The implant superstructures can be retained in 2 typical ways, screw-retained and cement-retained. Screw-retained crowns (SRCs) are more classical. Brånemark et al1 launched a series of pioneering studies on SRCs, and then, the screw retention design prevailed in our clinics considering the handling of complications, which occurred frequently at that time (1990s), and exhibited satisfactory clinical long-term outcomes.2–4 After that, SRC has been further developed to satisfy the needs for retrievability and the application in areas of limited interridge space.5 However, because of the existence of screw-access hole (SAH) or screw-access channel (SAC) on the occlusal surface of SRCs, occlusion and esthetics were sacrificed, and therefore, SRCs had some nonignorable disadvantages, such as screw loosening, inconvenience of fabrication, and poor passivity of fit.6 To overcome the esthetic and implant angulation problems, Lewis et al7,8 have performed a series of modifications to the UCLA abutment during 1988 to 1989, which led to the wide use of the cement-retained crowns (CRCs). The advantages of cement-retained restorations included easy fabrication, better esthetics, ideal occlusion, and passive fit.9,10 Nevertheless, the difficulties in retrievability and in removing excess cement were headaches for dentists.11,12 Many studies had indicated that excess cement from the peri-implant region might interfere with healing and implant integration.13–15 In addition, the restoration might be destroyed during the removal procedure, and excess cement might also increase the risk of abutment damage and of internal threads of implant.16
Rajan and Gunaseelan17 described a technique for fabricating a retrievable, cement- and screw-retained crown (CSRC), which harbored the advantages of both SRC and CRC. The major advantages of this approach were providing an overflow channel to reduce the residual cement from the margin of the crown, a choice to cement the crown on the abutment extraorally in the cases of thick gingival biotype, and a drilling direction to find the abutment screw when the superstructures need to be removed. However, CSRC has 2 problems in clinical use. First, because of the SAH on the occlusal surface, the crowns are not intact for both SRC and CSRC. However, SRC has been extensively studied and demonstrated that the SAH may form a weak point of the ceramic veneer,18,19 in contrast, the studies on CSRCs are very limited. Although several studies have been performed to evaluate the effect of the channel designed for these retrievable cemented crowns on fracture resistance of restoration, no consensus has been reached until now. Hussien et al20 and Derafshi et al21 have reported that the SAH in cement-retained implant restorations did not jeopardize fracture resistance. On the contrary, Mokhtarpour et al22 and Shadid et al23 have shown that the preparation of a screw-access channel led to decreased fracture resistance. Another problem is that, at present, there is no widely accepted guideline to suggest the diameter of the SAH. Wadhwani and Chung24 have proposed that excessive preparation of the screw-access channel might cause poor esthetics and a weaker restoration and described a process to minimize the dimension of the SAH, but the size of the SAH and its biomechanical effect was not included in this study because the object of this study was an interim screw-retained prosthesis.
Compared with the screw-retained crown, there are less restrictions of the dimension of the SAH of a cemented crown. The diameter can actually be reduced to a level that the screwdriver can insert into the abutment precisely; it can be even further minimized if we do not want to cement the crown on the abutment extraorally. These may be helpful in improving the strength and esthetics of the crown. Moreover, different diameters of the holes will lead to changes in the crown structure, the contact status between the crown and abutment, and the positional relation between the hole margin and occlusal site. Will these changes lead to adverse effects on the bite force conduction or local stress condition? Which diameter should be used to guarantee the optimum biomechanical states for every component of implant-support restoration (including crown, abutment, screw, and fixture) and surrounding cortical bone? What is the performance if different materials of crown and different load directions of force are applied? These questions have not been studied yet.
The aim of this finite element analysis (FEA) study was to investigate the stress change trend and stress distribution generated on each component of the implant prosthodontic system and surrounding bone when different diameters of the SAH are applied and to identify the optimum channel dimension.
Materials and Methods
Establishment of Fixture, Abutment, and Screw FEA Models
An internal hexagon cylindric fixture 4.5 mm in diameter and 10 mm in length (Semados S-Line; Bego, Bremen, Germany), a Sub-Tec titanium adhesive abutment (Semados S-Line; Bego), with a diameter of 4.5 mm, a total length of 6.0 mm, a total axial taper of 6 degrees and a 1-mm depth radial slight chamfer shoulder,19 and a titanium hexagonal retention screw (Semados S-Line; Bego) were used in this study. All models were created in a 3D modeling software (SolidWorks2014; SolidWorks Corp., Concord, MA) according to the data provided by the implant manufacturer.
The Establishment of the Partial Mandible FEA Model
The cone beam computed tomography (CBCT) image data of a healthy young person (male, aged 25 year) without oral disease were imported into Mimics 17.0 (Materialise, Leuven, Belgium) to reconstruct an stereolithography (STL) format mandibular model. A geometric model based on the STL model was then built with a reverse engineering software, Geomagic Studio 2013 (Raindrop, USA). In this model, the region from the second premolar to the second molar was cutoff and saved, followed by the postprocessing work including eliminating the teeth and smoothing the boundaries and surfaces. The cortical bone surrounding the implant position was modeled with a thickness of 0.9 to 1.2 mm, and the bone density was considered as type II according to the CBCT image.
The Establishment of the Crown FEA Models with Screw-Access Channels in Different Diameters
The abutment STL format model was sent to a denture manufacturer (Kangtailihua Dental Technology Co., Ltd., Xi'an, China) and was imported into 3 Shape 2014 Pro software (3 Shape, Copenhagen, Denmark) to develop a crown, which was matched to the abutment exclusively. The design of the crown respected the anatomical form of the final restoration, allowing a veneering thickness of 0.5 to 1.2 mm and an appropriate dimension (buccolingual: 8.5 mm and mesiodistal: 9.0 mm). The framework design had round edges and was anatomically customized with a minimum thickness of 0.6 mm. The SAHs were set from 0 to 4mm (φ = 0/1/2/3/4mm) in diameter and created along the implant axial in the SolidWorks software.
Two types of FC crowns based on zirconia and gold alloy, respectively, and 2 types of porcelain-fused-to-metal (PFM) crowns based on Co-Cr and Au-Pd alloy, respectively, were applied to evaluate the effects of CSRCs with or without a porcelain layer by means of importing various elastic modulus to the models. It is worthwhile mentioning that the material of the FC zirconia crown in this study was yttrium-oxide partially stabilized (Y-TZP) zirconia, which has been widely used currently.25 The thickness of the cement layer was not modeled.26,27
Preprocessing before the Finite Element Analysis
According to the materials of crowns, 4 model groups were established (Y-TZP, Gold Alloy, CO-CR, AU-PD), and each group contained 5 FEA models with a hole in different diameters (Φ = 0/1/2/3/4 mm). Therefore, a total of 20 FEA models were assembled in Solidworks 2014 and then imported into Ansys Workbench 13 finite element software (Ansys Inc., Canonsburg, PE). The model simulated an ideal osseointegration, so the fixtures were rigidly anchored along their entire interfaces. All materials were assumed to be homogeneous, isotropic, and linearly elastic.28–30 The mechanical properties of the materials were referred to the literature (Table 1).27,31–36 The finite element mesh was generated with tetrahedral elements (Fig. 1). The final model (SAH: Φ = 0 mm) had a total number of 1,362,433 nodes and 956,627 elements; the nodes were fixed in the x, y, and z axes.25,28,37 The boundary condition was set to fix the mesial and distal surfaces of the mandibular bone.31
To simulate mastication, 3 points were loaded on the outer inclines of the buccal cusps (occlusal area was 2.85 mm2).31,38 The applied force was 200 N axially (0°) and 100 N obliquely (45°s) (Fig. 2).39 A static-structural analysis was performed to generate the maximum equivalent von Mises stress (EQV) and the equivalent stress contour to study the linear elastic behaviors of the crown materials, abutment, screw, fixture, and surrounding cortical bones. Furthermore, as the occlusal surface and the margin of crown are often the sites where problems occur, they were evaluated separately in this study.
Considering the tremendous amount of experimental data and the purpose of investigating the trend in stress change, the change rate of maximum von Mises stress ([INCREMENT] [σmax]) was calculated for each SAH interval of 1 mm (0–1 mm, 1–2mm, 2–3 mm, 3–4 mm) according to the formula below:
σmax1: start value of each interval and σmax2: end value of each interval.
Four levels of stress change rate ([INCREMENT] [σmax]) were created to facilitate the analysis procedure and to exclude the result with a slight change, which might not be so dangerous compared with that with a higher change rate (I: [INCREMENT] [σmax] ≤ 10%, almost no change; II: 10% < [INCREMENT] [σmax] ≤ 20%, slight change; III: 20% < [INCREMENT] [σmax] ≤ 50%, obvious change; IV: 50% ≤ [INCREMENT] [σmax] ≤ 100%, remarkable change). The workflow of this study is summarized in Figure 3.
A total of 240 stress values (σmax) and 192 stress change rates ([INCREMENT][σmax]) were generated in this study (Table 2), and the results showed that (1) the stress (EQV) on the occlusal surface of the crowns was almost unchanged when the SAH diameter did not exceed 3 mm. However, when it reached 4 mm, the stress increased by 43.21% and 25.72% (Y-TZP), 42.64% and 25.65% (Gold Alloy), 19.68% and 32.6% (CO-CR), and 23.93% and 29.61% (AU-PD) both in vertical and oblique loads, respectively. Four groups showed a level III rise, except a nearly level III rise (19.68%) in the vertical load of the CO-CR group (Fig. 4). (2) The screw stress (EQV) in the vertical load was 70.029 Mpa (Y-TZP), 70.606 Mpa (Gold Alloy), 91.009 Mpa (Co-Cr), and 90.989 (Au-Pd) when the SAH diameter did not exceed 1 mm. After a level III rise of Y-TZP (28.0%) and Gold Alloy (26.92%) in the range of 1 to 2 mm, 4 groups showed the same rising trend till the end (Fig. 5). (3) The stress (EQV) on the other components (crown margin, abutment, fixture, and surrounding cortical bone) of all the 4 groups only showed level I or II rises (≤20%), no matter which type of load was applied. Thus, they will not be further investigated and discussed in this study.
Equivalent stress contours were also created to investigate the stress (EQV) distribution. For the occlusal surface of the crowns, it showed that EQV stresses were at their maximum in the loading area when the SAH diameter did not exceed 3 mm. After that, it changed to the margin of the SAH in the vertical loading when the diameter reached 4 mm, but it still remained unchanged in the oblique loading. Because the patterns of behavior were the same for all the 4 groups, the stress contours of the CO-CR group are shown as a representative (Fig. 6).
For the stress distribution of the screws (Fig. 7), it showed that the maximum stresses (EQV) were located at the beginning of the first thread, but for the entire model, it showed a uniform distribution without an obvious stress concentration.
FEA, adapted from the engineering arena, has been shown to be an effective computational tool in dental implant biomechanics. It has been used to examine and predict the stress behavior of the implant prosthodontic system to avoid potential problems before clinical application.40
This FEA-based study was designed to evaluate the effects of SAH diameters (Φ = 0/1/2/3/4 mm) on the biomechanical behavior of cement-retained implant prosthodontic system and on their surrounding cortical bones. In the present study, 2 loading directions, vertical and oblique, and 4 types of crowns, including 2 kinds of FC crowns and 2 types of PFM crowns, were applied to form a more comprehensive analysis and, moreover, to offer a preliminary evidence of the optimum SAH diameter in terms of the mechanical properties.
As mentioned earlier, dentists have not reached a consensus on the effect of the SAC on the fracture resistance of cement-retained restoration.20–22 In this study, we found that the maximum von Mises (EQV) stress on the occlusal surface of a crown was almost unchanged when the SAH was 0 to 3 mm; however, when the SAH reached 4 mm, the EQV stress showed an obvious increase. This might be because the location of the oversized SAH was too close to the bite force area. The results indicated that the positional relationship between the bite force area and the SAH might be a direct factor affecting the biomechanical properties of crowns. Considering that none of the in vitro studies above has described this positional relationship in their studies, it might be the reason why different studies drew largely different or even opposite conclusions. In addition, the results might further give a possible explanation for the Al-Omari41 and Torrado19 studies, which demonstrated that the position of the SAH within the occlusal surface did not significantly affect the porcelain fracture resistance, but these studies were all based on the SRCs with a fixed SAH diameter, thus, further researches comprising CSRCs with different diameters are necessary to make a more persuasive comparison.
As for the stress concentration zone, the stress was mainly located on the loading area when the SAH was within 3 mm; when the diameter reached 4 mm, the stress concentration zone was suddenly moved to the margin of the hole in the vertical loading, but it did not move in the oblique loading. A possible explanation to the difference between loading directions was that the oblique loading force was relatively too small to cause the change in the position of the stress concentration zone. This result indicated that vertical bite force might cause greater damage to the margin of the hole on a crown. However, no comparison data are available in the literature regarding the biomechanical effect of the loading directions on cement-retained implant-supported restorations with an occlusal SAH, so we invite further researches to more thoroughly validate our finding.
The results also showed that the maximum von Mises (EQV) stress on the screw in the T-TZP and Gold Alloy groups could remain at a relatively low level during the vertical loading when the diameter did not exceed 1 mm. This indicated that an FC crown with a small SAH (Φ = 0–1 mm) might have a protective effect on the screw; therefore, to retain the benefits of the CSRC, an SAH that is 1 mm in diameter should be recommended. Also, considering that there are a few related studies, the results need to be treated with caution.
Moreover, this study failed to reveal that the SAH had significant effect(s) on the biomechanical performances of other components, including crown margin, abutment, fixture, and surrounding cortical bone.
According to the results above, this FEA study provided a promising strategy: “1 mm in diameter” and “FC crown.” However, in our clinical practice, the diameter of the SAH on a traditional CSRC is usually slightly larger than the diameter of the screw head (about 2 mm, depends on the implant system) to ensure that the screw can be removed out of the abutment channel, or as Wadhwani24 reported, set the hole in the diameter of the shank of the screwdriver (about 1.2 mm, also depends on the implant system) to ensure that the screwdriver can just insert into the abutment. The value of these approaches is that the dentists can cement the crown on the abutment extraorally to completely clean the residual cement. But in this study, the minimized SAH with a diameter of 1 mm may not allow extraoral cementation anymore regarding the diameter of the screwdriver, which is usually larger than 1 mm. Thus, the cementation procedure can only be performed in a patient's mouth. Choosing the plan depends on possible complications. As Sailer et al6 described before, cemented reconstructions exhibited more serious biological complications, and screw-retained reconstructions exhibited more technical problems. To be more specific, dentists should systematically consider the above 2 aspects of the patient to figure out what kind of complication is more likely to happen. If the patient is in a poor periodontal condition or with a thick gingival biotype, which means complete removal of residual cement is difficult but necessary, then the traditional or Wadhwani-reported CSRC is worth recommending. And vice versa, if the dentist is confident of completely removing the residual cement, but more worried about the patient's occlusal force and contact, an FC crown with a 1-mm SAH was highly recommended.
The FEA method has been extensively used in dentistry because of its ability to evaluate the stress distribution in the inner regions of prosthetic components and surrounding bone tissues. However, this FEA study, as many other studies, had some limitations that needed to be addressed. First, all the materials investigated in this study were considered to be homogeneous, isotropic, and linearly elastic. However, some studies have shown that this is not always true. For example, the cortical bone tissue is not homogeneous and transversally isotropic.42 Second, the osseointegrated implant was considered to be bonded, however, the condition of 100% osseointegration on the implant interface is not represented.43 Furthermore, in the present study, the SAHs were left unfilled, whereas in clinical practice, they are often filled with various materials, although it remained largely controversial as to what material should be used.44 To date, most dentists prefer using resin, but because of its poor wear resistance compared with the materials of the crown, it might leave from the occlusal surface after a certain period of masticating. Hence, it is hard to accurately simulate the loading force and contact status of the resin in a FEA model; this represented another limitation to our FEA study that the computer simulation and clinical reality are difficult to compare. In our study, the results indicated that an FC crown with a 1-mm SAH was highly recommended in the posterior region, especially for patients who had excessive vertical bite forces. However, this result was only generated from the perspective of biomechanics; we did not evaluate the effect(s) of a minimized SAH on the cement residues. To address the above-mentioned concerns and limitations, a series of tests and experiments in future studies is guaranteed, including the fatigue test for filled crowns, the computational fluid dynamics simulation for the cement flow under the minimized SAH, and a multicenter randomized controlled clinical trial.
As mentioned before, the effect(s) of the SAH on the cement-retained implant restoration has been rarely studied. To improve the strength and esthetics of crowns, we tried to find a smaller diameter of the SAH for the cement-retained crowns and SRCs in the posterior region. This FEA study provided a promising strategy: “1 mm in diameter” and “FC crown,” and the results of this static mechanics analysis represented a reference for further studies. In our study, we found that the positional relationship between the SAH margin and the occlusal site could play a critical role in the biomechanical behavior of a crown; this represented a novel explanation of the contradictory results of previous studies. The analytical method in the present study was also interesting, as all data were converted into ranked data (I–IV), and data with a small change ([INCREMENT] [σmax] ≤ 20%) were dropped off the study, therefore we could pay more attention to the notable region and reduce the processing burden of a large-scale data, although there is no consensus on the classification for the maximum equivalent stress change rate. This data processing method might provide a novel analytical method for future studies.
The results of this FEA study indicated that an FC crown with a 1-mm screw-access hole (SAH) was a more promising choice for implant restoration in the posterior mandible because of lower stress behavior of the implant prosthodontic system and the absence of vulnerable porcelain, retaining the original advantages of the cement-retained crowns and SRCs (they provide a drilling direction and a cement overflow channel, but extraoral cementation may not be available anymore). In addition, the positional relationship between the screw-access hole (SAH) margin and occlusal site must be taken into account in future studies and clinical practice.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
Roles/Contributions by Authors
L. Du: contributed to model designs and finite element analyses. Z. Li: contributed to model designs and finite element analyses. X. Chang: played an important role in interpreting the results. O. Rahhal: contributed to manuscript preparation. B. Qin: performed the data analyses. X. Wen: performed the figure processing. D. Zhou: contributed to the conception and workflow of this study.
Drs. L. Du and Z. Li contributed equally to this work.
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