Marginal bone loss after dental implant placement is observed in many implant systems and after different surgical approaches. ¹ It usually begins at the neck of the implant and spreads to the first thread of the body or the first contact between the bone and the rough surface of the implant. ²

Implant crest module (i.e., transosteal region of dental implants that transfers stress to the adjacent crestal compact bone during loading) ³ is considered as one of the plausible etiologic factors that have been hypothesized for early crestal bone loss. ¹ Originally, a machined implant neck was proposed to prevent plaque accumulation. ⁴ However, later studies revealed a positive correlation between the length of the polished implant neck and the amount of crestal bone loss. ⁵ Interestingly, some finite element studies have shown that there is a concentration of greater stress contours at the crestal bone region, ⁶ which could potentially contribute to marginal bone loss. These new findings led to several modifications in crest module design. To diminish the marginal bone loss progression in the crestal region, Hansson in 1999, suggested considering retentive elements (e.g., a rough surface of suitable microarchitecture and/or a microthread) in the implant neck design from a biomechanical viewpoint. ⁷ Thereafter, numerous solutions were introduced with the aim of reducing the crestal bone resorption including platform switching, ⁸ different approaches of surface roughening, namely, TiOblast surface modification, ⁹ sandblast acid etched, ¹⁰ titanium plasma spray, ¹¹ and laser microtexturing. ¹² The incorporation of very small threads, so-called microthreads, with a favorable profile optimizes the stress distribution similar to commonly sized threads (i.e., macrothreads) ¹³ and its efficacy in preserving marginal bone has been documented in some animal studies ¹⁴ and clinical investigations. ² In a short-term human study evaluating the bone loss there was no significant difference between the implants with macro- and micro-thread in the neck after 1 year of loading. ¹⁵

Contrary to previous reports, a controversial publication by Schrotenboer et al. ¹⁶ indicated an increase in von Mises stress adjacent to microthreaded implant as compared to smooth neck. However, some questions regarding the condition of bone-implant interface, reliability of the material properties and precision of two-dimensional finite element modeling were raised. ¹⁷ Later, Hudieb et al. ¹⁸ stated that more compressive and less shear stress arising from a microthreaded implant neck clarifies the biomechanical aspect of this design.

Similar to extensive variability in main (macro) threads on the implant body the term “microthread” also could include myriads of design forms. In view of this great diversity and considering the controversy in previous finite element studies, we formulated this investigation to examine some of the microthread design parameters and introduce the most effective geometry which gives the biomechanical advantage of optimal stress distribution.

The aim of this study was to analyze the effect of microthread designs in crest module of the implant models under axial and oblique static loading to find out the optimal microthread design with the best stress transferring pattern.

Construction of three-dimensional models

Six three-dimensional implant models as illustrated in Figure 1were constructed with the ANSYS finite element analysis (FEA) program (ANSYS11.0, ANSYS, Canonsburg, PA). Figure 1

The solid models of six implant models and the design properties of main threads.

In both loading angles, the highest stress as a whole was concentrated in the cortical bone around the implant. Considering the differences between stress values in cortical and cancellous bone, the amounts and contours of stresses are discussed separately.

Cortical bone

Under axial load, the highest cortical bone stress was located buccally around the implant neck in model number 4 followed by 5. However, the least equivalent (EQV) stress was observed around model number 3. On the other hand, the model number 2 had the second most EQV stress under oblique load, after model number 4. The most favorable implant model in terms of EQV stress reduction in cortical bone was model number 3 Figure 3. Intragroup comparisons showed that under axial loading for each microthread profile the least stresses were generated around the 3 mm length of microthreaded neck. In other words, the more the length of the microthreaded neck, the less the maximum EQV stress in the cortical bone under 0° load. However, under oblique loads, although the same pattern was observed for fine microthreads; in the coarse group, the model number 2 generated the highest stress, followed by models number 3 and 1. Figure 3

Maximum equivalent stress in cortical bone under axial (a) and oblique (b) loads.

The role of unfavorable loading conditions in bone loss around dental implants that might lead to implant failure has been emphasized in many animal ²² and clinical studies. ²³ To cope with this resorption, macroscopic or microscopic modification of implant surface structure has been proposed. ²⁴ However, the effectiveness of different neck configurations in preserving the marginal bone could not be established in a systematic review by Bateli et al., ²⁵ due to lack of sufficient evidence.

Microthread design as a macro-roughness and retentive element ⁷ has been used by several manufacturers in the implant neck, primarily due to the biomechanical advantage of the threads in converting the potential deleterious shear forces into compressive, ¹ which the cortical bone withstands the best. ²⁶ There are variable designs of small threads in implant neck all under the same name of “microthread” and all with a postulate of maintaining the bone, that makes it of benefit to investigate stress in bone and its relation to different design parameters of implant neck. The present study used FEA for evaluating the influence of implant neck design particularly microthreads on stress distribution in bone. Although FEA offers multiple advantages over other methods in simulating the complexity of clinical situations, it is also sensitive to the assignment of proper material properties, loading and boundary conditions. ²⁷ Due to the simplifications made throughout the process of analysis, it only gives a general concept of stress variations under certain conditions, without resembling individual clinical situations. Thus, it is better to focus on qualitative comparison of stress distribution pattern rather than quantitative data. ²⁸

Previous FEA studies focusing on microthreads showed great variation in results. Some indicated an increase in marginal bone strain with the incorporation of microthreads in cervical portion of the implant, ²⁹ , ³⁰ while others emphasized on biomechanical advantage of microthreads due to the generation of compressive stresses. ¹⁸ , ³¹ This heterogeneity in studies could be the result of differences of the material properties, implant abutment connection designs, the fineness of mesh, element size and shape, and finally, the implant model. Although some studies precisely modeled rounded picks and valleys of the commercially available implants, others used models with sharp edges which could apparently cause stress concentration. We modeled the bone as a transversely isotropic and linearly elastic material which is a more realistic simulation of real bone properties and is believed to cause higher stress and strains in peri-implant bone. ²⁷ Nevertheless, the physical characteristics assigned to bone in studies by Schrotenboer et al. ¹⁶ and Karimi et al. ³¹ were indicative of a homogenous and isotropic material, which is far from actual condition. Earlier reports on microthreads have compared this design feature either with platform switching, ¹⁶ , ³⁰ or with smooth implant neck ¹⁸ or with macrothreads; ²⁹ but different designs of microthreads have not been yet compared under the conditions of a same investigation. This results from a simplistic postulate that categorizes all shapes of small threads under the name of microthreads. Thus, although different studies have modeled various commercial implant systems with inherently dissimilar designs, the results of all these researches are accounted as the performance of microthreads.

In the present study, we could clearly demonstrate the superiority of coarse profile to fine microthreads, since in similar lengths the stress magnitude was consistently lower in coarse group, especially in cortical bone that greater differences in generated values made the comparison reasonable. However, in cancellous bone, given the minor discrepancies among tested models, only the extremities of stress values for each loading direction were compared quantitatively. According to the results of our study, it appears that correlation between the length of the neck incorporating coarse microthreads and the magnitude of stresses in cortical bone does not follow a similar pattern under both loading conditions.

In all models, there was an average of 42% increase in compressive stress values from 0° to 30° axial loading. Nevertheless, the increase in shear component of stress had a range of 35%–52%. Model number 1 showed the most favorable performance in cortical bone under oblique load, considering its low shear stress. Considering the fact that cortical bone is 65% more susceptible to shear forces than compressive ones, an effective reduction in deleterious shear forces, especially under nonaxial loading would be a beneficial characteristic of implant design. ¹

When the microthreaded implants with both coarse and fine profiles were loaded along the long axis of the body, the level of both shear and compressive stresses were enhanced with the increase of the neck length. The same pattern was observed for fine microthreads under oblique load. However, conflicting results were observed for the coarse microthreads with oblique load, where model number 2 had the highest shear and compressive stresses. In other words, there was a distinction between optimal length of the neck with coarse microthreads under axial and oblique loads in cortical bone.

In cancellous bone, the stresses were higher for the 2 mm length of implant neck, in both loading conditions and for both microthread profiles. Interestingly, these two models were the ones with 2 mm length of microthreaded neck, exactly the same as the thickness of the cortical layer in bone models. However, since all surfaces in the study were considered to be bonded (i.e., with a frictional coefficient of 1) this finding could rather be attributed to the change in material properties from cortical to cancellous bone than concentration of surface stresses.

Frost ³² in 1989 suggested that microdamage arises in normal lamellar bone when the stress exceeds 45–60 MPa. In the present study, the maximum EQV stress values obtained for all models were below this threshold under axial loading. While under the oblique load of 30° all models showed higher stresses. To maximize the capacity of the implant to resist loads, it should be given such a design so that the peak stresses arising in the bone, as a result of a given load, are minimized. This is best addressed in model number 3 with the least amount of stress, especially less shear stress in comparison to other models.

Within the limitations of this study, this FEA suggested the following:

Oblique loading causes approximately 40% increase of the stress values in cortical bone compared to axial loading

Financial support and sponsorship

This paper was a Msc thesis project (n. 140) granted by Tabriz university of medical sciences.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.