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Evaluation of Thin Amorphous Calcium Phosphate Coatings on Titanium Dental Implants Deposited Using Magnetron Sputtering Sou Yokota, DDS,* Naruhiko Nishiwaki, MS,† Kyosuke Ueda, PhD,† Takayuki Narushima, PhD,† Hiroshi Kawamura, DDS, PhD,* and Tetsu Takahashi, DDS, PhD*
alcium phosphate (CaP) coatings are used extensively with dental implants to induce faster bone adaptation. These coatings also improve osseointegration because they are biocompatible and have osteoconductive properties, which result in the shortening of the healing process and an increase in the clinical success rate.1–4 Various methods have been used to coat the surfaces of implants, including plasma spraying, sputter deposition, and solgel coating. In recent times, the plasma-spraying method has become the most widely used one for the deposition of CaP coatings. Implants with plasma-sprayed coatings of CaP have shown faster bone apposition than did the uncoated implants, both in animal studies and in clinical practice.5 However, the plasma-spraying method has a number of disadvantages: the resultant coatings are highly porous, and there are high residual stresses at the implantcoating interfaces because of the high temperature involved in the process.6
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*Division of Maxillofacial Surgery, Department of Oral Medicine and Surgery, Graduate School of Dentistry, Tohoku University, Sendai, Japan. †Department of Materials Processing, Tohoku University, Sendai, Japan.
Reprint requests and correspondence to: Sou Yokota, DDS, Division of Maxillofacial Surgery, Department of Oral Medicine and Surgery, Graduate School of Dentistry, Tohoku University, 4-1 Seiryo, Aoba, Sendai 980-8575, Japan, Phone: +81-22-717-8350, Fax: +81-22-717-8354, E-mail: [email protected] ISSN 1056-6163/14/02303-343 Implant Dentistry Volume 23 Number 3 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000098
Objective: Calcium phosphate is used for dental material because of its biocompatibility and osteoconductivity. Amorphous calcium phosphate (ACP) coatings deposited by magnetron sputtering can control their thickness and absorbability. This study aimed to evaluate and characterize ACP coatings deposited via magnetron sputtering. It was hypothesized that ACP coatings would enhance bone formation and be absorbed rapidly in vivo. Methods: ACP coatings that are 0.5 mm in thickness were deposited via magnetron sputtering on dental implants. Uncoated implants served as controls. The effect of the ACP coatings in vivo was investigated in New Zealand white rabbit. To evaluate the effect of the ACP coatings on the bone response of the implants, the removal torque, implant stability quo-
tient, and histomorphometric analysis were performed on the implants at 1, 2, and 4 weeks after implantation. Results: Results of the x-ray diffraction analyses confirmed the deposition of ACP coatings. Images from the scanning electron microscopy revealed that the coatings were dense, uniform, and 0.5 mm in thickness and that they were absorbed completely. Mechanical stability and bone formation in the case of the ACP-coated implants were higher than those of control. Conclusion: These results suggest that implants coated with thin ACP layers improve implant fixation and accelerate bone response. (Implant Dent 2014;23:343–350) Key Words: animal experiments, amorphous calcium phosphate, dental implant coating, RF magnetron sputtering
Moreover, the plasma-spraying method is not effective for coating tiny dental implants that have complex shapes. In addition, it has been reported that CaP coatings on dental implants undergo delamination and CaP-coated implants fail at the implant-coating interface, when subjected to functional loading.7–10 Physical vapor deposition can be a suitable technique for obtaining uniform
and dense coatings of CaP on metal substrates. Radiofrequency (RF) magnetron sputtering has been used in a wide variety of areas for coating thin films with excellent adherence onto substrates and for applying coatings of CaP films on commercially pure titanium (Ti) implants. Its low processing temperature is also an advantage of the RF magnetron sputtering technique for depositing CaP coatings on Ti substrates
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because the mechanical properties of the substrates may be degraded at high processing temperatures.11–14 The thicknesses of the coating films can also be controlled more precisely and with greater ease by modulating the sputtering duration. RF magnetron sputtering can also be used to deposit coatings of amorphous calcium phosphate (ACP) on substrates. ACP is the precursor of bone tissue and is absorbed rapidly in vitro.12,15 In fact, the concentration of calcium and phosphorus ions from ACP coating films was higher than that from hydroxyapatite (Ca10(PO4)6(OH)2; HA) coating films, as determined by an immersion test. The use of CaP coatings as an alternative to plasma-sprayed HA coatings should help prevent the defects that result from the use of the latter. CaP coatings contribute to bone response in the early stages of implantation and disappear after osseointegration. Thus, adverse effects similar to those resulting from the use of plasma-sprayed HA coatings can be avoided. The aim of this study was to evaluate the biological behavior and characteristics of ACP coatings deposited on dental implants via RF magnetron sputtering using an in vivo model.
MATERIALS
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Fig. 1. A, XRD pattern showing the a-Ti peaks and halo peak associated with CaP phases. B, EXD spectra showing the presence of Ca, P, O, and Ti on the surface of the coated implant. C, Scanning electron micrograph of the ACP coating. The coated surface is dense and uniform and does not show any cracks or detachment. The thickness of the coating film was maintained at 0.5 mm.
METHODS
Preparation of the Implants and Their Coating With ACP
A total of 72 screw-shaped Ti (commercially pure titanium, grade 4) implants 3.75 mm in diameter and 8.5 mm in length (Nobel Biocare, Gothenburg, Sweden) were used in this study. All the implants were treated as having machined surfaces. The implants were divided into 2 groups: a coated group and an uncoated (control) group. In the case of the coated group, the ACP coatings were deposited via RF magnetron sputtering (MS-320; Universal Systems, Co, Ltd, Tokyo, Japan) using a target of hot-pressed b-tricalcium phosphate (Ca3(PO4)2) having a relative density of more than 99.6%. The implants were not heated intentionally during the sputtering, and their temperature was less than 373 K. The thickness of the coating films was maintained at 0.5 mm by controlling the sputtering
Fig. 2. A and C, SEM images of ACP-coated implants removed 1 week and 2 weeks after implantation in femurs, respectively. E and F, EDX spectra of the implants shown in (A) and (C), respectively. B and D are higher magnification images of the implants shown in (A) and (C), respectively.
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duration. The different phases of the coating films were confirmed using x-ray diffraction (XRD) analysis, and the chemical composition of the coating films was determined using energydispersive x-ray spectroscopy (EDX) (DX-4; EDAX, Mahwah, NJ). The topographies of the coatings and the crosssections of the implants were analyzed using scanning electron microscopy (SEM) (XL-30FEG; Philips, Amsterdam, the Netherlands). Animal and Surgical Procedures
Fig. 3. RFA was performed on the ACP-coated and control implants placed in the femurs (n ¼ 6). There was a significant difference in the 4-week ISQ values of the ACP-coated implants and those of the control implants (P , 0.05). The error bars represent the standard deviation.
All protocols for the animal experiments were reviewed and approved by the Animal Care and Use Committee of the Tohoku University. Eighteen male New Zealand white rabbits (18 weeks old; mean weight, 3150 g) were used in the present study. Before the surgical procedure, the animals were anesthetized with intramuscular injections of xylazine (20 mg/mL; Bayer, Leverkusen, Germany) and were administered pentobarbital (25 mg/kg; Kyoritsu Seiyaku, Tokyo, Japan) intravenously via the lateral veins in their ears. Infiltration anesthesia with 2% lidocaine containing epinephrine (Astra Zeneca, London, United Kingdom) in the ratio of 1:800,000 was administered at the surgical site. To place the implants, an incision was made on the medial surfaces of the femur and tibia, and the implantation sites were the proximal metaphyses of the femur and the tibia. Every animal received an implant in each proximal femur and proximal tibia. Muscle, fascia, and skin were sutured separately with resorbable sutures. All animals survived the procedures, and no pathological weight loss was seen in any of them. There were no signs of postoperative wound infection, fracture, or any other clinical anomalies that might have indicated poor healing during the postoperative period. At the time of the euthanization of the animals, all the implants had attained rigid fixation; infection and bone resorption were not observed. Resonance Frequency Analysis
Fig. 4. Removal torque force was measured at the femur. There was a significant difference between the 4-week removal torque force values of the ACP-coated implant and those of the control implants.
In all the rabbits, resonance frequency analysis (RFA) was performed for the femoral and tibial implants both at the time of implant
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Fig. 5. Histological evaluation after implantation. Undecalcified ground sections of implantfemur specimens stained with toluidine blue. The images in (A), (B), and (C) are of an uncoated implant. The images in (D), (E), and (F) are of an ACP-coated implant. Ti indicates titanium implant; CB, cortical bone; NB, new bone.
placement and euthanization. Before the analyses, the rabbits were anesthetized using the procedure described earlier, and the implants were accessed surgically. The area around each implant was gently debrided and cleared of granulation tissue. The abutment for RFA (SmartPeg; Osstell AB, Gothenburg, Sweden) was mounted on each implant before analysis and hand tightened. Then, the implant stability quotient (ISQ) value of each implant was measured using an Osstell stability meter (Osstell AB). Measurement of the Removal Torque Values
The values of the peak loosening torque for all the femoral implants were determined using a removal torque gauge (Tohnichi Mfg, Co, Ltd, Tokyo, Japan). Torque was applied in the counterclockwise direction until the implant
turned. This operation was performed under anesthesia, before the animal was euthanized. The removal torque reflects the strength of the bond between the bone tissue and the implant.16 Preparation of Bone-Implant Specimens and Their Histomorphometric Analysis
To perform histological and histomorphometric analyses, the bilateral tibial implants and the tissue surrounding the implants were removed en bloc, and the soft tissue adhering to the implants was dissected. The bone-implant specimens were immersed in 10% neutral buffered formaldehyde for 2 weeks and dehydrated in a graded series of ethanol solutions (70%–100%) under vacuum, washed with acetone, and then embedded in methyl methacrylate (Wako Pure Chemical Industries, Ltd, Tokyo, Japan). After polymerization, the blocks of the specimens were cut into
approximately 300-mm-thick sections using a diamond saw microsectioning system. Three slides were prepared for each implant by sectioning the implant perpendicular to the bone axis. All the sections of a particular implant were made sequentially through the same plane. Then, each sectioned slide was sequentially ground with sandpapers with grits of 800 to 4000 and stained using toluidine blue. To measure the bone-to-implant contact (BIC) values and to observe the bone response around the implants, images of the slides of the implants were taken using an optical microscope with a SPOT digital camera (Leica Microsystems, Wetzlar, Germany) and analyzed using an image analysis software (Image J; National Institutes of Health, Bethesda, MD). The BIC values were calculated only for the cortical bone regions. All the implants used in this study were inserted monocortically, and the length of the direct contact between the bone and the implant was measured along the cortical bone under 3100 magnification. The other half of the embedded and sectioned bone-implant specimens was evaluated via SEM (JSM-6500F; Nihondenshi, Tokyo, Japan) using backscattered electrons (BSE). The sectioned blocks were glued onto SEM stubs and coated with a thin layer of osmium via vapor deposition before being mounted in the microscope. Statistical Analysis
Statistical analysis was performed on the values of the removal torque and ISQ and also on those of the BIC. The differences between the ACP-coated implants and the controls were analyzed for each variable using the Student t test (n ¼ 6). Values with P , 0.05 were accepted as being statistically significant.
RESULTS Table 1. Mean BIC Values
Noncoated ACP coated P
Characteristics of the ACP Coating Film
1 Week (Mean 6 SD)
2 Week (Mean 6 SD)
2 Week (Mean 6 SD)
18.26 6 5.27 20.23 6 5.9 0.29
49.31 6 15.05 63.41 6 11.6 ,0.01
64.76 6 14.45 78.49 6 8.83 ,0.01
There was no significant difference between the 2 groups after 1 week. After 2 and 4 weeks, the ACP-coated group had significantly higher values than those of the control group (P , 0.01).
The phases of the ACP coating film were determined using XRD analysis, and its chemical composition was determined using EDX (DX-4; EDAX). The cross-sections of the implants were observed using SEM (XL-30FEG; Philips). The XRD pattern of the coating
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film is shown in Figure 1, A. No crystalline peaks were observed in the pattern. The halo peak indicates the formation of an ACP phase,17 which was confirmed by the results of the EDX as well (see Fig. 1, B), with the signals indicating the presence of P and Ca. The coating film was dense, uniform, and crack free, and no detachment between the coating film and the implant was observed in the SEM images (Fig. 1, C). The implants retrieved by applying a removal torque were also observed using EDX and SEM. The ACP coating films could be partially observed 1 week after implantation; however, they were not observed after 2 weeks (Fig. 2). Mechanical Stability of the Implants
The ISQ values 1, 2, and 4 weeks after implantation are shown in Figure 3. The ISQ values increased with the duration of implantation, suggesting that osseointegration increased with time. There was no significant difference in the 1- and 2-week values of the coated and control implants (P . 0.05); however, there was a significant difference in their 4-week values (P , 0.05). The removal torque values are shown in Figure 4. The removal torque increased with the duration of implantation, both in the case of the ACP-coated implants and in the case of the control implants; however, no statistical differences were found between the 1- and 2-week values of the 2 groups. On the other hand, a statistically significant difference was noticed between the 4-week values of the 2 groups (P , 0.05). Histological Analysis and Backscattered SEM of the Bone-Implant Specimens
Fig. 6. Backscattered SEM images. The images in A, C, and E are of an uncoated implant. The images in B, D, and F are of an ACP-coated implant. 1 week (A and B), 2 weeks (C and D), and 4 weeks after implantation (D and E). The black arrows indicate bone ingrowth.
The SEM images of the implants and bone tissue in the undecalcified sections, which were dyed with toluidine blue, are shown in Figure 5. Formation of bone directly on the implant surface was noticed in all the specimens, and there were no significant differences between the implants and the adjacent tissue. In most of the sections, the upper threads of the implant had integrated with the cortical bone, with the rest of the implant surface being in direct contact with the bone marrow. In addition, formation of new bone
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Table 2. Rate of Bone Augmentation in the Implant Threads
Noncoated ACP coated P
1 Week (Mean 6 SD)
2 Week (Mean 6 SD)
4 Week (Mean 6 SD)
52.34 6 5.36 64.11 6 5.14 ,0.01
82.03 6 4.08 85.30 6 6.09 0.30
80.40 6 5.20 93.56 6 2.63 0.11
There was a significant difference between the 2 groups after 1 week. On the other hand, after 2 and 4 weeks, the 2 groups did not show a significant difference (P , 0.01).
between the implant and the cortical bone was observed in all sections. No histological signs of adverse host responses, such as necrosis or osteolysis, were detected in any specimen. Three sections of each specimen were used for determining the BIC. The BIC values were calculated only over cortical bone. There was no significant difference between the ACP-coated and control implants after 1 week. On the other hand, after 2 and 4 weeks, the ACP-coated implants showed significantly higher BIC values compared with those of the control implants (Table 1). SEM of BSE showed that the implant threads contained mineralized lamellar bone and portions of newly formed bone with typical osteocyte lacunae and blood vessel spaces (Fig. 6). The area of bone surrounding the implant threads was calculated and labeled as the percent bone area (Table 2). The value of this parameter at 1 week in the case of the ACP-coated implants was higher than that in the case of the uncoated implants.
DISCUSSION Coatings deposited by the RF magnetron sputtering method are more uniform and bond better with substrates than those deposited by plasma-spraying methods, while not affecting the smoothness of the substrates. In addition, CaP layers deposited by this technique are also biocompatible and can enhance apatite formation.18–20 Moreover, the RF magnetron sputtering method is a low-temperature process and can thus be used to deposit layers of ACP. ACP is the precursor of bone apatite and tends to dissolve readily owing to its amorphous nature. However, ACP layers deposited by RF magnetron sputtering have not been previously
analyzed in vivo. The aim of this study was to elucidate the characteristics of ACP coatings deposited using RF magnetron sputtering. The CaP coating films investigated in this study were found to be dense and uniform on the basis of their SEM images. In addition, XRD analyses showed that they had an amorphous structure (Fig. 1), demonstrating that the CaP coatings deposited in this study were of ACP. ACP coating films deposited by RF magnetron sputtering exhibit high bonding stresses and have been found to be resorbable in vitro.14 A previous in vivo study has shown that thin ACP coating films disappear within 1 week of implantation.20 In the present study, SEM images of the ACP coatings showed that the coatings remained on the surfaces of the retrieved implants 1 week after implantation. On the other hand, the coatings were not observed 2 and 4 weeks after implantation (Fig. 2). These results were in accordance with those of previous studies and demonstrate the absorbability of the ACP coatings. It is likely that the dissolution rate of ACP coating films is dependent on the thickness of the films. The mechanical stability of the ACP-coated implants was evaluated on the basis of their removal torque and ISQ values. The 4-week values of both these parameters were higher than their 1-week values for both the ACPcoated implants and the uncoated controls, and this result demonstrated that osseointegration occurred in the case of both implant types. In addition, after 4 weeks, the values of both these parameters were higher in the case of the ACP-coated implants than in the case of the control implants. This result suggests that the ACP-coated implants led to faster bone formation than did the control implants, resulting in quicker osseointegration.
The results of the histomorphometric analysis showed that appositional bone healing occurred along the perimeters of all the implants and that an intimate contact between implant and bone was present in the cortical regions at 1, 2, and 4 weeks. These results confirmed that both the ACP-coated and the as-machined surfaces were biocompatible. Moreover, a significant difference was noticed in the 2- and 4week BIC values of the 2 implant surfaces, with higher degrees of bone organization being observed in cortical bone in the case of the ACP-coated implants than in the control group. Bone growth in the area of the thread, shown in the BSE images, indicates that there was a significant difference between the ACP-coated and control implants after 1 week (Table 2). The results of these analyses indicate that the ACP-coated implants promoted osseointegration. However, the significant increase during each analysis is not in accordance. We believe that this contradiction can be explained if one assumes that osseointegration takes place in several phases. In an experimental study on dogs, Berglundh et al21 found that initial and early wound-healing features resulted in osseointegration. They found that voids formed around implants immediately after their installation and that these voids were filled with a coagulum and subsequently a granulation tissue, which was replaced within a few days by a provisional matrix that consisted of vascular structures and a large number of mesenchymal cells. Furthermore, it was also reported that initial bone formation in such areas occurred within the first week of healing. This is the first phase of osseointegration and bone augmentation, a conclusion support by the results of the current study (Table 2). The ACP coating film was absorbed almost completely within the first 1 week and completely within 2 weeks. It was observed that there is a critical period during which the ACP coatings may affect the bone-to-implant interface. From a theoretical perspective, depositing nanoscaled Ca- and P-based coatings on implant surfaces, that is, incorporating these elements on implant
IMPLANT DENTISTRY / VOLUME 23, NUMBER 3 2014 surfaces should result in faster osseointegration, which should help prevent mechanical failures on clinical loading.22,23 In addition, alkaline phosphatase activity is enhanced by the use of Saos-2 cells when used along with CaP coatings.2 This is due to the release of CaP into the substrate region, resulting in the saturation of body fluids and the precipitation of proteins and a biological apatite onto the surface of the substrate.1,2 In this case, ACP plays the role of a supplier of calcium and phosphorus ions. The saturation of Ca and P ions in the periimplant region accelerates bone formation. The second phase of achieving osseointegration is the migration of newly formed bone along the implant surface. In this study, the values of BIC in the case of the ACP-coated implants were higher than those of the uncoated implants. This was true both for the 2- and 4-week values. Once the newly formed bone had migrated to the implant surface, bone formation occurs from the surface of the implant itself. After 1 week, the coating film dissolved and CaP was released into the periimplant region, and this increased the concentration of body fluids. Furthermore, owing to the dissolution of the coating layer, there was space between the implant surface and the bone for the formation of blood clots. This resulted in the precipitation of a biological apatite on the surface of the implant. The ACP coating accelerated bone healing around the implant and resulted in there being a direct contact between the implant and the bone. The final phase of osseointegration is bone maturation. Although the results of histological analyses showed that there was a significant difference between the 2 types of implants after 2 weeks, only the 4-week mechanical stability of the ACP-coated implants was higher than that of the uncoated implants. We speculate that this was due to bone maturation. The new bone that surrounds the implant is unable to withstand functional loading or mechanical stress. In addition, bone maturation requires time because apatite crystallization needs to take place. Therefore, for each period, bone formation and mechanical stability in the
case of the ACP-coated implants was higher than those in the case of the uncoated implants, with this difference being due to the initial bone response, which affects osseointegration. It has been reported that the overcompression of adjacent bone during implantation is a potential factor in implant failure.24 After the implantation of an ACP-coated implant, it was noticed that the coating film dissolved and was swiftly absorbed, resulting in a decrease in compressive stress on adjacent bone and in the creation of space for the saturation of body fluids. The current study found that dental implants coated with a layer of ACP deposited by RF magnetron sputtering increased biocompatibility and osteoconductivity, resulting in better hostimplant tissue response. However, there has been a report suggesting that ACP coatings on dental implants have no effect on osseointegration.25 This study used a coating layer of a thickness of 0.1 mm. It is speculated that ACP coating layers 0.1 mm in thickness are resorbed very quickly and are not thick enough to be able to release calcium and phosphorus ions. Further research to identify the optimal ACP coating thickness for efficient periimplant bone healing and osseointegration is therefore recommended. Moreover, no adverse inflammatory reactions or abnormal bone resorption in the long term has been reported for ACP-coated implants.26 This fact and the results indicate that ACP-coated implants can accelerate bone formation during the early stages of healing, and after the dissolution of the ACP coating, they behave as conventional dental implants. Poor bone quality, especially low bone density, has been cited as a major reason for prolonged bone-implant healing and even implant failure.27 Therefore, patients with limited bone availability, compromised health, such as those with uncontrolled diabetes or osteoporosis, or those at an elevated risk for implant failure could potentially benefit from the use of ACP-coated implants.28–30 In addition to their testing using animal models, future investigations of such implants should include their use in such patients as well.
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CONCLUSION The present study investigated the biological response of titanium implants that were coated with a 0.5-mm-thick layer of ACP. Significant improvements in biomechanical fixation, that is, the ISQ values, which were determined by RFA analysis, the removal torque values, and the BIC values were detected in an animal model 4 weeks after implantation. Keeping the limitations of the study in mind, coating dental implants with a layer of ACP using RF magnetron sputtering seems to be a valid and useful technique to improve osseointegration.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ACKNOWLEDGMENTS This study was financially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, under contract numbers 23390458 and 22360299. The authors thank Mr. Tamate and Mr. Kurihara of the Tohoku University for their assistance with the deposition of the ACP films and the animal procedures.
REFERENCES 1. Daculsi G, Laboux O, Malard O, et al. Current state of the art of biphasic calcium phosphate bioceramics. J Mater Sci Mater Med. 2003;14:195–200. 2. de Groot K, Wolke JG, Jansen JA. Calcium phosphate coatings for medical implants. Proc Inst Mech Eng H. 1998; 212:137–147. 3. Barrère F, van der Valk CM, Meijer G, et al. Osteointegration of biomimetic apatite coating applied onto dense and porous metal implants in femurs of goats. J Biomed Mater Res B Appl Biomater. 2003;67:655–665. 4. Jansen JA, van de Waerden JP, Wolke JG, et al. Histologic evaluation of the osseous adaptation to titanium and hydroxyapatite-coated titanium implants. J Biomed Mater Res. 1991;25: 973–989.
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5. Gottlander M, Johansson CB, Albrektsson T. Short- and long-term animal studies with a plasma-sprayed calcium phosphate-coated implant. Clin Oral Implants Res. 1997;8:345–351. 6. Filiaggi MJ, Coombs NA, Pilliar RM. Characterization of the interface in the plasma-sprayed HA coating/Ti-6Al-4V implant system. J Biomed Mater Res. 1991;25:1211–1229. 7. Chang YL, Lew D, Park JB, et al. Biomechanical and morphometric analysis of hydroxyapatite-coated implants with varying crystallinity. J Oral Maxillofac Surg. 1999;57:1096–1109. 8. Lee JJ, Rouhfar L, Beirne OR. Survival of hydroxyapatite-coated implants: A meta-analytic review. J Oral Maxillofac Surg. 2000;58:1372–1380. 9. Tinsley D, Watson CJ, Russell JL. A comparison of hydroxylapatite coated implant retained fixed and removable mandibular prostheses over 4 to 6 years. Clin Oral Implants Res. 2001;12:159–166. 10. Wheeler SL. Eight-year clinical retrospective study of titanium plasma-sprayed and hydroxyapatite-coated cylinder implants. Int J Oral Maxillofac Implants. 1996;11:340–350. 11. Wan T, Aoki H, Hikawa J, et al. RF-magnetron sputtering technique for producing hydroxyapatite coating film on various substrates. Biomed Mater Eng. 2007;17:291–297. 12. Yang Y, Agrawal CM, Kim KH, et al. Characterization and dissolution behavior of sputtered calcium phosphate coatings after different postdeposition heat treatment temperatures. J Oral Implantol. 2003;29:270–277. 13. Yonggang Y, Wolke JG, Yubao L, et al. In vitro evaluation of different heattreated radio frequency magnetron sputtered calcium phosphate coatings. Clin Oral Implants Res. 2007;18:345–353.
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14. Ueda K, Narushima T, Goto T, et al. Fabrication of calcium phosphate films for coating on titanium substrates heated up to 773 K by RF magnetron sputtering and their evaluations. Biomed Mater. 2007;2:S160–S166. 15. Kyosuke U, Yuuki K, Takayuki N, et al. Calcium phosphate films with/without hiat treatments fabricated using RF magnetron sputtering. J Biomech Sci Eng. 2009;3:392–403. 16. Johansson CB, Han CH, Wennerberg A, et al. A quantitative comparison of machined commercially pure titanium and titanium-aluminum-vanadium implants in rabbit bone. Int J Oral Maxillofac Implants. 1998;13:315–321. 17. Narushima T, Ueda K, Goto T, et al. Preparation of calcium phosphate films by radiofrequency magnetron sputtering. Mater Trans. 2005;46:2246–2252. 18. Hulshoff JE, van Dijk K, van der Waerden JP, et al. Biological evaluation of the effect of magnetron sputtered Ca/P coatings on osteoblast-like cells in vitro. J Biomed Mater Res. 1995;29: 967–975. 19. Ozeki K, Okuyama Y, Fukui Y, et al. Bone response to titanium implants coated with thin sputtered HA film subject to hydrothermal treatment and implanted in the canine mandible. Biomed Mater Eng. 2006;16:243–251. 20. Wolke JG, van der Waerden JP, Schaeken HG, et al. In vivo dissolution behavior of various RF magnetron-sputtered Ca-P coatings on roughened titanium implants. Biomaterials. 2003;24:2623– 2629. 21. Berglundh T, Abrahamsson I, Lang NP, et al. De novo alveolar bone formation adjacent to endosseous implants. Clin Oral Implants Res. 2003;14:251–262. 22. Coelho PG, Cardaropoli G, Suzuki M, et al. Histomorphometric evaluation of
a nanothickness bioceramic deposition on endosseous implants: A study in dogs. Clin Implant Dent Relat Res. 2009;11: 292–302. 23. Suh JY, Jeung OC, Choi BJ, et al. Effects of a novel calcium titanate coating on the osseointegration of blasted endosseous implants in rabbit tibiae. Clin Oral Implants Res. 2007;18: 362–369. 24. Bashutski JD, D’Silva NJ, Wang HL. Implant compression necrosis: Current understanding and case report. J Periodontol. 2009;80:700–704. 25. Ong JL, Bessho K, Cavin R, et al. Bone response to radio frequency sputtered calcium phosphate implants and titanium implants in vivo. J Biomed Mater Res. 2002;59:184–190. 26. Fügl A, Ulm C, Tangl S, et al. Longterm effects of magnetron-sputtered calcium phosphate coating on osseointegration of dental implants in non-human primates. Clin Oral Implants Res. 2009;20: 183–188. 27. Alsaadi G, Quirynen M, Komarek A, et al. Impact of local and systemic factors on the incidence of late oral implant loss. Clin Oral Implants Res. 2008;19: 670–676. 28. Glösel B, Kuchler U, Watzek G, et al. Review of dental implant rat research models simulating osteoporosis or diabetes. Int J Oral Maxillofac Implants. 2010; 25:516–524. 29. Herrmann I, Lekholm U, Holm S, et al. Evaluation of patient and implant characteristics as potential prognostic factors for oral implant failures. Int J Oral Maxillofac Implants. 2005;20:220–230. 30. Javed F, Romanos GE. Impact of diabetes mellitus and glycemic control on the osseointegration of dental implants: A systematic literature review. J Periodontol. 2009;80:1719–1730.
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Evaluation of Thin Amorphous Calcium Phosphate Coatings on Titanium Dental Implants Deposited Using Magnetron Sputtering Sou Yokota, DDS,* Naruhiko Nishiwaki, MS,† Kyosuke Ueda, PhD,† Takayuki Narushima, PhD,† Hiroshi Kawamura, DDS, PhD,* and Tetsu Takahashi, DDS, PhD*
alcium phosphate (CaP) coatings are used extensively with dental implants to induce faster bone adaptation. These coatings also improve osseointegration because they are biocompatible and have osteoconductive properties, which result in the shortening of the healing process and an increase in the clinical success rate.1–4 Various methods have been used to coat the surfaces of implants, including plasma spraying, sputter deposition, and solgel coating. In recent times, the plasma-spraying method has become the most widely used one for the deposition of CaP coatings. Implants with plasma-sprayed coatings of CaP have shown faster bone apposition than did the uncoated implants, both in animal studies and in clinical practice.5 However, the plasma-spraying method has a number of disadvantages: the resultant coatings are highly porous, and there are high residual stresses at the implantcoating interfaces because of the high temperature involved in the process.6
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*Division of Maxillofacial Surgery, Department of Oral Medicine and Surgery, Graduate School of Dentistry, Tohoku University, Sendai, Japan. †Department of Materials Processing, Tohoku University, Sendai, Japan.
Reprint requests and correspondence to: Sou Yokota, DDS, Division of Maxillofacial Surgery, Department of Oral Medicine and Surgery, Graduate School of Dentistry, Tohoku University, 4-1 Seiryo, Aoba, Sendai 980-8575, Japan, Phone: +81-22-717-8350, Fax: +81-22-717-8354, E-mail: [email protected] ISSN 1056-6163/14/02303-343 Implant Dentistry Volume 23 Number 3 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000098
Objective: Calcium phosphate is used for dental material because of its biocompatibility and osteoconductivity. Amorphous calcium phosphate (ACP) coatings deposited by magnetron sputtering can control their thickness and absorbability. This study aimed to evaluate and characterize ACP coatings deposited via magnetron sputtering. It was hypothesized that ACP coatings would enhance bone formation and be absorbed rapidly in vivo. Methods: ACP coatings that are 0.5 mm in thickness were deposited via magnetron sputtering on dental implants. Uncoated implants served as controls. The effect of the ACP coatings in vivo was investigated in New Zealand white rabbit. To evaluate the effect of the ACP coatings on the bone response of the implants, the removal torque, implant stability quo-
tient, and histomorphometric analysis were performed on the implants at 1, 2, and 4 weeks after implantation. Results: Results of the x-ray diffraction analyses confirmed the deposition of ACP coatings. Images from the scanning electron microscopy revealed that the coatings were dense, uniform, and 0.5 mm in thickness and that they were absorbed completely. Mechanical stability and bone formation in the case of the ACP-coated implants were higher than those of control. Conclusion: These results suggest that implants coated with thin ACP layers improve implant fixation and accelerate bone response. (Implant Dent 2014;23:343–350) Key Words: animal experiments, amorphous calcium phosphate, dental implant coating, RF magnetron sputtering
Moreover, the plasma-spraying method is not effective for coating tiny dental implants that have complex shapes. In addition, it has been reported that CaP coatings on dental implants undergo delamination and CaP-coated implants fail at the implant-coating interface, when subjected to functional loading.7–10 Physical vapor deposition can be a suitable technique for obtaining uniform
and dense coatings of CaP on metal substrates. Radiofrequency (RF) magnetron sputtering has been used in a wide variety of areas for coating thin films with excellent adherence onto substrates and for applying coatings of CaP films on commercially pure titanium (Ti) implants. Its low processing temperature is also an advantage of the RF magnetron sputtering technique for depositing CaP coatings on Ti substrates
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because the mechanical properties of the substrates may be degraded at high processing temperatures.11–14 The thicknesses of the coating films can also be controlled more precisely and with greater ease by modulating the sputtering duration. RF magnetron sputtering can also be used to deposit coatings of amorphous calcium phosphate (ACP) on substrates. ACP is the precursor of bone tissue and is absorbed rapidly in vitro.12,15 In fact, the concentration of calcium and phosphorus ions from ACP coating films was higher than that from hydroxyapatite (Ca10(PO4)6(OH)2; HA) coating films, as determined by an immersion test. The use of CaP coatings as an alternative to plasma-sprayed HA coatings should help prevent the defects that result from the use of the latter. CaP coatings contribute to bone response in the early stages of implantation and disappear after osseointegration. Thus, adverse effects similar to those resulting from the use of plasma-sprayed HA coatings can be avoided. The aim of this study was to evaluate the biological behavior and characteristics of ACP coatings deposited on dental implants via RF magnetron sputtering using an in vivo model.
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Fig. 1. A, XRD pattern showing the a-Ti peaks and halo peak associated with CaP phases. B, EXD spectra showing the presence of Ca, P, O, and Ti on the surface of the coated implant. C, Scanning electron micrograph of the ACP coating. The coated surface is dense and uniform and does not show any cracks or detachment. The thickness of the coating film was maintained at 0.5 mm.
METHODS
Preparation of the Implants and Their Coating With ACP
A total of 72 screw-shaped Ti (commercially pure titanium, grade 4) implants 3.75 mm in diameter and 8.5 mm in length (Nobel Biocare, Gothenburg, Sweden) were used in this study. All the implants were treated as having machined surfaces. The implants were divided into 2 groups: a coated group and an uncoated (control) group. In the case of the coated group, the ACP coatings were deposited via RF magnetron sputtering (MS-320; Universal Systems, Co, Ltd, Tokyo, Japan) using a target of hot-pressed b-tricalcium phosphate (Ca3(PO4)2) having a relative density of more than 99.6%. The implants were not heated intentionally during the sputtering, and their temperature was less than 373 K. The thickness of the coating films was maintained at 0.5 mm by controlling the sputtering
Fig. 2. A and C, SEM images of ACP-coated implants removed 1 week and 2 weeks after implantation in femurs, respectively. E and F, EDX spectra of the implants shown in (A) and (C), respectively. B and D are higher magnification images of the implants shown in (A) and (C), respectively.
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duration. The different phases of the coating films were confirmed using x-ray diffraction (XRD) analysis, and the chemical composition of the coating films was determined using energydispersive x-ray spectroscopy (EDX) (DX-4; EDAX, Mahwah, NJ). The topographies of the coatings and the crosssections of the implants were analyzed using scanning electron microscopy (SEM) (XL-30FEG; Philips, Amsterdam, the Netherlands). Animal and Surgical Procedures
Fig. 3. RFA was performed on the ACP-coated and control implants placed in the femurs (n ¼ 6). There was a significant difference in the 4-week ISQ values of the ACP-coated implants and those of the control implants (P , 0.05). The error bars represent the standard deviation.
All protocols for the animal experiments were reviewed and approved by the Animal Care and Use Committee of the Tohoku University. Eighteen male New Zealand white rabbits (18 weeks old; mean weight, 3150 g) were used in the present study. Before the surgical procedure, the animals were anesthetized with intramuscular injections of xylazine (20 mg/mL; Bayer, Leverkusen, Germany) and were administered pentobarbital (25 mg/kg; Kyoritsu Seiyaku, Tokyo, Japan) intravenously via the lateral veins in their ears. Infiltration anesthesia with 2% lidocaine containing epinephrine (Astra Zeneca, London, United Kingdom) in the ratio of 1:800,000 was administered at the surgical site. To place the implants, an incision was made on the medial surfaces of the femur and tibia, and the implantation sites were the proximal metaphyses of the femur and the tibia. Every animal received an implant in each proximal femur and proximal tibia. Muscle, fascia, and skin were sutured separately with resorbable sutures. All animals survived the procedures, and no pathological weight loss was seen in any of them. There were no signs of postoperative wound infection, fracture, or any other clinical anomalies that might have indicated poor healing during the postoperative period. At the time of the euthanization of the animals, all the implants had attained rigid fixation; infection and bone resorption were not observed. Resonance Frequency Analysis
Fig. 4. Removal torque force was measured at the femur. There was a significant difference between the 4-week removal torque force values of the ACP-coated implant and those of the control implants.
In all the rabbits, resonance frequency analysis (RFA) was performed for the femoral and tibial implants both at the time of implant
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Fig. 5. Histological evaluation after implantation. Undecalcified ground sections of implantfemur specimens stained with toluidine blue. The images in (A), (B), and (C) are of an uncoated implant. The images in (D), (E), and (F) are of an ACP-coated implant. Ti indicates titanium implant; CB, cortical bone; NB, new bone.
placement and euthanization. Before the analyses, the rabbits were anesthetized using the procedure described earlier, and the implants were accessed surgically. The area around each implant was gently debrided and cleared of granulation tissue. The abutment for RFA (SmartPeg; Osstell AB, Gothenburg, Sweden) was mounted on each implant before analysis and hand tightened. Then, the implant stability quotient (ISQ) value of each implant was measured using an Osstell stability meter (Osstell AB). Measurement of the Removal Torque Values
The values of the peak loosening torque for all the femoral implants were determined using a removal torque gauge (Tohnichi Mfg, Co, Ltd, Tokyo, Japan). Torque was applied in the counterclockwise direction until the implant
turned. This operation was performed under anesthesia, before the animal was euthanized. The removal torque reflects the strength of the bond between the bone tissue and the implant.16 Preparation of Bone-Implant Specimens and Their Histomorphometric Analysis
To perform histological and histomorphometric analyses, the bilateral tibial implants and the tissue surrounding the implants were removed en bloc, and the soft tissue adhering to the implants was dissected. The bone-implant specimens were immersed in 10% neutral buffered formaldehyde for 2 weeks and dehydrated in a graded series of ethanol solutions (70%–100%) under vacuum, washed with acetone, and then embedded in methyl methacrylate (Wako Pure Chemical Industries, Ltd, Tokyo, Japan). After polymerization, the blocks of the specimens were cut into
approximately 300-mm-thick sections using a diamond saw microsectioning system. Three slides were prepared for each implant by sectioning the implant perpendicular to the bone axis. All the sections of a particular implant were made sequentially through the same plane. Then, each sectioned slide was sequentially ground with sandpapers with grits of 800 to 4000 and stained using toluidine blue. To measure the bone-to-implant contact (BIC) values and to observe the bone response around the implants, images of the slides of the implants were taken using an optical microscope with a SPOT digital camera (Leica Microsystems, Wetzlar, Germany) and analyzed using an image analysis software (Image J; National Institutes of Health, Bethesda, MD). The BIC values were calculated only for the cortical bone regions. All the implants used in this study were inserted monocortically, and the length of the direct contact between the bone and the implant was measured along the cortical bone under 3100 magnification. The other half of the embedded and sectioned bone-implant specimens was evaluated via SEM (JSM-6500F; Nihondenshi, Tokyo, Japan) using backscattered electrons (BSE). The sectioned blocks were glued onto SEM stubs and coated with a thin layer of osmium via vapor deposition before being mounted in the microscope. Statistical Analysis
Statistical analysis was performed on the values of the removal torque and ISQ and also on those of the BIC. The differences between the ACP-coated implants and the controls were analyzed for each variable using the Student t test (n ¼ 6). Values with P , 0.05 were accepted as being statistically significant.
RESULTS Table 1. Mean BIC Values
Noncoated ACP coated P
Characteristics of the ACP Coating Film
1 Week (Mean 6 SD)
2 Week (Mean 6 SD)
2 Week (Mean 6 SD)
18.26 6 5.27 20.23 6 5.9 0.29
49.31 6 15.05 63.41 6 11.6 ,0.01
64.76 6 14.45 78.49 6 8.83 ,0.01
There was no significant difference between the 2 groups after 1 week. After 2 and 4 weeks, the ACP-coated group had significantly higher values than those of the control group (P , 0.01).
The phases of the ACP coating film were determined using XRD analysis, and its chemical composition was determined using EDX (DX-4; EDAX). The cross-sections of the implants were observed using SEM (XL-30FEG; Philips). The XRD pattern of the coating
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film is shown in Figure 1, A. No crystalline peaks were observed in the pattern. The halo peak indicates the formation of an ACP phase,17 which was confirmed by the results of the EDX as well (see Fig. 1, B), with the signals indicating the presence of P and Ca. The coating film was dense, uniform, and crack free, and no detachment between the coating film and the implant was observed in the SEM images (Fig. 1, C). The implants retrieved by applying a removal torque were also observed using EDX and SEM. The ACP coating films could be partially observed 1 week after implantation; however, they were not observed after 2 weeks (Fig. 2). Mechanical Stability of the Implants
The ISQ values 1, 2, and 4 weeks after implantation are shown in Figure 3. The ISQ values increased with the duration of implantation, suggesting that osseointegration increased with time. There was no significant difference in the 1- and 2-week values of the coated and control implants (P . 0.05); however, there was a significant difference in their 4-week values (P , 0.05). The removal torque values are shown in Figure 4. The removal torque increased with the duration of implantation, both in the case of the ACP-coated implants and in the case of the control implants; however, no statistical differences were found between the 1- and 2-week values of the 2 groups. On the other hand, a statistically significant difference was noticed between the 4-week values of the 2 groups (P , 0.05). Histological Analysis and Backscattered SEM of the Bone-Implant Specimens
Fig. 6. Backscattered SEM images. The images in A, C, and E are of an uncoated implant. The images in B, D, and F are of an ACP-coated implant. 1 week (A and B), 2 weeks (C and D), and 4 weeks after implantation (D and E). The black arrows indicate bone ingrowth.
The SEM images of the implants and bone tissue in the undecalcified sections, which were dyed with toluidine blue, are shown in Figure 5. Formation of bone directly on the implant surface was noticed in all the specimens, and there were no significant differences between the implants and the adjacent tissue. In most of the sections, the upper threads of the implant had integrated with the cortical bone, with the rest of the implant surface being in direct contact with the bone marrow. In addition, formation of new bone
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Table 2. Rate of Bone Augmentation in the Implant Threads
Noncoated ACP coated P
1 Week (Mean 6 SD)
2 Week (Mean 6 SD)
4 Week (Mean 6 SD)
52.34 6 5.36 64.11 6 5.14 ,0.01
82.03 6 4.08 85.30 6 6.09 0.30
80.40 6 5.20 93.56 6 2.63 0.11
There was a significant difference between the 2 groups after 1 week. On the other hand, after 2 and 4 weeks, the 2 groups did not show a significant difference (P , 0.01).
between the implant and the cortical bone was observed in all sections. No histological signs of adverse host responses, such as necrosis or osteolysis, were detected in any specimen. Three sections of each specimen were used for determining the BIC. The BIC values were calculated only over cortical bone. There was no significant difference between the ACP-coated and control implants after 1 week. On the other hand, after 2 and 4 weeks, the ACP-coated implants showed significantly higher BIC values compared with those of the control implants (Table 1). SEM of BSE showed that the implant threads contained mineralized lamellar bone and portions of newly formed bone with typical osteocyte lacunae and blood vessel spaces (Fig. 6). The area of bone surrounding the implant threads was calculated and labeled as the percent bone area (Table 2). The value of this parameter at 1 week in the case of the ACP-coated implants was higher than that in the case of the uncoated implants.
DISCUSSION Coatings deposited by the RF magnetron sputtering method are more uniform and bond better with substrates than those deposited by plasma-spraying methods, while not affecting the smoothness of the substrates. In addition, CaP layers deposited by this technique are also biocompatible and can enhance apatite formation.18–20 Moreover, the RF magnetron sputtering method is a low-temperature process and can thus be used to deposit layers of ACP. ACP is the precursor of bone apatite and tends to dissolve readily owing to its amorphous nature. However, ACP layers deposited by RF magnetron sputtering have not been previously
analyzed in vivo. The aim of this study was to elucidate the characteristics of ACP coatings deposited using RF magnetron sputtering. The CaP coating films investigated in this study were found to be dense and uniform on the basis of their SEM images. In addition, XRD analyses showed that they had an amorphous structure (Fig. 1), demonstrating that the CaP coatings deposited in this study were of ACP. ACP coating films deposited by RF magnetron sputtering exhibit high bonding stresses and have been found to be resorbable in vitro.14 A previous in vivo study has shown that thin ACP coating films disappear within 1 week of implantation.20 In the present study, SEM images of the ACP coatings showed that the coatings remained on the surfaces of the retrieved implants 1 week after implantation. On the other hand, the coatings were not observed 2 and 4 weeks after implantation (Fig. 2). These results were in accordance with those of previous studies and demonstrate the absorbability of the ACP coatings. It is likely that the dissolution rate of ACP coating films is dependent on the thickness of the films. The mechanical stability of the ACP-coated implants was evaluated on the basis of their removal torque and ISQ values. The 4-week values of both these parameters were higher than their 1-week values for both the ACPcoated implants and the uncoated controls, and this result demonstrated that osseointegration occurred in the case of both implant types. In addition, after 4 weeks, the values of both these parameters were higher in the case of the ACP-coated implants than in the case of the control implants. This result suggests that the ACP-coated implants led to faster bone formation than did the control implants, resulting in quicker osseointegration.
The results of the histomorphometric analysis showed that appositional bone healing occurred along the perimeters of all the implants and that an intimate contact between implant and bone was present in the cortical regions at 1, 2, and 4 weeks. These results confirmed that both the ACP-coated and the as-machined surfaces were biocompatible. Moreover, a significant difference was noticed in the 2- and 4week BIC values of the 2 implant surfaces, with higher degrees of bone organization being observed in cortical bone in the case of the ACP-coated implants than in the control group. Bone growth in the area of the thread, shown in the BSE images, indicates that there was a significant difference between the ACP-coated and control implants after 1 week (Table 2). The results of these analyses indicate that the ACP-coated implants promoted osseointegration. However, the significant increase during each analysis is not in accordance. We believe that this contradiction can be explained if one assumes that osseointegration takes place in several phases. In an experimental study on dogs, Berglundh et al21 found that initial and early wound-healing features resulted in osseointegration. They found that voids formed around implants immediately after their installation and that these voids were filled with a coagulum and subsequently a granulation tissue, which was replaced within a few days by a provisional matrix that consisted of vascular structures and a large number of mesenchymal cells. Furthermore, it was also reported that initial bone formation in such areas occurred within the first week of healing. This is the first phase of osseointegration and bone augmentation, a conclusion support by the results of the current study (Table 2). The ACP coating film was absorbed almost completely within the first 1 week and completely within 2 weeks. It was observed that there is a critical period during which the ACP coatings may affect the bone-to-implant interface. From a theoretical perspective, depositing nanoscaled Ca- and P-based coatings on implant surfaces, that is, incorporating these elements on implant
IMPLANT DENTISTRY / VOLUME 23, NUMBER 3 2014 surfaces should result in faster osseointegration, which should help prevent mechanical failures on clinical loading.22,23 In addition, alkaline phosphatase activity is enhanced by the use of Saos-2 cells when used along with CaP coatings.2 This is due to the release of CaP into the substrate region, resulting in the saturation of body fluids and the precipitation of proteins and a biological apatite onto the surface of the substrate.1,2 In this case, ACP plays the role of a supplier of calcium and phosphorus ions. The saturation of Ca and P ions in the periimplant region accelerates bone formation. The second phase of achieving osseointegration is the migration of newly formed bone along the implant surface. In this study, the values of BIC in the case of the ACP-coated implants were higher than those of the uncoated implants. This was true both for the 2- and 4-week values. Once the newly formed bone had migrated to the implant surface, bone formation occurs from the surface of the implant itself. After 1 week, the coating film dissolved and CaP was released into the periimplant region, and this increased the concentration of body fluids. Furthermore, owing to the dissolution of the coating layer, there was space between the implant surface and the bone for the formation of blood clots. This resulted in the precipitation of a biological apatite on the surface of the implant. The ACP coating accelerated bone healing around the implant and resulted in there being a direct contact between the implant and the bone. The final phase of osseointegration is bone maturation. Although the results of histological analyses showed that there was a significant difference between the 2 types of implants after 2 weeks, only the 4-week mechanical stability of the ACP-coated implants was higher than that of the uncoated implants. We speculate that this was due to bone maturation. The new bone that surrounds the implant is unable to withstand functional loading or mechanical stress. In addition, bone maturation requires time because apatite crystallization needs to take place. Therefore, for each period, bone formation and mechanical stability in the
case of the ACP-coated implants was higher than those in the case of the uncoated implants, with this difference being due to the initial bone response, which affects osseointegration. It has been reported that the overcompression of adjacent bone during implantation is a potential factor in implant failure.24 After the implantation of an ACP-coated implant, it was noticed that the coating film dissolved and was swiftly absorbed, resulting in a decrease in compressive stress on adjacent bone and in the creation of space for the saturation of body fluids. The current study found that dental implants coated with a layer of ACP deposited by RF magnetron sputtering increased biocompatibility and osteoconductivity, resulting in better hostimplant tissue response. However, there has been a report suggesting that ACP coatings on dental implants have no effect on osseointegration.25 This study used a coating layer of a thickness of 0.1 mm. It is speculated that ACP coating layers 0.1 mm in thickness are resorbed very quickly and are not thick enough to be able to release calcium and phosphorus ions. Further research to identify the optimal ACP coating thickness for efficient periimplant bone healing and osseointegration is therefore recommended. Moreover, no adverse inflammatory reactions or abnormal bone resorption in the long term has been reported for ACP-coated implants.26 This fact and the results indicate that ACP-coated implants can accelerate bone formation during the early stages of healing, and after the dissolution of the ACP coating, they behave as conventional dental implants. Poor bone quality, especially low bone density, has been cited as a major reason for prolonged bone-implant healing and even implant failure.27 Therefore, patients with limited bone availability, compromised health, such as those with uncontrolled diabetes or osteoporosis, or those at an elevated risk for implant failure could potentially benefit from the use of ACP-coated implants.28–30 In addition to their testing using animal models, future investigations of such implants should include their use in such patients as well.
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CONCLUSION The present study investigated the biological response of titanium implants that were coated with a 0.5-mm-thick layer of ACP. Significant improvements in biomechanical fixation, that is, the ISQ values, which were determined by RFA analysis, the removal torque values, and the BIC values were detected in an animal model 4 weeks after implantation. Keeping the limitations of the study in mind, coating dental implants with a layer of ACP using RF magnetron sputtering seems to be a valid and useful technique to improve osseointegration.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ACKNOWLEDGMENTS This study was financially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, under contract numbers 23390458 and 22360299. The authors thank Mr. Tamate and Mr. Kurihara of the Tohoku University for their assistance with the deposition of the ACP films and the animal procedures.
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