Introduction
The aim of this research was to verify that ultraviolet light (UV)–photofunctionalization improves the success rate and biomechanical stability of miniscrews regardless of length, and to evaluate the comparability of biomechanical stability between UV-photofunctionalized miniscrews with short lengths and untreated miniscrews with conventional lengths.
Methods
Eight male beagles (age, 12-15 months; weight, 10-13 kg) received a total of 64 miniscrews, including 7-mm and 4-mm untreated and UV-photofunctionalized, acid-etched miniscrews with the use of a random block design. The cumulative success rates were examined in all studied groups. The insertion and removal torques and screw mobility were measured. Microcomputed tomographic scans and histomorphometric analyses were performed at 8 weeks postoperatively.
Results
The success rates of 7-mm UV-untreated and UV-photofunctionalized miniscrews were 87.5% and 100%, respectively, vs 43.8% for the 4-mm UV-untreated and 4-mm UV-photofunctionalized miniscrews. The rates were significantly different in accordance with the length ( P <0.001). There were no differences in the insertion and removal torque and screw mobility between groups according to the length or UV treatment. However, the 4-mm UV-untreated miniscrews yielded a mean bone area ratio of 6.35 ± 7.43%, whereas the 7-mm UV-photofunctionalized miniscrew yielded a mean ratio of 32.17 ± 8.34% ( P = 0.037).
Conclusions
The UV-photofunctionalization significantly increased the biomechanical stability and led to increased bone and miniscrew contact area in dogs with miniscrews of the same length. However, the most important factor that affected the success rate of the miniscrew was the length.
Highlights
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The ultraviolet light–photofunctionalization increased biomechanical stability.
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The ultraviolet light–photofunctionalization increased bone and miniscrew contact area.
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Screw length was the factor that most affected the success rate.
Temporary anchorage devices, such as an orthodontic titanium miniscrews and miniplates, are frequently used for absolute anchorage control in orthodontic treatments as a way to avoid unwanted tooth movements. Specifically, miniscrews are extensively applied in clinical practice because they are not associated with additional procedures, are inexpensive, and can be placed immediately without patient discomfort. ,
However, it was reported that a failure rate of miniscrews was approximately 13.5%-14%. , Therefore, improvement of the success rate of miniscrew implants is important in daily practice as it increases the efficiency of clinical treatment, circumvents the burden associated with the need to have the miniscrew reinserted by the orthodontist, and increases posttreatment satisfaction by minimizing patient discomfort. The various etiologies for miniscrew failures are already known, but one of the most obvious reasons is the contacts between the root of the neighboring teeth and the miniscrews during their placements. , The root contact at the time of insertion led to failures that were more than 3 times higher than the rate without root contact. When the miniscrew is in contact with the root, a smaller volume of osseous tissue is formed around the miniscrew, and the root contact can create greater stresses that may result in surrounding bone resorption, thereby adversely affecting miniscrew stability.
Miniscrews, which are mainly used in orthodontic treatments, have lengths in the range of 6-8 mm. A reduction in the physical length of the miniscrew can eliminate the possibility of contact of the miniscrew with the root. The thickness of the cortical bone was important in the stability of the miniscrew, and the length of the miniscrew below the cortical bone was not considered to be significantly correlated with stability. Suzuki et al reported that the cutoff point for the minimum bone (total length of the miniscrew within the bone) required for miniscrew stabilization was approximately 3.8 mm, especially in the mandibular alveolar bone.
Ultraviolet light (UV)–mediated photofunctionalization is known to increase osseointegration on the surface of titanium dental implants. Titanium implants are constantly absorbing organic impurities while being transported and stored for commercialization. When UV is irradiated on the titanium implant surface, contaminants, such as hydrocarbons on the titanium surface, are eliminated, the deposition rate of blood plasma proteins is increased, and the adhesion of osteogenic cells to the titanium surface is achieved. , Funato et al , reported that the use of UV-photofunctionalization for dental implants allowed the faster execution of the loading protocol without compromising the success rate, even in various clinically challenging and compromised bone conditions. Soltanzadeh et al also reported that UV-functionalized surface treatments yielded a 2.4 times higher osseointegration strength than untreated implants in a rat model.
Therefore, this study aimed to verify that the UV-photofunctionalization improves the success rate and biomechanical stability of miniscrews regardless of length, and to evaluate the comparability of biomechanical stability between the UV-photofunctionalized miniscrews with short lengths and the untreated miniscrews with conventional lengths. This study hypothesized that the UV-functionalization improves the success rates and biomechanical stabilities of the miniscrews regardless of length, and the biomechanical stabilities of the UV-functionalized miniscrews with short lengths are comparable to those of the untreated miniscrews with conventional lengths, as demonstrated in a beagle model.
Material and methods
This study was designed in accordance with the guidelines for preclinical research, including minimizing the number of experimental animals, refining procedures for improving animal welfare, and replacing the use of animals with alternative protocols. The experiments were performed in an animal laboratory accredited by the International Association for Assessment and Accreditation of Laboratory Animals after receiving ethical approval from the institutional Animal Care and Use Committee of the Yonsei Medical Center, Seoul, South Korea (Permission no. 2018–0003). This study was designed in the form of a random block to avoid individual- and location-related discrepancies in 8 adult beagle dogs.
Eight male beagles (age, 12-15 months; weight, 10-13 kg) received 7-mm and 4-mm untreated and UV-photofunctionalized acid-etched miniscrews. A total of 64 miniscrews were used (8 miniscrews per dog). Among the 64 miniscrews used, 32 were used in the untreated groups (including 7-mm and 4-mm miniscrews), and the remaining 32 were equally divided in the UV-photofunctionalized groups (7-mm and 4-mm miniscrews). We used a self-drilled, tapered-type, orthodontic Ti-6Al-4V miniscrew (diameter, 1.8 mm; length, 7 mm and 4 mm; single-threaded) (ORLUS; Ortholution, Seoul, South Korea). UV-photofunctionalization was performed by treating samples with UV for 12 minutes with a photo device (TheraBeam Super Osseo; Ushio, Tokyo, Japan) immediately before insertion. On the basis of a pilot study, the sample size was calculated (G∗Power 3, University of Düsseldorf, Düsseldorf, Germany) using a significance level of 0.05, a power of 90%, and an effect size of 0.25, to detect mobility changes in each group over time. A minimum of 8 samples were used per group.
The experimental animals were injected subcutaneously with 0.05 mg/kg atropine (Kwang Myung Pharmaceutical, Seoul, South Korea), followed by intravenous xylazine 2 mg/kg (Rompun; Bayer Korea, Seoul, South Korea) and 10 mg/kg ketamine (Ketalar; Yuhan, Seoul, South Korea) to induce general anesthesia. Animals were maintained with inhalational isoflurane administered at 2% (Hana Pharm, Seoul, South Korea). Each animal’s temperature was maintained with a heating pad, and vital signs were monitored with an electrocardiogram.
Before placement, local anesthesia was administered using 2% lidocaine (1:100,000 epinephrine), and a 2 mm gingival incision was made with a no. 12 blade at the implantation site to prevent tissue impingement with concurrent saline solution irrigation. The insertion of each miniscrew adhered to a random block design, and the positions were equivalently distributed between the groups ( Table I ). The miniscrews were placed in the intraradicular spaces of the first molar, the fourth premolar, the third premolar, and the second premolar in the mandible ( Fig 1 ). The maxilla was not used in this study because of the possibility of sinus perforation. The placement was performed with the use of a motor-driven handpiece (e-Driver; Osstem Implant Co, Busan, South Korea) at a drilling speed of 60 revolutions per minute by 1 investigator (S.-H.C.), with the use of 90 angulations with respect to the gingival surface, in consideration of the buccolingual width of the alveolar bone in each experiment. Full insertion was confirmed for all the miniscrews by checking the bone contact of the final screw thread with a dental explorer. All the animals were killed 8 weeks after the initial operation.
| Tooth | Beagles 1 and 5 | Beagles 2 and 6 | Beagles 3 and 7 | Beagles 4 and 8 | ||||
|---|---|---|---|---|---|---|---|---|
| Left mandible: histology | Right mandible: removal torque | Left mandible: removal torque | Right mandible: histology | Left mandible: removal torque | Right mandible: histology | Left mandible: histology | Right mandible: removal torque | |
| PM2 | C7 | V4 | V7 | C4 | C7 | V4 | V7 | C4 |
| PM3 | C4 | C7 | V4 | V7 | C4 | C7 | V4 | V7 |
| PM4 | V7 | C4 | C7 | V4 | V7 | C4 | C7 | V4 |
| M1 | V4 | V7 | C4 | C7 | V4 | V7 | C4 | C7 |
The cumulative success rate of each group was examined during the experimental period. The criteria for the success of using the miniscrews included the absence of clinically detectable mobility (movement greater than 1 mm). ,
The insertion torque (N·cm) was maximized when the miniscrew was fully placed inside the bone. The maximum insertion torque was measured during the insertion by a motor-driven handpiece (e-Driver; Osstem Implant Co). The screw mobility of each miniscrew was measured twice immediately after insertion, at 7 days, 28 days, and immediately before the removal (56 days) using the Periotest M (Siemens AG, Munich, Germany) and AnyCheck devices (IMT-100; NeoBiotech Co, Seoul, South Korea). The stability increased, and the value of the periotest decreased (periotest value [PTV]). By contrast, the AnyCheck indicated increased stability when the measured value (initial stability test value [IST]) was increased.
Among the 64 miniscrews tested, the removal torque was measured immediately before sacrifice according to random block designs using a motor-driven handpiece (e-Driver; Osstem Implant Co) ( Table I ). The tissue blocks that contained the remaining miniscrews immediately after sacrifice were harvested and fixed with 10% formalin (pH = 7.4) for 2 weeks.
Before histologic processing, a microcomputed tomographic scan was performed (SkyScan 1173; Bruker-microCT, Kontich, Belgium) at a resolution of 35 μm (achieved using 130 kV and 60 μA) and reconstructed using image analysis software (CT analyzer; version 1.17.7.2; Bruker-microCT). The relevant conditions were as follows: tube voltage of 130 kVp, tube current of 60 μA, exposure time of 500 ms, pixel size of 24.15 μm, and a rotation step of 0.3° with an aluminum filter with a thickness of 1 mm. The pixel size was set to 24.15 μm, and a high-resolution image was obtained with a scanning angular rotation of 180°. The software NRecon (version 1.7.0.4; Bruker-microCT) was used for image reconstruction. A region-of-interest (2240 × 2240 pixels) was defined, wherein the bone area (BA) and bone volume/total volume (BV/TV) were calculated with the use of a concentric 0.5 mm radius ring which was centered on the miniscrew. The BA ratio (the composition of the total augmented area was identified, and the relative areas of bone were separately detected manually, calculated, and multiplied by 100%), and the BV/TV ratios (mineralized bone volume over total volume multiplied by 100%) in each group were estimated.
After microcomputed tomographic scanning, the specimens were dehydrated using increasing ethanol solution concentrations (70%, 80%, 95%, and 100%) and embedded in methyl methacrylate resin (Technovit 7200 VLC; Heraeus Kulzer, Wehrheim, Germany). For each miniscrew site, 1 buccolingual section parallel to the miniscrew axis was prepared that represented the central area of the site. The tissue slides were produced using EXAKT cutting (EXAKT 300; EXAKT, Norderstedt, Germany) and grinding (EXAKT 400CS; EXAKT) systems to a final thickness of 40 ± 5 μm, and were stained with Goldner trichrome. All the stained tissues were digitized using a Pannoramic 250 Flash III digital scanner (3DHISTECH, Budapest, Hungary) and gained optical images using CaseViewer (version 2.0; 3DHISTECH). The bone-implant contact (BIC) ratio (the length of the bone which was in direct contact with the bone divided by the total length of the implant interface multiplied by 100%) in each group was measured using an image analysis software (Image-Pro Plus; Media Cybernetics, Silver Springs, Md).
Statistical analysis
All statistical analyses were performed with the SPSS (version 21.0; IBM Korea, Seoul, South Korea). The Shapiro-Wilk test was applied to verify the data distribution and normality. The data were normally distributed. Kaplan-Meier survival curves were used to analyze the cumulative survival of the miniscrews in each group during the experimental period. Two-way analysis of variance (ANOVA) was used to compare the insertion torque, removal torque, BA, BV/TV, and BIC ratios, according to the surface treatment (untreated vs UV-photofunctionalized miniscrews) and the length of the miniscrew (7 mm vs 4 mm length). The time-dependent comparisons of the measured values (PTV and IST) of the mobilities of the miniscrews over time were on the basis of repeated measure ANOVA with Bonferroni correction. Statistical significance was considered when the P value was less than 0.05.
Results
The success rates of the 7-mm UV-untreated (n = 14/16) and UV-photofunctionalized (n = 16/16) miniscrews were 87.5% and 100%, respectively ( Fig 2 ). By contrast, the success rates of the 4-mm UV-untreated (n = 7/16) and UV-photofunctionalized (n = 7/16) miniscrews were the same at 43.8% (9 failures in each group). There was a statistically significant difference in the success rates between the 2 groups according to the length of the miniscrew ( P <0.001).
