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 Table of Contents  
ORIGINAL ARTICLE
Year : 2020  |  Volume : 17  |  Issue : 2  |  Page : 73-77

Stress analysis of short implants with different diameters in maxillary bilateral distal extension bases


Department of Prosthodontics, Faculty of Dentistry, Tanta University, Tanta, Egypt

Date of Submission30-Nov-2019
Date of Acceptance01-Jun-2020
Date of Web Publication26-Sep-2020

Correspondence Address:
MSc Amr A Azab
Prosthodontic Instructor, Faculty of Dentistry, Tanta University, Taha al-Hakim/Tanta Gharbiya, Tanta 31512
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tdj.tdj_55_19

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  Abstract 

Purpose
The aim of this study was to evaluate stress distribution around short implants with different diameters in maxillary bilateral distal extension bases.
Materials and methods
Strain gauge technology was used for strain analysis for two different diameters of short implant. On the seven epoxy resin models, two different short implant diameters were used as follow: Group A: on one side of the model, two implants with 4.8 mm diameter and 5 mm length were used. One implant was placed in the second premolar region and the other was placed in the second molar region. For group B: implants with 6.2 mm diameter and 5 mm length were used. For both groups, ceramometal screw retained fixed partial dentures were constructed on the transmucosal abutments. Stresses were measured at the buccal and lingual side of each implant under 200 N vertical load and 40 N oblique load. Data were collected, tabulated, and statistically analyzed.
Results
The present study revealed that minimum stresses were induced on vertical and oblique loads around the short implants that had wider diameters.
Conclusion
Within the limitation of this study, it could be concluded that, using short implants with wider diameter is a reliable method for restoring the distal extension bases.

Keywords: distal extension bases, implant diameter, short implant, strain gauge, stress analysis


How to cite this article:
Azab AA, El-Sheikh AM, Abd-Allah SM. Stress analysis of short implants with different diameters in maxillary bilateral distal extension bases. Tanta Dent J 2020;17:73-7

How to cite this URL:
Azab AA, El-Sheikh AM, Abd-Allah SM. Stress analysis of short implants with different diameters in maxillary bilateral distal extension bases. Tanta Dent J [serial online] 2020 [cited 2020 Oct 31];17:73-7. Available from: http://www.tmj.eg.net/text.asp?2020/17/2/73/296180


  Introduction Top


Posterior regions of the dental arches generally have less available bone height, poorer bone quality, while, at the same time, teeth in this region are exposed to greater occlusal loads than the anterior regions of the mouth [1].

Over the years, various strategies have been proposed to overcome the dimensional limitations of the bone available for implant placement. Several surgical interventions for bone augmentation have been proposed, including bone grafts, guided bone regeneration, distraction osteogenesis, sinus floor elevation, mandibular nerve transposition, and the use of tilted or zygomatic implants. Although these techniques have gained a degree of success through the years, with the exception of sinus floor elevation, there are insufficient data on their predictability [2].

Short dental implants are a more cost-effective alternative that reduces treatment time and rules out complications related to surgical and grafting procedures. Authors in their studies have quoted different lengths, however considering 10 mm as the standard length; an implant less than 10 mm in length is considered a short dental implant and is usually applied in alveolar ridges with decreased bone height [3].

The biomechanical basics for the use of short implants is that the crestal portion of the implant body is the most involved in supporting the load, while less stress is transmitted through the apical portion [4].

Moreover, the maximum bone stress is practically independent of the length of the implant, as arguably the width of the implant is more important than the length [5].

Despite all of the benefits of short implants, the one overwhelming reason why most clinicians are hesitant to use short implants is the fear that reduced implant length may overload the surrounding bone and lead to implant failure [6].

Petrie and Williams [7] showed that crestal bone stresses were dependent upon three interrelated parameters: diameter, length, and taper. According to them, diameter had the single most influence on crestal bone stresses.

The role of wide-diameter implants becomes more critical when the length of the implant is limited by the existence of alveolar nerve in the mandible and maxillary sinus in the maxilla [8].

Considering the need for additional researches to evaluate stress distribution with short implants and its association with its diameter, the aim of this study is to evaluate the influence of using different implant diameters on stress distribution around short implant in bilateral distal extension bases.


  Materials and Methods Top


Seven readymade maxillary Kennedy Class I epoxy resin models were used and the missing teeth were the second premolar, the first molar and the second molar in both sides of each model with the area representing the ridge and the vault covered by 2 mm of flexible polyurethane layer simulating oral mucosa research approval from the ethical committee for Faculty of Dentistry, Tanta University, were obtained.

Short implants that used, were had the same lengths which were 5 mm and had two different diameters 4.8 and 6.2 mm:

  1. Group A: implants with diameter 4.8 and 5 mm length.
  2. Group B: implants with diameter 6.2 and 5 mm length.


Implant installation

For each model, two implants 5 mm length with 4.8 mm diameter were placed in one side and on the other side of the same model, two implants 5 mm length with 6.2 mm diameter were placed.

The drilling of the implant site was done starting with the pilot drill and ending with final drill.

Firstly, a hole was carried out bilaterally in a point which lies 5 mm from the distal surface of the first premolar to leave a proper space between the implant and the first premolar. These points allow positioning of both implant for the second premolar in the same plane and another hole was carried out in the second molar region bilaterally in a point lies 15 mm from that of the second premolar.

Implants 4.8 mm diameter and 5 mm length for a side of each model and implant 6.2 mm diameter and 5 mm for the other side of each model were inserted in the prepared site and rotated clockwise until the plastic mount meeting resistance and achieved reasonable fixation of implant in the prepared sites, the plastic mount removed by gentle shaking motion upwards, the driver was inserted into the implant and the wrench connected to the head of the driver to complete implant insertion till the implant become flushed with the epoxy resin model.

Finally, the abutments tighted properly in their places. Theseabutments were for screw retained superstructure and with the same length [Figure 1].
Figure 1: The Model with the implants and the abutments.

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Superstructure construction

Screw retained fixed ceramometal partial denture had fabricated directly on the abutments using recognized rules of prosthodontics [9].

After complete insertion of the implants and their abutments, the burn-out caps are screwed on the abutments and the wax pattern of the partial denture had done directly on these caps and adjusted symmetrically on both sides then invested into cobalt chromium alloy. Finally, the ceramic facings were completed and finished with had rounded cusp tips and small cusp angle according to the principles of occlusion concepts in implant.

Strain gauge installation

Small channels (2 mm in diameter and 4 mm in length) were prepared in the epoxy model at the buccal and lingual surfaces of each implant to receive the strain gauge rosettes.

The channels were parallel to the long axis of the implants with depth to leave just 1 mm thickness of epoxy between the strain gauge rosettes and the implant. The channels will be prepared with flat walls especially the walls parallel to the implant on which the strain gauges will be mounted.

Every one of the holes was installed with a strain gauge in the epoxy resin on the surface which was toward the implant in the buccal and palatal surfaces to measure the microstrains in the medium surrounding the implant [Figure 2].
Figure 2: The model with strain gauge.

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Measurements of the microstrains transmitted to the model

Each model placed on the base of the loading device of fully digitalized testing machine (LLOYD instrument) and the post of the universal testing machine is directed to the central fossae of the pontic replacing the first molar [Figure 3]:
Figure 3: Universal testing machine with the model.

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  1. The vertical load measurement: every model was placed with the partial denture in its place in a horizontal plane of the base of the loading device base and static 200 N vertical load was applied [7],[10].
  2. The oblique load measurement: every model were placed with the partial denture in its place on the surface of an oblique wooden segment which made angle equal 45° with the applied load [11] and bilateral static 40 N oblique load was applied [7].


The mean of the values recorded from the strain gauges around the implants were used for stress evaluation using 200 N vertical load and 40 N oblique load.


  Results Top


The data of this study was collected and statistically analyzed. The test of significant had done using unpaired t test (P ≤ 0.05).

[Table 1] and [Table 2] showed the range, mean, and the SD of the microstrains around the implants in the buccal and palatal surfaces under vertical and oblique loading in group A and group B.
Table 1: Range, mean and the SD of the microstrains around the implants in group A and group B under vertical and oblique loading

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Table 2: Range, mean and the SD of the microstrains around the implants under vertical and oblique loading in group A and group B

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The mean microstrain that recorded under vertical load at the palatal surface of group A showed higher value than that of group B by 5.398 which was statistically significant.

The mean microstrains that recorded under vertical load at the buccal surfaces of group A showed higher value than that of group B by 7.445 which was statistically significant (P ≤ 0.05).

The palatal surface of group A showed significantly (P ≤ 0.05) higher value than that of group B under oblique load but in comparing the microstrains recorded in the buccal surfaces of both groups the difference was 0.124 which was not statistically significant (P > 0.05).

In comparing group A with group B: the mean microstrain that recorded from the oblique load of 40 N were greater in group A by 2.191 than in group B. P value was 0.036*. The difference was statistically significant (P ≤ 0.05).

Also, in comparing group A with group B: the mean microstrain that recorded from the vertical load of 200 N were greater in group A by 2.216 than in group B. P value was 0.038*. The difference was statistically significant (P ≤ 0.05).


  Discussion Top


In this study, the model was prepared from epoxy resin material which has an appropriate elastic modulus for a bone analog material (~20 grade point average) [12].

Installation of strain gauges was done in prepared flat surfaces in the epoxy resin parallel to the long axis of the implant and perpendicular to the crest of the ridge buccally and palatally instead of placing it directly on the implant surface because it is preferred to bond the strain gauge on completely flat surface to minimize the possibility of obtaining incremental apparent strain that result from mounting the strain gauge on curved surface [13],[14].

The load applicator used by the universal testing machine was adjusted to ensure fixed, accurate and definite loading points of contact over the center of the central fossae of the pontic which replacing the first molar occlusal surface to reproduce the masticatory area [15].

The abutment that used were transmucosal which allow formation of a thick protective mucosal-conjunction tissue joint [16] and screw retained for superstructure as the main advantage of screw-retained restorations is the predictable retrievability that can be achieved without damaging the restoration or fixture. Therefore, the prosthodontic components can be adjusted, the screws can be refastened, and the fractured components can be repaired with less time and at lower cost [17].

The loads were used in this study equal 200 N vertically and 40 N obliquely because the 200 N vertical load is the average force exerted from three units fixed partial denture and the value of the accompanying oblique force (45° inclined load) was calculated at 40 N and this agreed with Petrie and Williams [7] and Morneburg and Pröschel [10]. For the accuracy of the results, an interval of at least 5 min was elapsed between each successive loading to give a chance for heat dissipation from the strain gauge sensors and for rebounding of the deformation that occurred in the epoxy resin following load application [18].

According to Vasconcellos et al.[19] when an occlusal load is applied on an implant supported prostheses, the load is partially transferred to bone, with the highest stress occurring in the peri-implant area. Biomechanical studies showed that the occlusal forces are distributed primarily to the crestal bone and mostly in the first 4–6 mm [20].

The results of this study for group A and group B show that the palatal strain readings around both implants for each group were higher than buccal readings these results agree with the study of Qian et al.[21]. Their study was the interactions of implant diameter, insertion depth, and loading angle on stress/strain fields in a three-dimensional finite element implant/jawbone system and to determine the influence of the loading angle on stress/strain fields while varying the implant diameter and insertion depth. The results showed that the maximum stress in the bone always occurred at the upper edge of the cortical bone on the lingual side adjacent to the implant.

The results of group A and group B were within the normal range of strains according to the mechanical theory stated by Frost [22] and Himmlova et al.[23] which stated that bone strain above 3000 microstrains may be unfavorable for the bone leading to a hypertrophic response and bone strain above 4000 microstrains may cause overloading followed by bone loss.

The results of group A and group B agree with the study of Morneburg and Proschel [10]. Their study was two exchangeable implant abutments were equipped with strain gauges. In nine patients, the abutments were attached to implants supporting three-unit fixed partial dentures in one mandibular chewing center. The signals of the two abutments were summed to give a force reading that was independent of the location of force impact along the fixed partial dentures.

When comparing the induced microstrains with group A and group B under 200 N vertical load and 40 N oblique load were statistically analyzed and significant differences were found and this agreed with the study of Petrie and Williams [7] and Kong et al.[24] that concluded that the increase in width of the implant may decrease stress by increasing the surface area, which may also reduce the length requirement. The results also of this study agreed with Misch [25] that claimed implant diameter was more important than length once a minimum was reached, but the results disagreed with the study of Malo et al.[26] and Winkler et al.[27] which believed that the implant width was less important compared with implant length in functional loading.

The known advantages of using wide-diameter implants include providing more bone to implant contact, bicortical engagement, and immediate placement in failure sites and reduction in abutment stresses and strain. Therefore, more contact area provides increased initial stability and reduces the stresses. Improved implant strength and resistance to fracture can be attained by increasing the diameter of implant [28].


  Conclusion Top


  1. Short implants are a reliable treatment option to avoid sinus augmentation procedures and replace missing teeth in the posterior maxilla.
  2. The use of short implants reduces the need for complex surgeries. Therefore, they reduce the morbidity, treatment time, and financial cost.
  3. Regarding to the microstrains recorded on both diameters of short implant on vertical and oblique loads, using wider short implants is better as less microsrains were recorded.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Figures

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    Tables

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