[Measurements of the flux densities of static magnetic fields generated by two types of dental magnetic attachments and their retentive forces]

OBJECTIVE

To measure and analyze the flux densities of static magnetic fields generated by two types of commonly used dental magnetic attachments and their retentive forces, and to provide guidance for the clinical application of magnetic attachments.

METHODS

A digital Gaussmeter was used to measure the flux densities of static magnetic fields generated by two types of magnetic attachments, under four circumstances: open-field circuit; closed-field circuit; keeper and magnet slid laterally for a certain distance; and existence of air gap between keeper and magnet. The retentive forces of the magnetic attachments in standard closed-field circuit, with the keeper and magnet sliding laterally for a certain distance or with a certain air gap between keeper and magnet were measured by a tensile testing machine.

RESULTS

There were flux leakages under both the open-field circuit and closed-field circuit of the two types of magnetic attachments. The flux densities on the surfaces of MAGNEDISC 800 (MD800) and MAGFIT EX600W (EX600) magnetic attachments under open-field circuit were 275.0 mT and 147.0 mT respectively. The flux leakages under closed-field circuit were smaller than those under open-field circuit. The respective flux densities on the surfaces of MD800 and EX600 magnetic attachments decreased to 11.4 mT and 4.5 mT under closed-field circuit. The flux density around the magnetic attachment decreased as the distance from the surface of the attachment increased. When keeper and magnet slid laterally for a certain distance or when air gap existed between keeper and magnet, the flux leakage increased in comparison with that under closed-field circuit. Under the standard closed-field circuit, the two types of magnetic attachments achieved the largest retentive forces. The retentive forces of MD800 and EX600 magnetic attachments under the standard closed-field circuit were 6.20 N and 4.80 N respectively. The retentive forces decreased with the sliding distance or with the increase of air gap between keeper and magnet.

CONCLUSION

The magnetic attachments have flux leakages. When they are used in patients' oral cavities, if keeper and magnet are not attached accurately, the flux leakage will increase, and at the same time the retentive force will decrease. Therefore the keeper and magnet should be attached accurately in clinical application.

Hybrid gate design frameworks for the rehabilitation of the maxillectomy patient

A removable partial denture (RPD) for the restoration of maxillary defects must incorporate sound design principles to reduce potential damage to the abutment teeth and supporting periodontal tissue, while enabling acceptable functional levels of speech, mastication, and deglutition. These functional demands and potential stresses on the abutment teeth may exceed those encountered by patients without maxillary defects. This article describes the indications and design considerations for a hybrid gate design framework that incorporates both conventional cast direct retainers and the gate design concept in the same framework.

Comparison of cast Ti-Ni alloy clasp retention with conventional removable partial denture clasps

STATEMENT OF PROBLEM

The elastic property of Ti-Ni alloy may be a desirable property for cast removable partial dentures. However, little is known about the retentive properties of cast Ti-Ni alloy clasps.

PURPOSE

This in vitro study investigated the retentive force of various types of clasps during repeated cycles of placement and removal to determine whether Ti-Ni alloy clasps maintain their initial retentive force under varied conditions, including different retentive undercut depths and clasp size (thickness and width).

MATERIAL AND METHODS

The test models were developed using a mandibular dentiform with a horizontal plane of occlusion. Two complete metal abutment crowns were made for the left second molar, differing only in retentive undercut depths. A total of 98 clasps (n=7) were fabricated, including 14 wrought wire clasps and 28 clasps from each of the following alloys: Co-Cr alloy (Biosil), Type IV gold alloy (Cast-4), and Ti-Ni alloy (TN-10). Clasps were made to engage one of 2 retentive undercut depths (0.25 mm or 0.75 mm) and were made in 2 sizes (0.8 mm or 1.4 mm). Each clasp was radiographically examined for casting defects and porosity. The force (N) required to remove the clasps was measured using a universal testing machine with a crosshead speed of 10 mm/min. After measuring the retentive force, a masticatory simulator was used to cycle the clasps on and off the metal crown 500 times, simulating the insertion and removal of a removable partial denture clasp. The cycling sequence was repeated 10 times, and retention force was measured after each sequence. The internal surface of the clasp arms was examined for evidence of metal fatigue using scanning electron microscopy. Data were subjected to 2-way and 4-way analysis of variance, followed by Scheffe's multiple comparison test (alpha=.05).

RESULTS

Co-Cr alloy and gold alloy clasps in the 0.25-mm retentive undercut groups experienced a gradual decrease in retentive force measurements (P<.001). In contrast, the Ti-Ni alloy clasps maintained a retentive force of approximately 1.8 N and 2.6 N for the 0.8 mm and 1.4 mm clasp groups, respectively. Although Ti-Ni alloy clasps' retentive force was reduced slightly after the first cycling sequence, it was negligible compared with other clasps tested. The wrought wire clasps also retained their retentive force until the final cycling sequence. A similar trend was found in the clasp groups engaging 0.75-mm retentive undercuts.

CONCLUSION

Although the end-point retention for all the clasps was similar, there was less change in the retentive force of the cast Ti-Ni alloy clasps after repeated cycling sequences of simulated placement and removal.

O-ring attachments for transitional implant-retained overdentures

The lack of proper stability and retention is frequently found in mandibular complete dentures. The stability and retention of existing complete dentures may be improved by using transitional implants (TIs) incorporating the O-ring retention system. This article describes the procedures used to convert an existing conventional complete denture to a TI-retained complete overdenture with O-ring.

The mandibular labial bar major connector

The lower labial bar is rarely indicated as a major connector for a removable partial denture. It can be used satisfactorily when large mandibular tori interfere with conventional lingual bar placement or when the lower teeth are severely lingually tipped and placement of a lingual bar is not possible.

In vitro study on passive fit in implant-supported 5-unit fixed partial dentures

PURPOSE

Fabrication and retention methods have an influence on the passivity of superstructure fit. The objective of the study was to quantify the strain development of various cemented and screw-retained fixed partial dentures (FPDs).

MATERIALS AND METHODS

Forty samples of 4 different types of FPDs (10 of each type) were investigated. Each sample had 3 ITI implant abutments and 2 pontics. The 3 implants were anchored in a straight-line configuration in a measurement model simulating a real-life patient situation. Strain gauges were mounted close to the implants and on the pontics. The developing strains were recorded during cement setting and screw fixation. For statistical analysis, multivariate 2-sample tests were performed, with the level of significance set at P = .1.

RESULTS

All FPDs investigated revealed a considerable amount of strain, with no significant difference between cement and screw retention. Furthermore, no significant difference was found between the conventional fabrication modes for screw-retained FPDs. The lowest strains were found in prostheses that were intraorally bonded onto gold cylinders.

DISCUSSION

Because bonding of the superstructure in the oral cavity may compensate for impression and laboratory variables, restorations with the best possible passive fit can result from this retention technique. Before this technique can be recommended, the long-term stability of the adhesive layer should be investigated.

CONCLUSIONS

As an absolute passive fit of superstructures is not possible using conventional clinical and laboratory procedures, and as clinical fit-evaluation methods often do not detect "hidden" inaccuracies, the more sensitive strain-gauge technique should be utilized for an objective accuracy test. Reference strain values from implant-supported prostheses that have served without complications could help define a "biologically acceptable fit.

In vitro deformation of acetyl resin and metal alloy removable partial denture direct retainers

STATEMENT OF THE PROBLEM

Acetyl resin removable partial denture (RPD) direct retainers may provide an esthetic alternative to conventional metal direct retainers. The effect of repeated stress on acetyl resin direct retainers is unknown.

PURPOSE

This study compared deformation of acetyl resin and metal alloy RPD direct retainers after repeated dislodgments over a test die.

MATERIAL AND METHODS

Ten acetyl resin (Thermoflex) and 10 metal alloy (Ticonium Premium 100) RPD direct retainers, fabricated to manufacturers' specifications, were dislodged over a stainless steel die by means of a laboratory test apparatus for a simulated 3-year period (5000 cycles). Occlusal and facial digital images made before and after cycling were measured (mm) for direct retainer deformation by using computer-imaging software (Scion Image 1.62). Student t tests (alpha=.05) were performed for statistical comparisons.

RESULTS

A significant difference in deformation between acetyl resin and metal alloy direct retainers occurred in the occlusal view (P=.045), but not in the facial view (P=.832). Average deformation varied but was greatest in the occlusal view: 0.09 +/- 0.8 mm for acetyl resin direct retainers compared with 0.01 +/- 0.9 mm for metal alloy direct retainers. Average facial view deformations revealed no significant differences: 0.039 +/- 0.6 mm for metal alloy and 0.033 +/- 0.7 mm for acetyl resin direct retainers.

CONCLUSION

Within the limitations of this in vitro study, significantly greater deformation resulted with acetyl resin compared with metal alloy direct retainers after 3 years of simulated use.

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

It could be hypothesised that attachments, which provide more retention against vertical and horizontal dislodgement, will be associated with more favourable parameters of oral function. This in vivo study is designed to provide data regarding initial retention force, loss of retention force after 3 months of function and postinsertion maintenance and complications associated with the use of magnet, bar-clip and ball attachments in mandibular overdenture treatment. Eighteen edentulous subjects received two permucosal implants in the inter-foramina region of the mandible, a new denture and three successive suprastructure modalities (magnet-, bar-clip and ball attachments). The retention force of the attachments at baseline and after 3 months was measured in a standardised way. The amount and type of postinsertion maintenance that was related to the attachment were evaluated. No differences in retention force at baseline and after 3 months of loading were observed for all three attachment types. The mean retention forces of magnet attachments, bar-clip attachments and ball attachments were 8.1, 31.3 and 29.7 N respectively. Functional maintenance complications related to the attachments were predominantly observed in 11/36 magnet attachments. Functional problems in the ball attachment group were relatively rare, easily manageable and seen in 4/36 attachments. The bar-clip attachments exhibited no maintenance problems at all.

Esthetic considerations with removable partial dentures

The reduction in partial edentulism that has occurred due to successful preventive procedures and the predictable use of osseointegrated implants has reduced the need for removable partial dentures. However, for a variety of reasons, many patients can continue to benefit from partial denture therapy; and these patients deserve the best esthetic result possible. The primary esthetic objection to removable partial denture therapy is the unsightly display of the clasp assemblies. This article describes three strategies that can be used by the discriminating clinician to eliminate the display of the clasp assembly and provide an esthetic and functional removable prosthesis.

Esthetic designs of removable partial dentures

The increased emphasis on physical appearance in contemporary society has increased the demand for esthetic dental restorations. Although the success of implant dentistry has expanded the scope of esthetic fixed prostheses, many patients demand a removable partial denture (RPD) for health, anatomic, psychological, or financial reasons. Fabricating an esthetically pleasing RPD while avoiding the unsightly display associated with conventional clasp assemblies often presents a challenge to dentists. This article examines using lingual clasps, proximal undercuts (also known as rotational path insertion), and acetal resin clasps as simple and effective means of improving RPD esthetics.

Retrofitting implant overdenture attachments: a clinical report

The remake of implant-overdenture prostheses on preexisting implants can present the clinician with a challenge, especially when the prosthetic components used initially cannot be replaced. The difficulty of remaking the prosthesis may be further increased by implant attachments designed to be cemented to the implant itself--a feature that complicates future replacement. This clinical report describes the restoration of worn implant attachments using retrofit components.

A cost minimization analysis of implant treatment in mandibular edentulous patients

PURPOSE

This study aimed to determine if implant-supported overdentures are a long-term economically efficacious therapy for edentulous patients when compared to fixed osseointegrated prostheses.

MATERIALS AND METHODS

Clinical records of 25 patients from two long-term studies (fixed and overdenture) were included in this analysis. A cost minimization analysis from the patient perspective was employed. Direct clinical and time costs incurred over the 9-year period were deflated to 1995 Canadian dollars using the Consumer Price Index. National salary rates by occupation and gender were used to value patients' time, and a sensitivity analysis was carried out to assess the robustness of the results when an equal mean salary rate across treatment groups was assumed.

RESULTS

The mean total, clinical, and time costs were significantly higher (Ps .05) for the fixed restoration group (dollars CAD10,748, dollars CAD10,094, and dollars CAD654, respectively) when compared to the overdenture group (dollars CAD3,665, dollars CAD3,343, and dollars CAD322, respectively). The initial, maintenance, and clinical visit costs were also significantly higher (P < or = .05) in the fixed restoration group (dollars CAD7,567, dollars CAD2,527, dollars CAD542, respectively) than in the overdenture group (dollars CAD2,505, dollars CAD830, dollars CAD292, respectively). The sensitivity analysis demonstrated that the time cost for the fixed prosthodontic group (dollars CAD488 vs dollars CAD322) was still significantly higher (P = .002), even after an equal mean salary rate was assumed.

CONCLUSION

Overdenture therapy for edentulous patients is a more cost-effective treatment compared to fixed prosthodontic treatment.

Residual ridge resorption in the edentulous maxilla in patients with implant-supported mandibular overdentures: an 8-year retrospective study

PURPOSE

This retrospective study radiologically investigated alveolar bone resorption in the edentulous maxilla in patients with implant-supported mandibular overdentures.

MATERIALS AND METHODS

This study consisted of 35 healthy, completely edentulous patients with a mean age of 59.7 years. They had received two implants between the mental foramina. New bar-retained mandibular overdentures and maxillary complete dentures were fabricated. Standardized panoramic radiographs taken subsequent to loading and at annual recall visits for up to 8 years were measured for alveolar bone loss in the maxilla. Bone areas and areas of reference not subject to resorption were measured with a planimetry program. The proportional value between both was expressed as a ratio (R). Bone loss was expressed as a change in R between two time points. Differences in the resorption rate between the anterior and posterior parts of the maxilla were investigated.

RESULTS

Residual ridge resorption continued during the follow-up period and revealed high individual variability. With a range of 5% to 11% (median) loss in the original bone height, it was significantly (P < .031) more pronounced in the anterior than posterior maxilla (2% to 7%) from the second through eighth years. Regression analysis of the medians revealed a relatively high correlation between time and bone loss in both anterior and posterior parts of the maxilla.

CONCLUSION

The anterior anchorage of mandibular overdentures by means of two implants and an ovoid bar was associated with slightly higher resorption in the anterior than in the posterior part of the edentulous maxilla.

A randomized clinical trial comparing two mandibular implant overdenture designs: 3-year prosthetic outcomes using a six-field protocol

PURPOSE

This clinical trial tested the null hypothesis that there would be no difference in prosthetic maintenance for two-implant mandibular overdentures retained by either a bar-clip mechanism or ball attachments. Prosthetic outcomes are reported over 3 years using a six-field protocol.

MATERIALS AND METHODS

One hundred edentulous participants received new maxillary complete dentures and a mandibular two-implant overdenture (IOD), with random assignment to either a bar and metal clip or two ball attachments (titanium alloy matrix and spring) for retention. Eighty-seven subjects were available for follow-up after 3 years.

RESULTS

Almost three times as many bar-clip dentures (63%) were rated successful compared to the ball attachment design. Two percent of the participants in each group died over the course of the study, while 15% of the bar-clip and 8% of the ball IOD subjects were lost to follow-up. More than three times as many ball attachment IODs (60%) required retreatment in the form of excessive repairs, and twice as many of the ball attachment design (8%) required replacement. The ball attachment IOD was significantly more likely to require patrix tightening or matrix replacement, while the bar-clip design was more likely to require activation of the matrix.

CONCLUSION

Using the criteria of a six-field protocol for implant overdenture outcomes, the bar-clip IOD was a significantly more successful prosthesis, requiring less maintenance than the titanium alloy matrix and spring ball attachment IOD employed in this study. The null hypothesis was therefore defeated.

The distal extension base denture

The distal extension base denture may be indicated in situations in which the edentulous area to be restored is without a terminal abutment tooth. There may be significant challenges in providing a prosthesis with sufficient support and retention to make it comfortable without damaging the intra-oral tissues. This can be a greater problem in the mandible as the denture-bearing area is usually much smaller than in the maxilla. This paper considers how distal extension removable prostheses can be designed to restore edentulous spaces.

An overview of the O-ring implant overdenture attachment: clinical reports

The O-ring is used to increase retention of implant complete and partial overdenture prostheses. They possess a number of advantages, including ease of use and maintenance, low cost, and possible elimination of a superstructure bar. O-rings wear over time, gradually lose retention, and must be replaced periodically. It is essential that O-ring abutments be parallel to each other. Two cases are presented showing the use of O-rings with a complete mandibular implant overdenture and a maxillary removable partial denture.

In vitro study of a mandibular implant overdenture retained with ball, magnet, or bar attachments: comparison of load transfer and denture stability

PURPOSE

When implants are used to support a removable overdenture, the optimal stress distribution to minimize both forces on the implants and denture movement is desirable. This study compared the stress patterns generated around implants and denture movement among three retention systems.

MATERIALS AND METHODS

Two root-form implants were anchored in a mandibular model made of resin, and a removable overdenture on which all experiments were performed was fabricated. The surface of the model was covered with a layer of impression material to simulate oral mucosa. Ball/ O-ring, bar/clip, and magnetic attachments were used. A vertical force was applied to the left first molar and gradually increased from 0 to 50 N in 5-N steps. The resultant stress distribution and denture movement were evaluated.

RESULTS

The ball/O-ring attachment transferred the least stress to both implants and produced less bending moment than the bar/clip attachment. Vertical force applied to the bar/clip attachment created immediate stress patterns of greater magnitude and concentration on both implants.

CONCLUSION

This in vitro study suggested that the use of the ball/O-ring attachment could be advantageous for implant-supported overdentures with regard to optimizing stress and minimizing denture movement.

Improved design of long-span resin-bonded fixed partial dentures: three case reports

When single-tooth implants are not appropriate, the use of resin-bonded fixed partial dentures is a preferred treatment option when the abutments are relatively sound. However, the use of resin-bonded fixed partial dentures (RBFPDs) for replacing two or more missing teeth is considered to have a guarded prognosis, as long-span RBFPDs have been shown to be less successful than single pontic prostheses. The use of properly modified nonrigid connectors may well improve the success of long-span RBFPDs by reducing harmful interabutment stresses that appear to be responsible for retainer debonding in long-span RBFPDs. For such long-span prostheses, it is advised that the major retainer have wraparound on at least three surfaces of the abutment or have strategically placed opposing axial grooves or slots. It is considered essential that the connector allow interabutment movement in both the horizontal and vertical planes so that the retainer with the greater resistance and retention form does not stress and possibly debond the minor retainer. The upside-down positioning of the nonrigid connector and the matrix incorporated with the major retainer is considered important for successful maintenance, if a debond should occur due to greater loading on the major retainer, because it can be removed and recemented easily. Clinical cases are described that replace two or more missing teeth using fixed-movable RBFPDs with nonrigid connectors.

An alternate treatment method for a fixed-detachable hybrid prosthesis: a clinical report

The inability to consistently achieve a passive fit with screw-retained implant prostheses is well documented in the literature. Cement-retained implant frameworks have been advocated for implant-retained fixed partial dentures. However, cement retention has not been routinely advocated for fixed-detachable hybrid prostheses. This article describes a method for fabricating a cement-retained fixed-detachable hybrid prosthesis. The advantages and disadvantages are discussed.

[The ideal endodontic posts. A review of the literature]

Extensive research has been done regarding endodontic posts. In the majority of cases, only a few aspects are investigated. A literature review was performed regarding the various aspects that have to be considered when selecting a particular endodontic post for clinical application. The dentist has to realize that the ideal endodontic post (still) does not exist for all clinical situations. Most important is to achieve both a durable and a predictable result, sparing as much as possible of the root itself. Based on the literature, data presenting the requirements for an 'ideal' endodontic post are formulated.

An overview of the Lew attachment: clinical reports

Although the Lew attachment was originally developed to provide retention for overdentures fabricated in conjunction with subperiosteal implants, these attachments can also be used with other types of implants. Two cases are presented to describe the use of Lew attachments, one with a conventional subperiosteal implant, and the other with 4 endosteal implants and an implant connecting bar.