Implant Overdentures: Selections for Attachment Systems

INTRODUCTION
A consensus statement by McGill University, Montreal, Quebec, Canada concluded that a complete denture for the edentulous mandible should no longer be considered the first choice of treatment. Instead, the placement of 2 implants should and has now become the first choice, especially for the compromised atrophic mandibles. This among many other factors has led to an increase in the fabrication of implant overdentures for restoring the edentulous patients. Implant overdentures can be supported/retained by several different attachment systems; ie, a bar and clip or individual attachments for improved retention.
This article describes the rationale and the criteria for selecting the attachment systems optimally for implant overdentures. In addition, this article will provide an example case report describing a failed implant overdenture procedure due to lack of recording appropriate information at the time of the examination.

BACKGROUND
Dental implants are an integral part of most modern dental practices. Current research demonstrates that restorative approaches involving implants not only improve the denture bearing foundation, but also improve edentulous patients’ quality of life through improved prosthodontic service.1-8
In addition to conventional complete dentures, treatment options available for edentulous patients include removable implant-assisted complete dentures (ie, implant overdentures) and fixed implant-assisted complete dentures and implant-supported and retained fixed prosthesis (Figure 1).9 Implant-supported overdentures may use a variety of bar and clip attachment systems or incorporate a variety of individual, abutment-based attachments called stud attachments (ball, magnets, and resilient stud attachments such as Locators [Zest Anchors], ERA [Sterngold], and nonresilient stud attachments such as Ankylos Syncone [DENTSPLY Implants]) (Figures 2a to 3b).10-14 Selection of the most appropriate attachment system for the patient relates to a variety of factors that must be identified early in the treatment sequence. These factors include the following:
Implant position: The final location of the implant in relation to the bone and the prosthetic teeth will help decide the type of attachments; this should be determined at the diagnosis and treatment planning phase before the placement of implants. In order for the individual attachments to provide adequate retention, all the implants need to be placed as parallel to each other as possible.15-17 If the implants cannot be placed relatively parallel to each other, then a bar design would be our next choice to be fabricated for the patient. Additionally, a bar would be considered in cases where implants cannot be placed in ideal locations due to anatomic structures; eg, presence of mental foramen or ridge crest seen in patients with severely resorbed ridges.18-21

Figure 1. Maxillary retained fixed prosthesis (All-on-4 [Nobel Biocare]). Figures 2a and 2b. (a) Milled bar (ISUS [DENTSPLY Prosthetics]). (b) Micro stud abutments (ERA [Sterngold]).
Figures 3a and 3b. (a) Stud abutment; nonresilient (Syncone [DENTSPLY Implants]). (b) Stud abutment; resilient.

Desire for cross arch stabilization: In patients with shallow vestibules and atrophic ridges, bars are indicated to resist lateral loads by providing cross arch stabilization.22 They also help improve the stability of the prosthesis by providing a distal cantilever usually one to 2 teeth distal to the posterior most implant.22,23 In situations where the prosthesis is stable, and only improved prosthesis retention is required, the use of individual attachments can be utilized with predictable results.
Prosthesis size: When patients require minimum size of the final prosthesis, specifically designed milled bars are a good choice.24 However, ideal implant placement becomes a critical factor in the overall success of the final bar restoration. Utilizing the principles of anterior-posterior spread and cross arch stabilization, the size of the prosthesis can be decreased without increasing the lateral loads on the implants.23 Capturing the patient’s individualized muscle bound neutral zone recording will define the horizontal space available in determining the implant and attachment position.25 When defining the neutral zone, consideration must be given to the potential denture space; that space in the edentulous mouth vacated by the natural dentition and dental supporting tissues and bound by the tongue medially, and the lips and cheeks laterally. The neutral zone resides within this potential denture space. More specifically, the neutral zone is that region where forces imposed by the tongue directed outward are neutralized by inwardly directed forces originating from the cheeks and lips during normal neuromuscular function (Figure 4).25-27
Sore spots: It has been observed and reported that patients who are xerostomic and/or prone to soft-tissue sore spots are more comfortable with a bar, since the denture can be entirely bar supported without impinging on tissue surfaces.28 When using individual attachments, the denture is supported by the tissue bearing surfaces and compressive forces are present allowing soreness in the sensitive patient.29

Figure 4. Arrow depicts how lower prosthesis is bound by the lips, cheek, and tongue. Figure 5. Ten years after placement of bar tissues and bone had migrated beneath the bar due to very poor oral hygiene.
Figure 6. Lip ruler being secured on mandibular ridge crest; then, recording lower lip at repose. (Courtesy of Nobilium) Figure 7. Lip ruler being secured on maxillary ridge crest; then, recording lip at repose.
Figure 8. Five implants with abutment and cast bar. Figure 9. Patient forcefully moving lips together.
Figure 10. Measurement taken from ridge crest to top of cast bar. Figure 11. Note the excessive length of the mandibular anterior teeth.

Oral hygiene: Patients with bars who exhibit poor oral hygiene are prone to mucosal hyperplasia underneath the bar and inflammation of the soft tissue around the implants (Figure 5).29-36 Patients who will not dedicate appropriate time for oral hygiene should be cautioned and, at times, denied placement of bar attachments until they commit to an appropriate implant hygiene regimen.37
Economics: The cost of fabrication of the bar attachments in contrast to stud abutments will be much higher in most instances.9,20 In today’s economy, many times this may dictate the patient’s decision process. However, dedicated patients can be upgraded to bars if their financial situation improves over time. The author provides the optimal treatment recommendations and the option of upgrading in the future in detail in written form. However, in all cases, the interim or chosen treatment restoration must follow recognized guidelines conducive to the health and welfare of all patients. Treatment options should never solely be based on finances.
Restorative space: Dental restorative space may be defined as the 3-dimensional oral space available for prosthodontic restoration. In general terms, this space in edentulous patients is bound by the proposed occlusal plane, denture bearing tissues of the edentulous jaw, facial tissues (cheeks and lips), and the tongue.38 For implant overdenture patients, this space must accommodate a denture base of sufficient dimensions, appropriately positioned denture teeth, and an implant attachment system. Factors such as interocclusal rest space, phonetics, and aesthetics must also be considered when defining available restorative space. A reported minimum space requirement for implant-suported overdentures with Locator attachments is 8.5 mm of vertical space and 9 mm of horizontal space.39 A separate report on maxillary implant overdentures suggested that a minimum of 13 to 14 mm of vertical space is required for bar supported overdentures, and 10 to 12 mm for overdentures supported by other individual attachments.40 There are various techniques for evaluating restorative space in edentulous patients. These procedures should be implemented prior to implant placement, when treatment options are being considered.41
Aesthetic space: This is the space between the ridge crest and the corresponding lips at repose. Removable restorations supported by individual attachments will require less aesthetic space than those supported by a bar. The aesthetic space can be measured at the initial visit of the patient using the lip ruler (Nobilium [CMP Industries]) (Figures 6 and 7). The lip ruler can be utilized to determine the vertical distance between the ridge crests to the corresponding lip at repose. This vertical distance allows the dentist to determine the space allowed for the prosthesis (implant stud attachments, bars or fixed restorations). On average, to make an aesthetic and functional restoration, the prosthetic teeth should not extend 2 to 3.0 mm occlusal to this vertical distance. In the mandibular arch, this generally results in the incisal edges of the anterior teeth being positioned vertically 2 to 3.0 mm above the lower lip at repose.
Ease of fabrication/repair: Removable restorations supported by a bar are more challenging to fabricate and repair than removable restorations supported by individual stud attachments.11,41-45

Figure 12. Overview of the size of the prosthesis being dictated by the implant bar and design. Figures 13a and 13b. (a) Note the severely resorbed maxillary ridge. (b) Excess height of lower bar.
Figure 14. Converting bar to stud attachment can significantly reduce vertical height. Figure 15. Note the completed stud abutment overdenture strengthened with metal inner surface.
Figures 16a and 16b. (a) Before alteration of the vertical dimension of occlusion. (b) Completed case.

Opposing arch: It is necessary to identify the opposing arch in the decision making process. For example, if a patient is treatment planned to receive a complete maxillary denture and an implant-supported mandibular overdenture, it would be advised to treatment plan the mandibular implant overdenture with individual stud attachments as opposed to a bar to avoid excessive forces that can destabilize the maxillary denture. A common complaint reported by dentists, in this treatment scenario of a bar-retained mandibular overdenture opposing a complete denture, is that patients will eventually complain that their maxillary prosthesis feels loose in comparison to their previous maxillary denture.
Often attachments are chosen without considering the above listed factors that ultimately result in failed restorations and dissatisfied patients. The following is a case report of a patient poorly treated with implant-supported bar overdenture.

CASE REPORT
An 84-year-old female patient presented to our office. She was very unhappy with her mandibular complete denture due to its poor retention. She approached a local dentist who placed 5 mandibular implants and fabricated a bar-supported overdenture (Figure 8). Initially the patient was very happy with her retentive and stable prosthesis, but a few months later she started experiencing discomfort and facial pain associated with her prosthesis.
When the patient was presented to the author, she appeared stretched open. She was not able to get her lips together without manipulating them, and it was observed that the constant touching of her lip created a constant tick as you may note in patients with neurological twitching episodes (Figure 9). Upon the initial examination, it was observed that her rest vertical dimension was less than her occlusal vertical dimension (OVD) so the interocclusal distance was violated. Often OVD is increased to gain restorative space, but that must be done with caution, without impinging on the interocclusal distance and within the boundaries of the facial muscles of expression and mastication.38 The length of the implants measured 14 mm. The aesthetic space for the lower arch was measured to be 14 mm with the lip ruler. The distance between the crest of the ridge to the top of the bar was 20 mm (Figure 10). The accompanying prosthesis added 17 mm in length, resulting in the distance from the ridge crest to the lower lip at repose to measure 37 mm. This resulted in a vertical cantilever on the implants and violation of the aesthetic space by 20 mm vertically, when factoring in the incisor edge heights being placed 3 mm above the resting lip. The prosthetic teeth and the prosthesis were not centered over the ridge nor properly braced by the facial muscles. The opposing arch was edentulous, demonstrating severe bone resorption and this further compromised the overall forces exerted when chewing (Figures 11 to 13b).46
Having factored in the criteria set out in this article, along with the physical evaluation and patient-reported symptoms, it was concluded that the implant bar overdenture was not the optimal treatment for this patient. New restorations were fabricated for this patient using individual attachments, following current best-practices procedures. This resulted in a decreased requirement for restorative space and, as a result, the OVD could be established at the optimal physiological limits. This resulted in lowering the cantilever and making the restoration within the confines of the restorative, aesthetic space (Figures 14 to 16b) with optimal functional muscular bracing. The patient was satisfied with her new restorations.

CONCLUSION
In order for the prosthesis to function effectively and to also be aesthetic, careful attention must be given to diagnosis and treatment planning. The definitive prosthesis must be placed within the confines of the neutral zone/neutral space with particular attention to the implant position limitations, restorative space, the aesthetic space, and the condition of the opposing arches. The optimal prosthesis becomes the best guide for implant placement.


References

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Dr. Massad is an associate professor in the department of graduate prosthodontics at University of Tennessee Health Science Center, Memphis, Tenn, an associate Faculty at Tufts University School of Dental Medicine, Boston, Mass, and an adjunct associate faculty of the department of comprehensive dentistry, the University of Texas Health Science Center Dental School, San Antonio, Tex. He has a private practice in Tulsa, Okla. He can be reached at joe@joemassad.com.

Disclosure: Dr. Massad consults/has consulted for and receives/has received sponsorship from many companies, including the following: CMP Industries, DENTSPLY Caulk, Nobel Biocare, Sterngold, Zest Anchors, and more.

Dr. Ahuja is an assistant professor in the department of prosthodontics at University of Tennessee Health Science Center, Memphis Tenn. She can be reached at sahuja@uthsc.edu.

Dr. Ahuja reports no disclosures.

Dr. Cagna is an associate dean for postgraduate affairs at University of Tennessee Health Science Center (UTHSC) College of Dentistry, Memphis, Tenn. He is also a professor and director of the Advanced Prosthodontics Program at UTHSC College of Dentistry. He can be reached at dcagna@utmem.edu.

Dr. Cagna reports no disclosures.

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