Tanta Dental Journal

ORIGINAL ARTICLE
Year
: 2018  |  Volume : 15  |  Issue : 2  |  Page : 63--69

Effect of position of single implant with two different attachments on stress distribution of mandibular complete overdenture (in-vitro study)


Merhan F El-Abd1, Mohamed M El-Sheikh2, Mohammed N El-Gendy2,  
1 Dentist at Ministry of Health, El Mahalla El Kubra, Egypt
2 Prosthodontics Department, Faculty of Dentistry, Tanta University, Tanta, Egypt

Correspondence Address:
Merhan F El-Abd
Dentist at Ministry of Health, Elgharbia, El Mahalla El Kubra
Egypt

Abstract

Objective The aim of this in-vitro study was to evaluate the effect of position of single implant with two different attachments on stress distribution of mandibular complete overdenture using electrical resistance strain gauge. Materials and methods One acrylic mandibular edentulous model was constructed with 2 mm thickness of artificial mucosa. Four finished denture were obtained one denture for each group. Group 1: single implant at midline region with ball attachment. Group 2: single implant at midline region with locator attachment. Group 3: single implant at left canine region with ball attachment. Group 4: single implant at left canine region with locator attachment. Four strain gauges were cemented at distal, buccal, mesial, lingual aspect of midline and canine single implant to monitor the effect of the applied 50 N static. Unilateral vertical and oblique loads at three different sites central fossa of right first molar, midline and central fossa of left first molar over each implant inserted in the acrylic model. Results Midline single implant with locator attachment (group 2) during vertical and oblique load had lower stresses than ball attachment (group 1), but when implant at canine area with locator attachment (group 4) during vertical and oblique load had lower stresses than ball attachment (group 3). Comparing between midline and canine implant the lowest stresses obtained with (group 2). Conclusion (a) The locator attachment showed reducing stress on implant body ability to distribute the load than ball attachment in midline and canine region. (b) Single implant at midline region showed less stress than implant at canine region with both attachments. (c) Vertical load applied over mandibular overdenture retained by single implant showed less stress than oblique force for both attachments. Less strain value were recorded with the locator attachment in the midline region.



How to cite this article:
El-Abd MF, El-Sheikh MM, El-Gendy MN. Effect of position of single implant with two different attachments on stress distribution of mandibular complete overdenture (in-vitro study).Tanta Dent J 2018;15:63-69


How to cite this URL:
El-Abd MF, El-Sheikh MM, El-Gendy MN. Effect of position of single implant with two different attachments on stress distribution of mandibular complete overdenture (in-vitro study). Tanta Dent J [serial online] 2018 [cited 2018 Jul 18 ];15:63-69
Available from: http://www.tmj.eg.net/text.asp?2018/15/2/63/235132


Full Text

 Introduction



Severe atrophy of the alveolar process and underlying basal bone often results in problems with a lower denture. These problems include insufficient retention of the lower denture, intolerance to loading by the mucosa, pain, difficulties with eating and speech, loss of soft tissue support, altered facial appearance and reduced quality of life in this specific patient group. These problems are a challenge for the prosthodontist and surgeon [1].

Overdenture is any removable dental prosthesis that covers and rests on one or more remaining natural teeth, the roots of natural teeth, and/or dental implants [2].

The greatest available height of bone is located in the anterior mandible, between the mental foramina or anterior loops of the mandibular canal when present. This region usually presents with optimal bone density for implant support [3].

Implant supported overdentures considered an accurate and predictable treatment option and achieve a higher patients satisfaction. This type of treatment considered a cheaper treatment than fixed prostheses and in some patients with loss of lip support or with an interocclusal space larger than 15 mm, the choice of implant supported overdentures prevent future aesthetic or phonetic problems [4].

The number of implants used with mandibular overdenture may be one, two, three, four or more individual implants. Even though 2–4 implants are preferable, some patient still cannot afford them due to economical and anatomical restrictions. For these cases Cordioli et al. [5] and Krennmair and Ulm [6] suggested use of an overdenture with a single implant at the midline area.

Mandibular overdenture retained by single midline implant may be an alternative to the customary two implant overdenture for maladaptive denture patient and considered is an economical therapeutic alternative to conventional mandibular complete dentures, especially for older patients [7],[8],[9].

Single implant is less expensive than multiple implants and can be used to retain dentures temporarily before placing additional implants. Also there are potential surgical advantages of midline implant placement as it allows simplified imagine and flap design without concerns for the position of mental foramen or possible postoperative parathesia related to direct or indirect damage to branches of the inferior alveolar nerve, on the other hand single implant had significant reduction for surgical time to place implants [10],[11],[12].

High retention force is an essential requirement for a successful attachment to prevent overdenture displacement. Retentive force is gained from mechanical and frictional contacts or from magnetic forces of attachment between the patrices and matrices of various attachment systems [13],[14].

The advantages of ball attachment are simplicity in design, ease of use and maintenance, low cost, varying degrees of retention, wide range of movement, great patient satisfaction and used to increase retention of implant complete and partial overdenture prostheses with regard to optimizing stress and minimizing denture movement [15],[16].

Locator attachment is one of the newest attachment used with implant retain overdenture. It can be used with nonsplinted, free-standing implants. Locator attachments can compensate for angle corrections of up to 40°. They are indicated in cases of tissue supported complete removable overdenture on 2–4 implants, partially edentulous overdenture with one or more implants and limited interarch distance [17],[18].

Several in-vitro methods have been used to evaluate the stress on implants such as photoelastic, finite element and strain gauge stress analysis. Electrical strain gauge have been used widely for analysis of the stresses around implants supporting a mandibular overdenture [19],[20],[21].

The widely used types of electrical resistance strain gauge are the bonded wire and the metal foil strain gauges. The bonded wire strain gauges consist of a fine wire laid in zigzag fashion and sandwiched between two strips of paper. In the metal foil strain gauges, a very thin foil used instead of the fine wire which has greater heat dissipation properties. The strain gauges can only sense deformations of the surface to which they are bonded [22],[23].

If a wire insulated by a packing material is cemented to the structure for measuring strain, and the resistance change of the wire during loading is measured, this change in resistance can be converted into strain measurements [21].

Single implant will establish as a method of treatment to increase denture retention but the exact position of single implant still a point of controversy for this reason we will study the effect of different position of single implant.

 Materials and Methods



An in-vitro stress analysis study was conducted on different positions of single implant retained complete mandibular overdenture (single implant at midline/single implant at left canine region) placed on mandibular acrylic resin model simulating a completely edentulous mandibular arch using two different types of attachment (ball and socket/locator).

Approval for this project was obtained from Faculty of Dentistry, Tanta University Research Ethics Committee. The design and procedures of the present study were accomplished according to the research guidelines published by the Research Ethics Committee at Faculty of Dentistry, Tanta University.

Mandibular acrylic model construction

Heavy silicone impression (Putty c-silicone impression material; Zermack, Rovigo, Italy) for standard readymade, completely edentulous mandibular stone (Elite dental stones; Zhermack) models and then molten wax was poured into the impression to produce a wax cast. The waxed cast was then flasked and wax elimination was carried out. The created mold was packed with heat polymerized acrylic resin (Acrostone-Heat polymerized denture base material, England) and then cured, finished and polished.

Denture construction

A complete mandibular acrylic denture was constructed on the stone model. The finished denture was duplicated three times to create one for each group.

Group 1: Single implant at midline region with ball and socket attachmentGroup 2: Single implant at midline region with locator attachmentGroup 3: Single implant at left canine region with ball and socket attachmentGroup 4: Single implant at left canine region with locator attachment.

Simulation of the artificial mucosa

Two millimeter thickness of artificial mucosa was made with the light body silicon impression material (Zetaplus; Zhermack) was placed over the reduced edentulous area for the resin model after painted it with rubber adhesive (Universal tray adhesive for impression silicones Zhermack, 176898-Italy).

Implant installation

Dental implants (Implant Direct LLC, Calabasas Hills, California, USA) 3.7 mm in diameter and 13 mm in length were embedded in the midline and left canine region of the same mandibular acrylic model with artificial mucosa by using the transparent acrylic stent.

Abutment position

Ball abutment (Implant Direct LLC) and locator abutment (Implant Direct LLC) (3.5 mm) diameter with 2 mm gingival height similar to thickness of artificial mucosa were screwed in the implant according to its group with dentures have housing located in fitting surface according to their group [Figure 1] and [Figure 2].{Figure 1}{Figure 2}

Strain gauge installation

Four strain gauges (Kyowa Electronic Instrument Co. Ltd, Tokyo, Japan) around implant were used to monitored the effect of the applied loads vertical and oblique one strain gauge for each channel at acrylic model over flat surface of buccal, lingual, mesial and distal channel around the midline implant inserted in the acrylic model, were installed in the symphesial buccal, symphesial lingual, symphesial mesial and symphesial distal wall of implant at midline.

Four strain gauges fixed in position by using strain gauge adhesive (CC-33 strain gauge cement; Kyowa Electronic Instrument Co. Ltd) to avoid its movement during measurement which may affect accuracy of reading.

The same procedure was repeated when inserting implant at canine region of mandibular acrylic model at left side, four strain gauges were installed in the canine buccal, canine lingual, canine mesial and canine distal wall of implant [Figure 3].{Figure 3}

Strain meter

The wire of the strain gauges were connected to a digital multichannel strain meter (PCD-300 A; Kyowa Electronic Instrument Co. Ltd) was used to assess the strains induced by the load applied and transmitted to each strain gauge around the implant, while the machine was adjusted to apply loading.

Load application and strain recording measurement

According to Maeda et al. [24] compressive load was applied using universal testing machine (Lloyd LRX; Lloyd Instrument Ltd, Fareham, Hampshire, UK) during stress distribution measurements test with soft were Lloyd LR5K, version (4.1). The compressive load was 50 N static load, with cross-head speed 0.5 mm/s and the machine was computer controlled by the NEXYGEN software which permits the collection of data. A pointed load cell was attached to the cross-head to control its speed and direction.

Vertical loading

The overdenture was placed on acrylic model of placing implant at midline with ball abutment which placed on the lower plate of the universal testing machine and the denture contain housing of ball abutment at midline was vertically loaded over three points [right first molar (RT6), midline (MID) and left first molar (LT6)]. Universal testing machine was tested before measurement. A 50 N static load was applied by computer operating universal testing machine at these three points at speed of 0.5 mm/s.

After finished measurement the ball abutment was removed and replaced with locator abutment and using the prosthesis contain housing of locator abutment at midline then the measurement completed as before.

Oblique loading

After vertical load application, the model was secured to a custom made wedge shaped wooden base to apply oblique loading. The wooden base was constructed with a 40° slop. A hole was made in the center of the model so that the model could be fixed to the base using a long screw. The wooden base was stabilized by securing it to the custom made metal base.

The application of 50 N static loads was applied by computer operating universal testing machine at three different sites (RT6, LT6 and MID) over the model of placing implant at midline with ball abutment. The ball abutment was removed and replaced with locator abutment, the prosthesis was oblique loaded over three points A 50 N static load was applied by computer operating universal testing machine at three different sites (RT6, MID and LT6). The same procedures were repeated by using implant at left canine region of the model.

Statistical analysis

The microstrain data was collected to evaluate and the descriptive statistics analysis include mean and SD of different groups (group 1, group 2, group 3 and group 4) was performed using SPSS program, version 12 (SPSS Inc., Chicago, Illinois, USA).

t-Test was used to compare recorded microstrain values between (group 1, group 2, group 3 and group 4) with vertical and oblique forceAnalysis of variance and Tukey's were used to compare recorded microstrain values at distal (D), buccal (B), mesial (M), lingual (L) aspect of single implant to three different points of load (central fossa of RT6, MID and central fossa of LT6) of the occlusal surface of the denture.

 Result



Effect of type of attachment for comparison between (group 1, group 2, group 3 and group 4)

For group 1 (single implant at midline region with ball and socket attachment), group 2 (single implant at midline region with locator attachment), group 3 (single implant at left canine region with ball and socket attachment) and group 4 (single implant at left canine region with locator attachment), the mean value and SD of microstrain at D, B, M, L aspect for the implant when unilateral 50 N static load was applied, that presented in [Table 1] and [Figure 4].{Table 1}{Figure 4}

The lowest microstrain was obtained in L aspect of group 2. The difference was statistically significant between group 1, group 2 at D, L P value of less than 0.05 and nonsignificant between (group 1 and group 2 at B, M) also between (group 3 and group 4 at D, B, M, L) P value more than 0.05.

The results of this study showed that the stresses generated by the application of the static load varied according to the type of attachment, the angle of load application and implant site. Regarding the stress distribution of the attachment systems, locator attachment showed less stress values than the ball attachment with vertical load at midline implant as well as canine implant locator attachment showed less stress values.

Comparison between groups during loading at midline implant revealed that midline implant with locator attachment (group 2) during vertical load had lower stresses than ball attachment (group 1), but when implant at canine area with locator attachment during vertical load (group 4) had lower stresses than ball attachment during vertical load (group 3). When comparison between midline implant and canine implant lowest stresses obtained with (group 2).

Effect of point of load right first molar, midline and left first molar to implant retained mandibular overdentures with different types of attachments

Microstrain was measured at D, B, M, L aspect for the implant when unilateral 50 N static load was applied at three different sites (central fossa of RT6, MID and central fossa of LT6) of the occlusal surface of the denture, that presented in [Table 2].{Table 2}

The lowest microstrain was obtained in D aspect of implant when static load was applied at central fossa of RT6.

 Discussion



In-vitro stress analysis studies have been widely used to provide more accurate good understanding of the nature of stresses and strains acting on dental structures, even more than in-vivo studies [25]. The acrylic resin was used in the construction of experimental model because of the nearly in modulous of elasticity between the compact bone and acrylic resin. This was in agree with Cehreli and colleagues [26],[27],[28],[29].

A silicon resilient material covering the residual ridge had the lowest dimensional changes, thus providing close fit to the model. Besides, it maintained its resilient behavior throughout the study period providing an acceptable mucosal substitute [30]. The strain gauge system was used in this study as it was reported to be a stable and accurate system with few problems. The strain gauges assess strain induced into a loaded structure by converting the change in resistance of an electric wire into strain measurement [31].

A 50 N static load was applied at different sites of the occlusal surface of simulated denture base (RT6, right canine, MID, left canine, LT6) using each counterpart of the attachment [24],[27],[28],[32],[33].

Unilateral force was applied to central fossa of the first molar of mandibular overdenture retained by implant according to Celik and colleagues [20],[33].

These results were in agreement with Eltaflazani and colleagues [34] who concluded that less stresses were generated by the locator attachment on cortical and cancellous bone as well as around the implant than by ball attachment. This is possibly related to its low profile design and to rotational pivoting character of its abutment, that is also advocated in combination with close to parallel internal connection implant, to lower the rotational centre that potentially reduce the lateral forces. In addition, the vertical resilience of the locator attachment may also be a factor as it allows movements in both the vertical as well as the hing axis. The resilience is achieved through a space of 0.2 mm which is designed to allow for vertical resilience and hinging in any direction providing a higher amplitude movement of the prosthesis anteroposteriorly, laterally and intrusive [35],[36],[37].

When the stresses generated at the implant bodies were compared under all loading conditions of the study, it was found that the stresses were not only markedly less with the locator system than with the ball system, but this difference in stress values proportionally increased by increasing the angle of load application from 0° to 40°. The findings of the present study may support previous opinions that considered the locator attachment advantageous biologically and mechanically [17],[37],[38].

In terms of stability it is better to have taller attachments such as the ball which provide better bracing effects however these also increase risks of lateral forces with molar loading to the implant so fewer denture base movement with ball and has the advantage of providing better stress breaking capability causing less lateral force [24],[39].

According to the results a single midline implant showed less microstrain than single canine implant retained mandibular overdenture and that agree with study of Nascimento et al. [9], who mentioned that the load transferred to a single midline mandibular implant was distributed around the implant with low stress concentration, irrespective of the type of retention system.

The larger the distance between the implant and loading point such as the molar region, the smaller the lateral load to the implant and the larger the denture base movements. When loads were applied at midline, horizontal denture movements were transformed to rotational movements around the axis through the implant, increasing the lateral force and decreasing denture movements [24].

Microstrain values recorded for the edentulous area were minimum in case of midline implant with locator attachment and increased significantly in case of ball attachment under vertical load while under oblique load this increase were significant. These findings may be attributed to the manner of load application as the vertical and oblique loads were applied on the central fossa of both side at first molar and anterior area of denture tooth, so the majority of strains will be transmitted to the implant in the midline region so it may be preferable to simulate nearly the natural masticatory forces by loading at three locations for both right, left sides and anterior area. This method of loading can effectively distribute the stresses for the occlusal surfaces of the prosthesis resulting in reduced stresses for the supporting bone and implant [40].

 Conclusion



According to the results obtained from the study it could be concluded that. The locator attachment showed less stress on implant body and has ability to distribute the load than ball attachment in midline and canine region.

Single implant at midline region showed the less stress than implant at canine region with both attachments. Vertical load applied over mandibular overdenture retained by single implant showed less microstrain than oblique force for both attachments. Less strain value on the edentulous area were recorded with the locator attachment in the midline region.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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