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Table of Contents   
ORIGINAL RESEARCH  
Year : 2020  |  Volume : 31  |  Issue : 3  |  Page : 414-419
Impact of implants number and attachment type on the peri-implant stresses and retention of palateless implant-retained overdenture


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

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Date of Submission13-Oct-2018
Date of Decision26-Jun-2019
Date of Acceptance21-Aug-2019
Date of Web Publication06-Aug-2020
 

   Abstract 


Objectives: To evaluate the impact of implants number and attachments type on the peri-implant stresses and retention of maxillary palateless implant-supported overdenture. Materials and Methods: Four edentulous maxillary educational acrylic resin models were used. According to the implants number and type of attachment used, four groups were compared: Group I, 2-locator attachments in the canine area; Group II, 2- OT equator attachments in the canine area; Group III, 4-locator attachments in the canine, second premolar area and Group IV, 4-OT equator attachments in the canine, second premolar area. Implants retained palateless overdenture was constructed on each model. Four self-protected linear strain gauges were cemented on each implant. A digital loading device was used to apply compressive loads to measure the resulting peri-implant stresses. Forcemeter and Universal testing machines were used to test the retention of palateless overdenture. Results: A significant difference between the same implant number and distribution with different attachments was found (P = 0.003, P = 0.020), respectively. Least stresses amount was found around the 4-implant locator palateless overdenture, while the highest was found around the 2-implant OT equator palateless overdenture. Nevertheless; the result was that 2-implant locator palateless overdenture recorded insignificant higher retentive forces than the 4-implant OT equator one. Conclusions: It can be concluded that the implant-retained palateless overdenture with four locator attachments is considered a promising treatment option regarding stress distribution. Using locator attachments, for implant-retained palateless overdenture with either two or four implants considering their superior retentive properties, is advantageous when compared to OT equator attachments.

Keywords: Implants distribution, implants number, maxillary palateless overdenture

How to cite this article:
Hegazy SA, El Mekawy N, Emera RM. Impact of implants number and attachment type on the peri-implant stresses and retention of palateless implant-retained overdenture. Indian J Dent Res 2020;31:414-9

How to cite this URL:
Hegazy SA, El Mekawy N, Emera RM. Impact of implants number and attachment type on the peri-implant stresses and retention of palateless implant-retained overdenture. Indian J Dent Res [serial online] 2020 [cited 2020 Sep 23];31:414-9. Available from: http://www.ijdr.in/text.asp?2020/31/3/414/291498



   Introduction Top


Although studies have showed a decreasing prevalence of individuals suffering from complete edentulism, edentate status still remains a common health problem.[1] Edentulous patients usually have complaints with their complete dentures; mainly due to decrease in retention and stability of their denture and insufficient chewing efficiency.[2]

To enhance the retention of maxillary dentures, the denture base is usually extended distally to the vibrating line. This full coverage of the palate results in morphological changes of the palate that leads to disturbance of the oral motor functions.[3] To avoid this; a palateless denture is possibly an ideal design for a maxillary complete denture, to increase patient comfort, to positively enhance patient's sense of taste and to decrease the gagging tendency.[4],[5] However, this decrease in the tissue surface contacts causes lack of denture retention which needs to be overcome.[6]

Implant-retained maxillary overdenture (IRMO) is an option to improve retention. At least four implants are required for implant-retained palateless overdentures.[7] However, a less number of implants are preferable to minimise the surgical invasion for the patient in addition to the economic burden. This is done by placement of only two implants accompanied by reduction of mechanical stress on the tissues around the implant to achieve a longitudinally successful outcome.[4] Patients rehabilitated with a maxillary palateless overdenture (MPO) appear to require more maintenance visits when the prosthesis is retained by two free-standing implants in comparison to four implants.[8]

The attachment types must be taken into consideration to reduce the transferred force,[9] over contouring or prosthesis fracture which may occur due to inappropriate evaluation of limited inter-arch space.[10]

Therefore, a special group of attachments was introduced to be used within the available limited inter-arch space known as the low profile attachments.[11]

These attachments are known to disengage during excessive forces application, to avoid the harmful load from reaching the implants and implant-bone interface.[12]

Locator attachment is a low profile resilient attachment, with a universal hinge that allows free-floating movement between the nylon retention liner and its metal housing. Moreover, locator attachments are presented with different vertical heights, durability and can compensate implants angulation.[13],[14] OT equator attachment has the minimum diameter and vertical height for the overdenture abutments. Furthermore, it provides good retention and stability, reduced residual ridge resorption and improves aesthetics.[15]

Retention is a cornerstone in the removable appliances. Patient's satisfaction greatly depends on the prosthesis retention. By using a cross-over experimental design, Burns et al.[16] found a strong patient preference for the overdenture attachment with superior retention.

Therefore, the goal of this currentin vitro research was to compare the impact of implants number and type of two low profile attachments on the peri-implant stresses, and retention of implant palateless overdenture.


   Materials and Methods Top


Four standard educational clear acrylic resin models (Rapid simplified, Vertex-Dental B.V, The Netherlands) representing an edentulous maxilla with deep palatal vault were used in this study. Self-cured silicone layer (Softliner®, Promedica, GmbH, Neumünster, Germany) of 2 mm thickness was incorporated over the residual ridge of each model to simulate the oral mucosa. According to the number of implants installed, and type of the low profile attachment used, four models were constructed and classified as follow:

Group I: 2-implants (TioLogic; Dentaurum, Ispringen, Germany) were installed bilaterally at the region of the canine, the locator attachments (Zest Anchors, Escondido, California, USA) were used to retain the palateless overdenture.

Group II: 2-implants (TioLogic; Dentaurum, Ispringen, Germany) were installed bilaterally at canines region, the OT equator attachments (Rhein 83 OT equator attachment) were used to retain the palateless overdenture.

Group III: 4-implants (TioLogic; Dentaurum, Ispringen, Germany) were installed bilaterally at canines and second premolars regions, the locator attachments (Zest Anchors, Escondido, California, USA) were used to retain the palateless overdenture.

Group IV: 4-implants (TioLogic; Dentaurum, Ispringen, Germany) were installed bilaterally at canines and second premolars regions, the OT equator attachments (Rhein 83 OT equator attachment) were used to retain the palateless overdenture.

The four models were drilled at the implant sites with the aid of a surgical stent. Every implant fixture of 3.6 mm diameter and 12 mm length were wrapped by 1 mm thickness tin foil spacer before being installed in its place in the models. The locator and OT equator attachments abutments were inserted in the implants' internal hex by using the insertion key and torque wrench (30 N cm) [Figure 1] and [Figure 2].
Figure 1: (a) Group I model with two implants connected with two Locator attachments; (b) Group II model with two implants connected with OT Equator two attachments

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Figure 2: (a) Group III model with four implants connected with four Locator attachments; (b) Group IV model with four implants connected with OT Equator four attachments

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Construction of implants palateless overdentures

Undercuts of the clear acrylic resin model were blocked out; the acrylic resin model was duplicated using Agar-Agar (Gilvest, German.) to construct the refractory casts. Palatal bead line of 0.5 – 1 mm in width and depth was curved in the refractory cast. Cast chrome-cobalt frameworks were constructed to act as a denture base.[17] Record blocks were adjusted according to the standard measures and artificial teeth were arranged. Flasking, packing and curing of heat cure acrylic resin followed by finishing and polishing of the twenty palateless (five for each group) overdentures. The female housings for each group with the retentive male inserts corresponding to the moderate retention, (pink color for the locator attachment, yellow color for the OT equator attachment), were picked up to the intaglio surface of each implant palateless overdenture by using auto polymerized acrylic resin.[18]

Peri-implant stresses analysis procedures

Four self-protected linear strain gauges (Kyowa, Japan) with 1 mm lengths were luted to each implant at its different surfaces. By unwrapping the fixtures from the tin foil, the four strain gauges were glued to each implant fixture in the cervical one-third of the fixture using strain gauge cement.[19] The wire of each strain gauge was coded to be identified during measurements [Figure 3].
Figure 3: The strain gauge wires were marked with a code

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The OT equator and/or locator attachments were connected to each implant fixture. The palateless overdenture enclosed the attachments female housing was adapted to each model. Bilateral central loading with stainless steel hexagonal bars of a standardised 6 cm length and 5 mm diameter was applied on every palateless overdenture; the loading point was marked as the mid-point of the stainless bar that was fixed between first molar area bilaterally.

Compressive loads were applied by a digital loading device (Lloyd instrument, Ltd, UK, lrx plus.); to measure stress resulting around the implants for each type of attachments. The gradual central load (0–50 Newton) was applied on the predetermined loading point of the stainless steel bar. Measurements were collected and statistically analysed. Thereafter; the strain gauges were removed from the implant fixtures. Resin cement (Superbond CB; Sun Medical, Kyoto, Japan) was used to secure implants in their recess to simulate osseointegration.

Evaluation of retention forces

Fixation of a metal bar with a central ring of radius (3 mm), to the palateless overdenture palatal surface was done just below the occlusal plane at the region of first and second molar teeth using cold cure acrylic resin[20] [Figure 4]. The retention forces of palateless overdenture to the model was measured by a universal testing machine (Lloyd LRX) with a load cell of 5 KN. The maximum dislodging force was recorded at the moment of complete detachment of palateless overdenture from the model.
Figure 4: Metal bar with a central ring fixed to the palatal surface of implant palateless overdenture

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Statistical analysis

The data were analysed by SPSS version 20 (SPSS Inc., Chicago IL, USA).

Repeated measures of ANOVA were used to compare the recorded implant stress between different attachment (locator, OT equator attachment) with number and distribution of implants (two, four).

The significance of variables (attachment and number of the implant) was detected by two-factor ANOVA test. Tukey's post-hoc tests were performed to detect significance between subgroups. Student t- test was performed to detect the significance between the main groups. P values ≤ 0.05 are considered to be statistically significant in all tests.


   Results Top


Comparison between stresses on different implants for the model of 2-implants in canine site with locator attachments and the model of 2-implants in the canine site with OT equator attachments were presented in [Table 1]. There was a significant difference between the same implants number and distribution with different attachments as P = 0.003. The highest amount of stress was found around the surfaces of 2-implant OT equator palateless overdenture (114.55 ± 42.61).
Table 1: Comparison between stresses of 2-implants with OT equator attachments and 2-implants with locator attachments

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Comparison between stresses on different implants for the model of 4-implants in canine and premolar sites with locator attachments and model of 4-implants in canine and premolar sites with OT equator attachments were revealed in [Table 2]. There was a significant difference between the same implants number and distribution with different attachments as P = 0.020. The least amount of stress was found around the 4-implant locator palateless overdenture (5.83 ± 4.31).
Table 2: Comparison between stresses of 4-implants with the mean stress value with the locator attachments is OT equator attachments and 4-implants with locator attachments

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The results of a comparison of retention force for implant palateless overdenture as a function of attachment and implant number is shown in [Table 3] and [Figure 5]. The locator attachments with 4-implants palateless overdenture recorded the highest retention force mean values, followed by locator attachments with 2-implants palateless overdenture. Then, the OT equator attachments with 4-implants palateless overdenture, while, OT equator attachments with 2-implants palateless overdenture recorded the lowest retention force mean values.
Table 3: Comparison of retention force results for implant-retained palateless overdenture as function of attachment and implant number

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Figure 5: A column chart comparing retention force mean values between implant retained palateless overdenture as function of attachment and implant number ranked from higher to lower mean value

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One way analysis of variance ANOVA tests showed that the difference between groups was statistically significant (P = <0.0001). Tukey's post-hoc test showed non-significant (P > 0.05) difference between locator with 2-implant and OT equator with 4-implants which is a surprising result revealed by this study.


   Discussion Top


The OT equator attachments exhibited high-stress values on implants rather than locator attachments wherein the implants' distribution and number are the same. Furthermore, the stresses on 4-implants were lesser than stresses on 2-implants with the application of same attachments. Although two implants may provide reasonable retention to the palateless overdenture, the omission of hard palate coverage, which participates greatly in the support of the overdenture, increased stresses on the implants. This is in agreement with Damghmani et al.[21] who propositioned that when maxillary conventional complete denture was utilized (control group), nearly 37% of the stress was transmitted to the palate. While, upon utilisation of two implants retained overdenture with locator attachment, the value of stresses on the palate slightly declined that in turn increased stress on the implants.

Properly distributed 4-independent implants assisted maxillary overdentures were associated with great amount of load reduction compared to other overdentures assisted by only 2-independent implants.[22] This stands in line with this study result, where the stress on 2-implants with 2-locator attachments was higher than stress on 4-implants with 4-locator attachments. In addition, this result was the same with OT equator attachment.

In this study, the mean stress values of locator attachment implant palateless overdenture were less than the mean stress value recorded by OT equator attachment implants palateless overdenture with standard implant number. This decrease in the mean stresses was statistically significant. This can be attributed to the matchless design of locator attachment; the patrix (male) is the replaceable nylon inserted on the intaglio surface of the palateless overdenture and the matrix by virtue.[23] Furthermore, the nylon male element grabs the outside and the inside contours of the locator abutment which may be for good stress absorbtion and transmission. The OT equator attachment implants palateless overdenture combines the simplicity of ball attachments, with no stress absorbed and transmitted.

Moreover, in comparison to the previous results where the mean stress value with the locator attachments is less than that with OT equator attachment, it was found that the mean retention values of locator attachment implant palateless overdenture were higher than the mean retention value recorded by OT equator attachment implants palateless overdenture. This elevation in the retention values was statistically significant. Again; this may be due to the matchless design of the locator attachment where the nylon male element grabs the outside and the inside contours of the locator abutment. This characteristic feature redoubles the surface area for retentive contact. In addition, the OT equator attachment has the diversity of retention levels and easy replacement options of locator's attachment with no double surface area for retention and stress absorbtion and transmission. This result is in agreement with Greenbaum[24] who examined the retentive values of locator attachments with initially placed pink nylon-inserts.

It was found that locator attachments with 4-implants palateless overdenture recorded the highest retention force followed by locator attachments with 2-implants palateless overdenture, then OT equator attachments with 4-implants palateless overdenture while OT equator attachments with 2-implants palateless overdenture recorded the lowest retention force mean. This may be due to the increased friction between the nylon insert and the abutment. The dual retention feature of locator attachment system and the relatively wider surface area of the locator nylon insert must aid in magnifying the effect of friction between the two components, locator attachment system showed superior retentive properties when compared to OT equator attachments. This is in agreement with Satti[25] who compared the locator with OT equator attachment.

Finally, anin vitro research on an acrylic resin model rather than anin vivo research, was selected to perform the current research as studies in the laboratory are controlled easily, more practical and can yield further accurate results. Therefore, the laboratory experiments could be extra valid as the experience can be repeated under the same circumstances, where the object under the experimental study would be a single variable.In vitro studies are recommended to analyse stresses within the supporting structures in implant overdentures. Further studies are needed to measure the effectiveness of both attachments clinically either in the maxillary or mandibular arches.


   Conclusion Top


It can be concluded that the implant-retained palateless overdenture with four locator attachments is considered a promising treatment option related to stress distribution. Using locator attachments for implant palateless overdenture with either two or four implants regarding stress distribution pattern and the superior retentive properties is advantageous when compared to OT equator attachments.

Acknowledgements

The authors would like to thank Mr. Ismail Elbakoosh and, Mrs. Fatma Khalifa for their role in the collection and organization of the article data.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Correspondence Address:
Prof. Nesreen El Mekawy
68 El-Gomhoreya Street, Mansoura
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijdr.IJDR_772_18

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    Figures

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