Long and short term management of implant-supported mandibular overdentures

Outline of problem

As more patients chose to have ISMDs owing to their popularity and success rates, potential complications and maintenance need to be conveyed and readily managed. The two most common designs of ISMDs will be considered for this article. The first relies on plastic or metal clips in the denture base which engage a metal-alloy bar connected rigidly between two implants (bars). The second design relies on a rubber or a metal cap embedded in the denture base to engage an abutment attached directly onto each implant (studs or locators). All these options will require a degree of maintenance, regardless of the attachment system used. Current evidence indicates that most maintenance is required mainly in the first 5 years of service.9 This is usually due to adjustments needed to the denture base, or loosened abutments or components within the denture superstructure, such as retentive clips. Costs to maintain ISMDs are higher owing to the price of replacement components, and specialist settings in which patients are usually seen. However, patient satisfaction with this treatment option is much improved compared to conventional complete denture treatment. This is mainly due to improved masticatory ability and the overall improvement in mandibular denture stability.10

The aim of this article is to highlight the maintenance involved and potential complications of implant overdentures. It also demonstrates the need for meticulous case selection and treatment planning. It is also intended to provide an overview of the maintenance and clinical procedures that can be undertaken in a general practice setting to enable patients to care for their prosthesis. Training and education of general practitioners is required to enable them to provide clinical care for this popular, ‘gold-standard’ treatment for complete denture wearers.

Oral hygiene, ulceration and peri-implantitis

Patients do need to be aware that, although dental implants are resistant to carious attack, which would cause failure in conventional overdentures on retained roots, they are still at risk of failure caused by gingival inflammation, smoking and para-function.11 Continuous and thorough oral hygiene instruction, reinforced with regular recalls, are required to ensure patient compliance to prolong longevity of ISMDs. Poor oral hygiene resulting in bacterial plaque accumulation, smoking and overloading of the denture-supporting tissues can lead to inflammatory changes. If left untreated, the inflammation can lead to destruction of the implant-supporting tissues, called peri-implantitis.12^,13 (Figures 1–3).

It is essential that patients are given in depth oral hygiene instructions to maintain clean denture abutments, as well as care for their prosthesis to ensure longevity of the overdenture components and the dental implants themselves.13 Regular hygiene appointments should be encouraged to enable a visual assessment of the patient's home care of the implants and to reinforce continuing oral hygiene practices. It is a recommendation that patients treated with implant-retained prosthesis are seen at annual recall intervals. However, in line with NICE guidelines,14 this should be based on an individual basis as they will require routine hygienist treatment at 3- or 6-monthly intervals.15

Wearing of dentures, whether supported by implants or not, may lead to trauma of the tissues, if overextended, as they may rub or burrow into the mucosa during function. Any ulceration resulting from overextended dentures will require adjustment and recall to assess healing.

Bars

One of the methods of retaining ISMDs is to link the implants by a bar and use a clip retained within metal housings cured into the denture to retain the prosthesis. The bar is placed above the mucosa and the space underneath is cleansable and amenable to oral hygiene products. Milled bars are indicated where there is extensive ridge resorption. They require patient manual dexterity to clean effectively under the attachment, owing to its close contact with the underlying gingival tissue (Figure 4).

Retention is achieved by using plastic inserts with three different strengths (white for reduced retention, yellow for normal retention and red for increased retention) that snap into the housing, which allows for customized retention for each individual situation. The retentive clips within the denture provide rotation around the bar and help retain the prosthesis to the bar. However, these clips can deform or become worn out over time, making the overdenture less retentive over the bar. This can result from functional or para-functional loading or repeated insertion and removal of an implant overdenture.16 In this case, the clips will need to be replaced in order to maintain retention of the overdenture. This can be done in a lab or at chairside. These can be replaced simply in practice by pushing the clips laterally with a flat instrument. (Figures 5 and 6)

Retentive studs: ball and locator attachments

Where a mandibular overdenture is retained by two stand alone, non-splinted implants, the denture may be retained by resilient attachments incorporated within the denture to attach to the implants. This design allows for rotation and translation of the prosthesis.17 These are also known as retentive anchors which are an alternative to bars. They are less expensive and technique sensitive to place and maintain than bars. There is also less risk of fracture than with bars. However, the metal abutment can wear as well as the retentive housing in the overdenture.

There are several abutment designs, such as locator and ball abutments. Locators are made of plastic and have a low vertical height, which is ideal for overdenture patients, especially where there is reduced occlusal space. Locators also help to overcome implant angulation problems and allow correction of misaligned implants up to 45 degrees. They require fewer maintenance visits owing to their self-aligning properties. Locator inserts, which are housed in the denture superstructure, come in five colour-coded retentive holding force levels, with the minimum in blue having 680 grams and the maximum in green having 1361–1814 grams.18 The inserts are simple to replace with the appropriate male component removal tool or, if unavailable, a flat plastic instrument (Figures 7 and 8). The locator abutments can be hand-tightened and followed by final tightening with a torque wrench.18 Ball attachments are relatively easy to maintain too. The housing is cured within the denture super-structure, as with the locator attachment system, and customized retention (ranging from 750–1500 grams) is similarly achieved by colour-coded plastic inserts clipped into the housing.18 The plastic clips can be removed by using the tip of a spherical stainless steel burr or a hot instrument, ensuring that the metal housing is not damaged. A new plastic insert can be placed, ideally using an insertion tool or a stainless steel burr. This type of attachment is designed to eliminate wear on the ball abutment.

Evidence has shown that single attachments are changed more frequently than bar connections.19

Further complications to consider

Complete denture wearers develop relatively little bite force compared to the force generated with implant-supported restorations, therefore fracture of denture bases tend to occur more frequently with ISMDs,13 particularly in the anterior portion of bar-retained mandibular overdentures. This tendency to fracture is due to inadequate thickness of acrylic resin that results from the dimensions of the bars and clip attachments.20 Reinforcement of the denture base over the implants may increase resistance to this concentration of stress, and this approach has been suggested to prevent fractures occurring in ISMDs. Therefore, when repairing a fractured acrylic denture base, it may be worth considering reinforcement of the acrylic denture base with a metal substructure. An in vitro study using strain gauges suggested that acrylic resin, reinforced with a cast chrome-cobalt framework, strengthens the resistance to fracture of a denture base.21

A recent literature review found that a large number of mechanical complications have been reported. These are summarized in Table 1.22

Studies have found no difference in implant survival rate, health of peri-implant tissue, or marginal bone loss between different attachment systems retaining mandibular overdentures.23

With implant restorations becoming commonplace, and the provision and maintenance of prosthetics becoming a frequent procedure carried out in general practice, many of the above procedures are relatively straightforward for GDPs to manage in practice without the need for referral to specialist services.

Knowledge in the maintenance of conventional complete dentures, added to the basic ability to monitor implant abutments to prevent problems from occurring early on, will ensure patients with ISMDs can be cared for by their regular GDP. This is also likely to mean convenience for patients, who would not need to be put on lengthy waiting lists to be seen by specialists to rectify minor complications in maintenance.

Conclusion

Although ISMDs are efficient in providing high patient satisfaction and success rates, their maintenance is as important for their longevity as with most other implant-retained prostheses. The dental profession will increasingly use ISMDs as a popular treatment option, and it is necessary for the dental team to become familiar with the mostly basic procedures involved in their maintenance to ensure continued patient satisfaction.

This article highlights the need for general dental practitioners to become familiar with the different ISMD attachment systems in order that all GDPs, who have been trained to provide conventional complete dentures,10 can feel confident in providing and maintaining successful two-implant overdentures to their patients with minimal training in practice.