Primary stability and histomorphometric bone-implant contact of self-drilling and self-tapping orthodontic microimplants

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1 ORIGINAL ARTICLE Primary stability and histomorphometric bone-implant contact of self-drilling and self-tapping orthodontic microimplants Seçil Çehreli a and Ayça Arman- Ozçırpıcı b Ankara, Turkey Introduction: The aim of this study was to evaluate the primary stability and the histomorphometric measurements of self-drilling and self-tapping orthodontic microimplants and the correlations between factors related to host, implant, and measuring technique. Methods: Seventy-two self-drilling and self-tapping implants were placed into bovine iliac crest blocks after computed tomography assessments. Insertion torque values, subjective assessments of stability, and Periotest (Medizintecknik Gulden, Modautal, Germany) measurements were performed for each implant. Twelve specimens of each group were assigned to histologic and histomorphometric assessments. Results: The differences between insertion torque values, most Periotest values, and subjective assessments of stability scores were insignificant (P.0.05). The boneimplant contact percentage of the self-drilling group (87.60%) was higher than that of the self-tapping group (80.73%) (P \0.05). Positive correlations were found between insertion torque value, cortical bone thickness, and density in both groups (P \0.05). Negative correlations between insertion torque values and Periotest values were mostly observed in the self-drilling group (P \0.05). Positive correlations were found between bone-implant contact percentages, cortical bone densities, and insertion torque values in both groups (P \0.05). The differences between insertion torque values and corresponding subjective assessments of stability scores were different in both groups (P\0.05). Conclusions: The differences in insertion torque values, Periotest values, and subjective assessments of stability scores of self-drilling and self-tapping implants were insignificant. Self-drilling implants had higher bone-implant contact percentages than did self-tapping implants. Significant correlations were found between parameters influencing the primary stability of the implants. (Am J Orthod Dentofacial Orthop 2012;141:187-95) Orthodontic microimplants provide absolute anchorage with predictable survival rates in the low-compliance treatment of patients. 1,2 Because of the growing need toward immediate loading, the primary stability of orthodontic implants is of utmost importance. Primary implant stability is a prerequisite for achievement and maintenance of osseointegration. 3 It depends on the mechanical engagement of an implant with bone of the osteotomy. After implant surgery, this mechanical stability is gradually From the Department of Orthodontics, Faculty of Dentistry, Başkent University, Ankara, Turkey. a Postgraduate student. b Associate professor. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Ayça Arman- Ozçırpıcı, Başkent Universitesi, Diş Hekimligi Fak ultesi, Ortodonti Anabilim Dalı, Bahçelievler-Ankara, T urkiye; , ayca@baskent.edu.tr. Submitted, March 2011; revised and accepted, July /$36.00 Copyright Ó 2012 by the American Association of Orthodontists. doi: /j.ajodo replaced by biologic stability (secondary stability). Implant stability, an indirect sign of skeletal tissue integration, is a measure of clinical implant mobility. 4 Whereas optimum implant stability refers to the lack of clinical mobility, this does not necessarily imply the absence of micromotion or displacement in any direction at the microscale. Currently, Periotest (Medizintecknik Gulden, Modautal, Germany), cutting-torque, and resonance-frequency analysis are the most frequently used techniques for noninvasive quantitative assessment of implant stability. 5 Periotest is an electronic instrument, originally developed to measure the damping characteristics of the periodontal ligament around natural teeth. 6,7 It has been also used to assess mobility of dental and orthodontic implants. In essence, it measures predominantly the natural frequency and, to a lesser extent, the damping characteristics of the tooth or the bone-implant interface. The outcome is displayed digitally and audibly as a descriptive numeric value (Periotest value). This value is between 8 and 18 for implants. The force used to insert a dental implant is 187

2 188 Çehreli and Arman- Ozçırpıcı described as cutting or insertion torque. 8,9 The amount of insertion torque relies on site-specific properties of bone tissue, such as the thickness of the cortical bone and the density of the bone, the macro-design of the implant, and the difference between the diameters of the implant and the fresh surgical implant socket. Resonancefrequency analysis is also a noninvasive and nondestructive method for quantitative measurement of primary stability and osseointegration. 10,11 Resonance-frequency analysis measurements have been used to document timedependent healing changes along the dental implantbone interface by measuring the increase or decrease in its stiffness. Moreover, the technique has been used to determine the appropriate timing of loading and to identify at-risk implants. Nevertheless, there are no available transducers suitable for orthodontic microimplants; therefore, resonance-frequency analysis is only used in experimental orthodontic studies. 11 Beyond these techniques, subjective assessment of primary stability is another method that relies on the tactile sensation of the surgeon, who scores the primary stability ie, good, moderate, or poor with regard to the resistance of the implant being inserted. So far, this method has not been used extensively on implants, because it is not an objective method that provides a quantitative assessment. Its reliability and correlation with the outcome of noninvasive techniques is still under consideration. 12,13 The primary stability of orthodontic microimplants relies on several critical parameters. Because the subject-specific and site-specific structural and mechanical properties of bone tissue can exhibit great variations, the thickness of the cortical bone, the density of the cancellous bone, 14,15 the bone mineral content, 16 and the histomorphometric bone-implant contact 17,18 might affect implant stability. Moreover, the macro-design, diameter, and shape of the implant, the technique of implant placement (self-drilling vs self-tapping), the diameter of the pilot drill, and the intraosseous insertion angle of the implant are frequently cited as factors that influence the primary stability of implants and screws Since orthodontic mini-implants and microimplants that are 6 to 8 mm long are usually angulated in bone with 1 to 2 mm of cortical bone thickness, it is unclear whether it is the protocol of placement (self-drilling or self-tapping) or the macro-design of the implant (tapered vs cylindrical implant body) that influences the magnitude of insertion torque and the resultant primary stability. In addition, correlations between factors influencing the stability of orthodontic implants and measuring techniques have not been thoroughly explored. The purposes of this study were to compare the primary stability of self-drilling and self-tapping orthodontic microimplants by using insertion torque measurements and Periotests and to explore the validity of subjective assessment of primary stability. It was hypothesized that microimplants having a similar macro-design would lead to similar primary stability regardless of the method (self-drilling vs self-tapping) of placement. In addition, the histomorphometric bone-implant contacts of the self-drilling and the self-tapping orthodontic microimplants were compared to evaluate their impact on primary stability. Finally, possible correlations between factors relating to host, implant, and measuring technique that influence primary stability were explored. MATERIAL AND METHODS This study was approved by Başkent University institutional review board and ethics committee (project #D-DA10/01) in Ankara, Turkey, and supported by the Başkent University Research Fund. Sixteen bovine iliac crests were cut into cm bone blocks under profuse saline-solution cooling. To undertake quantitative x-ray computed tomography measurements on the implant sites (n 5 6 per each block), gutta-percha points (length, about 0.5 mm; diameter, 1 mm; number 15; SureDent, Kyeonggi-do, Korea) were bonded on the cortical surface of each bone block. The markers were used only to determine implant location and not as a comparative tool to measure bone thickness. Then, each bone block was scanned (150 mas and 120 kv; 2002 MSCT, Somatom Sensation 16; Siemens, Erlangen, Germany), and cortical bone thickness in millimeters and cortical and trabecular bone densities in Hounsfield units were measured in 1- mm thick images (Fig 1). 14,15 The density measurements were normalized by using water as a material with standard density. A pilot study was undertaken on 4 bone blocks by using several microimplants to become accustomed to using the manual torque application wrench and to train for correlating subjective assessment scores with corresponding insertion torque amplitudes of the implants. 12,13 Seventy-two self-drilling and self-tapping orthodontic microimplants (diameter mm; AbsoAnchor, Dentos, Daegu, Korea) were used. To place the implants at 30 to 40 insertion angles, the bone blocks were placed on the surveyor table of a milling machine (Paraskop; Bego, Bremen, Germany). 25,26 The self-tapping implants were inserted upon pilot hole drilling (diameter 1 mm; AbsoAnchor). Before placing the self-drilling implants, 0.5-mm deep dimples were created by the pilot drill, because high-insertion angles frequently result in slippage of self-drilling implants on the cortical bone. 27 The implants of both groups were inserted on the milling machine by using the implant driver until the thread next to the collar was visible. February 2012 Vol 141 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

3 Çehreli and Arman- Ozçırpıcı 189 Fig 1. The method of quantification of cortical and trabecular bone density. In the magnified image, diameter 0.5 mm (1) and diameter 2.5 mm (2) circular areas are shown for measurements in cortical and trabecular bone, respectively. Fig 2. The setup for measurement of implant primary stability. From left to right, the manual torque wrench connected to the data acquisition unit, the computer used to display and assess data, and the Periotest device are shown. The primary stability assessments were undertaken blindly. The insertion torque value of each implant was measured during final torque tightening of the implant by using a strain-gauged manual torque application wrench connected to a data acquisition device (ESAM Traveller 1; Vishay Micromeasurements Group, Raleigh, NC). The strain-gauge signals of the wrench were delivered to the data acquisition unit at 10 KHz and displayed in computer software (ESAM; ESA Messtechnik, Olching, Germany). The technical details and calibration experiments of the wrench are explained elsewhere. 28 In brief, the strain-gauge readings vs elicited torque were obtained, and a linear regression to data points was undertaken to determine the calibration constant, Ncm per microstrain (mε) with R The strain data were converted to torque units (Ncm) by using the general formula: torque 5 K 3 mε, where K is the calibration constant, and ε is the strain-gauge reading. Simultaneously with the insertion torque experiments, the subjective assessment of primary stability of each implant was scored as good, moderate, or poor by the same trained operator (S.C.). An insertion torque higher than or equal to 8 Ncm was regarded as good, $5 to\8 Ncm was moderate, and \5 Ncm was poor. The examiner was not supposed to detect small differences in torque output, because the pilot study suggested that the operator cannot detect small differences less than a few newton centimeters of torque. Then Periotest measurements were made on each implant from 3 directions, approximately 120 apart on the horizontal plane (Figs 2 and 3). Twelve randomly selected implants of each group were assigned to histologic and histomorhometric evaluations. Three-millimeter thick bone segments that contained the microimplants were removed and fixed by immersion in 4% buffered formalin solution for 48 hours. In preparation for the ground sections, the bone segments were dehydrated in an ascending series of American Journal of Orthodontics and Dentofacial Orthopedics February 2012 Vol 141 Issue 2

4 190 Çehreli and Arman- Ozçırpıcı Fig 3. The 3 directions of the Periotest measurements on the implant head (*). A represents the apex, and B represents the implant head with regard to the direction of angulation. ethanol (70%-99.5%) and embedded in autopolymerized methylmethacrylate. Because of the relatively low diameter of the microimplants compared with conventional dental implants, the thickness of the blocks was reduced by successive abrasive papers (numbers ) until midaxial sections of 50 mm were achieved. The sections were stained with trichrome and inspected in a light microscope for possible fractures of the periimplant bone and its presence in the threads as a sequel of placement. Finally, the bone-implant contact of each implant was measured to compare the impact of the placement protocol. 17,18,24 Statistical analysis To assess whether the data were normally distributed, the 1-sample Kolmogorov-Smirnov test was performed at the 95% CI level. Cortical bone thickness, Periotest 1, Periotest 2, and bone-implant contact percentage data of both groups were compared with the Mann- Whitney U test at the 95% CI level. Density of the cortical bone, density of the trabecular bone, insertion torque values, and Periotest 3 data of both groups were compared by independent t test at the 95% CI level. The subjective assessment data of both groups were compared with the chi-square test at the 95% CI level. To explore possible correlations between parameters, the Pearson and Spearman (rho) correlation tests were performed at the 95% CI level. To identify whether subjective asessment scores differed with regard to the corresponding insertion torque values of the implants, the Kruskal-Wallis test was performed at the 95% CI level. Because P was found, further asssessments were performed by using the 2-sample Mann-Whitney U test at the 95% CI level. RESULTS One-sample Kolmogorov-Smirnov test results are shown in Table I for intergroup comparisons. Bending or fracture of the implants was not observed in either group during placement. The implants of both groups were placed into bone blocks having similar characteristics. The differences in cortical bone thickness and trabecular bone density were insignificant in both groups (P and P , respectively), although slightly higher cortical bone density was observed in the self-tapping group (P ). The differences in insertion torque values, and the Periotest 1 and Periotest 2 values, were insignificant in both groups (P , P , and P , respectively), although the Periotest 3 was higher is the self-tapping group (P ). Likewise, the differences in subjective assessment scores were insignificant in both groups (P ) (Table II). As a sequel of placement, fracture of the upper part of the cortical bone was clinically discernable for some self-drilling implants. At the histologic level, however, it was observed that implants of both groups were associated with such fractures (Fig 4). All implants showed intimate contact with cortical bone, which was also fractured toward cancellous bone in many sections. In addition, the threads of all implants were partly filled with fractured local trabeculae. The apical zone of the selfdrilling implants showed more deposition of fractured trabecular bone than did the self-tapping implants. The mean and standard deviation of bone-implant contact percentage of the self-drilling group (87.60; SD, 6.20) were higher than those of the self-tapping group (80.73; SD, 8.58) (P ) (Fig 5). A positive correlation was found between insertion torque value and cortical bone thickness and cortical bone density in the self-drilling (r , P ; and r , P , respectively) and the self-tapping (r , P ; and r , P , respectively) groups. There were negative correlations between insertion torque value and Periotest 1 (r , P ), Periotest 2 (r , P ), and Periotest 3(r , P ) in the self-drilling group and only between Periotest 1 in the self-tapping group (r , P ). Significant correlations were found between bone-implant contact percentage and cortical bone density in the self-drilling (r , P ) and the self-tapping (r , P ) groups, and between bone-implant contact percentage and cortical bone thickness in the self-tapping group (r , P ). In addition, positive correlations were found between bone-implant contact percentage and insertion torque values in the self-drilling (r , P ) February 2012 Vol 141 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

5 Çehreli and Arman- Ozçırpıcı 191 Table I. One-sample Kolmogorov-Smirnov test results for self-drilling and self-tapping implants (n 5 72) Cort thick (mm) HU-cort HU-trab Torque (Ncm) PTV 1 PTV 2 PTV 3 Self-drilling Mean SD Kolmogorov-Smirnov Z P value * Self-tapping Mean SD Kolmogorov-Smirnov Z P value 0.000* * Cort thick, Thickness of the cortical bone (mm) measured by computed tomography; HU-cort, cortical bone density in Hounsfield units; HU-trab, trabecular bone density in Hounsfield units; Torque, insertion torque value (Ncm); PTV 1-3, Periotest values for the 3 measurements; Z, statistical representation of the Kolmogorov-Smirnov test. *P \0.05; data were not normally distributed. and self-tapping (r , P ) groups. Mann- Whitney U tests showed that, in the self-drilling and self-tapping groups, the differences between insertion torque values corresponding to good and moderate (P and P , respectively), good and poor (P and P , respectively), and moderate and poor (P and P , respectively) subjective scores were significant (Table III). DISCUSSION Previous studies on the insertion torque values of screws and implants did not reach consensus on which type of placement yields a higher torque output. Although many studies suggest that self-drilling screws or tapered designs have higher insertion torque values than self-tapping or cylindrical implant designs, 16,22,29,30 Lim et al 31 found higher insertion torque values for cylindrical implant designs. It is, therefore, tempting to question whether it is the protocol of placement (self-drilling vs self-tapping) or the macro-design of the implant (tapered vs cylindrical implant body) that determines the amplitude of the insertion torque. In this study, the mean insertion torque values for selfdrilling and self-tapping implants were 6.82 and 7.22 Ncm, respectively. The difference between the groups was less than 1 Ncm, and current evidence suggests that insertion torque values in this range do not cause local damage to surrounding bone, leading to implant failure. 9 Insertion torque value differences less than 1 Ncm would not be expected to worsen the placement of self-tapping implants. The small difference between the insertion torque values was possibly a consequence of slightly thicker cortical bone and higher bone density in the self-tapping group. Statistical analyses showed that the difference between insertion torque values for both types of implant was insignificant. This finding could be attributed to 2 factors. First, the implants of Table II. Distribution of subjective assessment scores in both groups Score Good Moderate Poor Total Self-drilling Count (n) Within implant (%) Self-tapping Count (n) Within implant (%) Total Count (n) Within implant (%) both groups were placed in bone blocks with similar cortical bone thicknesses, and, because of the implant length and 30 angled placement, approximately half of the implants were supported by cortical bone. 16,32 The other factor that led to this outcome was the implant design we used. Although the amount of taper might differ between implant systems, there was no drastic difference between the designs used in our study. 7,29,30,33 There seems to be an ideal torque range for placement of orthodontic mini-implants and microimplants between 7 and 10 Ncm. 9,34 This torque window probably does not lead to excessive micro-cracks that exceed the bone-healing capacity. Therefore, implants placed at 7 to 10 Ncm have survival rates over 80%. In this study, both types of implant yielded a mean torque output about 7 Ncm, which might be regarded as almost optimal. Longer implants and thicker cortical bone layers might increase the final insertion torque. The Periotest 1 and Periotest 2 values were similar in both groups, although the difference in Periotest 3 was significant. This finding suggests that Periotest values are, to some extent, in line with insertion torque test American Journal of Orthodontics and Dentofacial Orthopedics February 2012 Vol 141 Issue 2

6 192 Çehreli and Arman- Ozçırpıcı Fig 4. Histologic image of a self-drilling implant. The white arrows show fractured cortical bone toward the threads of the implant, and the yellow arrows denote the fractured trabecular bone in the implant threads. Table III. Descriptive statistics of insertion torque values corresponding to subjective assessment scores in both groups 95% CI for mean n Mean SD SE Lower bound Upper bound Minimum Maximum Self-drilling Good Moderate Poor Total Self-tapping Good Moderate Poor Total findings. Our findings are in contrast with the results of Kim et al, 35 who found remarkably lower Periotest values for self-drilling implants. The reason for the differences in the Periotest values and reports might be because the Periotest instrument can display unpredictable and inconsistent measurements on implants. 36,37 So far, subjective assessment of primary stability has been reported in only a few studies on dental implants. 12,13 Because it is a subjective method and does not provide quantitative information, it can only provide a general idea about primary stability. Clinical studies did not have high accuracy of prediction in primary stability by subjective assessments. Taking into account that there are already noninvasive instruments that provide quantitative information for dental implants, subjective assessment of primary stability will not gain popularity with clinicians. In the context of orthodontic implants, however, objective and quantitative information can only be obtained by using cutting-torque (insertion torque) measurements, since the Periotest instrument is reported to provide unpredictable results. 38 According to our knowledge, this study was the first on orthodontic microimplants. The reason that this parameter was included in this study was that instruments used to assess implant stability are not routinely used in clinical practice. Even though good scores in the self-tapping group were higher (Table II), the difference in the subjective assessments in both groups was insignificant. In addition, the differences between good and moderate, good and poor, and moderate and poor scores were significant in both groups. Studies on dental implants have reported February 2012 Vol 141 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

7 Çehreli and Arman- Ozçırpıcı 193 Fig 5. Histologic images of self-drilling (SD) and selftapping (ST) implants. low-to-moderate accuracy of subjective assessments of primary stability. 12,13 Because an insertion torque higher than or equal to 8 Ncm was regarded as good, 5to\8 Ncm was regarded as moderate, and \5 Ncm as poor as a sequel of training in this study, the operator was somewhat biased by objective measurements. Therefore, the validity of subjective assessments of primary stability of orthodontic microimplants is still an open field for investigation. The average bone-implant contact percentage exceeded 80% in this study. This outcome might seem unlikely compared with previous histologic studies, where bone-implant contact percentages measured in dog jawbones were about 30% and as low as 5%. 39 This information does not necessarily imply that a 30% boneimplant contact is the gold standard for dog jawbones. Freire et al 18 observed over 80% bone-implant contact for the control and test implants placed in dog mandibles after 12 weeks. In our study, about half of the length of the implants had contact with cortical bone, and the rest had contact with trabecular bone, which was fractured at some sites and filled the implant threads. Also, in this study, we provided an initial picture of how the interface looks. In other words, the fractured trabecula did not undergo remodeling, which might result in a decrease in the bone-implant contact percentage. There are few histologic and histomorphometric data on the immediate bone-implant relationship of selfdrilling and self-tapping orthodontic microimplants. Freire et al 18 found higher bone-implant contacts in the compression side of early-loaded orthodontic implants, but no difference in the tension side. We found higher bone-implant contacts in self-drilling implants than in self-tapping implants. This agrees with the findings of Heidemann et al, 24 who found more bone volume between the threads of self-drilling implants that led to higher bone-implant contact on remodeling of periimplant bone. Likewise, the amount of bone deposition was higher in the apical part of the self-drilling implants in our study. The relatively higher bone deposition around the apical part resulted in the difference between groups (approximately 7%). This difference was not high, because the length of the implant and the 30 angled placement resulted in high contact with cortical bone in both groups. The bone volume at the apical part that predominantly consisted of fractured trabeculae cannot have an effect on mechanical anchorage; therefore, the primary stability measurements in both groups were comparable. Correlation assessments showed positive relationships between insertion torque values and cortical bone thickness and cortical bone density in both groups. This finding is in line with previous reports. Alsaadi et al 12 found a significant correlation between insertion torque values of dental implants and bone quality scores (P\0.0001). Motoyoshi et al 33 found a positive correlation between insertion torque values of mini-implants and cortical bone thickness (r , P ). Wilmes et al 27 found higher correlations between these parameters at the in-vitro level (R ), and Cha et al 16 also detected correlations between insertion torque value and bone mineral density (0.58) and cortical bone thickness (0.48). The reason behind the negative correlation between insertion torque values and Periotest values is that lower Periotest values indicate higher implant stability. This finding suggests that both the increase in insertion torque values and the corresponding decrease in Periotest values indicate increased primary stability. Our findings agree with those of Cha et al, 16 who found a negative correlation ( 0.577) between insertion torque values and Periotest values. The positive correlations between bone-implant contact percentage and cortical bone density and cortical bone thickness suggest that cortical bone thickness and its density have great roles on the amount of bone-implant contact, and that implants should be placed at angles that allow the maximum contact with cortical bone. The increase in bone-implant contact percentage will eventually increase the primary stability, since a moderate-to-high level of correlation American Journal of Orthodontics and Dentofacial Orthopedics February 2012 Vol 141 Issue 2

8 194 Çehreli and Arman- Ozçırpıcı was also found between bone-implant contact percentages and insertion torque values. CONCLUSIONS Within the limits of this study, these conclusions were drawn. 1. Insertion torque value, Periotest value, and subjective assessment scores used to assess the primary stability of self-drilling and self-tapping implants were similar. 2. Self-drilling implants yielded a higher bone-implant contact percentage than did self-tapping implants. 3. Significant correlations were found between parameters influencing the primary stability of implants. REFERENCES 1. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31: Costa A, Raffaini M, Melsen B. Miniscrews as orthodontic anchorage: a preliminary report. Int J Adult Orthod Orthognath Surg 1998;13: Albrektsson T, Branemark PI, Hansson HA, Lindstrom J. Osseointegrated titanium implants. Requirements for ensuring a long-lasting direct bone-to-implant anchorage in man. 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