2D FEA of evaluation of micromovements and stresses at bone-implant interface in immediately loaded tapered implants in the posterior maxilla.

AIM: The aim of the study is to evaluate the influence implant length on stress distribution at bone implant interface in single immediately loaded implants when placed in D4 bone quality.
MATERIALS AND METHODS: A 2-dimensional finite element models were developed to simulate two types of implant designs, standard 3.75 mm-diameter tapered body implants of 6 and 10 mm lengths. The implants were placed in D4 bone quality with a cortical bone thickness of 0.5 mm. The implant design incorporated microthreads at the crestal part and the rest of the implant body incorporated Acme threads. The Acme thread form has a 29° thread angle with a thread height half of the pitch; the apex and valley are flat. A 100 N of force was applied vertically and in the oblique direction (at an angle of 45°) to the long axis of the implants. The respective material properties were assigned. Micro-movements and stresses at the bone implant interface were evaluated.
RESULTS: The results of total deformation (micro-movement) and Von mises stress were found to be lower for tapered long implant (10 mm) than short implant (6 mm) while using both vertical as well as oblique loading.
CONCLUSION: Short implants can be successfully placed in poor bone quality under immediate loading protocol. The novel approach of the combination of microthreads at the crestal portion and acme threads for body portion of implant fixture gave promising results.

INTRODUCTION

A dental implant serves as a load-bearing device that sustains masticatory forces and also transfers loads to peri-implant bone.[1] Initially, it was considered that the process of osseointegration requires on average an undisturbed healing of 3 months in the mandible and 6 months in the maxilla.[234] An increasing interest with regard to early and immediate loading of implants has been noticed, to expedite the restorative outcome. Immediately loaded dental implants have shown long-term success of removable and fixed prostheses using clinical and experimental animal trials.[5678910]

By using finite element analysis (FEA), it has been shown that the highest risk of bone resorption occurs in the neck region of an implant.[111213] Beginning at the crestal area of the cortical bone, bone loss, can progress toward the apical region, endangering the longevity of the implant and prosthesis.[14] In areas with low bone density, prerequisites for successful treatment include surgical procedures for adequate primary implant stability,[1516] prosthetic protocols with load control[1718] and tapered implant geometries with osseoconductive implant surfaces.[1920] Improvement of primary implant stability has been seen with the use of tapered implant geometry.[21]

Load transfer to implant depends on successful healing of the osteotomy and osseointegration. It is characterized as a direct structural and functional connection between the bone and the implant surface.[222324] Thus, to improve the osseointegration of implant to bone and establish implant stability, increasing the implant surface area (increasing the implant diameter or implant length) might help.[1] Increasing the length of an implant by 3 mm increases the surface area by more than 20%.[25] The increase in the surface area might help to improve implant support, implant stability and the odds of survival.[26] However, the jaw anatomy limits the choice of implant length. The presence of the inferior alveolar nerve and mental foramen in the mandible and the maxillary sinus in the maxilla, restricts the available bone height.[272829] In these situations, the use of short length implants is more appropriate.

Threads have been incorporated into implants to improve initial stability,[3031] enlarge implant surface area and distribute stress favorably.[3233] For a low density bone, implants should be selected on a bioengineering principle that the implant body has a thread profile, which maintains strain levels at the “steady state zone”[34] and stimulates bone preservation. The optimal load distribution that microthread offers, counteract marginal bone resorption.[35] Microthread preserves the bone better than an implant without microthread.[36] The acme thread form has a 29° thread angle with a thread height half of the pitch; the apex and valley are flat. This shape is easier to machine (faster cutting, longer tool life) than is a square thread. The tooth shape also has a wider base which means it is stronger (thus, the screw can carry a greater load) than a similarly sized square thread.[37]

Finite element model (FEM) analysis has been widely used to evaluate the stress in peri-implant bone. The 2D FEM is a very simple and schematic model. It is designed to clarify the “principal effect” that can be “hidden” in the 3D FEM as a result of the complex geometry of real 3D objects (e.g., tooth, maxilla, etc.,).[38] Therefore, the 2D model was only used for preliminary qualitative analysis in the present study.

Until date, there are no reports documenting the use of acme threads for dental implants as well as use of standard diameter short implants under immediate loading protocol. The microthreads used until now, are a form of triangular thread, but in this study, we are using the acme thread form as a microthread, which is again a novel approach.

The aim of the present study is to evaluate the biomechanical response of standard diameter tapered implants of varying lengths (6 mm and 10 mm) incorporating microthreads for crestal portion and acme threads for body portion placed in D4 bone under immediate loading protocol.

MATERIALS AND METHODS

2D FEMs of maxillary posterior section of bone were created. The bone was modeled as a cancellous core D4 bone surrounded by a 0.5 mm thick cortical layer. Tapered implants of length 6 mm and 10 mm with standard diameter of 3.75 mm were used for the study [Figures 1 and 2]. All the FEMs were created using a software program named ANSYS classic, version 11.

Figure 1

Standard diameter implant of 6 mm length in D4 bone with cortical bone thickness of 0.5 mm

Figure 2

Standard diameter implant of 10 mm length in D4 bone with cortical bone thickness of 0.5 mm

Material properties: All materials used in the models were considered to be homogeneous and linearly elastic. The elastic properties used were taken from the literature[3940] [Table 1].

Table 1

Material properties

Interface condition: To simulate the interface of an immediately loaded implant, a frictional coefficient of 0.6[41] was applied at bone implant interface.

Implant design: The implant design incorporates microthreads at the crestal part (2 mm) with 0.2 mm screw pitch and 29° thread angle. The rest of the implant body incorporates acme threads with 0.8 mm screw pitch and 29° thread angle[37] [Figure 3].

Figure 3

The 6 mm and 10 mm length implant design - microthreads at the crestal part (2 mm) with 0.2 mm screw pitch and 29° angles and rest implant body with acme threads with 0.8 mm screw pitch and 29° angle

Elements and nodes: The FEM model composed of 4195 nodes and 4087 elements for 6 mm implant and bone block of 10 mm height and 20 mm length, for 10 mm tapered implant the nodes were 5856 and elements were 5732 and bone block of 15 mm height and 20 mm length.

Loading conditions: Loads of 100 N were applied in the vertical direction (along the long axis of the implant) and an oblique direction (at an angle of 45° to the vertical) [Figure 4a and b].

Figure 4

(a and b) Force directed along long axis of implant and at 45° to long axis of implant

Parameters analyzed were:

Total deformation (Micro-movement)

Von mises stress.

RESULTS

The values for total deformation when using vertical loads (100 N) were 129 μm and 82.2 μm for 6 mm and 10 mm implant [Figure 5a and b] respectively and with oblique loads the values were 114 μm for 6 mm and 83.3 μm for 10 mm implant [Figure 6a and b].

Figure 5

(a) Total deformation for 6 mm implant with vertical (along long axis) forces; (b) total deformation for 10 mm implant with vertical (along long axis) forces

Figure 6

(a) Total deformation for 6 mm implant with oblique (45° along to long axis) forces; (b) total deformation for 10 mm implant with oblique (45° along to long axis) forces

Von mises stress values were 94.2 MPa for 6 mm implant and 47.2 MPa for 10 mm implant respectively when vertical loads were used [Figure 7a and b]. Stress values were more for oblique loading i.e., 306.6 MPa and 204.4 MPa for 6 mm and 10 mm implants respectively [Figure 8a and b]. The results of total deformation (micro-movement), Von mises stress were found to be lower for tapered long implant (10 mm) than short implant (6 mm) while using both vertical as well as oblique loading [Figures 9 and 10].

Figure 7

(a) Von Mises stress for 6 mm implant under vertical loading; (b) Von Mises stress for 10 mm implant under vertical loading

Figure 8

(a) Von Mises stress for 6 mm implant under oblique (45° to long axis) loading forces; (b) Von Mises stress for 10 mm implant under oblique (45° to long axis) loading

Figure 9

Graph showing total deformation for 6 mm and 10 mm implant under vertical and oblique (100 N) forces

Figure 10

Graph showing Von Mises stress for 6 mm and 10 mm implant under vertical and oblique (100 N) forces

DISCUSSION

The present study evaluates the effects of implant lengths (6 mm and 10 mm) on immediately loaded tapered implants placed in maxillary posterior region. In maxillary posterior region presence of poor bone quality and reduced bone height, makes short implants a favorable choice.

FEA, a computer based technique calculates the behavior of engineering structures and their strength numerically. In the FEM, a structure is broken down into many small simple blocks or elements. A simple set of equations describes the behavior of an individual element relatively. The structure will be built fully by joining together these set of elements, so the behavior of the whole structure will be described by extremely large set of equations, which were actually the equations describing the behavior of individual elements joined together. The behavior of individual elements is assessed by computer from the solutions. Hence, the stress and deflection of all parts of the structure can be calculated.[4243]

The most valid approach for implant placement is two-stage protocol. Recently, immediate functional loading has gained importance and comparable results were found in a single stage surgical procedure.[44] Good primary stability, controlled loading conditions and osseoconductive implant surface are the expected advantageous results for immediate functional loading.[45]

Careful consideration of fixture placement, prosthesis design, nature and magnitude of occlusal forces is essential, in order to achieve optimized biomechanical conditions in implant-supported suprastructures.[46] When applying FEM analysis to dental implants, it is important to consider oblique occlusal force because they represent more realistic occlusal directions and for a given force, will result in localized stress in cortical bone.[47] Loads of 100 N were applied in the vertical direction (along the long axis of the implant) and an oblique direction (at an angle of 45° to the vertical) in the present study.

The initial implant mobility does not necessarily prevent osseointegration.[48] In general, from uncontrolled masticatory forces; micro motion at the implant interface has to be distinguished. Cameron et al.[49] reported that osseointegration cannot be achieved with macro-movements. However, micro-movements are not a problem. There is a lack of a consistent terminology on the definition of micro and macro-movements. It has been suggested that a movement of 150 μm or more results in soft connective tissue apposition at bone implant interface and a movement of 30 μm or less has no adverse effect on integration.[505152] The results of the present study show all the micro-movement values within this range. The values are lower for long implant (10 mm) than for 6 mm implant.

Available bone is particularly important in implant dentistry. It describes the external architecture or volume of the edentulous area considered for implants. In addition, quality or density describes the internal structure of bone, which reflects the strength of the bone. Type 4 bone i.e., D4 bone has little cortical bone thickness and minimal internal strength.[53] As compared with more dense bone, increased clinical failure rates in poor quality, porous bone, have been well-documented.[5456] Jaffin and Berman,[57] in a 5-year analysis of Branemark implants, reported that out of 105 implants placed in Type 4 bones, 35% failed and among 949 implants placed in Types 1, 2 and 3 bones, only 3% of the implants were lost. Bass and Triplett[58] also revealed that bone quality four exhibited the greatest failure rate. In patients with implant retained overdentures, highest risk for implant failure (45%) was reported in dental arches with bone quality 4 by Hutton et al. in a prospective study of 510 Branemark implants.[59] Increase the number of implants or an implant design with greater surface area may be the choice to decrease the stress in D4 bone quality.[33536061]

Threaded implants increase the surface area for osseous integration and are generally preferred to smooth cylindrical ones.[62] The neck of the implant is called crest module. In the presence of a smooth neck, negligible forces are transmitted to the marginal bone leading to its resorption. However, the presence of retentive elements at the implant neck will dissipate some forces leading to the maintenance of the crestal bone height accordingly to Wolff's law.[35] A clinical trial[63] demonstrated possible preservation of crestal bone contact with implant systems using microthreads. Significantly lower amounts of bone loss, with an implant system that incorporates microthread retention elements at the implant neck was reported by Norton.[64] In the present study microthreads are used at the crestal module of the implant and the rest of the implant body consists of acme threads. Microthreads used previously were a form of triangular threads, but in our study, we used acme thread form with 29° thread angle as microthread.

When created prior to 1895, Acme screw threads were intended to replace square threads and a variety of threads of other forms used chiefly for the purpose of traversing motion on machines, tools, etc., Acme screw threads are now extensively used for a variety of purposes. Long-length acme threads are used for controlled movements on machine tools, testing machines, jacks, aircraft flaps and conveyors. Short-length threads are used on valve stems, hose connectors, bonnets on pressure cylinders, steering mechanisms and camera lens movement. They are best suited for applications that warrant large load bearing capacity and high accuracy.[37] We have used acme threads for the body of the implant to increase the load bearing characteristic of implant, particularly in the presence of weak bone (D4) and it gave promising results. The results of our study shows highest stress concentration at the cortical bone level of D4 bone where trabecular bone is sparse and cannot sustain loads.

Longer implants have been observed to score better than shorter ones.[6566] When a problem of severe atrophy of the jaws is encountered, there have been many different approaches in solving this condition by prosthetic reconstruction. In the presence of reduced alveolar bone height, the short dental implants have recently become available and offer the clinicians a pragmatic option to facilitate prosthetic restoration in the face of anatomic limitation.[67] Renouard and Nisand[46] reported a trend of increased failure rate due to the use of short or wide implants, when reviewing the effects of implant length and diameter. The survival rates of short-or wide-diameter implants were comparable to those of longer implants and implants with standard diameters, when the bone density, surface characteristics of the implant, operator's surgical skill and indications for treatment during the surgical preparation were considered. The results of our study are in accordance with other studies with lower stress values for long implant than short 6 mm implant. In our study, we found that stresses are 49.9% increased with vertical loading and 33.3% increased when oblique forces when used for 6 mm short implant as compared with 10 mm long tapered implant.

The presence of poor quality bone (D4) in the maxillary posterior region is a common finding. Usage of acme threads presents a good option to increase the load bearing characteristic of implant when poor bone quality is present. As stress values are more for oblique loading, eccentric loads have to be avoided during healing phases, particularly for short implants.

Limitations of this study are the simplified geometry of the bone model, material properties assigned and static occlusal force. Even though the strength of a bone block is similar to that of the jaw bone, the stress patterns might vary with the bone geometry. The material properties of the FEM maxillary were assumed to be isotropic and homogenous whereas, consideration of the anisotropic and inhomogeneous properties is still needed in future studies. Although oblique loading has been suggested to represent a realistic occlusal load,[68] chewing movement, especially with dynamic loading simulations, needs to be considered in future investigations.

SUMMARY AND CONCLUSION

The present study evaluates the effects of implant lengths (6 mm and 10 mm) on immediately loaded tapered implants placed in maxillary posterior region. Within the limitations of the 2D FEA study, the following conclusions can be drawn:

Short implants can be successfully placed in poor bone quality under immediate loading protocol

During the initial phases of implant healing, avoidance of oblique forces with optimally designed implant superstructures will help to reduce stresses around short implant

Incorporation of microthreads at the crestal portion helped in concentration of stresses in the cortical bone in D4 bone where trabecular bone is sparse

Acme threads are best suited for applications that warrant large load bearing capacity and high accuracy. Usage of Acme threads for body portion of the implants gave promising results.

Further randomized clinical trials are needed to validate results of FEM study.