Comparison of Retention of Two Different Attachment Systems Used in Implant-Supported Overdentures.

Aim: This study aims to compare the retentive capacity of two attachment systems after manual thermocycling. Settings and Design: In vitro study and Comparative trail. Materials and
Methods: An edentulous mandibular Polymethyl Methacrylate model was fabricated to receive the overdentures with the two attachment systems to be compared. Two dental implants were placed in the predetermined right and left mandibular canine regions of the model. A total number of eight overdentures, four per group, were fabricated over the two implants with two attachment systems to be compared; Bar and clip attachment system (Group-1) and Locator attachment system (Group-2). All the overdentures were subjected to 5000 alternating thermal cycles using manual thermocycling. Then the samples were subjected to 100 vertical pulls each in the anterior and posterior regions using a universal testing machine and the mean retentive forces were calculated for each sample in the anterior and posterior regions, respectively.
Results: The mean retentive forces after 100 vertical pulls, were calculated and tabulated for each sample in the anterior and posterior regions separately. Then, the cumulative mean of the anterior and the posterior regions were calculated for each group. The cumulative mean retentive forces of both the attachment groups were-Group-1 (Bar and clip attachment system) = 27.87 N ± 4.01 and Group-2 (locator attachment system) = 18.85 N ± 2.50 with a P value of 0.021. This difference was found to be statistically significant.
Conclusion: Within the limitations of the present in vitro study, the bar and clip attachment system offered better retention than the locator attachment system. Copyright:

INTRODUCTION

Edentulous patients present with many problems like loss of a major part of their facial structure and a compromised nutritional status as a result of improper mastication. Treating these problems has always been the primary goal of the dentist. A removable dental prosthesis like a complete denture has enabled the dentist to deliver a more predictable and satisfactory quality of life for these edentulous patients.[1]

With an increase in the aging population worldwide and also with more patients retaining their teeth, there has been a shift in rehabilitating such patients by adopting a preventive approach (Based on M M Devan statement)[2] An overdenture is defined as “A removable partial or complete denture that covers and rests on one or more remaining natural teeth, roots, and/or dental implants” (GPT 9).[2]

Introduction of the Principles of osseointegration in dentistry has improved the oral rehabilitation of the edentulous patients and the success rate of the implant rehabilitation seems to be higher as reported by several in vivo studies and it has become an acceptable alternative.[3] Implant overdentures demonstrate several advantages such as preservation of residual alveolar ridge, increased retention and stability, chewing efficiency, greater comfort and satisfaction, and a better quality of life. Hence, a dental professional should educate completely edentulous patients about the advantages of the implant therapy, thereby increasing their quality of life.[345]

Various attachment systems are available for retaining these implant-supported overdentures. Over the years, bar and ball attachments have been frequently and routinely used for implant-supported overdentures whereas the relatively new design-the locator attachment has not been explored sufficiently. This is the primary reason to carry out this study wherein bar attachment system and locator attachment system under implant-supported overdentures have been compared under strict conditions mimicking oral cavity.

MATERIALS AND METHODS

The present study was designed as an in vitro comparative study conducted in the Department of Prosthodontics and Implantology. The required sample size with 90% power is four per group and in total eight (Group 1-Bar and clip attachment system (n = 4); Group 2-Locator attachment system (n = 4)) (using G Power version 3.1.9.2 software).

This present in vitro study was carried out on a single heat-cured polymethyl methacrylate model (free of undercuts) to test for all the overdentures for the purpose of standardization.[6] Two dental implants (4.2 mm × 13 mm, BioLine) were placed conventionally in the predetermined left and right canine regions adhering to the implant placement protocol provided by the manufacturer. The two types of attachment systems that are to be compared in the current study are as follows.

Bar and clip attachment system

Hader bar and clip overdenture attachment (Rhein 83) consisting of a castable hader bar with a standard 13 gauge (1.8 mm) diameter and length of around 22 mm. This hader bar comes with a yellow plastic clip [Figure 1].

Figure 1

Prefabricated castable bar and clips

Locator attachment system

Locator overdenture attachment system (BioLine) along with metal cap for power click attachment and silicone sealed ring of standard softness (transparent color) [Figure 2].

Figure 2

Locator attachment system

After the implant placement, overdentures were fabricated with the Bar and Clip attachment system and the Locator attachment system. A total number of eight mandibular overdentures were fabricated (Group 1= 4 and Group 2= 4) [Figure 3].

Figure 3

Total number of eight mandibular overdentures (Group 1 = 4 and Group 2 = 4)

After the fabrication of the overdentures, all the eight samples along with the edentulous model were subjected to 5000 alternating thermal cycles (6 months of service in the oral cavity) to simulate artificial aging.[7] After thermocycling, though the dentures lost their polished surface and exhibited some degree of warpage, no failure was reported amongst the samples tested [Figure 4].

Figure 4

Warpage of the overdenture after thermocycling

Testing for retention

All the eight overdentures after thermocycling were subjected for testing of retention using “Instron® 8874” universal testing machine (UTM). All the eight overdentures were subjected to 100 pulls each in the anterior and posterior direction respectively in attempt to dislodge them from the acrylic model.[8]

The mean values of vertical force required to dislodge them in the anterior and posterior direction were recorded (”Instron® Bluehill” software). The following steps were employed to carry out the retention test for each of the eight models:

The edentulous mandibular resin model was stabilized on the surveying platform of the Instron® 8874 UTM

To dislodge the overdenture from the underlying model, separate fixtures were designed for the vertical dislodgement in the anterior and posterior direction as follows:

Anterior direction-A thin metal sheet of 1 mm thickness was placed such that it engages the anterior flange of the overdentures. This metal sheet was suspended from a metal hook in the crosshead of the UTM [Figure 5]

Figure 5

Metal sheet suspended from a metal hook in the crosshead of the UTM

Posterior direction-Two metal binder clips were engaged to the overdenture in the right and left molar regions where indentations were made on the buccal and lingual aspects of the molars using tungsten carbide bur. This was done to ensure proper engagement of the binder clips to the overdenture. These binder clips were in turn connected to two metal paper clips. These two paper clips were then attached to another paper clip suspended from the metal hook in the crosshead of the UTM [Figure 6]

Figure 6

Retention test performed in the posterior region of the mandibular overdenture with a pair of metal binder clips and paper clips that engage the posterior regions (right and left molar regions) of the overdenture

The overdentures were then attempted to dislodge from the mandibular acrylic model with a constant crosshead speed of 50.8 mm/min.[9] This speed was chosen since it is reportedly the approximate speed of movement of the overdenture away from the ridge during mastication[9]

This UTM was connected to the Instron® Bluehillsoftware to carry out the retention test and then the values were recorded and subjected to statistical analysis.

RESULTS

The data obtained were statistically analyzed using the SPSS (IBM SPSS Statistics for Windows, Version 19.0, IBM Corporation). The significance level was maintained at 5% (α = 0.05).

The retentive forces of group 1 (Bar and clip attachment system) in the anterior region ranged from 22.90 to 37.21 N and in the posterior region ranged from 22.15 to 27.80 N. The cumulative mean retentive force of Group 1 is 27.87 N [Table 1]. The retentive forces of Group 2 (Locator attachment system) in the anterior region ranged from 17.36 N to 24.74 N and in the posterior region ranged from 11.12 to 24.39 N. The cumulative mean retentive force of group 2 is 18.85 N [Table 2].

Table 1

Mean and cumulative mean retentive forces of the bar and clip attachment system in the anterior and posterior regions

Group 1	Anterior region (n)	Posterior region (n)	Average of anterior and posterior region (n)
S1	22.90	22.15	22.53
S2	27.14	27.80	27.47
S3	37.21	26.77	31.99
S4	35.48	23.50	29.49
Mean			27.87

Table 2

Mean and cumulative mean retentive forces of the locator attachment system in the anterior and posterior regions

Group 2	Anterior region (n)	Posterior region (n)	Average of anterior and posterior region (n)
S1	17.36	24.39	20.88
S2	24.74	17.55	21.15
S3	22.00	11.12	16.56
S4	19.98	13.66	16.82
Mean			18.85

On comparison of the mean retentive forces of group 1 and group 2 in the anterior region, bar and clip attachment system (30.68 N ± 6.80) showed higher retentive force values than the locator group (21.02 N ± 3.12) and this difference was found to be statistically significant (P < 0.05) [Table 3]. When comparing the mean retentive forces of Group 1 and Group 2 in the posterior region, bar and clip attachment system (25.06 N ± 2.67) showed higher retentive force values than the locator group (16.68 N ± 5.78), and this difference was found to be statistically insignificant (P > 0.05) [Table 4].

Table 3

Comparison of mean retentive force between bar and clip attachment system and locator attachment system in the anterior region

Anterior region	Mean±SD	Mann-Whitney test	P
Group 1	30.68±6.80	1.000	0.043
Group 2	21.02±3.12

SD: Standard deviation

Table 4

Comparison of mean retentive force between bar and clip attachment system and locator attachment system in the posterior region

Posterior region	Mean±SD	Mann-Whitney test	P
Group 1	25.06±2.67	2.000	0.083
Group 2	16.68±5.78

SD: Standard deviation

The final comparison was between the cumulative mean retentive forces of all the samples in the anterior and posterior regions of group 1 and 2. The cumulative mean retentive force of the bar and clip attachment system was 27.87 N ± 4.01 and for locator attachment system, it was 18.85 N ± 2.50. The results indicated that the bar and clip attachment system is superior to the locator attachment system in terms of offering better retentive forces (P = 0.021; P < 0.05) [Table 5 and Chart 1].

Table 5

Comparison of cumulative mean retentive force between bar and clip attachment system and locator attachment system

Average of anterior and posterior region	Mean±SD	Mann-Whitney test	P
Group 1	27.87±4.01	0.000	0.021
Group 2	18.85±2.50

SD: Standard deviation

Chart 1

Comparison of cumulative mean retentive force between bar and clip attachment system and locator attachment system

DISCUSSION

In patients presenting with soft and hard tissue defects, a simple fixed implant prosthesis cannot address all the requirements in regard to esthetics, phonetics, and maintenance of oral hygiene. In patients who have a tough time adjusting with inadequate complete denture function, other better treatment alternatives are the implant overdentures. Literature has reported significantly greater performance in terms of mastication, esthetics, and a better quality of life in patients treated with implant overdentures as compared to those edentulous patients rehabilitated with conventional complete dentures.[10]

A practical question that arises in opting for implant-supported overdentures is whether they are really worth their maintenance costs and technically demanding nature when the patient can be offered the alternative option of a fixed prosthesis. The answer to this is that, adequate oral hygiene measures and maintenance of the removable overdenture and its intraoral elements for retention elements are crucial for long-term clinical success.

Regarding the numbers required and position of placement of the implants, usage of two to four implants has been reported to provide exceptional functionally efficient prosthesis with appreciable comfort and satisfaction to the patients.[11] The presence of a sufficient amount of inter–arch spacing is essential to fabricate a prosthesis having adequate thickness thus offering fracture resistance.

The superstructure design and type of attachment systems used can also influence the biomechanics of the implant-supported overdentures as different types of connections allow various types of prosthesis movement to occur. In several previous studies, bar and ball attachments have been frequently and routinely used for implant-supported overdentures whereas the relatively newer design-the locator attachment has not been explored sufficiently.[12] The current study has also compared the bar and clip attachment system and the locator attachment system in an in vitro study design.

The retentive forces values of group 1 and group 2 obtained in this study are in close range with the study conducted by Elsyad et al.[12131415] The retentive force values obtained from the current study are greater than the required range of retention to obtain good patient comfort and satisfaction with the implant-supported overdentures (required range: “5–10 N”).[1415]

On comparing the retentive forces in the anterior and posterior regions of group 1and 2, the retentive forces obtained in the anterior region (Group 1 = 30.68 N ± 6.80; Group 2 = 21.02 N ± 3.12) seemed to be higher than that in the posterior region (Group 1 = 25.06 N ± 2.67; Group 2 = 16.68 N ± 5.78) in both the groups.

On comparison of the mean retentive forces of Group 1 and Group 2 in the anterior and posterior region, bar and clip attachment system (30.68 N ± 6.80 and 25.06 N ± 2.67) showed higher retentive force values than the locator group (21.02 N ± 3.12 and 16.68 N ± 5.78).

The results of the final comparison of the cumulative mean retentive force of group 1 and 2 (27.87 N ± 4.01 and 18.85 N ± 2.50) indicated that the bar and clip attachment system is superior to the locator attachment system in terms of offering better retentive forces with significant P value [Table 5]. The results of this study were similar to the previous studies (ELsyad et al., Shastry et al.), wherein the bar and clip attachment system showed superior retentive forces than the other types of attachment systems.[812]

According to Trakas et al., a retentive force of 20 N is considered sufficient to retain a mandibular overdenture.[16] Thus, the results of the present study indicate that the cumulative mean retentive value of group 1 (27.87 N ± 4.01) is greater than the sufficient range of retention required in an implant-supported overdenture and for group 2 it is slightly less than the normal range (18.85 N ± 2.50). This slight loss in the retention may be because of the surface changes in the silicone sealer ring after manual thermocycling and 100 vertical pulls using the UTM employed in this study.

Scherer et al. in 2013, reported that the effective retentive force values of the mandibular implant-supported overdentures may be in the range of 8–10 N.[15] Therefore it is evident that the present study shows that the cumulative mean retentive force of both the groups are greater than that of the normal range (Group 1-27.87 N ± 4.01, Group 2-18.85 N ± 2.50).

The limitations of the present in vitro study could include errors inherent during impression making and fabrication of the overdentures. The absence of simulation of saliva and a functional occlusal load may have had an influence on the wear processes in the attachments used. The addition of a 2 mm thick layer of autopolymerized silicone soft liner superficially to simulate the ridge mucosa may have had an additional influence on the retentive forces of the attachment systems.[1317]

Nonetheless, this current study has reported the retentive capacity of implant-supported overdentures in the vertical direction in two types of attachment systems both in the anterior and posterior regions, which is a significant merit to the study.

CONCLUSION

Within the limitations of the study, on comparing the retentive capacities of the widely used bar and clip attachment system and the relatively new locator attachment system, for an implant-supported overdenture in a controlled in vitro study model, the bar and clip attachment system offered better retention than the locator attachment system. However, the retentive capacities of both the attachment systems in the current study were well above the minimum values recommended.

Hence, based on the clinical scenario, patient's preference and cost factor, the prosthodontist should make informed decision in choosing the appropriate type of attachment system especially curated for that particular patient.

Financial support and sponsorship

Self-funded.

Conflicts of interest

There are no conflicts of interest.