3 equal milligrams (0, 75, 150) of calcium hydroxyapatite per cubic centimeter. between BMD and placement torque 14 and significant correlation between BMD and pull-out strength. 15,22,24 Seebeck et al 25 reported that in an axial pullout strength and cantilever bending test to the implanted screw, the screw s stability was mostly related to the CBT and cancellous bone density. It is also known that the strength and stiffness of trabecular bone are in linear proportion to bone density on the axial and bending loads. 26 This demonstrates that the density of trabecular bone and cortical bone makes a significant contribution to the stability of an implant. In a prosthodontic implant, most of the implant, either in the maxilla or the mandible, is Pearson correlation test was done between removal torque, mobility, CBT, and BMD, and the independent t test was performed for screw type and screw position. placed near cancellous bone, and the quality of cancellous bone is important in the stability of the implant. 27 However, our results indicated a poorer correlation between total bone density and placement torque compared with cortical bone density. It was observed that placement torque is related to screw position. This is due to thicker cortical bone and better bone quality in the mandible compared with the maxilla. 27 Despite the implant position, the stability of a prosthodontic implant is more influenced by its length and diameter in alveolar bone, rather than the thickness of cortical bone. 27 However, the stability of orthodontics miniscrews, with their relatively smaller diameter and length, is related to CBT. This study showed a significant correlation between placement torque and cortical bone density. Placement and removal torque also showed a positive correlation, which supports previously reported

6 78 Cha et al American Journal of Orthodontics and Dentofacial Orthopedics January 2010 Table III. Comparison of variables according to screw location and screw type Screw type Cylindrical Tapered Variable Position Mean SD Mean SD P value Removal torque (Ncm) Maxilla Mandible P value Mobility (PTV) Maxilla Mandible P value CBT (mm) Maxilla Mandible P value BMD cortical (mg of hydroxyapatite/cm 3 ) BMD total (mg of hydroxyapatite/cm 3 ) Maxilla Mandible P value Maxilla Mandible P value Table IV. Regression equation model to predict placement torque with CBT, BMD cortical, BMD total, screw type, and screw position Unstandardized coefficient Standardized coefficient Variable Beta SE Beta t P value CBT BMD cortical BMD total Screw type Screw position Corrected R Dependent value was placement torque (Ncm). studies, and they also had a correlation with the BMD of cortical bone. 28 However, the BMD of the total bone did not have a major effect on placement and removal torque. On the other hand, mobility was associated with the BMD of the total bone and CBT. Therefore, this suggests that BMD and CBT could affect a miniscrew s stability. Tapered miniscrews showed 65% higher placement torque than the cylindrical type. This coincides with the result from a previous study that used artificial bone. 8 As the tapered screw was placed, the torque along with the deformation of the surrounding bone increased with increasing diameter. In this study, a similar bone density was calculated at the placement area of each screw type. This meant that screw type affected the initial stability of the miniscrew because it was placed into some areas with low bone density or thinner cortical bone. A significant difference in placement torque was found according to the screw type, but no difference was found in removal torque. This suggested that the strain generated between the screw and the bone after placement decreased during healing of the surrounding bone. Further research involving histologic analysis is required to confirm this. A regression equation between placement torque, BMD, and screw type was derived. Screw type and position were major variables for the prediction of placement torque. This results from the difference in bone density and structure in the cortical bones of the maxilla and the mandible. The maxilla consists of more trabecular bone than the mandible, and its cortical bone is relatively thinner. The BMD cortical was also chosen as a significant variable, but the impact of the variables on the regression equation was smaller than those of screw type and position. These results suggest that it might be difficult to predict the primary stability of a miniscrew solely with BMD values, and screw design could play a major role in increasing the placement torque despite the quantity of bone. Because of the high radiation dose during CT scanning, there are limitations in using it for clinical purposes. However, dental CT such as cone-beam CT can be a useful technique for a noninvasive assessment of BMD in estimating the stability of a miniscrew at the preoperative stage. CONCLUSIONS The BMD of the cortical bone, screw position, and screw type have a compound influence on the primary stability of a miniscrew. Cone-beam CT might be a useful and noninvasive way to assess the BMD for estimating the stability of a miniscrew at the preoperative stage.

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