Influence of Matrix Attachment Installation Load on Movement and Resultant Forces in Implant Overdentures Takaharu Goto, DDS, PhD, Kan Nagao, DDS, PhD, Yuichi Ishida, DDS, PhD, Yoritoki Tomotake, DDS, PhD, & Tetsuo Ichikawa, DDS, PhD Department of Oral & Maxillofacial Prosthodontics and Oral Implantology, The University of Tokushima, Institute of Health Biosciences, Tokushima, Japan
Keywords Implant overdenture; attachment; installation load; denture movement. Correspondence Takaharu Goto, Department of Oral & Maxillofacial Prosthodontics and Oral Implantology, The University of Tokushima, Institute of Health Biosciences, 3-18-15 Kuramoto, Tokushima 770-8504, Japan. E-mail: [email protected] The authors deny any conflicts of interest. Accepted December 13, 2013 doi: 10.1111/jopr.12177
Abstract Purpose: This in vitro study investigated the effect of attachment installation conditions on the load transfer and denture movements of implant overdentures, and aims to clarify the differences among the three types of attachments, namely ball, Locator, and magnet attachments. Materials and Methods: Three types of attachments, namely ball, Locator, and magnetic attachments were used. An acrylic resin mandibular edentulous model with two implants placed in the bilateral canine regions and removable overdenture were prepared. The two implants and bilateral molar ridges were connected to three-axis load-cell transducers, and a universal testing machine was used to apply a 50 N vertical force to each site of the occlusal table in the first molar region. The denture movement was measured using a G2 motion sensor. Three installation conditions, namely, the application of 0, 50, and 100 N loads were used to install each attachment on the denture base. The load transfer and denture movement were then evaluated. Results: The resultant force decreased with increasing installation load for all attachments. In particular, the resultant force on implants on the loading side of the Locator attachment significantly decreased when the installation load was increased from 0 to 50 N, and that for magnetic attachment significantly decreased when the installation load was increased from 50 to 100 N. For the residual ridges on the loading side, the direction of the forces for all attachments changed to downward with increasing installation load. Furthermore, the yaw Euler angle increased with increasing installation load for the magnetic attachment. Conclusions: Subject to the limitations of this study, the use of any installation load greater than 0 N is recommended for the installation of ball and Locator attachments on a denture base. Regarding magnetic attachments, our results also recommend installation on a denture base using any installation load greater than 0 N, and suggest that the resultant force acting on the implant can be decreased by increasing the installation load; however, a large installation load of 100 N should be avoided when installing the attachment on the denture base to avoid increasing the denture movement.
Implant overdentures have become more popular since the McGill consensus statement determined that a two-implant overdenture should be the first choice of treatment for an edentulous mandible.1,2 Various types of attachments such as the splinted bar, unsplinted ball, and Locator have been used to append the structure of the implant overdenture.3-5 Clinical studies have shown the magnetic attachment to be inferior in terms of retention force, necessity of repair or adjustment, and patient satisfaction.6-8 The bar attachment has also been shown to be insufficient regarding mucosal incidents compared to ball and magnetic attachments. The ball attachment has been more
156
successful in terms of patient satisfaction than bar and magnetic attachments.9,10 Although numerous studies have been conducted to determine the ideal attachment for implant overdentures, the clinical procedure for selecting an attachment remains unclear. For proper attachment selection, the load transfer to the supporting tissues under the denture base, the denture movements, and the matrix attachment installation load must be considered. Previous studies have shown the load transfer to the supporting tissues to be closely associated with denture movements.11,12 The strain around the implant, which can be measured using
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
Goto et al
Implant Overdenture Matrix Attachment Installation Load
Figure 1 Three-axis load-cell transducer and experimental mandibular model with G2 motion sensor. Two implants placed in the canine regions and the bilateral molar ridges were connected to each three-axis load-cell
transducer. The complete overdenture was covered with a 2-mm-thick silicone material to mimic mucosal tissue. The denture movement was measured by the G2 motion sensor.
strain gauges, has been used to evaluate the lateral forces that act on the implant; however, little information is available on the resultant force as a function of the lateral and axial vectors to both the implants and the residual ridges.13-15 Moreover, few studies have evaluated the pressurized conditions for installing attachments on overdentures.11 We developed a novel experimental model using three-axis load-cell transducers for a 3D evaluation of the load transfer to implants and residual ridges. The purpose of this in vitro study was to investigate the influence of the matrix attachment installation load on load transfer and denture movements in implant overdentures, and to clarify the differences among the three types of attachments, namely ball, Locator, and magnet attachments.
force (vertical direction [Fz ]) and bending moments (mesialdistal [Mx ] and lingual-buccal [My ]) was used for the in vitro experiment (Fig 1A). In the experiment, two implants (3.75 × 13 mm, Br˚anemark Mk III; Nobel Biocare, G¨oteborg, Sweden) and a residual ridge block, each of which were connected to a three-axis load-cell transducer, were placed in the bilateral canine and first molar regions of a mandibular edentulous acrylic resin model (Fig 1B). To mimic mucosal tissue, the surface of the model was covered with 2 mm of a silicone rubber impression material (Exafine, Injection-type; GC Corp., Tokyo, Japan) (Fig 1C). A complete experimental denture with a flat occlusal table was fabricated on the model. The denture movements were measured using a G2 motion sensor (Aichi Steel Co., Tohkai, Japan) (Fig 1D). The sensor was attached to the artificial teeth in the anterior regions (Fig 1E).
Materials and methods Experimental mandibular model
Attachments
A three-axis load-cell transducer (PD3-32-10-040; Kyowa Electronic Co., Tokyo, Japan) that can be used to detect an axial
Three types of attachments were used: (1) ball attachment (Gold Cap; Nobel Biocare), (2) Locator attachment (pink; Zest
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
157
Implant Overdenture Matrix Attachment Installation Load
Goto et al
Figure 2 Three axes of the experimental system: (A) load-cell transducer, (B) G2 motion sensor.
The attachments were installed on the denture base using an autopolymerizing acrylic resin (Unifast II; GC Corp.) by applying a load. Three types of loading conditions were employed, namely, the application of 0 N (without loading), 50 N, and 100 N in installing each type of attachment on the denture base. Each load was applied to the bilateral molar regions on the flat occlusal table of the experimental denture model by a universal testing machine (AG-1kNX; Shimazu, Kyoto, Japan).
The output of the G2 motion sensor was calculated from the flexibility of the Euler angles (i.e., the pitch, yaw, and roll) using original software with a C-based synthesis system (Fig 2B). A 50 N static load was applied to the loading points of the first molar regions on the right side by a universal testing machine with a 2.0 mm/min crosshead speed. The magnitude of the applied load was based on the bite force of edentulous patients with complete dentures.16,17 Six complete experimental dentures were fabricated, and six artificial mucosal materials modified for the respective denture bases were also prepared. The recording was repeated five times for each experimental condition, allowing intervals of at least 5 minutes for recovery.
Experimental measurements
Statistical analysis
The original output of each transducer was integrated in the following manner. The X-axis was designated as the direction along the length, the Y-axis as the direction along the width, and the Z-axis as the vertical direction (Fig 2A). Twelve signals from the four transducers and one signal from the load cell were digitized by a digital data recorder (NR-2000; Keyence, Tokyo, Japan) with 14-bit accuracy at a rate of 50 Hz, and then transferred to a computer (PG-GV23352DE; NEC, Tokyo, Japan). In this study, the resultant force (FR ) was calculated using the following: FR = (M2x + M2y + F2z )
Multiple comparison analysis was performed with Bonferroni’s post hoc test using SPSS 19.0 (SPSS Co., Chicago, IL). In this study, a p-value of 5% was considered to be statistically significant.
Anchors Inc., Escondido, CA), and (3) magnetic attachment (Magfit IP-BFN30; Aichi Steel Co.). Attachment installation conditions
158
Results Time pattern
Figure 3 shows the time patterns of the resultant forces acting on the implant and residual ridges on the loading side. The time patterns were obtained by averaging the signal at the onset of the universal testing machine measurements for each condition. The resultant force acting on the implants on the loading side
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
Goto et al
Implant Overdenture Matrix Attachment Installation Load
Figure 3 Time pattern of resultant forces acting on the implant and residual ridges on the loading side.
Figure 4 Resultant forces acting on the implant and residual ridges.
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
159
Implant Overdenture Matrix Attachment Installation Load
Goto et al
of the magnetic attachment exhibited a two-phase pattern; the resultant force can be observed to increase twofold when the attachment was installed with 0 N. The resultant force acting on the implants on the loading side for the ball and Locator attachments transmitted homogeneous increases without a two-phase pattern. When the attachments were installed using a 50 N load, which was the same as the resultant force acting on the implants on the loading side, all the attachments transmitted a homogeneous increase without a two-phase pattern. The increase in the resultant force acting on the residual ridges on the loading side was greater for a 50 N installation load than for 0 N, especially for the Locator attachment. No distinctive pattern was observed in the resultant force acting on the implant residual ridges on the nonloading side. Resultant force
Figure 4 shows the resultant forces acting on the implant and the residual ridges. The resultant force acting on the implants on the loading side was within the range of 18.1 to 23.9 N for a 0 N installation load, 4.01 to 15.9 N for a 50 N installation load, and 0.44 to 11.0 N for a 100 N installation load. The resultant force decreased with increasing installation load for all attachments. In particular, the resultant force of the Locator attachment significantly decreased when the installation load was increased from 0 to 50 N, and that of the magnetic attachment significantly decreased when the installation load was increased from 50 to 100 N. The resultant force acting on the residual ridges on the loading side was within the range of 0.87 to 1.15 N for a 0 N installation load, 1.22 to 2.21 N for a 50 N installation load, and 1.65 to 2.43 N for a 100 N installation. The resultant force acting on the residual ridges on the loading side increased with increasing installation load for all the attachments. The resultant force acting on the implant on the nonloading side significantly decreased when the installation load was increased from 0 to 50 N for the ball and Locator attachments. The magnetic attachment had a significantly smaller resultant force than the ball and Locator attachments for all installation loads. The resultant force acting on the residual ridges on the nonloading side was not greater than that acting on the ridges on the loading side for all attachments. Direction of force
Figure 5 shows the direction of each force acting on the implant and the residual ridges on the loading side. For the implants on the loading side, when the installation load was 0 N, the forces were transmitted in the following directions: downward on the nonloading side for the ball attachment; backward and downward on the nonloading side for the Locator attachment; and forward and downward on the nonloading side for the magnetic attachment. However, the direction of the forces for all attachments changed to downward with increasing installation load. For the implants on the nonloading side, when the installation load was 0 N, the forces of the ball and Locator attachments were horizontal and large compared to that of the magnetic attachment. For the residual ridges on the loading side, the force of the ball attachment was transmitted downward for all instal160
lation loads. For the locator and magnetic attachments, the force on the nonloading side changed from downward to backward and downward with increasing installation load. When the installation load was 0 N, the force of the magnetic attachment was horizontal and large compared to those of the ball and Locator attachments. The direction of the forces of all the attachments changed to downward with increasing installation load. None of the forces of the attachments acting on the residual ridges on the nonloading side had a distinctive direction. Denture movements
Figure 6 shows the results of the denture movements. The magnetic attachment had a high yaw Euler angle compared to the ball and Locator attachments for all installation loads. The yaw Euler angle increased with increasing installation load for the magnetic attachment, and particularly increased significantly when the installation load was increased from 50 to 100 N. Simultaneously, the Euler angles of pitching and rolling slightly increased in the negative direction. The ball and Locator attachments did not exhibit this distinctive movement.
Discussion In general, mechanical characteristics such as support, stability, and retention are considered as critical factors in evaluating removable and fixed prostheses.18,19 These factors are also important for implant overdentures and were discussed in previous studies. The retentive force of the ball and Locator attachments in implant overdentures was reported to be significantly higher than that of the magnetic attachment because of the mechanical interlocking and frictional contact.20-23 To determine the support and stability of an implant overdenture, the stress distributions around the implants and the residual ridges were measured using strain gauges; however, there has been no report of a study on the resultant forces acting on the implant and the residual ridges, particularly with respect to the effect of the attachment installation load. This study aimed to clarify such issues. This in vitro study made use of one unique mandibular model. The limitations of the deductions made from the results should therefore be considered with regard to different types of residual ridge forms, residual ridge mucosae, and the geometry of the implant placements in clinical situations. The morphology of the experimental conditions of this study was typical; therefore, the matrix attachment installation conditions for every type of attachment could be predicted, taking into consideration the difference between experimental and actual morphologies. In this study, the threshold of the resultant force acting on the supporting tissue was configured to investigate the load transfer to the implant and the residual ridges. The threshold of the implant was estimated to be 20 N, which agrees with Frost’s report for bone microfracture around an implant.24 The threshold of the residual ridge was set to a value of 6 N, which agrees with the report of Kawano’s in vivo measurement of the load on residual ridges under a complete denture during peanut mastication.25 Consequently, each threshold value of the
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
Goto et al
Implant Overdenture Matrix Attachment Installation Load
Figure 5 Direction of each force acting on the implant and residual ridges on the loading side.
Figure 6 Denture movement.
resultant force (20 N for the implant and 6 N for the residual ridge) is employed in the following discussion. For all the attachments on the loading side, the resultant force acting on the implants decreased, and that acting on the residual ridges increased with increasing installation load. In particular, the resultant force acting on the implant for the Locator attachment substantially decreased on the loading side and that acting
on the residual ridges increased for installation loads of 0 and 50 N. For the magnetic attachment, the resultant force acting on the implants on the loading side significantly decreased, and that acting on the residual ridges increased for an installation load of 100 N. Regarding the threshold of the supporting tissue, those for the ball and Locator attachments exceeded the implant threshold of 20 N only when no installation load of
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
161
Implant Overdenture Matrix Attachment Installation Load
Goto et al
0 N was applied. Regarding the threshold of residual ridges, those for none of the attachments exceeded the threshold for all installation loads. These results indicate that ball and Locator attachments are suitable for installation on a denture base using any installation load above 0 N, whereas a magnetic attachment can be installed with or without loading; however, the resultant force acting on the implant for a magnetic attachment decreases with increasing installation load. In previous studies, a piezoelastic force transducer was used to determine the direction of the force acting on the implant for in vivo analyses. Mericske-Stern reported that the transverse force magnitude in a forward and downward direction was as high as 50% of the vertical magnitudes for bar and telescopic attachments.26 Yoda et al reported that the force exerted on an implant in a two-implant-supported overdenture acted in a lateral direction and was greater than that on a four-implantsupported overdenture.27 However, the direction of the force exerted on the residual ridges was not evaluated in either study. In the present study, a novel experimental model incorporating three-axis load-cell transducers was used to detect the direction of the force exerted on both the implants and the residual ridges. When no installation load was applied to the implants, the force acting on the nonloading side was transmitted as follows: downward for the ball attachment; backward and downward for the Locator attachment; and forward and downward for the magnetic attachment. However, the direction of all the forces changed to downward with increasing installation load. These results suggest that a resilient system that allows only vertical, horizontal, or rotational movement would produce a lateral force on the implant on the loading side when no installation load is applied. For implants on the nonloading side, the ball and Locator attachments transmitted a force in directions other than downward. This suggests a resilient system that allows no horizontal movement of ball and Locator attachments. The ball and Locator attachments thus exhibited a greater tendency to exert a transverse force on the implant than the magnetic attachment. With regard to the residual ridges on the loading side, the force was transmitted in a distinctive direction for each attachment. In particular, when no installation load was applied, the force exerted by the magnetic attachment was horizontal and large compared to that exerted by the ball and Locator attachments. This suggests that horizontal denture movement can occur in a situation like this. In a previous study, it was found that mucosa complication of decubitis ulcers occurred more often in implant overdentures with magnetic attachments compared to those with ball and Locator attachments.28 The decubitis ulcers were reported to be caused by a mechanical stimulus during horizontal denture movement. Based on the directions of the forces acting on the implant and the residual ridges observed in the present study, we recommend the installation of a magnetic attachment on a denture base with the application of an installation load above 0 N. In the analysis of load transfer on implant overdentures, the movement of the denture and the resultant force acting on the implant should be considered. In a previous in vitro analytical study, magnetic attachments were found to produce greater denture movement than bar and ball attachments.29 The small size and ease of attachment to the denture of the G2 motion 162
sensor used in the present study makes it applicable in clinical situations. Hence, the medical device can be used to detect denture movements intraorally, although it requires further development for application to in vivo studies. With regard to denture movement, the yaw Euler angle of the magnetic attachment was higher than those of the ball and Locator attachments for all installation loads. When no installation load of 0 N was applied, the result for the residual ridges was consistent with the consequence of a high horizontal force of the magnetic attachment compared to the ball and Locator attachments. Conversely, a large yaw Euler angle was observed in the magnetic attachment, particularly when the installation load was 100 N. This may be because the magnetic attachment, which provided no buffering to axial movements, produced gaps between the keeper and the magnetic assembly. This resulted in horizontal denture movement, especially for a 100 N installation load. In a nutshell, the displacement of the residual tissue was recovered. Moreover, the resultant force exhibited a two-phase time pattern that doubled when the magnetic attachment was installed without loading, resulting in horizontal denture movement due to separation of the magnetic assembly from the keepers. The results of the present experimental study, in which load transfer to supporting tissue and denture movement in implant overdentures were investigated, indicate that ball and Locator attachments are suitable for installation on a denture base using any installation load greater than 0 N. Regarding magnetic attachment, the resultant force acting on the implant can be decreased by increasing the installation load; however, a large installation load of 100 N increases the denture movement and should be avoided when installing an attachment on a denture base. The applicability of the results of our study is limited by the biomechanical model employed. In addition, patient satisfaction should be considered when selecting attachments for implant overdentures. Further in vivo study is necessary to investigate the correlation between patient satisfaction and the technical parameters, including installation conditions.
Conclusions We developed a novel experimental model for evaluating load transfer to implants and residual ridges. The effect of the attachment installation conditions on the load transfer and denture movements for implant overdentures was investigated. We clarified the differences among ball, Locator, and magnetic attachments. Subject to the limitations of our in vitro study, which used a sophisticated experimental system, we recommend the installation of ball and Locator attachments on denture bases using any installation load greater than 0 N. Based on our results, we recommend the installation of magnetic attachments on denture bases using any installation load greater than 0 N, and the decrease of the resultant force acting on the implant by increasing the installation load; however, a large installation load of 100 N should be avoided when installing the attachment on the denture base to avoid increasing the denture movement.
References 1. Fein JS, Carlsson GE, Awad MA, et al: The McGill consensus statement on overdentures. Int J Prosthodont 2002;15:413-414
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
Goto et al
2. Burns DR: Mandibular implant overdenture treatment: consensus and controversy. J Prosthodont 2000;9:37-46 3. MacEntee MI, Walton JN, Glick N: A clinical trial of patient satisfaction and prosthodontic needs with ball and bar attachments for implant-retained complete overdentures: three year results. J Prosthet Dent 2005;93:28-37 4. Andreiotelli M, Att W, Strub JR: Prosthodontic complications with implant overdentures: a systematic literature review. Int J Prosthodont 2010;23:195-203 5. Chikunov I, Doan P, Vahidi F: Implant-retained partial overdenture with resilient attachments. J Prosthodont 2008;17:141-148 6. van Kampen F, Cune M, van der Bilt, et al: Retention and postinsertion maintenance of bar-clip, ball and magnet attachments in mandibular implant overdenture treatment: an in vivo comparison after 3 months of function. Clin Oral Implants Res 2003;14:720-726 7. Naert I, Alsaadi G, Quirynen M: Prosthetic aspects and patient satisfaction with two-implant-retained mandibular overdentures: a 10-year randomized clinical study. Int J Prosthodont 2004;17:401-410 8. Cune M, van Kampen F, van der Bilt A, et al: Patient satisfaction and preference with magnet, bar-clip, and ball-socket retained mandibular implant overdentures: a cross-over clinical trial. Int J Prosthodont 2005;18:99-105 9. Naert I, Gizani S, Vuylsteke M, et al: A 5-year prospective randomized clinical trial on the influence of splinted and unsplinted oral implants retaining a mandibular overdenture: prosthetic aspects and patient satisfaction. J Oral Rehabil 1999;26:195-202 10. Goodacre CJ, Bernal G, Rungcharassaeng K, et al: Clinical complications with implants and implant prostheses. J Prosthet Dent 2003;90:121-132 11. Ichikawa T, Horiuchi M, Wigianto R, et al: In vitro study of mandibular implant-retained overdentures: the influence of stud attachments on load transfer to the implant and soft tissue. Int J Prosthodont 1996;9:394-399 12. Gonda T, Ikebe K, Ono T, et al: Effect of magnetic attachment with stress breaker on lateral stress to abutment tooth under overdenture. J Oral Rehabil 2004;31:1001-1006 13. Mericske-Stern R, Piotti M, Sirtes G: 3-D in vivo force measurements on mandibular implants supporting overdentures: a comparative study. Clin Oral Implants Res 1996;7:387-396 14. Heckmann SM, Winter W, Meyer M, et al: Overdenture attachment selection and the loading of implant and denture-bearing area. Part 2: a methodical study using five types of attachment. Clin Oral Implants Res 2001;12:640-647 15. Maeda Y, Horisaka M, Yagi K: Biomechanical rationale for a single implant-retained mandibular
Implant Overdenture Matrix Attachment Installation Load
16.
17.
18.
19.
20.
21.
22.
23.
24. 25.
26.
27.
28.
29.
overdenture: an in vitro study. Clin Oral Implants Res 2008; 19:271-275 De Boever JA, McCall WD Jr, Holden S, et al: Functional occlusal forces: an investigation by telemetry. J Prosthet Dent 1978;40:326-333 Fields HW, Proffit WR, Case JC, et al: Variables affecting measurements of vertical occlusal force. J Den Res 1986;65: 135-138 Kaires AK: A study of partial denture design and masticatory pressures in a mandibular bilateral extention case. J Prosthet Dent 1958;8:340-350 Jacobson TE, Krol AJ: A contemporary review of the factors involved in complete dentures. Part 3: support. J Prosthet Dent 1983;49:306-313 Evtimovska E, Masri R, Driscoll CF, et al: The change in retentive values of locator attachments and hader clips over time. J Prosthodont 2009;18:479-483 Alsabeeha NH, Payne AG, Swain MV: Attachment systems for mandibular two-implant overdentures: a review of in vitro investigations on retention and wear features. Int J Prosthodont 2009;22:429-440 Evtimovska E, Masri R, Driscoll CF, et al: The change in retentive value of locator attachments and hader clips over time. J Prosthodont 2009;18:479-483 Chung KH, Chung CY, Cagna DR, et al: Retention characteristics of attachment systems for implant overdenture. J Prosthodont 2004;13:221-226 Frost HM: Bone’s mechanostat: a 2003 update. Anat Rec A Discov Mol Cell Evol Biol 2003;275:1081-1101 Kawano F: A study on pressure of supporting tissues under complete denture during functions: concerning the effects of the arrangement of artificial posterior teeth. Ann Jpn Prosthodont Soc 1987;31:726-739 Mericske-Stern R: Three-dimensional force measurements with mandibular overdentures connected to implants by ball-shaped retentive anchors: a clinical study. Int J Oral Maxillofac Implants 1998;13:36-43 Yoda N, Ogawa T, Gunji Y, et al: The analysis of the load exerted on the implants supporting an overdenture based on in vivo measurement. Prosthodont Res Pract 2008;7: 258-260 Naert I, Gizani S, Vuylsteke M, et al: A randomised clinical trial on the influence of splinted and unsplinted oral implants in mandibular overdenture therapy—a 3-year report-. Clin Oral Investig 1997;1:81-88 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
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
163
Keywords Implant overdenture; attachment; installation load; denture movement. Correspondence Takaharu Goto, Department of Oral & Maxillofacial Prosthodontics and Oral Implantology, The University of Tokushima, Institute of Health Biosciences, 3-18-15 Kuramoto, Tokushima 770-8504, Japan. E-mail: [email protected] The authors deny any conflicts of interest. Accepted December 13, 2013 doi: 10.1111/jopr.12177
Abstract Purpose: This in vitro study investigated the effect of attachment installation conditions on the load transfer and denture movements of implant overdentures, and aims to clarify the differences among the three types of attachments, namely ball, Locator, and magnet attachments. Materials and Methods: Three types of attachments, namely ball, Locator, and magnetic attachments were used. An acrylic resin mandibular edentulous model with two implants placed in the bilateral canine regions and removable overdenture were prepared. The two implants and bilateral molar ridges were connected to three-axis load-cell transducers, and a universal testing machine was used to apply a 50 N vertical force to each site of the occlusal table in the first molar region. The denture movement was measured using a G2 motion sensor. Three installation conditions, namely, the application of 0, 50, and 100 N loads were used to install each attachment on the denture base. The load transfer and denture movement were then evaluated. Results: The resultant force decreased with increasing installation load for all attachments. In particular, the resultant force on implants on the loading side of the Locator attachment significantly decreased when the installation load was increased from 0 to 50 N, and that for magnetic attachment significantly decreased when the installation load was increased from 50 to 100 N. For the residual ridges on the loading side, the direction of the forces for all attachments changed to downward with increasing installation load. Furthermore, the yaw Euler angle increased with increasing installation load for the magnetic attachment. Conclusions: Subject to the limitations of this study, the use of any installation load greater than 0 N is recommended for the installation of ball and Locator attachments on a denture base. Regarding magnetic attachments, our results also recommend installation on a denture base using any installation load greater than 0 N, and suggest that the resultant force acting on the implant can be decreased by increasing the installation load; however, a large installation load of 100 N should be avoided when installing the attachment on the denture base to avoid increasing the denture movement.
Implant overdentures have become more popular since the McGill consensus statement determined that a two-implant overdenture should be the first choice of treatment for an edentulous mandible.1,2 Various types of attachments such as the splinted bar, unsplinted ball, and Locator have been used to append the structure of the implant overdenture.3-5 Clinical studies have shown the magnetic attachment to be inferior in terms of retention force, necessity of repair or adjustment, and patient satisfaction.6-8 The bar attachment has also been shown to be insufficient regarding mucosal incidents compared to ball and magnetic attachments. The ball attachment has been more
156
successful in terms of patient satisfaction than bar and magnetic attachments.9,10 Although numerous studies have been conducted to determine the ideal attachment for implant overdentures, the clinical procedure for selecting an attachment remains unclear. For proper attachment selection, the load transfer to the supporting tissues under the denture base, the denture movements, and the matrix attachment installation load must be considered. Previous studies have shown the load transfer to the supporting tissues to be closely associated with denture movements.11,12 The strain around the implant, which can be measured using
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
Goto et al
Implant Overdenture Matrix Attachment Installation Load
Figure 1 Three-axis load-cell transducer and experimental mandibular model with G2 motion sensor. Two implants placed in the canine regions and the bilateral molar ridges were connected to each three-axis load-cell
transducer. The complete overdenture was covered with a 2-mm-thick silicone material to mimic mucosal tissue. The denture movement was measured by the G2 motion sensor.
strain gauges, has been used to evaluate the lateral forces that act on the implant; however, little information is available on the resultant force as a function of the lateral and axial vectors to both the implants and the residual ridges.13-15 Moreover, few studies have evaluated the pressurized conditions for installing attachments on overdentures.11 We developed a novel experimental model using three-axis load-cell transducers for a 3D evaluation of the load transfer to implants and residual ridges. The purpose of this in vitro study was to investigate the influence of the matrix attachment installation load on load transfer and denture movements in implant overdentures, and to clarify the differences among the three types of attachments, namely ball, Locator, and magnet attachments.
force (vertical direction [Fz ]) and bending moments (mesialdistal [Mx ] and lingual-buccal [My ]) was used for the in vitro experiment (Fig 1A). In the experiment, two implants (3.75 × 13 mm, Br˚anemark Mk III; Nobel Biocare, G¨oteborg, Sweden) and a residual ridge block, each of which were connected to a three-axis load-cell transducer, were placed in the bilateral canine and first molar regions of a mandibular edentulous acrylic resin model (Fig 1B). To mimic mucosal tissue, the surface of the model was covered with 2 mm of a silicone rubber impression material (Exafine, Injection-type; GC Corp., Tokyo, Japan) (Fig 1C). A complete experimental denture with a flat occlusal table was fabricated on the model. The denture movements were measured using a G2 motion sensor (Aichi Steel Co., Tohkai, Japan) (Fig 1D). The sensor was attached to the artificial teeth in the anterior regions (Fig 1E).
Materials and methods Experimental mandibular model
Attachments
A three-axis load-cell transducer (PD3-32-10-040; Kyowa Electronic Co., Tokyo, Japan) that can be used to detect an axial
Three types of attachments were used: (1) ball attachment (Gold Cap; Nobel Biocare), (2) Locator attachment (pink; Zest
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
157
Implant Overdenture Matrix Attachment Installation Load
Goto et al
Figure 2 Three axes of the experimental system: (A) load-cell transducer, (B) G2 motion sensor.
The attachments were installed on the denture base using an autopolymerizing acrylic resin (Unifast II; GC Corp.) by applying a load. Three types of loading conditions were employed, namely, the application of 0 N (without loading), 50 N, and 100 N in installing each type of attachment on the denture base. Each load was applied to the bilateral molar regions on the flat occlusal table of the experimental denture model by a universal testing machine (AG-1kNX; Shimazu, Kyoto, Japan).
The output of the G2 motion sensor was calculated from the flexibility of the Euler angles (i.e., the pitch, yaw, and roll) using original software with a C-based synthesis system (Fig 2B). A 50 N static load was applied to the loading points of the first molar regions on the right side by a universal testing machine with a 2.0 mm/min crosshead speed. The magnitude of the applied load was based on the bite force of edentulous patients with complete dentures.16,17 Six complete experimental dentures were fabricated, and six artificial mucosal materials modified for the respective denture bases were also prepared. The recording was repeated five times for each experimental condition, allowing intervals of at least 5 minutes for recovery.
Experimental measurements
Statistical analysis
The original output of each transducer was integrated in the following manner. The X-axis was designated as the direction along the length, the Y-axis as the direction along the width, and the Z-axis as the vertical direction (Fig 2A). Twelve signals from the four transducers and one signal from the load cell were digitized by a digital data recorder (NR-2000; Keyence, Tokyo, Japan) with 14-bit accuracy at a rate of 50 Hz, and then transferred to a computer (PG-GV23352DE; NEC, Tokyo, Japan). In this study, the resultant force (FR ) was calculated using the following: FR = (M2x + M2y + F2z )
Multiple comparison analysis was performed with Bonferroni’s post hoc test using SPSS 19.0 (SPSS Co., Chicago, IL). In this study, a p-value of 5% was considered to be statistically significant.
Anchors Inc., Escondido, CA), and (3) magnetic attachment (Magfit IP-BFN30; Aichi Steel Co.). Attachment installation conditions
158
Results Time pattern
Figure 3 shows the time patterns of the resultant forces acting on the implant and residual ridges on the loading side. The time patterns were obtained by averaging the signal at the onset of the universal testing machine measurements for each condition. The resultant force acting on the implants on the loading side
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
Goto et al
Implant Overdenture Matrix Attachment Installation Load
Figure 3 Time pattern of resultant forces acting on the implant and residual ridges on the loading side.
Figure 4 Resultant forces acting on the implant and residual ridges.
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
159
Implant Overdenture Matrix Attachment Installation Load
Goto et al
of the magnetic attachment exhibited a two-phase pattern; the resultant force can be observed to increase twofold when the attachment was installed with 0 N. The resultant force acting on the implants on the loading side for the ball and Locator attachments transmitted homogeneous increases without a two-phase pattern. When the attachments were installed using a 50 N load, which was the same as the resultant force acting on the implants on the loading side, all the attachments transmitted a homogeneous increase without a two-phase pattern. The increase in the resultant force acting on the residual ridges on the loading side was greater for a 50 N installation load than for 0 N, especially for the Locator attachment. No distinctive pattern was observed in the resultant force acting on the implant residual ridges on the nonloading side. Resultant force
Figure 4 shows the resultant forces acting on the implant and the residual ridges. The resultant force acting on the implants on the loading side was within the range of 18.1 to 23.9 N for a 0 N installation load, 4.01 to 15.9 N for a 50 N installation load, and 0.44 to 11.0 N for a 100 N installation load. The resultant force decreased with increasing installation load for all attachments. In particular, the resultant force of the Locator attachment significantly decreased when the installation load was increased from 0 to 50 N, and that of the magnetic attachment significantly decreased when the installation load was increased from 50 to 100 N. The resultant force acting on the residual ridges on the loading side was within the range of 0.87 to 1.15 N for a 0 N installation load, 1.22 to 2.21 N for a 50 N installation load, and 1.65 to 2.43 N for a 100 N installation. The resultant force acting on the residual ridges on the loading side increased with increasing installation load for all the attachments. The resultant force acting on the implant on the nonloading side significantly decreased when the installation load was increased from 0 to 50 N for the ball and Locator attachments. The magnetic attachment had a significantly smaller resultant force than the ball and Locator attachments for all installation loads. The resultant force acting on the residual ridges on the nonloading side was not greater than that acting on the ridges on the loading side for all attachments. Direction of force
Figure 5 shows the direction of each force acting on the implant and the residual ridges on the loading side. For the implants on the loading side, when the installation load was 0 N, the forces were transmitted in the following directions: downward on the nonloading side for the ball attachment; backward and downward on the nonloading side for the Locator attachment; and forward and downward on the nonloading side for the magnetic attachment. However, the direction of the forces for all attachments changed to downward with increasing installation load. For the implants on the nonloading side, when the installation load was 0 N, the forces of the ball and Locator attachments were horizontal and large compared to that of the magnetic attachment. For the residual ridges on the loading side, the force of the ball attachment was transmitted downward for all instal160
lation loads. For the locator and magnetic attachments, the force on the nonloading side changed from downward to backward and downward with increasing installation load. When the installation load was 0 N, the force of the magnetic attachment was horizontal and large compared to those of the ball and Locator attachments. The direction of the forces of all the attachments changed to downward with increasing installation load. None of the forces of the attachments acting on the residual ridges on the nonloading side had a distinctive direction. Denture movements
Figure 6 shows the results of the denture movements. The magnetic attachment had a high yaw Euler angle compared to the ball and Locator attachments for all installation loads. The yaw Euler angle increased with increasing installation load for the magnetic attachment, and particularly increased significantly when the installation load was increased from 50 to 100 N. Simultaneously, the Euler angles of pitching and rolling slightly increased in the negative direction. The ball and Locator attachments did not exhibit this distinctive movement.
Discussion In general, mechanical characteristics such as support, stability, and retention are considered as critical factors in evaluating removable and fixed prostheses.18,19 These factors are also important for implant overdentures and were discussed in previous studies. The retentive force of the ball and Locator attachments in implant overdentures was reported to be significantly higher than that of the magnetic attachment because of the mechanical interlocking and frictional contact.20-23 To determine the support and stability of an implant overdenture, the stress distributions around the implants and the residual ridges were measured using strain gauges; however, there has been no report of a study on the resultant forces acting on the implant and the residual ridges, particularly with respect to the effect of the attachment installation load. This study aimed to clarify such issues. This in vitro study made use of one unique mandibular model. The limitations of the deductions made from the results should therefore be considered with regard to different types of residual ridge forms, residual ridge mucosae, and the geometry of the implant placements in clinical situations. The morphology of the experimental conditions of this study was typical; therefore, the matrix attachment installation conditions for every type of attachment could be predicted, taking into consideration the difference between experimental and actual morphologies. In this study, the threshold of the resultant force acting on the supporting tissue was configured to investigate the load transfer to the implant and the residual ridges. The threshold of the implant was estimated to be 20 N, which agrees with Frost’s report for bone microfracture around an implant.24 The threshold of the residual ridge was set to a value of 6 N, which agrees with the report of Kawano’s in vivo measurement of the load on residual ridges under a complete denture during peanut mastication.25 Consequently, each threshold value of the
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
Goto et al
Implant Overdenture Matrix Attachment Installation Load
Figure 5 Direction of each force acting on the implant and residual ridges on the loading side.
Figure 6 Denture movement.
resultant force (20 N for the implant and 6 N for the residual ridge) is employed in the following discussion. For all the attachments on the loading side, the resultant force acting on the implants decreased, and that acting on the residual ridges increased with increasing installation load. In particular, the resultant force acting on the implant for the Locator attachment substantially decreased on the loading side and that acting
on the residual ridges increased for installation loads of 0 and 50 N. For the magnetic attachment, the resultant force acting on the implants on the loading side significantly decreased, and that acting on the residual ridges increased for an installation load of 100 N. Regarding the threshold of the supporting tissue, those for the ball and Locator attachments exceeded the implant threshold of 20 N only when no installation load of
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
161
Implant Overdenture Matrix Attachment Installation Load
Goto et al
0 N was applied. Regarding the threshold of residual ridges, those for none of the attachments exceeded the threshold for all installation loads. These results indicate that ball and Locator attachments are suitable for installation on a denture base using any installation load above 0 N, whereas a magnetic attachment can be installed with or without loading; however, the resultant force acting on the implant for a magnetic attachment decreases with increasing installation load. In previous studies, a piezoelastic force transducer was used to determine the direction of the force acting on the implant for in vivo analyses. Mericske-Stern reported that the transverse force magnitude in a forward and downward direction was as high as 50% of the vertical magnitudes for bar and telescopic attachments.26 Yoda et al reported that the force exerted on an implant in a two-implant-supported overdenture acted in a lateral direction and was greater than that on a four-implantsupported overdenture.27 However, the direction of the force exerted on the residual ridges was not evaluated in either study. In the present study, a novel experimental model incorporating three-axis load-cell transducers was used to detect the direction of the force exerted on both the implants and the residual ridges. When no installation load was applied to the implants, the force acting on the nonloading side was transmitted as follows: downward for the ball attachment; backward and downward for the Locator attachment; and forward and downward for the magnetic attachment. However, the direction of all the forces changed to downward with increasing installation load. These results suggest that a resilient system that allows only vertical, horizontal, or rotational movement would produce a lateral force on the implant on the loading side when no installation load is applied. For implants on the nonloading side, the ball and Locator attachments transmitted a force in directions other than downward. This suggests a resilient system that allows no horizontal movement of ball and Locator attachments. The ball and Locator attachments thus exhibited a greater tendency to exert a transverse force on the implant than the magnetic attachment. With regard to the residual ridges on the loading side, the force was transmitted in a distinctive direction for each attachment. In particular, when no installation load was applied, the force exerted by the magnetic attachment was horizontal and large compared to that exerted by the ball and Locator attachments. This suggests that horizontal denture movement can occur in a situation like this. In a previous study, it was found that mucosa complication of decubitis ulcers occurred more often in implant overdentures with magnetic attachments compared to those with ball and Locator attachments.28 The decubitis ulcers were reported to be caused by a mechanical stimulus during horizontal denture movement. Based on the directions of the forces acting on the implant and the residual ridges observed in the present study, we recommend the installation of a magnetic attachment on a denture base with the application of an installation load above 0 N. In the analysis of load transfer on implant overdentures, the movement of the denture and the resultant force acting on the implant should be considered. In a previous in vitro analytical study, magnetic attachments were found to produce greater denture movement than bar and ball attachments.29 The small size and ease of attachment to the denture of the G2 motion 162
sensor used in the present study makes it applicable in clinical situations. Hence, the medical device can be used to detect denture movements intraorally, although it requires further development for application to in vivo studies. With regard to denture movement, the yaw Euler angle of the magnetic attachment was higher than those of the ball and Locator attachments for all installation loads. When no installation load of 0 N was applied, the result for the residual ridges was consistent with the consequence of a high horizontal force of the magnetic attachment compared to the ball and Locator attachments. Conversely, a large yaw Euler angle was observed in the magnetic attachment, particularly when the installation load was 100 N. This may be because the magnetic attachment, which provided no buffering to axial movements, produced gaps between the keeper and the magnetic assembly. This resulted in horizontal denture movement, especially for a 100 N installation load. In a nutshell, the displacement of the residual tissue was recovered. Moreover, the resultant force exhibited a two-phase time pattern that doubled when the magnetic attachment was installed without loading, resulting in horizontal denture movement due to separation of the magnetic assembly from the keepers. The results of the present experimental study, in which load transfer to supporting tissue and denture movement in implant overdentures were investigated, indicate that ball and Locator attachments are suitable for installation on a denture base using any installation load greater than 0 N. Regarding magnetic attachment, the resultant force acting on the implant can be decreased by increasing the installation load; however, a large installation load of 100 N increases the denture movement and should be avoided when installing an attachment on a denture base. The applicability of the results of our study is limited by the biomechanical model employed. In addition, patient satisfaction should be considered when selecting attachments for implant overdentures. Further in vivo study is necessary to investigate the correlation between patient satisfaction and the technical parameters, including installation conditions.
Conclusions We developed a novel experimental model for evaluating load transfer to implants and residual ridges. The effect of the attachment installation conditions on the load transfer and denture movements for implant overdentures was investigated. We clarified the differences among ball, Locator, and magnetic attachments. Subject to the limitations of our in vitro study, which used a sophisticated experimental system, we recommend the installation of ball and Locator attachments on denture bases using any installation load greater than 0 N. Based on our results, we recommend the installation of magnetic attachments on denture bases using any installation load greater than 0 N, and the decrease of the resultant force acting on the implant by increasing the installation load; however, a large installation load of 100 N should be avoided when installing the attachment on the denture base to avoid increasing the denture movement.
References 1. Fein JS, Carlsson GE, Awad MA, et al: The McGill consensus statement on overdentures. Int J Prosthodont 2002;15:413-414
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
Goto et al
2. Burns DR: Mandibular implant overdenture treatment: consensus and controversy. J Prosthodont 2000;9:37-46 3. MacEntee MI, Walton JN, Glick N: A clinical trial of patient satisfaction and prosthodontic needs with ball and bar attachments for implant-retained complete overdentures: three year results. J Prosthet Dent 2005;93:28-37 4. Andreiotelli M, Att W, Strub JR: Prosthodontic complications with implant overdentures: a systematic literature review. Int J Prosthodont 2010;23:195-203 5. Chikunov I, Doan P, Vahidi F: Implant-retained partial overdenture with resilient attachments. J Prosthodont 2008;17:141-148 6. van Kampen F, Cune M, van der Bilt, et al: Retention and postinsertion maintenance of bar-clip, ball and magnet attachments in mandibular implant overdenture treatment: an in vivo comparison after 3 months of function. Clin Oral Implants Res 2003;14:720-726 7. Naert I, Alsaadi G, Quirynen M: Prosthetic aspects and patient satisfaction with two-implant-retained mandibular overdentures: a 10-year randomized clinical study. Int J Prosthodont 2004;17:401-410 8. Cune M, van Kampen F, van der Bilt A, et al: Patient satisfaction and preference with magnet, bar-clip, and ball-socket retained mandibular implant overdentures: a cross-over clinical trial. Int J Prosthodont 2005;18:99-105 9. Naert I, Gizani S, Vuylsteke M, et al: A 5-year prospective randomized clinical trial on the influence of splinted and unsplinted oral implants retaining a mandibular overdenture: prosthetic aspects and patient satisfaction. J Oral Rehabil 1999;26:195-202 10. Goodacre CJ, Bernal G, Rungcharassaeng K, et al: Clinical complications with implants and implant prostheses. J Prosthet Dent 2003;90:121-132 11. Ichikawa T, Horiuchi M, Wigianto R, et al: In vitro study of mandibular implant-retained overdentures: the influence of stud attachments on load transfer to the implant and soft tissue. Int J Prosthodont 1996;9:394-399 12. Gonda T, Ikebe K, Ono T, et al: Effect of magnetic attachment with stress breaker on lateral stress to abutment tooth under overdenture. J Oral Rehabil 2004;31:1001-1006 13. Mericske-Stern R, Piotti M, Sirtes G: 3-D in vivo force measurements on mandibular implants supporting overdentures: a comparative study. Clin Oral Implants Res 1996;7:387-396 14. Heckmann SM, Winter W, Meyer M, et al: Overdenture attachment selection and the loading of implant and denture-bearing area. Part 2: a methodical study using five types of attachment. Clin Oral Implants Res 2001;12:640-647 15. Maeda Y, Horisaka M, Yagi K: Biomechanical rationale for a single implant-retained mandibular
Implant Overdenture Matrix Attachment Installation Load
16.
17.
18.
19.
20.
21.
22.
23.
24. 25.
26.
27.
28.
29.
overdenture: an in vitro study. Clin Oral Implants Res 2008; 19:271-275 De Boever JA, McCall WD Jr, Holden S, et al: Functional occlusal forces: an investigation by telemetry. J Prosthet Dent 1978;40:326-333 Fields HW, Proffit WR, Case JC, et al: Variables affecting measurements of vertical occlusal force. J Den Res 1986;65: 135-138 Kaires AK: A study of partial denture design and masticatory pressures in a mandibular bilateral extention case. J Prosthet Dent 1958;8:340-350 Jacobson TE, Krol AJ: A contemporary review of the factors involved in complete dentures. Part 3: support. J Prosthet Dent 1983;49:306-313 Evtimovska E, Masri R, Driscoll CF, et al: The change in retentive values of locator attachments and hader clips over time. J Prosthodont 2009;18:479-483 Alsabeeha NH, Payne AG, Swain MV: Attachment systems for mandibular two-implant overdentures: a review of in vitro investigations on retention and wear features. Int J Prosthodont 2009;22:429-440 Evtimovska E, Masri R, Driscoll CF, et al: The change in retentive value of locator attachments and hader clips over time. J Prosthodont 2009;18:479-483 Chung KH, Chung CY, Cagna DR, et al: Retention characteristics of attachment systems for implant overdenture. J Prosthodont 2004;13:221-226 Frost HM: Bone’s mechanostat: a 2003 update. Anat Rec A Discov Mol Cell Evol Biol 2003;275:1081-1101 Kawano F: A study on pressure of supporting tissues under complete denture during functions: concerning the effects of the arrangement of artificial posterior teeth. Ann Jpn Prosthodont Soc 1987;31:726-739 Mericske-Stern R: Three-dimensional force measurements with mandibular overdentures connected to implants by ball-shaped retentive anchors: a clinical study. Int J Oral Maxillofac Implants 1998;13:36-43 Yoda N, Ogawa T, Gunji Y, et al: The analysis of the load exerted on the implants supporting an overdenture based on in vivo measurement. Prosthodont Res Pract 2008;7: 258-260 Naert I, Gizani S, Vuylsteke M, et al: A randomised clinical trial on the influence of splinted and unsplinted oral implants in mandibular overdenture therapy—a 3-year report-. Clin Oral Investig 1997;1:81-88 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
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 24 (2015) 156–163
163
