While stress distribution at the marginal bone after implantation is critical, bone loss is the key criterion in determining the success rate of implantation after loading by connecting the prosthesis.

Adell et al. [1] reported for the first time in 1918 that bone loss, as a factor influencing implantation success, was 1.5 mm one year after loading. Smith et al. [2] also reported that bone loss ≤0.2 mm each year following the first year after loading indicated the success of implantation. Likewise, in the 1998 Toronto Conference, bone loss within 2.0 mm after the first year and within 0.2 mm in the subsequent years was reported to be the indicator of success [3]. Various other studies have also shown that bone loss for the first year after loading was in the range of 1.5–2 mm [1,2,4].

Numerous studies have continued to report on marginal bone loss. However, with the introduction of platform switching in 2005 and 2006, decreased marginal bone loss has been reported [5,6].

The main factors related to marginal bone loss include thermal damage due to the surgical procedure, overloading, peri-implantitis, micro-space in the fixture/abutment connection, biological width, and implant neck design. Among them, overloading is known to be the major cause of marginal bone loss.

Prendergast and Huiskes [7] conducted a finite element analysis (FEA) using micro-modelling and reported that bone overloading led to microstructural damages and that bone fragmentation or loss could be initiated as the fusion of damaged microstructures became more dominant than the healing of damaged bone via bone remodeling. Frost [8] reported that the strain of the microstructural damage at the cortical bone was approximately 3,000–4,000 microstrain, which can be converted to approximately 40–55 MPa of stress.

Notably, an overloading on the implant causes a high risk of local stress concentration in adjacent bones due to the lack of stress-dissipating tissues such as the periodontal ligament. Stress is expecially concentrated in the crestal marginal bone than in the other surrounding areas because crestal bone is composed of the cortical bone.

If marginal bone is lost due to uncontrolled stress concentration, the crown to root ratio of the implant becomes unfavorable and leads to increased level of stress, which may form a periodontal pocket around the implant and exacerate the risk of peri-implantitis.

Lim et al. [9] reported that the two factors that directly impact marginal bone loss are the implant neck design and marginal bone morphology. And it was said that the harmony between the geometric shape and physical properties of the two factors constituting the force transfer are important.

The neck of an implant is directly involved in stress distribution at the marginal bone, because it is the closest structure to the marginal bone in the route of force transfer between the implant and its surrouding bone. Many previous studies have investigated numerous implant neck designs such as platform switching, microthread [10], curvature design of the transmucosal area [11], and reverse slope on the neck [12], to explore any potential improvement in stress distribution.

However, these previous studies were limited in that they only analyzed the significance of a single design. Therefore, comparing the effects of multiple designs was necessary for asssessment of the advantages and disadvantages and for more comprehensive evaluation of their clinical outcomes and applications.

In this study, the stress distribution pattern of the blood pocket designed platform (Fig. 1) was analyzed using the FEA, and its clinical significance in marginal bone loss was evaluated with regard to implant neck design.

Fig. 1. Top view of blood pocket designed platform (A: drill hole, B: implant, C: implant+drill hole, D: implant+blood).

For geometric modeling, three internal type fixture designs of similar lengths and diameters but with different thread profiles were used: Bone Level Implant URIS (TruAbutment Inc., Irvine, CA, USA), TSIII (TSIII Co., Seoul, Korea), and Astra EV (Astra EV Tech AB, Mölndal, Sweden).

For clinical measurements of the marginal bone density and marginal bone loss, cone beam computed tomography (CBCT) and digital periapical radiographic imaging were performed on 114 blood pocket designed platform (URIS) implants placed in 48 patients (28 males and 20 females) (IRB No. CUDHIRB 2202 001 Q06). Patients with pronounced smoking habits, wasting diseases such as diabetes, and any other systemic diseases or abnormal habits that could affect the clinical outcomes were excluded from the study.

Finite element analysis methodology

An element model was produced with a submerged-type fixture 5.0 mm in length and 10 mm in diameter as instructed by the three companies. The geometrical model was produced with cortical bone at the neck of the fixture to which the abutments were connected, and cancellous bone in the areas below (Fig. 2).

Fig. 2. Schematic presentation of the implant/bone complex (A), and the element model of the three fixtures (URIS, TSIII, and Astra EV) (B). URIS: TruAbutment Inc., Irvine, CA, USA, TSIII: Osstem Co., Seoul, Korea, Astra EV: Astra Tech AB, Mölndal, Sweden.

The material properties were set as follows: Young’s modulus (MPa) of 135,000 for the implant (titanium), 13,700 for the cortical bone, and 1,370 for the cancellous bone and Poisson's ratio of 0.3 for all material types (Table 1).

Load (occlusal force)

The reported range of general occlusal force during mastication is 10–100 N. In this study, 150 N of axial force was applied at 0°, 45°, and 90° to the long axis of the implant to reproduce the masticatory forces in the oral cavity (Fig. 3).

Fig. 3. Diagram of occlusal forces. PN, Pilot Node.

To analyze the stress dispersion, the NX 11 software and NASTRAN (Siemens PLM Software Solutions, Plano, TX, USA) were used for the three models of three-dimensional (3D) FEA.

The FEA models were produced via meshing 3D tetrahedral elements (CTETRA10 of NX), while the total element size of 0.3 mm was used for the meshing of each bone. In addition, the element size was adjusted according to the morphological complexity.

In vivo bone density and crestal bone loss analysis

For the 114 blood pocket designed platform implants (URIS; TruAbutment Inc.) in 48 patients (28 males and 20 females), the bone density after one year of prosthesis loading was analyzed within 1–2 mm of the bucco-lingual and 2–3 mm mesio-distal parts of the implant platform and up to 2 mm below to the bony crestal part. For this, CBCT was taken for each patient, and the Implant Studio software (3shape, Copenhagen, Denmark) was used for bone density analysis (Fig. 4).

Fig. 4. Analysis of bone density using Implant Studio software (3shape, Copenhagen, Denmark). HU, Hounsfield units.

To measure the marginal bone loss one year after loading, the patients were divided into immediate and delayed loading groups, and digital periapical radiographs were taken for each implant. The data were analyzed with the measurement device of Adobe Photoshop CS5 (Adobe, San Jose, CA, USA) software; the bone loss was measured at the mesial and distal parts of the fixture platform and recorded as mesial vertical bone loss and distal vertical bone loss respectively (Fig. 5).

Fig. 5. Schematic presentation of the marginal bone loss. ML, mesial vertical bone loss; DL, distal vertical bone loss; L, fixture length.

Among the 114 implants, 67 were in the delayed loading group and 47 in the immediate loading group. The mean bone loss for both groups were also analyzed.

Three internal-type implants with identical sizes but different thread profiles were placed in the alveolar bone for the FEA models, and 150 N loading was applied at 0°, 45°, and 90° angles to the long axis of the implant to analyze the von Mises stress at the implant, cortical bone, and cancellous bone using the NX 11 and NASTRAN to perform the FEA (Fig. 6, Table 2).

Fig. 6. The FEA models presenting the stress upon 150 N loading for 0° (A: URIS, B: TSIII, C: Astra EV) and 90° (D) to the long axis of the three implants (from left, URIS, TSIII, and Astra EV). URIS: TruAbutment Inc., Irvine, CA, USA, TSIII: Osstem Co., Seoul, Korea, Astra EV: Astra Tech AB, Mölndal, Sweden. FEA, finite element analysis.

In addition, marginal bone loss was assessed from the digital periapical radiographs of the 114 blood pocket designed platform implants (Table 3).

Stress of vertical force on implant

The stress on each implant superstructure with 150 N loading along the major axis of implant was 75.32 MPa for Astra EV, 70.68 MPa for TSIII, and 50.44 MPa for URIS, indicating considerably low stress for URIS in comparison to the other two designs.

The stress on the cortical bone was 46.32 MPa for Astra EV, 21.06 MPa for TSIII, and 11.17 MPa for URIS, with URIS displaying the lowest stress.

The stress on the cancellous bone was 21.62 MPa for Astra EV, 8.88 MPa for TSIII, and 6.676 MPa for URIS, with URIS and TSIII showing lower stress than Astra EV (Table 2).

Stress of 45° angled vertical force on implant

The stress on each implant superstructure with 150 N loading at 45° to the long axis of the implant was 441.54 MPa for Astra EV, 262.73 MPa for TSIII, and 243.71 MPa for URIS, indicating considerably high stress for Astra EV in comparison to the other two designs.

The stress on the cortical bone was the highest at 76.03 MPa for Astra EV, followed by 29.97 MPa for TSIII and 37.84 MPa for URIS, with Astra EV showing higher stress than the other two implants.

The stress on the cancellous bone was the highest at 19.09 MPa for Astra EV, followed by 15.42 MPa for TSIII and 15.03 MPa for URIS, with no significant difference among the three implants (Table 2).

Stress of 90° angled vertical force to implant

The stress on each implant superstructure with 150 N loading at 90° to the long axis of the implant was 591.99 MPa for Astra EV, 344.55 MPa for TSIII, and 316.85 MPa for URIS, indicating considerably high stress for Astra EV in comparison to the other two designs.

The stress on the cortical bone was the highest at 83.03 MPa for Astra EV, followed by 36.21 MPa for TSIII and 48.42 MPa for URIS, with Astra EV showing higher stress than the other two implants.

The stress on the cancellous bone was the highest at 39.84 MPa for Astra EV, followed by 13.16 MPa for TSIII and 15.74 MPa for URIS, with Astra EV showing higher stress than the other two implants (Table 2).

Peripheral bone density of URIS fixtures

The bone density of the 114 URIS implants at the alveolar bone in patients based on the CBCT and Implant Studio results showed 91.8% D1 ossein of over 1,250 Hounsfield units (HU) and 8.2% D2 ossein of 850–1,250 HU.

Marginal bone loss of URIS fixture

The mean marginal bone loss measured for the 67 implants in the delayed loading was 0.46±0.30 mm, and for the 47 implants in the immediate loading group was 1.01±0.33 mm (Table 3).

For an efficient assessment of the stress applied on the marginal bone after implant placement, the FEA based on 3D modeling has been most widely used.

In the field of dental implant biomechanics, Geng et al. [13] reported that the FEA was the most effective in analyzing complex geometric structures of the implant placed at the bone.

Nevertheless, only a few studies to date were able to apply and compare the predictions that can be derived by simulating the tissues of the human body. Dávila et al. [14] reported the stress generated at the marginal bone using 3D FEA models with varying implant neck design and analyzed how such predictions compared to in vivo animal studies by placing the implants with the different designs in New Zealand rabbits.

The reported factors related to marginal bone loss include thermal damage due to surgery, overloading, peri-implantitis, micro-space in the fixture/abutment connection, biological width, and implant neck design; overloading was found to be the major cause of marginal bone loss.

As implant neck design is directly related to loading, numerous studies have investigated the effects of neck designs on marginal bone loss, and many companies have introduced various neck designs based on such studies.

Similarly in this study, the FEA with 3D modeling was performed on the recently introduced URIS fixture with blood pocket designed platform as well as TSIII and Astra EV fixures that are popularly used in clinical practice today. After simulating occlusal force with load application, the stress generated at the implant neck and the surrounding cortical bone was analyzed to determine the effectiveness of the novel neck design.

In order to model the effect of these loads on implants and the surrounding bone as three-dimensional finite elements, the loading corresponding to the occlusal pressure was applied at various angles and the respective effects were analyzed. The occlusal force during mastication has been reported to be in the range of 10–100 N [16]. In addition, studies have reported on stress responding to the occlusal force loaded at 0°, 45°, and 90° angles in order to reproduce the occlusal pressure [17].

Bozkaya et al. [18] conjectured that overloading had a greater influence on the implant and its surrounding bones than normal occlusal pressure and thus applied 1,000 N to reflect overloading and analyzed the stress on the neck of different types of implants (Ankylos, Astra EV, Bicon, ITI, and Nobel Biocare).

In this study, when a load of 150 N was appled along the implant long axis, the von Mises stress value applied to the implant fixture and cortical bone was the lowest in URIS implant. This signifies that the blood pocket designed platform is advantageous for stress distribution against vertical occlusal pressure, which is the main force of mastication.

However, when a load of 150 N was applied at 45° to the implant long axis, the von Mises stress value appeared in the order Astra EV>TSIII>URIS for implant fixuture and Astra EV>URIS>TSIII for cortical bone. Therefore, it can be persumed that with regards to the force applied laterally to the implant long axis, higher stress is more closely associated with the increasing number of threads than platform design.

In addition, when a load of 150 N was appled at 90° to the implant long axis, the von Mises value appeared in the order Astra EV>TSIII>URIS for implant fixture. Especially, Astra EV showed markedly higher stress than the other two designs. This result indicated again that the Astra EV design with many microtheads was more vulnerable to lateral force compared to the URIS and TSIII implants, which showed equivalent level of stress generation in response to the lateral force.

Because marginal bone loss is an indicator of successful implantation, it is a critical parameter in implant manufacturing. Thus, companies have attempted to prevent marginal bone loss and to retain marginal bone by creating different implant-abutment interfaces, surface treatments, thread use, and fixture shapes [19].

Preserving a stable marginal bone height around implants is crucial in success and survival of implantation. Studies have generally reported approximately 1.5–2 mm of marginal bone loss around for implants after the first year of functional loading in an edentulous area, followed by approximately 0.2 mm of bone loss after each subsequent year [1,2,4]. Various efforts have been made to protect the marginal bone. Al-Thobity et al. [20] found that microthreaded implants enabled higher marginal bone preservation than machined-surface or rough-surface implants. Since the introduction of the platform-switching principle, which is connecting abutments with diameters smaller than the diameter of the implant platform in 2005, the reported level of marginal bone loss has decreased.

Hürzeler et al. [5] compared crestal bone loss between platform-switched and non platform-switched implants and found the mean marginal bone loss to be 0.22 mm for the former and 2.02 mm for the latter. In another study by Cappiello et al. [6], the vertical bone loss was 0.6–1.2 mm (mean: 0.95±0.32 mm) for the platform-switched implants and 1.3–2.1 mm (mean: 1.67±0.37 mm) for the non platform-switched implants.

Other efforts have been made to identify the various causes of marginal bone loss.

da Rocha Ferreira et al. [21] have reported that the level of marginal bone adsorption was high when the crown-root ratio was high regardless of the implant fixture length. Kim et al. [22] investigated the factors influencing marginal bone adsorption, such as the surface design, loading time, immediate implant placement, staging, and dentoalveolar reconstructive procedure, but found no correlation of these factors with the 1.5–2.0 mm marginal bone loss that occurred within one year of implant placement.

The analysis of bone density for the 114 implants in this study based on the CBCT and Implant Studio software (3shape) results showed that 91.8% were D1 of ≥1,250 HU and 8.2% were D2 of 890–1,250 HU. The results confirmed the suitability of the bone density around the implants to study the effects of the blood pocket designed platform, which were to be tested around cortical bone and receiving the highest level of stress.

All of the implants used in this study were categorized as “immediate loading” and “delayed loading,” and the respective marginal bone loss was measured. The result showed that the mean bone loss was 1.01±0.33 mm for the immediate loading group and 0.46±0.30 mm for the delayed loading group.

In previous studies, the mean bone loss for the immediate loading group after implantation was 1.2–2.0 mm [23]. The mean bone loss in this study was 1.01±0.33 mm, indicating a more favorable outcome than that observed in previous studies. All blood pocket designed platform implants (URIS) in the patients in this study utilized the platform switching principle, and the marginal bone loss measured after one year of prosthesis loading was 0.46±0.30 mm, which is below the general implant bone loss reported. The outcome is also thought to be more favorable compared to that in other studies that applied the platform switching principle [5,6].

In this study, the 3D FEA methodology was used to predict the effect of a novel implant neck design with a blood pocket designed platform on the cortical bone, and the amount of marginal bone loss around the implants was measured in the patients.

The FEA result showed that the stress dispersion at the cortical bone was advantageous for vertical pressure, and the marginal bone loss analyzed for the implants placed in the patients showed a lower level of bone loss than that reported in previous studies.

Nevertheless, further studies should analyze implants placed in a larger number of patients.

In conclusion, the implant with the blood pocket designed platform showed minimized stress on the cortical bone. Based on the FEA stress analysis, the novel design led to favorable stress distribution in response to vertical occlusal pressure compared to the other implant designs and less marginal bone loss in patients with measurements lower than those reported in previous studies.

This study was supported by a research fund from Chosun University, 2022.

The authors declare that they have no competing interests.