Written by Samuel M. Strong, DDS Saturday, 31 December 2005 19:00
The use of implants to improve retention of removable prostheses has demonstrated success for many patients.1-3 Clinicians have found that conversion from conventional, nonimplant-retained appliances to implant-retained overdentures typically results in better function, retention, and overall satisfaction. Implants typically “retain” this type of appliance by one of 2 methods:
(1) Attachment retained—the removable prosthesis snaps into or is somehow secured into the implants by means of a maternal/paternal attachment system. In contemporary systems, one component portion threads into the implant body, and the other portion is secured into a metal housing, which is processed into the removable denture base (Figures 1 and 2).
(2) Bar retained—a cast metal bar, usually noble or high-noble alloy, is screw-retained into a series of 4 to 6 or more implants. In some cases as few as 2 implants can be used, although this is less effective than using a greater numbers of implants. The bar(s) will receive attachments in metal housings, which are processed into the acrylic denture base (Figures 3 and 4).
|Figure 1. Two o-ring abutments threaded into mandibular cuspid implant sites. (Courtesy of Dr. Charles English.)||Figure 2. Attachment-retained overdenture with metal housings and o-rings in denture base. (Courtesy of Dr. Charles English.)|
|Figure 3. Two maxillary cast bars for retention of an implant overdenture.||Figure 4. Maxillary overdenture retained by the 2 bars seen on master cast.|
The options for the use of attachments are numerous. One distributor alone lists more than 30 different attachments available for use with implants (Attachments International).4 Restorative dentists and commercial dental laboratories develop their own biases for which attachments to use based on their individual success and cost-effectiveness. Criteria for choosing a particular attachment may include the following:
• degree of retention available,
• ease of replacement,
• anticipated rate of replacement,
• availability of metal housing, and
• minimum height required to fit the attachment into edentulous space (also called the attachment profile height).
Many clinicians, including the author, have previously considered the bar-retained overdenture to be superior to the attachment-only-retained format. Some of the reasons put forth for this preference include the following:
(1) Implant longevity may be improved by splinting the implants together using a bar format.
(2) Attachment choices for bars are more numerous. Many “direct-to-implant” at-tachment choices are also available for use with a bar.
(3) Attachment sites can be more numerous with bar-retained format (eg, placement of attachments on ends, midsections, and sides of the bar).
(4) Implants with divergent angulations can have the attachment paths of insertion paralleled by us-ing the bar format.
However, the increased technique sensitivity and costs associated with bar-retained formats have driven some prosthetic dentists to seek alternative methods. Today, direct attachment technology has improved to the point that the attachment-only format appears increasingly attractive from clinician and consumer standpoints.
The following case will illustrate the conversion from a standard, bar-retained overdenture to an attachment-only retained format. We will compare the 2 options procedurally, economically, and preferentially from the patient’s point of view.
|Figure 5. Four Steri-Oss mandibular implants placed anterior to the mental foramina.||Figure 6. Cast noble alloy bar for retention of original overdenture.|
|Figure 7. Example of mandibular implants placed posterior to the mental foramina. This contraindicates cross-arch splinting due to mandibular flexure resulting in design here with 2 separate bars.||Figure 8. Original mandibular bar and overdenture with midline Hader clip and distal extension ERA attachments.|
The patient, a 60-year-old female at the time of the initial bar-retained overdenture delivery, had 4 Steri-Oss 3.8-mm nonhexed implants placed between the mental foramina (Figure 5). This common, “4-in-the-floor” ap-proach to a mandibular overdenture was restored with a single bar splinting all implants together (Figure 6). Cross-arch splinting of this type is usually considered acceptable as long as the implants are not positioned posterior to the foramina. Due to the nature of mandibular flexure observed on opening and closing, excess stresses on posteriorly placed implants could ad-versely affect the longevity of implants splinted in a cross-arch manner.5 Although valid evidence may currently be insufficient to confirm this concept, many clinicians avoid cross-arch mandibular bar splinting when the implants are positioned posterior to the foramina (Figure 7).
In the case report illustrated here the noble alloy cast bar incorporated a midsection Hader EDS bar and clip (Attachments International) with distal extension ERA attachments (Sterngold) cantilevered off the posterior-most implant sites (Figure 8). Many clinicians and laboratories have used this combination of attachments for bar retention with great success.
|Figure 9. Removal of worn Hader clip retained by a metal housing.||Figure 10. Replacement of new Hader clip into its metal housing. The housing is processed into the denture base and is therefore not removed.|
|Figure 11. Removal of worn ERA with metal housing retained in the denture base.||Figure 12. A new ERA positioned into the metal housing with a replacement tool.|
The nylon Hader clips and ERA male attachments were replaced every 12 months as they became worn, resulting in looseness of the overdenture. Having metal housings incorporated into the denture base greatly facilitates changing the attachments. Because the housings retain a precise position, the dentist or a trained auxiliary can secure new Hader and ERA attachments in minutes (Figures 9 to 12).
The patient was very satisfied with her original mandibular bar overdenture and continued to use it for 5 years without complications or problems. At that time, the denture base fractured, requiring either repair or replacement of the overdenture. The denture base was repaired, but concerns about future breakage coupled with improved technology allowed the author to offer the option of a new overdenture retained by attachments only. One advantage of this choice was that it allowed the patient to continue using her bar-retained overdenture throughout the construction of the new prosthesis.
The new overdenture fabrication was initiated with a transfer impression of the implant sites using Aquasil Ultra material (DENTSPLY Caulk). A master cast was poured by the dental laboratory, and Locator abutments (Zest Anchors) of the appropriate heights were selected for placement into the implant analogs (Figure 13). These abutments are available in heights up to 5.0 mm for most implant systems and are selected according to how deep subgingivally the top of the implant is positioned. The most coronal portion of the Locator abutments should be positioned slightly supragingivally to facilitate reception of the attachments and cleaning by the patient (Figure 14).
At the second appointment, a standard baseplate with wax rim on the master cast was returned from the lab to the prosthetic dentist. Using a face-bow transfer and centric bite registration the case was mounted on a semiadjustable articulator (SAM 3, Great Lakes Pros-thodontics). The mounted models were then returned to the lab for a denture tooth setup. The mounted models can be removed from the dentist’s articulator and shipped separately to the dental lab. They can then be attached to another articulator of the same type that is calibrated to mount them in exactly the same position. In this fashion, the dentist does not have to purchase multiple articulators and can handle many cases with just one calibrated articulator in the prosthetic office. This not only saves the significant expense of multiple articulator purchases, but it simplifies packing and shipping costs to and from the commercial laboratory.
|Figure 13. The patient’s new overdenture utilized Locator abutments rather than a bar for retention.||Figure 14. Locator abutments of various heights selected according to subgingival placement of the implants.|
|Figure 15. Pink Locator attachments originally used in the processed denture were too retentive.||Figure 16. Four blue (1 lb) attachments used for “customized” effective retention while still allowing ease of removal by the patient.|
|Figure 17. Final mandibular overdenture in occlusion.||Figure 18. Case completion smile illustrates a pleasing conformity of the mandibular removable appliance with the maxillary natural dentition.|
At the third appointment the denture setup was evaluated and modified to satisfy occlusal, phonetic, and aesthetic concerns. If these modifications require significant movement of the denture teeth, a new bite registration and return for reset to the lab may be required. The intaglia of the baseplate with setup included processing attachments. This permits a limited preview of the retention available and, more importantly, removes the distraction of a loose baseplate during evaluation intraorally. Following patient and dentist approval of the setup, the case was returned to the lab for final processing.
The completed attachment-retained overdenture was delivered at the fourth appointment. This represents a savings of at least 1 appointment compared to a bar overdenture format due to removal of the bar try-in appointment. In some cases the bar may require sectioning and a laser weld or solder joint repositioning, which would add another appointment to the entire bar overdenture sequence.
The first Locator attachments used for this case employed the pink (3-lb retention) attachments (Figure 15). These proved to be excessively retentive to the point that the patient could not remove the overdenture once seated. This situation was easily remedied by experimenting with the various retention levels available with the Locator to produce good retention during use while still allowing the patient to remove the appliance without undue effort. The use of 4 blue (1-lb retention) Locator attachments satisfied these criteria (Figure 16).
This particular case is unique in that the patient was able to evaluate, from personal experience, the advantages and disadvantages of both a bar-retained and attachment-retained overdenture. Surprisingly to the author, she noted that the Locator (attachment-retained) overdenture felt more secure during insertion and function than her previous bar overdenture. In addition, she noted a strong preference for the ease of cleaning around the individual Locator abutments versus the more laborious technique of cleaning the bar.
Intraoral views of the completed overdenture in occlusion and with the patient’s smile are seen in Figures 17 and 18.
As a result, a second, or spare, Locator attachment overdenture was fabricated. This is a valuable appliance to have when (not if) a denture tooth or denture base is fractured. It can be offered at a significant cost reduction compared to the “primary” overdenture if both are constructed at the same time. We have found that patients are often surprised or feel that the denture has been inadequately constructed when these fractures occur unless they have been previously advised that such contingencies are common. By proactively anticipating and advising the patient as to how repairs will be accomplished in a timely manner, dissatisfaction and resentment can usually be defused or avoided. As with all areas of dentistry, time spent preoperatively with the patient educating and documenting potential problems during and after treatment is time very well spent.
Historically the author has advocated bar-retained rather than attachment-only overdentures primarily due to a preference for splinting implants together. Empirically, it seems that splinting provides a structurally better support system, similar to some of our traditional perioprosthetic rationales with natural teeth. However, successful anecdotal reports from clinicians who have utilized attachment-only formats are increasingly being reported.
According to Green Dental Laboratory, the attachment-only overdenture reduces the clinician’s laboratory and component costs by about $800. The 2 primary items that reduce the cost of the attachment-only format are seen in the elimination of bar cost and fewer numbers of office appointments to complete the procedure. With the significant overhead costs found in most dental practices today, the reduction of 1 or more appointments translates into an appreciable savings to the patient. Factoring in this overhead reduction with the attachment-only format results in an overall cost savings of about $1,200 or more. (Note that these figures are generalized and depend on the particular laboratory fees incurred on a specific case and the clinician-dependent overhead involved.)
Continued long-term comparisons of attachment and bar retained formats are needed. Because of more experience with bar retention, the author still tends to consider this as the superior option. However, the Locator attachment has significant economic advantages. This attachment provides more than ample retention while eliminating the technique sensitivity and costs associated with producing a passively fitted metal casting. From discussions with many entry-level to moderately experienced dentists, eliminating the intimidation factor perceived with bar overdentures also may encourage more prosthetic dentists to offer the alternative option to their patients.
In turn, the option of a highly retentive over-denture appears within the financial grasp of a much larger population group. Continued success with attachments such as the Locator could cause a major shift in prosthetic preferences associated with removable implant-retained appliances. It is exciting to see these innovative approaches continue the rich history of dentistry finding ways to improve our patients’ lives with more cost-effective methods and components.
1. Strong S, Callan D. Combining maxillary bar overdenture attachment systems: a case report. Dent Today. Jan 2001;20:78-84.
2. Misch CE. Treatment options for mandibular implant overdentures: an organized approach. In: Contemporary Implant Dentistry. Philadelphia, Pa: Mosby; 1993:223-240.
3. Strong S, Callan D. A new approach to implant-supported overdentures. Dent Today. Jan 1997;16:58-65.
4. Staubli P, Bagley D. Attachments and Implants Reference Manual. San Mateo, Calif: Attachments International; 2002:76-119.
5. English CE. Biomechanical concerns with fixed partial dentures involving implants. Implant Dent. 1993;2:221-242.
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