Influence of High Insertion Torque on Implant Placement-An Anisotropic Bone Stress Analysis

Over the last 30 years, clinical studies with osseointegrated implants have shown excellent longterm results, with over 90% success rate (1,2). However, early failures may occur during the healing process affecting osseointegration (3). These failures may have biological causes, such as periimplantitis and systemic diseases. In addition, biochemical factors can negatively influence implant success; for instance, bone over heating during the surgical procedure, occlusal overload, besides the effects of tensile strength, shear and compressive stresses in the peri-implant bone tissue (4). The osseointegration process requires ideal stress Influence of High Insertion Torque on Implant Placement An Anisotropic Bone Stress Analysis


INTRODUCTION
Over the last 30 years, clinical studies with osseointegrated implants have shown excellent longterm results, with over 90% success rate (1,2).However, early failures may occur during the healing process affecting osseointegration (3).These failures may have biological causes, such as periimplantitis and systemic diseases.In addition, biochemical factors can negatively influence implant success; for instance, bone over heating during the surgical procedure, occlusal overload, besides the effects of tensile strength, shear and compressive stresses in the peri-implant bone tissue (4).
The osseointegration process requires ideal stress

Influence of High Insertion Torque on Implant
Placement -An Anisotropic Bone Stress Analysis levels to maintain normal bone repair (5).Excessive tension may cause irreversible damage to the periimplant bone tissue (6).Conversely, too low tension may unsatisfactorily stimulate the bone repair process (4).
Recent computerized simulations have suggested that compressive stresses and hydrostatic tensions of the interstitial liquid may modulate tissue differentiation and bone remodeling (4,7,8).Byrne et al. (7), using mathematical models about cellular differentiation in bone repair, verified that the stress increase changes the bone repair process, reducing the amount of newly formed bone tissue by 23% and increasing the amount of cartilage by 21%.Similarly, Checa and Prendergast (8) verified less newly formed bone and greater connective tissue formation under elevated compressive stresses.
During placement of osseointegrated implants, the insertion torque may result in varied levels of compressive stresses transmitted to the adjacent bone, given that the implant bed is slightly narrower than the diameter of the implant to be placed in order to optimize primary stabilization (9,10).Clinical studies have demonstrated a close relationship between initial stabilization and the success of an osseointegrated implant (11)(12)(13), which can be measured by the insertion torque during implant placement (12).The insertion torque must exceed 30 Ncm to obtain predictable success rates (12,14), aiming at avoiding implant micromovement and consequent connective tissue formation (6,13).However, an excessively high insertion torque, above 50 Ncm (15), can occur during dense bone implant placement (12,16,17), resulting in the transmission of high compressive stresses to the adjacent bone, in addition to compromising osseointegration success (3).
This way, the understanding of the high values of insertion torque that can be used during implant placement, without causing excessive stress in the bone tissue, would be helpful for the success of implant osseointegration.
Considering the lack of studies associating compressive stress and strain with the high values of the insertion torques during implant placement, the aim of the present study was to evaluate the influence of different insertion torques on stress and strain distribution in cortical and cancellous bones, using the three-dimensional finite element method.

MATERIAL AND METHODS
After approval by the local Research Ethics Committee and signature of an informed consent form, a computed tomography (CT) scan was taken from a patient to obtain dicom format images.A representative mathematical model of the anterior segment of the maxilla was built using Mimics 11.11 software (Interactive Medical Image Control System, Materialise Inc., Leuven, Belgium) and Solid Works 2010 software (Dassault Systèmes SolidWorks Corporation, Concord, MA, USA).
The geometry of an external hexagonal implant of 4.5 x 11.5 mm (Strong SW model; Sistema de Implantes-SIN, São Paulo, SP, Brazil) was used to build the implant design (Fig. 1A) with the aided of the Solid Works 2010 software.This implant was adapted to the bone segment corresponding to the region of the maxillary right central incisor (Fig. 1B).
After building, the initial model was imported to the finite element program Ansys Workbench 10.0 (Swanson Analysis Systems Inc., Houston, PA, USA) to determine the regions and generate the finite element mesh.Each model received an implant with one of the following insertion torques: 30, 40, 50, 60, 70 or 80 Ncm on the external hexagon.These values were applied using six forces on the implant external hexagon, perpendicular to the long axis and tangent to the implant platform, determined by the following equation (Fig. 2): F= T / 6 x D x senθ Where T is the insertion torque; D is a point of force application to the rotating axis; and θ is the angle formed by the direction of the force applied and the plan of force application.
Parabolic tetrahedral elements of 0.8 mm were used for the mesh.The mesh refinement was established by the convergence of analysis (6%).The models presented 170,504 nodes and 112,507 elements.The implant presented 92,432 nodes and 62,483 elements.The cancellous and cortical bones presented 58,463 and 19,609 nodes and 39,442 and 10,582 elements, respectively.A zero-displacement boundary condition was applied to the three Cartesian axes (x=y=z=0).
Maximum principal stress (σ max ) and maximum principal strain (ε max ) were obtained for the cortical and cancellous bones around the implant.Pearson's correlation test was used to determine the relationship between the insertion torques and stress concentrations, considering the significance level at 5%.

RESULTS
In general, there was higher stress in the cancellous and cortical bones after increasing the insertion torques.The σ max was smaller for the cancellous bone, with greater stress variation among the insertion torques.Theε max was higher in the cancellous bone than in the cortical bone.

Cancellous Bone
Stress distribution in cancellous bone is presented in figure 3. The σ max for different insertion torques of 30, 40 and 50 cm were 0.114, 0.144 and 0.168 MPa, respectively.There was an increase in the main stresses between insertion torques of 50 and 60 Ncm, varying from 0.168 to 1.09 MPa, maintaining a linear increase with 70 and 80 Ncm, exhibiting 1.17 and 1.34 MPa, respectively.The insertion torques showed a significant correlation with σ max and ε max (r=1.0,p=0.001).

DISCUSSION
An adequate stability of the dental implant in the surrounding bone plays an important role in the bone healing processes, avoiding micromovement and damage to the bone healing process (17,19).Clinical assessment of primary stability can be performed by the implant insertion torque at the moment of placement (19).
Considering the importance of the insertion torque for the osseointegration process, this study was performed by mimicking a dental implant placement using the anisotropic finite element technique.It was hypothesized that high values of the insertion torque can generate overcompressive stress to the peri-implant tissues and compromising osseointegration process.According to Ottoni et al. ( 14) and Irinakis and Wiebe (12), the insertion torque must exceed 30 Ncm, especially for immediate load implants.However, insertion torques above 50 Ncm are considered high (15) and generally induce excessive compressive stresses to the bone periimplant.Some studies have reported that the increase of compressive stress in the bone tissue may lead to failure in the bone healing (5) and osseointegration process (3).
The present study showed similar results to those of a previous study (4), in which the increase of the insertion torque generates higher compressive stress concentrations to the peri-implant bone tissue.In the present study, for the cancellous bone tissue, the increase of insertion torque (from 30 to 80 Ncm) showed an increase of 1175% for the maximum principal stress and 266% for the maximum principal strain.In the cortical bone tissue both stresses were higher (267%).Studies based on mathematical analyses of the bone healing process presented a correlation between the compressive stress and the type of tissue formed during bone remodeling in vivo (8).It is important to draw attention to the fact that under high stresses, significant alterations occurred in the angiogenesis dynamics impairing the formation of new blood vessels, causing hypoxia in the periimplant tissues, thus inhibiting bone formation and favoring the formation of cartilage and connective tissue (8).It has also been emphasized the model of bone tissue formed by a complex threedimensional tubule network filled with an interstitial fluid that supplies bone cells (20).This fluid would be able to transmit external stresses to bone cells through a mechanism known as mechanotransduction, which refers to the conversion of mechanical energy from external stresses into bioelectrical and biochemical signals that modulate the bone cell metabolism (20).Therefore, when this mechanical energy is too high, osteocytes are induced to death, followed by recruitment of osteoclasts and bone destruction (8,20).
The results of the present study demonstrated that the maximum principal stress increased by 648%, from 0.168 to 1.09 MPa, between torques of 50 and 60 Ncm in the cancellous bone.High insertion torques above 50 Ncm (15) can generate high compressive stresses to the peri-implant tissues causing blood supply deficiency and bone necrosis during the osseointegration phase and early implant failure (3) usually within the first month after placement (3).A high insertion torque may occur during implant placement in high density bone tissue (12,16,17).This observation has been demonstrated in a study that evaluated the relationship between bone density and the maximum insertion torque supported by the bone tissue, using computer tomography images and Hounsfield scale, and found a significant correlation between bone density, insertion torque and primary stability (17).
The cortical bone tissue had lower capacity to dissipate stress as well as a more uniform increase of the insertion torque, showing higher principal maximum stress in comparison with the cancellous tissue.These results are similar to those of previous studies (9,10), which assessed the influence of the osteotomy diameter for implant placement and the stress concentration on implant threads.These results are explained by the different mechanical properties between cancellous and cortical bones.
The computational analysis by finite elements shows great versatility in the analysis of complex models.This analytical method allowed identifying the homogeneity between different models with varied insertion torques that are difficult to obtain in an experimental study with physical models, as well as the same mechanical properties and dimensions for cancellous and cortical bones.The anisotropy found in the bone tissue was reproduced in the present study for the cortical and cancellous bones, being characterized by different stress responses under forces applied in varied directions (9).Although the results of the present study can add data to the implant/bone behavior influenced by the high values of the insertion torque, further animal and clinical investigation studies are needed to confirm these findings.
According to the methodology used and within the limitations of the present study, it may be concluded that high insertion torques can generate higher tensile and compressive stresses to the periimplant bone tissue.

Figure 1 .
Figure 1.Implant dimensions (A) and model dimensions with cortical and cancellous bones (B) expressed in millimeters.

Figure 2 .
Figure 2. Torque applied to implant on implant external hexagon.

Table 1 .
Material properties used in anisotropic model.Material axes correspond to global coordinate system shown in figure 1.E=Young's modulus.G=shear modulus.ν ij =Poisson's ratio for strain in j-direction when loaded in i-direction.