journal of prosthodontic research 58 (2014) 115–120
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/jpor
Original article
Pressure distribution of implant-supported removable partial dentures with stress-breaking attachments Kentaro Kono DMDa,*, Daisuke Kurihara DMD, PhDa, Yasunori Suzuki DMD, PhDb, Chikahiro Ohkubo DMD, PhDa a
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine, 2-1-3, Tsurumi, Tsurumi-ku, Yokohama, Kanagawa, Japan b Division of Oral Maxillofacial Implantology, Tsurumi University School of Dental Medicine, 2-1-3, Tsurumi, Tsurumi-ku, Yokohama, Kanagawa, Japan
article info
abstract
Article history:
Purpose: This in vitro study investigated the pressure distribution of the implant-supported
Received 5 November 2013
removable partial dentures (RPDs) with the stress-breaking attachments under the occlusal
Received in revised form
force.
30 December 2013
Methods: The experimental model of bilateral missing premolars and molars was modified
Accepted 27 January 2014
from a commercial simulation model. Five pressure sensors were embedded near the
Available online 11 March 2014
bilateral first molars, first premolars, and medio-lingual alveolar crest. Two implants were placed near the second molars, and they were connected to the denture base using the
Keywords:
following conditions: complete separation between the denture base and implant with
Implant-supported removable
cover screws (CRPD), flexible connection with a stress-breaking ball (SBB) attachment, and
partial denture
rigid connection without stress breaking with healing caps (HC). The pressure at five
Stress-breaking ball (SBB)
different areas of the soft tissue and the displacement of the RPDs were simultaneously
attachment
measured, loading up to 50 N. The coefficient of variation (CV) for each connection was
Pressure distribution
calculated from all data of the pressure at five areas to evaluate the pressure distribution.
Implant
Results: The pressure on medio-lingual alveolar crest and molars of the HC was less than SBB and CRPD. In contrast, the pressure on premolars of SBB was greater than for the HC and CRPD. The CV of SBB was less than that of HC and CRPD. Denture displacement of HC and SBB was less than for CRPD. Conclusions: Within the in vitro limitations, precise denture settlements and pressure distribution under the denture base could be controlled using an SBB attachment. An SBB attachment might be able to protect the implant from harmful force. Crown Copyright # 2014 Published by Elsevier Ireland on behalf of Japan Prosthodontic Society. All rights reserved.
* Corresponding author. Tel.: +81 45 580 8420; fax: +81 45 573 9599. E-mail address: [email protected] (K. Kono). 1883-1958/$ – see front matter. Crown Copyright # 2014 Published by Elsevier Ireland on behalf of Japan Prosthodontic Society. All rights reserved. http://dx.doi.org/10.1016/j.jpor.2014.01.002
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1.
journal of prosthodontic research 58 (2014) 115–120
Introduction
Distal-extension removable partial dentures (RPDs) were associated with several problems related to their limited stability, retention, aesthetics, and masticatory efficiency [1– 4]. The rotational movements of RPDs might produce terminal torque forces against the abutment teeth and the soft tissue [5,6]. Ill-fitting retainers, occlusal disharmony, and pain in the soft tissue under the connector or denture base are frequently observed after long-term use [7–10]. In addition, constant pressure from the denture base gradually causes ridge resorption under the denture base [11,12]. To prevent the rotational movements of RPDs, precise attachments or telescope systems have been used on the remaining teeth, and an altered cast technique has been applied to offset different displacement between the remaining tooth and soft tissue during function [13–15]. However, the rotational change of RPDs cannot be completely prevented in long-term use even if the above techniques are used. Particularly, these phenomenon are remarkably occurred for short term in the cases of Eichner classification C1 [10]. A bilateral implant placement at the distal extension of the denture base will minimize the resultant denture displacement. Consequently, less ridge resorption, fewer numbers of relining, and minimum decrease of retentive force of precision attachments would be leaded [16–18]. Moreover, the survival rate of this treatment option within 10 years is comparable to other implant therapy [19,20]. However, there are extraordinary differences in settling during a chewing load between the implant and mucosa under the denture base [21,22]. In addition, horizontal forces and rotational moments would also be applied to the implants depending on the occlusal contact, location, and number of implants in the dental arch. Therefore, excessive and harmful occlusal forces might be applied to the implant. To protect implants from excessive force, stress-breaking attachments, such as an ERA attachment, a locater attachment, or a cushion-type magnetic attachment, have been manufactured as conventional commercial attachments. However, these attachments cannot exactly compensate for the different amounts of pressure displacement of the mucosa in individual patients [23,24]. A stress-breaking ball attachment, hereafter called ‘‘SBB attachment,’’ for implants was developed with the Department of Prosthodontics, Gerodontology and Oral Rehabilitation, Osaka University Graduate School of Dentistry to prevent excessive and harmful occlusal forces. The SBB attachment can distribute the occlusal force equally between the alveolar ridge and the implant [25]. Because these attachments are covered with a silicone housing with three amounts of space to allow three kinds of settlement, i.e., 0.3 mm, 0.5 mm, and 0.7 mm, they can be selected individually according to the thickness or pressure displacement of the mucosa and the occlusal forces. The aim of this study was to assess the differences in denture displacement and pressure distribution on the soft tissue under the denture base of a distal extension implant supported by removable partial dentures with and without stress-breaking support.
Fig. 1 – Experimental model with five pressure sensors.
Fig. 2 – Experimental bilateral metal base denture on the simulation model.
2.
Materials and methods
2.1.
Preparation of simulation model
A commercial simulation model (E50-550, Nissin, Tokyo, Japan) with mandibular bilateral missing premolars and molars was modified to prepare an experimental model. The edentulous ridge area of the commercial model was cut, and a 2-mm thickness of artificial soft tissues was created with silicone impression material (Fit Checker, GC, Tokyo, Japan). A 0.3 mm thickness of an artificial periodontal membrane of the six remaining anterior teeth was also made with the same silicone impression material (Fit Checker, GC). Five pressure sensors (PS-10K, Kyowa, Tokyo, Japan) were attached near the left and right first molars (#36 and #46), first premolars (#34 and #44), and medio-lingual alveolar crest (ML) (Fig. 1).
2.2.
Fabrication of experimental dentures
After the definitive impression of the experimental model was taken using the silicone impression material (Exaflex, GC,
journal of prosthodontic research 58 (2014) 115–120
117
Fig. 3 – Three connecting conditions: healing screw (CRPD) for separating between the denture base and the implant, healing cap (HC) for a rigid connection between the denture base and implant, and SBB attachment for a flexible connection between them.
Fig. 5 – Loading point on the simulation model. Load was applied at the intersection of the median line and the right and left mesial first molars using the loading apparatus. Fig. 4 – The structure and size of the SBB attachment.
Tokyo, Japan), a master cast was conventionally made with hardened stone. Bilateral distal extension RPDs forming an occlusion rim without the denture teeth with a lingual bar and Akers clasps on both canines were designed. Cobalt-Chromium (Wisil, Austenal, Chicago, IL, USA) frameworks were conventionally cast, and heat-cured denture base resin (Acron, GC, Tokyo, Japan) was then packed and polymerized according to the manufacture’s instructions. After deflasking, finishing, and polishing, five metal-base RPDs were completed (Fig. 2). After two implants (ITI, SLA 8 mm, Straumann, Basel, Switzerland) were placed in the right and left second molar regions, healing caps (HC) (4.5 mm, Straumann, Basel, Switzerland) and SBB attachments (GC, Tokyo, Japan) were mounted (Fig. 3). HC was selected as a control to the attachment of implant-supported RPD [26]. For a conventional RPD (CRPD), healing screws put in place without connecting the denture base to the implants. The SBB attachment consists of both a flat-top ball-head male and an O-ring rubber female, as shown in Fig. 4. The male is 2.9 mm high, and the ball head is 1.75 mm in diameter with a 0.2 mm undercut. The male was milled from a Ti-6Al-4 V alloy and completed using barrel polishing. The O-ring (1.35 mm inner diameter) in the female was made of nitrile-butadiene rubber. It was covered with a
silicone housing with three amounts of space to allow three kinds of settlement: 0.3 mm, 0.5 mm, and 0.7 mm [27]. The SBB attachment can be selected individually according to the thickness or pressure displacement of the mucosa and the occlusal forces. SBB attachment with 0.3 mm displacement was chosen in this study because it was confirmed that the denture displacement was less than 300 mm in the preliminary study (Fig. 4). To fabricate the implant-supported RPDs, the denture bases were fitted to the HC and SBB with autopolymerized resin (Unifast III, GC, Tokyo, Japan) to support the RPDs.
2.3.
Measurement of the pressure distribution
After a brass plate was attached for one-point loading to the occlusion rim of each RPD, loads up to 50 N were applied at the intersection of the median line and the right and left mesial first molars using the loading apparatus (Seiki, Tokyo, Japan) (Fig. 5). The displacement sensor (DT-A30, Tech, Tokyo, Japan) and load cell (LM-20KA, Yokosawa, Tokyo, Japan) were set up on the loading rod in the apparatus. The pressure at five different regions of the soft tissue and the displacement of the RPDs (n = 5) were simultaneously measured using a sensor interface (PCD-300B, Kyowa, Tokyo, Japan) and personal computer (Dynabook T350/56AB, Toshiba, Tokyo, Japan). All data were analyzed using a one-way ANOVA and Tukey multiple comparison test at a significance level of a = 0.05. The
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journal of prosthodontic research 58 (2014) 115–120
Fig. 6 – The pressure values at five regions for each connecting condition.
Fig. 7 – Amount of total pressure for each connecting condition.
Table 1 – Mean (s.d.) CV for each connecting condition.
Coefficient of variation
HC
CRPD
SBB
0.37
0.41
0.28
coefficient of variation (CV) for each connecting condition was calculated from all data on the pressure at five regions to evaluate the pressure distribution.
3.
Results
3.1.
Pressure distribution
The pressure values at five regions for each connecting condition are shown in Fig. 6. The pressures on the first molar region for the HC (#36: 6.17 kPa and #46: 6.73 kPa) were significantly less than those for the CRPD (#36: 12.3 kPa and #46 14.8 kPa) and SBB (#36: 11.1 kPa and #46: 13.1 kPa) ( p < 0.05). There was no statistical difference on the ML between the CRPD and SBB ( p > 0.05). Similarly, the HC (8.88 kPa) at the ML showed the least pressure, followed by the CRPD (14.9 kPa) and SBB (11.3 kPa). In contrast, the SBB (#34: 10.5 kPa and #44: 7.43 kPa) condition at the first premolar regions was larger than those at the CRPD (#34: 8.32 kPa and #44: 6.86 kPa) and HC (#34: 5.55 kPa and #44: 4.15 kPa). The CVs for each connecting condition are indicated in Table 1. The CV of the SBB (0.28) was less than that of the HC (0.37) and CRPD (0.41). The total pressure values at five regions for each connecting condition are shown in Fig. 7. The total pressure value for the HC (32.9 kPa) was significantly less than that for the CRPD (57.3 kPa) and SBB (53.6 kPa) ( p < 0.05).
3.2.
Displacement of the experimental denture
The denture displacement of the HC, CRPD, and SBB is displayed in Fig. 8. The HC (118 mm) showed significantly smaller denture displacements than those of the CRPD (154 mm) ( p < 0.05). However, there was no statistical difference between the CRPD and SBB (137 mm) ( p > 0.05).
Fig. 8 – Denture displacement of three attachments.
4.
Discussion
The implant-supported RPDs are one optional treatment for partially edentulous patients who were treated using conventional RPDs. The literature suggests that the association of RPDs with implants improves the prosthetic biomechanics, resulting in greater patient satisfaction [28]. Mijiritsky and Karas [29] reported that greater retention, stability, and function were obtained and that patient satisfaction, aesthetics, and function were also improved. Of the implant removable prostheses, the implant survival rate has been found to range from 71.3% to 83.7% in the maxilla and 83% to 100% in the mandible [30–35]. Hutton et al. [36] reported that the three-year implant failure rate of implantsupported overdentures was 6% in the mandible and 28% in the maxilla, the difference resulting mainly from the density and mechanical properties of the bone, amount of denture displacement during biting, and direction of the occlusal force in each jaw. When a static load of axial direction (0–5000 gf) was applied to the implant superstructure, the vertical displacement of the implant was 3.6 mm [37,38]. Similarly, the displacement of alveolar mucosa was 300 mm when a static load (0–400 gf) was applied [39]. Moreover, an alveolar
journal of prosthodontic research 58 (2014) 115–120
mucosa shows two movement phases (viscoelastic characteristics), but an implant shows one movement phase (idealized elastic characteristic). Moustafa et al. [40] reported that there would be a possibility of cumulative overload on the implant. To protect implants from excessive force, stress-breaking attachments have been manufactured as conventional commercial attachments. However, these attachments cannot exactly compensate for the different amounts of pressure displacement of the mucosa in individual patients [23,24]. The limitation of this study was that loading was only applied vertically. Thus, the retention of the denture, bracing effectiveness of the implant, and lateral movements of the extension base could not be definitively confirmed in this study. The thickness of the tissues covering the ridge will undoubtedly affect the amount of denture movement and will be an important factor in the direction of force transmitted to the supporting structure. The thickness of the residual ridge mucosa in this study was determined based on the results of previous studies [41–45]. Similar to previous studies [21,46], the results of this study indicated that implant placement at the distal edentulous ridge can prevent the movement of the distal extension bases. Note that the pressure at the distal regions (#36 and #46) with implant support (HC, SBB) decreased compared to the pressure at the distal regions where the denture base was not connected to the implant (CRPD). The tendency of the pressure distribution to the soft tissue would change from Kennedy Class I situations to those of Kennedy Class III, which is similar to the results of the previous study. Of the pressure on first molar region (#36 and #46), mediolingual alveolar crest and the total amount of pressure at five regions, SBB showed intermediate value between CRPD and HC by the effect of stress-breaking attachment. Likewise, the result of the displacement of the experimental denture for the SBB was a mean value between those for the CRPD and HC. The CV of pressure distribution for the SBB showed less than for the CRPD and HC. Because the far greater force than load of 50 N was applied in the mouth, the results of this study suggest that use of the SBB attachment can protect implant fixtures from harmful occlusal force and prevent the rotational movement of distal extension RPDs by vertical loads. Several studies reported that the implant survival rate of implant-supported RPDs whose implant length ranged from 8 to 13 mm was 98–99% [47–51]. However, the survival rate would decreased in the cases which the short length and small diameter of implant fixture must be placed by anatomical reason, and unexpectedly harmful forces were applied to the implant by bruxism and tooth contact habit. Stress-breaking attachments, such as the SBB attachment, should probably be used to protect implant fixtures in the above cases. Further in vivo study is needed to evaluate the patient’s satisfaction, bite force, and masticatory performance with implant-supported distal extension RPDs with the SBB attachment.
5.
Conclusion
The implant placement at the distal edentulous ridge can prevent movement of the distal extension bases. The pressure
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distribution on the alveolar ridge, the SBB attachment tended to be greater than with the Healing Cap. Within the in vitro limitations, precise denture settlements and pressure distribution under the denture base could be controlled using an SBB attachment and might be able to protect the implant from harmful force.
Conflict of interest This study was funded by GC Corp (Tokyo, Japan). The sponsor of the study had no role in the study design, conduct of the study, data collection, data interpretation or preparation of the report.
references
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/jpor
Original article
Pressure distribution of implant-supported removable partial dentures with stress-breaking attachments Kentaro Kono DMDa,*, Daisuke Kurihara DMD, PhDa, Yasunori Suzuki DMD, PhDb, Chikahiro Ohkubo DMD, PhDa a
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine, 2-1-3, Tsurumi, Tsurumi-ku, Yokohama, Kanagawa, Japan b Division of Oral Maxillofacial Implantology, Tsurumi University School of Dental Medicine, 2-1-3, Tsurumi, Tsurumi-ku, Yokohama, Kanagawa, Japan
article info
abstract
Article history:
Purpose: This in vitro study investigated the pressure distribution of the implant-supported
Received 5 November 2013
removable partial dentures (RPDs) with the stress-breaking attachments under the occlusal
Received in revised form
force.
30 December 2013
Methods: The experimental model of bilateral missing premolars and molars was modified
Accepted 27 January 2014
from a commercial simulation model. Five pressure sensors were embedded near the
Available online 11 March 2014
bilateral first molars, first premolars, and medio-lingual alveolar crest. Two implants were placed near the second molars, and they were connected to the denture base using the
Keywords:
following conditions: complete separation between the denture base and implant with
Implant-supported removable
cover screws (CRPD), flexible connection with a stress-breaking ball (SBB) attachment, and
partial denture
rigid connection without stress breaking with healing caps (HC). The pressure at five
Stress-breaking ball (SBB)
different areas of the soft tissue and the displacement of the RPDs were simultaneously
attachment
measured, loading up to 50 N. The coefficient of variation (CV) for each connection was
Pressure distribution
calculated from all data of the pressure at five areas to evaluate the pressure distribution.
Implant
Results: The pressure on medio-lingual alveolar crest and molars of the HC was less than SBB and CRPD. In contrast, the pressure on premolars of SBB was greater than for the HC and CRPD. The CV of SBB was less than that of HC and CRPD. Denture displacement of HC and SBB was less than for CRPD. Conclusions: Within the in vitro limitations, precise denture settlements and pressure distribution under the denture base could be controlled using an SBB attachment. An SBB attachment might be able to protect the implant from harmful force. Crown Copyright # 2014 Published by Elsevier Ireland on behalf of Japan Prosthodontic Society. All rights reserved.
* Corresponding author. Tel.: +81 45 580 8420; fax: +81 45 573 9599. E-mail address: [email protected] (K. Kono). 1883-1958/$ – see front matter. Crown Copyright # 2014 Published by Elsevier Ireland on behalf of Japan Prosthodontic Society. All rights reserved. http://dx.doi.org/10.1016/j.jpor.2014.01.002
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journal of prosthodontic research 58 (2014) 115–120
Introduction
Distal-extension removable partial dentures (RPDs) were associated with several problems related to their limited stability, retention, aesthetics, and masticatory efficiency [1– 4]. The rotational movements of RPDs might produce terminal torque forces against the abutment teeth and the soft tissue [5,6]. Ill-fitting retainers, occlusal disharmony, and pain in the soft tissue under the connector or denture base are frequently observed after long-term use [7–10]. In addition, constant pressure from the denture base gradually causes ridge resorption under the denture base [11,12]. To prevent the rotational movements of RPDs, precise attachments or telescope systems have been used on the remaining teeth, and an altered cast technique has been applied to offset different displacement between the remaining tooth and soft tissue during function [13–15]. However, the rotational change of RPDs cannot be completely prevented in long-term use even if the above techniques are used. Particularly, these phenomenon are remarkably occurred for short term in the cases of Eichner classification C1 [10]. A bilateral implant placement at the distal extension of the denture base will minimize the resultant denture displacement. Consequently, less ridge resorption, fewer numbers of relining, and minimum decrease of retentive force of precision attachments would be leaded [16–18]. Moreover, the survival rate of this treatment option within 10 years is comparable to other implant therapy [19,20]. However, there are extraordinary differences in settling during a chewing load between the implant and mucosa under the denture base [21,22]. In addition, horizontal forces and rotational moments would also be applied to the implants depending on the occlusal contact, location, and number of implants in the dental arch. Therefore, excessive and harmful occlusal forces might be applied to the implant. To protect implants from excessive force, stress-breaking attachments, such as an ERA attachment, a locater attachment, or a cushion-type magnetic attachment, have been manufactured as conventional commercial attachments. However, these attachments cannot exactly compensate for the different amounts of pressure displacement of the mucosa in individual patients [23,24]. A stress-breaking ball attachment, hereafter called ‘‘SBB attachment,’’ for implants was developed with the Department of Prosthodontics, Gerodontology and Oral Rehabilitation, Osaka University Graduate School of Dentistry to prevent excessive and harmful occlusal forces. The SBB attachment can distribute the occlusal force equally between the alveolar ridge and the implant [25]. Because these attachments are covered with a silicone housing with three amounts of space to allow three kinds of settlement, i.e., 0.3 mm, 0.5 mm, and 0.7 mm, they can be selected individually according to the thickness or pressure displacement of the mucosa and the occlusal forces. The aim of this study was to assess the differences in denture displacement and pressure distribution on the soft tissue under the denture base of a distal extension implant supported by removable partial dentures with and without stress-breaking support.
Fig. 1 – Experimental model with five pressure sensors.
Fig. 2 – Experimental bilateral metal base denture on the simulation model.
2.
Materials and methods
2.1.
Preparation of simulation model
A commercial simulation model (E50-550, Nissin, Tokyo, Japan) with mandibular bilateral missing premolars and molars was modified to prepare an experimental model. The edentulous ridge area of the commercial model was cut, and a 2-mm thickness of artificial soft tissues was created with silicone impression material (Fit Checker, GC, Tokyo, Japan). A 0.3 mm thickness of an artificial periodontal membrane of the six remaining anterior teeth was also made with the same silicone impression material (Fit Checker, GC). Five pressure sensors (PS-10K, Kyowa, Tokyo, Japan) were attached near the left and right first molars (#36 and #46), first premolars (#34 and #44), and medio-lingual alveolar crest (ML) (Fig. 1).
2.2.
Fabrication of experimental dentures
After the definitive impression of the experimental model was taken using the silicone impression material (Exaflex, GC,
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Fig. 3 – Three connecting conditions: healing screw (CRPD) for separating between the denture base and the implant, healing cap (HC) for a rigid connection between the denture base and implant, and SBB attachment for a flexible connection between them.
Fig. 5 – Loading point on the simulation model. Load was applied at the intersection of the median line and the right and left mesial first molars using the loading apparatus. Fig. 4 – The structure and size of the SBB attachment.
Tokyo, Japan), a master cast was conventionally made with hardened stone. Bilateral distal extension RPDs forming an occlusion rim without the denture teeth with a lingual bar and Akers clasps on both canines were designed. Cobalt-Chromium (Wisil, Austenal, Chicago, IL, USA) frameworks were conventionally cast, and heat-cured denture base resin (Acron, GC, Tokyo, Japan) was then packed and polymerized according to the manufacture’s instructions. After deflasking, finishing, and polishing, five metal-base RPDs were completed (Fig. 2). After two implants (ITI, SLA 8 mm, Straumann, Basel, Switzerland) were placed in the right and left second molar regions, healing caps (HC) (4.5 mm, Straumann, Basel, Switzerland) and SBB attachments (GC, Tokyo, Japan) were mounted (Fig. 3). HC was selected as a control to the attachment of implant-supported RPD [26]. For a conventional RPD (CRPD), healing screws put in place without connecting the denture base to the implants. The SBB attachment consists of both a flat-top ball-head male and an O-ring rubber female, as shown in Fig. 4. The male is 2.9 mm high, and the ball head is 1.75 mm in diameter with a 0.2 mm undercut. The male was milled from a Ti-6Al-4 V alloy and completed using barrel polishing. The O-ring (1.35 mm inner diameter) in the female was made of nitrile-butadiene rubber. It was covered with a
silicone housing with three amounts of space to allow three kinds of settlement: 0.3 mm, 0.5 mm, and 0.7 mm [27]. The SBB attachment can be selected individually according to the thickness or pressure displacement of the mucosa and the occlusal forces. SBB attachment with 0.3 mm displacement was chosen in this study because it was confirmed that the denture displacement was less than 300 mm in the preliminary study (Fig. 4). To fabricate the implant-supported RPDs, the denture bases were fitted to the HC and SBB with autopolymerized resin (Unifast III, GC, Tokyo, Japan) to support the RPDs.
2.3.
Measurement of the pressure distribution
After a brass plate was attached for one-point loading to the occlusion rim of each RPD, loads up to 50 N were applied at the intersection of the median line and the right and left mesial first molars using the loading apparatus (Seiki, Tokyo, Japan) (Fig. 5). The displacement sensor (DT-A30, Tech, Tokyo, Japan) and load cell (LM-20KA, Yokosawa, Tokyo, Japan) were set up on the loading rod in the apparatus. The pressure at five different regions of the soft tissue and the displacement of the RPDs (n = 5) were simultaneously measured using a sensor interface (PCD-300B, Kyowa, Tokyo, Japan) and personal computer (Dynabook T350/56AB, Toshiba, Tokyo, Japan). All data were analyzed using a one-way ANOVA and Tukey multiple comparison test at a significance level of a = 0.05. The
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Fig. 6 – The pressure values at five regions for each connecting condition.
Fig. 7 – Amount of total pressure for each connecting condition.
Table 1 – Mean (s.d.) CV for each connecting condition.
Coefficient of variation
HC
CRPD
SBB
0.37
0.41
0.28
coefficient of variation (CV) for each connecting condition was calculated from all data on the pressure at five regions to evaluate the pressure distribution.
3.
Results
3.1.
Pressure distribution
The pressure values at five regions for each connecting condition are shown in Fig. 6. The pressures on the first molar region for the HC (#36: 6.17 kPa and #46: 6.73 kPa) were significantly less than those for the CRPD (#36: 12.3 kPa and #46 14.8 kPa) and SBB (#36: 11.1 kPa and #46: 13.1 kPa) ( p < 0.05). There was no statistical difference on the ML between the CRPD and SBB ( p > 0.05). Similarly, the HC (8.88 kPa) at the ML showed the least pressure, followed by the CRPD (14.9 kPa) and SBB (11.3 kPa). In contrast, the SBB (#34: 10.5 kPa and #44: 7.43 kPa) condition at the first premolar regions was larger than those at the CRPD (#34: 8.32 kPa and #44: 6.86 kPa) and HC (#34: 5.55 kPa and #44: 4.15 kPa). The CVs for each connecting condition are indicated in Table 1. The CV of the SBB (0.28) was less than that of the HC (0.37) and CRPD (0.41). The total pressure values at five regions for each connecting condition are shown in Fig. 7. The total pressure value for the HC (32.9 kPa) was significantly less than that for the CRPD (57.3 kPa) and SBB (53.6 kPa) ( p < 0.05).
3.2.
Displacement of the experimental denture
The denture displacement of the HC, CRPD, and SBB is displayed in Fig. 8. The HC (118 mm) showed significantly smaller denture displacements than those of the CRPD (154 mm) ( p < 0.05). However, there was no statistical difference between the CRPD and SBB (137 mm) ( p > 0.05).
Fig. 8 – Denture displacement of three attachments.
4.
Discussion
The implant-supported RPDs are one optional treatment for partially edentulous patients who were treated using conventional RPDs. The literature suggests that the association of RPDs with implants improves the prosthetic biomechanics, resulting in greater patient satisfaction [28]. Mijiritsky and Karas [29] reported that greater retention, stability, and function were obtained and that patient satisfaction, aesthetics, and function were also improved. Of the implant removable prostheses, the implant survival rate has been found to range from 71.3% to 83.7% in the maxilla and 83% to 100% in the mandible [30–35]. Hutton et al. [36] reported that the three-year implant failure rate of implantsupported overdentures was 6% in the mandible and 28% in the maxilla, the difference resulting mainly from the density and mechanical properties of the bone, amount of denture displacement during biting, and direction of the occlusal force in each jaw. When a static load of axial direction (0–5000 gf) was applied to the implant superstructure, the vertical displacement of the implant was 3.6 mm [37,38]. Similarly, the displacement of alveolar mucosa was 300 mm when a static load (0–400 gf) was applied [39]. Moreover, an alveolar
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mucosa shows two movement phases (viscoelastic characteristics), but an implant shows one movement phase (idealized elastic characteristic). Moustafa et al. [40] reported that there would be a possibility of cumulative overload on the implant. To protect implants from excessive force, stress-breaking attachments have been manufactured as conventional commercial attachments. However, these attachments cannot exactly compensate for the different amounts of pressure displacement of the mucosa in individual patients [23,24]. The limitation of this study was that loading was only applied vertically. Thus, the retention of the denture, bracing effectiveness of the implant, and lateral movements of the extension base could not be definitively confirmed in this study. The thickness of the tissues covering the ridge will undoubtedly affect the amount of denture movement and will be an important factor in the direction of force transmitted to the supporting structure. The thickness of the residual ridge mucosa in this study was determined based on the results of previous studies [41–45]. Similar to previous studies [21,46], the results of this study indicated that implant placement at the distal edentulous ridge can prevent the movement of the distal extension bases. Note that the pressure at the distal regions (#36 and #46) with implant support (HC, SBB) decreased compared to the pressure at the distal regions where the denture base was not connected to the implant (CRPD). The tendency of the pressure distribution to the soft tissue would change from Kennedy Class I situations to those of Kennedy Class III, which is similar to the results of the previous study. Of the pressure on first molar region (#36 and #46), mediolingual alveolar crest and the total amount of pressure at five regions, SBB showed intermediate value between CRPD and HC by the effect of stress-breaking attachment. Likewise, the result of the displacement of the experimental denture for the SBB was a mean value between those for the CRPD and HC. The CV of pressure distribution for the SBB showed less than for the CRPD and HC. Because the far greater force than load of 50 N was applied in the mouth, the results of this study suggest that use of the SBB attachment can protect implant fixtures from harmful occlusal force and prevent the rotational movement of distal extension RPDs by vertical loads. Several studies reported that the implant survival rate of implant-supported RPDs whose implant length ranged from 8 to 13 mm was 98–99% [47–51]. However, the survival rate would decreased in the cases which the short length and small diameter of implant fixture must be placed by anatomical reason, and unexpectedly harmful forces were applied to the implant by bruxism and tooth contact habit. Stress-breaking attachments, such as the SBB attachment, should probably be used to protect implant fixtures in the above cases. Further in vivo study is needed to evaluate the patient’s satisfaction, bite force, and masticatory performance with implant-supported distal extension RPDs with the SBB attachment.
5.
Conclusion
The implant placement at the distal edentulous ridge can prevent movement of the distal extension bases. The pressure
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distribution on the alveolar ridge, the SBB attachment tended to be greater than with the Healing Cap. Within the in vitro limitations, precise denture settlements and pressure distribution under the denture base could be controlled using an SBB attachment and might be able to protect the implant from harmful force.
Conflict of interest This study was funded by GC Corp (Tokyo, Japan). The sponsor of the study had no role in the study design, conduct of the study, data collection, data interpretation or preparation of the report.
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