12. Influence of the arrangement of the magnetic attachments in the remaining dentition upon the denture mobility
M.Hideshima, M.Tokoh, M.Fukumoto, T.Andoh, A. Nishiyama＊, M. Sato, H. Mizutani, Y. Igarashi
Section of Removable Prosthodontics, *Temporomandibular Joint and Occlusion, Graduate School, Tokyo Medical and Dental University
Introduction
It is well known that dental magnetic attachments are generally applied to prosthodontic treatments as one of the most useful retainers and comprehensive clinical data are obtained
However there is not sufficient indication how we arrange magnetic attachments in the remaining dentition in order to acquire effective retentive force with the overdenture. Therefore we investigated the movements and retentive forces of the experimental denture plates in various arrangement of the magnetic attachments with the simulated model.
Objective
The purpose of this study was to clarify the effective arrangement of the magnetic attachments in the simulated model with experimental base plate when changed the numbers of attachments and location of tensile loading.
Materials and Methods
1. Simulated maxillary denture base model
Eight magnetic root keepers (Magfit RKR,φ3.6mm; Aichi Steel Co. Ltd, Japan) were symmetrically arranged in the regions of both second incisors, canines, first and second molars on the flat acrylic base supposing maxillary dentition (Fig.1).
Each keeper was fixed with translucent auto-polymerizing resin. Corresponding acrylic plate was fabricated as the experimental denture base plate and 8 magnetic attachments were fixed in the opposing area of each corresponding root keeper. Four metal hooks were attached in the regions of both between second incisors and canines (Anterior:A), and first and second molars (Posterior:P) and lifted chain was attached to each hook (Fig.2).
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Each magnetic attachment was covered with auto-polymerizing resin and the magnetic assembly was designed to intercuspate with the experimental base plate not to dislodge while lifting the base plate (Fig.3). Handling knob was attached on top of each assembly so that the arrangement of the magnetic attachments was easily changed.
2. Arrangement of the magnetic attachments and lifted region
Arrangement of the magnetic attachments was performed from 2 to 4 abutments (Fig.4). At each arrangement the experimental plate was pulled upward with 4 lifted chains attached to antero-postero (AP) hooks and with 2 chains attached to anterior (A) hooks and posterior (P) hooks.
3. Measurement of retentive force at each magnetic arrangement
Using universal testing machine AGS-H (Shimazu Co.Ltd, Japan), experimental base plate was subjected to tensile loading and lifted upward by means of S-shaped metal hook and chains perpendicularly to the occlusal plane at a cross-head loading speed of 1mm per minute (Fig.5).
The load required to dislodge the base plate was recorded. The retentive force at each magnetic arrangement was determined as the maximum load in a dislodgement cycle. The test was repeated 5 times when the arrangement of the attachments and location of the lifting hooks were changed. Average retentive force and relationship between tensile loading and vertical displacement were analyzed.
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Results and Discussion
1. Retentive force of a single magnetic attachment
In order to ascertain retentive force of the individual magnetic attachment in this system, single magnetic assembly without experimental denture plate was measured with 2 lifting methods. Table 1 shows retentive forces when lifted with a metal stick attached to tensile rod with auto-polymerizing resin and when lifted with a suspended chain same with this experiment.
Retentive force with suspended chain represented about 65% of the indicated maximum force of the production company; 5.6 Newton(N).
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Lifting method |
Retentive force |
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Attached to tensile rod |
4.2±0.1Ｎ |
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Suspended with chain |
3.7±0.1Ｎ |
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2. Retentive force with 8 magnetic attachments
In order to ascertain maximum retentive force of the fully 8 magnetic attachments in this system, experimental denture plate was pulled with 3 lifted regions of antero-postero (AP) 4 chains, anterior (A) 2 chains and posterior (P) 2 chains (Fig.6). It was resulted in 21.7N with AP chains, whereas 9.9N with A and 9.0N with P.
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3. Comparative analysis among 2, 3 and 4 magnets with 4 AP chains
Fig.7 shows retentive force at each arrangement from 2 to 4 magnets with 4 AP chains.
Symmetrical arrangement at right and left canine and first molar with 4 magnets represented highest force ;13.0N and symmetrical at right and left canine showed highest; 6.7N in 2 magnets, while asymmetrical adjacent canine and lateral incisor and contra-lateral first molar arrangement represented highest; 10.3N in 3 magnets.
Comparing average retentive force among the number of abutments, 4 magnets showed 10.5N and 3 magnets 8.6N and 2 magnets 5.3N.
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4. Comparative analysis in 4 magnets among 3 lifted methods
Fig.8 shows retentive forces in 4 magnetic attachments among 3 lifted regions of AP, A and P. Arrangement at right and left canine and first molar represented highest force; 13.0N with AP, while right and left canine and lateral incisor showed highest; 11.6N with A. Although it was different in 3 lifted methods in 2 groups of arrangements, average retentive force of 3 lifted methods in both right and left canine and first molar showed 8.3N and in both canine and lateral incisor showed 7.8N.
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5. Comparative analysis in 2 magnets among 3 lifted regions
Fig.9 shows retentive forces in 2 magnetic attachments among 3 lifted regions of AP, A and P. Symmetrical arrangement at right and left canine showed highest force; 6.8N with AP and A, and asymmetrical arrangement at canine and lateral incisor showed same inclination with AP and A but the highest value 4.3N was lower than that of both canines. Asymmetrical arrangement at first and second molar showed highest force; 6.3N with P.
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6. Comparative analysis in 3 magnets among 3 lifted regions
Fig.10 shows retentive forces in 3 magnetic attachments among 3 lifited regions of AP, A and P. Arrangement at adjacent canine and lateral incisor and contra-lateral first molar showed highest force; 10.6N with AP, and represented same inclination with the isolated arrangement of both canines and first molar. But each value of the 3 lifited regions in the isolated arrangement was approximately 2N lower than that of the canine, lateral incisor and contra-lateral molar arrangement.
7. Increased retentive forces at each arrangement from 2 to 3 magnets
Fig.11 shows average increased retentive forces in 3 lifted regions when added 1 magnetic attachment to 2 magnetic arrangement. In adjacent canine and lateral incisor arrangement, added contra-lateral canine and first molar showed more than 3N whereas 0.5N in both canines added with first molar.
8. Load-vertical displacement curves in 3 magnets
Fig.12 shows load-vertical displacement curves of representative 3 magnetic attachments in each lifted region. Load-vertical displacement curve in arrangement at canine, lateral incisor and contra-lateral first molar or canine showed linier increase and single peak at 0.45mm so that the effective retention was supposed to be obtained. On the other hand, there were 2 or 3 peaks in the load-vertical displacement curves of the isolated canine and both first molars so that the dislodgment was occurred and effective retention was not supposed to be obtained.
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Conclusions
It was suggested that symmetrical arrangement in even-numbered 2 or 4 magnetic attachments represent effective retentive force, whereas adjacent 2 abutments and contra-lateral 1 abutment arrangement in odd-numbered 3 magnets represent effective retentive force.
1. Maximum retentive force showed more than 22N with 8 magnets, 8N with 4 ones, 7N with 3 ones and 4N with 2 ones when lifted with 4 antero-postero(AP) chains attached to the corresponding hooks.
2. When 2 magnet attachments were applied, symmetrical arrangement showed more retentive force than that of the asymmetrical adjacent arrangement.
3. When 3 magnets were applied, adjacent 2 abutments and contra-lateral arrangement showed more retentive force than that of the 3 isolated abutment arrangement.
References
1. Geering A, Kundert M, Kelsey C: Complete denture and overdenture prosthetics, 1-262, Thieme Medical Publishers, New York, 1993.
2. Ishikawa S: Mechanical studies on dental magnetic attachments. J Stomatologic Soc 60: 98-120, 1993.
3. Rutkunas V, Mizutani H, Takahashi H: Influence of attachment wear on retention of mandibular overdenture. J Oral Rehabil 34: 41-51, 2007.