- Email: [email protected]
Effect of PEEK and PTFE coatings in fatigue performance of dental implant retaining screw joint: An in vitro study
journal of the mechanical behavior of biomedical materials 103 (2020) 103530
Contents lists available at ScienceDirect
Journal of the Mechanical Behavior of Biomedical Materials journal homepage: http://www.elsevier.com/locate/jmbbm
Effect of PEEK and PTFE coatings in fatigue performance of dental implant retaining screw joint: An in vitro study Xi Chen a, b, Ruiyang Ma a, Jie Min a, Zhi Li a, Ping Yu a, Haiyang Yu a, * a
State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, West China Hospital of Stomatology, Sichuan University, 610041, Chengdu, Sichuan Province, China b Beijing Stomatological Hospital, Capital Medical University, 100050, Beijing, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Dental implant Screw loosening PEEK coating PTFE coating Fatigue
Objectives: Mechanical complications play a key role in failure of dental implants. Retaining screw loosening was one of the most commonly encountered. This study investigated the effect of PEEK and PTFE coatings on dental implant screw thread joint. Methods: Retaining screws were coated with PEEK and PTFE in thickness of 30 μm and 60 μm. Friction coefficient and clamping force of screw thread pair were measured, single load-to-fracture (SLF) test and dynamic fatigue life (DFL) test were done to test the stability of implant thread connection. After that, screw fracture mode and erosion morphology of screw surface and implant internal thread were observed. Results: The results showed that both PEEK and PTFE coatings could reduce friction coefficient, and consequently increase clamping force, especially PTFE coatings. PEEK coatings had no significant effect on fracture load, while 30 μm PTFE coating reduced fracture load. PEEK coatings also elongated fatigue life and improved the antiloosening property under dynamic load, while 30 μm PTFE coating shortened fatigue life. Most of the screw fracture happened at the first thread of the retaining screws. The fracture-end of PEEK coated screws were loosed and could easily remove, but fracture-end of PTFE screws could not. Internal thread observation showed that both PEEK and PTFE coatings could reduce wear of implant internal thread. Conclusion: PEEK coatings could effectively improve the stability of implant threaded connection, and reduce wear of implant internal thread. PEEK coating may be a suitable way to prevent screw loosening.
1. Introduction Dental implant-supported restorations are used as a predictable treatment option for partially or completely edentulous patients, and present high success rates (Adell et al., 1990; Jemt and Lekholm, 1995). The clinical success of dental implants and related reconstructions not only depends on surgical survival, but also biological and mechanical complications occurring during clinical function (Sailer et al., 2012). It was reported that more than half of implants experience at least one mechanical complication (Shemtov-Yona and Rittel, 2015). Among these complications, screw loosening is one of the most commonly encountered, with a 5-year rate of 3.1–10.8% (Pjetursson et al., 2014). Screw loosening can lead to prosthetic failure, occlusal overload of the implant or even internal thread damage, causing irreversible recon struction failure. Therefore, efforts are required to prevent screw
loosening. Preloading retaining screws during screw tightening causes elonga tion and helps keep them under tension (Al Jabbari et al., 2008). This tension represents a clamping force between prosthetic components and the implant (Haack et al., 1995). Torque control is the most common method employed to tighten the abutment retaining screws (Guda et al., 2008). While tightening a screw, the torque on its joint is not entirely transferred to the preload (Parmley Robert, 1977). According to Shigley, only 10% of the tightening torque is used to generate screw tension, and most of the applied torque is used to overcome the friction between the screw head and the abutment surface, as well as to counter thread friction (Shigley, 2011). Therefore, the effective preload is far less than the applied torque. The exact mechanism of screw loosening in fixed prostheses is likely to be complex (Sakaguchi and Borgersen, 1995). After screw tightening, other parameters such as fatigue cycling,
* Corresponding author. State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University, 610041, Chengdu, Sichuan Province, China. E-mail addresses: [email protected], [email protected], [email protected] (H. Yu). https://doi.org/10.1016/j.jmbbm.2019.103530 Received 20 August 2019; Received in revised form 10 November 2019; Accepted 11 November 2019 Available online 14 November 2019 1751-6161/© 2019 Elsevier Ltd. All rights reserved.
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
chewing patterns and loads also affect the mechanical features of dental implants (Wu et al., 2017). Besides, it is inevitable that small-scale machine tolerances are between the implant-screw thread pair surface. Meanwhile, masticatory loading leads to micromotion and dynamic fa tigue of the implant-screw interface, and the preload decrease could significantly increase micromotion (Binon, 1994; Gratton et al., 2001), causing a further preload loosening and finally leading to screw loos ening or even fracture. Screw friction is increased by material hardness and surface rough ness, and decreased by lubricants (Burguete et al., 1994). To minimize friction, kinds of lubricats were used. Although the chemical composi tion and physical characteristics of saliva samples differ among in dividuals (Tzenakis et al., 2002), the presence and quantity of lubricant (saliva, peri-implant fluid, and/or blood) between the implant’s mating components, which is clinically unpredictable, can also affect the coef ficient of friction (Barbosa et al., 2011; Weiss et al., 2000). Nigro er al. found that the wet condition (the inner threads of the implants filled with artificial saliva) generated higher preload values of zirconia abut ment screws than the dry condition (Nigro et al., 2010). But according to Tzenakis et al., artificial saliva has potentially different lubrication characteristics with human saliva. It has been reported that lubrication of implant gold prosthetic screws with saliva had no apparent effect on tensile strength (Al Rafee et al., 2002) and the ability of saliva to provide any significant lubrication is doubtful (Byrne et al., 2006). Wu et al. (2017) investigated the stability of implant-abutment connection under varying lubrication: graphite powder and medical vaseline. The results showed that lubricated screws made the joint easier to loosen. Solid film lubricants were also used. Martin et al. (2001) tested four types of screw: Gold-Tite (gold alloy screw coated with 0.76 μm of pure gold, 3i Implant Innovations, Inc.), TorqTite (titanium alloy screw coated with Tiodize and amorphous diamond coatings, Nobel Biocare UK, Ltd.), gold alloy and titanium alloy. The results showed that gold-coated screw had higher preloads and TorqTite gave the higher rotational angles when compared with the conventional gold alloy or titanium alloy screws. Elias et al. (2006) measured the opening torque (N cm) of implant abutment screws coated with four different materials, TiN (titanium nitride), TiCN (titanium carbonitride), Teflon and Parylene. Their re sults showed that screws without coating had the highest opening torque for a given applied tightening torque (35N cm), titanium screws without coating were more stable than Teflon coated screws under cyclic loading. Choe et al. (2011) coated the abutment screws with 1–3 μm TiN and WC (tungsten carbide), found that the fatigue life of implant system with TiN and WC coated abutment screws was relatively longer compared to non-coated abutment screw at 420N load. In summary, graphite powder, medical vaseline and Teflon made the joint easier to loosen. TiN, TiCN and WC are extremely hard ceramic materials, coated screws could ineluctably lead to implant internal thread wear under masticatory loading. From the perspective of clinical strategy, prostheses need to be replaced regularly under normal circumstances; however, the implant cannot be removed once the osseointegration occurs, and is not as easy to replace as the retaining screw. It is also crucial to protect the implant’s internal thread from abrasion. Therefore, coatings are required, which could increase the preload, prevent loosening under cyclic stress, and protect the internal thread. Polytetrafluoroethylene (PTFE) or Teflon resin is a waxy and smooth synthetic polymer material broadly used in many industrial and medical applications. It has good chemical, thermal, and electrical stability as well as low friction (Sajid and Ilyas, 2017). PTFE can be applied to the human body as an artificial blood vessel, a heart valve, and so on. Pol yetheretherketone (PEEK) is a biocompatible polymer with good me chanical properties. It has good resistance to wear and fatigue, resulting in low coefficient of friction for a large range of sliding conditions (Friedrich et al., 2011; Schwitalla and Müller, 2013). In addition, PEEK has reliable biosafety, and bacterial colonization of its surface is reduced compared to pure titanium. Besides, its composites have similar elastic
modulus to human bone tissue (Skinner, 1988), which makes it widely used in the medical field. Studies have been performed with PEEK retaining screws, but the results showed insufficient strength for PEEK or reinforced PEEK screws (Neumann et al., 2014; Schwitalla et al., 2016). In this study, we hypothesized that appropriate PEEK and PTFE coatings could realize all required goals, including friction reduction, stability improvement and internal thread prevention. The aim of this study was to investigate the stability of dental implant screw thread connection with PEEK and PTFE coated titanium alloy screws, by determining the clamping force, fracture load, fatigue life, loosening torque and surface wear under static and dynamic loads. 2. Materials and methods 2.1. Sample preparation Commercially available titanium dental implants (Ø4.3 � L11 mm) and straight abutments (Ø4.5 � Gh1.5 � h6.0) manufactured by WEGO (WEGO, Jericom Biomaterilas Co., Ltd. Weihai, Shandong, China) were used. Abutment retaining screws (1.5 � L10 mm) were used as a tita nium substrate. PEEK (VICOTE 700, Victrex, England) and PTFE (Zhangjiagang Youcheng High-tech Materials Co. Ltd. Zhangjiagang, Jiangsu, China) were selected as coating polymers. PEEK and PTFE coatings with different thicknesses (30 or 60 μm) were deposited on the screw thread through thermal spraying (Zhangjiagang Youcheng Hightech Materials Co. Ltd.) within the machining tolerance of WEGO implant system (Fig. 1). Totally 300 specimen sets were prepared in this study. Specimens were divided into 5 groups based on screw surface condition, including the control (C; screw without coating), PEEK-1 (screw coated with 30 μm PEEK), PEEK-2 (screw coated with 60 μm PEEK), PTFE-1 (screw coated with 30 μm PTFE), and PTFE-2 (screw coated with 60 μm PTFE) groups. 2.2. Friction coefficient measurement A bolt fastener test and analysis system (5413-2777, Schatz-Analyze, SCHZTA AG, Cologne, Germany) was used according to the GB/T 16823.3-2010/ISO 16047:2005 standard for screw joint clamping force and friction coefficient measurements. To fit the bolt fastener system, implants and abutments were embedded into a titanium block, and fit in a holder, respectively. A schematic diagram is exhibited in Fig. 2. After the holders were set in place, the screw was fastened into the implant by a screw driver through the abutment at a speed 4 rpm. During the pro cedure, the sensor would record the torque and clamping force, and calculate the friction coefficient. When the tightening torque reached 20 Ncm, the driver stopped, and the test was completed. A total of 25 implant and abutment pairs were assessed. The deviation of the di mensions of titanium blocks was less than 10%. 2.3. Fracture resistance test An in vitro model was stablished under the ISO/FDIS 14801:2014 guidance (Fig. 3). A specimen set was assembled by an implant, straight abutment, retaining screw and simplified zirconia crown (Lava, 3M ESPE, St. Paul, MN, USA). Each implant was embedded into a cylindrical acrylic resin (NISSIN, Tokyo, Japan) measuring 30 mm in diameter and 20 mm in height (Fig. 4). After tightening and loosening the retaining screw 5 times by hand, the straight abutment was tightened to 20.0 Ncm using an electric tor que meter (LutronTQ8800, Taipei, Taiwan China) according to the manufacturer’s recommendations, and retorqued to 20.0 Ncm after 10 min. Then, the abutment was sealed, with the simplified crown adhered to the abutment using cement (Rely X Luting, 3M ESPE, St. Paul, MN, USA). Specimens were divided into 5 groups based on retaining screw 2
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 1. Dental implant abutment screws of different test groups: A, Control group (C); B, PEEK-1 group; C, PEEK-2 group; D, PTFE-1 group; E, PTFE-2 group.
2.4. Screw loosening test Specimens were assembled and embedded as described in 2.3 above. A total of 150 specimens were prepared, randomly divided into 5 groups (same as above). First, 5 specimens in each group were selected to un dergo screw loosening 10 min after tightening (without loading). The loosening torque was recorded as Ti, and average Ti and percentage decline in torque (θ; θ¼(20-Ti)/20 � 100%) were calculated. Secondly, dynamic asymmetric sine wave load, 25–250 N was applied at 15 Hz, with different numbers of loading cycles to simulate functional masti cation (1.0 � 105, 3.0 � 105, 6.0 � 105, 9.0 � 105 and 1.2 � 106 cycles). After the test, the detorque value (Tm) of each specimen was recorded. 2.5. Morphologic observation All fractures of the retaining screws were classified according to fracture location. After the DFL test, the fractured screw-ends were removed from the implant. In the subgroups of 90% F0 of each test group, fracture surface was observed under a scanning electron micro scope (SEM, Quanta200, FEI, Hillsboro, OR, USA). After screw loosening test, surface of the retaining screw and implant internal thread in the subgroups of 1.2 � 106 cycle for each test group were also observed under SEM.
Fig. 2. Schematic diagram of the clamping force and friction coefficient test. 1, principal axis; 2, total torque sensor; 3, screwdriver holder; 4, screwdriver; 5, holder 1; 6, abutment and titanium block; 7, holder 2; 8, implant and titanium block; 9, retaining screw; 10, clamp force sensor.
coating. Each specimen was fixed into a dynamic fatigue testing ma chine (BOSS ELF-3330,Eden Prairie, USA). The loading force was pro vided by the testing machine through a fixed zirconia indenter with a flat contact surface, at a speed of 1.0 mm/min.
2.6. Statistical analysis Statistical analyses were performed with SAS (SAS Institute Inc., Cary, USA, Version 5.0.1). Data were analyzed by one-way analysis of variance (ANOVA) with Fisher’s PLSD test. P < 0.05 was considered statistically significant.
2.3.1. Single load-to-fracture (SLF) test A gradually increased load was applied to the specimen until frac ture. Then, the maximum load value was recorded and defined as the fracture load F0. A total of 25 specimens were tested, and the average F0 was determined.
3. Results 3.1. Friction coefficient and clamping force
2.3.2. Dynamic fatigue life (DFL) test The DFL test was performed after the SLF test. Four levels of dynamic load were selected, including 90%, 80%, 70%, and 60% of the average F0 for each group (C, PEEK-1, PEEK-2, PTFE-1, and PTFE-2). The load varied within 10% of the nominal peak value at 15 Hz. The fatigue life was defined as the loading cycle to fracture; the fatigue limit was set as cycle limit 5 � 106. A total of 100 specimens were prepared. Each dy namic loading was tested for a total of 5 times.
The friction coefficients and clamping forces of implant-screw thread connection are shown in Fig. 5. Significant differences were found be tween each group. Among test groups, group C had the largest friction coefficient, followed by the PEEK-2, PEEK-1 and PTFE-2 groups; the PTFE-1 group had the smallest value. Accordingly, the PTFE-1 group had the highest clamping force, followed by the PTFE-2, PEEK-1 and PEEK-2, with group C showing the smallest value.
3
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 3. Schematic of the test setup (A. 1, loading device; 2, nominal bone level; 3, abutment; 4, hemispherical loading member; 5, dental implant body; 6, specimen holder; line AB, loading axis; C, loading center; line DE, axis of the implant; F, loading force; ∠ACD ¼ 30� ); Loading apparatus (B, C).
Fig. 4. Schematic of a set of specimen (A. 1, simplified zirconia crown; 2, straight abutment; 3, retaining screw; 4, implant; 5, acrylic resin); The specimen embedded in acrylic resin (B).
3.2. Fracture load and fatigue life
group was 880.93 � 3.65 N, i.e. reduced by 50.47 N (5.42%), while that of PTFE-2 group was 995.17 � 47.13 N, i.e. increased by 63.77 N (6.85%).
3.2.1. Fracture load The average fracture load of group C was 931.40 � 32.78 N. The PEEK groups had fracture loads comparable with that of group C, including 936.70 � 9.51 N and 937.60 � 30.86 N for the PEEK-1 and PEEK-2 groups, respectively (Fig. 6). It can be found that PEEK coating had no significant effect on the fracture load of the implant-abutmentscrew composite. Significant statistical differences were found be tween the PTFE groups and group C. The fracture load of the PTFE-1
3.2.2. Dynamic fatigue life According to SLT results, the four levels of fatigue force were 90%, 80%, 70%, and 60% of the mean F0, i.e. 84 N–838 N, 75 N–745 N, 65 N–652 N, and 56 N–559 N, respectively. Representative load-cycle curves of the DFL test are shown in Fig. 7. The results showed that the larger the load on a specimen, the shorter 4
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
the corresponding fatigue life, and coating did not affect this tendency. The numbers of cycles in each group were relatively close at high loads. As the load decreases, the differences among groups became more sig nificant; the lower the value, the greater the difference. At a load of �559 N, except for the PTFE-1 group, the other groups reached the fa tigue limit (5 � 106 cycles) without fracture. This indicated that the fatigue limit of the PEEK-1, PEEK-2 and PTFE-2 groups was 559N. As shown in Fig. 7, various coatings had different effects on fatigue life, with PEEK and PTFE-2 coatings prolonging the fatigue life and the PTFE1 coating shortening it, compared with group C. 3.2.3. Fracture mode and fractography The fracture mode was divided into two types according to the fracture location of retaining screws. The most common type was the fracture occurring at the first thread root. Specimens of the C, PEEK-1, PTFE-1, and PTFE-2 groups belonged to this type. Meanwhile, speci mens of the PEEK-2 group were mainly fractured at the fourth thread root. Fracture-ends without coating or with PEEK coating were loose and easy to remove, while those with PTFE coating remained tight, and could not be removed manually. Screw fractographs in group C are shown in Fig. 8. The fracture surface was irregular (Fig. 8A) but could be recognized as a fatigue fracture. According to the different fractal features, typical fatigue fracture characteristics such as quasi-cleavage facets (Fig. 8C), dimples (Fig. 8D and E) and fatigue striations (Fig. 8F) were observed. The fractographs of coating groups are shown in Fig. 9. PEEK-1 group had similar fractographs as group C. The initiation and crack propagation area of the fracture was not obvious in PEEK-2 and PTFE-2 group, while a large number of dimples observed (Fig. 9E–H, M-P). The fractograph of the PTFE-1 group was different (Fig. 9I–K). Some dimples were found (Fig. 9I and j), while most of the fracture surface was flat and the grainy crystal structure was observed (Fig. 9J and K).
Fig. 5. Friction coefficients and clamping forces at 20 Ncm tightening torque of screw thread connection in various test groups. *, þsignificant difference be tween groups.
3.3. Loosening torque and surface microtopography 3.3.1. Loosening torque The initial loosening torque Ti and the torque loss rate (θ) in each group are shown in Table 1. Ti in each group was lower than the tightening torque of 20 Ncm, indicating that after tightening, torque loss occurred without cyclic loading. Ti in group C was 14.06 � 1.47 Ncm, indicating a 30.0% decrease of the tightening torque. Compared with group C, all coating groups had higher Ti values and reduced torque loss. Among the coating groups, the PEEK-2 group had the highest Ti (15.17 � 0.77 Ncm) and the lowest θ (24.37%), followed by the PEEK-1 (Ti ¼ 14.98 � 0.51 Ncm, θ ¼ 25.26%) and PTFE-2 (Ti ¼ 14.72 � 0.77 Ncm, θ ¼ 26.57%) groups, and the PTFE-1 group had the smallest Ti (14.29 � 0.75 Ncm) and largest θ (28.69%). The detorque values after cyclic loading are shown in Fig. 10. It was lower than Ti in all groups, and detorque values in all test groups were higher than those of group C. Significant differences were found be tween the PEEK-1 group and group C at T9 and T12. In the PEEK-1 group, the detorque value gradually increased with the cyclic load number. On the contrary, the detorque value gradually decreased in the PTFE-2 group, while significant differences were found between the PTFE-2 group and group C at T1 and T12. After 1.2 � 106 cycles of loading, the detorque values were significantly higher than the group C value in the PEEK-1 and PTFE-2 groups. The PEEK-1 group had the highest value, followed by the PEEK-2 and PTFE-2 groups.
Fig. 6. Fracture load (F0) values of the test groups. *significant difference be tween groups.
3.3.2. Surface microtopography of retaining screws Retaining screws were removed and observed by SEM. As shown in Fig. 11, the surface of the new screw was smooth, and mechanical cut ting marks were visible (Fig. 11A and B). A lot of wear debris were found on the screw surface in group C (Fig. 11C and D, white arrow). In the PEEK-1 group, coating was peeled off from the fifth thread, and stripping was on one side of the thread flank (flank oriented to the screw head),
Fig. 7. Representative load-cycle curves of different groups in the DFL test. 5
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 8. Fractographs of the abutment screw in group C. Figure A, Integral morphology, with a, b, c, and d representing different regions of the fracture surface. “a” was the initiation and crack propagation area of the fracture. The arrow shows the direction in which the fracture extended. B, Area shown by “b” in Figure A; a boundary between the crack propagation and catastrophic rupture area is visible. C, Area shown by “e” in Figure B, i.e. the crack propagation area of the fatigue fracture, with visible quasicleavage facets. D, Catastrophic rupture area of the fatigue fracture, with a large number of dimples observed. E, Higher magnification of D. F, Higher magnification of Figure C, with the box showing fatigue strips surrounded by small dimples.
while the last thread was peeled off on both sides (Fig. 11E). Several grinding debris were observed on the screw surface (Fig. 11F), and PEEK scraps were found around the grinding debris (Fig. 11F, black arrow). In the PEEK-2 group, the stripping area was larger. From the surface of the third thread, the coating peeled off and a limited amount of debris were visible (Fig. 11H). In the PTFE-1 group, coating was peeling from the fourth thread, and only coating on the thread flank surface oriented to the screw head peeled off. Scratches were found on the coating surface (Fig. 11J). The peeled coating adhered to the screw surface. In the PTFE2 group, coating in thread crests was irregularly fractured and peeled off from the fifth thread, and the metal substrate was exposed less than in other coating groups. PTFE scraps were observed (Fig. 11K and L). In contrast to group C, metal wear debris were sharply reduced on the thread surface in the coating groups.
dissipation while only a fraction is stored as elastic deformation or preload (Barbosa et al., 2011). Therefore, reducing friction could in crease the preload. Coating is one of the methods for reducing the fric tion coefficient. As shown in Fig. 5, all the four coatings could reduce the friction coefficient and increase the clamping force (PTFE-1>PT FE-2>PEEK-1>PEEK-2). For the same materials, the friction coefficient of thin layer coating (30 μm) was smaller, with a larger clamping force. This may be due to the more intimate contact of thicker coatings compared with thinner ones. At the same material thickness, the friction coefficient of PTFE coating was lower than that of PEEK coating. This could result from the outstanding lubricant property of PTFE. The PTFE-1 group had the highest clamping force, but was fractured at the lowest load, significantly lower than that of group C. This may be due to screw deformation. The increased clamping force acted on the weak point of the thread interface, caused stress concentration, and resulted in damage and fracture. Although coatings increase the preload, it is also important that the preload would remain stable instead of being lost quickly, especially under masticatory stress. Therefore, we also assessed the effect of coating on torque loss before and after dynamic load. Studies have shown that after tightening, the immediate torque loss rate of the thread joint is about 3–20% (Nigro et al., 2010), and tightening-unscrewing the retaining screw repeatedly could lead to an increase in torque loss. In the Brånemark system, the torque is lost by approximately 20% after the first tightening of the screw, about 31% after repeated tightening-unscrewing for 5 times, and approximately 36% after 15 times (Weiss et al., 2000). The reason for the short-term relaxation may be embedment relaxation, localized plastic deformation or torsional relaxation of the screw (Cantwell and Hobkirk, 2004). Our results were consistent with previous studies. The initial loosening torque was lower than the tightening torque (Table 1), and torque loss in the coating groups was reduced. These findings indicated that application of coating reduces the torque loss rate and prevents torque loss. After dynamic cyclic load, the detorque value was also decreased in each group. The detorque value in the coating groups was higher than that of group C. It could be considered that coating may improve the anti-loosening performance of the implant thread joint. Except for the PTFE-1 group (Fig. 10), the other three coating groups showed signifi cantly increased detorque value at 1.2 � 106 cycles of load. Coating changes the surface topography of the screw, and further affects the mechanical behavior of the thread joint of the implant. However, whether it affects the long-term life of the implant is un known. The Dental implant-abutment complex is subjected to dynamic
3.3.3. Wear microtopography of internal thread As shown in Fig. 12, irregular corrugation (Fig. 12D, white arrow) was visible at the interface between the corresponding surface of the screw root (Fig. 12A, a) and the flank (Fig. 12A and b, Fig. 12B and c), representing metal surface peeling off due to abrasion. According to morphological features, the surface in thin layer coating groups (Fig. 12E, F, I, J) had more wear debris, and that of thick coating groups (Fig. 12G, H, K, L) was cleaner and flatter. Specifically, the PEEK coating groups showed slighter internal thread wear compared with group C (Fig. 12C and D), exhibiting sporadic tiny grinding debris (Fig. 12F, red arrow) and PEEK scraps (Fig. 12F, black arrow). Compared with the PEEK-1 group, more wear debris were observed in the PTFE-1 group. Interface metal stripping was more obvious in the PTFE-2 group compared with the PEEK-2 group. 4. Discussion Screw loosening is one of the most common complications of dental implants in clinical practice. An effective anti-loosening method is ex pected to be developed. This study investigated the effect of PEEK and PTFE coatings on implant threaded connection. The results showed that PEEK coating was effective in improving connection stability and pro tecting the implant’s internal thread. For screw joint, the clamping force (preload) should be high enough in case the retaining screws loosen easily under repeated bite forces. The process of screw tightening is also an energy conversion process. Tightening torque converts into kinetic, frictional, and elastic compo nents (Breeding et al., 1993). Most of the torque converts into frictional 6
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 9. Fractographs of the abutment screw in coating groups. PEEK-1 group (Figure A, B, C, D. Figure A, Integral morphology, with a, b and c representing different regions of the fracture surface. “a” was the initiation and crack propagation area of the fracture. B, Area shown by “b” in Figure A; a boundary between the crack propagation and catastrophic rupture area is visible. C, Area shown by “d” in Figure B. D, Higher magnification of “c” in Figure A). PEEK-2 group (Figure E, F, G, H. Figure E, Integral morphology, with e and f representing different regions of the fracture surface. F, Area shown by “e” in Figure E; G, Area shown by “f” in Figure E. H, Higher magnification of F). PTFE-1 group (Fig. I, J, K, L. Fig. I, Integral morphology, with i and j representing different regions of the fracture surface. J, Area shown by “i” in Fig. I; K, Higher magnification of J. L, Higher magnification of area shown by “j” in Fig. I). PTFE-2 group (Figure M, N, O, P. Figure M, Integral morphology, with m and n representing different regions of the fracture surface. N, Area shown by “m” in Figure M; O, Area shown by “n” in Figure M. P, Higher magnification of N). The arrow shows the direction in which the fracture extended.
the number of external load cycles experienced during fatigue damage. According to Fig. 7, PEEK and PTFE-2 coatings prolonged the fatigue life, which was shortened by PTFE-1 coating. The fatigue limit (at 5 � 106 cycles) in the PEEK-1, PEEK-2 and PTFE-2 groups was 559N, which was greater than the human average occlusal and masticatory force (Binon and McHugh, 1996; Paphangkorakit and Osborn, 1997; Raadsheer et al., 1999; Van Eijden, 1991). Thicker coating resulted in prolonged fatigue life, which may be due to the thicker coating making the thread contact more intimate or the surface stress distribution of the thread pair more uniform. At the same coating thickness, fatigue life with PEEK coating was longer than that of PTFE, which may be due to the larger friction coefficient of PEEK. The cold flow property of PTFE may also contribute to this difference. The effect of coatings on the implant screw fracture characteristics have received relatively little investigation. According to Fig. 6, PEEK
Table 1 Loosening torque (Ti) values and torque loss rates (θ) of test groups without loading. Groups
Ti (Ncm)
θ (%)
C PEEK-1 PEEK-2 PTFE-1 PTFE-2
14.06 � 1.47 14.98 � 0.51*y 15.17 � 0.77* 14.29 � 0.75y 14.72 � 0.77*
30.00 25.26 24.37 28.69 26.57
*, yP < 0.05 comparing Ti values among test groups.
occlusal force during its function. Although the occlusal force is much lower than the fracture load of the material itself, it could be destroyed after a certain number of cycles, i.e. fatigue damage. The Fatigue life is 7
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 10. Detorque values (Tm) after corresponding cyclic loads in the test groups. T1 ¼ detorque value at 1.0 � 105 cycles, T3 ¼ detorque value at 3.0 � 105 cycles, T6 ¼ detorque value at 6.0 � 105 cycles, T9 ¼ detorque value at 9.0 � 105 cycles, T12 ¼ detorque value at 1.2 � 106 cycles. *, þ,◆,●significant difference between groups at the same cyclic load; α, β, δ, ε, ϕ, χ significant difference between groups at different cyclic loads.
groups had comparable fracture load with group C. The fractograph in PEEK groups (Fig. 9A–H) was similar with group C (Fig. 8). These results indicated that PEEK coatings had little effect on fracture. For PTFE-1 group, the fracture load was decreased, and the fractograph showed different structure, which may be deduced low resistance of fracture. For PTFE-2 group, the fracture load was increased and similar fractograph with group C was found, which may be inferred a better performance of fracture resistance than group C. These results indicated that PEEK and PTFE coating have influence on the fatigue characteristics of the screws. This may because coating surface and thickness affect the distribution of stress and then have influence on crack formation. Most of the retaining screw was broken at the first thread, corrobo rating previous findings (Wu et al., 2017). In the study, a three-dimensional model of the implant-abutment-retaining screw was established by FEA, and the stress distribution of each component was analyzed after using different lubricants. The results showed that the maximum stress of the retaining screw is concentrated at the first thread regardless of whether the lubricant was used or not. Stress levels were more obvious in the first four threads than in the subsequent one. In addition, stress level was higher in the lubricant group compared with the control group. Our results were consistent with these findings. In this study, fracture-ends of screws in group C, PEEK-1 and PEEK-2 were mobile and easy to remove, while fracture-ends in the PTFE-1 and PTFE-2 groups remained clamped. In clinical practice, once a retaining screw with PTFE coating is fractured, removing the fracture-end may be difficult. The surface topography and micro-roughness of implant-screw mating surfaces change throughout the lifetime of the implanted pros thesis (Guzaitis et al., 2011). The retaining screw and internal thread surface of the implant should be assessed for manufacturer machining irregularities before use; meanwhile, because of micromotion caused by external load, the screw and internal thread surface would inevitably wear. This can cause deformation, lead to preload loss or even screw slipping. Effective coatings should protect internal threads and not cause excessive wear. Unfortunately, few studies have focused on internal thread wear. As shown in Figs. 11 and 12, internal thread surface in each group was worn out. Peeling off of the internal thread metal surface occurred mostly at the intersection of the screw root and the crest, corresponding to the position where the central screw was broken, and stress concentration and plastic deformation started. In this study, PEEK and PTFE coatings could both reduce internal thread damage to some
Fig. 11. Microtopography of abutment screws. A and B, Blank controls. C and D, group C. E and F, PEEK-1 group. G and H, PEEK-2 group. I and J, PTFE-1 group. K and L, PTFE- 2 group. White arrows, metal wear debris; black ar rows, coating scraps. Scratches were highlight in the boxes.
8
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
extent while preventing loosening. In further studies, different implantation systems produced by other manufacturers should be tested. In the dynamic cyclic test, the number of loading cycles will be increased to mimic a greater clinic longevity. Additionally, different test conditions could simulate changeable masticatory circumstances. 5. Conclusion In summary, PEEK and PTFE coatings improve the immediate fastening and long-term anti-loosening performances, and could reduce abrasion of implant internal thread. However, since the PTFE-1 group showed no improved fracture resistance, and the fracture-end obtained with PTFE coating was difficult to remove, we consider PEEK coating an ideal method for improving the stability of implant screw thread connection. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank the staff of WEGO Jericom Bio materilas Co., Ltd. for assistance in this study. This company had no role in study design, data collection and analysis, decision to publish, or manuscript preparation. This study was supported by the National Natural Science Foundation of China (No. 81771113). References Adell, R., Eriksson, B., Lekholm, U., Brånemark, P.I., Jemt, T., 1990. A Long- term followup study of osseointegrated implants in the treatment of totally edentulous jaws. Int. J. Oral Maxillofac. Implant. 5, 13. Al Jabbari, Y.S., Fournelle, R., Ziebert, G., Toth, J., Iacopino, A.M., 2008. Mechanical behavior and failure analysis of prosthetic retaining screws after long-term use in vivo. Part 1: characterization of adhesive wear and structure of retaining screws. J. Prosthodont. 17, 168–180. Al Rafee, M.A., Nagy, W.W., Fournelle, R.A., Dhuru, V.B., Tzenakis, G.K., Pechous, C.E., 2002. The effect of repeated torque on the ultimate tensile strength of slotted gold prosthetic screws. J. Prosthet. Dent 88, 176–182. Barbosa, G.S., Silva-Neto, J.P.d., Simamoto-Júnior, P.C., Neves, F.D.d., Mattos, M.d.G.C. d., Ribeiro, R.F., 2011. Evaluation of screw loosening on new abutment screws and after successive tightening. Braz. Dent. J. 22, 51–55. Binon, P., 1994. The role of screws in implant systems. Int. J. Oral Maxillofac. Implant. 9, 48–63. Binon, P.P., McHugh, M.J., 1996. The effect of eliminating implant/abutment rotational misfit on screw joint stability. Int. J. Prosthod. 9. Breeding, L.C., Dixon, D.L., Nelson, E.W., Tietge, J.D., 1993. Torque required to loosen single-tooth implant abutment screws before and after simulated function. Int. J. Prosthod. 6. Burguete, R.L., Johns, R.B., King, T., Patterson, E.A., 1994. Tightening characteristics for screwed joints in osseointegrated dental implants. J. Prosthet. Dent 71, 592–599. Byrne, D., Jacobs, S., O’Connell, B., Houston, F., Claffey, N., 2006. Preloads generated with repeated tightening in three types of screws used in dental implant assemblies. J. Prosthodont. 15, 164–171. Cantwell, A., Hobkirk, J.A., 2004. Preload loss in gold prosthesis-retaining screws as a function of time. Int. J. Oral Maxillofac. Implant. 19. Choe, H., Lee, C., Jeong, Y., Ko, Y., Son, M., Chung, C., 2011. Fatigue fracture of implant system using TiN and WC coated abutment screw. Procedia Eng. 10, 680–685. Elias, C., Figueira, D., Rios, P., 2006. Influence of the coating material on the loosing of dental implant abutment screw joints. Mater. Sci. Eng. C 26, 1361–1366. Friedrich, K., Sue, H., Liu, P., Almajid, A., 2011. Scratch resistance of high performance polymers. Tribol. Int. 44, 1032–1046. Gratton, D.G., Aquilino, S.A., Stanford, C.M., 2001. Micromotion and dynamic fatigue properties of the dental implant–abutment interface. J. Prosthet. Dent 85, 47–52. Guda, T., Ross, T.A., Lang, L.A., Millwater, H.R., 2008. Probabilistic analysis of preload in the abutment screw of a dental implant complex. J. Prosthet. Dent 100, 183–193. Guzaitis, K.L., Knoernschild, K.L., Viana, M.A., 2011. Effect of repeated screw joint closing and opening cycles on implant prosthetic screw reverse torque and implant and screw thread morphology. J. Prosthet. Dent 106, 159–169. Haack, J.E., Sakaguchi, R.L., Sun, T., Coffey, J.P., 1995. Elongation and preload stress in dental implant abutment screws. Int. J. Oral Maxillofac. Implant. 10, 529–536.
Fig. 12. Wear microtopography of the implant internal thread. A and B, Blank controls. “a”, corresponding surface to the screw root; “b”, surface corre sponding to the screw crest; “c”, flank surface. C and D, group C. E and F, PEEK1 group. G and H, PEEK-2 group. I and J, PTFE-1 group. K and L, PTFE-2 group. White arrow, metal surface peeling; red arrow, metal wear debris; black arrow, coating scraps. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 9
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530 Sajid, M., Ilyas, M., 2017. PTFE-coated non-stick cookware and toxicity concerns: a perspective. Environ. Sci. Pollut. Control Ser. 24, 23436–23440. Sakaguchi, R.L., Borgersen, S.E., 1995. Nonlinear contact analysis of preload in dental implant screws. Int. J. Oral Maxillofac. Implant. 10. Schwitalla, A., Müller, W.-D., 2013. PEEK dental implants: a review of the literature. J. Oral Implantol. 39, 743–749. Schwitalla, A.D., Abou-Emara, M., Zimmermann, T., Spintig, T., Beuer, F., Lackmann, J., Muller, W.D., 2016. The applicability of PEEK-based abutment screws. J. Mech. Behav. Biomed. Mater. 63, 244–251. Shemtov-Yona, K., Rittel, D., 2015. An overview of the mechanical integrity of dental implants. Biomed. Res. Int. 2015. Shigley, J.E., 2011. Shigley’s Mechanical Engineering Design. Tata McGraw-Hill Education. Skinner, H.B., 1988. Composite technology for total hip arthroplasty. Clin. Orthop. Relat. Res. 235, 224–236. Tzenakis, G.K., Nagy, W.W., Fournelle, R.A., Dhuru, V.B., 2002. The effect of repeated torque and salivary contamination on the preload of slotted gold implant prosthetic screws. J. Prosthet. Dent 88, 183–191. Van Eijden, T., 1991. Three-dimensional analyses of human bite-force magnitude and moment. Arch. Oral Biol. 36, 535–539. Weiss, E.I., Kozak, D., Gross, M.D., 2000. Effect of repeated closures on opening torque values in seven abutment-implant systems. J. Prosthet. Dent 84, 194–199. Wu, T., Fan, H., Ma, R., Chen, H., Li, Z., Yu, H., 2017. Effect of lubricant on the reliability of dental implant abutment screw joint: an in vitro laboratory and three-dimension finite element analysis. Mater. Sci. Eng. C 75, 297–304.
Jemt, T., Lekholm, U., 1995. Implant treatment in edentulous max- illae: a 5-year followup report on patients with different degrees of jaw resorption. Int. J. Oral Maxillofac. Implant. 10, 9. Martin, W.C., Woody, R.D., Miller, B.H., Miller, A.W., 2001. Implant abutment screw rotations and preloads for four different screw materials and surfaces. J. Prosthet. Dent 86, 24–32. Neumann, E.A.F., Villar, C.C., FRANÇA, F.M.G., 2014. Fracture resistance of abutment screws made of titanium, polyetheretherketone, and carbon fiber-reinforced polyetheretherketone. Braz. Oral Res. 28, 1–5. Nigro, F., Sendyk, C.L., Francischone Jr., C.E., Francischone, C.E., 2010. Removal torque of zirconia abutment screws under dry and wet conditions. Braz. Dent. J. 21, 225–228. Paphangkorakit, J., Osborn, J., 1997. The effect of pressure on a maximum incisal bite force in man. Arch. Oral Biol. 42, 11–17. Parmley Robert, O., 1977. Standard Handbook of Fastening and Joining. McGraw-Hill Book Co, New York, NY. Pjetursson, B., Asgeirsson, A., Zwahlen, M., Sailer, I., 2014. Improvements in implant dentistry over the last decade: comparison of survival and complication rates in older and newer publications. Int. J. Oral Maxillofac. Implant. 29, 308–324. Raadsheer, M., Van Eijden, T., Van Ginkel, F., Prahl-Andersen, B., 1999. Contribution of jaw muscle size and craniofacial morphology to human bite force magnitude. J. Dent. Res. 78, 31–42. Sailer, I., Mühlemann, S., Zwahlen, M., H€ ammerle, C.H., Schneider, D., 2012. Cemented and screw-retained implant reconstructions: a systematic review of the survival and complication rates. Clin. Oral Implant. Res. 23, 163–201.
10
Contents lists available at ScienceDirect
Journal of the Mechanical Behavior of Biomedical Materials journal homepage: http://www.elsevier.com/locate/jmbbm
Effect of PEEK and PTFE coatings in fatigue performance of dental implant retaining screw joint: An in vitro study Xi Chen a, b, Ruiyang Ma a, Jie Min a, Zhi Li a, Ping Yu a, Haiyang Yu a, * a
State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, West China Hospital of Stomatology, Sichuan University, 610041, Chengdu, Sichuan Province, China b Beijing Stomatological Hospital, Capital Medical University, 100050, Beijing, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Dental implant Screw loosening PEEK coating PTFE coating Fatigue
Objectives: Mechanical complications play a key role in failure of dental implants. Retaining screw loosening was one of the most commonly encountered. This study investigated the effect of PEEK and PTFE coatings on dental implant screw thread joint. Methods: Retaining screws were coated with PEEK and PTFE in thickness of 30 μm and 60 μm. Friction coefficient and clamping force of screw thread pair were measured, single load-to-fracture (SLF) test and dynamic fatigue life (DFL) test were done to test the stability of implant thread connection. After that, screw fracture mode and erosion morphology of screw surface and implant internal thread were observed. Results: The results showed that both PEEK and PTFE coatings could reduce friction coefficient, and consequently increase clamping force, especially PTFE coatings. PEEK coatings had no significant effect on fracture load, while 30 μm PTFE coating reduced fracture load. PEEK coatings also elongated fatigue life and improved the antiloosening property under dynamic load, while 30 μm PTFE coating shortened fatigue life. Most of the screw fracture happened at the first thread of the retaining screws. The fracture-end of PEEK coated screws were loosed and could easily remove, but fracture-end of PTFE screws could not. Internal thread observation showed that both PEEK and PTFE coatings could reduce wear of implant internal thread. Conclusion: PEEK coatings could effectively improve the stability of implant threaded connection, and reduce wear of implant internal thread. PEEK coating may be a suitable way to prevent screw loosening.
1. Introduction Dental implant-supported restorations are used as a predictable treatment option for partially or completely edentulous patients, and present high success rates (Adell et al., 1990; Jemt and Lekholm, 1995). The clinical success of dental implants and related reconstructions not only depends on surgical survival, but also biological and mechanical complications occurring during clinical function (Sailer et al., 2012). It was reported that more than half of implants experience at least one mechanical complication (Shemtov-Yona and Rittel, 2015). Among these complications, screw loosening is one of the most commonly encountered, with a 5-year rate of 3.1–10.8% (Pjetursson et al., 2014). Screw loosening can lead to prosthetic failure, occlusal overload of the implant or even internal thread damage, causing irreversible recon struction failure. Therefore, efforts are required to prevent screw
loosening. Preloading retaining screws during screw tightening causes elonga tion and helps keep them under tension (Al Jabbari et al., 2008). This tension represents a clamping force between prosthetic components and the implant (Haack et al., 1995). Torque control is the most common method employed to tighten the abutment retaining screws (Guda et al., 2008). While tightening a screw, the torque on its joint is not entirely transferred to the preload (Parmley Robert, 1977). According to Shigley, only 10% of the tightening torque is used to generate screw tension, and most of the applied torque is used to overcome the friction between the screw head and the abutment surface, as well as to counter thread friction (Shigley, 2011). Therefore, the effective preload is far less than the applied torque. The exact mechanism of screw loosening in fixed prostheses is likely to be complex (Sakaguchi and Borgersen, 1995). After screw tightening, other parameters such as fatigue cycling,
* Corresponding author. State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University, 610041, Chengdu, Sichuan Province, China. E-mail addresses: [email protected], [email protected], [email protected] (H. Yu). https://doi.org/10.1016/j.jmbbm.2019.103530 Received 20 August 2019; Received in revised form 10 November 2019; Accepted 11 November 2019 Available online 14 November 2019 1751-6161/© 2019 Elsevier Ltd. All rights reserved.
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
chewing patterns and loads also affect the mechanical features of dental implants (Wu et al., 2017). Besides, it is inevitable that small-scale machine tolerances are between the implant-screw thread pair surface. Meanwhile, masticatory loading leads to micromotion and dynamic fa tigue of the implant-screw interface, and the preload decrease could significantly increase micromotion (Binon, 1994; Gratton et al., 2001), causing a further preload loosening and finally leading to screw loos ening or even fracture. Screw friction is increased by material hardness and surface rough ness, and decreased by lubricants (Burguete et al., 1994). To minimize friction, kinds of lubricats were used. Although the chemical composi tion and physical characteristics of saliva samples differ among in dividuals (Tzenakis et al., 2002), the presence and quantity of lubricant (saliva, peri-implant fluid, and/or blood) between the implant’s mating components, which is clinically unpredictable, can also affect the coef ficient of friction (Barbosa et al., 2011; Weiss et al., 2000). Nigro er al. found that the wet condition (the inner threads of the implants filled with artificial saliva) generated higher preload values of zirconia abut ment screws than the dry condition (Nigro et al., 2010). But according to Tzenakis et al., artificial saliva has potentially different lubrication characteristics with human saliva. It has been reported that lubrication of implant gold prosthetic screws with saliva had no apparent effect on tensile strength (Al Rafee et al., 2002) and the ability of saliva to provide any significant lubrication is doubtful (Byrne et al., 2006). Wu et al. (2017) investigated the stability of implant-abutment connection under varying lubrication: graphite powder and medical vaseline. The results showed that lubricated screws made the joint easier to loosen. Solid film lubricants were also used. Martin et al. (2001) tested four types of screw: Gold-Tite (gold alloy screw coated with 0.76 μm of pure gold, 3i Implant Innovations, Inc.), TorqTite (titanium alloy screw coated with Tiodize and amorphous diamond coatings, Nobel Biocare UK, Ltd.), gold alloy and titanium alloy. The results showed that gold-coated screw had higher preloads and TorqTite gave the higher rotational angles when compared with the conventional gold alloy or titanium alloy screws. Elias et al. (2006) measured the opening torque (N cm) of implant abutment screws coated with four different materials, TiN (titanium nitride), TiCN (titanium carbonitride), Teflon and Parylene. Their re sults showed that screws without coating had the highest opening torque for a given applied tightening torque (35N cm), titanium screws without coating were more stable than Teflon coated screws under cyclic loading. Choe et al. (2011) coated the abutment screws with 1–3 μm TiN and WC (tungsten carbide), found that the fatigue life of implant system with TiN and WC coated abutment screws was relatively longer compared to non-coated abutment screw at 420N load. In summary, graphite powder, medical vaseline and Teflon made the joint easier to loosen. TiN, TiCN and WC are extremely hard ceramic materials, coated screws could ineluctably lead to implant internal thread wear under masticatory loading. From the perspective of clinical strategy, prostheses need to be replaced regularly under normal circumstances; however, the implant cannot be removed once the osseointegration occurs, and is not as easy to replace as the retaining screw. It is also crucial to protect the implant’s internal thread from abrasion. Therefore, coatings are required, which could increase the preload, prevent loosening under cyclic stress, and protect the internal thread. Polytetrafluoroethylene (PTFE) or Teflon resin is a waxy and smooth synthetic polymer material broadly used in many industrial and medical applications. It has good chemical, thermal, and electrical stability as well as low friction (Sajid and Ilyas, 2017). PTFE can be applied to the human body as an artificial blood vessel, a heart valve, and so on. Pol yetheretherketone (PEEK) is a biocompatible polymer with good me chanical properties. It has good resistance to wear and fatigue, resulting in low coefficient of friction for a large range of sliding conditions (Friedrich et al., 2011; Schwitalla and Müller, 2013). In addition, PEEK has reliable biosafety, and bacterial colonization of its surface is reduced compared to pure titanium. Besides, its composites have similar elastic
modulus to human bone tissue (Skinner, 1988), which makes it widely used in the medical field. Studies have been performed with PEEK retaining screws, but the results showed insufficient strength for PEEK or reinforced PEEK screws (Neumann et al., 2014; Schwitalla et al., 2016). In this study, we hypothesized that appropriate PEEK and PTFE coatings could realize all required goals, including friction reduction, stability improvement and internal thread prevention. The aim of this study was to investigate the stability of dental implant screw thread connection with PEEK and PTFE coated titanium alloy screws, by determining the clamping force, fracture load, fatigue life, loosening torque and surface wear under static and dynamic loads. 2. Materials and methods 2.1. Sample preparation Commercially available titanium dental implants (Ø4.3 � L11 mm) and straight abutments (Ø4.5 � Gh1.5 � h6.0) manufactured by WEGO (WEGO, Jericom Biomaterilas Co., Ltd. Weihai, Shandong, China) were used. Abutment retaining screws (1.5 � L10 mm) were used as a tita nium substrate. PEEK (VICOTE 700, Victrex, England) and PTFE (Zhangjiagang Youcheng High-tech Materials Co. Ltd. Zhangjiagang, Jiangsu, China) were selected as coating polymers. PEEK and PTFE coatings with different thicknesses (30 or 60 μm) were deposited on the screw thread through thermal spraying (Zhangjiagang Youcheng Hightech Materials Co. Ltd.) within the machining tolerance of WEGO implant system (Fig. 1). Totally 300 specimen sets were prepared in this study. Specimens were divided into 5 groups based on screw surface condition, including the control (C; screw without coating), PEEK-1 (screw coated with 30 μm PEEK), PEEK-2 (screw coated with 60 μm PEEK), PTFE-1 (screw coated with 30 μm PTFE), and PTFE-2 (screw coated with 60 μm PTFE) groups. 2.2. Friction coefficient measurement A bolt fastener test and analysis system (5413-2777, Schatz-Analyze, SCHZTA AG, Cologne, Germany) was used according to the GB/T 16823.3-2010/ISO 16047:2005 standard for screw joint clamping force and friction coefficient measurements. To fit the bolt fastener system, implants and abutments were embedded into a titanium block, and fit in a holder, respectively. A schematic diagram is exhibited in Fig. 2. After the holders were set in place, the screw was fastened into the implant by a screw driver through the abutment at a speed 4 rpm. During the pro cedure, the sensor would record the torque and clamping force, and calculate the friction coefficient. When the tightening torque reached 20 Ncm, the driver stopped, and the test was completed. A total of 25 implant and abutment pairs were assessed. The deviation of the di mensions of titanium blocks was less than 10%. 2.3. Fracture resistance test An in vitro model was stablished under the ISO/FDIS 14801:2014 guidance (Fig. 3). A specimen set was assembled by an implant, straight abutment, retaining screw and simplified zirconia crown (Lava, 3M ESPE, St. Paul, MN, USA). Each implant was embedded into a cylindrical acrylic resin (NISSIN, Tokyo, Japan) measuring 30 mm in diameter and 20 mm in height (Fig. 4). After tightening and loosening the retaining screw 5 times by hand, the straight abutment was tightened to 20.0 Ncm using an electric tor que meter (LutronTQ8800, Taipei, Taiwan China) according to the manufacturer’s recommendations, and retorqued to 20.0 Ncm after 10 min. Then, the abutment was sealed, with the simplified crown adhered to the abutment using cement (Rely X Luting, 3M ESPE, St. Paul, MN, USA). Specimens were divided into 5 groups based on retaining screw 2
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 1. Dental implant abutment screws of different test groups: A, Control group (C); B, PEEK-1 group; C, PEEK-2 group; D, PTFE-1 group; E, PTFE-2 group.
2.4. Screw loosening test Specimens were assembled and embedded as described in 2.3 above. A total of 150 specimens were prepared, randomly divided into 5 groups (same as above). First, 5 specimens in each group were selected to un dergo screw loosening 10 min after tightening (without loading). The loosening torque was recorded as Ti, and average Ti and percentage decline in torque (θ; θ¼(20-Ti)/20 � 100%) were calculated. Secondly, dynamic asymmetric sine wave load, 25–250 N was applied at 15 Hz, with different numbers of loading cycles to simulate functional masti cation (1.0 � 105, 3.0 � 105, 6.0 � 105, 9.0 � 105 and 1.2 � 106 cycles). After the test, the detorque value (Tm) of each specimen was recorded. 2.5. Morphologic observation All fractures of the retaining screws were classified according to fracture location. After the DFL test, the fractured screw-ends were removed from the implant. In the subgroups of 90% F0 of each test group, fracture surface was observed under a scanning electron micro scope (SEM, Quanta200, FEI, Hillsboro, OR, USA). After screw loosening test, surface of the retaining screw and implant internal thread in the subgroups of 1.2 � 106 cycle for each test group were also observed under SEM.
Fig. 2. Schematic diagram of the clamping force and friction coefficient test. 1, principal axis; 2, total torque sensor; 3, screwdriver holder; 4, screwdriver; 5, holder 1; 6, abutment and titanium block; 7, holder 2; 8, implant and titanium block; 9, retaining screw; 10, clamp force sensor.
coating. Each specimen was fixed into a dynamic fatigue testing ma chine (BOSS ELF-3330,Eden Prairie, USA). The loading force was pro vided by the testing machine through a fixed zirconia indenter with a flat contact surface, at a speed of 1.0 mm/min.
2.6. Statistical analysis Statistical analyses were performed with SAS (SAS Institute Inc., Cary, USA, Version 5.0.1). Data were analyzed by one-way analysis of variance (ANOVA) with Fisher’s PLSD test. P < 0.05 was considered statistically significant.
2.3.1. Single load-to-fracture (SLF) test A gradually increased load was applied to the specimen until frac ture. Then, the maximum load value was recorded and defined as the fracture load F0. A total of 25 specimens were tested, and the average F0 was determined.
3. Results 3.1. Friction coefficient and clamping force
2.3.2. Dynamic fatigue life (DFL) test The DFL test was performed after the SLF test. Four levels of dynamic load were selected, including 90%, 80%, 70%, and 60% of the average F0 for each group (C, PEEK-1, PEEK-2, PTFE-1, and PTFE-2). The load varied within 10% of the nominal peak value at 15 Hz. The fatigue life was defined as the loading cycle to fracture; the fatigue limit was set as cycle limit 5 � 106. A total of 100 specimens were prepared. Each dy namic loading was tested for a total of 5 times.
The friction coefficients and clamping forces of implant-screw thread connection are shown in Fig. 5. Significant differences were found be tween each group. Among test groups, group C had the largest friction coefficient, followed by the PEEK-2, PEEK-1 and PTFE-2 groups; the PTFE-1 group had the smallest value. Accordingly, the PTFE-1 group had the highest clamping force, followed by the PTFE-2, PEEK-1 and PEEK-2, with group C showing the smallest value.
3
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 3. Schematic of the test setup (A. 1, loading device; 2, nominal bone level; 3, abutment; 4, hemispherical loading member; 5, dental implant body; 6, specimen holder; line AB, loading axis; C, loading center; line DE, axis of the implant; F, loading force; ∠ACD ¼ 30� ); Loading apparatus (B, C).
Fig. 4. Schematic of a set of specimen (A. 1, simplified zirconia crown; 2, straight abutment; 3, retaining screw; 4, implant; 5, acrylic resin); The specimen embedded in acrylic resin (B).
3.2. Fracture load and fatigue life
group was 880.93 � 3.65 N, i.e. reduced by 50.47 N (5.42%), while that of PTFE-2 group was 995.17 � 47.13 N, i.e. increased by 63.77 N (6.85%).
3.2.1. Fracture load The average fracture load of group C was 931.40 � 32.78 N. The PEEK groups had fracture loads comparable with that of group C, including 936.70 � 9.51 N and 937.60 � 30.86 N for the PEEK-1 and PEEK-2 groups, respectively (Fig. 6). It can be found that PEEK coating had no significant effect on the fracture load of the implant-abutmentscrew composite. Significant statistical differences were found be tween the PTFE groups and group C. The fracture load of the PTFE-1
3.2.2. Dynamic fatigue life According to SLT results, the four levels of fatigue force were 90%, 80%, 70%, and 60% of the mean F0, i.e. 84 N–838 N, 75 N–745 N, 65 N–652 N, and 56 N–559 N, respectively. Representative load-cycle curves of the DFL test are shown in Fig. 7. The results showed that the larger the load on a specimen, the shorter 4
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
the corresponding fatigue life, and coating did not affect this tendency. The numbers of cycles in each group were relatively close at high loads. As the load decreases, the differences among groups became more sig nificant; the lower the value, the greater the difference. At a load of �559 N, except for the PTFE-1 group, the other groups reached the fa tigue limit (5 � 106 cycles) without fracture. This indicated that the fatigue limit of the PEEK-1, PEEK-2 and PTFE-2 groups was 559N. As shown in Fig. 7, various coatings had different effects on fatigue life, with PEEK and PTFE-2 coatings prolonging the fatigue life and the PTFE1 coating shortening it, compared with group C. 3.2.3. Fracture mode and fractography The fracture mode was divided into two types according to the fracture location of retaining screws. The most common type was the fracture occurring at the first thread root. Specimens of the C, PEEK-1, PTFE-1, and PTFE-2 groups belonged to this type. Meanwhile, speci mens of the PEEK-2 group were mainly fractured at the fourth thread root. Fracture-ends without coating or with PEEK coating were loose and easy to remove, while those with PTFE coating remained tight, and could not be removed manually. Screw fractographs in group C are shown in Fig. 8. The fracture surface was irregular (Fig. 8A) but could be recognized as a fatigue fracture. According to the different fractal features, typical fatigue fracture characteristics such as quasi-cleavage facets (Fig. 8C), dimples (Fig. 8D and E) and fatigue striations (Fig. 8F) were observed. The fractographs of coating groups are shown in Fig. 9. PEEK-1 group had similar fractographs as group C. The initiation and crack propagation area of the fracture was not obvious in PEEK-2 and PTFE-2 group, while a large number of dimples observed (Fig. 9E–H, M-P). The fractograph of the PTFE-1 group was different (Fig. 9I–K). Some dimples were found (Fig. 9I and j), while most of the fracture surface was flat and the grainy crystal structure was observed (Fig. 9J and K).
Fig. 5. Friction coefficients and clamping forces at 20 Ncm tightening torque of screw thread connection in various test groups. *, þsignificant difference be tween groups.
3.3. Loosening torque and surface microtopography 3.3.1. Loosening torque The initial loosening torque Ti and the torque loss rate (θ) in each group are shown in Table 1. Ti in each group was lower than the tightening torque of 20 Ncm, indicating that after tightening, torque loss occurred without cyclic loading. Ti in group C was 14.06 � 1.47 Ncm, indicating a 30.0% decrease of the tightening torque. Compared with group C, all coating groups had higher Ti values and reduced torque loss. Among the coating groups, the PEEK-2 group had the highest Ti (15.17 � 0.77 Ncm) and the lowest θ (24.37%), followed by the PEEK-1 (Ti ¼ 14.98 � 0.51 Ncm, θ ¼ 25.26%) and PTFE-2 (Ti ¼ 14.72 � 0.77 Ncm, θ ¼ 26.57%) groups, and the PTFE-1 group had the smallest Ti (14.29 � 0.75 Ncm) and largest θ (28.69%). The detorque values after cyclic loading are shown in Fig. 10. It was lower than Ti in all groups, and detorque values in all test groups were higher than those of group C. Significant differences were found be tween the PEEK-1 group and group C at T9 and T12. In the PEEK-1 group, the detorque value gradually increased with the cyclic load number. On the contrary, the detorque value gradually decreased in the PTFE-2 group, while significant differences were found between the PTFE-2 group and group C at T1 and T12. After 1.2 � 106 cycles of loading, the detorque values were significantly higher than the group C value in the PEEK-1 and PTFE-2 groups. The PEEK-1 group had the highest value, followed by the PEEK-2 and PTFE-2 groups.
Fig. 6. Fracture load (F0) values of the test groups. *significant difference be tween groups.
3.3.2. Surface microtopography of retaining screws Retaining screws were removed and observed by SEM. As shown in Fig. 11, the surface of the new screw was smooth, and mechanical cut ting marks were visible (Fig. 11A and B). A lot of wear debris were found on the screw surface in group C (Fig. 11C and D, white arrow). In the PEEK-1 group, coating was peeled off from the fifth thread, and stripping was on one side of the thread flank (flank oriented to the screw head),
Fig. 7. Representative load-cycle curves of different groups in the DFL test. 5
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 8. Fractographs of the abutment screw in group C. Figure A, Integral morphology, with a, b, c, and d representing different regions of the fracture surface. “a” was the initiation and crack propagation area of the fracture. The arrow shows the direction in which the fracture extended. B, Area shown by “b” in Figure A; a boundary between the crack propagation and catastrophic rupture area is visible. C, Area shown by “e” in Figure B, i.e. the crack propagation area of the fatigue fracture, with visible quasicleavage facets. D, Catastrophic rupture area of the fatigue fracture, with a large number of dimples observed. E, Higher magnification of D. F, Higher magnification of Figure C, with the box showing fatigue strips surrounded by small dimples.
while the last thread was peeled off on both sides (Fig. 11E). Several grinding debris were observed on the screw surface (Fig. 11F), and PEEK scraps were found around the grinding debris (Fig. 11F, black arrow). In the PEEK-2 group, the stripping area was larger. From the surface of the third thread, the coating peeled off and a limited amount of debris were visible (Fig. 11H). In the PTFE-1 group, coating was peeling from the fourth thread, and only coating on the thread flank surface oriented to the screw head peeled off. Scratches were found on the coating surface (Fig. 11J). The peeled coating adhered to the screw surface. In the PTFE2 group, coating in thread crests was irregularly fractured and peeled off from the fifth thread, and the metal substrate was exposed less than in other coating groups. PTFE scraps were observed (Fig. 11K and L). In contrast to group C, metal wear debris were sharply reduced on the thread surface in the coating groups.
dissipation while only a fraction is stored as elastic deformation or preload (Barbosa et al., 2011). Therefore, reducing friction could in crease the preload. Coating is one of the methods for reducing the fric tion coefficient. As shown in Fig. 5, all the four coatings could reduce the friction coefficient and increase the clamping force (PTFE-1>PT FE-2>PEEK-1>PEEK-2). For the same materials, the friction coefficient of thin layer coating (30 μm) was smaller, with a larger clamping force. This may be due to the more intimate contact of thicker coatings compared with thinner ones. At the same material thickness, the friction coefficient of PTFE coating was lower than that of PEEK coating. This could result from the outstanding lubricant property of PTFE. The PTFE-1 group had the highest clamping force, but was fractured at the lowest load, significantly lower than that of group C. This may be due to screw deformation. The increased clamping force acted on the weak point of the thread interface, caused stress concentration, and resulted in damage and fracture. Although coatings increase the preload, it is also important that the preload would remain stable instead of being lost quickly, especially under masticatory stress. Therefore, we also assessed the effect of coating on torque loss before and after dynamic load. Studies have shown that after tightening, the immediate torque loss rate of the thread joint is about 3–20% (Nigro et al., 2010), and tightening-unscrewing the retaining screw repeatedly could lead to an increase in torque loss. In the Brånemark system, the torque is lost by approximately 20% after the first tightening of the screw, about 31% after repeated tightening-unscrewing for 5 times, and approximately 36% after 15 times (Weiss et al., 2000). The reason for the short-term relaxation may be embedment relaxation, localized plastic deformation or torsional relaxation of the screw (Cantwell and Hobkirk, 2004). Our results were consistent with previous studies. The initial loosening torque was lower than the tightening torque (Table 1), and torque loss in the coating groups was reduced. These findings indicated that application of coating reduces the torque loss rate and prevents torque loss. After dynamic cyclic load, the detorque value was also decreased in each group. The detorque value in the coating groups was higher than that of group C. It could be considered that coating may improve the anti-loosening performance of the implant thread joint. Except for the PTFE-1 group (Fig. 10), the other three coating groups showed signifi cantly increased detorque value at 1.2 � 106 cycles of load. Coating changes the surface topography of the screw, and further affects the mechanical behavior of the thread joint of the implant. However, whether it affects the long-term life of the implant is un known. The Dental implant-abutment complex is subjected to dynamic
3.3.3. Wear microtopography of internal thread As shown in Fig. 12, irregular corrugation (Fig. 12D, white arrow) was visible at the interface between the corresponding surface of the screw root (Fig. 12A, a) and the flank (Fig. 12A and b, Fig. 12B and c), representing metal surface peeling off due to abrasion. According to morphological features, the surface in thin layer coating groups (Fig. 12E, F, I, J) had more wear debris, and that of thick coating groups (Fig. 12G, H, K, L) was cleaner and flatter. Specifically, the PEEK coating groups showed slighter internal thread wear compared with group C (Fig. 12C and D), exhibiting sporadic tiny grinding debris (Fig. 12F, red arrow) and PEEK scraps (Fig. 12F, black arrow). Compared with the PEEK-1 group, more wear debris were observed in the PTFE-1 group. Interface metal stripping was more obvious in the PTFE-2 group compared with the PEEK-2 group. 4. Discussion Screw loosening is one of the most common complications of dental implants in clinical practice. An effective anti-loosening method is ex pected to be developed. This study investigated the effect of PEEK and PTFE coatings on implant threaded connection. The results showed that PEEK coating was effective in improving connection stability and pro tecting the implant’s internal thread. For screw joint, the clamping force (preload) should be high enough in case the retaining screws loosen easily under repeated bite forces. The process of screw tightening is also an energy conversion process. Tightening torque converts into kinetic, frictional, and elastic compo nents (Breeding et al., 1993). Most of the torque converts into frictional 6
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 9. Fractographs of the abutment screw in coating groups. PEEK-1 group (Figure A, B, C, D. Figure A, Integral morphology, with a, b and c representing different regions of the fracture surface. “a” was the initiation and crack propagation area of the fracture. B, Area shown by “b” in Figure A; a boundary between the crack propagation and catastrophic rupture area is visible. C, Area shown by “d” in Figure B. D, Higher magnification of “c” in Figure A). PEEK-2 group (Figure E, F, G, H. Figure E, Integral morphology, with e and f representing different regions of the fracture surface. F, Area shown by “e” in Figure E; G, Area shown by “f” in Figure E. H, Higher magnification of F). PTFE-1 group (Fig. I, J, K, L. Fig. I, Integral morphology, with i and j representing different regions of the fracture surface. J, Area shown by “i” in Fig. I; K, Higher magnification of J. L, Higher magnification of area shown by “j” in Fig. I). PTFE-2 group (Figure M, N, O, P. Figure M, Integral morphology, with m and n representing different regions of the fracture surface. N, Area shown by “m” in Figure M; O, Area shown by “n” in Figure M. P, Higher magnification of N). The arrow shows the direction in which the fracture extended.
the number of external load cycles experienced during fatigue damage. According to Fig. 7, PEEK and PTFE-2 coatings prolonged the fatigue life, which was shortened by PTFE-1 coating. The fatigue limit (at 5 � 106 cycles) in the PEEK-1, PEEK-2 and PTFE-2 groups was 559N, which was greater than the human average occlusal and masticatory force (Binon and McHugh, 1996; Paphangkorakit and Osborn, 1997; Raadsheer et al., 1999; Van Eijden, 1991). Thicker coating resulted in prolonged fatigue life, which may be due to the thicker coating making the thread contact more intimate or the surface stress distribution of the thread pair more uniform. At the same coating thickness, fatigue life with PEEK coating was longer than that of PTFE, which may be due to the larger friction coefficient of PEEK. The cold flow property of PTFE may also contribute to this difference. The effect of coatings on the implant screw fracture characteristics have received relatively little investigation. According to Fig. 6, PEEK
Table 1 Loosening torque (Ti) values and torque loss rates (θ) of test groups without loading. Groups
Ti (Ncm)
θ (%)
C PEEK-1 PEEK-2 PTFE-1 PTFE-2
14.06 � 1.47 14.98 � 0.51*y 15.17 � 0.77* 14.29 � 0.75y 14.72 � 0.77*
30.00 25.26 24.37 28.69 26.57
*, yP < 0.05 comparing Ti values among test groups.
occlusal force during its function. Although the occlusal force is much lower than the fracture load of the material itself, it could be destroyed after a certain number of cycles, i.e. fatigue damage. The Fatigue life is 7
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
Fig. 10. Detorque values (Tm) after corresponding cyclic loads in the test groups. T1 ¼ detorque value at 1.0 � 105 cycles, T3 ¼ detorque value at 3.0 � 105 cycles, T6 ¼ detorque value at 6.0 � 105 cycles, T9 ¼ detorque value at 9.0 � 105 cycles, T12 ¼ detorque value at 1.2 � 106 cycles. *, þ,◆,●significant difference between groups at the same cyclic load; α, β, δ, ε, ϕ, χ significant difference between groups at different cyclic loads.
groups had comparable fracture load with group C. The fractograph in PEEK groups (Fig. 9A–H) was similar with group C (Fig. 8). These results indicated that PEEK coatings had little effect on fracture. For PTFE-1 group, the fracture load was decreased, and the fractograph showed different structure, which may be deduced low resistance of fracture. For PTFE-2 group, the fracture load was increased and similar fractograph with group C was found, which may be inferred a better performance of fracture resistance than group C. These results indicated that PEEK and PTFE coating have influence on the fatigue characteristics of the screws. This may because coating surface and thickness affect the distribution of stress and then have influence on crack formation. Most of the retaining screw was broken at the first thread, corrobo rating previous findings (Wu et al., 2017). In the study, a three-dimensional model of the implant-abutment-retaining screw was established by FEA, and the stress distribution of each component was analyzed after using different lubricants. The results showed that the maximum stress of the retaining screw is concentrated at the first thread regardless of whether the lubricant was used or not. Stress levels were more obvious in the first four threads than in the subsequent one. In addition, stress level was higher in the lubricant group compared with the control group. Our results were consistent with these findings. In this study, fracture-ends of screws in group C, PEEK-1 and PEEK-2 were mobile and easy to remove, while fracture-ends in the PTFE-1 and PTFE-2 groups remained clamped. In clinical practice, once a retaining screw with PTFE coating is fractured, removing the fracture-end may be difficult. The surface topography and micro-roughness of implant-screw mating surfaces change throughout the lifetime of the implanted pros thesis (Guzaitis et al., 2011). The retaining screw and internal thread surface of the implant should be assessed for manufacturer machining irregularities before use; meanwhile, because of micromotion caused by external load, the screw and internal thread surface would inevitably wear. This can cause deformation, lead to preload loss or even screw slipping. Effective coatings should protect internal threads and not cause excessive wear. Unfortunately, few studies have focused on internal thread wear. As shown in Figs. 11 and 12, internal thread surface in each group was worn out. Peeling off of the internal thread metal surface occurred mostly at the intersection of the screw root and the crest, corresponding to the position where the central screw was broken, and stress concentration and plastic deformation started. In this study, PEEK and PTFE coatings could both reduce internal thread damage to some
Fig. 11. Microtopography of abutment screws. A and B, Blank controls. C and D, group C. E and F, PEEK-1 group. G and H, PEEK-2 group. I and J, PTFE-1 group. K and L, PTFE- 2 group. White arrows, metal wear debris; black ar rows, coating scraps. Scratches were highlight in the boxes.
8
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530
extent while preventing loosening. In further studies, different implantation systems produced by other manufacturers should be tested. In the dynamic cyclic test, the number of loading cycles will be increased to mimic a greater clinic longevity. Additionally, different test conditions could simulate changeable masticatory circumstances. 5. Conclusion In summary, PEEK and PTFE coatings improve the immediate fastening and long-term anti-loosening performances, and could reduce abrasion of implant internal thread. However, since the PTFE-1 group showed no improved fracture resistance, and the fracture-end obtained with PTFE coating was difficult to remove, we consider PEEK coating an ideal method for improving the stability of implant screw thread connection. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank the staff of WEGO Jericom Bio materilas Co., Ltd. for assistance in this study. This company had no role in study design, data collection and analysis, decision to publish, or manuscript preparation. This study was supported by the National Natural Science Foundation of China (No. 81771113). References Adell, R., Eriksson, B., Lekholm, U., Brånemark, P.I., Jemt, T., 1990. A Long- term followup study of osseointegrated implants in the treatment of totally edentulous jaws. Int. J. Oral Maxillofac. Implant. 5, 13. Al Jabbari, Y.S., Fournelle, R., Ziebert, G., Toth, J., Iacopino, A.M., 2008. Mechanical behavior and failure analysis of prosthetic retaining screws after long-term use in vivo. Part 1: characterization of adhesive wear and structure of retaining screws. J. Prosthodont. 17, 168–180. Al Rafee, M.A., Nagy, W.W., Fournelle, R.A., Dhuru, V.B., Tzenakis, G.K., Pechous, C.E., 2002. The effect of repeated torque on the ultimate tensile strength of slotted gold prosthetic screws. J. Prosthet. Dent 88, 176–182. Barbosa, G.S., Silva-Neto, J.P.d., Simamoto-Júnior, P.C., Neves, F.D.d., Mattos, M.d.G.C. d., Ribeiro, R.F., 2011. Evaluation of screw loosening on new abutment screws and after successive tightening. Braz. Dent. J. 22, 51–55. Binon, P., 1994. The role of screws in implant systems. Int. J. Oral Maxillofac. Implant. 9, 48–63. Binon, P.P., McHugh, M.J., 1996. The effect of eliminating implant/abutment rotational misfit on screw joint stability. Int. J. Prosthod. 9. Breeding, L.C., Dixon, D.L., Nelson, E.W., Tietge, J.D., 1993. Torque required to loosen single-tooth implant abutment screws before and after simulated function. Int. J. Prosthod. 6. Burguete, R.L., Johns, R.B., King, T., Patterson, E.A., 1994. Tightening characteristics for screwed joints in osseointegrated dental implants. J. Prosthet. Dent 71, 592–599. Byrne, D., Jacobs, S., O’Connell, B., Houston, F., Claffey, N., 2006. Preloads generated with repeated tightening in three types of screws used in dental implant assemblies. J. Prosthodont. 15, 164–171. Cantwell, A., Hobkirk, J.A., 2004. Preload loss in gold prosthesis-retaining screws as a function of time. Int. J. Oral Maxillofac. Implant. 19. Choe, H., Lee, C., Jeong, Y., Ko, Y., Son, M., Chung, C., 2011. Fatigue fracture of implant system using TiN and WC coated abutment screw. Procedia Eng. 10, 680–685. Elias, C., Figueira, D., Rios, P., 2006. Influence of the coating material on the loosing of dental implant abutment screw joints. Mater. Sci. Eng. C 26, 1361–1366. Friedrich, K., Sue, H., Liu, P., Almajid, A., 2011. Scratch resistance of high performance polymers. Tribol. Int. 44, 1032–1046. Gratton, D.G., Aquilino, S.A., Stanford, C.M., 2001. Micromotion and dynamic fatigue properties of the dental implant–abutment interface. J. Prosthet. Dent 85, 47–52. Guda, T., Ross, T.A., Lang, L.A., Millwater, H.R., 2008. Probabilistic analysis of preload in the abutment screw of a dental implant complex. J. Prosthet. Dent 100, 183–193. Guzaitis, K.L., Knoernschild, K.L., Viana, M.A., 2011. Effect of repeated screw joint closing and opening cycles on implant prosthetic screw reverse torque and implant and screw thread morphology. J. Prosthet. Dent 106, 159–169. Haack, J.E., Sakaguchi, R.L., Sun, T., Coffey, J.P., 1995. Elongation and preload stress in dental implant abutment screws. Int. J. Oral Maxillofac. Implant. 10, 529–536.
Fig. 12. Wear microtopography of the implant internal thread. A and B, Blank controls. “a”, corresponding surface to the screw root; “b”, surface corre sponding to the screw crest; “c”, flank surface. C and D, group C. E and F, PEEK1 group. G and H, PEEK-2 group. I and J, PTFE-1 group. K and L, PTFE-2 group. White arrow, metal surface peeling; red arrow, metal wear debris; black arrow, coating scraps. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 9
X. Chen et al.
Journal of the Mechanical Behavior of Biomedical Materials 103 (2020) 103530 Sajid, M., Ilyas, M., 2017. PTFE-coated non-stick cookware and toxicity concerns: a perspective. Environ. Sci. Pollut. Control Ser. 24, 23436–23440. Sakaguchi, R.L., Borgersen, S.E., 1995. Nonlinear contact analysis of preload in dental implant screws. Int. J. Oral Maxillofac. Implant. 10. Schwitalla, A., Müller, W.-D., 2013. PEEK dental implants: a review of the literature. J. Oral Implantol. 39, 743–749. Schwitalla, A.D., Abou-Emara, M., Zimmermann, T., Spintig, T., Beuer, F., Lackmann, J., Muller, W.D., 2016. The applicability of PEEK-based abutment screws. J. Mech. Behav. Biomed. Mater. 63, 244–251. Shemtov-Yona, K., Rittel, D., 2015. An overview of the mechanical integrity of dental implants. Biomed. Res. Int. 2015. Shigley, J.E., 2011. Shigley’s Mechanical Engineering Design. Tata McGraw-Hill Education. Skinner, H.B., 1988. Composite technology for total hip arthroplasty. Clin. Orthop. Relat. Res. 235, 224–236. Tzenakis, G.K., Nagy, W.W., Fournelle, R.A., Dhuru, V.B., 2002. The effect of repeated torque and salivary contamination on the preload of slotted gold implant prosthetic screws. J. Prosthet. Dent 88, 183–191. Van Eijden, T., 1991. Three-dimensional analyses of human bite-force magnitude and moment. Arch. Oral Biol. 36, 535–539. Weiss, E.I., Kozak, D., Gross, M.D., 2000. Effect of repeated closures on opening torque values in seven abutment-implant systems. J. Prosthet. Dent 84, 194–199. Wu, T., Fan, H., Ma, R., Chen, H., Li, Z., Yu, H., 2017. Effect of lubricant on the reliability of dental implant abutment screw joint: an in vitro laboratory and three-dimension finite element analysis. Mater. Sci. Eng. C 75, 297–304.
Jemt, T., Lekholm, U., 1995. Implant treatment in edentulous max- illae: a 5-year followup report on patients with different degrees of jaw resorption. Int. J. Oral Maxillofac. Implant. 10, 9. Martin, W.C., Woody, R.D., Miller, B.H., Miller, A.W., 2001. Implant abutment screw rotations and preloads for four different screw materials and surfaces. J. Prosthet. Dent 86, 24–32. Neumann, E.A.F., Villar, C.C., FRANÇA, F.M.G., 2014. Fracture resistance of abutment screws made of titanium, polyetheretherketone, and carbon fiber-reinforced polyetheretherketone. Braz. Oral Res. 28, 1–5. Nigro, F., Sendyk, C.L., Francischone Jr., C.E., Francischone, C.E., 2010. Removal torque of zirconia abutment screws under dry and wet conditions. Braz. Dent. J. 21, 225–228. Paphangkorakit, J., Osborn, J., 1997. The effect of pressure on a maximum incisal bite force in man. Arch. Oral Biol. 42, 11–17. Parmley Robert, O., 1977. Standard Handbook of Fastening and Joining. McGraw-Hill Book Co, New York, NY. Pjetursson, B., Asgeirsson, A., Zwahlen, M., Sailer, I., 2014. Improvements in implant dentistry over the last decade: comparison of survival and complication rates in older and newer publications. Int. J. Oral Maxillofac. Implant. 29, 308–324. Raadsheer, M., Van Eijden, T., Van Ginkel, F., Prahl-Andersen, B., 1999. Contribution of jaw muscle size and craniofacial morphology to human bite force magnitude. J. Dent. Res. 78, 31–42. Sailer, I., Mühlemann, S., Zwahlen, M., H€ ammerle, C.H., Schneider, D., 2012. Cemented and screw-retained implant reconstructions: a systematic review of the survival and complication rates. Clin. Oral Implant. Res. 23, 163–201.
10