Effects of New Modification in the Design of the Attachments Retaining Distal Extension Partial Denture on Stress Distribution Around the Abutments And Residual Ridges: An In Vitro Study

Mahabad Mahmud Saleh; Dhia Aldori

doi:10.4103/denthyp.denthyp_38_20

ORIGINAL RESEARCH

Year : 2020 | Volume : 11 | Issue : 4 | Page : 112-120

Effects of New Modification in the Design of the Attachments Retaining Distal Extension Partial Denture on Stress Distribution Around the Abutments And Residual Ridges: An In Vitro Study

Mahabad Mahmud Saleh¹, Dhia Aldori²
¹ Department of Prosthodontics, College of Dentistry, Hawler Medical University, Erbil, Iraq
² Department of Prosthodontics, College of Dentistry, Al Kitab University, Altun Kupri, Kirkuk, Iraq

Date of Submission	11-Apr-2020
Date of Decision	02-Jun-2020
Date of Acceptance	08-Jul-2020
Date of Web Publication	18-Nov-2020

Correspondence Address:
Mahabad Mahmud Saleh
Department of Prosthodontics, College of Dentistry, Hawler Medical University, Erbil,
Iraq

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/denthyp.denthyp_38_20

Abstract

Introduction: The aim of this in vitro study was to evaluate the effect of modification of the attachment on stress transmitted to the abutment tooth and residual ridges in lower unilateral distal extension partial dentures. Materials and Methods: An acrylic lower unilateral distal extension cast with the first premolar as the main abutment was constructed. Three types of commonly used extracoronal castable attachments were selected, namely: (1) Preci-vertix standard, CEKA attachment, (2) Preci-sagix mini size, CEKA attachment, (3) OT–cap normal, Rhein 83. They underwent a simple new modification and their effect on stress distribution was studied. Six attachment retained removable partial dentures were constructed: among them, three with nonmodified attachments, and three with modified attachments. Four strain gauges were installed on the acrylic cast to measure the microstrain induced around the abutment tooth and residual ridges. A unilateral static vertical load of 300 Newton was applied on the first premolar and the first molar at a crosshead speed of 2 mm/min and microstrain was recorded using specific miniature Universal Testing Machine. Data were collected and analyzed using the Wilcoxon test for comparison between attachments before and after modification. Results: The highest microstrain was recorded for modified OT cap and modified Preci-sagix attachment around the abutment tooth and residual ridges respectively. While modified PV attachment showed the lowest microstrain around abutment and residual ridges. Conclusion: Maximum strain induced around the tooth and residual ridges in cases of OT cap and Preci-sagix attachments. Among all attachments, the use of Preci-vertix showed better stress distribution around both abutment and residual ridges.

Keywords: Attachment, distal extension, microstrain, strain gauge, stress

How to cite this article:
Saleh MM, Aldori D. Effects of New Modification in the Design of the Attachments Retaining Distal Extension Partial Denture on Stress Distribution Around the Abutments And Residual Ridges: An In Vitro Study. Dent Hypotheses 2020;11:112-20

How to cite this URL:
Saleh MM, Aldori D. Effects of New Modification in the Design of the Attachments Retaining Distal Extension Partial Denture on Stress Distribution Around the Abutments And Residual Ridges: An In Vitro Study. Dent Hypotheses [serial online] 2020 [cited 2023 Jun 2];11:112-20. Available from: http://www.dentalhypotheses.com/text.asp?2020/11/4/112/300865

Introduction

Loss of posterior teeth can reduce oral function and affect nutritional status, prosthetic treatment options for partially edentulous patients include fixed and removable dental prosthesis.^[1] A well-constructed removable partial denture (RPD) can be an adequate treatment option for the partially edentulous patient.^[2] Cases classified as Kennedy class II are among the most challenging cases^[3] for RPDs. The restoration of free end saddle requires planning based on biomechanical principles.^[4] Therefore, the selection of a suitable retainer is an important factor for a long-term successful restoration.^[5]

Extracoronal attachment direct retainers are mechanical devices that stay completely outside the normal clinical contours of the abutments. They provide a rigid, movable, or resilient connection between abutments and an RPD.^[6] The proper use of attachments provides satisfactory retention^[7] and stability to the prosthesis, thus eliminating the need for facial clasp arms. The result will be more esthetic, thus increasing the psychological acceptance of the denture.^[3],[6]

Numerous stress analysis techniques assessed the occlusion forces transmitted to the oral tissues, such as photoelasticity, two-dimensional 2D or three-dimensional 3D finite element analysis, and strain gauge analysis.^[8] The main advantage of the three-dimensional technology is that it provides a visual feedback of the treatment progress.^[9]

A strain gauge is a tool designed to measure the strain of an entity.^{[3],[4],[8],[10]} Strain gauge evaluation is a method for measuring microstrains, which consist of the use of electrical resistance. Strain gauges are primarily based on the concept that certain materials have different electrical resistivities when subjected to a force. The electrical resistivity can be measured at the site where the strain gauge is attached, using a Wheatstone’s bridge circuit.^[11] This method has been proposed to evaluate strains in implant-supported prostheses in vitro, in vivo^[12] and under static and/or dynamic forces.^[11]

The aim of this in vitro study was to evaluate the strain induced to the abutment tooth and residual ridges under occlusal vertical loading in lower unilateral distal extension cast (Kennedy class II) retained by different attachments using strain gauges technology.

Materials and Methods

Strain gauges technology was used to assess and compare the amount of stress transmitted to the abutment tooth and underlying residual ridges.

Fabrication of lower acrylic model

Commercially available rubber maxillary and lower models (Nissin Dental Products Inc, Kyoto Japan) with acrylic teeth were used. This model contained anatomically shaped teeth with roots that can be inserted and removed from the model.^[4] Lower second premolar, first, and second molars were removed unilaterally for construction of a class II Kennedy cast. The area of the root sockets was covered with adhesive tape to prevent the entrance of the duplicating material into the empty sockets. Silicon duplicating materials were used for duplication of the rubber model. After the setting of silicon, the rubber model was removed from the silicon mold. Remaining teeth were removed from the rubber model and their roots were covered with 0.3 mm thickness aluminum tin foil material to simulate the dimensions of the periodontal ligaments and then inserted in their positions in the silicon mold. Acrylic resin material (vertex, self-polymerizing pour denture base material, curing time, 30 minutes at 55°C and 2.5 bar) was mixed and poured into the silicon mold. After complete polymerization, the acrylic resin cast was removed from the silicon mold. The teeth with tin foil surrounding the roots of the teeth were removed from the acrylic cast. The tin foil was removed, and the sockets and roots were cleaned from remnants and returned to their positions on the acrylic cast. Light body silicon rubber base impression material was mixed and inserted in the sockets of the teeth and teeth were repositioned in their sockets inside the light body material and pressed till it set.^[4],[13] [Figure 1] showed the process for making the acrylic cast.

Figure 1 A. silicon mold with remaining teeth wrapped with tin foil. B. Acrylic cast after cleaning and reshaping of the ridge. C. Acrylic cast with light body silicon around the roots of the teeth.

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Design and construction of prosthesis

The acrylic cast was restored with attachment-retained RPD (aRPD) instead of conventional clasp retained RPD. For this purpose, an abutment of first premolar was prepared to receive a full metal ceramic crown and unilateral conventional Co-Cr RPD for restoring the edentulous area. Two parts were connected through an extracoronal attachment. Three different types of extracoronal castable attachments were used in this study, as shown in [Figure 2]: (1) Preci-vertix (PV) standard, CEKA attachment, (2) Preci-sagix (PS) mini-size, CEKA attachment, (3) OT–cap (OT) normal, Rhein 83. Each attachment underwent simple modification and six aRPD were constructed, three with nonmodified attachments remaining with modified attachments.

Figure 2 Attachments used in this study: A. Preci-vertix, B. Preci-sagix and C. OT-cap.

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Modifications of attachments

For all attachments, a modification was done on the male part which is casted in a hard dental alloy. The Preci-vertix (PV) attachment consisted of two pieces; (1) PV plastic female part (yellow) which was placed within the RPD, (2) PV plastic male (blue) part which was cast in hard dental alloy and attached to the crown of the abutment tooth, as shown in [Figure 2]. In the nonmodified attachment, the male part was cast parallel to the long axis of the abutment tooth and was vertically oriented. In the modified attachment, the male part was placed parallel to the residual ridge and was horizontally oriented, as shown in [Figure 3]. This modification was done in order to optimize the retention by increasing the surface area of the attachment; at the same time, this modification reduced the amount of stresses transmitted to the underlying supporting structures.

Figure 3 A. attachment retained RPD with non-modified Preci-vertix (NPV). B. Attachment retained RPD made with modified one.

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The Preci-sagix (PS) attachment was composed of two pieces: (1) PS female part (yellow) was fixed inside the RPD, (2) spherical hard dental alloy male part was attached to the crown of the abutment tooth. The RHEIN 83 OT cap is an extracoronal attachment used at the distal end of the abutments and cast with the crowns. The male component design is a sphere with a flat head (green), and female component is retentive nylon caps (pink) which are colour-coded according to different retentive properties [Figure 2].^[3] The same modifications were done for the male parts of both PS and OT attachments. A round disk with 0.4 mm thickness was used to prepare a slot in the spherical plastic male part of both attachments. The slot was created in the center of the spherical male part. [Figure 4] showed the phases of construction with modifications for both attachments.

Figure 4 A. attachment retained RPD (aRPD) with non-modified Preci-sagix (NPS) attachment. B. The modification done on Preci-sagix attachment. C. aRPD with non-modified OT-cap (NOT) attachment. D. The modification done on OT-cap attachment.

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Simulation of mucosa covering the residual ridge

According to previous studies,^{[2],[13],[14]} light body rubber base impression material was used to simulate the oral mucosa covering the residual ridges. For this purpose, the residual ridge area was reduced by about two mm. Then a baseplate wax of two mm thickness was adapted as a spacer on the residual ridge area covering the retromolar pad. The stone index was poured and extended buccally and lingually on the cast to act as a stopper for correct repositioning. The wax spacer was then replaced by light body silicon impression material to mimic the oral mucosa covering the residual ridge areas.

After fabrication of the stone index, a final impression was made using silicone rubber base impression materials. The impression was sent to the dental laboratory and instructions were given for the construction of the aRPD. A total of six aRPD were constructed from this impression according to the design mentioned.

Installation of the strain gauges

Four strain gauges (Hottinger Baldwin Messtechnik GMBH, gauge type 1.5/120LG11, gauge resistance 120 ± 0.2 %, gauge factor 1.97 ± 0.5 %) were installed on the prepared acrylic resin cast after preparing the stone index and recording the final impression. The wax spacer was removed, and the surface of the acrylic model was cleaned and prepared for fixation of the strain gauges. A total of four strain gauges were fixed on the acrylic cast as shown in [Figure 5]. Two strain gauges were fixed to the surface of acrylic resin at buccal and lingual aspects of the first premolar abutment parallel to its long axis^[15] and labeled as SG-1 and SG-2 respectively. A third strain gauge was installed on the buccal aspects of the residual ridge in the area of the lower first molar (SG-3), and the last strain gauge was mounted on the residual ridge beneath the central fossa of lower first molar (SG-4).

Figure 5 A. Atrain gauges were installed on the selected area on the acrylic model. B. Miniature universal testing machine. C. Unilateral vertical static load of 300 N applied on the distal fossa of first premolar.

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Areas covering the strain gauge should be prepared correctly according to the standard of strain gauge installation. Acrylic bur was used for flattening the selected areas first, and then fine sandpaper used for smoothing. The areas were scrubbed with clean paper moistened with acetone several times until it was completely cleaned. Epoxy resin glue was applied to fix the strain gauge to the selected surface of the acrylic cast. The tail wire of the gauge was grasped by tweezers to avoid any damage to the instrument. The gauge was placed on an adhesive tape with its sensitive part facing up and fixed to the selected area on the cast with resin glue. The adhesive tape was removed after the glue was completely set. The same procedure was performed for the fixation of all strain gauges. The wiring procedure was completed with 0.5 mm thickness tinned copper-based wires. The wiring accuracy of the strain gauges was measured with a multimeter (MultiMate). Then, the strain gauges wires were soldered to the multichannel strain meter (Keithley 236, US) which electrically amplified the small signals of the strain gauge and converted them into a voltage output. The voltage output then converted to microstrain through the following formula:^[2]

με = microstrain

V1 = unstrained voltage

V2 = strained voltage

The voltage output was recorded with an interactive characterization software (ICS), version 3.5.0

Load application and strain measurement

Strain gauges were calibrated to assess the repeatability of force measurements.^[16] A cyclic load ranging from 20 to 80 Newton (N) was applied on the occlusal surface of the lower denture using a loading device instrument to age the strain gauges. The load was applied three times in ten N steps. The loading device used in this study was a miniature universal testing machine specifically made for this study; its accuracy was 0.1 N [Figure 5].

The acrylic cast was secured on the universal testing machine with the occlusal plane placed parallel to the floor. A miniature universal testing machine was used to apply a unilateral vertical static load of 300 N for each prosthesis.^{[10],[16],[17],[18]} The load was applied in compression mode at a cross head speed of two mm/min.

Light body silicon impression material was mixed and poured into the empty sockets of the remaining teeth. Then the teeth were repositioned. The light body around the roots was used to simulate the PDL in natural dentition. Concerning the mucosa, the stone index was filled with light body silicon impression material and was put over the residual ridge area. Pressure was applied to provide close contact between the ridge and the index. This light body was used to simulate the oral mucosa. All the samples were positioned on the acrylic resin cast for strain gauge study. The load was applied in two points on the distal occlusal fossa of the lower first premolar and central occlusal fossa of the lower first molar [Figure 5]. In order to prevent any movement, the point of load application on the teeth surfaces was notched with a bur to accommodate the tip of the loading pin. Measurements for each denture were repeated five times and between each measurement, they were given a minimum of five minutes for heat dissipation.^[10],[19]

Data analysis

Statistical analysis was performed with SPSS (statistical package for social science) version 26 (SPSS Inc., Chicago, IL, USA). Before analysis Shapiro-Wilk test was used to test the normality of the recorded data. Therefore, the Wilcoxon signed rank test was performed for comparison between nonmodified and modified groups.

If p-Value of ≥ 0.05 indicated a not significant difference (NS).
If p-Value of < 0.05 indicated a significant difference (S).
If p-Value of ≤0.01 indicated a highly significant difference (HS).

Results

Under unilateral premolar loading conditions, the mean microstrain recorded for all SG demonstrated in [Figure 6]. As the point of load application closer to the strain gauges, they demonstrated greater strain. It was obvious for SG-1 and SG-2 that measured greater strain compared to the SG-3 and SG-4. The recorded microstrain was mainly negative which is compressive in nature. Maximum compressive microstrain was recorded by SG-1 for the sample with modified OT attachment of (-2165.28) and maximum tensile microstrain was recorded by SG-4 for the sample with modified PS attachment of (154.31). The lowest compressive microstrain was (−16.24) recorded by SG-3 for RPD retained by the modified PV attachment. Among all groups, the PV attachment showed a reduction of microstrain after modification. Among all SG, the greater microstrain was recorded by SG-1 which is positioned opposite to the root of the first premolar abutment on the buccal side; on the other side, SG-3 had the lowest microstrain.

Figure 6 Mean micro-strain recorded by different strain gauges (SG) for all samples under premolar loading.

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The mean microstrain recorded by four SG for all samples under unilateral molar loading conditions demonstrated on [Figure 7]. Under unilateral molar loading, the microstrain recorded by SG-1 and SG-3 were mainly compressive (negative) strain, while SG-2 and SG-4 demonstrated tensile (positive) strain. The RPDs retained with PV attachments showed lower microstrain in all SG; the only exception was SG-4, which demonstrated greater strain as compared with the other groups.

Figure 7 Mean micro-strain recorded by different strain gauges (SG) for all samples under molar loading.

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The Wilcoxon signed rank test was used for comparing the amount of microstrain recorded around the abutment tooth and residual ridges of different samples. For this purpose, we considered that SG-1 and SG-2 recorded the microstrain around the abutment tooth and SG-3 and SG-4 recorded the microstrain around the residual ridges.

[Table 1] demonstrated the microstrain comparison around the abutment tooth under premolar and molar loading conditions. The microstrain recorded around the abutment tooth for all samples were compressive (negative) in nature. It revealed that statistically there was no significant differences of recorded microstrain around the abutment tooth for all samples before and after modifications. Except for the RPD retained with PS attachment which showed significant reduction of compressive microstrain after modification from (−471.07) to (−363.45) under molar loading conditions.

Table 1 Statistical results comparing the micro-strain recorded around the abutment tooth

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The highest microstrain around the abutment tooth was recorded for modified OT attachment of (−1027.41) and nonmodified PS of (−1104.57) under premolar loading. While the lowest microstrain was recorded for modified PV attachment of (−144.16) and nonmodified PV attachment of (−154.31) under molar loading conditions. For all groups the microstrain recorded around the abutment tooth were lower under molar loading conditions than premolar.

[Table 2] demonstrated the microstrain comparison around the residual ridges of all samples under premolar and molar loading conditions. The microstrain recorded around the residual ridges for all samples was tensile (positive) in nature. The microstrain recorded under premolar loading conditions was lower than molar loading. The highest microstrain was recorded for modified PS of (427.61) and nonmodified OT attachment of (412.18) under molar loading conditions. While the lowest microstrain was recorded for modified PV attachment of (16.24) and nonmodified PS attachment of (32.49) under premolar loading conditions.

Table 2 Statistical results comparing the micro-strain recorded around the residual ridges

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Discussion

Distal extension bases have a problem of support due to the absence of the distal abutment. Rotation of the distal extension base in a tissue ward direction occurs during function under occlusal force. Due to the mismatch of the viscoelastic responses between mucosa and the teeth this rotation potential always exists. To protect the natural abutment from torqueing during function, resilient extracoronal attachment may be used for distal extension bases allowing for a limited degree of distal rotation. It also decreases stresses transmitted to the abutments as the forces are or near the long axis of the abutment.^[1]

The present study compared different attachment designs retaining distal extension RPDs. Static vertical load of 300 N was applied unilaterally on the first premolar and first molar areas. Strain gauges fixed on the model recorded the microstrain under different loading conditions around abutment tooth and residual ridges. The stresses transmitted to the abutment tooth (first premolar) and residual ridge by aRPD were evaluated before and after modifications of the attachments. To the authors knowledge no available studies were found using the exact experimental protocol, so it is presumed indirect comparison may be possible.

Under unilateral premolar and molar loading conditions, the microstrain recorded around the abutment teeth were compressive in nature and the microstrain recorded around the residual ridges were mainly tensile in nature. The presence of the rigid connection between the abutment teeth and the RPD provided by the attachments could influence the obtained stress pattern, as well as the resiliency and morphology of the underlying soft tissues distally. This outcome supported by an in vitro study that was done by Elsyad et al.^[16] who analyzed the strains around distally inclined implants used for retaining lower overdentures with locator attachments. They found that all mesial peri-implant sites experienced compressive (negative) strains, while distal implant sites showed tensile (positive) strains. They stated that obtained stress patterns may be due to the morphology of the residual ridge that had an upward slope towards the ramus. When the vertical load was applied to the first molar area it may lead to a mesial shift of the denture base.^[19]

Sometimes bacteria associated with periodontitis are found around the implants, even in the absence of obvious signs of inflammation. The presence of such bacterial species has also been linked with an increasing number of diseases affecting other organs beyond the mouth.^[20] Certainly, both the abutment material, and the surface microtopography has been shown to influence soft and hard peri-implant tissue response. A modified prosthetic abutment surface seems to promote the creation of a more strong collagen fiber attachment to the abutment, and it is thought to play an important role in cellular reactions, tissue healing, and implant stability.^[21]

In the present study, the microstrain recorded around the abutment teeth were much higher than the microstrain recorded around the residual ridges. Those findings are in agreement with the study of Dahab et al.^[4] They studied the effect of different prosthetic treatments for rehabilitating the lower distal extension saddle on stress distribution. They found that the abutment tooth received a higher amount of load inside the RPD plate. Many authors confirmed that unilateral distal extension RPD exhibited a considerable amount of displacement during function which increases stresses on tooth abutment.^[4]

Upon unilateral loading conditions, the stresses transmitted to the abutment teeth were increased up on premolar loading, as the point of load application was closed to the abutment teeth. Similarly stresses transmitted to the residual ridges were increased as the point of load application nearest to the residual ridges in the molar region. The results are consistent with the study of Elsayed, who investigated the stress analysis around distal extension cases rehabilitated by two different treatments. He found that during load application on the second molar, more stresses were transmitted to the residual ridge.^[13] For all groups of the study under premolar and molar loading conditions, the strain gauges on the buccal surface of the abutment tooth were associated with the greater strain. This finding was supported by the study done by Mohamed and his co-workers. They did an in vitro study evaluating the effect of unilateral lower implant-tooth supported telescopic prosthesis on the supporting structures. They found that during force application on the loading side denture shifted in the buccal direction because it had a lesser resistance to lateral movement. “The lateral movements cause a compressive strain to the upper edges of the buccal cortical bone plate, which had the potential tendency to be displaced outward in the horizontal plane.”^[10]

Among all samples, RPD retained with modified OT attachment demonstrated the highest microstrain that recorded around the abutment teeth while modified PS attachment showed the greatest microstrain recorded around the residual ridges. Between all attachment groups, modified PV attachment showed the lowest microstrain recorded around the abutment teeth and on the residual ridges and therefore demonstrated the better distribution of strain transmitted to the abutments and residual ridges.

Conclusion

Under the limitation of this study the following conclusions were drawn:

As the point of load application was closer to the strain gauges, the strain recorded was increased.
Under premolar loading conditions RPD retained by modified PV attachment demonstrated better stress distribution.
For all groups of the study microstrain recorded around the abutment tooth were lower under molar loading conditions than premolar.
Maximum strain induced around tooth and residual ridges in cases of RPD retained by OT cap and PS attachments.
Among all groups in this study, RPD retained by PV attachment demonstrated better distribution of microstrain around both abutment and residual ridges.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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