Biomechenical influence of the different attachments on implant-retained overdentures
作者：1Wang Yining, 2Per-Olof Glantz, 2Krister Ninler
单位：1Wang Yining, 2Per-Olof Glantz, 2Krister Ninler. 1College of Stomatology, Hubei Medical University, Wuhan 430079；2Faculty of Odontology, Lund University, Sweden
关键词：覆盖义齿；附着体；应变片；生物力学
口腔医学纵横990316 Abstract Objective：To evaluate biomechanical influence of the bar, ball and magnet attachments on the implants when loaded with mandibular overdenture. Methods：Two implant replicas were inserted in regions of canines bilaterally in acrylic resin edentulous mandibular model. Identical model and overdenture were used apart from the different attachments and implant replicas. Four linear strain gauges were attached on the overdenture. The strain was recorded under 4 different loading magnitudes (50 N, 100 N, 150 N and 200 N) and 5 different directions (vertically, forward, backward, towards the left and towards the right) for overdenture without attachment and with bar attachment, ball attachments and magnet attachments respectively. Results：deformation of overdenture in midline was most apparent in all tests and the deformations occurred from the greatest to least were in magnet attachments, ball attachments, without attachment and in bar attachment on the whole. The strain pattern with solitary attachments presented more similarity to without attachment than that with bar attachment. Conclusion：Implants with bar attachment bear more transverse force, ball attachment is more in accordance with the concept of mucosal- and implant- in common support for occlusal force, and magnet attachment can absorb more transverse force.
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Key words Implant-retained overdenture Attachment Strain gauge Biomechenical
摘要 目的：评价使用杆式、球形和磁性附着体对种植体支持式下颌覆盖义齿及其种植体的生物力学影响。 方法：将两颗种植体代用品植入丙烯酸酯无牙颌模型的双侧尖牙区，在覆盖义齿的基托上粘附4块微型应变片，测量分析无附着体，杆式附着体和球形附着体覆盖义齿在4种力加载，5种力加载方向条件下的综合应变。 结果：覆盖义齿变形在中线处最明显，球形附着体覆盖义齿与无附着体覆盖义齿的应变型相似，综合分析受力发生的应变与分别分析的结果基本一致。 结论：杆式附着体覆盖义齿对种植体施加较大的水平力；球形附着体的覆盖义齿更符合粘膜和种植体共同支持牙 合力的观点；磁性附着体覆盖义齿能吸收较多的水平分力。
Clinical and radiographic findings showed results similar to those experienced for fixed prostheses supported by more implants. However the limited number of patients and length of the observation period do not permit a definite assessment of success rate in relation to different attachment systems. Up to now, there remains insufficient evidence to prove that one type of attachment is superior to another in the distribution of stress. Biomechanical influences may play an important role in the longevity of bone around implants[1]. Forces derived from functional activity are first transferred to attachment through the overdenture and subsequently to the implants. In this study, a strain analysis on overdenture retained by two implants with no attachment, bar attachment, ball attachments and magnet attachments was made in mandibular model in different loading magnitudes and directions. The purpose was to evaluate biomechenical influence of the different attachments on the implants when loaded with overdentures.
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Materials and methods
A transparent acrylic resin ( Paladur , Kulzer , Germany) edentulous mandibular model was duplicated from a moderate ridge resorption plaster cast. Two implant replicas were inserted in regions of canines bilaterally. The overdenture was manufactured using conventional laboratory procedures. An about 2 mm thick layer of silicone material (Gingifast, Zhermack, Italy) between the denture base and the acrylic resin model was used as a substitute for the alveolar mucosa. A plaster lid was designed and fabricated to fit over the tooth cups and ridges, which was used to transfer loading force to whole cups and ridges evenly. Four linear strain gauges (type N11-FA-5-120-11, Showa, Measuring Instruments, Co., Ltd. Japan) were attached to the oral surface of the overdenture in the regions of 35-36 buccal and lingual, 31-41 lingual and 45-46 buccal surface, respectively represented strain gauge 1, 2, 3, and 4.[2]
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Strain gauge 1, 3 and 4 were oriented mes-distally, while strain gauge 2 occlu-gingivally. Further details of the used lead wires and coupling boxes are given by Glantz & Stafford[3]and Glantz et al,[4,5]and so are the instruments used for signal conditioning, amplification and recording.
After conditioning and calibration of the instruments according to Glantz et al.[4,5] for each overdenture strain gauge signals were recorded when loads were applied on the following identical model apart from the different situations.
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1. No attachment overdenture served as a comparison Space was created in overdenture base around implant abutment to accommodate implant abutment placement.
2. Overdenture with a bar attachment (Br a nemark System, DCB 077, DCA 072, 075, and 156) The gold-alloy bar of round was curved an angular at midline with straight bucaal part above the alveolar ridge and soldered to the mesial aspects of gold-alloy cylinders secured to the replicas. Two clips were positioned in the overdenture base corresponding to the straight round gold bar with a spacer beside the midline (left and right). The round bar system allows for rotation and vertical movement of overdenture.
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3. Overdenture with ball attachments (Br a nemark System, DCB 113, DCC 111 and 112) The caps corresponding to replica ball attachment are made of a plastic material and includes a rubber O-ring. Retention is obtained through the O-ring. Following the test of bar attachment, the replicas and bar was replaced by two ball replicas and two caps were installed in the overdenture base corresponding to balls with spacers. The use of the ball attachments permits vertical and rotational movements of the overdenture.
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4. Overdenture with magnet attachments (Astra Tech Implants Dental System) The magnet keeper, which is located on the 45(uni-abutment, is made from a titanium-coated iron-neodymium-boron magnet. After the test of ball attachment, the two keepers and two magnets were used to substitute for ball replicas and caps in the model. According to the normal procedure, the overdenture was fabricated with a non-resilient situation over the attachments.
The loading was applied on the center of plaster lid and distributed to whole overdenture evenly through it. Under the loading machine the model was placed in five different positions, i.e., tiled 0° (vertical loading), tilted 30° forward (forward loading), tilted 30° backward (backward loading), tilted 30° left (towards the left loading ) and tilted 30° right (towards the right loading ) respectively. The loading forces applied were separately 50, 100, 150, and 200 Newton (N). The same procedures were undertaken for each design. The microstrain signals were collected 15 times per second under loading condition and two second data from all 4 strain gauges were recorded. All recordings were repeated six times and a total of 1920 recordings were made.
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Results
Fig. 1 shows mean deformations of overdenture without attachment and with various attachments under all loading directions and magnitudes. The patterns of strain in gauges for each overdenture are markedly consistent with a bit variance as loading magnitude changed. There are more similarities among overdenture without attachment and with solitary attachments than with bar attachment in the patterns and magnitudes of strain.
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The types of strain recorded by the individual gauges on the mandibular overdenture without and with various attachments under all loading conditions including 4 different loading magnitudes and 5 different loading directions are presented in Table I (for each gauge on a overdenture n=120). As can be seen from this table all of the strain in gauge 3 except with bar attachment exhibited tensile type that was also dominant in overdenture with bar attachment. The major type of deformation in gauge 2 was in compression for overdentures without attachment and with ball and magnet attachments, but in tension for that with bar attachment. There was no a consistent major type of strain in gauge 1 and 4 for most of overdentures; however, it can be seen that among overdentures with 3 different types of attachment the type of strain in ball attachments (+=75%, -=25% in gauge 1; +=50, -=50 in gauge 4) most approached that without attachments (+=65%, -=35% in gauge 1; +=53%, -=47% in gauge 4), more in magnet attachments (+=33%, -=67% in gauge 1; +=77%, -=23% in gauge 4) and least in bar attachment (+ =100%, -=0 in gauge 1; +=92%, -=8% in gauge 4).
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(A)Loading=50N (B)Loading=100N
(c)Loading=150N (D)Loading=200N
Fig. 1. Mean strains in overdentures without attachment (No), and with bar attachment (Bar), ball attachments (Ball) and magnet attachments (Mag.) for gauge 1 (G1), gauge 2 (G2), gauge 3 (G3) and gauge 4 (G4) in total different 5 loading directions under (A) 50N, (B) 100N, (C) 150 N and (D) 200 N loading. Positive strain (+) indicating tension and negative strain (-) compression.
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Table I. Types of strain (percentage) recorded by four linear stain gauges (n=1920 for all gauges)
Positive (%)
Negative (%)
No
Bar
Ball
Magnet
No
Bar
Ball
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Magnet
Gauge 1
65
100
75
33
35
0
25
67
Gauge 2
17
75
, 百拇医药
0
22
83
25
100
78
Gauge 3
100
85
100
100
0
15
, 百拇医药 0
0
Gauge 4
53
92
50
77
47
8
50
23
Discussion
Significance of the design In the strict sense, overdenture supported by implants consists of mucosal- and implant-supported overdenture and implant-supported overdenture. For the former, occlusal forces are shared between the implants and the overdenture-bearing mucosa, whilst for the latter, occlusal forces are mainly supported by implants and therefore independent of the resilience and gradual changes in the soft tissues. The rigid connection of the implant-supported overdenture requires four or more implants and the use of U-shaped, rectangular or milled bars. The present study was about mucosal- and implant-supported overdenture, with bar-clip attachments, ball attachments and magnet attachments that were connected to two implants. The main function of these attachments is to assist the retention and stability of overdenture. The resilient connection acts as stress-breaking to absorb and avoid unfavorable lateral forces, bending moment and overloading to implants.
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Owing to the complexity of oral functional activities, the loading experiments only in a direction and a fixed magnitude of loading are obviously not enough to evaluate accurately strain distribution on overdenture and effect on implants. In order to simulate the real oral cavity functional condition, the present study designed test in 6 different loading directions and 4 different loading magnitudes. In addition, a silicon material was used as "mucosa". When measurements of strain are to be made with linear strain gauges, it is not only necessary to place a number of gauges, but also to place them in the relevant parts of the denture and also to locate them in the direction of principal strain[6]. Gauge 3, oriented mesi-distally in the midline between two implants and closest to implants in distance, can best reflect the strain changes in overdenture and the effect on implants. Gauges 1 and 4, oriented mesi-distally, were symmetrically positioned in posterior buccal flange of two sides to monitor the deformation of overdenture in these regions. Gauge 2 was located in lingual flange of the overdenture base and oriented occlu-gingivaly so as to detect how the base fits the mucosa. With the microstrain recorded from all the gauges, the stress distribution, overdenture deformation and possible effect on implants can be ascertained when forces were exerted.
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Without attachment overdenture It is evident that there was tensile strain concentration in gauge 3 region, which could result in overdenture deformation to trend to "open" or "stretch" the horse-shoe shaped denture. Subsequently mandibular lingual mucosa suffered from the traverse forces outward, and the forces increased with the increase of loading
Gauge 2 presented compressive strain because the base sank and pressed the mucosa downward and buccally, which was in agreement with the effect of tensile strain found in gauge 3 on overdenture.
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The strains received from gauge 1 and 4 were rather inconsistent, which reflected the complexity of strain distribution in related areas of overdenture in oral functional activities. Moreover, asymmetry of the alveolar ridge two sides anatomically was also responsible for this variation. Nevertheless, though there was variation in strain gauge 1 and 4, the results from certain gauge under identical testing condition might save as a comparison with those from attachment overdentures.
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In bar attachment overdenture Although gauge 3 exhibited same strain pattern, i.e., in tension as shown in no attachment overdenture, it is evident that values of the former were much lower than the latter on the whole and did not increase with the increase of loading, even decreased in tendency. It is believed that rigid structure of the bar attachment limited the deformation or "stretch" of horse-shoe shaped overdenture and formed fulcrums. Consequently the implants suffered from the traverse force towards outsides transferred by attachment. This force could be divided into two separate forces :one towards the lateral side and the other towards the frontal side due to the horse-shoe shaped structure of overdenture and mandible. The lateral forces might be offset each other with the bar connection completely or partially; whilst the forces towards frontally might not be. The traverse force is unfavorable to bone around implants. It is attributed to effect of the spacer that lower loading (50 N) produced higher tensile strain in gauge 3 because of existence of the spacer. As loading increased, the spacer decreased and disappeared the bar attachment closed together and restricted the deformation of overdenture, and then implants bore more traverse forces. The noted inconsistency in recorded directions and magnitudes of gauge 1, 2, and 4, comparing with no attachment overdenture under the same loading conditions was pointing out the differences between them in deformation of overdenture and mucosal support pattern. In gauge 2 relative lower compressive strains and even tensile strains indicated the corresponding mucosal support area did not receive as much force as in no attachment overdenture. Higher strain values recorded in gauge 1 and 4 situated in main mucosal support areas meant greater overdenture deformation in those areas, which is considered unfavorable for even distribution of force and may result in resorption of corresponding alveolar bone due to stress concentration.
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In ball attachment overdenture The higher values of tensile strain were observed in gauge 3 comparing with overdenture without attachment and with a bar attachment. Because of the resilient characteristic of plastic caps, the ball attachments allowed overdenture deformation in a greater extent and relieved the transverse force to implants. With the increase of loading the overdenture sank, approached the position where no attachment overdenture was and fit better so that mandibular alveolar bone assisted to restrict the "stretch" of overdenture further. Therefore it is not surprising that the magnitudes of strains decreased with the increase of loading. On the basis of the analysis above, there is an implication that using rigid lingual Co-Cr alloy framework instead of acrylic base will be favorable for decreasing deformation in that area, consequently decreasing the transverse forces towards implants.
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Unlike overdenture without attachment, gauge 2 presented consistent compressive strains in all testing conditions, which could be explained as the result of ball attachments securely holding the overdenture on the alveolar ridge mucosa in angular loading namely, being of good stability. These findings was pointing out the alveolar ridge mucosa shared support imparted by implants, and bar attachment implants seem to share more loading than ball ones. Guerra[7]suggested a bar attachment be used in case of excessive resorption, while solitary attachment be selected in case of mild to moderate ridge reduction.
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In gauge 1 and 4, the directions and magnitudes of the strains were more similar to in no attachment overdenture than in bar attachment overdenture. This is because ball attachment is more flexible than bar attachment.
In magnet attachment overdenture The high tensile strain values in gauge 3 suggested overdenture in that area deformed more as magnet slipped on the keeper when lateral forces were applied on them. This "stress-breaking" action might make the implant avoid suffering from unfavorable lateral forces. Strains in gauge 2 were in compressive and most of them showed the highest values, but it not believed that this overdenture fit best in those situations according to the analysis to vertical loading conditions above. The explanation for the high compressive strain values should be that the slip characteristic of magnet attachments gave the overdenture more freedom to "stretch" resulting in the increase of lateral pressure in gauge 2 against alveolar ridge mucosa. This can be verified by the highest tensile values in gauge 3. Using rigid lingual Co-Cr alloy framework is of the same importance as in overdenture with ball attachments.
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The movability between magnet attachment components is responsible for similarity of the strain pattern and values in gauge 1 and 4 to those in no attachment and ball attachment overdentures.
In fact, it is impossible for a denture to receive the loading force from only one direction while mastication. Usually a principal vertical force is accompanied by several forces from other different directions. Fig 1 illustrates mean strain values from total sum of 5 different directions under certain loading magnitude and Table I reveals strain types of each gauge in all testing conditions for different kind of overdentures. On the whole it is fundamentally in consistence with independent strain patterns and rank orders of the deformation in various overdentures with a few exceptions. In clinical experiments, the strain values will only be record in a form of mean from multiple directions. Therefore it is practical and important to understand and analyze the mean values and the outline of distribution of strain value pattern.
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For the point of biomechanical view, the axial force is more favorable, as it distributes stress more evenly throughout the implant[8]. However, traverse force is inevitable due to the position of arrangement of teeth in denture and motion of mandible. What we need to do is to reduce the traverse force exerted in implants as well as in the bone. According to this study, plastic ball attachments and "stress-breaking" magnet attachments seem to be favorable for overdenture because the stress distribution through it is more conformity with the mucosal- and implant- in common supported concept. Magnet attachments can prevent implants from suffer lateral force more effectively than ball attachments, but in keeping stability of overdenture and distributing loading force evenly on alveolar ridge mucosa, the latter is superior to the former. Bar attachment may result in unfavorable effect on implant as well as bone, which is in agreement with Meijer et al[9]. Because what role the bar connection plays in offsetting traverse forces between two implants is currently unavailable and what is the limited strains within the physiologic zone is not known[10]，further studies in vivo are needed.
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REFERENCES
1 Skalak R. Biomechanical considerations in osseointegrated prostheses. J Prosthet Dent 1983，49:843～848
2 王贻宁，Glantz P-O, Ninler K. 杆式与球形附着体对种植体支持式覆盖义齿影响的应力分析. 中国口腔种植学杂志，1999，4(2)：57
3 Glantz P-O, Stafford GD. Clinical deformation of maxillary complete denture. J Dent 1983，11:224～230
4 Glantz P-O, Nyman S, Standman E, et al. On functional strain in fixed mandibular reconstructions. II. An in vivo study. Acta Odontol Scand 1984,42:269～276
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5 Glantz P-O, Standman E, Svensson SA, et al. On functional strain in fixed mandibular reconstructions. I. An in vitro study. Acta Odontol Scand 1984,42:241-249
6 Stafford GD, Glantz P-O. Intraoral strain gauge measurements on complete dentures: a methodological study. J Dent 1991;19:80～84
7 Guerra LR, Finger IM, Block MS. Tissue-supported implant overdentures. Implant Dent 1992，1:69～77
8 Rangert B, Jemt T, J o arneus L. Forces and moments on Br a nemark implants. Int J Oral Maxillofac Implants 1989;4:241～247
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9 Meijer HJA, Starmans FJM, Bosman F, et al. A comparison of three finite element models of an edentulous mandible provided with implants. J Oral Rehabilitation 1993,20:147～157
10 Clelland NL, Gilat A, McGlumphy EA, et al. A photoelastic and strain gauge analysis of angled abutments for an implant system. Int J Oral Maxillofac Implants 1993,541～548
[收稿：1999-06-20], http://www.100md.com(1Wang Yining, 2Per-Olof Glantz, 2Krister Ninler)
单位：1Wang Yining, 2Per-Olof Glantz, 2Krister Ninler. 1College of Stomatology, Hubei Medical University, Wuhan 430079；2Faculty of Odontology, Lund University, Sweden
关键词：覆盖义齿；附着体；应变片；生物力学
口腔医学纵横990316 Abstract Objective：To evaluate biomechanical influence of the bar, ball and magnet attachments on the implants when loaded with mandibular overdenture. Methods：Two implant replicas were inserted in regions of canines bilaterally in acrylic resin edentulous mandibular model. Identical model and overdenture were used apart from the different attachments and implant replicas. Four linear strain gauges were attached on the overdenture. The strain was recorded under 4 different loading magnitudes (50 N, 100 N, 150 N and 200 N) and 5 different directions (vertically, forward, backward, towards the left and towards the right) for overdenture without attachment and with bar attachment, ball attachments and magnet attachments respectively. Results：deformation of overdenture in midline was most apparent in all tests and the deformations occurred from the greatest to least were in magnet attachments, ball attachments, without attachment and in bar attachment on the whole. The strain pattern with solitary attachments presented more similarity to without attachment than that with bar attachment. Conclusion：Implants with bar attachment bear more transverse force, ball attachment is more in accordance with the concept of mucosal- and implant- in common support for occlusal force, and magnet attachment can absorb more transverse force.
, http://www.100md.com
Key words Implant-retained overdenture Attachment Strain gauge Biomechenical
摘要 目的：评价使用杆式、球形和磁性附着体对种植体支持式下颌覆盖义齿及其种植体的生物力学影响。 方法：将两颗种植体代用品植入丙烯酸酯无牙颌模型的双侧尖牙区，在覆盖义齿的基托上粘附4块微型应变片，测量分析无附着体，杆式附着体和球形附着体覆盖义齿在4种力加载，5种力加载方向条件下的综合应变。 结果：覆盖义齿变形在中线处最明显，球形附着体覆盖义齿与无附着体覆盖义齿的应变型相似，综合分析受力发生的应变与分别分析的结果基本一致。 结论：杆式附着体覆盖义齿对种植体施加较大的水平力；球形附着体的覆盖义齿更符合粘膜和种植体共同支持牙 合力的观点；磁性附着体覆盖义齿能吸收较多的水平分力。
Clinical and radiographic findings showed results similar to those experienced for fixed prostheses supported by more implants. However the limited number of patients and length of the observation period do not permit a definite assessment of success rate in relation to different attachment systems. Up to now, there remains insufficient evidence to prove that one type of attachment is superior to another in the distribution of stress. Biomechanical influences may play an important role in the longevity of bone around implants[1]. Forces derived from functional activity are first transferred to attachment through the overdenture and subsequently to the implants. In this study, a strain analysis on overdenture retained by two implants with no attachment, bar attachment, ball attachments and magnet attachments was made in mandibular model in different loading magnitudes and directions. The purpose was to evaluate biomechenical influence of the different attachments on the implants when loaded with overdentures.
, 百拇医药
Materials and methods
A transparent acrylic resin ( Paladur , Kulzer , Germany) edentulous mandibular model was duplicated from a moderate ridge resorption plaster cast. Two implant replicas were inserted in regions of canines bilaterally. The overdenture was manufactured using conventional laboratory procedures. An about 2 mm thick layer of silicone material (Gingifast, Zhermack, Italy) between the denture base and the acrylic resin model was used as a substitute for the alveolar mucosa. A plaster lid was designed and fabricated to fit over the tooth cups and ridges, which was used to transfer loading force to whole cups and ridges evenly. Four linear strain gauges (type N11-FA-5-120-11, Showa, Measuring Instruments, Co., Ltd. Japan) were attached to the oral surface of the overdenture in the regions of 35-36 buccal and lingual, 31-41 lingual and 45-46 buccal surface, respectively represented strain gauge 1, 2, 3, and 4.[2]
, 百拇医药
Strain gauge 1, 3 and 4 were oriented mes-distally, while strain gauge 2 occlu-gingivally. Further details of the used lead wires and coupling boxes are given by Glantz & Stafford[3]and Glantz et al,[4,5]and so are the instruments used for signal conditioning, amplification and recording.
After conditioning and calibration of the instruments according to Glantz et al.[4,5] for each overdenture strain gauge signals were recorded when loads were applied on the following identical model apart from the different situations.
, 百拇医药
1. No attachment overdenture served as a comparison Space was created in overdenture base around implant abutment to accommodate implant abutment placement.
2. Overdenture with a bar attachment (Br a nemark System, DCB 077, DCA 072, 075, and 156) The gold-alloy bar of round was curved an angular at midline with straight bucaal part above the alveolar ridge and soldered to the mesial aspects of gold-alloy cylinders secured to the replicas. Two clips were positioned in the overdenture base corresponding to the straight round gold bar with a spacer beside the midline (left and right). The round bar system allows for rotation and vertical movement of overdenture.
, 百拇医药
3. Overdenture with ball attachments (Br a nemark System, DCB 113, DCC 111 and 112) The caps corresponding to replica ball attachment are made of a plastic material and includes a rubber O-ring. Retention is obtained through the O-ring. Following the test of bar attachment, the replicas and bar was replaced by two ball replicas and two caps were installed in the overdenture base corresponding to balls with spacers. The use of the ball attachments permits vertical and rotational movements of the overdenture.
, http://www.100md.com
4. Overdenture with magnet attachments (Astra Tech Implants Dental System) The magnet keeper, which is located on the 45(uni-abutment, is made from a titanium-coated iron-neodymium-boron magnet. After the test of ball attachment, the two keepers and two magnets were used to substitute for ball replicas and caps in the model. According to the normal procedure, the overdenture was fabricated with a non-resilient situation over the attachments.
The loading was applied on the center of plaster lid and distributed to whole overdenture evenly through it. Under the loading machine the model was placed in five different positions, i.e., tiled 0° (vertical loading), tilted 30° forward (forward loading), tilted 30° backward (backward loading), tilted 30° left (towards the left loading ) and tilted 30° right (towards the right loading ) respectively. The loading forces applied were separately 50, 100, 150, and 200 Newton (N). The same procedures were undertaken for each design. The microstrain signals were collected 15 times per second under loading condition and two second data from all 4 strain gauges were recorded. All recordings were repeated six times and a total of 1920 recordings were made.
, 百拇医药
Results
Fig. 1 shows mean deformations of overdenture without attachment and with various attachments under all loading directions and magnitudes. The patterns of strain in gauges for each overdenture are markedly consistent with a bit variance as loading magnitude changed. There are more similarities among overdenture without attachment and with solitary attachments than with bar attachment in the patterns and magnitudes of strain.
, http://www.100md.com
The types of strain recorded by the individual gauges on the mandibular overdenture without and with various attachments under all loading conditions including 4 different loading magnitudes and 5 different loading directions are presented in Table I (for each gauge on a overdenture n=120). As can be seen from this table all of the strain in gauge 3 except with bar attachment exhibited tensile type that was also dominant in overdenture with bar attachment. The major type of deformation in gauge 2 was in compression for overdentures without attachment and with ball and magnet attachments, but in tension for that with bar attachment. There was no a consistent major type of strain in gauge 1 and 4 for most of overdentures; however, it can be seen that among overdentures with 3 different types of attachment the type of strain in ball attachments (+=75%, -=25% in gauge 1; +=50, -=50 in gauge 4) most approached that without attachments (+=65%, -=35% in gauge 1; +=53%, -=47% in gauge 4), more in magnet attachments (+=33%, -=67% in gauge 1; +=77%, -=23% in gauge 4) and least in bar attachment (+ =100%, -=0 in gauge 1; +=92%, -=8% in gauge 4).
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(A)Loading=50N (B)Loading=100N
(c)Loading=150N (D)Loading=200N
Fig. 1. Mean strains in overdentures without attachment (No), and with bar attachment (Bar), ball attachments (Ball) and magnet attachments (Mag.) for gauge 1 (G1), gauge 2 (G2), gauge 3 (G3) and gauge 4 (G4) in total different 5 loading directions under (A) 50N, (B) 100N, (C) 150 N and (D) 200 N loading. Positive strain (+) indicating tension and negative strain (-) compression.
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Table I. Types of strain (percentage) recorded by four linear stain gauges (n=1920 for all gauges)
Positive (%)
Negative (%)
No
Bar
Ball
Magnet
No
Bar
Ball
, 百拇医药
Magnet
Gauge 1
65
100
75
33
35
0
25
67
Gauge 2
17
75
, 百拇医药
0
22
83
25
100
78
Gauge 3
100
85
100
100
0
15
, 百拇医药 0
0
Gauge 4
53
92
50
77
47
8
50
23
Discussion
Significance of the design In the strict sense, overdenture supported by implants consists of mucosal- and implant-supported overdenture and implant-supported overdenture. For the former, occlusal forces are shared between the implants and the overdenture-bearing mucosa, whilst for the latter, occlusal forces are mainly supported by implants and therefore independent of the resilience and gradual changes in the soft tissues. The rigid connection of the implant-supported overdenture requires four or more implants and the use of U-shaped, rectangular or milled bars. The present study was about mucosal- and implant-supported overdenture, with bar-clip attachments, ball attachments and magnet attachments that were connected to two implants. The main function of these attachments is to assist the retention and stability of overdenture. The resilient connection acts as stress-breaking to absorb and avoid unfavorable lateral forces, bending moment and overloading to implants.
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Owing to the complexity of oral functional activities, the loading experiments only in a direction and a fixed magnitude of loading are obviously not enough to evaluate accurately strain distribution on overdenture and effect on implants. In order to simulate the real oral cavity functional condition, the present study designed test in 6 different loading directions and 4 different loading magnitudes. In addition, a silicon material was used as "mucosa". When measurements of strain are to be made with linear strain gauges, it is not only necessary to place a number of gauges, but also to place them in the relevant parts of the denture and also to locate them in the direction of principal strain[6]. Gauge 3, oriented mesi-distally in the midline between two implants and closest to implants in distance, can best reflect the strain changes in overdenture and the effect on implants. Gauges 1 and 4, oriented mesi-distally, were symmetrically positioned in posterior buccal flange of two sides to monitor the deformation of overdenture in these regions. Gauge 2 was located in lingual flange of the overdenture base and oriented occlu-gingivaly so as to detect how the base fits the mucosa. With the microstrain recorded from all the gauges, the stress distribution, overdenture deformation and possible effect on implants can be ascertained when forces were exerted.
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Without attachment overdenture It is evident that there was tensile strain concentration in gauge 3 region, which could result in overdenture deformation to trend to "open" or "stretch" the horse-shoe shaped denture. Subsequently mandibular lingual mucosa suffered from the traverse forces outward, and the forces increased with the increase of loading
Gauge 2 presented compressive strain because the base sank and pressed the mucosa downward and buccally, which was in agreement with the effect of tensile strain found in gauge 3 on overdenture.
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The strains received from gauge 1 and 4 were rather inconsistent, which reflected the complexity of strain distribution in related areas of overdenture in oral functional activities. Moreover, asymmetry of the alveolar ridge two sides anatomically was also responsible for this variation. Nevertheless, though there was variation in strain gauge 1 and 4, the results from certain gauge under identical testing condition might save as a comparison with those from attachment overdentures.
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In bar attachment overdenture Although gauge 3 exhibited same strain pattern, i.e., in tension as shown in no attachment overdenture, it is evident that values of the former were much lower than the latter on the whole and did not increase with the increase of loading, even decreased in tendency. It is believed that rigid structure of the bar attachment limited the deformation or "stretch" of horse-shoe shaped overdenture and formed fulcrums. Consequently the implants suffered from the traverse force towards outsides transferred by attachment. This force could be divided into two separate forces :one towards the lateral side and the other towards the frontal side due to the horse-shoe shaped structure of overdenture and mandible. The lateral forces might be offset each other with the bar connection completely or partially; whilst the forces towards frontally might not be. The traverse force is unfavorable to bone around implants. It is attributed to effect of the spacer that lower loading (50 N) produced higher tensile strain in gauge 3 because of existence of the spacer. As loading increased, the spacer decreased and disappeared the bar attachment closed together and restricted the deformation of overdenture, and then implants bore more traverse forces. The noted inconsistency in recorded directions and magnitudes of gauge 1, 2, and 4, comparing with no attachment overdenture under the same loading conditions was pointing out the differences between them in deformation of overdenture and mucosal support pattern. In gauge 2 relative lower compressive strains and even tensile strains indicated the corresponding mucosal support area did not receive as much force as in no attachment overdenture. Higher strain values recorded in gauge 1 and 4 situated in main mucosal support areas meant greater overdenture deformation in those areas, which is considered unfavorable for even distribution of force and may result in resorption of corresponding alveolar bone due to stress concentration.
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In ball attachment overdenture The higher values of tensile strain were observed in gauge 3 comparing with overdenture without attachment and with a bar attachment. Because of the resilient characteristic of plastic caps, the ball attachments allowed overdenture deformation in a greater extent and relieved the transverse force to implants. With the increase of loading the overdenture sank, approached the position where no attachment overdenture was and fit better so that mandibular alveolar bone assisted to restrict the "stretch" of overdenture further. Therefore it is not surprising that the magnitudes of strains decreased with the increase of loading. On the basis of the analysis above, there is an implication that using rigid lingual Co-Cr alloy framework instead of acrylic base will be favorable for decreasing deformation in that area, consequently decreasing the transverse forces towards implants.
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Unlike overdenture without attachment, gauge 2 presented consistent compressive strains in all testing conditions, which could be explained as the result of ball attachments securely holding the overdenture on the alveolar ridge mucosa in angular loading namely, being of good stability. These findings was pointing out the alveolar ridge mucosa shared support imparted by implants, and bar attachment implants seem to share more loading than ball ones. Guerra[7]suggested a bar attachment be used in case of excessive resorption, while solitary attachment be selected in case of mild to moderate ridge reduction.
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In gauge 1 and 4, the directions and magnitudes of the strains were more similar to in no attachment overdenture than in bar attachment overdenture. This is because ball attachment is more flexible than bar attachment.
In magnet attachment overdenture The high tensile strain values in gauge 3 suggested overdenture in that area deformed more as magnet slipped on the keeper when lateral forces were applied on them. This "stress-breaking" action might make the implant avoid suffering from unfavorable lateral forces. Strains in gauge 2 were in compressive and most of them showed the highest values, but it not believed that this overdenture fit best in those situations according to the analysis to vertical loading conditions above. The explanation for the high compressive strain values should be that the slip characteristic of magnet attachments gave the overdenture more freedom to "stretch" resulting in the increase of lateral pressure in gauge 2 against alveolar ridge mucosa. This can be verified by the highest tensile values in gauge 3. Using rigid lingual Co-Cr alloy framework is of the same importance as in overdenture with ball attachments.
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The movability between magnet attachment components is responsible for similarity of the strain pattern and values in gauge 1 and 4 to those in no attachment and ball attachment overdentures.
In fact, it is impossible for a denture to receive the loading force from only one direction while mastication. Usually a principal vertical force is accompanied by several forces from other different directions. Fig 1 illustrates mean strain values from total sum of 5 different directions under certain loading magnitude and Table I reveals strain types of each gauge in all testing conditions for different kind of overdentures. On the whole it is fundamentally in consistence with independent strain patterns and rank orders of the deformation in various overdentures with a few exceptions. In clinical experiments, the strain values will only be record in a form of mean from multiple directions. Therefore it is practical and important to understand and analyze the mean values and the outline of distribution of strain value pattern.
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For the point of biomechanical view, the axial force is more favorable, as it distributes stress more evenly throughout the implant[8]. However, traverse force is inevitable due to the position of arrangement of teeth in denture and motion of mandible. What we need to do is to reduce the traverse force exerted in implants as well as in the bone. According to this study, plastic ball attachments and "stress-breaking" magnet attachments seem to be favorable for overdenture because the stress distribution through it is more conformity with the mucosal- and implant- in common supported concept. Magnet attachments can prevent implants from suffer lateral force more effectively than ball attachments, but in keeping stability of overdenture and distributing loading force evenly on alveolar ridge mucosa, the latter is superior to the former. Bar attachment may result in unfavorable effect on implant as well as bone, which is in agreement with Meijer et al[9]. Because what role the bar connection plays in offsetting traverse forces between two implants is currently unavailable and what is the limited strains within the physiologic zone is not known[10]，further studies in vivo are needed.
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[收稿：1999-06-20], http://www.100md.com(1Wang Yining, 2Per-Olof Glantz, 2Krister Ninler)