Retentive force of three attachments for root-supported overdentures

X. Wang, C. Ohkubo¹, T. Hosoi¹, H. Shimpo¹, D. Kurihara¹ and T. Murata¹

Faculty of Stomatology, Jilin University, Chngchun, China

¹Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine, Yokohama, Japan

Introduction

Many kinds of stud attachments have been used as retainers for root-retained overdentures to obtain appropriate retention. “O” ring Post Anchor (OPA) attachments are composed of nitrile-butadiene rubber (NBR), an “O” ring, and a ball head patrix. In the Inoue Attachment (Tokyo, Japan) system, plastic anchor patterns with a 0.21 - 0.30 mm undercut on the post coping are cast for the patrix.

Recently, magnetic attachments have been used for root- or implant-retained overdentures and maxillofacial prostheses. The magnetic keeper is usually fabricated using the cast-on technique so that it is located on the post coping. However, the casting procedure causes a contaminated layer to form on the keeper surface. Because of this phenomenon, the surface changes from being completely flat to an uneven surface when it is sanded using abrasive paper to remove the contamination layer. The direct bonding technique was recently developed to resolve this problem. In this technique, the keeper is bonded using resin bond cement on the cast coping where the housing patterns with post coping are cast. Although the direct bonding technique maintains stable retentive force similar to that of a newly manufactured keeper, the height of the coping with the keeper is greater than with the cast-on technique because the housing used for direct bonding adds to the overall height.

The coping telescope was modified from a Konus telescope for easy fabrication and adjustment. The shape of the cone used as an inner crown is the same (6 degree taper) for both telescopes. However, the cone of the Konus telescope is covered with an outer metal crown, and the cone of the coping telescope is covered with autopolymerized polymethyl methacrylate (PMMA) resin.

Although many experiments have been performed on Konus telescopes, little is known about coping telescopes.

Objective

The purpose of this study was to evaluate the retentive force of three retainers, such as, O-ring post anchor (OPA) attachment direct bonded magnetic attachment and coping telescopes for root-retained overdentures (Fig. 1 a-c). The in vitro retentive forces were measured in cases of both single retainers using one root and double retainers using two roots. Additionally, the wet conditions were also investigated in the same way as the dry conditions.

Materials and Methods

Specimen preparation

The materials tested in this study are shown in Table I.

For the OPA and magnetic attachments, artificial mandibular canines (Nissin, Japan) were cut for the root preparation. A ball anchor pattern for OPA and a housing pattern for the magnetic keeper with post coping pattern to accommodate the root were cast conventionally with Ag-Au-Pd alloy. An “O&rdquo ring (No. 2) was fit to the ball anchor and then covered with autopolymerized polymethyl methacrylate. The keeper was cemented in the housing on the post coping with resin cement so that the keeper surface was positioned horizontal to the occlusal surface. Metal primer was applied to the magnet, which was then attached on the keeper covered with PMMA.

To prepare the artificial canines for the coping telescope, the inner crown of the coping telescope was waxed up (Flat top surface, 8.0 mm high, 6 degree taper) on the prepared tooth using a milling machine (F3, Degussa, Germany). After the wax pattern was then cast, the outer crown with 0.5 mm relief for the PMMA layer on the inner crown was waxed up and then cast with the same metal. After autopolymerized PMMA was applied to the outer crown using the brush-on technique, the outer crown fit to the proper position on the inner crown using a positioning index.

Ten paired patrix-matrix specimens were made for each attachment for a total of 30 specimens.

Measurement of retentive force

Using a constant load-press machine (Seiki, Tokyo, Japan), the patrix and matrix (except for the magnetic attachment) were joined using 1 kg of pressure. The joined patrix and matrix were mounted on a screw-driven mechanical testing machine (Instron 5565, Instron, Japan), and the retentive force (N) was evaluated as the tensile force obtained when the patrix and matrix were separated at a crosshead speed of 20 mm/min. This measurement was repeated 5 times for each attachment. After tensile testing was performed under dry conditions, all specimens were soaked for more than 1 hour in distilled water maintained 37°C. The wet specimens were also measured in the same way as the dry specimens.

This study measured the retentive force for both a single retainer using one root and a double retainer using two roots. To measure the double retainers, two combined attachments were simultaneously measured as shown in Fig 2.

The data were analyzed using SPSS statistical software (Version 10.0, SPSS Inc., Chicago, IL) by one-way analysis of variance (ANOVA) and Tukey's HSD multiple comparisons test at a significance level of α=0.05.

Results

The retentive force of the single and double retainers is shown in Table II and Fig. 3. The retentive force of the single retainer for all materials under dry conditions ranged from 1.96 to 6.76 N. The coping telescope had the significantly greatest force (p < 0.05), and there were significant differences in retentive force among all the attachments tested (p < 0.05). However, the coping telescope indicated a high coefficient of variation (51.5 %) in its retentive force.

The retentive force of OPA and the coping telescope under the wet condition decreased (82.4% and 90.1%, respectively) compared to the dry condition, except for the magnetic attachment (101.8%). There were significant differences only for the OPA attachment between dry and wet conditions (p < 0.05).

The retentive force of the double retainer ranged between 3.80 N and 14.20 N. All of the attachments had retentive force approximately two times greater (1.84 times for OPA, 1.92 times for the magnet, and 2.10 times for the coping telescope) than for the single retainer (p < 0.05). Similar to the single retainer, the coping telescope had a higher coefficient of variation (70.1 %) compared to the OPA and magnet attachments.

Discussions

This study was performed to determine the retentive force of three attachments for root-retained overdentures since there seem to be no studies in the literature on a comparison of their retentive force under uniform conditions, particularly the retention of double retainers for two remaining roots

Magnetic attachments are now fabricated using either the cast-on technique or the direct bonding technique. Suminaga et al. reported that the retention of the cast-on attachments was less than the direct-bonded attachments because the keeper surface using the cast-on technique was rough and required finishing and polishing for removal of the contamination layer. Therefore, the direct bonding technique is recommended if there is enough space between the remaining root and the opposing teeth. Theoretically, a magnetic attachment using the direct bonding technique can provide the amount of retentive force that the manufacturers claim because there is no alteration of the keeper surface. However, the retentive force in this study was lower than the manufacturer's data, which might have been obtained by tensile testing that exceeded a strictly perpendicular tensile direction to the magnet surface. One of characteristics of magnetic attachments is that the retentive force remarkably decreases from only a slight deviation from the perpendicular direction to the magnet. Therefore, the retentive force in the patient's mouth may be lower than the retentive forces obtained in this study. In contrast, the advantage of the magnetic attachment is that the retentive force is not affected by wet conditions.

The retentive force of the coping telescope varied widely, the lack of constant force was a problem. Because a ready-made pattern (i.e., the keeper, “O&rdquo ring, and magnet) was used for the OPA and magnet attachments, more consistent retentive values were obtained than for the coping telescope.

When there are more than two retainers in the same jaw, the retention of the overdentures is usually similar to the total retentive force of each retainer. This study found that the force of the double retainers was approximately two times higher than the force of a single one. The total retentive force can be estimated by calculating the number of attachments used and their retention.

Conclusions

The retentive force under dry and wet conditions was evaluated for both single and double retainers, i.e., the OPA attachment, magnetic attachment, and coping telescope. Within the limits of this study, the following results were obtained:

1. The retentive force of the coping telescopes with both the single and double retainers was significantly greater (p < 0.05) than for the OPA and the magnetic attachments.

2. Excluding the magnetic attachments, the wet OPA attachments and wet coping telescopes had lower retentive force compared to the dry attachments.

3. The coefficient of variation of the coping telescope was also greater than for OPA and the magnetic attachment because a ready-made was used.

4. The retentive force of all the attachments on the double retainers was approximately two times greater than for those on the single retainers.

Acknowledgement

The authors would like to thank Mrs. Jeanne Santa Cruz, Baylor College of Dentistry, TX, USA, for editing assistance. This study was partially supported by a research grant from the Japan-Chinese Medical Association, Sasagawa Foundation (project No. 2841)

References

1. Suzuki Y, Experimental studies on the retentive force of OPA-attachment, Tsurumi University Dental Journal 8: 13-33, 1982.

2. Shirato K, Experimental studies on the accuracy and the retentive force of coping telescope, Tsurumi University Dental Journal 17: 131-150, 1991.

3. Suminaga Y, et al, Surface analysis of keepers on dental magnetic attachments: comparison of cast-bonding technique and direct-bonding technique, Prosthodontic Research & Practice 3: 62-68, 2004.

4. Kobayashi M, Ohkubo C, Suzuki Y, Aoki T, Sato J, Hosoi T, Retentive force of “O” ring attachment to use immediate provisional implant (IPI)-retained overdenture, European Journal of Prosthodontic Restorative Dentistry 13: 147-149, 2005.

Discussion Board