Comparative evaluation of retentive capacity of three different attachment systems for implant retained overdentures: An in vitro study

Abstract

BACKGROUND

The primary issue in managing edentulous patients is the severely resorbed mandibular ridge, particularly in older individuals with diminished adaptive capacities. This compromised situation leads to the fabrication of inadequate dentures that lack retention and stability, potentially causing psychosocial issues.

AIM

To determine the difference in retentive capacity between three attachment systems in implant-retained overdentures.

METHODS

Three edentulous mandibular models were fabricated using heat-cured polymethacrylate resin, with two implant replicas placed in the intra-foraminal region of each model. 30 acrylic resin mandibular overdentures were fabricated with provisions for three different overdenture attachment systems: A prefabricated ball/O-ring attachment, a locator attachment system, and an equator attachment system. Each model was subjected to 15000 pulls using a universal testing machine to remove the overdenture from the acrylic model and the force data were recorded.

RESULTS

The ball/O-ring attachment system demonstrated superior retentive capacity for 15 years, while the locator and equator attachment systems maintained excellent retentive capacity for 5 years.

CONCLUSION

The ball/O-ring attachment system outperformed better than the other two attachment systems regarding retentive capacity. The locator and equator attachment systems presented sufficient retentive abilities until 15000 cycles. After 7500 cycles, significant differences in retentive force between the systems evolved.

Key Words: Dentures; Overdenture attachment systems; Equator; Dislodging cycles; Retentive capacity

Core Tip: The extensively resorbed mandibular ridge is the most common issue when treating edentulous patients, particularly as they age and lose some of their adaptive abilities. This often leads to the fabrication of unsatisfactory dentures with low retention and stability, potentially exacerbating psychological issues. In contrast, patients with maladaptive dentures showed excellent clinical outcomes with implant-supported overdentures. For edentulous patients, the mandibular 2-implant overdenture is the preferred treatment approach.

INTRODUCTION

Reduced adaptive capabilities are the most common problem with severely resorbed mandibular ridges in complete dentures, resulting in poor retention and stability[1-3]. With high success rates, edentulous patients have received various prosthetic treatment options using osseointegrated implants[4-6]. An implant-supported overdenture improves aesthetics and oral hygiene, simplifies fabrication, and is more cost-effective[7,8]. The long-term functionality of implant-supported overdentures relies significantly on the attachment system’s retentiveness[9]. Nowadays, implant-supported overdentures use a variety of attachment systems to improve their functionality[10,11]. Clinical applications with tooth-supported or implant-supported overdentures use ball attachments, and locator attachments are the most accessible types of attachments[12-16]. When prosthetic space is limited, a novel attachment method called the “OT Equator” with the smallest vertical height and diameter was designed[17]. An implant-retained overdenture attachment should have appropriate retentive characteristics that improve prosthesis retention while allowing for the patient’s simple placement and removal[18].

Physical nature and retention, such as frictional contacts, mechanical interlocking, or magnetic forces, may influence the degree of retentive force[19-21]. Therefore, consuming food and liquids while chewing, wearing, and removing the prosthesis is likely to affect the retentive capacity of the attachment systems. During mastication and insertion, micro and macro movement happens between the retentive surfaces of an attachment system. Over time, taking out the overdenture causes wear and lowers the retentive pressure[22]. The purpose of this in vitro study was to compare and evaluate the retentive capacity of three different attachment systems for implant-supported overdentures, i.e., the ball, the locator, and the equator system. The retentive capacity of each system was evaluated after multiple simulated insertion-removal cycles. The retentive values remained constant after many simulated insertion and removal cycles.

The objectives include: (1) Measure the retentive capacity of three different implant-retained overdenture attachment systems at the 1^st, 7500^th, and 15000^th levels; (2) Compare the retentive capacity of the three implant-retained overdenture attachment systems at the 1^st, 7500^th, and 15000^th; (3) Measure the retentive strength of three implant-retained overdenture attachment systems changes after being put in and taken out several times in a simulated manner at the 1^st, 7500^th, and 15000^th stages; and (4) Compare the changes in the retentive capacity of three different attachment systems after multiple simulated insertion-removal cycles at the 1^st, 7500^th, and 15000^th stages.

MATERIALS AND METHODS

The mandibular edentulous acrylic resin models were made with heat-polymerized polymethyl methacrylate resin (DPI Heat Cure, DPI, Mumbai, Maharashtra, India). The two implant replicas (Collagen Meniscus Implant) have a diameter of 3.75 mm and a length of 10 mm were placed. The acrylic resin mandibular overdentures were fabricated using heat-polymerized polymethyl methacrylate resin (DPI Heat Cure, DPI, Mumbai, Maharashtra, India). A prefabricated ball/O-ring attachment (Bioline dental implant series), OT Equator^® attachment (Rhein 83, Bologna, Italy), Locator^® attachment (Zest Anchor, Escondido, CA, United States), resin cement (RelyxTM, 3M ESPE, United States), universal testing machine (Instron 5567), manual thermocycling unit (two S-U-Polytub, Schuler Dental, Germany), and surveyor table and metallic clips were used in this study.

This study, which adhered strictly to all pertinent ethical principles, received approval from the ethical scientific committee of the local institution (Mahatma Gandhi Dental College and Hospital, Jaipur, India). The study model was fabricated using three wax patterns of standard mandibular edentulous models, which were made using modeling wax (Figure 1). Each wax model inserted two implant replicas in a parallel direction within the osteotomy site in the mandible at sites B and D. According to Misc et al[23], the two implants were separate, running alongside each other, positioned at the same horizontal level, perpendicular to the occlusal plane, and equidistant from the midline. We assessed their parallelism at the implant site by employing paralleled pins. Acrylic wax models were subsequently created using the compression molding technique (Figure 2).

Figure 1 Wax models. A: Ball/O-ring attachment; B: Locator attachment system; C: Equator attachment system.

Figure 2 Attachments. A: Ball/O-ring attachment; B: Locator attachment system; C: Equator attachment system.

Group 1 used the pre-fabricated ball/O-ring attachment (Bioline dental implant series). In pre-fabricated ball/O-ring attachment, a diameter of 2 mm, a metallic housing with a rubber O-ring component was used. Group 2 used the OT Equator^® attachment (Rhein 83, Bologna, Italy). In pre-fabricated OT equator attachments, a diameter of 2 mm and a metallic housing with a nylon insert were used. It has a castable Hader bar with a length of 22 mm, a diameter of 1.8 mm, and a gauge of 13. The nylon rider measures 5 mm in length and 2.6 mm in width, with a moderate retention rate. The nylon rider measures 5 mm in length and 2.6 mm in width, with a moderate retention rate. Group 3 used the Locator^® attachment (Zest Anchor, Escondido, CA, United States). The implant replicas (Collagen Meniscus Implant, 3.75 mm × 10 mm) were put into the acrylic models using a physio dispenser, similar to an implant that would be put into an osteotomy site in the mandible. Locator^® attachment (Zest Anchor, Escondido, CA, United States) has Tissue cuff length = 1.0 mm, diameter = 3.86 mm. The retention force for the male blue locator inserts is 6.7 N, and maximum convergence = 20°. The models were sealed with resin cement (RelyxTM, 3M ESPE, United States). Each attachment system was secured into the implant replicas on the acrylic resin model, placed the overdentures with the corresponding housing on it, and tightened to 35 N/cm.

Experimental setup

Acrylic-made edentulous mandibles hold acrylic overdentures with their respective attachment systems (Figure 3). Some metallic clips were attached to the dentures and secured with clear auto-polymerized acrylic resin (Figure 4). The edentulous acrylic models were fixed in place using a surveyor table (Figure 5 and Figure 6).

Figure 3 Attachment assembly. A: Ball/O-ring attachment; B: Locator attachment system; C: Equator attachment system.

Figure 4 Mandibular dentures fabricated in a conventional manner using heat-cured acrylic resin.

Figure 5 Acrylic resin overdenture. A: O-ring housing for ball attachment; B: Equator metallic housing and nylon insert; C: Locator metallic housing and male insert.

Retention force testing

The Instron 8874 universal testing machine was set to perform 15000 insertion and de-insertion cycles on each denture specimen. There were cycles of a 2 mm upward movement at a crosshead speed of 50 mm/minute, followed by a downward movement with the same characteristics created in universal testing machine. The test machine was programmed for frequency and recorded retention strength data for each specimen over 15000 cycles. The readings were recorded from the start of the test, along with the retention force data for each cycle, ranging from 1 to 15000. The means of thirty values from the 1^st, 500^th, 1500^th, 3000^th, 4500^th, 6000^th, 7500^th, and 9000^th cycles were calculated for statistical analysis, using a significant level P of 0.05.

Figure 6 Mandibular overdenture models. A: O-ring housing for ball attachments; B: Equator metallic housing and nylon insert; C: Locator metallic housing and male insert.

RESULTS

Table 1 and Figure 7 display descriptive statistics for changing retention strengths in the three study groups throughout the cycle sequence outlined below. In group 3, average retention decreased from 22.460 ± 2.2 N at baseline to 11.79 ± 1.4 N after 15000 cycles. The equator system reduced retention from 19.2 ± 2.7 N to 11.2 ± 1.7 N, while the ball attachment system decreased from 22.62 ± 2.1 N to 13.78 ± 3.1 N at the end of the cycle series. The graph depicts the distribution of retention values across the three groups. All three groups showed statistically significant differences in the retentive value at the 7500^th and 15000^th cycles. Three systems demonstrated enhanced retention strength after the initial 1500 cycles. Until the 4500-cycle, the locator and equator systems showed similar retention levels. However, there was more fluctuation in the ball attachment group. Up to the 4500^th cycle, the locator and equator groups had higher median values, with the most significant disparity observed in the ball attachment system.

Figure 7 Mean values. A: Mean values of the retentive capacity of three attachment systems at cycle 1; B: Mean values of the retentive capacity of three attachment systems at cycle 7500; C: Mean values of the retentive capacity of three attachment systems at cycle 15000.

Table 1 Evolution of retention strength in all groups according to number of insertion/de-insertion cycle, ball attachment system, equator attachment system, and locator attachment system.

Cycles	Min	Max	Mean	Standard deviation	Mean differences	Change, %	P value	P value vs 1 cycle
Ball attachment system
1	21.8	23.9	22.620	0.7899			0.001 (S)
7500	12.0	15.1	17.390	0.7430	5.23	23.1		0.001 (S)
15000	15.9	18.5	13.78	0.79	8.84	39.1		0.001 (S)
Equator attachment system
1	17.4	20.1	19.200	0.7318			0.001 (S)
7500	12.5	14.2	13.22	0.48	5.98	31.1		0.001 (S)
15000	10.3	12	11.2	0.53	8	41.6		0.001 (S)
Locator attachment system
1	21.4	23.6	22.460	0.7230			0.001 (S)
7500	13.5	14.8	14.3	0.46	8.16	36.3		0.001 (S)
15000	11.3	12.7	11.79	0.37	10.67	90.5		0.001 (S)

The Kruskal-Wallis test was performed. Once again, there were notable disparities in retention values across the whole cycle, except the 500^th cycle. Compared to the baseline value, the locator group experienced a mean percentage change of 36.3% and 90.5% at the 7500^th and 15000^th cycles, respectively. In the equator group, 31.1% and 46.1% had a change in value from the baseline at the 7500^th and 15000^th cycles, respectively. The percentage change in the ball attachment group was 23.1% and 39.1% at the same cycles of the baseline value, respectively. A nonparametric statistical method known as the Kruskal-Wallis test compares the medians of two or more groups of data. This test evaluates whether the medians of the groups are identical. We randomly select the samples to ensure the independence of the observations and to maintain a minimum ordinal measurement scale for the dependent variable. When we violate the assumptions of normality and homogeneity of variance, this method becomes useful. This method proves advantageous when analyzing data that deviates from the normal distribution, such as microbiome data in health research. The test establishes and ranks the data from smallest to largest, sums the ranks in each subgroup, and determines the statistic H value. The null hypothesis states that the medians of both groups are equal. We reject the null hypothesis if the H statistic value demonstrates significance[20]. We used the Kruskal-Wallis test (Table 2, Figure 8) to compare the initial and end retention, revealing substantial disparities in median values. This suggests that the ball attachment method experienced reduced retention loss during the fatigue testing cycles. The mean retentive force for the ball attachment system was 22.620 N, 17.390 N, and 13.88 N from the baseline at the 7500^th and 15000^th cycles, respectively. The Locator^® attachment system came in second with 22.46 N (base), 14.3 N (cycle 7500), and 11.79 N (cycle 15000). And the OT Equator^® attachment system came in third with 19.2 N (base), 13.22 N (cycle 7500), and 11.2 N (cycle 15000). All three groups showed statistically significant differences in the retentive value at the 7500^th and 15000^th cycles.

Figure 8 Kruskal-Wallis test results (P < 0.05) and mean values.

Table 2 Kruskal-Wallis test results (P < 0.05) and mean values.

Circles	P value
Initial	0.001
500	0.2
1500	0.001
3000	0.001
4500	0.001
6000	0.001
7500	0.001
9000	0.001
15000	0.001

DISCUSSION

Complete dentures pose a challenge for retention because of the complex oral musculature and its attachments. In most edentulous patients, decreased retention in prostheses due to bone resorption causes mastication discomfort and dissatisfaction with conventional complete dentures. Implant placement in a completely edentulous mandibular arch is one of the treatment options for retaining or supporting long-term restorations[24]. Mandibular implant-retained prostheses outperform conventional dentures in each aspect, potentially overshadowing the challenge of excellent retention[2]. The purpose of this in vitro study was to compare and rate how well the ball, equator, and locator attachment systems held their shape during fatigue testing, which involved putting and taking out the attachments up to 15000 times. This in vitro study simulated two implant-retained overdentures by placing two implant analogs 23 mm apart at the canine eminence. For completely edentulous patients, this is considered the minimum standard of care. The procedure involved vertically separating the denture from the base. Fatigue or failure of overdenture attachments adversely affect their function, maintenance, and patient satisfaction. We generated 15000 pulls of separation for each specimen to assess the retentive capacity of the attachment systems. In their study, Al-Ghafli et al[25] found that if someone used the prosthesis every day for ten years, it would separate 15000 times, assuming they took out and put back in the overdenture four times a day.

The most common instrument for vertical separation and peak load force testing in in vitro studies is the Instron universal testing machine. We used an upward movement of 2 mm at a crosshead speed of 50 mm/minute, which approximates the rate at which patients remove the implant overdenture[9]. Although the speed of over-denture removal remains unproven, Sarnat et al[26] proposed that it could approximate the speed at which a real overdenture detaches from its holding elements under vertical force. We subjected the obtained results to statistical analysis and conducted comparisons to assess the retentive ability during different cycles corresponding to previous years of denture usage. The results indicated differences in peak load-to-dislocation between the three types of attachments. The ball attachment (28.4 ± 5.86 N) showed the maximum amount of retention, followed by the equator attachment (26.9 ± 7.75 N) and the locator attachment (26.6 ± 4.14 N).

Further data was analyzed to determine the percentage changes in the three attachment systems. The ball and locator systems showed similar characteristics at baseline, with a mean retention capacity of 22.6 N and 22.4 N, respectively, and the Equator system showed a retention capacity of 19.2 N. However, at the 7500^th cycle, there was a reduction of 23.1% for the ball attachment system, 31.1% for the equator, and 36.3% for the locator attachment. After that, the decline in retentive values continued for all three attachment systems until the 15000^th cycle, when the ball attachment system showed a mean retention of 13.7 ± 8.84 N (reduction by 39.1%) and the locator system demonstrated a mean retention of 11.79 ± 10.67 N (reduction by 90.5%). In contrast, the OT equator had a mean value of 11.2 ± 8 N (a decrease of 41.6%).

The results indicated that the three attachment systems maintained or increased retention until around three years of usage. Ball attachment systems showed the highest retention until one year, when the patient inserts an overdenture four times daily. Equator and locator showed their highest retention for 3 years. There is a direct association between adequate retention, improved patient satisfaction, and increased quality of life. Various studies[27-30] have shown that forces varying between 10 and 20 N are sufficient for minimum retention in mandibular ODs, ensuring stability. Mínguez-Tomás et al[31] in their study evaluated retentive capacity in different attachment systems and found sufficient retention capacities after 14600 cycles and provided more precise data on attachment wear and retention loss.

Al-Ghafli et al[25] suggested some of the factors influencing retention loss, like the number and position of implants, the type of material used to fabricate the attachments, prosthetic design, and forces of different magnitudes in different directions. Conversely, Rutkunas et al[9] proposed that friction between the male and female retention elements, resulting from deformation or dimensional changes in the inner diameter of attachment nylon inserts, causes wear during the simple daily use of prosthesis. Different groups of researchers did an in vitro study that measured retention force in several different areas. These included fatigue (after 100, 200, 500, 1000, and 5000 dislodgement cycles), thermal undulation (10000 cycles at 5 and 55 degrees), implant angulation (0, 5, and 10 degrees), and disinfectants (three different agents). Repeated dislodging and thermal undulation did not alter the retention forces. Locator attachments showed a notable reduction in retention force of up to 58%. Implant angulation did not induce any significant changes in retention forces[24-27,31].

This study’s result suggests that the ball attachment system in OD-1 maintains its superior retentive capacity until it begins to decline one year later. Still, the equator and locator maintain excellent retention for 3-4 years and then show rapid decline thereon. After 5 years, all three maintain good retention - even up to 10 years in the testing scenario. This could be due to the ball attachment system, which consists of a titanium male unit and a rubber-ring female unit. This system transfers stress to the abutments and provides an excellent shock-resorbing effect during operation. This can be due to the design of the ball attachment, which provides a bigger mechanical undercut. We performed this study in a controlled experimental setting to evaluate the retentive capacity of three different systems used in implant-supported overdentures, but we only applied mono-directional forces during the evaluation. This approach does not represent a realistic model for a clinical situation with overdentures. Therefore, the first molars generate the main forces, which in turn generate rotational forces on the attachments through leverage[32-34].

CONCLUSION

Given the limitations of this in vitro study, the following conclusion can be drawn: (1) The ball/O-ring attachment system showed superior retentive capacity among all three attachment systems; (2) Locator, OT equator, and ball attachment systems maintain clinically acceptable retention after 10 years; (3) Retention increases from baseline values until around the 1500^th cycle mark in the ball attachment system; (4) Retention values were similar for the locator and equator attachment systems until the 4500^th cycle; and (5) All three groups showed statistically significant differences in the retentive value at 7500^th and 15000^th. The ball attachment and locator attachment systems maintain their retentive capacity longer than the OT equator attachment systems. All three attachment systems showed a statistically significant decrease in the retention force. Further research is required to understand the loss in the retention force of various overdenture attachment systems.