Rehabilitation of an edentulous jaw presents a number of challenges. Complete dentures have a longstanding record for rehabilitation of edentulous jaws, but they are not without functional shortcomings. They only partially restore function, and treatment challenges increase when the residual alveolar ridges are deficient or resorbed, particularly in the mandible. The advent and evolution of osseointegrated implants has greatly enhanced prosthodontic and patient-centered outcomes for treatment of the completely edentulous patient. A recent systematic review has reconfirmed the recommended two-implant mandibular overdenture as the minimum standard of care in complete denture therapy.

A complete implant overdenture is defined as a complete removable dental prosthesis that is supported by dental implants, individual or splinted, and related tissue structures. The three elements of an implant overdenture are the denture prosthesis itself, the implant configuration, and the attachments. The first step in assessing a patient for implant overdentures is to review the existing prostheses to see if they are adequately designed and constructed. Design elements of the complete denture prostheses are outlined in a separate Learning Module titled 'Design Principles for Complete Removable Dentures'. The learner should complete that module as a prerequisite to the present module. The aim of the present module is to give an overview of the recommended number and configuration of implants and to review the different types of attachments so that the most appropriate option may be selected.

After completing this ITI Academy Module, you should be able to outline recommendations for configuration of implants supporting a complete overdenture, identify the degree of prosthesis support according to implant configuration, and select appropriate attachment types for denture retention.

In spite of the optimal construction principles set out in the prerequisite Learning Module 'Design Principles for Complete Removable Dentures', there may be specific patient requirements and/or persisting functional limitations of the denture prostheses that are indications for implant assistance. Examples of such indications are retention and support to overcome looseness, lack of stability, and pain. This learning objective will examine recommendations for implant configurations to address these patient requirements and resolve functional limitations.

In addition to the patient's denture requirements and persisting functional limitations, there are a number of factors to be considered in planning the implant configuration. The factors include the patient's general health, physical autonomy, and attitude towards treatment. A socially and physically active patient is likely to have different denture needs than a dependent elderly patient. Financial considerations are also likely to play a role, as are any behavioral risk factors associated with the implant treatment in general. The difference in the nature of bone in the edentulous maxilla and mandible has an impact on recommended minimum implant numbers, together with the patient's occlusal force and the resulting masticatory load. The mandible is a flexible bone, and the deformation that takes place under masticatory loads and during opening may also influence implant positioning. Anatomical risk factors and available bone volume further impact the implant positions. Selection of implant number and positions will be discussed in more detail in the following slides.

There is consensus that a two-implant tissue-supported mandibular complete overdenture should be regarded as the minimum standard of care for the edentulous mandible. This professional consensus was first agreed upon in 2002. Supported by both scientific and clinical evidence, this consensus was confirmed in 2009 and again in a systematic literature review published in 2012. The subjective and objective improvement in retention and stability of the overdenture and quality of life of the patient are significant when compared to conventional mandibular removable dental prostheses. Moreover, if minimally invasive treatment is required, a two-implant overdenture is the first choice. Implants should be placed in the farthest anterior and lateral position. Depending on the shape of the ridge, this will most often be the canine region. Implants placed too far distally may result in a rocking movement of the overdenture shortly after insertion. Implant positions may vary depending on the shape of the ridge; however, the positions selected should still adhere to these stated guidelines.

Survival of a single implant placed in the mandibular midline is well documented. However, evidence on functional and patient-centered outcomes remains insufficient. In very curved and narrow anterior ridges, where placing two implants would ultimately result in an anterior lever arm, three implants may be useful, with the middle implant acting as an indirect rest seat providing indirect rather than direct retention. Placing an attachment on such a third implant would lead to premature fatigue and fracture of the female part.

As a general rule, an implant overdenture is unlikely to require more than four implants in the mandible. Limiting the number of implants to four ensures the positioning of all implants in the interforaminal region, thereby avoiding the inferior alveolar nerve and minimizing risks associated with mandibular flexure during mastication. The four interforaminal implants should be distributed such that the posterior implants are placed as distally as possible while staying mesial to the mental foramen. The anterior implants should be positioned as far mesially from the posterior implants - and as far laterally from the midline - as possible. In other words, an ideal four-implant configuration for the mandible is 5 - 3 - 3 - 5, in which the posterior implants are favorably placed in the second premolar region and the anterior implants in the canine region. However, due to anatomical limitations and available bone, this ideal arrangement may be suitably modified to a 4 - 2 - 2 - 4 configuration in which the posterior implants are placed in the first premolar region, and the anterior implants in the lateral incisor region. In no circumstances should the implants be placed in close proximity to one another, as this may result in catastrophic peri-implant bone loss and lead to implant failure.

Although the two-implant-supported maxillary overdenture has been documented in the literature, it is considered a risky treatment approach because of the maxillary bone quality and the soft tissue condition. The mucosa covering the hard palate is considerably more resilient than the mucosa covering the mandibular alveolar ridge. This results in inevitable rocking of the overdenture on the fulcrum line created by the two implants. Thus, the placement of only two implants to support a maxillary removable complete prosthesis must be approached with caution.

As a rule therefore, a minimum of four implants is recommended, regardless of palatal coverage by the prosthesis. The anterior implants should be placed in the canine region, and the posterior implants as close as possible to the center of the posterior occlusal table or chewing center. This area corresponds to the first molar region. In a geriatric patient in whom a lack of vertical bone height prevents implant placement in the first molar region, the implant may instead be inserted just anterior to the maxillary sinus. This prevents the need for a sinus lift procedure. The functional benefit in terms of greater prosthesis stability does not outweigh the risk of an intervention such as a sinus lift in a frail geriatric patient. The four-implant concept also enhances phonetics by allowing for reduced palatal coverage and eliminating the need for a bulky superstructure in the phonetic zone.

The degree of support provided by the implants will depend upon their configuration. Overdentures will be either implant/tissue-supported or fully implant-supported. For implant/tissue-supported overdentures, the implants and tissue share the occlusal load, with the denture-bearing tissue area providing the larger part of support. This clinical image, with two implants placed in the anterior area, shows a configuration of this type. The implants provide localized retention and support from either individual attachments or a splinted implant superstructure, but the denture still moves along with the underlying resilient tissue. Conversely, implant-supported overdentures rest almost entirely on individual attachments or splinted implants, as seen in this clinical image. There is therefore minimal to no pressure on the underlying tissues, and very minimal denture movement. This distinction between implant/tissue support versus full implant support is relevant to the selection of attachments.

As noted on the previous slide, the implant/tissue-supported prosthesis is prone to movement during function. This movement is limited to a large extent by the implant attachments or splinted superstructure but remains a factor when considering how to address case-specific indications for implant assistance. In the two clinical examples seen here, the movement is reduced to an anterior-posterior rotation of the overdenture around the fulcrum axis formed by the two implants in the mandibular canine region. These two implant configurations can therefore assist in resolving issues of retention and, to some extent, stability. They would not, however, fully resolve issues of persisting pressure-induced soreness or progressive bone atrophy in the posterior denture-bearing areas.

In implant-supported overdentures, the design of the superstructure restricts movement to a minimum. The extension of the superstructure, as shown in these clinical images, limits rotational movement in all dimensions. The superstructure, as shown in the occlusal view on the left, follows the contours of the existing ridge anatomy. The prosthesis rests entirely on the superstructure, which may resolve issues related to persisting pressure-induced soreness as well as protect the posterior denture-bearing areas against atrophy. This could be particularly relevant in patients with sensitive mucosa.

Recommendations for Implant Configuration, Key Learning Points: The ideal number and position of implants will vary according to denture requirements, functional limitations, and additional factors such as bone volume and occlusal forces. For a mandibular overdenture, the minimum standard of care is two implants, although a three-implant or four-implant configuration may be indicated. For a maxillary overdenture, the minimum recommended number of implants is four. In a tissue/implant-supported overdenture, the tissue provides most of the denture support, and the prosthesis is prone to movement around the axis formed by the implants. An implant-supported overdenture places minimal to no pressure on the underlying tissues, and there is minimal denture movement. Fully implant-supported overdentures may be appropriate for patients with pressure-induced soreness or atrophy of posterior denture-bearing areas or patients with sensitive mucosa.

An implant overdenture attachment can be defined as an element that is incorporated into the design of a prosthesis to aid retention and/or stability. The attachment is comprised of two parts, a matrix and a patrix. The matrix is the receptacle or female component, and the corresponding patrix or male component fits closely within the matrix. Retention is achieved either by undercuts or frictional fit.

Attachment systems vary in design. The literature does not recommend one ideal attachment system, but there are certain mechanical features that are desirable. Attachments should be able to compensate for a certain amount of axial divergence between implants. An ideal attachment is also low in volume to avoid bulky superstructures. The attachments should be smooth in shape to avoid injury to surrounding oral structures when the denture is not worn - for example, at night. Finally, it is important that attachment components are wear resistant to avoid premature maintenance and replacement.

There are also a number of clinical requirements that affect the selection of suitable overdenture attachments. First, the clinician should be able to adjust the retentive force of the attachment system towards either an increase or a decrease in retention. Second, it is advantageous to be able to perform chairside repairs and placement or replacement of the female component.

Attachments should be easy for the patient to handle and keep clean. This is particularly important for the geriatric patient with reduced vision and dexterity. The geriatric patient will also benefit from a low need for attachment maintenance.

In the event of functional decline of the geriatric patient, it is beneficial if the male and female components can both be easily removed. This allows for adoption of a 'back-off' strategy, in which more retentive attachment components are replaced with less retentive counterparts as dictated by the capabilities of the patient.

The distinction between implant/tissue-supported and fully implant-supported configurations, as discussed in the previous learning objective, is also a factor in attachment design and therefore attachment selection. Attachments used in connection with two-implant configurations must allow for the unavoidable rotational movement of the implant/tissue-supported mandibular overdenture, as illustrated in these two images showing individual and splinted attachments. The rotational axis requirement limits the choice of attachments when only two implants are placed. On the other hand, the larger area of implant support in four-implant configurations allows for more options when selecting an attachment system.

A wide range of attachment systems are available in the market, but this module will concentrate on only the most commonly used types. These can be divided into individual attachments and splinted attachments. The individual attachments can be further divided into generic categories of stud attachments, telescopic crowns, and magnets. The splinted attachments are usually referred to simply as 'bars'. These can be divided by their mode of manufacture into standard bars made from prefabricated components, cast and milled bars, and CAD/CAM-produced bars.

Stud attachments are indicated for two- and four-implant overdenture configurations. Among their advantages are a low initial cost and convenient dimensions for treating cases of limited interarch space. There is also easier access for oral hygiene, and the process of fabrication is relatively easy including the option of chairside fitting and repairs. Stud attachments are available in different designs that offer a variety of retentive strengths. More recent designs specifically offer the ability to accommodate angular discrepancies between implants of up to 60 degrees. Amongst the general disadvantages are long-term maintenance and repair costs. Moreover, not all designs can accommodate angular discrepancies. Specific disadvantages of some designs are food and biofilm accumulation as well as high variability in retentive forces and early loss of retention.

Telescopic crowns are indicated for both two- and four-implant configurations. There are very few studies on these types of attachments, but the available reports suggest little need for maintenance and good patient satisfaction. On the other hand, the attachment design requires parallel implants and laboratory expertise, and cost of the attachments is high. Magnet attachments are also indicated for both two- and four-implant configurations, and they are suitable for the geriatric population because they are easier to handle due to their limited retentive force.

Bars are indicated for two- and four-implant configurations, and they are available in a variety of cross-sectional shapes. Bars can be designed to accommodate the rotational axis of two-implant configurations. Conversely, cantilever extensions can be added to bars to increase the area of overdenture support. Bars offer good retention, and there is usually little need for maintenance. On the other hand, there is a high initial cost for fabrication, and bars require substantial vertical space. They also pose a challenge to effective plaque removal; moreover, mucosal hypertrophy can be an issue under bar segments.

Appropriate Attachment Types, Key Learning Points: Attachments are comprised of a male and a female component that provide retention by mechanical or frictional fit. Ideal attachments allow for some implant divergence and are low in volume, smooth, wear-resistant, and easy to adjust and replace. Geriatric patients need attachments that are easy to handle, maintain, and clean. Individual attachments include studs of differing designs, telescopic crowns, and magnets. Splinted attachments include standard bars made from prefabricated components, cast and milled bars, and CAD/CAM-produced bars.

Complete Removable Overdentures: Implant Configuration and Retention, Module Summary: Well-designed, well-fitting dentures are retentive and stable and provide the patient with satisfactory oral health-related quality of life. Complete dentures, however, can have functional deficiencies that are most often related to resorption of the residual ridge. When complete dentures are no longer ideal, a patient can opt for implant-supported complete dentures. The ideal implant configuration to support a removable prosthesis depends on the patient's denture requirements and persisting functional limitations. The configuration differs according to the jaw being rehabilitated. The difference in the nature of bone in the edentulous maxilla and mandible has an impact on recommended minimum implant numbers. Other factors such as occlusal force, anatomical risk, and available bone volume further impact the implant configuration.

For a mandibular overdenture, the recommendation is two to four implants in the interforaminal region. Overdenture attachments should be selected according to the desired type of overdenture support: whether the forces of occlusion are shared between implants and tissue, or fully borne by the implants. For tissue/implant-supported overdentures, individual non-splinted attachment choices include stud attachments, magnets, and telescopic attachments that do not provide complete distal support. If splinted attachments are desired, a bar without complete distal support can be supported by two implants. For fully implant-supported overdentures with individual non-splinted attachments, telescopic crowns on four or more implants will afford complete distal support. If splinted attachments are required, a bar providing complete distal support should be used.

For a maxillary overdenture, the minimum recommended number of implants is four. The anterior implants should be placed in the canine region, and the posterior implants as close as possible to the chewing center or first molar region. Overdenture attachments should be selected according to the desired type of overdenture support: whether the forces of occlusion are shared between implants and tissue, or fully borne by the implants. With four implants placed anteriorly to the maxillary sinuses, individual non-splinted attachment choices such as stud attachments, magnets, and telescopic attachments will not provide complete distal support and the forces of the occlusion are therefore shared between tissues and implants. Equally, when a splinted attachment in the form of a bar is without distal support the overdenture is also supported by a combination of tissues and implants. With more than four implants in both anterior and posterior positions, individual non-splinted attachments and telescopic crowns will offer distal and therefore complete implant support. Similarly for splinted attachments a bar extending distally to the first molar region will offer complete occlusal support.