Self-drilling Miniscrews Research Articles

Comparative study of the primary stability of self-drilling and self-tapping orthodontic miniscrews

The purpose of this study was to determine the influence of self-tapping and self-drilling placement techniques on the stability of miniscrews. We included 70 orthodontic patients who received miniscrews (140 screws). Miniscrews measuring 1.6 mm in diameter and 8 mm in length were placed using the self-tapping (35 patients, 70 screws) and self-drilling (35 patients, 70 screws) methods. We examined the success rates, placement torque values, Periotest (Medizintechnik Gulden, Bensheim, Germany) values, rates of root contact, and influence of root contact on mobility. Cone-beam computed tomography was used to evaluate root contact. Miniscrews that endured an orthodontic force for 6 months or more were considered successful. The success rates of the miniscrews were approximately 96% with either placement technique. The placement torques were 7 and 7.5 N cm in the self-tapping and self-drilling miniscrews, respectively (P>0.05). The Periotest values of the self-drilling method were significantly greater than those of the self-tapping method. The Periotest values of the self-drilling miniscrews with root contact were significantly greater than those with no root contact. Both placement techniques showed high stability in the maxillary bone, although the self-drilling miniscrews with root contact had greater mobility. Special attention to root contact further improves the stability of the self-drilling miniscrews.

Torque Analysis of Self-drilling Mini-screws placed with and without a Pilot Drill: A Canine Study

ABSTRACTPurposeThe use of temporary anchorage devices (TADs), such as mini-screws, palatal implants and mini-plates, has become a popular treatment option in orthodontics. The objectives of this study were: (1) to compare the success rates of self-drilling mini-screws placed with or without use of a pilot drill, (2) to evaluate the implant placement torque (IPT) of self-drilling miniscrews and (3) to measure the removal torque of self-drilling mini-screws.Materials and methodsSix American foxhounds served as subjects in this study. Three OsteoMed self-drilling mini-screws (1.6 mm diameter and 8 mm length) were planned to be placed in each side of the mandible between the roots of the teeth canine/premolar 1 (PM1), PM2/PM3 and PM3/PM4. The sites were allocated randomly in: (1) control group—mini-screws placed with pilot drill and (2) test group—mini-screws placed without pilot drill (self-drilling). The implant placement torque (IPT) and the number of turns performed by the mini-screws during placement were measured. After a healing period of 6 weeks, the mini-screws were removed, and the removal torque was measured.ResultsA total of 33 mini-screws were placed in six animals. After 6 weeks, the success rate for the control and test groups were 46.7 and 80% respectively (difference not statistically significant). There was no statistically significant difference in the terminal mean IPT values and the mean number of turns for failed and successful mini-screws. The difference between the mean IPT values at the 3rd turn during mini-screw placement in the successful and failed mini-screws (5.56 and 14.03 Ncm respectively) was statistically significant (p < 0.01).ConclusionThere was no statistically significant difference in the success rate between self-drilling mini-screws that did or did not have osteotomies (46vs80% respectively). However, the IPT at the third revolution for successful mini-screws averaged 5.56 Ncm (p < 0.01).How to cite this articleZambon-Fagundes DA, Kerns DG, Hallmon WW, Solomon ES. Torque Analysis of Self-Drilling Mini-screws placed with and without a Pilot Drill: A Canine Study. J Contemp Dent 2014;4(2):77-86.

A comparison of tapered and cylindrical miniscrew stability

This study compared the stability of tapered miniscrews with cylindrical miniscrews. One hundred and five tapered and 122 cylindrical self-drilling miniscrews were placed into the maxillary and mandibular buccal alveolar areas of 132 patients (43 males and 89 females). The insertion torque and removal torque were measured and Periotest values (PTVs) were recorded at implantation. The success rates of the tapered and cylindrical miniscrews examined were similar. In the maxilla, the insertion torque of the tapered miniscrews (8.3 Ncm) was significantly higher than that of the cylindrical miniscrews (6.3 Ncm) (P < 0.05). The PTVs of the tapered miniscrews were statistically significantly lower in the maxilla (P < 0.05). The removal torque values showed no significant difference between the tapered and cylindrical miniscrews in the upper and lower buccal areas (P > 0.05). Tapered miniscrews had higher initial stability when compared to cylindrical miniscrews, whereas the clinical success rates and removal torques were similar between the two designs. The long-term stability is not directly affected by the miniscrew design.

A new skeletal retention system for retaining anterior open bites

Objectives: Relapse of anterior open bite after treatment poses a challenge to orthodontists and warrants finding new methods. We aimed to compare the effect of a skeletal retention (SR) system to the conventional retention (CR) commonly used. Materials and Methods: Twenty patients participated in this study. SR group ten patients (five females and five males) with mean age of 16.2 years, CR group ten patients (five females and five males) with mean age of 17.1 years in pretreatment stage. The SR system is comprised of four self-drilling miniscrews and vacuum retainers with interarch elastics where the CR group is comprised of removable or fixed retainers. Pretreatment (T1), posttreatment (T2), and 1-year follow up (T3) lateral cephalograms were taken and analyzed to compare the stability of both retention modalities. Results: The overbite in the CR group showed more relapse in the form of significant reduction when compared to the SR group ( P P Conclusion: Skeletal retention using miniscrews and vertical elastic is an effective method for retention of anterior open bite cases.

Effects of thread shape on the pullout strength of miniscrews

The aim of this study was to determine the effects of variations in thread shape on the axial pullout strength of orthodontic miniscrews. A total of 35 miniscrews, 7 of each design being considered, were tested by performing pullout tests on a synthetic bone support. We used self-tapping and self-drilling miniscrews having a diameter of 2 mm and a thread shaft length of 12 mm (the longest and the largest supplied by the manufacturer). A buttress reverse thread shape served as the control design and was tested against 4 experimental designs, each manufactured with a modification in thread shape while maintaining all other characteristics. The experimental groups had the following thread designs: buttress, 75° joint profile, rounded, and trapezoidal. The control group with a buttress reverse thread shape had consistently higher pullout strength values than did the other designs. A statistically significant reduction in pullout force was found between the buttress reverse and the buttress thread miniscrews. Thread design influenced the resistance to pullout of the orthodontic miniscrews. The buttress reverse thread shape provided the greatest pullout strength.

Bone damage associated with orthodontic placement of miniscrew implants in an animal model

The purposes of this study were to quantify bone damage associated with insertion of 2 types of miniscrew implants and to relate the amount of bone damage to monocortical plate thickness. Nondrilling (n = 28) and self-drilling (n = 28) miniscrew implants (6 × 1.6 mm, Dentaurum, Newtown, Pa), and pilot holes (n = 26) were placed bilaterally in the maxillae and the mandibles of 5 adult dogs immediately after death. Bone blocks were cut, bulk stained with 1% basic fuchsin, embedded in methyl methacrylate, sectioned, and mounted. Monocortical plate thickness was measured adjacent to the miniscrew implant insertion site. Damage amounts were quantified at distances of 0 to 0.5 mm (adjacent region) and 0.5 to 1 mm (distant region) from the bone-implant interface. Total fractional damaged area (%), fractional microcracked area (%), and fractional diffuse damaged area (%) were quantified by using standard histomorphometric methods. The mean monocortical plate thickness of the specimens from the mandible (2.2 mm) was significantly (P <0.001) greater than that of the maxillary specimens (0.9 mm). In the mandible, the 3 damage parameters were greater with self-drilling miniscrew implants than with nondrilling miniscrew implants; however, there were no differences in the damage parameters in the maxilla. Bone damage accumulation is related to the type of miniscrew implant and the thickness of the bone.

Accuracy of orthodontic miniscrew implantation guided by stereolithographic surgical stent based on cone-beam CT–derived 3D images

To develop surgical stents for cone-beam computed tomography (CBCT) 3-dimensional (3D) image-based stent-guided orthodontic miniscrew implantation and to evaluate its accuracy. Ten surgical stents were fabricated with stereolithographic appliances (SLAs) according to 3D CBCT image-based virtual implantation plans. Thirty self-drilling miniscrews were implanted at two to three positions on each side of the maxillary or mandibular posterior arches in three phantoms: 20 guided by 10 surgical stents in two phantoms (stent group) and 10 guided freehand in one phantom (freehand group). Six parameters (mesiodistal and vertical deviations at the corona and apex and mesiodistal and vertical angular deviations) were measured to compare variations between the groups. No root damage was found in the stent group, whereas four of 10 miniscrews contacted with roots in the freehand group. In the stent group, deviations in the mesiodistal and vertical directions were 0.15 ± 0.09 and 0.19 ± 0.19 mm at the corona, respectively, and 0.28 ± 0.23 and 0.33 ± 0.25 mm at the apex, respectively; angular deviations in the mesiodistal and vertical directions were 1.47° ± 0.92° and 2.13° ± 1.48°, respectively. In the freehand group, the corresponding results were 0.48 ± 0.46 mm and 0.94 ± 0.87 mm (corona), 0.81 ± 0.61 mm and 0.78 ± 0.49 mm (apex), and 7.49° ± 6.09° and 6.31° ± 3.82°. Significant differences were found in all six parameters between the two groups (Student's t-test, P < .05). 3D CBCT image-based SLA-fabricated surgical stents can provide a safe and accurate method for miniscrew implantation.

Three-dimensional finite element analysis of strength, stability, and stress distribution in orthodontic anchorage: A conical, self-drilling miniscrew implant system

The aims of this study were to analyze the stress distribution and displacement patterns that develop in an orthodontic miniscrew implant and its surrounding osseous structures for 2 implant materials under horizontal and torsional loading, with no ossseointegration. A numeric approach was adopted. The finite element method was used to determine the stress and displacement of the various components at a given time after miniscrew implant application, when, due to viscoelastic relaxation effects, the only remaining stress field was from the application of the orthodontic load. Stress distribution was not significantly different between the 2 types of implant material. Increased stress values were located at the necks of the implants and the surrounding cortical bone. Bending of the titanium miniscrew was observed in the neck region under horizontal traction. The differences between the values of stress and displacement we obtained for the 2 types of miniscrew were too small to be clinically significant. Optimization of the miniscrew implant composed of the titanium alloy might be achieved by increasing the bulk (quantity) of the material in the neck region. The miniscrew implant can be immediately loaded and used for group movement of teeth.

Is miniscrew primary stability influenced by bone density?

Primary stability is absence of mobility in the bone bed after mini-implant placement and depends on bone quality among other factors. Bone quality is a subjective term frequently considered as bone density. The aim of this preliminary study was to evaluate bone density in two bovine pelvic regions and verify the primary stability of miniscrews inserted into them. Forty bone blocks were extracted from bovine pelvic bones, 20 from iliac and 20 from pubic bone, all of them containing cortical bone about 1 mm thick. Half of the sections extracted from each bone were designated for histological evaluation of bone density (trabecular bone area - TBA) and the other half for bone mineral density (BMD) evaluation by means of central dual-energy X-ray absorptiometry (DEXA). Then, twenty self-drilling miniscrews (INP®, São Paulo, Brazil) 1.4 mm in diameter and 6 mm long were inserted into the bone blocks used for BMD evaluation. Peak implant insertion torque (IT) and pull-out strength (PS) were used for primary stability evaluation. It was found that iliac and pubic bones present different bone densities, iliac bone being less dense considering BMD and TBA values (P > 0.05). However, the miniscrew primary stability was not different when varying the bone type (P < 0.05). IT and PS were not influenced by these differences in bone density when cortical thickness was about 1 mm thick.

Bone mineral density

To verify whether bone mineral density (BMD) of cortical bone, trabecular bone, and total bone influence the primary stability of orthodontic miniscrews and to verify whether there is a correlation between the measurement of BMD by cone-beam computed tomography (CBCT) and central dual-energy x-ray absorptiometry (DEXA). Twenty bovine bone sections were extracted from the pubic and iliac bones from regions with cortical thicknesses of approximately 1 mm. The BMD of the total bone block was evaluated using two methods: CBCT and DEXA. The BMD of cortical, trabecular, and total bone in the region of interest (ROI) were also evaluated by CBCT. After scanning the bone blocks, 20 self-drilling miniscrews (INP®) 1.4 mm in diameter and 6 mm long were inserted into them. The peak implant insertion torque (IT) was registered. After this, the pull-out test (PS) was performed and the maximum force registered. The Pearson correlation test was applied to verify the correlations between variables. The BMD of the total bone block verified by CBCT and DEXA showed a positive and strong correlation (r = 0.866, P = .000). The BMD of the ROI for cortical bone influenced the IT (r = 0.518, P = .40) and the PS of miniscrews (r = 0.713, P = .001, Table 2). However, the total bone BMD (verified by CBCT and DEXA) and trabecular bone BMD presented weak and not statistically significant correlations with primary stability. There was a positive correlation between total bone block BMD measured by DEXA and CBCT. The cortical BMD influenced the IT and PS.

Placement and removal torque values of orthodontic miniscrew implants

The purpose of this study was to analyze the maximum insertion torque (MIT) and maximum removal torque (MRT) values of orthodontic miniscrews. Two hundred eighty titanium miniscrews were placed in several sites in the maxillae and mandibles of orthodontic patients to provide skeletal anchorage. Two types of miniscrews were used: predrilling, cylindrical miniscrews, and self-drilling, conical miniscrews. A force of 50 g was applied with nickel-titanium coil springs 2 weeks after placement. MIT and MRT values were assessed with a digital torque gauge. Torque values were subjected to the paired t test, Student t test, chi-square test, and 1-way analysis of variance (ANOVA) (P <0.05). MIT values were significantly higher for the self-drilling miniscrews (mean, 14.5 Ncm; 95% CI, 13.6-15.3) than for the predrilling miniscrews (mean, 9.2 Ncm; 95% CI, 8.6-9.9) in all implant sites. For both predrilling and self-drilling miniscrews, the highest MIT values were observed at the midpalatal suture site followed by the dentoalveolar bones of the mandible and maxilla, respectively. In contrast, MRT values were significantly higher for the predrilling miniscrews (mean, 22.6 Ncm; 95% CI, 22.0-23.3) than for the self-drilling miniscrews (mean, 17.6 Ncm; 95% CI, 16.8-18.4). Accordingly, the mean torque ratio of predrilling miniscrews was twice that of the self-drilling miniscrews. An inverse relationship between MIT and MRT values was observed. The results suggest that relatively lower MIT values were more favorable to osseointegration than higher values.

Orthodontic miniplate with tube as an efficient tool for borderline cases

An orthodontic miniplate tube device, the C-tube, was designed for use in patients for whom a conventional miniscrew is not suitable, such as those with narrow interradicular spaces, extended maxillary sinuses, dilacerated roots, or severe alveolar bone loss. After local anesthesia, 2 parallel horizontal incisions are made in the area of placement, and the periosteum is elevated. The C-tube is slipped under the mucosal flap and fixed with self-drilling miniscrews (diameter, 1.5 mm; length, 4 mm). Because the screws are short, there is adequate retention in the alveolar plate, and the clinician can avoid the increased morbidity of anchoring to the zygomatic buttress. This makes placement possible with superficial anesthesia. A small rolled tube at the head part can act as an orthodontic tube and accommodate archwires or as a hook to attach orthodontic elastics. However, in some patients with pneumatization or systemic diseases, such as diabetes mellitus, or in heavy smokers, cross-type C-tubes with longer miniscrews are recommend for better stability. This new type of orthodontic miniplate can be an effective alternative to conventional 1-component screws or miniplates in complex situations.

Friction heat during self-drilling of an orthodontic miniscrew

The aim of this study was to measure the heat generated when using a self-drilling miniscrew at speeds of 50, 100, 150, and 250 rpm. Specimens were classified into two groups: in the thin group the cortical bone thickness was 1.2 ± 0.02 mm on average and in the thick group it was 2.0 ± 0.03 mm on average. The thin group was used to model maxillary bone and the thick group to model mandibular bone in humans. The temperature in the 1.2-mm and 2.0-mm cortical bone specimens was measured according to revolution speed. As the revolution speed increased, the temperature significantly increased in both bone thicknesses. The temperature increased significantly more in the thicker cortical bone. The temperature increase in the 2.0-mm thick bone at 250 rpm exceeded 10°C, regarded as the threshold for bone damage in this study; other temperature increases were below this threshold. Installing self-drilling screws at high speeds with an implanter is not recommended; low speeds of less than 150 rpm should be used.

Assessment of Miniscrew Implant Stability by Resonance Frequency Analysis: A Study in Human Cadavers

Resonance frequency analysis (RFA) is a reliable, noninvasive method of measuring dental implant stability in vivo. However, the possibility of using this method to assess the stability of miniscrew implant has not been investigated. The purpose of this study was to evaluate the possibility of using RFA to assess the stability of miniscrew implants placed in different sites in the maxillae and mandibles of cadavers. Ten pairs of dentate maxillas and mandibles were retrieved from human cadavers. Two hundred self-drilling miniscrew implants of 1.5-mm diameter and 8-mm length were systematically inserted into the dentoalveolar bones aided by a 3-dimensional surgical guide. Maximum insertion torque value was recorded during implantation procedures. Assessment of primary stability was carried out immediately after implantation using the RFA method, and implant stability quotient (ISQ) values were recorded. Analysis of variance, the Student t test, and generalized estimating equation regression analyses were used to analyze the data. A strong correlation was found between maximum insertion torque and ISQ values (r = 0.873). Maximum insertion torque and ISQ values were significantly higher in the mandible (15.8 ± 3.3 N-cm and 71.7 ± 12.1) than in the maxilla (11.8 ± 3.8 N-cm and 56.0 ± 15.8). Miniscrews inserted in the posterior region of the maxilla exhibited the lowest ISQ values. No significant difference among ISQ values of miniscrews inserted in the anterior, middle, or posterior regions of the mandible was observed. Estimates of implant stability using resonance frequency analysis are highly correlated with maximum insertion torque. Further studies to assess the primary stability of miniscrew implants in the clinical situation are necessary.

Accurate pre-surgical determination for self-drilling miniscrew implant placement using surgical guides and cone-beam computed tomography

Miniscrew implants have proven to be effective in providing absolute orthodontic anchorage. However, as self-drilling miniscrew implants have become more popular, a problem has emerged, i.e. root contact, which can lead to perforation and other root injuries. To avoid possible root damage, a surgical guide was fabricated and cone-beam computed tomography (CBCT) was used to incorporate guide tubes drilled in accordance with the planned direction of the implants. Eighteen patients (5 males and 13 females; mean age 23.8 years; minimum 10.7, maximum 45.5) were included in the study. Forty-four self-drilling miniscrew implants (diameter 1.6, and length 8 mm) were placed in interradicular bone using a surgical guide procedure, the majority in the maxillary molar area. To determine the success rates, statistical analysis was undertaken using Fisher's exact probability test. CBCT images of post-surgical self-drilling miniscrew implant placement showed no root contact (0/44). However, based on CBCT evaluation, it was necessary to change the location or angle of 52.3 per cent (23/44) of the guide tubes prior to surgery in order to obtain optimal placement. If orthodontic force could be applied to the screw until completion of orthodontic treatment, screw anchorage was recorded as successful. The total success rate of all miniscrews was 90.9 per cent (40/44). Orthodontic self-drilling miniscrew implants must be inserted carefully, particularly in the case of blind placement, since even guide tubes made on casts frequently require repositioning to avoid the roots of the teeth. The use of surgical guides, fabricated using CBCT images, appears to be a promising technique for placement of orthodontic self-drilling miniscrew implants adjacent to the dental roots and maxillary sinuses.

Clinical Study of Temporary Anchorage Devices for Orthodontic Treatment

The aim of this retrospective study was to determine factors that might cause complications in use of temporary anchorage devices (TADs) for orthodontic anchorage. We investigated 904 TADs in 455 patients. Clinical diagnoses requiring orthodontic treatment were malocclusion, jaw deformity, various syndromes, cleft lip and palate and impacted teeth. All patients underwent surgery at Tokyo Dental College Chiba Hospital between November 2000 and June 2009. Three kinds of titanium screw of different diameter and length were used: self-drilling mini-screws (Dual Top Autoscrew® and OSAS®), pre-drilling micro-screws (K1 system®) and palatal screws (PIAS®). Mini-plates fixed with 2 or 3 screws (SAS system®) were also used for skeletal anchorage. Patients were aged between 8 and 68 years (25.7±9.8 years). A total of 460 screw-type and 444 plate-type TADs were used. These comprised the following: mini-plates, 444; self-drilling mini-screws, 225; pre-drilling micro-screws, 83; and palatal screws, 152. Each type of implant had a high success rate of over about 90%. Failure rates were as follows: micro-screws, 7%; mini-screws, 6%; palatal implants, 11%; and mini-plates, 6%. Inflammation rate occurring in soft tissue surrounding TADs was follows: plate-type, 7.6%; mini-screws, 1.3%; micro-screws, 0%; and palatal implants, 2.5%. Inflammation frequencies depended on degree of mucosal penetration. Granulation rate in soft tissue surrounding TADs occurred as follows: micro-screws, 5.7%; self-drilling mini-screws, 0%; palatal screws, 0.6%; plate-type, 0.9%. Both plate- and screwtype orthodontic implants showed excellent clinical performance.

Biomechanical Properties of Orthodontic Miniscrews. An In-vitro Study

Comparing five commercially-available miniscrew types for skeletal anchorage with regard to the biomechanical properties influencing their primary stability. Included in this study was a total of 196 self-tapping and self-drilling miniscrews having a diameter of 2 mm (or the largest possible diameter of the manufacturer in question), a thread shaft length of 10 mm, or the longest miniscrew supplied by the manufacturers. The screw types tested were the FAMI 2, Orlus mini-implant, T.I.T.A.N. Pin, tomas-pin and Vector TAS. Insertion and loosening torque measurements, and pullout tests in axial (0 degrees), 20 degrees and 40 degrees directions, as well as test series with and without pilot hole drilling were performed. Bovine femoral heads having the same bone mineral density (BMD) were used as bone-testing material. Higher insertion torques were found for the cylindrical FAMI 2 screw, the conical Orlus mini-implant and the Vector TAS screw (with mean values of 39.2 Ncm, 32.1 Ncm and 49.5 Ncm) than for the cylindrical T.I.T.A.N. pin and tomas-pin. Insertion without predrilling led the insertion torques of all five screws to rise significantly. We noted statistically significant differences among the five screws in the pullout tests. Those highly significant differences at axial (0 degrees) and 20 degrees angles were not apparent at the 40 degrees pullout angle. Compared with the pullout forces (load) in the axial direction, the cylindrical screws' load values decreased markedly according to the angle (by up to -46.6%). The reduction in pullout force in conjunction with an increasing angle was much less pronounced in the conical screws (-0.8% to -29.0%). The tomas-pin demonstrated the highest pullout force and stiffness values throughout the tests. A total of five tomas-pins, two Orlus mini-implants and one FAMI 2 screw fractured during the pullout tests. Results from our insertion torque measurements suggest that a conical screw design will provide greater primary stability than cylindrical screw types. The cylindrical screw design's superiority was evident in the pullout tests. All the miniscrews' primary stability rose after drill-free insertion. The tomas-pin screws, although biomechanically superior to the other screws, were most prone to fracture.

Stability of miniplates and miniscrews used for orthodontic anchorage: experience with 492 temporary anchorage devices

The aim of this retrospective study was to evaluate systematically the potential factors that influence failure rates of temporary anchorage devices (TADs) used for orthodontic anchorage. Data on 492 TADs (miniplates, pre-drilling miniscrews, and self-drilling miniscrews) in 194 patients were collected. The factors related to TAD failure were evaluated using univariate analysis and multivariate forward stepwise logistic regression analysis. There were no significant differences in failure rates among the TADs for the following variables: gender, type of malocclusion, facial divergency, implantation site (buccal, lingual, or crestal/midpalatal), location (anterior or posterior), method of force application (power chain or Ni-Ti coil spring), arch (upper or lower), type of soft tissue (attached gingiva or removable mucosa), and most of the cephalometric measurements that reflect dento-cranio-facial characteristics. An increased failure rate was noted for the self-drilling miniscrew type of TAD, TADs used for tooth uprighting, those inserted on bone with lower density, those associated with local inflammation of the surrounding soft tissue, those loaded within 3 weeks after insertion, and those placed in patients with greater mandibular retrusion. Failure rates of the self-drilling miniscrews installed by an oral surgeon and by an orthodontist did not differ significantly. Inflammation of soft tissue surrounding a TAD and early loading within 3 weeks after insertion were the most significant factors predicting TAD failure. Both orthodontists and oral surgeons who install orthodontic TADs must undergo sufficient training to achieve clinical excellence.

Comparison of the loading behavior of self-drilling and predrilled miniscrews throughout orthodontic loading

A predrilled miniscrew, when used as a temporary anchorage device in the infrazygomatic crest of the maxilla, can be displaced under orthodontic loading. The purpose of this retrospective cephalometric study was to compare the loading behavior of predrilled and self-drilling miniscrews placed in the infrazygomatic crest of the maxilla. The subjects were 32 women who had miniscrews in the infrazygomatic crest of the maxilla as skeletal anchorage for en-masse anterior retraction and intrusion; 16 had predrilled miniscrews, and 16 had the self-drilling type. The miniscrews were all 2 mm in diameter and 10 to 17 mm long. They were loaded with nickel-titanium closed-coil springs 2 weeks after placement. All the miniscrews remained stable, without detectable mobility or loosening, throughout the treatment period. Cephalometric radiographs were taken immediately before force application (T1) and at least 5 months later (T2). The T1 and T2 cephalometric tracings were superimposed to determine whether any displacement of the miniscrews had occurred. The predrilled and self-drilling miniscrews were all significantly displaced in accordance with the force direction of the nickel-titanium coil springs. The amounts of miniscrew displacement were similar between the predrilled and self-drilling miniscrews, and were correlated to the length of the loading period. The displacements were 0.0 to 1.6 mm with extrusion, 1.5 mm with forward or backward tipping at the screw tail, and 1.5 mm with forward tipping at the screw head. The loading behaviors of predrilled and self-drilling miniscrews were similar in the infrazygomatic crest of the maxilla. These miniscrews were all subject to displacement under orthodontic loading, and the amount of the displacement was correlated to the length of the loading period, although they remained stable without detectable mobility or loosening.

Mechanical Characteristics of Various Orthodontic Mini-screws in Relation to Artificial Cortical Bone Thickness

To evaluate the effect of cortical bone thickness on the maximum insertion and removal torque of different types of self-drilling mini-screws and to determine if torque depends on the screw design. Three different types of self-drilling mini-screws (cylindrical type [Cl], taper type [Ta], taper type [Tb]) were inserted with the use of a driving torque tester at a constant speed of 3 rotations per minute. Experimental bone blocks with different cortical bone thicknesses were used as specimens. Differences in the cortical bone thickness had little effect on the maximum insertion and removal torque in Cl. However, with Ta and Tb, the maximum insertion torque increased as the cortical bone thickness increased. The maximum insertion torque of Tb was highest in all situations, followed by Ta and Tb, in that order. Cl showed less torque loss in all cortical bone thicknesses and a longer removal time compared to Ta or Tb. There were significant relationships between cortical bone thickness, maximum insertion and removal torque, and implantation time in each type of self-drilling mini-screw. Since different screw designs showed different insertion torques with increases in cortical bone thickness, the suitable screw design should be selected according to the cortical thickness at the implant site.