The O-rings in ball retained overdentures deteriorate with time and need replacement to restore the retentive quality. We evaluated retrospectively the mechanical properties of O-rings after 3 years in function in 1 and 2-piece implant-supported overdentures. The O-rings were retrieved from one-piece (Myriad snap, Equinox-Straumann, 3.3 × 13 mm) and 2-piece (Neo Biotech, 3.3 × 13 mm) implant-supported overdenture patients. A total of 16 pairs of matrices were tested for wear, type of damage, and elasticity using Pin on Disc method, USB Digital Camera in 30× zoom and Universal Tensile Machine, respectively. The statistical analysis for independent groups were done with the Mann-Whitney U test. Assessment of used O-rings showed 84% more wear in the 2-piece system with an abrasive type of damage while 46% wear in the 1-piece system with a compressive type of damage. The O-rings in 1-piece system showed increase in elongation and maximum displacement to 2% and 7%, respectively, whereas the 2-piece system showed decrease in elongation and maximum displacement by 13% and 6%, respectively. In 1-piece system, the loss of retention was more with slow wear rate, and in 2-piece system, the wear resistance of O-rings decreased due to increased stiffness. Further studies to evaluate the changes in O-ring with increased sample size and at interval 1 year will pave way for insight into the progressive changes in the mechanical properties of an O-ring.

The implant retained overdenture is considered an effective treatment plan for achieving retention and stability in complete dentures.14  With the availability of various resilient attachment systems, the most used in implant-retained overdenture are stud attachment consisting of a metal housing with the matrices.5  The matrix inserts are synthetic elastomeric polymers usually made of silicone, that has an inherent quality of elasticity and is compressed inside a metal housing for retention. Literature claims that the O-ring and ball attachment offers less stress to implant-bone interface compared with the bar and clip.6,7  However, the ball type of attachment system has mechanical failures such as (1) wear of matrix, (2) loss of elasticity, (3) fracture associated with the attachment system leading to loss of retention over time.8,9 

The initial retentive force of the matrices has been found to be 4N and gradually decreases with time10  due to a variety of forces involving rotation, flexion, and removal/dislodgement.12  Fromentin et al11  enumerated that several factors affect the retentive quality of the prosthesis, such as mechanical properties of the prosthetic components and the patient's masticatory force. Among the different prosthetic failures in the attachment system, retentive loss and/or tear of matrices is the most common failure leading to patient's dissatisfaction. Stress on the O-ring leads to elongation and maximum displacement that alters its physical property. The magnitude of force with which a material opposes this motion at rest is termed the frictional force, and that increases until the maximum limit (Fmax) after which the material does not oppose the motion. This lead to the wear of elastic material and loss of retention. The knowledge of physical properties that alters the retention and longevity of the prosthesis is essential to achieve prosthetic success.

In the treatment of mandibular edentulous arches for implant-supported overdenture, 1-piece and 2-piece implants are widely used and have comparable survival rates. The 1-piece implant system has the expediency of immediate loading without second-stage surgery, whereas the 2-piece implant system has an implant-abutment junction with the convenience of delayed loading. The implant-supported overdenture is assessed in various aspects such as bone loss, systemic effects, and prosthetic failure due to denture fracture. Literature suggest that ball attachment wear was found to be maximal after 3 years of usage.11  However, O-ring matrices have not yet been assessed in vivo for its change in elastic property and wear resistance. Null hypothesis was proposed that there was an alteration in the elastic property that did not have an effect on the wear resistance of matrix component. Hence, we conducted an in vivo retrospective cross-sectional study to evaluate the elastic property, wear-resistance, and the type of failure in silicone matrices of 1-piece and 2-piece ball retained dentures after 3 years of prosthetic loading.

Subjects

A cross-sectional, observational, cohort study was conducted that followed the declaration of Helsinki-Ethical principles for medical research involving human subjects. The participants included were clinically healthy individuals and treated either with 1- or 2-piece implant-retained overdentures with ball and O-ring attachments that were in use for 3 years after initial loading (Figures 1 and 2). The patients were given new dentures with balanced occlusion using semianatomic teeth and the matrices were secured in position by chairside closed-mouth relining procedure during the rehabilitation stage. Patients were recalled based on the existing records stored in the database and the participants with parafunctional habits, neurological disorder, and any systemic disease that triggered bruxism were excluded. A nonrandom sampling with 16 patients were recruited as test group based on inclusion and exclusion criteria. The samples belonged to medium built south Indian males aged between 50 and 60 years. All the participants were informed of the study protocol and their consent was obtained. The participants had informed that they were using the prosthesis only during the daytime and avoided nighttime wear during sleep. The used O-rings were replaced with new O-rings and no patients were deprived of treatment. The retrieved O-rings, 8 pairs in each system, were taken for the study.

Figure 1.

One-piece implant system.

Figure 1.

One-piece implant system.

Close modal
Figure 2.

Two-piece implant system.

Figure 2.

Two-piece implant system.

Close modal

Materials

The O-rings retrieved after 3 years of function from 1-piece (Myriad snap, Equinox-Straumann, 3.3 × 13 mm) and 2-piece (Neo Biotech, 3.3 × 13 mm) implant retained overdenture wearers were the test samples. The control was the unused silicone O-rings from both the 1-piece and 2-piece implant systems.

Instrumentation/measurement

Universal Tensile Machine was utilized for testing the elasticity, and Pin on Disc method (American Society of Testing of Material [ASTM G99]) assessed the wear of retrieved silicone rings.13  To assess the type of damage 3 years postloading, the images were captured with USB Digital Camera in 30× zoom.

Procedures

Universal Tensile machine holds the O-ring at both ends to measure the elongation and maximum displacement of the material. Pin on disc measured the frictional force and wear with the O-ring placed on the testing disc. The property of unused silicone ring was tested as a control for both the 1-piece and 2-piece implant systems. For the validity of the results, a single examiner measured the readings for the elasticity and wear tests. For reliability of the results, the examiner operated the testing machine in standardized protocol.

Statistical analysis

The methodology was reviewed by an independent statistician as per requirement of the Journal. The collected data were analyzed with IBM SPSS statistics software (version 23.0). To describe the data, descriptive statistics, mean, and standard deviation were tabulated. The statistical analysis was done for the percentage deterioration from control value. To evaluate the significant difference between the bivariate samples for rate of deterioration in independent groups (1-piece and 2-piece systems), the Mann-Whitney U test was used and probability value 0.05 was considered as significant level.

The results revealed that the retrieved O-rings from both systems showed a significant difference from the control value in the elasticity and wear tests (P < .05) (Table 1). In the 1-piece implant system, the Fmax and stress values were reduced by 23% and 65%, respectively, whereas the elongation increased by 2% in the retrieved O-rings. Two-piece system showed a reduction in Fmax and elongation by 4% and 13%, respectively, whereas the stress values increased by 16%. The maximum displacement of O-ring increased by 7% in 1-piece system while reduced by 6% in 2-piece system. The wear in O-ring of the 1-piece system was increased by 46% while in the 2-piece system, it was increased by 84%. Similarly, the frictional force also increased by 59.26% and 66.88% in 1- and 2-piece systems, respectively. This indicates that the percentage deterioration in elastic property was higher for 1-piece system and the wear of O-ring was more for a 2-piece implant system (Table 2). In comparison, the rate of deterioration in both systems, there was a significant difference in the elastic and wear properties, except in maximum displacement in elasticity (P < .05) (Table 3). The type of damage in O-ring was compressive in 1-piece (Figure 3) and abrasive in 2-piece systems (Figure 4).

Table 1

Mechanical property of O-ring after 3 years of usage

Mechanical property of O-ring after 3 years of usage
Mechanical property of O-ring after 3 years of usage
Table 2

Rate of deterioration from control value in property of O-ring

Rate of deterioration from control value in property of O-ring
Rate of deterioration from control value in property of O-ring
Table 3

Test statistics using Mann-Whitney U between the groups for rate of deterioration*†

Test statistics using Mann-Whitney U between the groups for rate of deterioration*†
Test statistics using Mann-Whitney U between the groups for rate of deterioration*†
Figure 3.

Compressive type of damage in 1-piece system.

Figure 3.

Compressive type of damage in 1-piece system.

Close modal
Figure 4.

Abrasive type of damage in 2-piece system.

Figure 4.

Abrasive type of damage in 2-piece system.

Close modal

Consequences of biomechanical overload on implant-supported prosthesis were crestal bone loss, screw loosening, component fracture, and implant failure. The functional and parafunctional forces rotate the denture around the ball attachment and ultimately causes the matrix to wear and deform leading to retention loss even with controlled loading.1417  The specific mechanical property that determines the amount of matrix deformation with the loading of implant has not yet been ascertained in the literature. The literature search on wear of attachment system documented only the wear on the ball or patrix component, and the difference in wear properties of the titanium, gold, and nylon inserts,8,1719  but there was no study available to evaluate the property of silicone O-ring. Hence, the present study was performed to evaluate the mechanical property of silicone O-rings of 1- and 2-piece implants after 3 years of implant loading from the baseline to observe the durability of the material. Homogeneity in population was ensured by selecting medium statured male participants within the age group of 50 to 60 years treated with overdentures in balanced occlusion, which also eliminated age and gender bias. Literature also reveals that the body mass index has influence on bite force and obese individuals had less masticatory performance than individuals with normal body mass index.20,21 

In 2-piece implant system, with 3 years of loading, the stress was increased by 16%, while the strain measured as the elongation and maximum displacement decreased by 13% and 6%, respectively, compared with the control. The results depict that more force per unit area was required to displace the silicone ring that was stiffening with usage due to reduced elasticity. This result was in consensus with Botega et al22  who observed an increased retention of O-ring attachment after repeated removal and insertion in vitro. An in vitro study by Branchi et al8  suggested that the progressive loss of retentive component was more in O-ring matrix in comparison with titanium matrix. Our study disclosed that there was a relative resistance to movement due to increased frictional force by 66%, and accentuated wear of O-ring by 84%, suggesting that the deformation was due to the altered plastic property and increased surface roughness. Moreover, the increased wear rate observed was an abrasive type in the 2-piece O-rings were apparently due to reciprocating oscillation and rotary movement between the junctions of an implant to the superstructure.23  Rotatory movement of the abutment also happens due to inherent nature of micromotion in the implant-abutment junction of a 2-piece implant system.24,25  Though the range of micromotion was not measured in the present retrospective study, higher wear rate in a 2-piece system when compared with the 1-piece system, indicated the micromovement at implant-abutment junction contributed more to the deterioration of the O-ring.

In contrast with the 2-piece system, the 1-piece implant system showed the stress value to decrease by 65%, and the elongation and maximum displacement increased to 2% and 7%, respectively, compared with the control O-ring. The results suggested that a minimal force per unit area was required to elongate the silicone in 1-piece implant system. Also, the O-ring of 1-piece implant system was observed to have Fmax decreased by 23% and the frictional force increased by only 59%. The property of this material to elongate or displace more easily had caused a decrease in Fmax. We also observed the wear rate increased by only 46% after 3 years of loading when compared with 2-piece implant. Though the wear resistance had reduced similar to 2-piece system, it had slower wear rate compared with the 2-piece system. Literature reveals slower wear rate is associated with the higher modulus of elasticity of the matrix.8  Moreover, the reduced matrix thickness in macroscopic image of 1-piece O-ring depicted compression set type of damage. Absence of implant-abutment junction and micromovement in 1-piece implant26  could have compressed the O-ring toward the stable ball attachment during denture movement.27 

The present study rejected the null hypothesis and postulated that a material with an increased elastic property has an improved wear resistance. The housing in both the systems utilized silicone in their matrices, but our study revealed that the mechanical properties differed in the elasticity, wear and type of damage. In our present study, none of the selected patients had reported earlier than 3 years with the complain of retention loss of the overdenture and we attributed the prolonged retention provided by the attachment system to the difference in material quality of matrix and patrix component. Fromentin et al11  suggested the clinical wear of ball attachment was more in 3- and 8-years postimplant loading. The authors claimed that patient subjective opinion of retentive loss reported earlier than 3 years was not statistically related to wear of ball attachment. Bayer et al17  also suggested that matrix and patrix component made of different materials compensates wear of the attachment with altering retentive force on the overdenture. Our study had utilized titanium ball attachment patrix with the silicone matrix component. We found that the O-rings of 1- and 2-piece implant systems demonstrated changes in the mechanical properties after 3 years postloading. However, further evaluation of O-ring in 1 and 2 years of loading can provide knowledge over the frequency of silicone ring replacement to reduce the wear of ball attachment.

The changes in the elastic properties depend upon several factors such as the oral temperature, the contact fluids, and the mechanical friction, which eventually cause loss of retention. We found that 1-piece O-ring had the tendency to displace and more wear-resistant after 3 years of loading, whereas the 2-piece O-ring had increased rate of wear. Literature suggests the micromovement of implant-abutment junction causes less strain in the bone by a 2-piece implant system.28  However, we found that the rotatory movement or micromotion along with the reduced elasticity of O-ring in 2-piece system had increased the wear rate. But 1-piece implant system without an implant abutment junction had the O-ring with the high elastic property, resulted in compression of the O-ring within the housing and slower wear rate.

The limitation of our retrospective study was the sample size, and individual variation in oral environment that affected the elastic property and the range of micromovement. Further studies with larger sample size to evaluate relationship between the wear of ball attachment and silicone O-ring at 1-, 2- and 3-years interval are required. Assessment of the range of micromovement, occlusal force, and oral environmental factors that influence on the O-ring elastic property would be future scope of the present study.

We conclude that in the 1-piece system with high elastic property, had loss of retention with a slow wear rate, and in the 2-piece system with lower elastic property, the wear resistance of O-rings decreased due to increased stiffness.

The methodology was reviewed by independent statistician, Dr S. Venkatesan, MSc, MPhil, PhD, Proprietor, Zigma Analytics, Chennai.

1. 
Feine
JS,
Carlsson
GE,
Awad
MA,
et al
The McGill consensus statement on overdentures. Mandibular two-implant overdentures as the first-choice standard of care for edentulous patients
.
Gerodont
.
2002
;
19
:
3
4
.
2. 
Kim
SM,
Choi
JW,
Jeon
YC,
et al
Comparison of changes in retentive force of three stud attachments for implant overdentures
.
J Adv Prosthodont
.
2015
;
7
(4)
:
303
311
.
3. 
Bergendahl
T,
Engquist
B.
Implant supported overdentures: a longitudinal prospective study
.
Int J Oral Maxillofac Implants
.
1998
;
13
:
253
262
.
4. 
Gotfredsen
K,
Holm
B.
Implant-supported mandibular overdentures retained with ball or bar attachments: a randomized prospective 5-year study
.
Int J Prosthodont
.
2000
;
13
(2)
:
125
130
.
5. 
Mensor
MC
Jr.
Attachment fixation of the overdenture: part II
.
J Prosthet Dent
.
1978
;
39
(1)
:
16
20
.
6. 
Kenney
R,
Richards
MW.
Photoelastic stress patterns produced by implant-retained overdentures
.
J Prosthet Dent
.
1998
;
80
:
559
564
.
7. 
Tokuhisa
M,
Matsushita
Y,
Koyano
K.
In vitro study of a mandibular implant overdenture retained with ball, magnet, or bar attachments: comparison of load transfer and denture stability
.
Int J Prosthodont
.
2003
;
16
:
128
134
.
8. 
Branchi
R,
Vangi
D,
Virga
A,
Guertin
G,
Fazi
G.
Resistance to wear of four matrices with ball attachments for implant overdentures: a fatigue study
.
J Prosthodont
.
2010
;
19
(8)
:
614
619
.
9. 
Vafaee
F,
Fotovat
F,
Firuz
F,
et al
The amount of wear in attachment of Implant-supported overdentures in mandible
.
J Dent Mater Tech
.
2016
;
5
(4)
:
181
188
.
10. 
Kim
SM,
Choi
JW,
Jeon
YC,
et al
Comparison of changes in retentive force of three stud attachments for implant overdentures
.
J Adv Prosthodont
.
2015
;
7
:
303
311
.
11. 
Fromentin
O,
Lassauzay
C,
Nader
SA,
Feine
J,
Rubens
F.
Wear of matrix overdenture attachments after one to eight years of clinical use
.
J Prosthet Dent
.
2012
;
107
(3)
:
191
198
.
12. 
Sadig
W.
A comparative in vitro study on the retention and stability of implant-supported overdentures
.
Quintessence Int
.
2009
;
40
:
313
319
.
13. 
Raza
FB,
Vaidyanathan
AK,
Veeravalli
PT,
Ravishankar
S,
Ali
AS.
Analysis of crestal bone loss around single piece ball attachment implant placed bilaterally in canine region and wear of O-ring in implant supported overdenture: three year follow-up
.
Clin Implant Dent Relat Res
.
2018
;
20
(3)
:
403
409
.
14. 
Trakas
T,
Michalakis
K,
Kang
K,
Hirayama
H.
Attachment systems for implant retained overdentures: a literature review
.
Implant Dent
.
2006
;
15
(1)
:
24
34
.
15. 
Rutkunas
V,
Mizutani
H,
Takahashi
H.
Influence of attachment wear on retention of mandibular overdenture
.
J Oral Rehabil
.
2007
;
34
(1)
:
41
51
.
16. 
Bayer
S,
Grüner
M,
Keilig
L,
et al
Investigation of the wear of prefabricated attachments: an in vitro study of retention forces and fitting tolerances
.
Quintessence Int
.
2007
;
38
(5)
:
e229
e237
.
17. 
Bayer
S,
Steinheuser
D,
Grüner
M,
et al
Comparative study of four retentive anchor systems for implant supported overdentures–retention force changes
.
Gerodontology
.
2009
;
26
(4)
:
268
272
.
18. 
Büttel
AE,
Lüthy
H,
Sendi
P,
Marinello
CP.
Wear of ceramic and titanium ball attachments in subjects with an implant-retained overdenture: a controlled clinical trial
.
J Prosthet Dent
.
2012
;
107
(2)
:
109
113
.
19. 
Alsabeeha
NH,
Swain
MV,
Payne
AG.
Clinical performance and material properties of single-implant overdenture attachment systems
.
Int J Prosthodont
.
2011
;
24
(3)
:
247
254
.
20. 
Isabel
CA,
Moysés
MR,
van der Bilt
A,
Gameiro
GH,
Ribeiro
JC,
Pereira
LJ.
The relationship between masticatory and swallowing behaviors and body weight
.
Physiol Behav
.
2015
;
151
:
314
319
.
21. 
Araujo
DS,
Salomé Marquezin
MC,
de Souza Barbosa
T,
Duarte Gavião
MB,
Castelo
PM.
Evaluation of masticatory parameters in overweight and obese children
.
Eur J Orthod
.
2016
;
38
(4)
:
393
397
.
22. 
Botega
DM,
Mesquita
MF,
Henriques
GE,
Vaz
LG.
Retention force and fatigue strength of overdenture attachment systems
.
J Oral Rehabil
.
2004
;
31
(9)
:
884
889
.
23. 
Misch
CE.
Available bone and dental implant treatment plans
.
Dent Implant Prosthetics
.
2014
;
21
:
315
.
24. 
Trisi
P,
Perfetti
G,
Baldoni
E,
Berardi
D,
Colagiovanni
M,
Scogna
G.
Implant micromotion is related to peak insertion torque and bone density
.
Clin Oral Implants Res
.
2009
;
20
(5)
:
467
471
.
25. 
Liu
Y,
Wang
J.
Influences of microgap and micromotion of implant-abutment interface on marginal bone loss around implant neck
.
Arch Oral Biol
.
2017
;
83
:
153
160
.
26. 
Winter
W,
Klein
D.
Micromotion of dental implants: basic mechanical considerations
.
J Med Eng
.
2013
;
2013
:
265412
.
27. 
Raza
FB,
Vaidyanathan
AK,
Veeravalli
PT,
Ravishankar
S,
Ali
AS.
Analysis of crestal bone loss around single piece ball attachment implant placed bilaterally in canine region and wear of O-ring in implant supported overdenture: three year follow-up
.
Clin Implant Dent Relat Res
.
2018
;
20
(3)
:
403
409
.
28. 
Wu
AY,
Hsu
JT,
Chee
W,
et al
Biomechanical evaluation of one-piece and two-piece small-diameter dental implants: in-vitro experimental and three-dimensional finite element analyses
.
J Formos Med Assoc
.
2016
;
115
:
794
800
.