journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
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Research Paper
Analysis and comparison of clutch techniques of two dental implants Giovanni Zonfrilloa, Sara Matteolia,n, Andrea Ciabattinia, Maurizio Dolfib, Lorenzo Lorenzinib, Andrea Corvia a
Department of Industrial Engineering, University of Florence, Via di S. Marta 3, 50139 Florence, Italy Leone S.p.A.,Via P. a Quaracchi 50, 50019 Sesto Fiorentino, Firenze, Italy
b
art i cle i nfo
ab st rac t
Article history:
From the clinical point of view, primary implant stability is a fundamental requirement.
Received 29 November 2013
The aim of the present work was to investigate the primary stability of two types of dental
Received in revised form
implants, with truncated cone (TC) and cylindrical (CL) geometry, by evaluating their
21 January 2014
performance by means of pull-out tests. Moreover, several samples were tested by varying
Accepted 23 January 2014
surgical preparation method as well as the material where the implant was housed in
Available online 30 January 2014
order to assess whether primary stability could be affected by these factors. A critical load
Keywords:
which corresponds to a displacement of 0.2 mm in pull-out test was chosen as indicator of
Implant housing
the implant primary stability. CL implants had the advantage of requiring lower torques
Implant preparation method
during the installation phase, and thus, applying less local stresses on the bone. Among
Primary stability
the housing preparation methods investigated in the present study, the housings realized
Pull-out tests
by using two mill cutters of different diameters for different depths implied higher primary
Screwing torque
stability for TC implant.
1.
Introduction
From the clinical point of view, primary implant stability is a fundamental requirement (Adell et al., 1981; Misch, 2007; McElhaney, 1966) particularly when the implant is subjected to loads in a short time after implantation. It depends on the macro-implant design, the correspondence between the system geometry and the implant site, and the bone characteristics which are responsible for the implant deformations when subjected to external loads (Salles et al., 2010). These deformations are very important: some authors have found relationship between the maximum tolerable displacement and implant failure (Szmukler-Moncler et al., 1998).
n
Corresponding author. Tel.: þ39 055 479 6430. E-mail address: sara.matteoli@unifi.it (S. Matteoli).
1751-6161/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2014.01.017
& 2014 Elsevier Ltd. All rights reserved.
Primary stability is also determined by the mechanical interference fitting between implant and bone, and thus by the stresses that the implant transmits to the bone (Zmudzki et al., 2008; Albrektsson et al., 1986). Other studies (Carlos, 2011; Irinakis and Wiebe, 2009; Duyck et al., 2010; Turkyilmaz et al., 2009) evaluated that the torque necessary for inserting the implant in its housing and keeping an adequate primary stability may vary between 0.32 and 0.45 Nm. Some authors (Trisi et al., 2011) argue that by increasing the insertion torque it is possible to improve an implant's primary stability. Otherwise, high implant insertion torque can produce compression and distortion on the peri-implant bone. This has been claimed to induce deleterious effects on the local
2
journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
microcirculation, which may lead to bone necrosis and possibly to failure of the implant. In order to achieve good primary stability without creating excessive compression in the peri-implant bone, some studies have suggested (Testori et al., 2003) that implants be inserted with a torque of at least 0.3 Nm for immediately loaded full-arch prostheses in the mandible or partial prostheses in either arch. This aspect is controversial: in fact, other authors (Grandi et al., 2013), after having carried out a study on 156 implants, claiming that the use of high insertion torque (up to 0.8 Nm) did not prevent osseo-integration and did not increase bone resorption around implants early loaded up to 12 months after implant placement In the last years, with the increasing request of procedures for immediate loading, the recommended values (or at least acceptable) of insertion torque has therefore gone progressively increasing compared to the past (up to 0.8 Nm). During the development of a dental implant it may be useful to evaluate how the implant itself will behave in the mouth. In order to assess the mechanical characteristics of the implant itself (Zonfrillo and Pratesi, 2008), some methods for assessing primary stability have already been proposed, as insertion torque and resonance frequency, but no methods show completely precise results. Specifically, it is important to investigate the main characteristics (i.e. torque applied during insertion and deformation of the system) related to its primary stability. Medical orthopedics proposed a method to simulate this analysis, the pull-out tests which, despite the need for specific devices for each type of implant, are established in this medical specialty (Hoshaw and Brunski, 1989; Abid et al., 2009; Oliscovicz et al., 2013). Furthermore, it is necessary to select materials to be used as a substrate for these tests in view of the disadvantages found in selecting similar substrates to the human bone (Al-Nawas et al., 2006; Cristofolini et al., 1996; Cristofolini and Viceconti, 2000) as well as the difficult access to good in vivo quality bone, which is a prerequisite for the initial fixation (Carvalho et al., 2008). Indeed, by varying the specimen geometry as well as preparation methods of implant housing, it is possible to investigate different scenarios and optimize the system performance. Results derived from these experimental tests may provide information in terms of primary implant stability to be taken into consideration when evaluating the performance of the implant itself. The aim of the present work was to investigate the primary stability of two types of dental implants, produced by Leone S.p.A. (Firenze – Italy) by means of pull-out tests. These implants have two different external geometries: cylindrical (CL) and truncated cone (TC) shape. CL implant is the classical model used in most clinical cases (Lindh et al., 1998; Albrektsson et al., 1986), whereas TC implant has a tapered shape so as to be self-threading, and has more protrusive threads to have a better grip on the housing. The housing arrangements were chosen by taking into account their plausibility with the operations that may be performed in real cases. The performance of both implants was evaluated by means of pull-out tests when varying the preparation method of the housings. Furthermore, in order to
evaluate the performance obtained from implants when varying the method of housing preparation, PVC was used as well. Such material, although its characteristics are not similar to those of bone, has been used as its greater stiffness compared to polyurethane makes any errors due to clearance or incorrect fixtures less influential, thus reducing the dispersion of the results caused by these imperfections arises from the insertion phase. So, test results obtained using these materials can be very useful in a comparative way to evaluate the behavior of the various housings.
2.
Material and methods
2.1.
Samples
The two types of dental implants investigated are shown in Fig. 1. Both implants were made in titanium alloy Ti6Al4V grade 5 with a collar suitable for the cortical seal (Donachie, 1989). Specifically, CL implant had a core diameter equal to 3.4 mm, whereas TC had the same core diameter only in proximity of the collar and then progressively a lower diameter. The height of TC threading was 0.3 mm higher than that of the CL implant. Both threads were in accordance with ISO 5835. The implants were modified by providing a cylindrical shape on the upper part of the crown, with a hole in which a pin was inserted, so that it could provide the connection with the test machine. The lower part of the implant, with a length of 10 mm, was inserted in the housing. The material used for the housings was chosen in such a way to have properties comparable with those of the jaw bone. Specifically, according to ASTM F1839-08, the selected materials were polyurethane grade 20 (PU20) and grade 40 (PU40), as they can simulate both low and high density spongy bones. Furthermore, for each of the three materials used (PVC, PU 20 and PU40) the housings of the implants were cylinders with external diameter of 12 mm and height 40 mm. The preparation of the hole in the polymeric housings was made by using a surgical kit and following the same procedures applied during real surgery (Adell et al., 1981). A pilot
Fig. 1 – TC implant (on the left) and CL implant (on the right).
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journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
hole with diameter of 2.8 mm was produced by using an end mill, in order to ensure its concentricity with the housing and provide a guide for the subsequent processing. Then, the hole was bored for a depth of 10 mm with mills of increasing diameter up to 3.5 mm or 3.8 mm. In order to investigate the possibility of using cylindrical cutters even for TC implants, aiming to obtain a greater adherence, some housing were prepared with a pilot hole enlarged only to a depth of 8 mm, so that the last 2 mm of the hole was in contact with the tip of TC implant, as shown in Fig. 2. The screwing of the implant on the polymer was done manually, by using a fixing pin as a lever, and ensuring the coaxiality of the two pieces. During this phase, the screwing torque was measured. Ten samples were made for each condition (Table 1), for a total of 120 samples.
2.2.
manufactured for this study. As shown in Fig. 3, the polymeric housing and the implant were fixed at the basement and the moving cross head of the machine, respectively. Specifically, the housing was inserted in a hole of equal size made in a steel block which was tightened in the grip. A plate, perforated in order to allow the passage of the implant and
Equipment
Pull-out tests were conducted on all samples with an Instron machine (type 3365) by using a gripping system designed and
Fig. 2 – Section of the TC implant-housing assembly. On the right, hole of constant diameter; and on the left, preparation of the hole with the terminal part of smaller diameter.
Fig. 3 – Scheme of the device used for experimental tests. 1¼steel block, 2 ¼ plate, 3 ¼ grip for implant housing, 4¼linear displacement transducer, 5 ¼ implant, and 6 ¼ load cell.
Table 1 – Characteristics of the specimen used for pull-out tests. Specimen designation
Implant
Material
Housing
Tapping
Hole
1 2 3 4 5 6 7 8 9 10 11 12
TC TC CL TC TC CL TC TC TC TC TC TC
PU20 PU20 PU20 PU40 PU40 PU40 PVC PVC PVC PVC PVC PVC
Diameter [mm]
Length [mm]
3.5 3.5 3.5 3.5 3.5 3.5 3.8 3.8 3.8 3.8 3.5 3.5
10 10 10 10 10 10 8 8 10 10 8 8
No Yes Yes No Yes Yes No Yes No yes No Yes
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journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
connected to the steel block by four screws, closes the top of the housing and prevents it from moving. The Instron machine was equipped with a linear displacement transducer, so that the elongation of the samples could be measured during the test. Specifically, the transducer was fixed at the bottom of the steel block and connected to the other end to a pin rigidly linked to the cylindrical part of the implant. Moreover, a load cell allowed measuring of the applied load.
2.3.
Experimental text
Pull-out tests are carried out in order to investigate the behavior of implant systems installed by applying different procedures, summarized in Table 1. Once the samples were connected to the machine test, they were subjected to five loading–unloading cycles with axial forces, very much lower than those achieved during tests that allowed keeping samples in the elastic field and settling the connection between the housing and implant. Then, pull-out tests were performed. During these tests, the applied force as well as the displacement detected by the transducer and the displacement of the crosshead of the machine were measured and recorded. A digital torsiometer (Model MGT12, C.I.S.A.M. sas, Italy) was used to measure the torque applied to the implant during the screwing phase on the sample.
3.
Results
A first consideration can be made by analyzing the clamping forces reported in Tables 2 and 3. In some configurations the measured values showed an appreciable variability, as they could be influenced either by shavings located inside the hole
or possible misalignments during screwing. In real cases the variability is likely to be even greater than the one obtained in this study, so further suitable tests are necessary to investigate the probability of exerting excessive stresses on the bone during assembly before using these configurations. The literature reports different values for the maximum insertion torque depending on various factors (e.g. bone characteristics, type of installation, etc.); for the implants investigated, the manufacturer suggested to use a maximum torque equal to 0.6 Nm. The values of the torques measured showed that CL implants with tapered hole complied with this maximum bearable limit. TC implant could be installed on both lowdensity and high-density bone, but in the latter case the tapping of hole was necessary, in order to avoid exceeding the limit 0.6 Nm. When assembled in equal conditions, CL implants showed lesser resistance to screwing due to the lower height of their threads. TC implants, although quantitative data may vary from case to case, implied an increase of about 35% on torques average values in housings with a hole of 3.5 mm, while in those with a hole of 3.8 mm the increase of the torque was lower. Regarding the pull-out tests, the same phenomenology was found in all configurations. Specifically, when load was applied the implants elongated with greater deformations in the polymeric material. Indeed, increasing the load, the housing enlarged diametrically until it yielded, allowing a progressive slippage of the implant. The main results can be displayed on graphs where the applied load is plotted as a function of the displacement of the specimen, detected by the transducer. For any configuration, the load–displacement curves showed the same behavior for the 10 kinds of samples tested. Therefore, data obtained from tests performed on nominally equal samples
Table 2 – Maximum load, stiffness and torque, obtained on PU20 and PU40 housings, expressed as average plus/minus one standard deviation. Sample designation
Maximum load [N]
Stiffness [N/mm]
Torque [Nm]
Lc [N]
1 2 3 4 5 6
29475.71 241717.1 21874.38 968725.6 846724.7 811739.5
11257175 7977159 6587188 33337964 31987450 21517336
0.1470.60 0.0970.40 0.0370.70 0.6777.00 0.4573.50 0.1571.10
225735 160732 132738 6677193 640790 430767
Lc ¼Critical load in correspondence of 0.2 mm.
Table 3 – Maximum load, stiffness and torque obtained on PVC housings, expressed as average plus/minus one standard deviation. Sample designation
Maximum load [N]
Stiffness [N/mm]
Torque [Nm]
Lc [N]
7 8 9 10 11 12
29717101.8 2835779.36 2959755.9 2832726.50 3016751.62 2856792.14
1255171601 1164572025 796571658 781871971 1275971512 1177272607
0.75711.7 0.67715.4 0.4974.00 0.4777.20 1.1175.90 0.7179.40
25107321 23297405 15937331 15647394 25527304 23547523
journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
were processed in order to obtain average values, and a single graph (representing the typical behavior of the implants during the pull-out tests) was built and shown. However, the determination of the parameters chosen for assessing the primary stability – the maximum load reached in the tests, and the angular coefficient (representing the stiffness of the system) calculated in the linear part of the loading curve which – was carried out for each test. The higher these parameters, the greater the stability, as the implants will be able to bear higher loads and show lower deformations at the same applied load.
3.1.
Housings in PU 20 and PU40
Figs. 4 and 5 show curves obtained from the pull-out tests, while Table 2 shows the main results obtained for samples in PU 20 and PU 40. These two materials used for the housing, excluding the obvious differences due to different mechanical properties of the two polymers, showed similar behavior when subjected to the same conditions. It can be noted that the higher the stiffness, the higher the maximum load. The most stable implant was therefore TC implant with not tapered housing. Specifically, when comparing this case with a TC implant inserted in a tapered housing, there was an increase of approximately 20% of both stiffness and load.
5
Unlike what expected, no marked differences were found between CL and TC implants with tapered housings. Considering an increase of 85.7% of the height of the thread, the maximum load that could be reached was only greater of about 13%. By analyzing the shape of the housings after the test, it can be observed that in those lodging CL implants the hole had a smooth surface, whereas in those lodging TC implants the hole had a more jagged shape (Fig. 6). It can be assumed that the stresses in the housing are quite localized, mainly involving the material located between the threads. This portion of material deforms in such a way to increase the size of the hole, and – in case of CL – allowing the removal of the implant. TC implant, having the first threads with greater outer diameter, would cause some rips in the polymeric material during the extraction.
3.2.
Housings in PVC
Fig. 7 shows the curves obtained from the pull-out tests while and Table 3 shows the main results found for TC implants with PVC housings when varying the housing preparation methods. PVC housings led to values of both stiffness and load considerably higher than those observed in tests performed on PU housings. The focus, however, is directed to the method of realization of the hole in which the system is housed and not to the comparison of the results with those previously described. The curves shown in Fig. 7, although without any marked differences in terms of maximum load or angular coefficient,
Fig. 4 – Average behavior of the load vs. displacement curves of PU 20 housings.
Fig. 6 – Comparison between samples after a pull-out test. CL (on the right) and TC implant (on the left).
Fig. 5 – Average behavior of the load vs. displacement curves of PU 40 housings.
Fig. 7 – Average behavior of the load vs. displacement curves of PVC housings when varying the implant preparation method.
6
journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
give indications of the different stabilities obtained according to the method of preparation used. In particular, the cases with higher stability were those with non-tapered hole and made with two cylindrical portions of different diameters (samples 7 and 11, Table 1). This type of preparation implies a greater contact surface between the housing and the implant, also evidenced by the significantly higher values of the tightening torques. Indeed, compressive residual stresses on both implant and housing were generated during assembling, all threads were engaged and the system was stiffer during pull-out. The tapping of the hole, instead, makes the system less stable; when screwing the implant, the higher torque required to tighten up at the bottom of the implant leads to higher compressive load on the threads of the nut and generate some gap. This will lead to behavior similar to that obtained for the system with the hole of constant diameter. Finally, no relevant differences were observed when using a hole of 3.8 mm or 3.5 mm.
4.
Discussion and conclusions
In the present study the primary stability of both TC and CL implants was evaluated by performing laboratory pull-out tests. The material used for the housings were polyurethane grade 20 (PU20) and grade 40 (PU40), as they can simulate both low and high density spongy bones. The structural contribution of cortical bone was not considered due to the negligible thickness. From these tests various variables were obtained. The parameter that appears to be more directly related to the stability is the micromotion between the implant and bone surfaces; literature reports a threshold for tolerated micromotion between 50 and 100 mm (Carlos, 2011) for keeping osteoblasts in high activity, above which fibrous encapsulation prevails over osseointegration. Preliminary finite element analyses have shown that the implant displacement during loading must not exceed 0.2 mm in order to ensure bone–implant mutual displacements less than 100 mm. Therefore, in the present study we have considered a critic load (Lc) which corresponds to a displacement of 0.2 mm (Dc). Lc can be evaluated on the basis of the value of stiffness of the system. A first consideration can be made comparing Lc with the maximum load (Lmax) achieved during the tests; as Lc is lower than Lmax in all tests, it was correct to look at the critical load for further analysis. When looking at the trends of the load–displacement curves obtained from the pull-out tests on PU housings (Figs. 4 and 5), it can be noticed that when both TC and CL implants were built with the same preparation method, the geometry of TC implant led always to a higher primary stability. TC implant reached higher loads with less deformation of the system, and the value of Lc increased by more than 50%. Indeed, the particular design of the thread of TC implants distributes the stress over a greater amount of material, and thus the implant is more stable. The material on which the implant is screwed plays a crucial role on the implant stability, varying Lc withstood by
the system by a factor of 3 or more. Specifically, Lc was around 130–230 N for PU20, while Lc grew over 400 N for PU40. In order to analyze whether the presence of tapping was affecting the TC primary stability, taking into account that TC implant is self-threading, only half of TC housings were tapered by using the same tapping procedure applied on CL housings. Indeed, the tapping is a very important operation, because in this phase a quantity of material – greater than that strictly required to realize the thread – could be removed, making this process both critical and decisive in terms of the implant stability. Therefore, a gap between tapped housings and implant may be present, reducing the force necessary to obtain Dc. In case of CL implants the gap magnitude was considerable, whereas for TC implants the gap was significantly reduced or absent in correspondence of the first threads (due to their greater height), while it was comparable to that present in the CL in the last threads (due to the tapering of the geometry). This may explain the different stiffness observed in the cases examined. Results obtained for PVC housings, useful only for comparative analysis, showed that the best technique for installing a TC implant is to realize the hole with two end mills of different diameters without any tapping (sample 11). The absence of tapping allows the implant to adapt on the housing in function of its geometry. Furthermore, the reduction in size of the hole leads to a tight linkage between the implant and housing in its terminal portion. The installation without tapping is clearly possible only for the geometry of TC implant. Moreover, there was no a significant difference in terms of stability between an implant installed in a hole of 3.5 mm or 3.8 mm. The choice of the implant to be installed cannot be made only on the basis of which one withstands higher loads, but must also take into account the implant tightening torques. In some cases, a strong increment of the assembly stress corresponds to a small increase in performance during pullout tests. The value of torque is, however, very much influenced by both material and type of implant preparation. In general the increase of housing density and the adhesion with the implant correspond to an increase of the screwing torque necessary to insert the implant. By observing the average values of torque when varying the tests, it is evident that the material is the most crucial feature that influences torque, followed by the presence or absence of the tapping. It is possible to obtain a relationship between screwing torque and Lc or stiffness, and hence between torque and system stability. A roughly linear trend, with flattening in correspondence of both the highest and lowest values, is obtained from our data, even though the plot was not shown for lack of a useful number of tests and cases. In conclusion, in this study it was assessed that the housing preparation technique has a great influence on primary stability. We have also given a quantitative indication of how the primary stability varies with the different techniques by relating it with the value of Lc. As TC support loads more than CL, it would always be convenient to use a TC implant, especially for relatively weaker bones, represented by the behavior of the PU 20.
journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
On the other hand, the use of a CL implant presents clear advantages considering the simplicity of the surgical phase. The present work gives quantitative indications for choosing the appropriate installation, taking into account both the values of the maximum torque – directly related to the stresses on the bone due to the screwing phase – and the critical load. Obviously, this information has to be integrated by surgeons with both instrumental and direct observations on the patient during surgery in order to identify the optimal surgical strategy.
r e f e r e n c e s
Abid, M., Kharrat, M., Dammak, M., Maalej, A., 2009. Load transfer analysis at the interface between a steel post and polyethylene matrix using pull-out test: experimental and theoretical parametric study. J. Adhes. Sci. Technol. 23 (2), 259–267. Adell, R., Lekholm, U., Rockler, B., Bra˚nemark, P.I.A., 1981. 15-year Study of osseointegrated implants in the treatment of the edentulous jaw. Int. J. Oral Surg. 10 (6), 387–416. Al-Nawas, B., Wagner, W., Gro¨tz, K.A., 2006. Insertion torque and resonance frequency analysis of dental implant systems in an animal model with loaded implants. Int. J. Oral Maxillofac. Implants 21 (5), 726–732. Albrektsson, T., Zarb, G., Worthington, P., Eriksson, A.R., 1986. The long term efficacy of currently used implants: a review and proposed criteria of success. Int. J. Oral Maxillofac. Implants 1 (1), 11–25. Carlos Nelson Elias, 2011. Factors affecting the success of dental implants, implant dentistry—a rapidly evolving practice. Ilser Turkyilmaz (Ed.), ISBN: 978-953-307-658-4, InTech, DOI: 10.5772/18746. Available from: 〈http://www.intechopen.com/ books/implant-dentistry-a-rapidly-evolving-practice/ factors-affecting-the-success-of-dental-implants〉. Carvalho, M.A., Queiroz, C.M., Molena, C.C.L., Rezende, C.P., Rapoport, A., 2008. Clinical study of the relationship between the implant insertion torque and the osseointegration. Rev. Bra. Cir. Cabec¸a Pescoc¸o 37 (4), 202–205. Cristofolini, L., Viceconti, M., 2000. Mechanical validation of whole bone composite tibia models. J. Biomech. 33 (3), 279–288. Cristofolini, L., Viceconti, M., Cappello, A., Toni, A., 1996. Mechanical validation of whole bone composite femur models. J. Biomech. 29 (4), 525–535. Donachie, 1989. Titanium, A Technical Guide. S.l: ASM. Duyck, J., Corpas, L., Vermeiren, S., Ogawa, T., Quirynen, M., Vandamme, K., et al., 2010. Histological, histomorphometrical,
7
and radiological evaluation of an experimental implant design with a high insertion torque. Clin. Oral Implants Res. 21, 877–884. Grandi, T., Guazzi, P., Samarani, R., Grandi, G., 2013. Clinical outcome and bone healing of implants placed with high insertion torque: 12-month results from a multicenter controlled cohort study. Int. J. Oral Maxillofac. Surg. 42 (4), 516–520. Hoshaw, S.J., Brunski, J.B., Cochran, G.V.B., 1989. Pull-out and fatigue failure of bone–dental implant interfaces. Am. Soc. Mech. Eng., Appl. Mech. Div., AMD 98, 05–208. Irinakis, T., Wiebe, C., 2009. Initial torque stability of a new bone condensing dental implant. A cohort study of 140 consecutively placed implants. J. Oral Implantol. 6, 277–282. Lindh, T., Gunne, J., Tillberg, A., et al., 1998. A meta-analysis of implant in partial edentulism. Clin. Oral Implants Res. 9 (2), 80–90. McElhaney, J.H., 1966. Dynamic response of bone and muscle tissue. J. Appl. Physiol. 21 (4), 1231–1236. Misch, C.E., 2007. Contemporary implant dentistry 3e, Maryland Heights. Mosby Elsevier, Missouri1102. Oliscovicz N.F., Shimano A.C., Marcantonio Junior E., Lepri C.P., Dos Reis A.C., 2013. Analysis of primary stability of dental implants inserted in different substrates using the pullout test and insertion torque. International Journal of Dentistry. 2013:194987, http://dx.doi.org/10.1155/2013/194987. Epub Jan 22, 2013. Salles, B., Passos, E., Oliveira, E., Chagas, A., Bruniera, R., Antoninha, A., 2010. Influence of high insertion torque on implant placement—an anisotropic bone stress analysis. Brazilian Dent. J. 21 (6), 508–514. Szmukler-Moncler, S., Salama, H., Reingewirtz, Y., Dubruille, J.H., 1998. Timing of loading and effect of micromotion on bone– dental implant interface: review of experimental literature. J. Biomed. Mater. Res. 43 (2), 192–203. Testori, T., Del Fabbro, M., Szmukler-Moncler, S., Francetti, L., Weinstein, R.L., 2003. Immediate occlusal loading of osseotite implants in the completely edentulous mandible. Int. J. Oral Maxillofac. Implants 18, 544–551. Trisi, P., Todisco, M., Consolo, U., Travaglini, D., 2011. High versus low implant insertion torque: a histologic, histomorphometric, and biomechanical study in the sheep mandible. Int. J. Oral Maxillofac. Implants 26 (4), 837–849. Turkyilmaz, I., Sennerby, L., McGlumphy, E.A., Tozum, T.F., 2009. Biomechanical aspects of primary implant stability: a human cadaver study. Clin. Implant Dent. Relat. Res. 2, 113–119. Zmudzki, J., Walke, W., Chladek, W., 2008. Stress present in bone surrounding dental implants in FEM model experiments. J. Achiev. Mater. Manuf. Eng. 27 (1), 71–74. Zonfrillo, G., Pratesi, F., 2008. Mechanical strength of dental implants. J. Appl. Biomater. Biomech. 6 (2), 110–118.
Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Research Paper
Analysis and comparison of clutch techniques of two dental implants Giovanni Zonfrilloa, Sara Matteolia,n, Andrea Ciabattinia, Maurizio Dolfib, Lorenzo Lorenzinib, Andrea Corvia a
Department of Industrial Engineering, University of Florence, Via di S. Marta 3, 50139 Florence, Italy Leone S.p.A.,Via P. a Quaracchi 50, 50019 Sesto Fiorentino, Firenze, Italy
b
art i cle i nfo
ab st rac t
Article history:
From the clinical point of view, primary implant stability is a fundamental requirement.
Received 29 November 2013
The aim of the present work was to investigate the primary stability of two types of dental
Received in revised form
implants, with truncated cone (TC) and cylindrical (CL) geometry, by evaluating their
21 January 2014
performance by means of pull-out tests. Moreover, several samples were tested by varying
Accepted 23 January 2014
surgical preparation method as well as the material where the implant was housed in
Available online 30 January 2014
order to assess whether primary stability could be affected by these factors. A critical load
Keywords:
which corresponds to a displacement of 0.2 mm in pull-out test was chosen as indicator of
Implant housing
the implant primary stability. CL implants had the advantage of requiring lower torques
Implant preparation method
during the installation phase, and thus, applying less local stresses on the bone. Among
Primary stability
the housing preparation methods investigated in the present study, the housings realized
Pull-out tests
by using two mill cutters of different diameters for different depths implied higher primary
Screwing torque
stability for TC implant.
1.
Introduction
From the clinical point of view, primary implant stability is a fundamental requirement (Adell et al., 1981; Misch, 2007; McElhaney, 1966) particularly when the implant is subjected to loads in a short time after implantation. It depends on the macro-implant design, the correspondence between the system geometry and the implant site, and the bone characteristics which are responsible for the implant deformations when subjected to external loads (Salles et al., 2010). These deformations are very important: some authors have found relationship between the maximum tolerable displacement and implant failure (Szmukler-Moncler et al., 1998).
n
Corresponding author. Tel.: þ39 055 479 6430. E-mail address: sara.matteoli@unifi.it (S. Matteoli).
1751-6161/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2014.01.017
& 2014 Elsevier Ltd. All rights reserved.
Primary stability is also determined by the mechanical interference fitting between implant and bone, and thus by the stresses that the implant transmits to the bone (Zmudzki et al., 2008; Albrektsson et al., 1986). Other studies (Carlos, 2011; Irinakis and Wiebe, 2009; Duyck et al., 2010; Turkyilmaz et al., 2009) evaluated that the torque necessary for inserting the implant in its housing and keeping an adequate primary stability may vary between 0.32 and 0.45 Nm. Some authors (Trisi et al., 2011) argue that by increasing the insertion torque it is possible to improve an implant's primary stability. Otherwise, high implant insertion torque can produce compression and distortion on the peri-implant bone. This has been claimed to induce deleterious effects on the local
2
journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
microcirculation, which may lead to bone necrosis and possibly to failure of the implant. In order to achieve good primary stability without creating excessive compression in the peri-implant bone, some studies have suggested (Testori et al., 2003) that implants be inserted with a torque of at least 0.3 Nm for immediately loaded full-arch prostheses in the mandible or partial prostheses in either arch. This aspect is controversial: in fact, other authors (Grandi et al., 2013), after having carried out a study on 156 implants, claiming that the use of high insertion torque (up to 0.8 Nm) did not prevent osseo-integration and did not increase bone resorption around implants early loaded up to 12 months after implant placement In the last years, with the increasing request of procedures for immediate loading, the recommended values (or at least acceptable) of insertion torque has therefore gone progressively increasing compared to the past (up to 0.8 Nm). During the development of a dental implant it may be useful to evaluate how the implant itself will behave in the mouth. In order to assess the mechanical characteristics of the implant itself (Zonfrillo and Pratesi, 2008), some methods for assessing primary stability have already been proposed, as insertion torque and resonance frequency, but no methods show completely precise results. Specifically, it is important to investigate the main characteristics (i.e. torque applied during insertion and deformation of the system) related to its primary stability. Medical orthopedics proposed a method to simulate this analysis, the pull-out tests which, despite the need for specific devices for each type of implant, are established in this medical specialty (Hoshaw and Brunski, 1989; Abid et al., 2009; Oliscovicz et al., 2013). Furthermore, it is necessary to select materials to be used as a substrate for these tests in view of the disadvantages found in selecting similar substrates to the human bone (Al-Nawas et al., 2006; Cristofolini et al., 1996; Cristofolini and Viceconti, 2000) as well as the difficult access to good in vivo quality bone, which is a prerequisite for the initial fixation (Carvalho et al., 2008). Indeed, by varying the specimen geometry as well as preparation methods of implant housing, it is possible to investigate different scenarios and optimize the system performance. Results derived from these experimental tests may provide information in terms of primary implant stability to be taken into consideration when evaluating the performance of the implant itself. The aim of the present work was to investigate the primary stability of two types of dental implants, produced by Leone S.p.A. (Firenze – Italy) by means of pull-out tests. These implants have two different external geometries: cylindrical (CL) and truncated cone (TC) shape. CL implant is the classical model used in most clinical cases (Lindh et al., 1998; Albrektsson et al., 1986), whereas TC implant has a tapered shape so as to be self-threading, and has more protrusive threads to have a better grip on the housing. The housing arrangements were chosen by taking into account their plausibility with the operations that may be performed in real cases. The performance of both implants was evaluated by means of pull-out tests when varying the preparation method of the housings. Furthermore, in order to
evaluate the performance obtained from implants when varying the method of housing preparation, PVC was used as well. Such material, although its characteristics are not similar to those of bone, has been used as its greater stiffness compared to polyurethane makes any errors due to clearance or incorrect fixtures less influential, thus reducing the dispersion of the results caused by these imperfections arises from the insertion phase. So, test results obtained using these materials can be very useful in a comparative way to evaluate the behavior of the various housings.
2.
Material and methods
2.1.
Samples
The two types of dental implants investigated are shown in Fig. 1. Both implants were made in titanium alloy Ti6Al4V grade 5 with a collar suitable for the cortical seal (Donachie, 1989). Specifically, CL implant had a core diameter equal to 3.4 mm, whereas TC had the same core diameter only in proximity of the collar and then progressively a lower diameter. The height of TC threading was 0.3 mm higher than that of the CL implant. Both threads were in accordance with ISO 5835. The implants were modified by providing a cylindrical shape on the upper part of the crown, with a hole in which a pin was inserted, so that it could provide the connection with the test machine. The lower part of the implant, with a length of 10 mm, was inserted in the housing. The material used for the housings was chosen in such a way to have properties comparable with those of the jaw bone. Specifically, according to ASTM F1839-08, the selected materials were polyurethane grade 20 (PU20) and grade 40 (PU40), as they can simulate both low and high density spongy bones. Furthermore, for each of the three materials used (PVC, PU 20 and PU40) the housings of the implants were cylinders with external diameter of 12 mm and height 40 mm. The preparation of the hole in the polymeric housings was made by using a surgical kit and following the same procedures applied during real surgery (Adell et al., 1981). A pilot
Fig. 1 – TC implant (on the left) and CL implant (on the right).
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journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
hole with diameter of 2.8 mm was produced by using an end mill, in order to ensure its concentricity with the housing and provide a guide for the subsequent processing. Then, the hole was bored for a depth of 10 mm with mills of increasing diameter up to 3.5 mm or 3.8 mm. In order to investigate the possibility of using cylindrical cutters even for TC implants, aiming to obtain a greater adherence, some housing were prepared with a pilot hole enlarged only to a depth of 8 mm, so that the last 2 mm of the hole was in contact with the tip of TC implant, as shown in Fig. 2. The screwing of the implant on the polymer was done manually, by using a fixing pin as a lever, and ensuring the coaxiality of the two pieces. During this phase, the screwing torque was measured. Ten samples were made for each condition (Table 1), for a total of 120 samples.
2.2.
manufactured for this study. As shown in Fig. 3, the polymeric housing and the implant were fixed at the basement and the moving cross head of the machine, respectively. Specifically, the housing was inserted in a hole of equal size made in a steel block which was tightened in the grip. A plate, perforated in order to allow the passage of the implant and
Equipment
Pull-out tests were conducted on all samples with an Instron machine (type 3365) by using a gripping system designed and
Fig. 2 – Section of the TC implant-housing assembly. On the right, hole of constant diameter; and on the left, preparation of the hole with the terminal part of smaller diameter.
Fig. 3 – Scheme of the device used for experimental tests. 1¼steel block, 2 ¼ plate, 3 ¼ grip for implant housing, 4¼linear displacement transducer, 5 ¼ implant, and 6 ¼ load cell.
Table 1 – Characteristics of the specimen used for pull-out tests. Specimen designation
Implant
Material
Housing
Tapping
Hole
1 2 3 4 5 6 7 8 9 10 11 12
TC TC CL TC TC CL TC TC TC TC TC TC
PU20 PU20 PU20 PU40 PU40 PU40 PVC PVC PVC PVC PVC PVC
Diameter [mm]
Length [mm]
3.5 3.5 3.5 3.5 3.5 3.5 3.8 3.8 3.8 3.8 3.5 3.5
10 10 10 10 10 10 8 8 10 10 8 8
No Yes Yes No Yes Yes No Yes No yes No Yes
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journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
connected to the steel block by four screws, closes the top of the housing and prevents it from moving. The Instron machine was equipped with a linear displacement transducer, so that the elongation of the samples could be measured during the test. Specifically, the transducer was fixed at the bottom of the steel block and connected to the other end to a pin rigidly linked to the cylindrical part of the implant. Moreover, a load cell allowed measuring of the applied load.
2.3.
Experimental text
Pull-out tests are carried out in order to investigate the behavior of implant systems installed by applying different procedures, summarized in Table 1. Once the samples were connected to the machine test, they were subjected to five loading–unloading cycles with axial forces, very much lower than those achieved during tests that allowed keeping samples in the elastic field and settling the connection between the housing and implant. Then, pull-out tests were performed. During these tests, the applied force as well as the displacement detected by the transducer and the displacement of the crosshead of the machine were measured and recorded. A digital torsiometer (Model MGT12, C.I.S.A.M. sas, Italy) was used to measure the torque applied to the implant during the screwing phase on the sample.
3.
Results
A first consideration can be made by analyzing the clamping forces reported in Tables 2 and 3. In some configurations the measured values showed an appreciable variability, as they could be influenced either by shavings located inside the hole
or possible misalignments during screwing. In real cases the variability is likely to be even greater than the one obtained in this study, so further suitable tests are necessary to investigate the probability of exerting excessive stresses on the bone during assembly before using these configurations. The literature reports different values for the maximum insertion torque depending on various factors (e.g. bone characteristics, type of installation, etc.); for the implants investigated, the manufacturer suggested to use a maximum torque equal to 0.6 Nm. The values of the torques measured showed that CL implants with tapered hole complied with this maximum bearable limit. TC implant could be installed on both lowdensity and high-density bone, but in the latter case the tapping of hole was necessary, in order to avoid exceeding the limit 0.6 Nm. When assembled in equal conditions, CL implants showed lesser resistance to screwing due to the lower height of their threads. TC implants, although quantitative data may vary from case to case, implied an increase of about 35% on torques average values in housings with a hole of 3.5 mm, while in those with a hole of 3.8 mm the increase of the torque was lower. Regarding the pull-out tests, the same phenomenology was found in all configurations. Specifically, when load was applied the implants elongated with greater deformations in the polymeric material. Indeed, increasing the load, the housing enlarged diametrically until it yielded, allowing a progressive slippage of the implant. The main results can be displayed on graphs where the applied load is plotted as a function of the displacement of the specimen, detected by the transducer. For any configuration, the load–displacement curves showed the same behavior for the 10 kinds of samples tested. Therefore, data obtained from tests performed on nominally equal samples
Table 2 – Maximum load, stiffness and torque, obtained on PU20 and PU40 housings, expressed as average plus/minus one standard deviation. Sample designation
Maximum load [N]
Stiffness [N/mm]
Torque [Nm]
Lc [N]
1 2 3 4 5 6
29475.71 241717.1 21874.38 968725.6 846724.7 811739.5
11257175 7977159 6587188 33337964 31987450 21517336
0.1470.60 0.0970.40 0.0370.70 0.6777.00 0.4573.50 0.1571.10
225735 160732 132738 6677193 640790 430767
Lc ¼Critical load in correspondence of 0.2 mm.
Table 3 – Maximum load, stiffness and torque obtained on PVC housings, expressed as average plus/minus one standard deviation. Sample designation
Maximum load [N]
Stiffness [N/mm]
Torque [Nm]
Lc [N]
7 8 9 10 11 12
29717101.8 2835779.36 2959755.9 2832726.50 3016751.62 2856792.14
1255171601 1164572025 796571658 781871971 1275971512 1177272607
0.75711.7 0.67715.4 0.4974.00 0.4777.20 1.1175.90 0.7179.40
25107321 23297405 15937331 15647394 25527304 23547523
journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
were processed in order to obtain average values, and a single graph (representing the typical behavior of the implants during the pull-out tests) was built and shown. However, the determination of the parameters chosen for assessing the primary stability – the maximum load reached in the tests, and the angular coefficient (representing the stiffness of the system) calculated in the linear part of the loading curve which – was carried out for each test. The higher these parameters, the greater the stability, as the implants will be able to bear higher loads and show lower deformations at the same applied load.
3.1.
Housings in PU 20 and PU40
Figs. 4 and 5 show curves obtained from the pull-out tests, while Table 2 shows the main results obtained for samples in PU 20 and PU 40. These two materials used for the housing, excluding the obvious differences due to different mechanical properties of the two polymers, showed similar behavior when subjected to the same conditions. It can be noted that the higher the stiffness, the higher the maximum load. The most stable implant was therefore TC implant with not tapered housing. Specifically, when comparing this case with a TC implant inserted in a tapered housing, there was an increase of approximately 20% of both stiffness and load.
5
Unlike what expected, no marked differences were found between CL and TC implants with tapered housings. Considering an increase of 85.7% of the height of the thread, the maximum load that could be reached was only greater of about 13%. By analyzing the shape of the housings after the test, it can be observed that in those lodging CL implants the hole had a smooth surface, whereas in those lodging TC implants the hole had a more jagged shape (Fig. 6). It can be assumed that the stresses in the housing are quite localized, mainly involving the material located between the threads. This portion of material deforms in such a way to increase the size of the hole, and – in case of CL – allowing the removal of the implant. TC implant, having the first threads with greater outer diameter, would cause some rips in the polymeric material during the extraction.
3.2.
Housings in PVC
Fig. 7 shows the curves obtained from the pull-out tests while and Table 3 shows the main results found for TC implants with PVC housings when varying the housing preparation methods. PVC housings led to values of both stiffness and load considerably higher than those observed in tests performed on PU housings. The focus, however, is directed to the method of realization of the hole in which the system is housed and not to the comparison of the results with those previously described. The curves shown in Fig. 7, although without any marked differences in terms of maximum load or angular coefficient,
Fig. 4 – Average behavior of the load vs. displacement curves of PU 20 housings.
Fig. 6 – Comparison between samples after a pull-out test. CL (on the right) and TC implant (on the left).
Fig. 5 – Average behavior of the load vs. displacement curves of PU 40 housings.
Fig. 7 – Average behavior of the load vs. displacement curves of PVC housings when varying the implant preparation method.
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journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
give indications of the different stabilities obtained according to the method of preparation used. In particular, the cases with higher stability were those with non-tapered hole and made with two cylindrical portions of different diameters (samples 7 and 11, Table 1). This type of preparation implies a greater contact surface between the housing and the implant, also evidenced by the significantly higher values of the tightening torques. Indeed, compressive residual stresses on both implant and housing were generated during assembling, all threads were engaged and the system was stiffer during pull-out. The tapping of the hole, instead, makes the system less stable; when screwing the implant, the higher torque required to tighten up at the bottom of the implant leads to higher compressive load on the threads of the nut and generate some gap. This will lead to behavior similar to that obtained for the system with the hole of constant diameter. Finally, no relevant differences were observed when using a hole of 3.8 mm or 3.5 mm.
4.
Discussion and conclusions
In the present study the primary stability of both TC and CL implants was evaluated by performing laboratory pull-out tests. The material used for the housings were polyurethane grade 20 (PU20) and grade 40 (PU40), as they can simulate both low and high density spongy bones. The structural contribution of cortical bone was not considered due to the negligible thickness. From these tests various variables were obtained. The parameter that appears to be more directly related to the stability is the micromotion between the implant and bone surfaces; literature reports a threshold for tolerated micromotion between 50 and 100 mm (Carlos, 2011) for keeping osteoblasts in high activity, above which fibrous encapsulation prevails over osseointegration. Preliminary finite element analyses have shown that the implant displacement during loading must not exceed 0.2 mm in order to ensure bone–implant mutual displacements less than 100 mm. Therefore, in the present study we have considered a critic load (Lc) which corresponds to a displacement of 0.2 mm (Dc). Lc can be evaluated on the basis of the value of stiffness of the system. A first consideration can be made comparing Lc with the maximum load (Lmax) achieved during the tests; as Lc is lower than Lmax in all tests, it was correct to look at the critical load for further analysis. When looking at the trends of the load–displacement curves obtained from the pull-out tests on PU housings (Figs. 4 and 5), it can be noticed that when both TC and CL implants were built with the same preparation method, the geometry of TC implant led always to a higher primary stability. TC implant reached higher loads with less deformation of the system, and the value of Lc increased by more than 50%. Indeed, the particular design of the thread of TC implants distributes the stress over a greater amount of material, and thus the implant is more stable. The material on which the implant is screwed plays a crucial role on the implant stability, varying Lc withstood by
the system by a factor of 3 or more. Specifically, Lc was around 130–230 N for PU20, while Lc grew over 400 N for PU40. In order to analyze whether the presence of tapping was affecting the TC primary stability, taking into account that TC implant is self-threading, only half of TC housings were tapered by using the same tapping procedure applied on CL housings. Indeed, the tapping is a very important operation, because in this phase a quantity of material – greater than that strictly required to realize the thread – could be removed, making this process both critical and decisive in terms of the implant stability. Therefore, a gap between tapped housings and implant may be present, reducing the force necessary to obtain Dc. In case of CL implants the gap magnitude was considerable, whereas for TC implants the gap was significantly reduced or absent in correspondence of the first threads (due to their greater height), while it was comparable to that present in the CL in the last threads (due to the tapering of the geometry). This may explain the different stiffness observed in the cases examined. Results obtained for PVC housings, useful only for comparative analysis, showed that the best technique for installing a TC implant is to realize the hole with two end mills of different diameters without any tapping (sample 11). The absence of tapping allows the implant to adapt on the housing in function of its geometry. Furthermore, the reduction in size of the hole leads to a tight linkage between the implant and housing in its terminal portion. The installation without tapping is clearly possible only for the geometry of TC implant. Moreover, there was no a significant difference in terms of stability between an implant installed in a hole of 3.5 mm or 3.8 mm. The choice of the implant to be installed cannot be made only on the basis of which one withstands higher loads, but must also take into account the implant tightening torques. In some cases, a strong increment of the assembly stress corresponds to a small increase in performance during pullout tests. The value of torque is, however, very much influenced by both material and type of implant preparation. In general the increase of housing density and the adhesion with the implant correspond to an increase of the screwing torque necessary to insert the implant. By observing the average values of torque when varying the tests, it is evident that the material is the most crucial feature that influences torque, followed by the presence or absence of the tapping. It is possible to obtain a relationship between screwing torque and Lc or stiffness, and hence between torque and system stability. A roughly linear trend, with flattening in correspondence of both the highest and lowest values, is obtained from our data, even though the plot was not shown for lack of a useful number of tests and cases. In conclusion, in this study it was assessed that the housing preparation technique has a great influence on primary stability. We have also given a quantitative indication of how the primary stability varies with the different techniques by relating it with the value of Lc. As TC support loads more than CL, it would always be convenient to use a TC implant, especially for relatively weaker bones, represented by the behavior of the PU 20.
journal of the mechanical behavior of biomedical materials 34 (2014) 1 –7
On the other hand, the use of a CL implant presents clear advantages considering the simplicity of the surgical phase. The present work gives quantitative indications for choosing the appropriate installation, taking into account both the values of the maximum torque – directly related to the stresses on the bone due to the screwing phase – and the critical load. Obviously, this information has to be integrated by surgeons with both instrumental and direct observations on the patient during surgery in order to identify the optimal surgical strategy.
r e f e r e n c e s
Abid, M., Kharrat, M., Dammak, M., Maalej, A., 2009. Load transfer analysis at the interface between a steel post and polyethylene matrix using pull-out test: experimental and theoretical parametric study. J. Adhes. Sci. Technol. 23 (2), 259–267. Adell, R., Lekholm, U., Rockler, B., Bra˚nemark, P.I.A., 1981. 15-year Study of osseointegrated implants in the treatment of the edentulous jaw. Int. J. Oral Surg. 10 (6), 387–416. Al-Nawas, B., Wagner, W., Gro¨tz, K.A., 2006. Insertion torque and resonance frequency analysis of dental implant systems in an animal model with loaded implants. Int. J. Oral Maxillofac. Implants 21 (5), 726–732. Albrektsson, T., Zarb, G., Worthington, P., Eriksson, A.R., 1986. The long term efficacy of currently used implants: a review and proposed criteria of success. Int. J. Oral Maxillofac. Implants 1 (1), 11–25. Carlos Nelson Elias, 2011. Factors affecting the success of dental implants, implant dentistry—a rapidly evolving practice. Ilser Turkyilmaz (Ed.), ISBN: 978-953-307-658-4, InTech, DOI: 10.5772/18746. Available from: 〈http://www.intechopen.com/ books/implant-dentistry-a-rapidly-evolving-practice/ factors-affecting-the-success-of-dental-implants〉. Carvalho, M.A., Queiroz, C.M., Molena, C.C.L., Rezende, C.P., Rapoport, A., 2008. Clinical study of the relationship between the implant insertion torque and the osseointegration. Rev. Bra. Cir. Cabec¸a Pescoc¸o 37 (4), 202–205. Cristofolini, L., Viceconti, M., 2000. Mechanical validation of whole bone composite tibia models. J. Biomech. 33 (3), 279–288. Cristofolini, L., Viceconti, M., Cappello, A., Toni, A., 1996. Mechanical validation of whole bone composite femur models. J. Biomech. 29 (4), 525–535. Donachie, 1989. Titanium, A Technical Guide. S.l: ASM. Duyck, J., Corpas, L., Vermeiren, S., Ogawa, T., Quirynen, M., Vandamme, K., et al., 2010. Histological, histomorphometrical,
7
and radiological evaluation of an experimental implant design with a high insertion torque. Clin. Oral Implants Res. 21, 877–884. Grandi, T., Guazzi, P., Samarani, R., Grandi, G., 2013. Clinical outcome and bone healing of implants placed with high insertion torque: 12-month results from a multicenter controlled cohort study. Int. J. Oral Maxillofac. Surg. 42 (4), 516–520. Hoshaw, S.J., Brunski, J.B., Cochran, G.V.B., 1989. Pull-out and fatigue failure of bone–dental implant interfaces. Am. Soc. Mech. Eng., Appl. Mech. Div., AMD 98, 05–208. Irinakis, T., Wiebe, C., 2009. Initial torque stability of a new bone condensing dental implant. A cohort study of 140 consecutively placed implants. J. Oral Implantol. 6, 277–282. Lindh, T., Gunne, J., Tillberg, A., et al., 1998. A meta-analysis of implant in partial edentulism. Clin. Oral Implants Res. 9 (2), 80–90. McElhaney, J.H., 1966. Dynamic response of bone and muscle tissue. J. Appl. Physiol. 21 (4), 1231–1236. Misch, C.E., 2007. Contemporary implant dentistry 3e, Maryland Heights. Mosby Elsevier, Missouri1102. Oliscovicz N.F., Shimano A.C., Marcantonio Junior E., Lepri C.P., Dos Reis A.C., 2013. Analysis of primary stability of dental implants inserted in different substrates using the pullout test and insertion torque. International Journal of Dentistry. 2013:194987, http://dx.doi.org/10.1155/2013/194987. Epub Jan 22, 2013. Salles, B., Passos, E., Oliveira, E., Chagas, A., Bruniera, R., Antoninha, A., 2010. Influence of high insertion torque on implant placement—an anisotropic bone stress analysis. Brazilian Dent. J. 21 (6), 508–514. Szmukler-Moncler, S., Salama, H., Reingewirtz, Y., Dubruille, J.H., 1998. Timing of loading and effect of micromotion on bone– dental implant interface: review of experimental literature. J. Biomed. Mater. Res. 43 (2), 192–203. Testori, T., Del Fabbro, M., Szmukler-Moncler, S., Francetti, L., Weinstein, R.L., 2003. Immediate occlusal loading of osseotite implants in the completely edentulous mandible. Int. J. Oral Maxillofac. Implants 18, 544–551. Trisi, P., Todisco, M., Consolo, U., Travaglini, D., 2011. High versus low implant insertion torque: a histologic, histomorphometric, and biomechanical study in the sheep mandible. Int. J. Oral Maxillofac. Implants 26 (4), 837–849. Turkyilmaz, I., Sennerby, L., McGlumphy, E.A., Tozum, T.F., 2009. Biomechanical aspects of primary implant stability: a human cadaver study. Clin. Implant Dent. Relat. Res. 2, 113–119. Zmudzki, J., Walke, W., Chladek, W., 2008. Stress present in bone surrounding dental implants in FEM model experiments. J. Achiev. Mater. Manuf. Eng. 27 (1), 71–74. Zonfrillo, G., Pratesi, F., 2008. Mechanical strength of dental implants. J. Appl. Biomater. Biomech. 6 (2), 110–118.
