Comparing the Maximum Load Capacity and Modes of Failure of Original Equipment Manufactured and Aftermarket Titanium Abutments in Internal Hexagonal Implants

: The purpose of this in vitro study is to compare the maximum load capacity and modes of failure under static loading in three types of titanium abutments (n = 3) with di ﬀ erent processes or manufacturers. The Pre-Ti group consists of prefabricated titanium abutments from original equipment manufacturers (OEM), the CAD-Ti group consists of OEM titanium abutments fabricated with computer-assisted design / manufacturing (CAD / CAM) technique, and the AM-Ti group is CAD / CAM titanium abutment made by aftermarket manufacturers. A full zirconia crown was fabricated and cemented to each abutment. An all-electric dynamic test instrument was used to place loading on the zirconia crown with a crosshead speed set at 1 mm / min. The mean maximum load capacity of both OEM titanium abutments was signiﬁcantly higher than the aftermarket titanium abutments. All these three types of implant–abutment complexes exhibited similar modes of failure, which included deformation of the abutment and implant, fracture of the abutment and retentive screw.


Introduction
Although replacing a missing tooth with a dental implant is a predictable and frequently used treatment, implant-supported prosthesis has proven to be an effective method for oral reconstruction [1][2][3]. The long-term success of dental implants has been confirmed by several studies, with a 5-year survival rate of 94.5%, and a 10-year survival rate of 89.4% [4,5]. In the aesthetic area, the hue of the metal abutment may be visible through the surrounding soft tissue. However, due to its excellent biocompatibility and mechanical properties, titanium abutment is still often utilized in clinical practice [6,7]. Among the formulations of titanium alloys, titanium-aluminum-vanadium (Ti-6Al-4V) is the most widely used in dentistry due to its better physical and mechanical properties and it being more fatigue-resistant compared to commercially pure titanium [8,9].
In the past two decades, computer-assisted design and computer-assisted manufacturing (CAD/CAM) have become an increasingly popular part of dentistry. In routine clinical practice, physicians use implants and its prosthetic components, including abutments and screws, from the same manufacturer, which is known as the original equipment manufacturer (OEM). Sailer et al. [10] and Zembic et al. [11] investigated the survival and complication rates of CAD/CAM OEM zirconia and Metals 2020, 10, 556 2 of 9 titanium abutments in a randomized-controlled clinical trial. All patients received regular platform (RP) Nobel Biocare implant therapy. A total of 20 Procera zirconia abutments and 20 Procera titanium abutments were used in the study. During a 36-month follow-up, all implants and abutments in both groups showed a 100% survival rate and no technical complications were seen in either group. A systematic review by Kapos et al. [12] also reported that the short-to medium-term survival rate of CAD/CAM OEM abutments was 100%. They concluded that CAD/CAM technology can provide results comparable to conventional techniques in terms of implant survival, prosthesis survival, and technical and biologic complications.
With the rapid development of CAD/CAM technology in the dental field, aftermarket manufacturers have begun to introduce their prosthetic components. In daily practice, physicians, dental technicians, or patients may choose the aftermarket abutments for financial concerns. These aftermarket manufacturers attempt to replicate the design of prefabricated original equipment manufacturing abutments [13]. However, aftermarket manufacturers might change the design of the abutment to avoid infringing OEM patents, and the impact of these design changes on the performance of the implant prosthesis is unknown. Yilmaz et al. [14] investigated the load to the failure of five different titanium abutments, including one OEM abutment and four aftermarket abutments. The results showed that the OEM abutment (Zimmer PSA) was the only abutment that showed no fracture of any component before implant failure. On the other hand, all the aftermarket abutments demonstrated screw fracture at an average load of 649.17 N. They concluded that the screw fracture that occurred in aftermarket abutments could result in further clinical prosthetic complications. However, the dimensions of these titanium abutments were not identical, and the crowns were not made to simulate clinical situation in the study.
Owing to the fact that studies on aftermarket abutments are scarce and lack consistency in specimen design, the present study aims to compare the differences in maximum load capacity and modes of failure of the three different titanium abutments, including prefabricated OEM, CAD/CAM OEM and CAD/CAM aftermarket titanium abutments. To the best of our knowledge, this is the first study to compare OEM and aftermarket titanium abutments with identical external dimensions for a static loading test. In this manner, the errors caused by the difference in external dimensions of the abutments could be minimized. The null hypothesis was that no differences would be found in the maximum load capacity and modes of failure of the three different titanium abutments.

Materials and Methods
Three groups of titanium implant abutments (n = 3) with different compositions or manufacturers were investigated. The Pre-Ti group consisted of prefabricated OEM titanium abutments (Esthetic Abutment NobelReplace, Nobel Biocare, Yorba-Linda, CA, USA), the CAD-Ti group consisted of OEM titanium abutments fabricated with CAD/CAM technique by original manufacturers (NobelProcera Titanium Abutment, Nobel Biocare, Yorba-Linda, CA, USA), and the AM-Ti group consisted of CAD/CAM titanium abutments made by aftermarket manufacturers (JingGang, Tainan, Taiwan). These three types of titanium abutment were all made by titanium alloy (Ti-6Al-4V).
One abutment from the Pre-Ti group ( Figure 1) was designated as the prototype abutment for the maxillary central incisors and scanned with a scanner (NobelProcera 2G scanner, Nobel Biocare, Kloten, Switzerland). OEM titanium abutments were then obtained from Nobel Biocare, and abutments from the AM-Ti group were milled by the milling machines (Arum 5X−150, DoowonID Co, Daejeon, Korea). In this way, all the abutments in nine pieces were manufactured in such a way that their external dimensions were identical, with a 0.5 mm-deep circumferential chamfer margin and an incisogingival height of 9.5 mm ( Figure 2).
Nine implant fixtures (NobelReplace Conical Connection PMC RP 4.3 × 10 mm, Yorba-Linda, CA, USA) with standard diameter internal hexagonal connections were used for this study. A 0.35 Nm preload was applied to all the abutments to tighten them to the implant fixtures according to the manufacturer's instructions. Zirconia crowns (VITA In-Ceram®YZ DISC Color medium, Bad Sackingen, Germany) with an incisogingival height of 11 mm and a mesiodistal width of 8.5 mm were fabricated for all 9 abutments by scanning each abutment (Activity 880, Smart Optics, Bochum, Germany). The intaglio surface of the zirconia crowns and the surface of the titanium abutments were sandblasted and a ceramic primer (Cera-Resin Bond, CRB, Shofu Dental, Kyoto, Japan) was applied over the intaglio surface of the zirconia crowns. The crowns were bonded to the abutments using a dual-polymeric composite resin cement (RelyX U200 Self-adhesive Resin Cement, 3M ESPE, St. Paul, MN, USA). Excess cement was removed at ×10 magnification.
The maximum load capacity of the titanium abutments was measured using the all-electric dynamic test instrument (ElectroPulsTM E3000, Instron, Norwood, MA, USA). A metal jig was fabricated to hold the specimen in a position so that the long axis of the implant fixture was tilted in a 30 ° angulation to simulate a Class I incisor relationship ( Figure 3) [15][16][17][18]. After placing the specimen in the jig, a metal rod, 6 mm in diameter, was used to place loading at 2 mm lingual to the incisal edge of the zirconia crown. The speed of the universal testing machine was set at 1 mm/min, and the load was placed on the zirconia crown. The crosshead motion was stopped after the load started to decrease because of the plastic deformation, or fracture, of the implant-abutment complex, and the value of the maximum load capacity was recorded. In addition, the modes of failure of the titanium abutment and implant fixture were examined and analyzed under a digital microscope (VHX−950F, Keyence, Belgium). The data were statistically analyzed by variation analysis (ANOVA) using SPSS® software (version 22.0, SPSS Inc., Chicago, IL, USA) to compare the maximum load capacity between the 3 groups of titanium abutments. Since the average of the maximum incisive force in the anterior region reported in the studies was 370 N [19,20], an expected load capacity of 740 N (with safety factor 2.0) was determined to evaluate the reliability of the titanium abutments.   dynamic test instrument (ElectroPulsTM E3000, Instron, Norwood, MA, USA). A metal jig was fabricated to hold the specimen in a position so that the long axis of the implant fixture was tilted in a 30 ° angulation to simulate a Class I incisor relationship (Figure 3) [15][16][17][18]. After placing the specimen in the jig, a metal rod, 6 mm in diameter, was used to place loading at 2 mm lingual to the incisal edge of the zirconia crown. The speed of the universal testing machine was set at 1 mm/min, and the load was placed on the zirconia crown. The crosshead motion was stopped after the load started to decrease because of the plastic deformation, or fracture, of the implant-abutment complex, and the value of the maximum load capacity was recorded. In addition, the modes of failure of the titanium abutment and implant fixture were examined and analyzed under a digital microscope (VHX−950F, Keyence, Belgium). The data were statistically analyzed by variation analysis (ANOVA) using SPSS® software (version 22.0, SPSS Inc., Chicago, IL, USA) to compare the maximum load capacity between the 3 groups of titanium abutments. Since the average of the maximum incisive force in the anterior region reported in the studies was 370 N [19,20], an expected load capacity of 740 N (with safety factor 2.0) was determined to evaluate the reliability of the titanium abutments.   The maximum load capacity of the titanium abutments was measured using the all-electric dynamic test instrument (ElectroPulsTM E3000, Instron, Norwood, MA, USA). A metal jig was fabricated to hold the specimen in a position so that the long axis of the implant fixture was tilted in a 30 • angulation to simulate a Class I incisor relationship (Figure 3) [15][16][17][18]. After placing the specimen in the jig, a metal rod, 6 mm in diameter, was used to place loading at 2 mm lingual to the incisal edge of the zirconia crown. The speed of the universal testing machine was set at 1 mm/min, and the load was placed on the zirconia crown. The crosshead motion was stopped after the load started to decrease because of the plastic deformation, or fracture, of the implant-abutment complex, and the value of the maximum load capacity was recorded. In addition, the modes of failure of the titanium abutment and implant fixture were examined and analyzed under a digital microscope (VHX−950F, Keyence, Belgium). The data were statistically analyzed by variation analysis (ANOVA) using SPSS®software (version 22.0, SPSS Inc., Chicago, IL, USA) to compare the maximum load capacity between the 3 groups of titanium abutments. Since the average of the maximum incisive force in the anterior region reported in the studies was 370 N [19,20], an expected load capacity of 740 N (with safety factor 2.0) was determined to evaluate the reliability of the titanium abutments.

Results
The mean maximum load capacity was 803.95 N for the Pre-Ti group, 911.05 N for the CAD-Ti group, and 460.96 N for the AM-Ti group (Table 1). There were significant differences in maximum load capacity among the three groups (one-way ANOVA, P < 0.005). The mean maximum load capacity of both OEM titanium abutments was significantly higher than the aftermarket titanium abutments (P < 0.05; Tukey, HSD). The difference in maximum load capacity between the prefabricated OEM and the CAD/CAM OEM titanium abutments was not statistically significant ( Figure 4). The power analysis (G-Power version 3.1.9.2, Heinrich Heine University, Dusseldorf, Germany) for one-way ANOVA revealed the power of β = 0.99.
The maximum load capacity of the Pre-Ti and CAD-Ti abutments were 8.64% and 23.11% higher than the expected load capacity (740 N), respectively. However, the aftermarket abutment showed 37.71% lower than expected load capacity.
All these three types of implant-abutment complexes exhibited similar modes of failure, which included deformation of the abutment and implant, fracture of the abutment and retentive screw. Only one specimen in the prefabricated OEM titanium abutments demonstrated failure without abutment fracture. The fracture surface of most screws was at the level of the implant platform ( Figure 5).

Results
The mean maximum load capacity was 803.95 N for the Pre-Ti group, 911.05 N for the CAD-Ti group, and 460.96 N for the AM-Ti group (Table 1). There were significant differences in maximum load capacity among the three groups (one-way ANOVA, P < 0.005). The mean maximum load capacity of both OEM titanium abutments was significantly higher than the aftermarket titanium abutments (P < 0.05; Tukey, HSD). The difference in maximum load capacity between the prefabricated OEM and the CAD/CAM OEM titanium abutments was not statistically significant ( Figure 4). The power analysis (G-Power version 3.1.9.2, Heinrich Heine University, Dusseldorf, Germany) for one-way ANOVA revealed the power of β = 0.99.   The maximum load capacity of the Pre-Ti and CAD-Ti abutments were 8.64% and 23.11% higher than the expected load capacity (740 N), respectively. However, the aftermarket abutment showed 37.71% lower than expected load capacity.
All these three types of implant-abutment complexes exhibited similar modes of failure, which included deformation of the abutment and implant, fracture of the abutment and retentive screw. Only one specimen in the prefabricated OEM titanium abutments demonstrated failure without abutment fracture. The fracture surface of most screws was at the level of the implant platform ( Figure 5).

Discussion
In the present study, both OEM titanium abutments showed significantly higher maximum load capacity than the aftermarket titanium abutments. The null hypothesis, that all titanium abutments would have similar load-to-fracture values, was rejected. The maximum load capacity of the three different titanium abutments were all higher than the physiological incisive force in the anterior region reported in studies (90-370 N) [21]. However, in some cases, such as trauma-related dental injuries or patients with parafunction habits, (e.g., bruxism, clenching, etc.), the incisive force could be much higher than the physiological range [22]. Waltimo et al. [23] reported that in men with severe dental attrition, the maximum incisive force in the anterior region may reach 569 N or higher. If excessive force causes catastrophic damage to the implant-abutment complex, physicians may need

Discussion
In the present study, both OEM titanium abutments showed significantly higher maximum load capacity than the aftermarket titanium abutments. The null hypothesis, that all titanium abutments would have similar load-to-fracture values, was rejected. The maximum load capacity of the three different titanium abutments were all higher than the physiological incisive force in the anterior region reported in studies (90-370 N) [21]. However, in some cases, such as trauma-related dental injuries or patients with parafunction habits, (e.g., bruxism, clenching, etc.), the incisive force could be much higher than the physiological range [22]. Waltimo et al. [23] reported that in men with severe dental attrition, the maximum incisive force in the anterior region may reach 569 N or higher. If excessive force causes catastrophic damage to the implant-abutment complex, physicians may need to remove or replace it surgically, which usually leads to extensive damage to the alveolar bone. Therefore, in high-risk patients, the use of aftermarket titanium abutments should be more cautious.
Jarman et al. [13] investigated the maximum load capacity of OEM and aftermarket zirconia abutments under static load, and reported that aftermarket zirconia abutments had significantly lower static load values than their counterpart OEM abutments. Kelly et al. [24] confirmed that under cyclic loading study, OEM abutments (Straumann) clearly outperformed aftermarket abutments (Atlantis). They concluded that the design and fabrication differed by aftermarket manufacturers could significantly influence performance. These results could also raise concerns about the use of aftermarket abutments.
Both OEM abutment groups were manufactured by the original equipment manufacturer (Nobel Biocare) in the current study. Since both titanium abutments may be fabricated by the same block material and equipment, it seems reasonable that they possess comparable maximum load capacity. By contrast, aftermarket abutments showed a significantly lower maximum load capacity. This difference may result from block material, manufacturing process accuracy, and quality control. The mechanical properties of titanium alloys are affected by the microstructure of the raw materials. Various manufacturing processes, such as thermomechanical and heat treatment processes, impart various titanium alloy microstructures, which in turn, leads to varying mechanical properties [25]. Aftermarket manufacturers using different raw materials from original manufacturers may affect the performance of the maximum load capacity of titanium abutments. Besides, titanium is a difficult metal to machine. Machining guidelines include cutting at lower speeds, sharpening tools, using sturdy set-ups, and never interrupting the cut, and these guidelines should be followed [26]. If aftermarket manufacturers fail to follow the guidelines, the mechanical performance of titanium abutments could be significantly influenced. For example, the surface discontinuity caused by poor processing can reduce the ductility and fatigue properties of the metal. Overheating of the surface can result in interstitial pickup of oxygen and nitrogen, making the metal hard and brittle [27].
The difference of maximum load capacity between OEM and aftermarket titanium abutments may also result from misfit between the implant fixture, abutment and retentive screw. The connection between the implant fixture and the abutment plays an important role in the stability of the system [28]. The improper connection between the two parts may cause compression of the implant screw, resulting in torque loss, loosening of the screw, or fracture of the screw [29][30][31][32]. Several studies have suggested that aftermarket abutments differ in the design of connection surfaces and materials exhibiting higher rotational misfit and microgap [33,34]. Gigandet et al. [35] evaluated the accuracy of the connection between the original and non-original abutments and concluded that non-original abutments differed in the design of the connection surfaces and materials showed higher rotational misfit, which may lead to unexpected failures. In addition, the microgaps and cavities between the implant, abutment and screw may become a bacterial reservoir [36][37][38], the colonization of microorganisms could trigger chronic inflammatory infiltration. As implant-abutment interface is located near the alveolar bone crest, these inflammatory host responses could lead to peri-implant soft tissue inflammation and marginal bone loss [39][40][41][42][43].
All titanium abutments in the present study showed similar modes of failure, including deformation of the implants and abutments and eventually causing screw fracture. Alqahtani et al. [44] assessed the modes of failures of titanium abutments under static load, and found that all the titanium abutments experienced screw fracture and abutment bending at the apical part. Titanium has settling effects which could tolerate a certain degree of elastic deformation and withstand plastic deformation caused by friction between different components [45,46]. However, when the load is increased beyond the yield limit of the titanium abutment, it will deform and bend, eventually causing the weakest part to fracture, which is usually the abutment screw.
This study has several limitations, such as the small number of samples and the use of static rather than cyclic loads, which may not fully replicate the intraoral conditions. However, the results of this study should provide relative comparisons to guide clinicians until more definitive laboratory tests are reported. Future studies should include a large number of samples and simulate intraoral conditions to obtain more definitive laboratory results.

Conclusions
In the present investigation, three types of titanium abutments with different processes or manufacturers were tested, and the critical conclusions are summarized as follows: (1) Prefabricated OEM and CAD/CAM OEM titanium abutments have a comparable maximum load capacity (803.95 N and 911.05 N, respectively) and are significantly higher than CAD/CAM aftermarket titanium abutments (460.96 N). (2) When the safety factor 2.0 was used to evaluate the reliability of titanium abutments, the maximum load capacity of prefabricated OEM and CAD/CAM OEM titanium abutments were 8.64% and 23.11% higher than the expected load capacity (740 N), respectively. However, the aftermarket abutments showed 37.71% lower than the expected load capacity. (3) All these three types of implant-abutment complexes exhibited similar modes of failure, which included deformation of the abutment and implant, fracture of the abutment and retentive screw.