Allen L. Schneider, DDS
Gregori M. Kurtzman, DDS
Reprinted from General Dentistry/March-April 2001

Abstract Implant-retained overdentures are a restorative option for both the fully and partially edentulous arches. A new attachment, the Locator, which features a reduced interarch requirement and the advantage of built-in guide planes providing precise insertion, is described. The Locator is an advancement in attachment technology, with an improved design combined from the best features of a ball attachment, an ERA attachment, and a cap attachment.
Received: October 13, 2000 Last revisions: November 16, 2000 Accepted: January 8, 2001
Bar overdentures have been used with implants since the 1950s. Little variation has occurred in the design of the bar; innovations have been in the methods of attaching the removable prosthesis to the implant-retained bar. The attachments utilized for overdenture retention have improved, as has the practitioner’s understanding of the function of the removable component. This article examines a new attachment for bar-retained overdentures that overcomes many of the limitations of earlier designs.
Cast bars traditionally are designed either with round, ovoid, or square cross-sections to accommodate the particular brand of attachment being utilized and, more importantly, to provide the strength necessary to avoid flexing of the bar during function.1 Excessive flexure of the bar may lead to screw loosening, casting fracture, implant cervical bone loss, and implant failure.
Ease of patient insertion is another important factor of any overdenture system, as is the lifespan of the attachments. More often than not, patients place the removable prosthesis into the mouth, move it around with the tongue, and bite the components into place.
This causes excessive forces on the attachments, leading to premature wear or deformation of the resilient portion of the attachment. The addition of guide planes on the superior aspect of the bar with corresponding depressions within the removable prosthesis ensures that when the patient inserts the overdenture, it will seat correctly each time. In the past, this has been the alternative to an expensive milled bar. The guide planes are trapezoidal in shape and are placed in three locations around the arch on the bar.2 The guide planes align the denture as it seats onto the bar so that the attachment components connect properly. The reward to the patient is the desired benefit of an appliance that is easier to place and requires less maintenance.
Attachment can be divided into three groups with regard to bar designs: intra-bar, extra-bar, and circum-bar.3-14
Extra-bar attachments require more interarch space than do intra-bar attachments, which may restrict their use in some cases. The extra-bar attachments that have been on the market require the following minimum of male/female total height over the bar: ERA (Sterngold-ImplaMed, Attleboro, MA; 800/243/9942), 4.85 mm; Dal-Ro (3I (Implant Innovations, Inc.), Palm Beach Gardens, FL; 800/342-5454), 5.82 mm; O-Ring (various manufactures), 6.14mm; and EDS (Essential Dental Systems, Inc., South Hackensack, NJ; 800/223/-5394), 6.22mm (Fig. 1). One benefit of extra-bar attachments is that they are placed on the superior aspect of the bar rather than within the bar. This results in a cast bar of higher strength due to the fact that there is a greater bulk of metal at the cross-section adjacent to the attachment.
The main benefit of intra-bar attachments is that the connection between the two components directs the forces of mastication closer to the crest of the ridge, thus decreasing the lever arm mechanics on the supporting implant structure. Inter-arch space constraints are overcome best by the use of intra-bar attachments; however, incorporating these within the bar may compromise bar strength due to insufficient cast metal around the attachment.
Circum-bar attachments are described as an attachment that wraps around the bar (for example, Hader clips and Dolder clips). When these types of attachments are placed properly parallel to each other along the anterior portion of a bar, they allow functional rotation of the prosthesis around the bar. With improper placement of the attachments along the posterior distal extension of a bar, rotational ability is greatly minimized and stresses are placed on the attachments, leading to increased wear. Circum-bar attachments also must be perfectly parallel in the vertical orientation or proper seating of the denture is hampered.
One aspect that often is not addressed when selecting which attachment to use is the lifespan of the resilient portion of the attachment. With repeated insertion and removal of the attachment, wear of the nylon (ZAAG (Zest Anchors, Escondido, CA; 800/262-2310), ERA, Dal-Ro, EDS and Locator (Zest Anchors) or rubber (O-Ring) component eventually compromises the retentive ability of the attachment. Ideally, an attachment should have as long a lifespan as possible, with minimal chair time required to replace the resilient part when it is worn out.
Bench testing the lifespan of an attachment typically is reported in the number of cycles before wear necessitates replacement of the resilient part. A study conducted by Delsen Testing laboratories (Glendale, CA; 888/433-5736) comparing various attachments found that the ERA attachment lasted 3,000 cycles (equivalent to 1-2- years of clinical use) and the ZAAG attachment lasted 12,000 cycles (equivalent to 2-3 years of clinical use).15
The Locator attachment was developed as a combination of an ERA attachment, an O-Ring attachment, and a cap attachment. The benefits of both the intra-bar attachments (minimal interarch height requirement) and extra-bar attachments (greater metal cross-section for bar strength) are present in this new design.
The male self-aligns with the female, simulating a built-in guide plane. As the skirt on the male contacts the outer rim of the female, the center portion of the male is guided into the female and correctly aligned seating occurs (Fig. 2). The Locator attachment features a combination of external and internal retentive mating surfaces, a concept known as “dual retention”, which creates more than twice the retentive surface area of other attachments. The nylon male pivots in its stainless steel cap, permitting 8-degrees of rotation without any resulting loss of retention.
The Locator has a total male/female vertical height requirement of only 2.5mm. This permits its use in tight inter-arch cases without the compromise in bar strength that normally would occur when the height of the cast bar is reduced to create more occlusal space. Independent lifespan testing at Delsen testing Laboratories found that the Locator attachment survived 60,000 cycles, equivalent to many years of normal wear, prior to failure of the resilient portion. Retention of the Locator attachment is 4.0-5.0 lb, compared with 3.0-4.0 lb for the ERA (depending on which color insert is used).15
Fig 1. Vertical height comparison of various implant attachments that can be used either for stud attachments or in bar overdenture applications. (From left to right: Locator, ERA, Dal-Ro, O-Ring, and EDS.) Fig 2. Illustration demonstrating mating of the male and female components of the Locator attachment. Fig 3. Mandibular arch prior to implant placement, demonstrating lack of available ridge for denture retention.
A 67-year-old woman with an edentulous mandibular arch (Fig. 3) wanted an implant-stabilized restoration. Four 4.0mm threaded RBM implants (Lifecore Biomedical, Chaska, MN; 800/752.2663) were placed around the arch to support a bar overdenture (Fig. 4). Following an appropriate healing period, the implants were uncovered and closed tray impression heads were placed (Fig. 5 and 6). Polyvinyl putty impression material (GC America, Alsip, IL; 800/323-7063) was used to take a master impression in a stock tray. The impression posts were removed from the implants, analogs were attached to each impression head, and a master cast was fabricated (Fig. 7).
Titanium temporary abutment heads were affixed to each analog to prepare for fabricating a verification stent. Dental floss was wound loosely from temporary abutment head to temporary abutment head to act as a scaffold for the Triad gel (Dentsply Trubyte, York, PA; 800/877-0020). Triad sheet material was placed over the dental floss to fabricate an acrylic bar joining all the implants. To eliminate stress from shrinkage in the acrylic, a cut was made with a thin disk midway between each temporary abutment head (Fig. 8).
The portions of the verification stent were taken intraorally. Passive seating of each portion was verified. Triad gel was flowed over each cut in the stent and light cured to orient each implant rigidly in relation to the others (Fig. 9). The verification stent was removed from the mouth and reinserted to verify passive fit of the stent. The verification stent was tried on the master model and passive fit on the analogs was checked. This step ensures that what is present intraorally is reflected on the master cast.
Fig 4. Panoramic radiograph showing placement of implants in the anterior mandible. Fig 5. Intraoral view showing implants at second stage surgery. Healing screws have been removed. Fig 6. Impression posts placed on the implants.
Fig 7. Final impression showing impression heads with analogs attached. Fig 8. Verification stent fabricated on the master cast with Triad gel and titanium temporary abutment heads. The stent has been sectioned between each abutment head to eliminate polymer shrinkage. Fig 9. Try-in of the verification stent to check for passive fit of the portions on each implant. Additional Triad gel is flowed over each section in the verification stent to lock the pieces together in a stable, accurate position.
Analogs were attached to each temporary titanium abutment head on the verification stent and then inserted into a patty of fast set plaster, with the analog side down (Fig.10). After setting up, this verification model was used later in the office to verify passive fit of the cast framework when it was returned from the laboratory. Should a discrepancy be present between the cast framework and the verification model, the casting can be sectioned prior to the patient’s arrival in preparing a soldering index. A bite registration was made, using an acrylic baseplate (Dentsply Trubyte) formed on the master model with a wax rim, and taken intraorally (Fig. 11).
The laboratory mounted the articulated models using the bite registration and set the teeth. When the denture was returned to the office, both fit and occlusion were verified in the mouth (Fig. 12). The approved wax setup of the teeth was returned to the lab to begin the bar fabrication phase. A silicone matrix was fabricated to show the position of the teeth and buccal extent of the denture. This aids in fabricating the bar so that it lies within the approved wax setup of the denture. Gold UCLA abutments (Lifecore Biomedical, Chaska, MN; 800-752-2663) were placed on the analogs and a bar was waxed. The stainless steel Locator females were waxed to the superior aspect of the bar and cast (Fig. 13). The finished cast bar was tried in on the master model and checked for passive fit and any discrepancies (Fig. 14). A black processing male with stainless steel denture cap was placed on each of the female Locator attachments and an acrylic record base was fabricated. The teeth were reset in wax using the silicone matrix.
Fig 10. Verification model based on verification stent taken intraorally. This will be used to verify passive fit of the cast framework. Fig 11. Bite Registration taken to articulate the upper and lower casts. Fig 12. Wax try-in of the denture to ensure proper position and occlusion.
Fig 13. A silicone matrix of the teeth is shown on the cast with the waxed bar. The female portion of the attachments is shown in silver. The plastic waxing sleeves of the gold UCLA abutment are shown in white. Fig 14. The final cast bar on the master cast with the female portion of the Locator attachment cast within the bar. Fig 15. The cast bar inserted on the implants.
The cast bar was returned to the dental office and tried on the plaster verification model for passive fit. Upon approval of the bar on the verification model, the bar was tried intraorally and inspected for passive fit (Fig. 15). The denture with black processing males was snapped onto the bar so that occlusion, fit, and retention could be verified. Both denture and bar were returned to the lab to process the denture. After denture processing, the black male liner was removed from the stainless steel cap with the Locator core tool and the final nylon male liner was inserted (Fig. 16). The completed denture and bar were returned to the dentist for final insertion. The bar was inserted and a torque wrench was used to achieve the proper preload on the abutment screws. The denture was seated and occlusion was checked. The denture was seated and occlusion was checked.
Fig 16. The underside of the finished denture illustrating the male component of the Locator attachment.
The Locator attachment is an advancement in implant bar attachments. It requires significantly less interarch space than other attachments presently in use for implant bar overdentures. The built-in guide plane of the attachment ensures proper mating of the removable overdenture component to the fixed portion on the bar, thereby, increasing the lifespan of the resilient portion of the attachments. The Locator male is able to pivot in its stainless steel metal denture housing, creating a resilient connection for the prosthesis. The retentive nylon male remains in static contact with the female component, allowing its metal housing to have a full range of rotational movement over the male. This particular design of the pivoting Locator male allows a resilient connection for the overdenture without any resulting loss of retention. Minimal dexterity is required by the patient to insert or remove the denture.
The authors thank Hermanson Dental Laboratory (St. Paul, MN for fabrication of the prosthetics and Dr. Adrian Patterson (Burke, VA) for the surgical phase of the case presented.
The views expressed in this article are those of the authors; the studies were not supported by the manufacturer of any commercial product mentioned in this article.
Dr. Schneider is in private practice in Springfield, Virginia and is an international speaker and consultant to various dental companies. Dr. Kurtzman is in private practice in Silver Spring, Maryland and is a clinical instructor at the University of Maryland School of Dentistry; Department of Restorative Dentistry.
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