International Journal of Prosthodontics and Restorative Dentistry
Volume 13 | Issue 4 | Year 2023

An In Vitro Study to Evaluate Retention of Ball and Locator Attachments in Straight and Angulated Implant-supported Overdenture

Neha Gaur1https://orcid.org/0000-0002-8595-0245, Shefali Singla2, Lalit Kumar3, Sharique Rehan4

1–4Department of Prosthodontics and Crown & Bridge, Dr Harvansh Singh Judge Institute of Dental Sciences and Hospital, Panjab University, Chandigarh, India

Corresponding Author: Shefali Singla, Department of Prosthodontics and Crown & Bridge, Dr Harvansh Singh Judge Institute of Dental Sciences and Hospital, Panjab University, Chandigarh, India, Phone: +91 9417194185, e-mail: shefali_singla@yahoo.com

Received: 01 November 2023; Accepted: 08 December 2023; Published on: 30 December 2023


Purpose: To compare retention and retention loss exhibited by a pair of ball or locator attachments individually or in combination in straight and angulated two implant-retained overdentures (IRODs) over a period equivalent to 5 years of denture insertion and removal.

Materials and methods: The bilateral canine region of the edentulous mandibular model was used to receive two implants at 0 and 15°. Overdenture bases were retained by ball or locator attachments individually or in combination and grouped as group BB (retained by a pair of ball attachments); group LL (retained by a pair of locator attachments); group BL (retained by ball attachment on straight and locator attachment on 15° angulated implant); and group LB (retained by locator attachment on straight and ball attachment on 15° angulated implant). Respective housings were picked up in overdenture matrices (seven per group). After cyclic loading with 70 N of load for 100,000 cycles, retention values were recorded at various intervals of time during 7,200 insertion-removal cycles. Data was analyzed using one-way analysis of variances (ANOVA) and Tukey’s multiple comparison test.

Results: A significant difference was found for intergroup comparison of mean retention. Group LB maintained the highest initial and final retentive force, that is, 182.44 ± 63.59 N and 108.64 ± 52.03 N, respectively. Group LB exhibited the highest mean loss of retention of 42.42 ± 9.88%, followed by BB (41.38 ± 4.25%), BL (35.57 ± 5.78%), and lastly LL (24.65 ± 6.60%). There was a significant decrease in mean retention for all the groups during the 1st year and subsequently from 2 to 5 years except LL.

Conclusion: There exists a significant loss in mean retention in all combinations of ball and locator attachments at straight and angulated implants. The combination of a locator at 0° and ball attachment at 15° exhibited the highest retention, in spite of the maximum percentage loss of retention. Therefore, the combination of a locator on a straight and ball attachment on an angulated implant seems suitable and can ensure adequate retention during long-term clinical use.

How to cite this article: Gaur N, Singla S, Kumar L, et al. An In Vitro Study to Evaluate Retention of Ball and Locator Attachments in Straight and Angulated Implant-supported Overdenture. Int J Prosthodont Restor Dent 2023;13(4):235–241.

Source of support: Nil

Conflict of interest: None

Keywords: Ball attachment, Implant angulation, Implant overdenture, Locator attachment, Retention force


Implant-retained overdentures (IRODs) have been established as the gold standard treatment modality for the edentulous mandible. Their cost is relatively less compared to fixed implant-supported prostheses. They also provide superior results to conventional dentures in terms of patient satisfaction, comfort, chewing ability, and quality of life.15

The overdentures are retained over the underlying implants via precision attachments. By reducing the lateral movement of the prosthesis, these attachments provide superior effectiveness without adding undue stress to the implants.6,7 They impart retentiveness, support, and stability to these prostheses along with longevity and ease of maintenance at a reasonable cost.8 Bar and clip-like attachments splint the implants whereas others like magnets, telescopic crowns, ball types, and locators keep them unsplinted. Bar attachments are disadvantageous in terms of high cost, cumbersome fabrication, poor maintenance of oral hygiene, and requirement of greater interocclusal space.3

Unsplinted attachments offer the benefit of better cleansing, simplified fabrication procedure, and adequate retention along with adaptability to the changes in oral conditions.9 Ball attachments provide varying degrees of resiliency in both vertical and horizontal directions and are commonly used in two-IROD in the anterior region.10 A prefabricated, self-aligning locator attachment is reportedly an alternative to ball attachments in situations where there is less interarch space and requires low-profile attachments.11 Angulated implants can be managed by using the pivoting ability of an extended range locator that can accommodate divergence from 10 to 40° between two implants.12

According to Goodacre et al.,13 the most common (33%) prosthodontic complication of IROD is loss of retention of the attachments due to wear of resilient housing over time.13 Wear is a complex process that involves adhesion, abrasion, surface fatigue, and corrosion. Both the direct factors like position, number, angulation, and interimplant distance, and the indirect factors such as ridge resorption influence the retention and wear of overdentures.14

It is recommended that multiple implants should be placed parallel to each other in order to better distribute occlusal stresses and prevent early wear. However, this parallelism is often hard to achieve due to local anatomic factors or operator skills and could jeopardize the success of the prosthesis.15 One or more implants may have to be angled buccolingually or mesiodistally in accordance with available bone morphology. Angulation of implants along with nonaxial insertion-removal of the prostheses retained over them is the major cause of wear of nylon housing and loss of retention necessitating frequent replacement of the resilient housing.9

Sometimes clinical situations enforce a combination of straight and angulated implants, which might accelerate the wear of its attachment due to the nonparallel path of insertion and removal.14,15 The behavior of a combination of ball and locator attachments has not been explored in terms of long-term retention in a two-IROD, where one implant is straight and another is angulated.

Enough information regarding the suitability of an attachment in a particular clinical situation is not available in previous literature.1,14,16 The selection of appropriate attachments in terms of the amount of retentive strength and how the attachment maintains that strength is an important consideration.

Locator attachments are supposedly self-aligning and provide better retention with angulated implants but have shown more retention loss due to wear as compared to ball attachment.15 In light of the above, the purpose of this in vitro study was to compare rétention and retention loss exhibited by a pair of ball or locator attachments individually or in combination in straight and angulated two-IROD over a period equivalent to 5 years of denture insertion and removal. The null hypothesis was that there would be no difference in retentive capability and retention loss due to the wear of a pair of ball or locator attachments or a combination of the two attachments in an overdenture supported by a straight and an angulated implant.


The present in vitro study was conducted at the Department of Prosthodontics and Crown and Bridge of Dr Harvansh Singh Judge Institute of Dental Sciences and Hospital, Panjab University, Chandigarh, and Indian Institute of Technology, New Delhi. The sample size of seven overdenture bases/matrices per study group was calculated assuming a type I error to be 5%, type II error of 20, and 80% power of study with reference to previous studies.15,16

A heat cure polymethylmethacrylate resin (DPI, Mumbai, India) model of a completely edentulous mandible was used in this study. Two implants (Pivot Implant, Pivot Fabrique Inc, Mohali, India) of dimension 3.7 × 10 mm were placed at 0 and 15° bilaterally in the canine region using physiodispenser (ST-923, W&H Implantmed, Burmoos, Austria). The implant was placed using an implant surgical guide maintaining the desired angulation during the drilling procedure. The resin model ridge was covered with a 2 mm thickness autopolymerized silicone resilient liner (Esthetic Mask Automix, DETAX GmbH, Ettlingen, Germany). Ball and locator attachments (Fig. 1) were tightened with the hex driver for each sample based on the group specifications as follows (Fig. 2):

Figs 1A and B: (A) Ball attachment with its nylon cap and housing; (B) Locator attachment with its nylon cap and housing

Figs 2A and D: Study groups. (A) Group BB; (B) Group LL; (C) Group BL; (D) Group LB

Nylon caps were inserted into the metal housings and the combination of these two was aligned on each abutment on the patrix (Fig. 3A). Petroleum jelly (Vaseline, Unilever, Mumbai, India) was coated all over the model except for the housings, which were picked up along with matrix before cyclic loading and insertion/removal cycles. A matrix of light cure resin sheet (Plaque Photo, Willmann & Pein GmbH¸ Barmstedt, Germany) was adapted over the patrix, 2 mm short of vestibular depth. A T-shaped bar was attached joining at the midline and in the molar region and the tray was polymerized in the polymerizing unit (Sibari SR 620-Sirio Dental SRL, Meldola, Italy) for 5 minutes. A hole was drilled in the center of the T-shaped bar to facilitate the attachment of the matrix to the universal testing machine (UTM) (ZwickRoell HC 25 Servohydraulic testing machine, Ulm, Germany). Separate matrices (n = 28) were fabricated with metal housings for each of the seven samples of the four study groups (Fig. 3B).

Figs 3A and B: (A) Nylon caps within metal housings aligned on each abutment on patrix; (B) Overdenture matrix with picked-up nylon caps within metal housings and T-shaped bar

A pair of circular assemblies with a vertical rod was customized so as to connect the model along with the overdenture matrix to the UTM (Fig. 4A). In order to secure the model firmly, an adjustable L-shaped plate was incorporated with one of the circular component, which was further attached to the lower member of UTM (Fig. 4B). The vertical rod, however, was fastened with a nut to the T-shaped bar of overdenture matrices (n = 28) on one side and the second circular component on the other which was further secured with the upper member of UTM.11,16,17

Figs 4A to D: (A) Customized assembly with upper and lower circular parts to be attached with upper and lower members of UTM, respectively, and a vertical rod connecting the upper circular component with the overdenture matrix; (B) Lower assembly depicting the model firmly secured to the base via L-shaped plate; (C) Experimental setup for water bath; (D) Arrangement of customized assembly on UTM

For cyclic loading, the overdenture assembly (matrix + patrix) was placed in a water bath (Fig. 4C). A vertical force of 70 N was applied for 100,000 repeated cyclic loadings in 37°C deionized water to simulate oral conditions.17 Upon completion, matrix was connected to the upper crosshead of UTM to facilitate 7,200 insertion-removal cycles (Fig. 4D). The cycle consisted of an upward movement of 2 mm at a crosshead speed of 50 mm/minute and a downward movement of the same characteristics.1723 The UTM was programmed to perform upward and downward movements at a frequency of 2 Hz. It recorded retention strength data for each of the 7,200 cycles for each sample of all four groups. However, data evaluation was done at 120 (1 month), 720 (6 months), 1,440 (1 year), 2,880 (2 years), 4,320 (3 years), 5,760 (4 years), and 7,200 (5 years) cycles. 24,25

Statistical software [IBM Corp. Released 2012. IBM Statistical Package for the Social Sciences (SPSS) Statistics for Windows, Version 21.0. Armonk, New York: IBM Corp.] was used for the statistical analysis. One-way analysis of variance (ANOVA) was done for intergroup comparison between all the groups at different intervals. Post hoc Tukey test done for pair-wise comparison of various groups.


Shapiro–Wilk test and Kolmogorov–Smirnov test depicted that the data was normally distributed (p > 0.05) for the majority of data in all four study groups. The statistical analysis of the data established the supremacy of group LB design over others in terms of mean initial rétentionn (182.44 N) at the end of the simulated period of 1 year (144.74 N) and then after 5 years (108.64 N). Reported retention values for each study group were significantly different (p < 0.05) at each study interval as per one-way ANOVA (Table 1). The study reported a reduction in mean retention provided by each combination of attachments under study at the end of 1-year and after 5-year intervals. One-way ANOVA for intergroup comparison gave significant differences in percentage retention loss in all study groups during the 1st year and overall 5-year study period (Table 2). However, during the 2–5 years of study period, percentage retention loss was not significantly different between study groups.

Table 1: Intergroup comparison of retention (using one-way ANOVA)
Number of cycles (simulating time period) Mean ± standard deviation (Newton) p-value
Group BB Group LL Group BL Group LB
120 (1 month) 113.09 ± 45.74 56.63 ± 19.47 58.17 ± 15.67 182.44 ± 63.59 0.0001*
720 (6 months) 92.99 ± 38.39 53.86 ± 18.82 51.62 ± 17.47 152.04 ± 57.80 0.0002*
1,440 (1 year) 86.32 ± 35.70 51.31 ± 18.39 47.99 ± 17.44 144.74 ± 57.06 0.0003*
2,880 (2 years) 78.29 ± 32.73 48.59 ± 16.81 44.01 ± 14.84 131.13 ± 56.15 0.0006*
4,320 (3 years) 73.26 ± 32.70 45.83 ± 16.63 42.12 ± 13.59 120.26 ± 50.30 0.0009*
5,760 (4 years) 70.54 ± 31.45 44.37 ± 16.34 40.74 ± 13.14 112.32 ± 52.55 0.0028*
7,200 (5 years) 67.58 ± 28.81 42.93 ± 16.15 37.68 ± 11.65 108.64 ± 52.03 0.0024*

*, Statistically significant at p < 0.05; group BB, retained by a pair of ball attachments; group LL, retained by a pair of locator attachments; group BL, retained by ball attachment on straight and locator attachment on 15° angulated implant; group LB, retained by locator attachment on straight and ball attachment on 15° angulated implant

Table 2: Intergroup comparison of mean percentage loss in study groups (%) using one-way ANOVA
Number of cycles (simulating time interval) Mean ± standard deviation (%) f-value p-value
Group BB Group LL Group BL Group LB
120 (1 month) – 1,440 (1 year) 22.95 ± 7.47 9.91 ± 3.77 19.13 ± 8.80 21.33 ± 5.11 4.702 0.0101*
2,880 (2 years) – 7,200 (5 years) 14.89 ± 10.42 12.33 ± 4.16 13.42 ± 6.43 18.68 ± 7.13 0.8441 0.4832
120 (1 month) – 7,200 (5 years) 41.38 ± 4.25 24.65 ± 6.60 35.57 ± 5.78 42.42 ± 9.88 8.273 0.0006*

*, Statistically significant at p < 0.05

Further, Tukey’s multiple comparison tests (Table 3) determined that percentage retention loss during 1st year was significantly higher in BB (p = 0.011) and LB groups (p = 0.029). Group LL had the least percentage retention loss during the 1st year and subsequent periods. The difference in percentage retention loss was statistically insignificant between the four groups from 2 to 5 years (p > 0.05). Overall percentage retention loss was also significantly higher in BB (p = 0.0018) and LB (p = 0.0009) groups as compared to group LL (p = 0.0535) and BL (p = 0.3414). However, effective retention loss was more in the LB and BB groups in spite of higher initial and total retention loss.

Table 3: Tukey’s multiple comparison test for percentage retention loss
Number of cycles No of cycles (simulating time interval) Tukey’s multiple comparison test p-value for % mean loss
120 (1 month) – 1,440 (1 year) Groups BB vs LL 0.011*
Groups BB vs BL 0.748
Groups BB vs LB 0.9735
Groups LL vs BL 0.099
Groups LL vs LB 0.0292*
Groups BL vs LB 0.9373
2,880 (2 years) – 7,200 (5 years) Groups BB vs LL 0.9314
Groups BB vs BL 0.9856
Groups BB vs LB 0.8108
Groups LL vs BL 0.9941
Groups LL vs LB 0.4603
Group BL vs LB 0.6126
120 (1 month) – 7,200 (5 years) Groups BB vs LL 0.0018*
Groups BB vs BL 0.4825
Groups BB vs LB 0.9937
Groups LL vs BL 0.0535
Groups LL vs LB 0.0009*
Groups BL vs LB 0.3414

*, Statistically significant at p < 0.05


The present study reported a significant difference in retention capacity and wear-induced retention loss of a pair of ball or locator attachments or a combination of the two attachments in an overdenture supported by a straight and an angulated implant. Thus, the null hypotheses were rejected.

Literature reports a variable range of retention provided by ball and stud attachments in two IRODs at various implant angulations.7,10,20,24,26 These studies vary in terms of their clinical or experimental setup, number of insertion-removal cycles, frequency or direction of dislodgement force, and type and material of attachment system.

The present study compared retention provided by ball and locator attachments in two IROD experimental setups with a straight implant in the mandibular right canine region and a 15° angulated implant in the left canine region. Due to variation in retention and wear of ball and locator attachments in relation to angulated implants, two of the study groups evaluated retention and retention loss of a combination of ball and locator attachment (BL and LB group).

A significant difference was found in the mean retention values of all study groups at all study intervals in the present study. The BB group using ball attachments on both implants exhibited significantly higher initial mean retention than the LL group. These results are consistent with studies of Kurtulus and Gurbulak15 and de Albuquerque et al.27 who in their respective in vitro and in vivo studies reported superior initial mean retention with ball attachments in two IRODs. Out of the combination groups, the LB group with ball attachment on a 15° angulated implant showed significantly higher retention than the BL group and also than the BB and LL groups. This difference might be because, on a 15° angulated implant, the patrix ball abutment exhibited greater undercut on the distal than on the mesial side (Figs 1A and 2D), thus forming a challenging path of removal for matrices in this group as compared to BL and other study groups (Fig 2A2BC).17 LL and BL groups performed poorly in the 0 and 15° angulated implant setup possibly because locator matrix housings could not fully engage the accentuated undercut on the distal side of the locator patrix on the angulated implant (Fig 2B and C). Also, the pivot locator attachment used in the study lacked an inner undercut contributing to a dual retention mechanism (Fig. 1B).17

Initial few insertion-removal cycles were ignored in all study groups to allow an adaptation period for the attachments in the experimental setup so as to achieve optimum function.21 Baseline retention forces for the four study groups in this study ranged from 56.63 N (LL group) to 182.44 N for the LB group with significant difference (p < 0.00001) thus rejecting the first null hypothesis.

Higher retention values than previous studies15-2831 can be explained on account of differences in the test setup, testing equipment, implant attachment system, clear nylon housing with maximum retention, interimplant distance, location in an arch location in the arch, and resilient silicone layer on model ensuring better marginal seating of denture base in this study. Initial retention forces were significantly greater than at the subsequent study intervals, consistent with the findings of previous studies.15,2831 By the end of 7,200 cycles simulating a 5-year period (four per day), mean retention was significantly reduced in all study groups (p = 0.024) with the LB group exhibiting the highest (108.64 N) and BL group exhibiting the least (37.68 N) retention but all in acceptable range for clinical use as per earlier studies.15,32

This loss of retention is a major limitation of IROD and can be attributed to wear of the matrix–patrix interface caused by insertion-removal cycles. Few samples in various groups exhibited an increase in retention during some intervening cycles. This may be due to plastic deformation of nylon leading to increased hardness and surface roughness which thereby increases the retentive properties17,33 or it may be due to thermal expansion.2834 The configuration of the ball and locator varies significantly. A ball abutment has only one retentive constriction at the outer surface of its base as compared to an inner and outer undercut in locators (dual retention mode). Since ball abutments allow movement in all spatial planes, the amount of wear is higher than what is exhibited by locators which permit movement in only one plane.35 The results also demonstrated that the loss of retention in the initial 1 year of usage was significantly higher (p = 0.0001) than subsequent wear activity in 2–5 years of use in BB, BL, and LB groups. This can be attributed to the fact that the maximum wear occurs during the 1st year after which the intimate contact of the nylon insert was lost with the abutments. Due to this, friction decreased between the surfaces between 2 and 5 years of span and the percentage loss of retention became more or less constant for all combinations of ball and locator attachments and at all the angulations.11

Percentage loss of retention in the LL group with locator attachment on both implants during all study intervals was significantly lower than the other three groups as per Tukey’s multiple comparison test. This can be explained on account of their self-aligning property in nonparallel implants which reduced wear.36 The percentage loss in retention between all study groups for the simulated period of initial 1 year (p = 0.0101) and baseline to 5 years (p = 0.0006) was significant. Thus, the second null hypothesis stating that there is no difference in loss of retention in the study groups after simulated insertion-removal cycles for the study period was also rejected. The study also reports that percentage retention loss was much reduced after an initial 1 year period of use in all study groups. This may be due to adaptive changes in the surfaces of matrix and patrix components after initial wear in function.11,17

Most in vitro studies in the literature were performed using implant analogs embedded in dental stone,37 metal rings,26 ultra-high-molecular-weight polyethylene blocks,11 or acrylic blocks.16,38 In the present study, an edentulous mandibular model was fabricated using heat-cure acrylic resin which has higher endurance for all types of forces than self-cure acrylic resin. 2 mm of resilient liner simulating the mucosa was also applied in order to make the model as close to the clinical scenario as possible and enhance the marginal seal of the overdenture base during function. The same master model was used in all the groups. All the matrices and the attachments were of the same material, that is, nylon and titanium, respectively, both belonging to the same manufacturer (Pivot Implant, Pivot Fabrique Inc).

Cyclic loading for 100,000 cycles was performed in a water bath with deionized water at 37°C to simulate oral conditions.17 The 7,200 cycles of insertion/removal were calculated based on the assumption that on an average patients remove their dentures four times per day.15 In order to obtain consistent results, retention was evaluated under standardized conditions during linear dislodgement cycles.17,18

In spite of the high percentage retention loss in group LB at the end of 1st year (21.33%) and at the end of 5 years (42.42%), the mean retention value for a combination of ball attachment on 15° angulated implant and locator attachment on straight implant was significantly higher at end of 1st year (144.74 N) and also at end of 5 years (108.64 N) compared to all other groups. Group BB using a pair of ball attachments on both implants performed better than the LL and BL groups, though its percentage retention loss was higher. This study also revealed that cyclic dislodgement in two-IROD with 0 and 15° angulation using different combinations of ball and locator attachment caused significant loss in retention during 1st year and thereafter.

The limitation of this study was that the overdenture dislodgement pattern could not completely replicate movements during masticatory function, insertion-removal path, and oral habits of the patients. Only unidirectional vertical forces were applied throughout the experiment, while in a clinical situation with a two-IROD, a rotational movement will inevitably be present in the region of the first molars and therefore create multidirectional forces, which will exacerbate the wear of the attachment components. The effect of wear under moisture, temperature, and denture cleansers was not considered. Further research needs to be conducted to validate the results of retention and wear in this study in an oral environment.


Considering the limitations of the present in vitro study, it can be concluded that for optimum long-term clinical use in two IRODs supported by a straight and 15° angulated implant, a combination of locator attachment on straight and ball attachment on the angulated implant can be used. This combination exhibited significantly higher mean retention at all intervals in spite of the highest percentage retention loss. In cases where the parallel placement of implants in the mandibular arch is restrained by some reason a combination of two stud attachments in a two-IROD can be considered.


Neha Gaur https://orcid.org/0000-0002-8595-0245


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