This study investigated the influence of thread pitch, helix angle, and compactness on micromotion in immediately loaded implants in bone of varying density (D2, D3, and D4). Five models of the three-dimensional finite element (0.8 mm pitch, 1.6 mm pitch, 2.4 mm pitch, double-threaded, and triple-threaded implants) in three types of bone were created using Pro/E, Hypermesh, and ABAQUS software. The study had three groups: Group 1, different pitches (Pitch Group); Group 2, same compactness but different helix angles (Angle Group); and Group 3, same helix angle but different compactness (Compact Group). Implant micromotion was assessed as the comprehensive relative displacement. We found that vertical relative displacement was affected by thread pitch, helix angle, and compactness. Under vertical loading, displacement was positively correlated with thread pitch and helix angle but negatively with compactness. Under horizontal loading in D2, the influence of pitch, helix angle, and compactness on implant stability was limited; however, in D3 and D4, the influence of pitch, helix angle, and compactness on implant stability is increased. The additional evidence was provided that trabecular bone density has less effect on implant micromotion than cortical bone thickness. Bone type amplifies the influence of thread pattern on displacement.
In the conventional protocol for implant-based dental repair, an undisturbed 3- to 6-month healing period is suggested for successful implant osseointegration [
Restoration that allows immediate loading of implants in edentulous areas is increasing because of the advantages of regained chewing functions and aesthetics. Although studies have found that the survival rate of immediately loaded implants is acceptable [
Consequently, modifications in implant body design have been suggested to increase the success of immediate loading by gaining better initial stability and restricting micromovement. Threads are used to maximize initial contact, improve initial stability, enlarge implant surface area, and favor the dissipation of interfacial stress [
Among the implant design variables, the pitch is considered to have a significant effect on stability, because of its effect on surface area [
Primary stability of immediately loaded dental implants is related to micromotion. Clinically, it is impossible to introduce a device into an implant-bone interface to investigate the level of micromotion between the bone and the implant under masticatory force. Several parameters have been used in past experimental and clinical studies to represent implant primary stability in experimental or clinical studies. These parameters include insertion torque [
In this study, the effects of single-threaded, double-threaded, and triple-threaded implants on micromotion at the bone-implant interface were examined using the 3D FEA method.
The 3D models of a mandibular bone block and a screw-shaped dental implant with a healing abutment were constructed using computer assisted designing (CAD) system (PRO/E) on a personal computer. To study the thread helix angel and the compactness of the single-threaded, double-threaded, and triple-threaded implants, five different V-shaped threaded implants were designed: single-threaded implants with pitches of 0.8 mm, 1.6 mm, and 2.4 mm; a double-threaded implant with a thread pitch of 1.6 mm and thread spacing of 0.8 mm; and a triple-threaded implant with a thread pitch of 2.4 mm and thread spacing of 0.8 mm (Figure
Experiment grouping.
Group number | Group name | Description | Component |
---|---|---|---|
1 | Pitch Group | Different pitches | Single 0.8 mm pitch |
Single 1.6 mm pitch | |||
Single 2.4 mm pitch | |||
|
|||
2 | Angle Group | Same thread compactness but different helix angles | Double-threaded |
Triple-threaded | |||
Single 0.8 mm pitch | |||
|
|||
3 | Compactness Group | Same helix angle but different thread compactness | Double-threaded and Single 1.6 mm pitch |
Triple-threaded and Single 2.4 mm pitch |
Five different configurations of implants with abutment.
The five 3D solid V-threaded implants were modeled under similar conditions. The bone block had dimensions of 15 mm × 25 mm × 20 mm representing buccolingual × mesiodistal × inferosuperior surfaces. As per the classification of Lekholm and Zarb [
Configuration of the dental implant/bone system. In the D2 model, a core of dense cancellous bone was covered by a thick layer of compact bone with a width of 2 mm. The geometric configurations of D3 and D4 models were similar to those of the D2 model, but the width of the compact bone layers was reduced to 1 mm.
The combined solid model was transferred to Hypermesh 7.0 (Hypermesh 7.0 Inc., Providence, RI, USA) to create a finite element meshed model for later analysis. To guarantee the comparability of the model, four coordinate points were defined to the collar, tip, and corresponding bone type (compact and cancellous) in each style of implant model, and the points in all of the implant models were the same for the collar and the tip, respectively. The four coordinate points are nodes in the model meshing, with each point located in the same position in each model (Figure
Four coordinate points were defined to the collar, tip, and corresponding bone quality of each type of implant model. Four coordinate points were nodes in model meshing, with each point located in the same position in each model.
The mechanical properties of the models were assumed to be homogeneous, isotropic, and linearly elastic. The specific values of the properties were adopted from previous studies [
Mechanical properties of the finite element models.
Materials | Young’s modulus ( |
Poisson’s ratio ( |
---|---|---|
Compact bone | 14.7 [ |
0.3 |
Dense trabecular bone (for D2, D3 bone) | 1.47 [ |
0.3 |
Low-density trabecular bone (for D4 bone) | 0.231 [ |
0.3 |
Titanium | 110 [ |
0.35 |
Nonlinear frictional contact elements were used to simulate the adaptation between the bone and the implant. A frictional coefficient of 0.3 was assumed for all contact surfaces [
The boundary condition of total fixation on the nodes of the three faces (the mesial, distal, and bottom faces) of the bone block was chosen [
Convergence testing of the 3D finite element models was performed to verify the accuracy of the mesh; this resulted in a convergence criterion of less than 3% change in the maximum displacement of bone between the elements at a given point (Figure
Convergence test results in Single 0.8 mm pitch model in D2 bone.
For the displacement of the five implants under vertical load, please refer to Table
Displacement of five implants under vertical load (
Thread pattern | D2 | D3 | D4 | |||
---|---|---|---|---|---|---|
VD | CD | VD | CD | VD | CD | |
Single-threaded | ||||||
Single 0.8 mm pitch | 4.797 | 4.8 | 8.537 | 8.774 | 18.329 | 18.543 |
Single 1.6 mm pitch | 7.146 | 7.236 | 14.401 | 14.549 | 32.711 | 32.782 |
Single 2.4 mm pitch | 7.29 | 7.353 | 15.873 | 15.916 | 35.785 | 35.940 |
Double-threaded | 5.676 | 5.739 | 11.026 | 11.245 | 24.663 | 24.765 |
Triple-threaded | 5.758 | 5.832 | 12.174 | 12.219 | 26.773 | 26.931 |
VD: the vertical relative displacement; CD: the comprehensive relative displacement; D2–D4: varying types of bone (D2 has the highest density; see Section
Comprehensive relative displacement among the Pitch Group, the Angle Group, and the Compactness Group under vertical load.
(1) In the Pitch Group, the 0.8 mm pitch single-threaded implant had a minimum comprehensive relative displacement in the collar region of the implant, whereas the 2.4 mm pitch single-threaded implant had the maximum. Compared with the 0.8 mm pitch single-threaded implant, the comprehensive relative displacement of the 1.6 mm pitch single-threaded implant in the collar part increased by 50.75% in D2, 65.82% in D3, and 76.79% in D4, whereas that for the 2.4 mm pitch single-threaded implant in the collar region increased by 53.19% in D2, 81.40% in D3, and 93.82% in D4.
Under the conditions of the same thread pattern, the comprehensive relative displacement of the 0.8 mm pitch single-threaded implant in collar part increased by 82.79% in D3 bone and 286.31% in D4 bone as compared with that for the same pitch in D2 bone. The comprehensive relative displacement of a 1.6 mm pitch single-threaded implant in the collar part increased by 101.06% in D3 bone and 353.04% in D4 bone, whereas that for a 2.4 mm pitch single-threaded implant in the collar part increased by 116.46% in D3 bone and 388.78% in D4 bone.
(2) In the Angle Group, the 0.8 mm pitch single-threaded implant had minimum comprehensive relative displacement in the collar part, whereas the triple-threaded implant had the maximum. Compared with the 0.8 mm pitch single-threaded implant, the comprehensive relative displacement of the double-threaded implant in the collar part increased by 19.56% in D2 bone, 28.16% in D3 bone, and 33.55% in D4 bone, whereas that for the triple-threaded implant in the collar part increased by 21.50% in D2 bone, 39.26% in D3 bone, and 45.24% in D4 bone.
Under the conditions of the same thread pattern, the comprehensive relative displacement of the double-threaded implant in the collar part increased by 95.94% in D3 bone and 331.52% in D4 bone, whereas that for the triple-threaded implant increased by 109.52% in D3 bone and 361.78% in D4 bone in the same region of the implant.
(3) In the Compactness Group, compared with the 1.6 mm pitch single-threaded implant, the comprehensive relative displacement of the double-threaded implant in the collar part decreased by 20.69% in D2 bone, 22.71% in D3 bone, and 22.46% in D4 bone. Furthermore, compared with the 2.4 mm pitch single-threaded implant, the comprehensive relative displacement of the triple-threaded implant in the collar part decreased by 20.69% in D2 bone and by 23.23% and 25.07% in both D3 and D4 bone.
For the displacement of five implants under horizontal load, please refer to Table
Displacement of five implants under horizontal load (
Thread pattern | D2 | D3 | D4 | ||||||
---|---|---|---|---|---|---|---|---|---|
VD | HD | CD | VD | HD | CD | VD | HD | CD | |
Single-threaded | |||||||||
Single 0.8 mm pitch | 20.028 | 18.856 | 27.512 | 29.712 | 25.748 | 38.192 | 31.370 | 26.448 | 41.056 |
Single 1.6 mm pitch | 21.388 | 18.984 | 28.598 | 37.142 | 30.053 | 48.278 | 39.620 | 31.405 | 52.410 |
Single 2.4 mm pitch | 22.284 | 19.022 | 29.314 | 40.736 | 32.090 | 51.941 | 43.119 | 33.200 | 56.644 |
Double-threaded | 20.282 | 18.850 | 27.84 | 33.497 | 26.838 | 43.124 | 35.257 | 28.193 | 46.436 |
Triple-threaded | 20.490 | 19.006 | 27.956 | 35.833 | 27.121 | 45.943 | 38.035 | 29.641 | 49.804 |
VD: the vertical relative displacement; HD: the horizontal relative displacement; CD: the comprehensive relative displacement; D2–D4: varying density of bone (D2 has the highest density; see Section
Comprehensive relative displacement among the Pitch Group, the Angle Group, and the Compactness Group under horizontal load.
(1) In the Pitch Group, the 0.8 mm pitch, single-threaded implant had minimum comprehensive relative displacement in the collar part, whereas the 2.4 mm pitch single-threaded implant had the maximum. With the same pitch, compared with the 0.8 mm pitch single-threaded implant, the comprehensive relative displacement of the 1.6 mm pitch single-threaded implant in the collar part increased by 3.95% in D2 bone, 26.41% in D3 bone, and 27.66% in D4 bone, whereas that for the 2.4 mm pitch single-threaded implant increased by 6.55% in D2 bone, 36.00% in D3 bone, and 37.97% in D4 bone.
Under the conditions of the same thread pattern, the comprehensive relative displacement of 0.8 mm pitch single-threaded implant in the collar part increased by 38.81% in D3 bone and 49.23% in D4 bone as compared with that for the same pitch in D2 bone. The comprehensive relative displacement of the 1.6 mm pitch single-threaded implant in the collar part increased by 68.82% in D3 bone and 83.26% in D4 bone. The comprehensive relative displacement of the 2.4 mm pitch single-threaded implant in the collar part increased by 77.19% in D3 bone and 93.23% in D4 bone.
(2) In the Angle Group, the 0.8 mm pitch single-threaded implant had the minimum comprehensive relative displacement in the collar part, whereas the triple-threaded implant had the maximum. Compared with 0.8 mm pitch single-threaded implant, the comprehensive relative displacement of the double-threaded implant in the collar part increased by 1.19% in D2 bone and by 12.91% and 13.10% in both D3 and D4 bones, whereas that for the triple-threaded implant increased by 1.61% in D2 bone and by 20.29% and 21.31% in both D3 and D4 bones.
Under the conditions of the same thread pattern, the comprehensive relative displacement of the double-threaded implant in the collar part increased by 54.90% in D3 bone and 66.80% in D4 bone. The comprehensive relative displacement of the triple-threaded implant in the collar part increased by 64.34% in D3 bone and 78.15% in D4 bone.
(3) In the Compactness Group, compared with the 1.6 mm pitch single-threaded implant, the comprehensive relative displacements of the double-threaded implant in the collar part decreased by 2.65%, 10.68%, and 11.40% in D2, D3, and D4 bones, respectively. Compared with the 2.4 mm pitch single-threaded implant, the comprehensive relative displacement of the triple-threaded implant in the collar part decreased by 4.63%, 11.55%, and 12.08% in D2, D3, and D4 bones, respectively.
The use of the FEA method in this mechanical analysis of dental implants has been described by many authors [
Comprehensive relative displacement of the implant (in
In the FEA method, the mesh division is its essence. Therefore, a high-quality mesh division can greatly improve the calculation accuracy. In previous work [
In our study, we observed a relative displacement in the vertical direction of the implant under vertical load. In comparison, under horizontal load, the relative displacements occurred in both vertical and horizontal directions. At the same osteotomy site, the vertical displacement was determined to be more than the horizontal displacement. The comprehensive relative displacement at the implant collar under horizontal load was significantly more than that under vertical load, which is in accordance with the clinical presumption of the higher influence of horizontal loading on implant stability [
The micromotion of implants with different pitches (the Pitch Group) was compared. Under vertical and horizontal load, the 0.8 mm pitch single-threaded implant had the minimum comprehensive relative displacements at the collar, whereas the 2.4 mm pitch implant had the maximum displacement in all three-bone types. This demonstrates that variation in pitch affects the vertically and horizontally loaded implant in terms of its stability and that as the pitch increases, the implant resistance to vertical and horizontal load diminishes. Our result is consistent with the results of previous studies mentioned above [
There are two major differences in implants with different pitches—the helix angle and the compactness—which both affect the stability of the implant. To date, the direct elucidation of the effect of the helix angle and compactness on the micromotion, however, has not been reported. In the present study, we sought to illustrate the effect of the helix angle and compactness on micromotion. Micromotion of identical implants, with a constant pitch of 0.8 mm but different thread helices (Angle Group), was compared. The double- and triple-threaded implants had twice and triple the thread helix of the single-threaded implant, respectively. Irrespective of the load, the 0.8 mm pitch single-threaded implant showed minimum comprehensive relative displacement at the collar, whereas the triple-threaded implant had the maximum. Overall, these results demonstrate that helix angle affects the stability of a vertically and horizontally loaded implant and that, as the thread helix angle increases, the implant resistance to vertical and horizontal load reduces.
The introduction of double- and triple-threaded implants (where two or three threads run parallel to one another [
To date, a direct elucidation of stress relief with implant placement in bone has not been reported. However, some authors [
In this study, another important pattern emerged for implants with the same helix angle but different thread compactness (Compactness Group). Irrespective of bone quality or load, the comprehensive relative displacement in the collar part of double-threaded implants was smaller than that for 1.6 mm pitch single-threaded implants. Additionally, the comprehensive relative displacement in the collar part of the triple-threaded implants was smaller than that for 2.4 mm pitch single-threaded implants. These findings demonstrate that, with the same threaded helix angle, the resistance of the implant to vertical and horizontal loads enhances as the compactness increases. This is likely to be attributed to the improved mechanic interlocking caused by the increase in the thread compactness, which, in turn, leads to less micromotion and better stability.
We also noticed that, under vertical load and as compared with the 0.8 mm pitch single-threaded implant, the comprehensive relative displacement of the 1.6 mm pitch and 2.4 mm pitch single-threaded implants in the collar part increased by 50.75% and 53.19% in D2, 65.82% and 81.40% in D3, and 76.79% and 93.82% in D4, respectively (Figure
Increasing rates of comprehensive relative displacement between the Pitch Group and the Angle Group under vertical load (comparing the 0.8 mm pitch single-threaded implant). Decreasing rates of comprehensive relative displacement in the Compactness Group (double-threaded implant compared with the 1.6 mm pitch single-threaded implant, and the triple-threaded implant compared with the 2.4 mm-pitch single-threaded implant).
Increasing rates of comprehensive relative displacement between the Pitch Group and the Angle Group under horizontal load (comparing the 0.8 mm pitch single-threaded implant). Decreasing rates of comprehensive relative displacement in the Compactness Group (double-threaded implant compared with the 1.6 mm pitch single-threaded implant, and the triple-threaded implant compared with the 2.4 mm pitch single-threaded implant).
In our study, the comprehensive relative displacement of the 2.4 mm pitch single-threaded implant in the collar part of the implant in D4 increased by 388.78% and 93.23%, respectively, under vertical and horizontal loads as compared with that of the same implant in D2. However, the comprehensive relative displacement of the 0.8 mm pitch single-threaded implant in the collar part of the implant in D4 increased by 286.31% and 49.23% under vertical and horizontal loads, respectively, compared with that for the same implant in D2. Compared with the Pitch Group, similar results were also found in the Angle Group and the Compactness Group. Overall, our results indicate that the influence of thread pitch, helix angle, and compactness on the comprehensive relative displacement is amplified along with the variation of the bone quality.
In this numerical study, several assumptions were made in the development of the model in the present study. The structures in the models were all assumed to be homogenous and isotropic and to possess linear elasticity. The properties of the materials modeled in this study, particularly the living tissues, however, are different. For instance, it is well documented that the cortical and cancellous bones are neither homogeneous nor isotropic. A frictional contact of bone-implant interface was assumed. Therefore, the results of this study must be interpreted cautiously, and the inherent limitations of 3D FEA studies shall also be considered.
In conclusion, vertical relative displacement is affected by thread pitch, helix angle, and compactness. Displacement is positively correlated with thread pitch and helix angle but negatively correlated with compactness under vertical loading in variable types of bone density. Under horizontal loading in higher density D2 bone, the thicker cortical component would be more effective in providing implant stability than thread pattern; however, in D3 and D4, with reduced density and increased cancellous structure, the density of trabecular bone has less effect on implant micromotion than the thickness of cortical bone. The influence of thread pitch, helix angle, and compactness on the comprehensive relative displacement is amplified along with the variation of the bone quality under vertical and horizontal loads.
All authors declare to have no conflict of interests.
This study was supported by Beijing Capital Medical University Fund for Basic and Clinical Medicine Research (no. 13JL83) and Beijing Stomatological Hospital of Capital Medical University Fund for Basic Research (no. 14-09-09). The authors are grateful to B. R. Wang for her support in the preparation of this paper.