INTRODUCTION

Anchorage is one of the main factors for determining the success of orthodontic treatment. Headgears and Nance appliances are routinely used to establish anchorage during clinical treatment. However, many patients reject headgear wear because of social and esthetic concerns, and the success of this treatment greatly depends on patient cooperation.1 In most of the studies on Nance appliances, anchorage loss was unavoidable, and reduced hygiene under the acrylic resin button led to inflammation of soft tissues.23

Advances in implants in general dentistry have made it possible to use them as a means of anchorage in adult orthodontic patients. Some studies45 have shown that dental implants placed in the alveolar bone were resistant to the orthodontic force. However, there is no available site for implant placement because orthodontic patients generally have complete dentition. Thus, some other anatomic locations such as the palatal region have been used as alternative sites.

Block and Hoffman6 introduced a subperiosteal disc of 10 mm diameter. Keles et al7 used a screw-type implant with a height of eight mm and a width of 4.5 mm, whereas Wehrbein et al89 introduced a small 3.3-mm-diameter implant with a low to medium four and six mm length. Almost all these studies showed that a palatal implant could offer sufficient anchorage effect.

However, these implants must be loaded after a period of approximately 12 to 24 weeks to allow healing and osseointegration, and waiting for osseointegration is inconvenient for both the orthodontist and patients.

Some studies1011 have shown the number of days from implantation to force application is not associated with stability of the implant and recommended immediate loading to the implant. The possibility of such immediate loading is attributed to the successful Mechanical interdigitation between the implant and the bone. However, it is questionable whether the implant loaded before osseointegration has the same anchorage effect as the implant loaded after osseointegration because the degree of implant osseointegration and direction of force applied have not been well documented in these studies.

It is generally accepted that anchorage is related to periodontal stresses;12 for an implant to modify anchorage, it must also modify the stress of the periodontal ligament (PDL) of the premolar connected with the palatal implant. Such modifications could result in the redistribution of stress values to a level below the physiologic threshold at which movement is believed to occur. However, it is virtually impossible to measure accurately periodontal stress distribution in vivo.

Finite element analysis (FEA) has become an increasingly useful tool for prediction of the effects of stress on the tissues pertinent to orthodontic force. FEA is a mathematical method in which the shape of complex geometric objects and their physical properties are computer constructed. Physical interactions of various components of the model are then calculated in terms of stress and strain.

The purpose of this study was to compare the anchorage effect of osseointegrated and nonosseointegrated palatal implants (NOPI), using FEA.

MATERIALS AND METHODS

Model

Two models were created in this study. Model 1 (Figure 1) was composed of two maxillary premolars, PDL, alveolar bone, a palatal implant, palatal bone, a bracket, band, and transpalatal arch (TPA). A maxillary second premolar was created by manually designing the tooth according to the dimensions and morphology found in a standard dental anatomy textbook.13 The outermost boundary of the tooth was defined two dimensionally at first, and sectioning the tooth into cross sections created the third dimension. Three-dimensional coordinates were input into the Unigraphics NX 1.0 (Unigraphic Solutions Inc, Cypress, Calif) to create a CAD model of the tooth. Next, the PDL, alveolar bone, palatal implant, palatal bone, bracket, band, and TPA were created. The bracket, band, and TPA were combined as one connected device to simulate a bracket and TPA welded to the band in the clinic. The PDL width was 0.25 mm, and the alveolar cortical bone was 1.0 mm. A cylinder implant was 3.3 mm in diameter and nine mm in length, and the abutment was three mm long. The subperiosteal part with a thread surface was six mm in length. The TPA was a 0.8 × 0.8–mm rectangular arch wire;89 the distance between the centers of the two premolars was 42.8 mm. The palatal bone had a cortical surface thickness of 2.0 mm for the oral-palatal cortical bone, a cancellous thickness of 5.5 mm, and a cortical surface of 1.0 mm in the direction of the nasal floor.14 Model 2 was the same as model 1 but had neither an implant nor palatal bone.

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Elements and nodes

Elements and nodes were created by Unigraphics NX volume mesher. Tetrahedral three-dimensional elements were used in this study. Four-node linear cells were used because they are good at meshing arbitrary geometries.15 Small elements of similar size were used to mesh the area of interest (PDL, implant) uniformly for the stress analysis (Table 1).

TABLE 1.  Nodes and Elements in the Study^a

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A tooth-band interface and implant-TPA interface were created as a fully bonded surface to simulate the cemented band and fixed contact between the TPA and the implant. Bone-implant interfaces were treated either as fully bonded (osseointegrated implant) or frictional (nonosseointegrated implant) surfaces. A fully bonded surface was achieved by creating common faces at the interface to simulate a condition where the bodies were “welded” or “glued” together, which ensured that the connectivity would be maintained at the interface.16

The frictional surface was modeled using nonlinear frictional contact elements, which allow minor displacements between the implant and the bone. The contact zone thus transferred pressure and tangential forces (ie, friction) but no tension. The surface texture of the implants was considered in the analyses by including a Coulomb frictional interface with a coefficient of 0.3. This frictional surface was proved consistent in in vivo data.17

Material properties

Each material was defined as homogenous and isotropic. The physical properties of the constituent materials comprising the model were based on a review of the literature151618 (Table 2).

TABLE 2.  Material Properties for the Constituent Materials

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RESULTS

Because there was no significant difference on stress magnitude and distribution between the right and left PDL in all models, the left premolar was extracted to compare PDL stress. Stress magnitudes were denoted by a series of colors, as shown in the spectrum display to the right of the plot. In all cases, the greatest point of interest lay in the stresses produced in the PDL in the direction of the force application.

Figure 2 shows PDL stress when the premolars were subjected to the five-N distomesial horizontal force. In each model, the highest stress was in the PDL at the cervical margin. The location of the maximum stress was not changed by the implant. The stress decreased toward the apex, and it was almost the same in the models with the implant. The maximum PDL stress (MPS) and average PDL stress (APS) were 607.67 and 151.72 kPa, respectively, in model 2. The stress was reduced more than 13% by the implant (Table 3). APS is the average stress of all nodes in the PDL (Unigraphic software can calculate average stress in a solid body).

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TABLE 3.  Maximum PDL Stress (MPS) and Average PDL Stress (APS)^a

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Figure 3 shows PDL stress when the premolars were subjected to the five-N buccolingual horizontal force. In each model, the highest stress was in the PDL at the cervical margin. The stress decreased sharply toward the apex, and again it was almost the same in the models with implant. MPS and APS for model 2 were 321.62 and 88.41 kPa, respectively. The APS was reduced about 60% by the implant (Table 3).

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Figure 4 shows PDL stress when the premolars were subjected to the five-N vertical intrusive force. In each model, the highest stress was in the PDL at the cervical margin. The stress decreased sharply toward the apex, and again it was almost the same in the models with the implant. MPS and APS for model 2 were 582.51 and 84.77 kPa, respectively. The stress was reduced about 16% by the implant (Table 3).

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Figure 5 shows stress in the implant-bone interface. The difference in the maximum stress in the NOPI and the osseointegrated palatal implant (OPI) was less than 5% for the horizontal force, whereas the maximum stress in NOPI was far higher than OPI for the vertical force. The average stress in NOPI was far higher than that in OPI (Table 4).

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TABLE 4.  Maximum Stress and Average Stress in the Implant

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DISCUSSION

The purpose of this investigation was to use a finite element analysis to compare the anchorage effect of palatal osseointegrated and nonosseointegrated implants, under a horizontal and vertical force. To accomplish this analysis, osseointegrated and nonosseointegated bone-implant interfaces were constructed separately in the same model. The same boundary condition was used for alveolar bone; the same size and type element were created for the same material; the same mesh refiner was performed in the same place until the percentage error of the resultant stress was lower than 5%, which is the widely accepted level of confidence for the stress percentage error.16 The resultant stress in a model without an implant was compared with stress produced in the model with implants.

The limitations of our model included an approximation in the material behavior and shapes of the tissues. As in previous studies,2123 the PDL was modeled as a 0.25 mm layer of uniform thickness and was treated as linear elastic and isotropic, although the PDL exhibits anisotropy and nonlinear viscoelastic behavior because of tissue fluids.24 The PDL value was selected because it agreed with human tooth movement.15 The tooth was simplified as a homogeneous body without tips because the force transmitted to the PDL was not significantly affected by adding the internal and external tooth structure.

Palatal-bone height (two mm in oral-palatal and one-mm nasal-palatal cortical bone, and 5.5-mm cancellous bone thickness) has been proved suitable for biomechanical investigations with regard to anatomical structures and deformation under stress.14 In this study, the bone size, 12 × 12 mm, was decided by trial runs. Modeling the palatal bone greater than 12 × 12 mm did not result in any significant change of the stress (5% as the criteria).

OPI assumed that a 100% implant-bone interface was established.21 However, the percentage of direct bone-to-implant contact varied from 34% to 93%, with an average value of 75.5%.9 A 100% bone apposition was almost never obtained at the surface of the dental implant.25 The boundary condition was assumed fixed at the base of the alveolar bone2021 and all nodes on the lateral edges of the palatal bone19 because there was no agreement for giving a boundary condition for bone segments.

In each model, the highest stress concentration in the PDL was localized at the cervical margin. This might be because of the fact that the orthodontic force was applied in the buccal bracket of each premolar. Because the line of force was not through the center of resistance of the tooth, the tooth exhibited a tipping movement. These findings were essentially consistent with previous studies.1826

The implant markedly reduced the stress of the PDL. In engineering terms, an implant acts like a bar elastically supported by the surrounding bone. The anchorage loads were transmitted from the tooth to the implant because of the rigid connection of the TPA. In this study, the differences in stress were less than 2% between the model with OPI and the model with NOPI, and the stress distribution was the same for the same direction of force. If one accepts the premise that orthodontic anchorage is based on periodontal stress and 5% as a criterion to indicate significance, then the results of this study suggest that osseointegration does not serve to enhance the anchorage effect.

Osseointegration can lead to lower stress in the implant-bone interface (Figure 5). This can be explained by the observation that the stress was withstood by more bone in OPI than that in NOPI. It is interesting to note that the MPS in the implant-bone interface is almost same for horizontal forces. This might be because of the local shape of the element. Although the stress in the NOPI surface is higher than that in the OPI surface, it is important to point out that both of these are of such low magnitude that they are unable to produce a failure in the implant.18 Therefore, the NOPI is able to function as adequate anchor units as is the OPI.

An orthodontic load is usually made up of continuous, horizontal forces of low value (from 20 or 40 g to a few hundred). The orthodontic implant is used only in the early stages of treatment. According to our results, it is reasonable to conclude that there is no need to wait for osseointegration. The anchorage effect of a palatal implant might be related more to the diameter and the length of the implant, the quality of the bone, and the rigidity of TPA rather than the degree of osseointegration.

Mellal et al17 compared osseointegrated and nonosseointegrated implants, using models which included only bone and implants; they also found that the stress in the nonosseointegrated implant surface was higher than that in the osseointegrated implant when the implant was subjected to the same force.

Histologic analyses2728 also found that nonosseointegration did not compromise the clinical stability of the implant during treatment. The authors also consider nonosseointegration favorable because it can facilitate surgical removal of the implant at the end of treatment.

Some points, such as the biomechanical nature of the osseointegrated lamellar bone and the process of bone remodeling, remain to be determined when using a palatal implant to provide anchorage. This study is only a primary one. In the future, additional accurate models should be created. However, this analysis does provide quantitative results of the complex three-dimensional stress caused by different orthodontic loads. It allows comparisons of the anchorage effect of OPI with NOPI primarily, and these are considered to be relevant in understanding the implant as an anchorage unit. It should be noted that this theoretical study, which has no empirical basis for clinical application, involved some assumptions; the findings may be changed if the assumptions were unrealistic. Therefore, the resulting values should be interpreted only as a reference to aid clinical judgment.

CONCLUSIONS

A model with a different implant-bone interface was compared with a model without an implant. According to the FEA, the implant significantly lowered the PDL stress. OPI and NOPI showed the same anchorage effect. It is suggested that waiting for osseointegration might be unnecessary for an orthodontic implant.