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An In Vitro Biomechanical Evaluation of a New Commercial Titanium-Zirconium Alloy Dental Implant: A Pilot Study Aaron Yu-Jen Wu, DDS, PhD,* Jui-Ting Hsu, PhD,† and Heng-Li Huang, PhD‡
ommercial pure (CP) titanium (Ti) and its alloys are widely used as metallic biomaterials in bone screws and plates, and also as the orthopedic and dental replacements due to their excellent biocompatibility and mechanical properties, including low modulus, high corrosion resistance, and lightness.1,2 However, pure Ti and Ti alloys still fail to meet some of the requirements for implant biomaterials in clinical applications. The tensile strength of Ti is insufficient for its use in orthopedic joints and bone plates and screws,3 and its poor wear resistance4 also prevents its widespread application in many medical fields. A Ti-aluminum (Al)-vanadium (V) alloy is the most commonly used Ti-based material for medical implants. Although Ti-Al-V alloys have very good corrosion resistance and biocompatibility, some concerns remain about the release of toxic Al and V ions into the surrounding tissue during long-term implantation. Zirconium (Zr) has been considered as an excellent alternative biomaterial
C
*Assistant Professor and Director, Department of Dentistry, Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Kaohsiung, Taiwan. †Associate Professor, Biomechanics Res. Lab, School of Dentistry, China Medical University, Taichung, Taiwan. ‡Professor, Biomechanics Res. Lab, School of Dentistry, China Medical University, Taichung, Taiwan.
Reprint requests and correspondence to: Heng-Li Huang, PhD, School of Dentistry, China Medical University, 91 Hsueh-Shih Road, Taichung 40402, Taiwan, Phone: 1-886-4-22053366 ext. 2306, Fax: 1-886-4-22014043, E-mail: [email protected] ISSN 1056-6163/14/02305-534 Implant Dentistry Volume 23 Number 5 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000108
Background: The study compared the implant mobility and surrounding bone strain between the titanium-zirconium (Ti-Zr) alloy and the commercial pure (CP) Ti implants. Methods: The mobilitydquantified as the implant stability quotient (ISQ) and Periotest value (PTV)dof implants constructed from Ti-Zr alloy and CP Ti placed into artificial type-2 jawbone models were measured. Specimens were tested by applying 190 N vertically or at 30 degrees laterally. Peak values of the principal strains of bone were recorded by rosette strain gauges with a data acquisition system and were analyzed statistically using Wilcoxon rank-sum test. Results: PTV and ISQ values did not differ significantly between the
Ti-Zr and CP Ti implants (P . 0.01). Under vertical loading, the peak bone strains did not differ significantly between the Ti-Zr and CP Ti specimens (P . 0.006). However, the peak strains were 52% lower around the Ti-Zr implant than around the Ti implant on the buccal side of bone under lateral loading (P , 0.001). Conclusions: The implant material (Ti-Zr alloy vs CP Ti) had no effect on the mobility of small-diameter dental implants. However, using Ti-Zr alloy as an implant material decreased the periimplant bone strain under lateral loading in this pilot study. (Implant Dent 2014;23:534–538) Key Words: small-diameter implant, Ti-Zr alloy, CP Ti, implant mobility, bone strain
to Ti. Not only is Zr a nontoxic metal, but a bone-like apatite layer can form on its surface that attracts the growth of bone tissue.5 There is in vivo evidence of Zr exhibiting good osseointegration with bone in animal studies.6–8 Additionally, Zr-based materials have advantages of outstanding corrosion resistance9 and high bending strength and fracture toughness.10,11 The increasing use of computer-aided design and computer-aided manufacturing techniques in recent decades has also increased the general acceptance of Zr-based materials in dental applications.12
Ti and Zr are the group 4 elements in the periodic table and hence they have similar chemical properties. Some researchers have selected Zr as an alloying element to improve the properties of CP Ti and to create different types of Ti-Zr–based alloys for medical uses, especially for implant applications.3,13–16 A novel alloy based on Ti that contained 13% to 17% Zr was recently developed for use in small-diameter implants. Decreasing the diameter of an implant increases the risk of implant fracture because of the lower mechanical fatigue strength.17 However, the superior tensile and fatigue
IMPLANT DENTISTRY / VOLUME 23, NUMBER 5 2014 strengths of the new Ti-Zr alloy compared with CP Ti18 resulted in it exhibiting acceptable mechanical strength for dental implant. Additionally, its biocompatibility in enhancing osseointegration has been confirmed in both animal tests and a clinical study.18–21 Since 2010, the mechanical strength and biocompatibility of Ti-Zr implants have been demonstrated in animal studies and a 1-year clinical observation.18–21 However, the effects of Ti-Zr implants relative to Ti implants on the biomechanical characteristics of the surrounding bone remain to be determined. This study applied a strain-gauge analysis to artificial jawbone samples to compare the biomechanical effects of Ti-Zr and CP Ti implants on induced bone strains. In addition, the mobilities of Ti-Zr and CP Ti implants were compared by measuring the implant stability quotient (ISQ) and Periotest value (PTV).
elastic modulus of 759 MPa (model 1522–05; Pacific Research Laboratories, Vashon Island, WA) was prepared to attach to a 3-mm-thick commercially available synthetic cortical shell (model 3401–02; Pacific Research Laboratories) with an elastic modulus of 16.7 GPa. The trabecular bone simulated in this study was type-2 (D2) bone22 according to the bone-density classification of Misch.23 The 3-mm thickness of the cortical bone was taken from Hahn’s study,24 which indicated that D2 bone was associated with a cortical bone height of 2.5 to 4.0 mm. The synthetic bone had a rectangular shape with dimensions of 410 3 300 3 435 mm. Three specimens of artificial foam bone were prepared for each implant system. Implant Mobility Measurement
535
implant mobility was measured using the Periotest M device (Medizintechnik Gulden, Modautal, Germany) (Fig. 2, B). The tip of the measurement device was positioned perpendicularly at 2 mm from the abutment, and it impacted the implant 4 times per second for 4 seconds.25 PTV values were also measured 4 times in 4 different directions for each model. Strain Gauge Measurement
A self-developed jig was designed with an adjustable rotational screwing device so that both a vertical load and a 30-degree lateral force in the buccallingual direction could be applied in the experiments. Each loading procedure involved applying a force of 190 N26 to the cylindrical abutment using a universal testing machine (JSVH1000; Japan Instrumentation System, Nara, Japan) with a head speed of 1 mm/min (Fig. 3). Rectangular rosette strain gauges (KFG-1-120-D17–11 L3M3S; Kyowa, Tokyo, Japan) were attached to the buccal and lingual sides of the crestal cortical region around the implant using cyanoacrylate cement (CC-33A; Kyowa). Signals corresponding to the 3 independent strains (ea, eb, and ec) measured by the rosette strain gauge were sent to a data acquisition system (NI CompackDAQ; National Instruments, Austin, TX) and analyzed by the associated software (LabVIEW SignalExpress 3.0; National Instruments). After repeating each measurement 3 times for each specimen, the maximum (emax ) and minimum (emin ) principal strains were obtained as follows:
Implants constructed from 2 kinds of materials were selected for analysis: (1) Ti implant (SLActive; Institute Straumann AG, Basel, Switzerland) and (2) Ti-Zr implant (Roxolid; Institute Straumann AG) (Fig. 1). These 2 implant types had hydrophilic sandblasted and acid-etched surfaces, a diameter of 3.3 mm, and a length of 12 mm. A Sawbone model of trabecular bone with a density of 0.64 g/cm3 and
After placing the implant, a wireless resonance frequency analyzer (Osstell ISQ; Osstell AB, Gothenburg, Sweden) was used to measure the ISQ value. Before the measurement, SmartPegs for SLActive implant system (Type 41; Osstell AB) were placed onto the top of the implants. Each SmartPeg has a magnetic material attached to its upper part that causes the probe of the Osstell ISQ instrument to vibrate when it is near to the SmartPeg, which allows the Osstell ISQ instrument to determine the resonance frequency (Fig. 2, A) and convert it into the ISQ value. For each specimen, 4 ISQ values were measured for 4 different directions (ie, buccal, lingual, mesial, and distal directions). After connecting Ti abutments, the
Fig. 1. CP Ti (left) and Ti-Zr alloy (right) implants.
Fig. 2. A, The wireless Osstell ISQ instrument used to measure ISQ value after the model was fixed to the jig. B, The PTV value was acquired after the tip of the Periotest M device touched the abutment.
MATERIALS
AND
METHODS
Implant Design Parameters and Bone Specimen Preparation
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Fig. 3. The universal loading machine applied vertical and lateral forces to the cylindrical abutment on the top of each implant individually.
Table 1. PTV and ISQ Values for Ti and Ti-Zr Implants Evaluation Approaches, Median (IQR) Implant Ti Ti-Zr P†
ISQ
PTV
71.00 (1.00) 71.00 (1.00) 0.03
−2.50 (1.00) −2.00 (0.25) 0.216
There is no significant difference in both the PTV and ISQ values between Ti and Ti-Zr implants. †Multiple comparison with Wilcoxon rank-sum test. IQR indicates interquartile ranges.
1 emax 5 ðea þ ec Þ 2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi iffi 1 h 2 2 ðea −ec Þ þð2eb −ea −ec Þ ; þ 2 (1) 1 emin 5 ðea þ ec Þ 2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i 1 h − ðea −ec Þ2 þð2eb −ea −ec Þ2 : 2
Fig. 4. Medians and interquartile ranges of maximum and minimum principal microstrain values on jawbone models around the Ti-Zr and CP Ti implants under vertical loading (A) and lateral loading (B). Max. P and Min. P indicate the maximum and minimum principal strains, respectively; B and L, the buccal and lingual sides, respectively.
Table 2. Peak Values of the Principal Strain of Bone Around Ti and Ti-Zr Implants Microstrain, Median (IQR) Location Buccal side P* Lingual side P*
(2)
Correlation and Statistical Analysis
The measured implant mobility and the peak minimum principal strains under vertical and lateral loadings for the designed scenarios of 2 implant models (Ti-Zr alloy vs CP Ti) were summarized as medians and interquartile
Model
Vertical Loading
Lateral Loading
Ti Ti-Zr
−682.63 (105.16) −579.08 (123.83) 0.006 −615.34 (64.12) −614.11 (144.97) 0.566
−2356.91 (212.59) −1111.37 (212.48) ,0.001 −51.52 (56.7) −73.04 (49.19) 0.233
Ti Ti-Zr
No significant difference is shown between Ti and Ti-Zr implants under vertical loading; however, the bone strain on the buccal side is 52.8% lower around the Ti-Zr implant than around the Ti implant under lateral loading. *Multiple comparison with Wilcoxon rank-sum test. IQR indicates interquartile ranges.
ranges. The parameters were compared between the 2 implant models using Wilcoxon rank-sum test. All analyses were performed using the commercial statistical software (SPSS 12.0; SPSS Inc., Chicago, IL) with an alpha value of 0.001.
RESULTS Implant Mobility
Although the PTV values were slightly lower but with higher stability on the models with a Ti implant than in those with a Ti-Zr implant, analysis
IMPLANT DENTISTRY / VOLUME 23, NUMBER 5 2014 using Wilcoxon rank-sum test revealed that both PTV and ISQ values did not differ significantly between Ti and Ti-Zr implants (P . 0.001) (Table 1). Strain Gauge Analysis
Under vertical loading, the peak bone strains (minimum principal strains) around implants did not differ significantly between the Ti and Ti-Zr implants on both the buccal side (P ¼ 0.006) and the lingual side (P ¼ 0.566) of the bone (Fig. 4, A and Table 2). Under lateral loading, the peak bone strains on the lingual side also did not differ significantly between the 2 types of models (P ¼ 0.233), but the bone strain on the buccal side was 52.8% lower around the Ti-Zr implant than around the Ti implant (P , 0.001) (Fig. 4, B and Table 2).
DISCUSSION This study investigated the use of a new alloy in dental implants that contains Ti with 13% to 17% Zr in mainly an a-type structure (hexagonal closed-packed phase) but with up to 10% in an a + b-type structure (bodycentered cubic phase).18,20 This new alloy has been claimed to exhibit superior material propertiesdespecially increased fatigue strength. Researchers have also recently confirmed that the biocompatibility of a Ti-Zr implant can compete with that of a CP Ti implant.18–21 It is well known that Ti-Zr alloys can exhibit different material properties from Ti15,27; however, no previous study has investigated the biomechanical performance of bone when using a Ti-Zr implant. Search of research on this topic found that this is the first study to have used experimental strain-gauge analysis to compare the mechanical effects of Ti-Zr and Ti implants on the jawbone model. Most dentists have been taught to suggest bone grafting including autogenous bone or other available bonesubstitute materials when the width of the alveolar bone is inadequate for placing a standard-diameter implant. A small-diameter implant is also an option for patients who prefer to have implant surgery without undergoing additional bone augmentation procedures. At
present, there is no clear definition of small-diameter implants; some researchers have defined a diameter of 3.0 to 3.4 mm as small,28 whereas others have stated that small-diameter implants or mini-implants are widely believed to be those with diameters of 3 mm and below.29 The present study performed biomechanical investigations of bone for implants with a diameter of 3.3 mm. Implant mobility is considered to strongly influence the success of osseointegration. The primary implant stability at the time of placement is related to the local bone quality and quantity, the type of implant, and the placement techniques used.30 The present study used only noninvasive methods (eg, PTV and wireless ISQ) to evaluate implant mobility; the results revealed that PTV and ISQ do not differ between Ti-Zr and Ti implants, hence indicating that using either of these materials will have no effect on the mobility of a small-diameter implant. Small-diameter implants are typically recommended for narrow edentulous ridges and limited interdental spaces that are not suitable for the use of standard-diameter implants. Although grafted bone augmentation has been demonstrated to successfully enhance the width of alveolar bone for placing standard-diameter implants, avoiding the use of invasive bone augmentation associated with donorsite morbidity might enhance patient acceptance of dental implant surgery and also reduce the treatment cost. However, the use of a small-diameter implant may also increase apprehension about implant fracture due to the limited tensile strength of pure Ti for the use of orthopedic joints and medical implants.3 Another concern is that a smaller implant diameter will decrease the contact area between the implant and bone and hence may increase the risk of implant failure due to the increased stress/strain in surrounding bone31,32 associated with possible excessive bone resorption.33 The present study found no significant difference in bone strain between Ti-Zr and CP Ti implants under vertical loading. However, a particularly interesting finding was that under lateral loading the bone strain was lower for the Ti-Zr implant than
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for the CP Ti implant. This difference might be related to mechanical properties such as the bending modulus.15 As the bending modulus of Ti-Zr alloy is higher than that of pure Ti, it is reasonable to assume that strains will be lower adjacent to a Ti-Zr implant during lateral loading and that this will reduce the strains induced in the surrounding bone. This might in turn reduce the loading of the crestal bone and thereby decrease the marginal bone loss. Nevertheless, the results of this study do not prove that using a Ti-Zr implant will decrease the likelihood of bone loss or implant failure. As the results of in vitro studies cannot reliably be transferred to the clinic without also performing clinical trials, additional investigations involving clinical studies of Ti-Zr implants are required. The limitations of this study are related to (1) the trabecular structure of bone and (2) the loading conditions. The type of bone influences the implant mobility,34 and more accurate results might be obtained when using bone models with a cellular structure than with solid-bone blocks, because their architecture is more similar to that of trabecular bone. Although a static vertical load and a lateral load were suggested to represent a realistic occlusal load,35 other loading conditions combined with implant positions should also be considered in future investigations.
CONCLUSIONS Within the limitations of this study, the following conclusions can be drawn: 1. The implant mobility did not differ significantly between Ti-Zr and Ti implants, and hence using either of these implants will not influence the mobility of smalldiameter implants. 2. The strains in the jawbone model were lower for the Ti-Zr implant than for the CP Ti implant (although only under lateral loading). 3. Additional studies are needed to either corroborate or refute these conclusions.
DISCLOSURE The authors claim to have no financial interest, either directly or
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indirectly, in the products or information listed in the article.
ACKNOWLEDGMENTS The authors thank Yun-Te Lin for the technical assistance on the strain gauge experiments. This study was supported by a grant (NSC 101–2314B-039–022-MY3) from the National Science Council, Taiwan, and partly, by a research project (CMRPG8A0761) from Chang Gung Memorial Hospital, Taiwan.
REFERENCES 1. Long M, Rack HJ. Titanium alloys in total joint replacementdA materials science perspective. Biomaterials. 1998;19: 1621–1639. 2. Niinomi M. Metallic biomaterials. J Artif Organs. 2008;11:105–110. 3. Geetha M, Singh AK, Asokamani R, et al. Ti based biomaterials, the ultimate choice for orthopaedic implantsdA review. Prog Mater Sci. 2009;54:397–425. 4. Miller PD, Holladay JW. Friction and wear properties of titanium. Wear. 1958;2: 133–140. 5. Kokubo T, Kim HM, Kawashita M. Novel bioactive materials with different mechanical properties. Biomaterials. 2003;24:2161–2175. 6. Thomsen P, Larsson C, Ericson LE, et al. Structure of the interface between rabbit cortical bone and implants of gold, zirconium and titanium. J Mater Sci Mater Med. 1997;8:653–665. 7. Sherepo KM, Red’ko IA. The use of zirconium for implants in traumatology and orthopedics [in Russian]. Med Tekh. 2004; 2:22–24. 8. Gahlert M, Roehling S, Sprecher CM, et al. In vivo performance of zirconia and titanium implants: A histomorphometric study in mini pig maxillae. Clin Oral Implants Res. 2012;23:281–286. 9. Kasemo B, Lausmaa J. Biomaterials and interfaces. In: Naert I, van Steenberghe D, Worthington P, eds. Osseointegration in Oral Rehabilitation. London, United Kingdom: Quintessence publishing; 1993:63–75. 10. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials. 1999; 20:1–25. 11. Gehrke P, Dhom G, Brunner J, et al. Zirconium implant abutments: Fracture
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strength and influence of cyclic loading on retaining-screw loosening. Quintessence Int. 2006;37:19–26. 12. Kapos T, Ashy LM, Gallucci GO, et al. Computer-aided design and computer-assisted manufacturing in prosthetic implant dentistry. Int J Oral Maxillofac Implants. 2009;24(suppl):110–117. 13. Niinomi M. Mechanical properties of biomedical titanium alloys. Mater Sci Eng A. 1998;243:231–236. 14. Ikarashi Y, Toyoda K, Kobayashi E, et al. Improved biocompatibility of titanium-zirconium (Ti-Zr) alloy: Tissue reaction and sensitization to Ti-Zr alloy compared with pure Ti and Zr in rat implantation study. Mater Trans. 2005; 46:2260–2267. 15. Ho WF, Chen WK, Wu SC, et al. Structure, mechanical properties, and grindability of dental Ti–Zr alloys. J Mater Sci Mater Med. 2008;19:3179–3186. 16. Hsu HC, Wu SC, Hsu SK, et al. Effects of heat treatments on the structure and mechanical properties of Zr– 30Ti alloys. Mater Charact. 2011;62: 157–163. 17. Allum SR, Tomlinson RA, Joshi R. The impact of loads on standard diameter, small diameter and mini implants: A comparative laboratory study. Clin Oral Implants Res. 2008;19:553–559. 18. Gottlow J, Dard M, Kjellson F, et al. Evaluation of a new titanium‐zirconium dental implant: A biomechanical and histological comparative study in the mini pig. Clin Implant Dent Relat Res. 2012;14:538– 545. 19. Al‐Nawas B, Brägger U, Meijer HJ, et al. A double‐blind randomized controlled trial (RCT) of titanium‐13zirconium versus Titanium Grade IV small‐diameter bone level implants in edentulous mandibles–results from a 1‐year observation period. Clin Implant Dent Relat Res. 2012;14:896–904. 20. Barter S, Stone P, Brägger U. A pilot study to evaluate the success and survival rate of titanium–zirconium implants in partially edentulous patients: Results after 24 months of follow‐up. Clin Oral Implants Res. 2012;23:873–881. 21. Thoma DS, Jones AA, Dard M, et al. Tissue integration of a new titanium–zirconium dental implant: A comparative histologic and radiographic study in the canine. J Periodontol. 2011;82:1453– 1461. 22. Chong L, Khocht A, Suzuki JB, et al. Effect of implant design on initial
stability of tapered implants. J Oral Implantol. 2009;35:130–135. 23. Misch CE. Density of bone: Effect on surgical approach and healing. In: Misch CE, ed. Contemporary Implant Dentistry. 2nd ed. St. Louis, MO: Mosby; 1999:372–379. 24. Hahn J. Clinical uses of osteotomes. J Oral Implantol. 1999;25:23–29. 25. Alsaadi G, Quirynen M, Michiels K, et al. A biomechanical assessment of the relation between the oral implant stability at insertion and subjective bone quality assessment. J Clin Periodontol. 2007;34: 359–366. 26. Steiner M, Mitsias ME, Ludwig K, et al. In vitro evaluation of a mechanical testing chewing simulator. Dent Mater. 2009;25:494–499. 27. Wang BT, Li WD, Zhang P. Firstprinciples calculations of phase transition, elasticity, and thermodynamic properties for TiZr alloy. J Nucl Mater. 2012;420: 501–507. 28. Quek CE, Tan KB, Nicholls JI. Load fatigue performance of a single-tooth implant abutment system: Effect of diameter. Int J Oral Maxillofac Implants. 2006; 21:929–936. 29. Christensen GJ. The increased use of small-diameter implants. J Am Dent Assoc. 2009;140:709–712. 30. Meredith N. Assessment of implant stability as a prognostic determinant. Int J Prosthodont. 1998;11:491–501. 31. Hsu JT, Fuh LJ, Lin DJ, et al. Bone strain and interfacial sliding analyses of platform switching and implant diameter on an immediately loaded implant: Experimental and three-dimensional finite element analyses. J Periodontol. 2009; 80:1125–1132. 32. Huang HL, Fuh LJ, Ko CC, et al. Biomechanical effects of a maxillary implant in the augmented sinus: A threedimensional finite element analysis. Int J Oral Maxillofac Implants. 2009;24: 455–462. 33. Isidor F. Influence of forces on peri‐ implant bone. Clin Oral Implants Res. 2006;17(suppl 2):8–18. 34. Huang HL, Tu MG, Fuh LJ, et al. Effects of elasticity and structure of trabecular bone on the primary stability of dental implants. J Med Biol Eng. 2010; 30:85–89. 35. Geng JP, Tan KB, Liu GR. Application of finite element analysis in implant dentistry: A review of the literature. J Prosthet Dent. 2001;85:585–598.
TI-ZR ALLOY DENTAL IMPLANT
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An In Vitro Biomechanical Evaluation of a New Commercial Titanium-Zirconium Alloy Dental Implant: A Pilot Study Aaron Yu-Jen Wu, DDS, PhD,* Jui-Ting Hsu, PhD,† and Heng-Li Huang, PhD‡
ommercial pure (CP) titanium (Ti) and its alloys are widely used as metallic biomaterials in bone screws and plates, and also as the orthopedic and dental replacements due to their excellent biocompatibility and mechanical properties, including low modulus, high corrosion resistance, and lightness.1,2 However, pure Ti and Ti alloys still fail to meet some of the requirements for implant biomaterials in clinical applications. The tensile strength of Ti is insufficient for its use in orthopedic joints and bone plates and screws,3 and its poor wear resistance4 also prevents its widespread application in many medical fields. A Ti-aluminum (Al)-vanadium (V) alloy is the most commonly used Ti-based material for medical implants. Although Ti-Al-V alloys have very good corrosion resistance and biocompatibility, some concerns remain about the release of toxic Al and V ions into the surrounding tissue during long-term implantation. Zirconium (Zr) has been considered as an excellent alternative biomaterial
C
*Assistant Professor and Director, Department of Dentistry, Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Kaohsiung, Taiwan. †Associate Professor, Biomechanics Res. Lab, School of Dentistry, China Medical University, Taichung, Taiwan. ‡Professor, Biomechanics Res. Lab, School of Dentistry, China Medical University, Taichung, Taiwan.
Reprint requests and correspondence to: Heng-Li Huang, PhD, School of Dentistry, China Medical University, 91 Hsueh-Shih Road, Taichung 40402, Taiwan, Phone: 1-886-4-22053366 ext. 2306, Fax: 1-886-4-22014043, E-mail: [email protected] ISSN 1056-6163/14/02305-534 Implant Dentistry Volume 23 Number 5 Copyright © 2014 by Lippincott Williams & Wilkins DOI: 10.1097/ID.0000000000000108
Background: The study compared the implant mobility and surrounding bone strain between the titanium-zirconium (Ti-Zr) alloy and the commercial pure (CP) Ti implants. Methods: The mobilitydquantified as the implant stability quotient (ISQ) and Periotest value (PTV)dof implants constructed from Ti-Zr alloy and CP Ti placed into artificial type-2 jawbone models were measured. Specimens were tested by applying 190 N vertically or at 30 degrees laterally. Peak values of the principal strains of bone were recorded by rosette strain gauges with a data acquisition system and were analyzed statistically using Wilcoxon rank-sum test. Results: PTV and ISQ values did not differ significantly between the
Ti-Zr and CP Ti implants (P . 0.01). Under vertical loading, the peak bone strains did not differ significantly between the Ti-Zr and CP Ti specimens (P . 0.006). However, the peak strains were 52% lower around the Ti-Zr implant than around the Ti implant on the buccal side of bone under lateral loading (P , 0.001). Conclusions: The implant material (Ti-Zr alloy vs CP Ti) had no effect on the mobility of small-diameter dental implants. However, using Ti-Zr alloy as an implant material decreased the periimplant bone strain under lateral loading in this pilot study. (Implant Dent 2014;23:534–538) Key Words: small-diameter implant, Ti-Zr alloy, CP Ti, implant mobility, bone strain
to Ti. Not only is Zr a nontoxic metal, but a bone-like apatite layer can form on its surface that attracts the growth of bone tissue.5 There is in vivo evidence of Zr exhibiting good osseointegration with bone in animal studies.6–8 Additionally, Zr-based materials have advantages of outstanding corrosion resistance9 and high bending strength and fracture toughness.10,11 The increasing use of computer-aided design and computer-aided manufacturing techniques in recent decades has also increased the general acceptance of Zr-based materials in dental applications.12
Ti and Zr are the group 4 elements in the periodic table and hence they have similar chemical properties. Some researchers have selected Zr as an alloying element to improve the properties of CP Ti and to create different types of Ti-Zr–based alloys for medical uses, especially for implant applications.3,13–16 A novel alloy based on Ti that contained 13% to 17% Zr was recently developed for use in small-diameter implants. Decreasing the diameter of an implant increases the risk of implant fracture because of the lower mechanical fatigue strength.17 However, the superior tensile and fatigue
IMPLANT DENTISTRY / VOLUME 23, NUMBER 5 2014 strengths of the new Ti-Zr alloy compared with CP Ti18 resulted in it exhibiting acceptable mechanical strength for dental implant. Additionally, its biocompatibility in enhancing osseointegration has been confirmed in both animal tests and a clinical study.18–21 Since 2010, the mechanical strength and biocompatibility of Ti-Zr implants have been demonstrated in animal studies and a 1-year clinical observation.18–21 However, the effects of Ti-Zr implants relative to Ti implants on the biomechanical characteristics of the surrounding bone remain to be determined. This study applied a strain-gauge analysis to artificial jawbone samples to compare the biomechanical effects of Ti-Zr and CP Ti implants on induced bone strains. In addition, the mobilities of Ti-Zr and CP Ti implants were compared by measuring the implant stability quotient (ISQ) and Periotest value (PTV).
elastic modulus of 759 MPa (model 1522–05; Pacific Research Laboratories, Vashon Island, WA) was prepared to attach to a 3-mm-thick commercially available synthetic cortical shell (model 3401–02; Pacific Research Laboratories) with an elastic modulus of 16.7 GPa. The trabecular bone simulated in this study was type-2 (D2) bone22 according to the bone-density classification of Misch.23 The 3-mm thickness of the cortical bone was taken from Hahn’s study,24 which indicated that D2 bone was associated with a cortical bone height of 2.5 to 4.0 mm. The synthetic bone had a rectangular shape with dimensions of 410 3 300 3 435 mm. Three specimens of artificial foam bone were prepared for each implant system. Implant Mobility Measurement
535
implant mobility was measured using the Periotest M device (Medizintechnik Gulden, Modautal, Germany) (Fig. 2, B). The tip of the measurement device was positioned perpendicularly at 2 mm from the abutment, and it impacted the implant 4 times per second for 4 seconds.25 PTV values were also measured 4 times in 4 different directions for each model. Strain Gauge Measurement
A self-developed jig was designed with an adjustable rotational screwing device so that both a vertical load and a 30-degree lateral force in the buccallingual direction could be applied in the experiments. Each loading procedure involved applying a force of 190 N26 to the cylindrical abutment using a universal testing machine (JSVH1000; Japan Instrumentation System, Nara, Japan) with a head speed of 1 mm/min (Fig. 3). Rectangular rosette strain gauges (KFG-1-120-D17–11 L3M3S; Kyowa, Tokyo, Japan) were attached to the buccal and lingual sides of the crestal cortical region around the implant using cyanoacrylate cement (CC-33A; Kyowa). Signals corresponding to the 3 independent strains (ea, eb, and ec) measured by the rosette strain gauge were sent to a data acquisition system (NI CompackDAQ; National Instruments, Austin, TX) and analyzed by the associated software (LabVIEW SignalExpress 3.0; National Instruments). After repeating each measurement 3 times for each specimen, the maximum (emax ) and minimum (emin ) principal strains were obtained as follows:
Implants constructed from 2 kinds of materials were selected for analysis: (1) Ti implant (SLActive; Institute Straumann AG, Basel, Switzerland) and (2) Ti-Zr implant (Roxolid; Institute Straumann AG) (Fig. 1). These 2 implant types had hydrophilic sandblasted and acid-etched surfaces, a diameter of 3.3 mm, and a length of 12 mm. A Sawbone model of trabecular bone with a density of 0.64 g/cm3 and
After placing the implant, a wireless resonance frequency analyzer (Osstell ISQ; Osstell AB, Gothenburg, Sweden) was used to measure the ISQ value. Before the measurement, SmartPegs for SLActive implant system (Type 41; Osstell AB) were placed onto the top of the implants. Each SmartPeg has a magnetic material attached to its upper part that causes the probe of the Osstell ISQ instrument to vibrate when it is near to the SmartPeg, which allows the Osstell ISQ instrument to determine the resonance frequency (Fig. 2, A) and convert it into the ISQ value. For each specimen, 4 ISQ values were measured for 4 different directions (ie, buccal, lingual, mesial, and distal directions). After connecting Ti abutments, the
Fig. 1. CP Ti (left) and Ti-Zr alloy (right) implants.
Fig. 2. A, The wireless Osstell ISQ instrument used to measure ISQ value after the model was fixed to the jig. B, The PTV value was acquired after the tip of the Periotest M device touched the abutment.
MATERIALS
AND
METHODS
Implant Design Parameters and Bone Specimen Preparation
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Fig. 3. The universal loading machine applied vertical and lateral forces to the cylindrical abutment on the top of each implant individually.
Table 1. PTV and ISQ Values for Ti and Ti-Zr Implants Evaluation Approaches, Median (IQR) Implant Ti Ti-Zr P†
ISQ
PTV
71.00 (1.00) 71.00 (1.00) 0.03
−2.50 (1.00) −2.00 (0.25) 0.216
There is no significant difference in both the PTV and ISQ values between Ti and Ti-Zr implants. †Multiple comparison with Wilcoxon rank-sum test. IQR indicates interquartile ranges.
1 emax 5 ðea þ ec Þ 2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi iffi 1 h 2 2 ðea −ec Þ þð2eb −ea −ec Þ ; þ 2 (1) 1 emin 5 ðea þ ec Þ 2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i 1 h − ðea −ec Þ2 þð2eb −ea −ec Þ2 : 2
Fig. 4. Medians and interquartile ranges of maximum and minimum principal microstrain values on jawbone models around the Ti-Zr and CP Ti implants under vertical loading (A) and lateral loading (B). Max. P and Min. P indicate the maximum and minimum principal strains, respectively; B and L, the buccal and lingual sides, respectively.
Table 2. Peak Values of the Principal Strain of Bone Around Ti and Ti-Zr Implants Microstrain, Median (IQR) Location Buccal side P* Lingual side P*
(2)
Correlation and Statistical Analysis
The measured implant mobility and the peak minimum principal strains under vertical and lateral loadings for the designed scenarios of 2 implant models (Ti-Zr alloy vs CP Ti) were summarized as medians and interquartile
Model
Vertical Loading
Lateral Loading
Ti Ti-Zr
−682.63 (105.16) −579.08 (123.83) 0.006 −615.34 (64.12) −614.11 (144.97) 0.566
−2356.91 (212.59) −1111.37 (212.48) ,0.001 −51.52 (56.7) −73.04 (49.19) 0.233
Ti Ti-Zr
No significant difference is shown between Ti and Ti-Zr implants under vertical loading; however, the bone strain on the buccal side is 52.8% lower around the Ti-Zr implant than around the Ti implant under lateral loading. *Multiple comparison with Wilcoxon rank-sum test. IQR indicates interquartile ranges.
ranges. The parameters were compared between the 2 implant models using Wilcoxon rank-sum test. All analyses were performed using the commercial statistical software (SPSS 12.0; SPSS Inc., Chicago, IL) with an alpha value of 0.001.
RESULTS Implant Mobility
Although the PTV values were slightly lower but with higher stability on the models with a Ti implant than in those with a Ti-Zr implant, analysis
IMPLANT DENTISTRY / VOLUME 23, NUMBER 5 2014 using Wilcoxon rank-sum test revealed that both PTV and ISQ values did not differ significantly between Ti and Ti-Zr implants (P . 0.001) (Table 1). Strain Gauge Analysis
Under vertical loading, the peak bone strains (minimum principal strains) around implants did not differ significantly between the Ti and Ti-Zr implants on both the buccal side (P ¼ 0.006) and the lingual side (P ¼ 0.566) of the bone (Fig. 4, A and Table 2). Under lateral loading, the peak bone strains on the lingual side also did not differ significantly between the 2 types of models (P ¼ 0.233), but the bone strain on the buccal side was 52.8% lower around the Ti-Zr implant than around the Ti implant (P , 0.001) (Fig. 4, B and Table 2).
DISCUSSION This study investigated the use of a new alloy in dental implants that contains Ti with 13% to 17% Zr in mainly an a-type structure (hexagonal closed-packed phase) but with up to 10% in an a + b-type structure (bodycentered cubic phase).18,20 This new alloy has been claimed to exhibit superior material propertiesdespecially increased fatigue strength. Researchers have also recently confirmed that the biocompatibility of a Ti-Zr implant can compete with that of a CP Ti implant.18–21 It is well known that Ti-Zr alloys can exhibit different material properties from Ti15,27; however, no previous study has investigated the biomechanical performance of bone when using a Ti-Zr implant. Search of research on this topic found that this is the first study to have used experimental strain-gauge analysis to compare the mechanical effects of Ti-Zr and Ti implants on the jawbone model. Most dentists have been taught to suggest bone grafting including autogenous bone or other available bonesubstitute materials when the width of the alveolar bone is inadequate for placing a standard-diameter implant. A small-diameter implant is also an option for patients who prefer to have implant surgery without undergoing additional bone augmentation procedures. At
present, there is no clear definition of small-diameter implants; some researchers have defined a diameter of 3.0 to 3.4 mm as small,28 whereas others have stated that small-diameter implants or mini-implants are widely believed to be those with diameters of 3 mm and below.29 The present study performed biomechanical investigations of bone for implants with a diameter of 3.3 mm. Implant mobility is considered to strongly influence the success of osseointegration. The primary implant stability at the time of placement is related to the local bone quality and quantity, the type of implant, and the placement techniques used.30 The present study used only noninvasive methods (eg, PTV and wireless ISQ) to evaluate implant mobility; the results revealed that PTV and ISQ do not differ between Ti-Zr and Ti implants, hence indicating that using either of these materials will have no effect on the mobility of a small-diameter implant. Small-diameter implants are typically recommended for narrow edentulous ridges and limited interdental spaces that are not suitable for the use of standard-diameter implants. Although grafted bone augmentation has been demonstrated to successfully enhance the width of alveolar bone for placing standard-diameter implants, avoiding the use of invasive bone augmentation associated with donorsite morbidity might enhance patient acceptance of dental implant surgery and also reduce the treatment cost. However, the use of a small-diameter implant may also increase apprehension about implant fracture due to the limited tensile strength of pure Ti for the use of orthopedic joints and medical implants.3 Another concern is that a smaller implant diameter will decrease the contact area between the implant and bone and hence may increase the risk of implant failure due to the increased stress/strain in surrounding bone31,32 associated with possible excessive bone resorption.33 The present study found no significant difference in bone strain between Ti-Zr and CP Ti implants under vertical loading. However, a particularly interesting finding was that under lateral loading the bone strain was lower for the Ti-Zr implant than
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for the CP Ti implant. This difference might be related to mechanical properties such as the bending modulus.15 As the bending modulus of Ti-Zr alloy is higher than that of pure Ti, it is reasonable to assume that strains will be lower adjacent to a Ti-Zr implant during lateral loading and that this will reduce the strains induced in the surrounding bone. This might in turn reduce the loading of the crestal bone and thereby decrease the marginal bone loss. Nevertheless, the results of this study do not prove that using a Ti-Zr implant will decrease the likelihood of bone loss or implant failure. As the results of in vitro studies cannot reliably be transferred to the clinic without also performing clinical trials, additional investigations involving clinical studies of Ti-Zr implants are required. The limitations of this study are related to (1) the trabecular structure of bone and (2) the loading conditions. The type of bone influences the implant mobility,34 and more accurate results might be obtained when using bone models with a cellular structure than with solid-bone blocks, because their architecture is more similar to that of trabecular bone. Although a static vertical load and a lateral load were suggested to represent a realistic occlusal load,35 other loading conditions combined with implant positions should also be considered in future investigations.
CONCLUSIONS Within the limitations of this study, the following conclusions can be drawn: 1. The implant mobility did not differ significantly between Ti-Zr and Ti implants, and hence using either of these implants will not influence the mobility of smalldiameter implants. 2. The strains in the jawbone model were lower for the Ti-Zr implant than for the CP Ti implant (although only under lateral loading). 3. Additional studies are needed to either corroborate or refute these conclusions.
DISCLOSURE The authors claim to have no financial interest, either directly or
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indirectly, in the products or information listed in the article.
ACKNOWLEDGMENTS The authors thank Yun-Te Lin for the technical assistance on the strain gauge experiments. This study was supported by a grant (NSC 101–2314B-039–022-MY3) from the National Science Council, Taiwan, and partly, by a research project (CMRPG8A0761) from Chang Gung Memorial Hospital, Taiwan.
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