The effect of varying the position of implant screw access opening on the stress distribution of sealing composite resin: a 3D-FEA study

Abstract

Background

The aim of this study was to assess the mechanical differences of composite resin sealing the screw access channel with different-positioned openings by a three-dimensional finite element analysis.

Methods

A 3D finite element model of dental implant at the site of mandibular first molar was established using computer-aided design (CAD). Models of sealing composite resin with different thicknesses were applied under 200 N static loading, and the optimal thickness was then selected via stress distribution analysis of composite resin models. Angulated screw channel (ASC) was used to vary the position of implant screw access opening. 200 N static loading was then applied to composite resin models with the selected thickness at different stages of a chewing cycle, and the stress of sealing composite resin in screw access channel with different-positioned openings was compared.

Results

The stress within composite resin model was stably at a lower level when the thickness was no less than 3 mm. Varying the position of implant screw access opening influenced stress distribution of sealing composite resin. In contrast with the opening located at central fossa, both maximum von Mises stress and shear stress of sealing composite resin (4.503 MPa and 1.032 MPa) was reduced when the opening was on the lingual side of central fossa (3.974 MPa and 0.9464 MPa) but was increased when the opening was on the buccal side of central fossa (5.695 MPa and 2.162 MPa).

Conclusions

The stress was significantly lower at the site of mandibular molar when the position of screw access opening was designed on the lingual side of central fossa. Within the limits of the present in silico investigation, this study provided a new design strategy for screw-retained implant prosthesis to reduce the mechanical complications caused by the incomplete sealing of screw access opening.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-026-07696-x.

Background

Implant-supported prostheses have been a well-established treatment for patients who are partially or totally edentulous [1–3]. There are two retention types for implant-supported fixed prosthesis: cement and screw retained. As for cement-retained restorations, the difficulty of removing excess cement adds a potential risk for the development of peri-implant diseases such as peri-implant mucositis and peri-implantitis [4]. Moreover, the removal of cemented crowns on implants is not only time consuming but also uncomfortable for patients [5]. In contrast, screw-retained restorations are retrievable, and the incidences of biological complications were significantly lower [6]. With the advent of angulated screw channel (ASC), screw-retained prosthesis can also be applied in the aesthetic zone with the access opening placed toward the palatal aspect [7]. As a key component involved in the implant-abutment connection, the loosening or fracture of retaining screw caused by the incomplete sealing of screw access opening disrupts the stability of the implant-supported restoration and subsequently causes masticatory dysfunction and soft tissue injury [8, 9]. To avoid mechanical complications caused by overload, an optimized biomechanical design of screw access opening is crucial for minimizing the failure of screw-retained implant prostheses.

The restoration of screw access opening is inevitably required to maintain occlusal function and esthetic results. Dental filling composite resin exhibits ease of use and favorable mechanical properties [10–12], thus it is generally used as restoration material to seal the screw access opening. In clinical practice, fatigue under long-term mastication may cause abrasion, subsidence or dislocation of composite resin in screw access channel [13]. Incomplete sealing of screw access opening deforms the occlusal shape, reduces the fracture resistance of central screw, and even leads to the failure of implant protheses [14]. To date, studies involved in the analysis of stress distribution within composite resin restoration of screw access opening are lacking, though it is important to prevent screw fracture or loosening by alleviating masticatory stress on composite resin sealing the screw access openings.

Despite the limitations of numerical methods, finite element analysis (FEA) is widely used for stress distribution analysis of complicated structures such as implant-supported prothesis, peri-implant alveolar bone, etc [15–17]. FEA has been carried out to examine the mechanical behavior of implant components and bone under different stimulation conditions [16, 18, 19]. Occlusal forces on a protheses vary during chewing [20]. To mimic the dynamic changes of loading time, loading direction, and loading position, one chewing cycle was divided into five phases, and the loading force was equal in each phase (200 N) [21]. Hence, this study focused on assessing the stress distribution of composite resin sealing different-positioned screw access openings under static loading with different directions on different contact points in different chewing stages via FEA.

During chewing, different regions of the tooth experience different occlusal forces. For posterior teeth, the central fossa is a stress concentration zone, thus, this part is more susceptible to abrasion under mastication. Likewise, composite resin sealing the screw access opening around the central fossa is also prone to wear or fracture due to excessive stresses. Accordingly, we speculated that appropriate alteration in the position of the screw access opening would avoid the failure of screw-retained implant prostheses by decreasing the risk of subsidence, deformation or even dislocation of sealing composite resin. ASC designed by Nobel Biocare (Sweden) allows the screw access channel to be placed up to 25° off the center axis of the implant within a 360° radius [7, 22]. Thus, we applied ASC to change the position of screw access opening, then examined stress distribution of composite resin sealing the access opening by means of FEA under static loading, and attempted to provide some theoretical basis for an optimized design of screw retention. The null hypothesis of this study was that the characteristics of composite resin filling and sealing the screw access channel was insusceptible to the position of screw access opening.

Methods

Experimental equipment

Geomagic Studio 2014 (Geomagic, United States) was applied to materialize the components of FE model; SolidWorks 2022 (Dassault Systems S.A, France) was applied to draw and assemble the components of FE model; HyperMesh 2021(Altair, United States) was applied to paint FE model meshes; ABAQUS CAE 2021 (SIMULIA, United States) was applied to solve FE models.

Grouping

Five groups were established based on the thickness of composite resin restoration in the screw access channel—1 mm, 2 mm, 3 mm, 4 mm, and 5 mm—to identify the optimal thickness for subsequent analyses. To investigate the effect of varying the position of the screw access opening on the biomechanical behavior of sealing composite resin, three sub-groups were created: (1) control group, which traditionally located screw access channel with the opening at central fossa; (2) lingual group, which located screw access channel with the opening on the lingual side of central fossa using ASC; (3) buccal group, which located screw access channel with the opening on the buccal side of central fossa using ASC.

Modeling

According to the data provided by implant product manual (Nobel Biocare, Sweden), a three-dimensional (3D) finite element model consisting of cortical bone, cancellous bone, implant, abutment, central screw, crown, polytetrafluoroethylene and composite resin (Z100, 3 M, United States), was established on SolidWorks 2022 (Fig. 1). In this FE model, the implant was placed 0.5 mm below the alveolar ridge, and the composite resin directly filled and sealed the screw access channel without the use of resin bonding system. All the components except the crown were designed from scratch. The design of the crown at the site of left mandibular first molar (buccolingual diameter of 10.35 mm, mesio-distal diameter of 10.48 mm, and occluso-gingival height of 7.95 mm) was in line with clinical demands, and the data of the crown was supported by the dental laboratory center (Nanjing Jinyi Denture Co., Ltd., China) as the format of STL. The generated STL file of the crown was imported into a reverse engineering software Geomagic Studio, which optimized the crown by using tools such as nail removal, feature removal, mesh doctor, etc., and eventually materialized the crown using the precise surface command. Next the materialized crown data was imported back to SolidWorks, and a straight screw channel was placed by using the stretch cut command to cut along the central screw axially, while ASC was placed by using the scan cut command. Posterior to the design of screw access channel, two components (composite resin and polytetrafluoroethylene) were obtained by Boolean operation and segmentation command. To maintain the resistance to occlusal forces, ASC was designed with the opening at a safety range: at least 2 mm away from buccal or lingual side of the crown, meanwhile avoiding buccal or lingual cusps [23].

Fig. 1.

a 3D finite element model: internal diagram of screw-retained implant prothesis at the site of mandibular first molar. b Three groups were divided according to the position of screw access opening. From left to right: control group, lingual group and buccal group

All the components were assembled in SolidWorks and formatted to STP file, which was then imported into Hypermesh for mesh division of tetrahedral units. The mesh size of abutment, central screw, crown, polytetrafluoroethylene and composite resin was 0.2 mm, while the rest components were 0.5 mm in mesh size. The convergence test was available in Supplementary material. Mesh parameters for components in different groups were shown in Table 1. The grid model data was pre-processed in ABAQUS CAE 2021. Mechanical property parameters (Young’s modulus and Poisson’s ratio) are shown in Table 2. The interface between bone tissue and implant was set to be completely bonded to simulate ideal osteointegration [24, 25]. The contact condition between components was set to bonded. Mandible was totally restrained in mesio-distal direction to mimic its real situation. According to reported studies, the stress values of sealing composite resin with different thickness in straight screw channel at the center of occlusal plane were compared under static loading [26]. To show the stress distribution of sealing composite resins in screw channels with different-positioned openings, static loading with dynamic changes of loading time, direction and position was introduced for subsequent analyses. A simulated masticatory cycle was divided into five stages, in which the loading positions (occlusal contact points) and the directions were both varied with time [27–29] (Table 3). In this study, loading force was uniformly set at 200N [30, 31].

Table 1.

Mesh parameters for components in different groups

	Control group	Lingual group	Buccal group
Number of elements	886,066	855,509	811,506
Number of nodes	172,012	166,491	157,821

Table 2.

Mechanical properties of finite element model components

Material	Young’s modulus (MPa)	Poisson’s ratio	Reference
Cortical Bone	13,700	0.3	[32]
Cancellous Bone	1370	0.3	[33]
Implant, abutment, center screw	115,000	0.35	[34]
Crown(zirconia)	210,000	0.31	[35]
Composite resin	20,000	0.24	[12, 36]
Teflon	300	0.41	[37]

Table 3.

Loading time, direction, position and magnitude of force at different stages of a chewing cycle

Stage	Time (s)	Loading direction	Loading position	Magnitude of force (N)
Stage I	0.0-0.13	-	-	-
Stage II	0.13–0.15	Perpendicular to the occlusal plane	Buccal cusps, lingual cusps	200
Stage III	0.15–0.26	From the buccal side to the lingual side with 45°	Buccal slopes of the buccal cusps	200
Stage IV	0.26–0.30	From the lingual side to the buccal side with 45°	Lingual slopes of the buccal cusps	200
Stage V	0.30-0.875	-	-	-

- indicates no loading

Finite element calculation

All components of the model were assumed to be homogeneous, isotropic and elastic material behavior, and analyzed by the FEA linear analysis [38]. The optimal resin thickness was selected via maximum von Mises stress and shear stress of sealing composite resin with different thickness, and these stress values were then compared among sealing composite resin with selected thickness at different-positioned openings of screw channels.

Results

Stress distribution of sealing composite resins with different thickness

This study mainly compared the maximum von Mises stress and shear stress of sealing composite resin in screw access channel. As shown in Fig. 2, stress concentration area of composite resin models with different thickness was all located at the upper part of models, at the junction of the sealing composite resin and the crown. Although the thickness made no difference on the position of stress concentration zone, the values of maximum von Mises stress and shear stress both decreased with the increase of composite resin thickness when the thickness was less than 3 mm, and tended to be stable at a lower level when the thickness was equal to or more than 3 mm (Fig. 3). Accordingly, the thickness of sealing composite resin was uniformly set at 3 mm for the subsequent study.

Fig. 2.

Von Mises stress and shear stress nephogram of composite resin models with different thickness (A: 1 mm; B: 2 mm; C: 3 mm; D: 4 mm; E: 5 mm) under static loading

Fig. 3.

Variation trend of a maximum von Mises stress and b shear stress of composite resin models with different thickness under static loading

Stress distribution of sealing composite resins with selected thickness at different-positioned openings of screw access channels

According to the stress nephogram in Figs. 4 and 5, stress concentration zone of sealing composite resins varied with the position of screw access openings under static loading with dynamic changes of loading time, direction and position in a chewing cycle. In Stage II, the area of stress concentration was all located on the lingual side of sealing composite resin at the top, close to its border on the crown. In Stage III, stress concentration zone in both control group and lingual group was located at the bottom of sealing composite resins, whereas in buccal group, stress concentration zone was located on the buccal side of sealing composite resin at the top. In Stage IV, the area of stress concentration was located both on the buccal side of the top and at the bottom of sealing composite resin in control group, while in lingual group and buccal group, stress concentration zone was located at the bottom, top of sealing composite resins respectively.

Fig. 4.

Von Mises stress nephogram of sealing composite resins at different-positioned openings of screw access channels in different stages of a chewing cycle

Fig. 5.

Shear stress nephogram of sealing composite resins at different-positioned openings of screw access channels in different stages of a chewing cycle. Note: The shear stress is a vector, and the positive and negative symbols represent the direction

The value of the maximum von Mises stress of sealing composite resin at the different-positioned opening was presented in Table 4. In contrast with the control group, the value decreased from 4.503 MPa to 3.974 MPa when the screw access opening was positioned to the lingual side of central fossa. On the contrary, the value increased to 5.695 MPa when the opening was positioned to the buccal side. In other words, the value of maximum von Mises stress of sealing composite resin was the highest in the buccal group but the lowest in the lingual group. Similarly, the value of shear stress was decreased from 1.032 MPa to 0.946 MPa when the position of screw access opening was on the lingual side of control group, but it was increased to 2.162 MPa when the opening was located on the buccal side of control group (Table 5). The value of maximum shear stress was largest in buccal group but smallest in lingual group as well. In addition, the peak value of von Mises stress occurred at Stage IV in all groups. As for the peak value of shear stress, it appeared at Stage IV in control group and buccal group but at Stage III in lingual group.

Table 4.

Values of maximum von mises stress of sealing composite resins in different stages of a chewing cycle (MPa)

Model	Stage II	Stage III	Stage IV	Peak value
Control group	1.464	3.733	4.503	4.503
Lingual group	1.339	3.927	3.974	3.974
Buccal group	0.9419	3.728	5.695	5.695

Table 5.

Values of maximum shear stress of sealing composite resins in different stages of a chewing cycle (MPa)

Model	Stage II	Stage III	Stage IV	Peak value
Control group	0.2101	0.6308	1.032	1.032
Lingual group	0.3422	0.9464	0.8296	0.9464
Buccal group	0.2631	0.7387	2.162	2.162

Collectively, the change of stress distribution and peak values unveiled that varying the position of screw access opening would affect the stress of sealing composite resin. In contrast with the screw access channel vertically at central fossa, both von Mises stress and shear stress of sealing composite resin could be significantly reduced when the channel was tilted to the lingual side using ASC, whereas both stresses would be increased when the channel was oblique buccally (Fig. 6).

Fig. 6.

Variation trend of a maximum von Mises stress and b shear stress of sealing composite resins in different position groups at different stages of a chewing cycle

Discussion

For implant-supported prostheses, the connection of final restoration to the implant via the abutment directly affects the achievement of a functional and stable treatment outcome [39]. The restorative connection can be either screw- or cement-retained. Overall, screw retained dental protheses have the advantage of more predictable retrievability and require a minimal amount of interocclusal space, only 4 mm. This retention type is therefore preferable for clinicians. However, sealing composite resin in screw channel may be abrased, subsident, or even dislocated under long-term chewing, and it limits the use of screw-retained retention in clinical application. Usually, the abutment screw is protected with polytetrafluoroethylene tape and then sealed with composite resin. The position of screw channel is traditionally parallel to the long axis of implant and through the occlusal plane near the central fossa of the crown at posterior region. Nevertheless, the central fossa area normally tends to bear the maximal occlusal force during chewing, and this enlightened us a potential strategy for screw retention design: changing the path of screw access channel and averting its opening from the central fossa area may effectively reduce the stress of sealing composite resin under mastication. The position of screw access opening in this study was thereby divided into three groups: control group (near the central fossa), lingual group (on the non-functional side of central fossa), buccal group (on the functional side of central fossa), and then the stress distribution of sealing composite resin was compared among the three groups.

The finite element (FE) method is a numerical technique that relies on mathematical models to quantify various physical phenomena such as mechanical stress [40]. It has been extensively used for optimizing designs in dentistry [41–43], and is particularly useful for studying complex biochemical systems to predict their mechanical response during functional loads. This study aimed to assess the stress in a complete implant prosthesis that consisted of different components, including implant, abutment, central screw, crown, polytetrafluoroethylene, and composite resin. Using FEA to examine the mechanical behavior of such a complicated structure may provide accurate and reproducible results. Moreover, reverse engineering software was applied in this study to draw and process the FE model, and finally establish an idealized 3D solid model of zirconia crown, implant and mandible for a more accurate and reliable analysis. Afterwards, the optimal thickness of sealing composite resin was selected, and the stress values were stable at a lower level when the thickness was no less than 3 mm. Thus, 3 mm was chosen as composite resin thickness for the following study. It is noteworthy that occlusal forces on protheses is variable under mastication [44, 45]. This study thereby used static loading with dynamic changes of loading time, direction and position, to accurately simulate the variation of occlusal force in a chewing cycle. According to the process of a chewing cycle, the loading pattern was divided into five stages. No occlusion occurred in Stage I and V, and occlusal force was loaded on cusp tips, buccal slopes of buccal cusp and lingual slopes of buccal cusp at Stage II, III, Ⅳ, respectively. Loadings on cusp tips were vertically, while on slopes of buccal cusp were obliquely with a 45° angle, which is standardly and commonly used as the angle of oblique loadings on posterior teeth [46]. The application of static loading with dynamic changes made the results of this study more reliable.

Von Mises stress is a scalar quantity obtained from the combination of tensile stress, compressive stress, and shear stress acting on any object. It helps us to assess the yielding (or failure) of a ductile material [47]. As one of the most commonly used evaluation indicators in FEA, von Mises stress can quickly predict the area of stress concentration in a model [48, 49]. Reportedly, fracture occurs more easily by shear stress. When a force is obliquely applied, shear stress at restorative/dental interface significantly degrades the resistance of restoration materials [50, 51]. Thus, von Mises stress and shear stress were used as evaluation indicators in this study. Stress nephogram of sealing composite resin at different-positioned openings of screw access channels showed that the maximum stress of composite resins was ranked as buccal group> control group> lingual group. It may be related to the fact that buccal cusp is the functional cusp of mandibular first tooth [52]. Functional cusps of posterior teeth play a key role in masticatory movements, and mainly responsible for food cutting and grinding. When filling composite resin is close to functional cusps, it may bear more stresses, due to the region around functional cusps participating in major part of mastication. The increase of these stresses adversely affects the stability of the composite resin, and influence the long-term durability and function of composite resin restorations. On the contrary, filling composite resin away from functional cusps may be subject to less stresses, and thereby exhibit better stability and longer service life. According to Tables 4 and 5, values of maximum von Mises stress and shear stress were both larger in buccal group than in the rest groups. This result suggested that the opening of screw access channel could be placed as far away from functional cusps (buccal cusp for mandibular molars) as possible to alleviate the stress of sealing composite resin under the permission of channel-drilling conditions. This design of screw retention may reduce the occurrence of composite resin abrasion, subsidence or dislocation, and eventually lead to a successful outcome of implant restoration.

In the anterior maxilla, implants for screw retained restorations are not always placed in favorable position due to centripetal resorption pattern and concave bone configuration [53, 54]. In this esthetic zone, screw retention compromises esthetics unless the screw access opening in the crown is concealed to the palatal of the incisal edge [55]. To avoid labial or incisal screw access opening, ASC was designed to ensure the access opening placed toward the palatal aspect. The ASC design uses a screw with a hexalobular head shape that can be engaged with a hexagonal faceted sphere screwdriver at various angles between 0 ~ 25°with 360° freedom of rotation [56]. This allowed the screw access channel at an orientation different from the center axis of the implant and the access opening away from the area of concern. Inspired by its benefits, ASC was used to reposition the opening of screw access channel in this study. Then the stress of sealing composite resin at different-positioned openings was compared to demonstrate that varying the opening of screw access channel would improve the long-time survival of sealing composite resin in screw-retained implant restorations.

In this study, all components of the model were assumed to be homogeneous, isotropic, and linearly elastic, which was different from actual conditions in clinical practice on aspects of contact conditions, boundary conditions, load variability, numerical approximations, etc. Moreover, the oral environment is quite complicated under the effect of different biochemical factors such as saliva, and needs to be more realistically stimulated for further studies. Additionally, masticatory movement is also difficult to stimulate relying on mathematical models. Aside from the occlusion of teeth, it involves the coordination of chewing muscles and the regulation of nervous system as well [57]. The actual process of mastication will be well studied in further clinical research. In spite of the limitations above, this study still provided solid results of mechanical analysis for the following clinical investigations.

Conclusion

In clinical practice, the thickness of the sealing composite resin within screw access channel is recommended as 3 mm on the consideration of the convenience for clinical operation. The application of ASC could locate the opening of screw access channel at the non-functional side, then alleviate the stress of sealing composite resin, and eventually improve the long-time survival of screw-retained implant restorations.

Abbreviations

3D

Three-dimensional

FEA

Finite element analysis

CAD

Computer-aided design

ASC

Angulated screw channel

Authors’ contributions

Conceptualization, Ming Shen; Methodology, Zhixuan Zhou; Software, Feifan Li; Formal analysis, Meiqi Wu; Resources, Xiya Zhang; Data curation, Mengfei Bi; Writing—original draft preparation, Feifan Li&Meiqi Wu; Writing—Zhixuan Zhou; Supervision, Ming Shen; Project administration, Ming Shen&Zhixuan Zhou; Funding acquisition, Ming Shen&Zhixuan Zhou. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by grants from the National Natural Science Foundation of China (82471017), the Project of Jiangsu Provincial Health Commission (M2021018), the Special Project Funded by the Health and Technology Development of Nanjing (YKK24290), Jiangsu Province Capability Improvement Project through Science, Technology and Education-Jiangsu Provincial Research Hospital Cultivation Unit (Grant: YJXYYJSDW4), Jiangsu Provincial Medical Innovation Center (Grant: CXZX202227).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable. Clinical trial number: not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.