The Outcome of Implant Overdentures from a Prosthodontic Perspective: Proposal for a Classification Protocol

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Alan G. T. Payne, BDS, M Dent, FCD(SA), Senior Lecturer/Prosthodontist, Department of Oral Rehabilitation, School of Dentistry, University of Otago, Dunedin, New Zealand. Terry R. Walton, BDS, MDSc, MS, FRACDS, Specialist/Prosthodontist, Private Practice; and Clinical Associate Professor, Faculty of Dentistry, University of Sydney, Australia. Joanne N. Walton, DDS, Cert Pros, FRCD, Associate Professor, Faculty of Dentistry, University of British Columbia, Vancouver, Canada. Yvette F. Solomons, BDS, M Dent, Senior Specialist/Lecturer, Department of Prosthetic Dentistry, University of the Witwatersrand, Johannesburg, South Africa.
Reprint requests: Dr Alan G. T. Payne, Senior Lecturer/Prosthodontist, Department of Oral Rehabilitation, School ofDentistry, PO Box 647, University of Otago, Dunedin, New Zealand. e-mail: alan.payne@dent.otago.ac.nz
Purpose: This article proposes a classification protocol for reporting the outcome of implant overdentures. Materials and Methods: Review of the literature pertaining to postinsertion prosthodontic procedures for maxillary and mandibular implant overdentures revealed a wide range in the terminology used and methods of reporting outcome, usually as a result of the use of varying designs among different implant systems. A need for comparative evaluation of the prosthodontic success of implant overdentures was identified. Results: An existing classification protocol for implant-fixed prostheses containing six objectively defined fields was modified for implant overdentures. This new protocol, which also uses the descriptive fields of success, survival, unknown, dead, and retreatment (repair or failure), provides a method whereby postinsertion prosthodontic procedures and subsequent time to retreatment can be quantified regardless of design, method of attachment, or implant system. A ratio method for identifying prosthodontic treatment cost effectiveness of implant overdentures is included. Conclusion: This protocol could assist clinicians in their choices of implant overdenture therapy by providing accurate comparisons of varying implant overdenture systems and designs, and by identifying prosthodontic treatment cost effectiveness. Int J Prosthodont 2001;14:27–32.

References

One measure of prosthodontic treatment success is how long a prosthesis survives.1–3 One measure of treatment longevity extends from the time of prosthesis insertion until any additional prosthodontic treatment is required, hence the term "time to retreatment." Postinsertion prosthodontic procedures can be attributed to either mechanical or biologic causes,1 but for implant overdentures, specific details of possible procedures, including routine maintenance, minor adjustments, relines, major corrections, and remakes, are pertinent. Unfortunately, reviews of treatment outcomes4,5 lack sufficient information about these procedures for mandibular and maxillary implant overdentures. Confusion appears to exist between what constitutes accepted maintenance and what constitutes repairs, and whether either of these categories should be considered retreatment. Furthermore, outcomes of maxillary and mandibular implant overdentures have been grouped together,6 and there is a lack of detailed information on the status of the opposing jaw, which could have implications on the prosthodontic follow-up of the implant overdenture. The multitude of implant systems and attachment mechanisms available in practice lack uniform categorization,5,7 possibly because of the plethora of overdenture designs and implant components.8,9 Overall, variations in philosophy of prosthetic design, implant systems, and maintenance requirements have made it difficult to compare success rates of either removable10 or fixed11 implant prostheses.

There is a need for a standardized consensus on effective categorization of postinsertion prosthodontic procedures. This should include the development of objective criteria within fields of prosthesis outcome that are applicable to varying implant systems and the number of implants used, thus allowing an accurate prediction of the time to retreatment.9 In addition to mechanical and biologic complications associated with implant prostheses, patient factors related to subjective complaints about phonetics, esthetics, and neurologic impairment are part of the psychosocial costs of treatment12–14 and must also be considered.

The economic cost analysis of any prosthodontic intervention is a balance between initial capital outlay (professional fees, laboratory fees, implant component cost13) and the maintenance and repair costs over time,15 together with any hidden costs of the service.16 Authors have retrospectively questioned the use of implant overdentures rather than implant-fixed prostheses given the number of postinsertion procedures required,17,18 even though the latter involve significantly higher initial capital cost. However, compared to conventional mucosa-supported dentures with their associated maintenance costs, there is a perception that over the long term, implant overdentures are an equally economic, cost-effective treatment modality.15,19 The complexity of different systems and designs on different numbers of implants, and disagreement on the costs associated with similar designs,13,20 does make effective economic cost comparison difficult when compared to the more standarized method of constructing conventional mucosa-supported dentures. Attempts at such comparison have been made,13 but they have also been determined to require further research.20 Application of the Resource-Based Relative Value Scale21 to implant overdentures is not relevant for most practicing clinicians.

Any economic cost comparison must be weighed against overall cost effectiveness, which includes the impact of the treatment modality on the patient's quality of life over the long term.

Given that well-defined fields for assessing the outcome of fixed prostheses on implants have been proposed,11 the same should be possible for implant overdentures. The six-field outcome protocol described for implant-fixed prostheses enhances the measurement of prosthodontic success compared to a previously proposed four-field protocol for measuring implant success.22–24 Intuitive expansion of fields into success, survival, unknown, dead, retreatment (repair), and retreatment (failed) is more meaningful to clinicians in evaluating the actual burden of prosthodontic follow-up. Because implant overdentures vary according to the design, terminology, method of attachment, and support received from the differing numbers of implants and the surrounding mucosa, the fixed prosthodontic outcome protocol must be modified to encompass these boundaries.

Proposed Classification for Implant Overdenture Outcomes

Terminology

Terminology describing conventional fixed and removable dental prostheses has been applied to implant prostheses,25 resulting in some confusion and ambiguity. A fixed prosthesis is luted to the teeth and is not removable by either the operator or the patient. A conventional removable prosthesis, on the other hand, can be supported by teeth alone or by a combination of teeth and mucosa, but is removable by both the operator and the patient. The exception is the "telescopic" prosthesis (a type of overdenture26) that may be provisionally luted and therefore only removable by the operator, or frictionally/magnetically retained and then both operator and patient removable. Conventional fixed prostheses and most telescopic prostheses are entirely tooth supported.

Fixed prostheses that are supported only by implants can be either cemented or screw retained (operator removable). However, there are instances in which it is preferable that the implant only–supported prostheses be patient removable--in the maxilla5,9,10 for esthetic, phonetic, hygiene-related reasons, or patient preference, and in the mandible for arch shape or residual bone height considerations.4,10 These prostheses consist of either a metal superstructure (with or without cantilevers) joining the implants or copings with or without attachments on the individual implants, and usually a metal substructure in the overlying denture. Hence, they closely resemble tooth-supported telescopic prostheses. Conversely, by far the majority of reports4,5,7,10,17–19,27–34 on implant overdentures consider them as being both implant and mucosa supported, albeit to varying degrees.

To avoid confusion, and with guidance from The Glossary of Prosthodontic Terms,26 it is therefore proposed that the following definitions apply to implant prostheses:

  1. A partial or complete "implant fixed denture/prosthesis" is supported entirely by implants and is not patient removable.
  2. A partial or complete "implant overdenture" derives support entirely from implants or derives support from both the implants and the mucosa and is patient removable.

Table 1 Terminology: Examples of Matrix and Patrix Components
Implant component Matrix Patrix
Bar-clip systems: round, ovoid, parallel sided, spark eroded, or milled bars •Metal alloy clips
•Plastic Hader clips (APM Sterngold)with or without metal housing in denture base
Bar itself + the gold cyllinders and the retaining screws
Ball or stud systems with patrix attached to the implant and matrix housed in implant overdenture (eg, Ceka) Retentive ring or cap with or without housing in base of implant overdenture Ball abutment or stud itself and the retaining screw
Ball or stud systems with matrix attached to the implant and patrix housed in denture (eg, ERA/Zaag, APM Sterngold) Retentive ring or cap + gold cylinder and retaining screw Ball abutment or stud housing
itself with or without housing in
base of implant overdenture
Magnetic attachment systems with patrix attached to the implant and associated matrix in implant overdenture Split-pole magnet in the base of the implant overdenture Magnetic keeper that is threaded onto the abutment

 

It is accepted that retention is always obtained from the implants (regardless of the number), their attachments, or interconnecting bars/superstructures. A variation in the number and position of supporting implants will determine whether the overdenture has both implant and mucosa support or is entirely implant supported.

It is necessary to also achieve consensus on any terminology used to describe postinsertion prosthodontic procedures5,17,27–34 before categorization and grouping into fields. The Glossary of Prosthodontic Terms26 defines the matrix of the retentive component as "the portion of an attachment system that receives the patrix" and the patrix as "the extension of a dental attachment system that fits into the matrix." Thus, in implant overdentures, irrespective of whether the implants are splinted or unsplinted and regardless of the materials used, the matrix is always considered the "negative," or receptive, portion of the attachment mechanism, while the patrix is always the "positive" portion.25 For simplicity, any implant components that are attached to a matrix or patrix, holding them either to the implants or in the denture, are included as part of the matrix or patrix for the purposes of reporting postinsertion prosthodontic maintenance (Table 1).

Categorization

For the purposes of this protocol, categories of postinsertion prosthodontic procedures for implant overdentures have been defined and then grouped into fields. These procedures can be divided into accepted maintenance of the components and repairs that are caused by biologic and mechanical complications. Interpretation of accepted maintenance among the variety of designs and system-specific components can be controversial and can overly complicate any attempt at categorization of these procedures. The result should be a simplified rather than an exhaustive list of proposed categories. An attempt has therefore been made to combine the perceived maintenance of the respective patrices and matrices into similar categories for all unsplinted and splinted overdenture designs to allow accurate comparison between systems:

  1. Patrix loose (Patrix here pertains to the patrix [ball abutment, retentive anchor, stud attachment, magnetic keeper] and/or its component screws as well as the patrix component screws of any related gold cylinders and all interabutment and cantilever bars/superstructures [round, ovoid, U shaped, milled, spark eroded].)
  2. Patrix activated (number of occasions)
  3. Patrix replaced (number of occasions unilaterally or bilaterally)
  4. Patrix fractured
  5. Dislodged, worn, or loose matrix or its respective housing (Matrix here pertains to the matrix components [O ring, resilient cap, titanium spring, gold cap attachment, magnets] as well as all types of metal alloy or plastic retention clips [single sleeve or multiple sleeve] or permanent resilient lining material, connecting to interabutment or cantilevered bars/superstructures.)
  6. Matrix activated (number of occasions)
  7. Matrix replaced (number of occasions unilaterally or bilaterally)
  8. Matrix fractured
  9. Fractured implant overdenture, puncture fracture of acrylic resin over patrix, or fractured denture teeth
  10. Reline of implant overdenture
  11. New implant overdenture constructed
  12. Periimplant or interabutment mucosal enlargement

There is consensus in prospective19,29,31–34 and retrospective17,27,28,30 studies and reviews5,7 that the adjustment of the contour and selective grinding/occlusal reshaping26 of overdentures are common during the first year of service. As these modifications are easily effected using the principles of conventional complete denture treatment, they are considered part of separate routine prosthodontic follow-up and therefore are excluded from this categorization. For the proposed categorization in this article, the first four categories relate to accepted maintenance of the patrix components, while categories 5 to 8 relate to the accepted maintenance of the matrix components. Fractures of the overdenture base or teeth are included in category 9. Relining and remaking are designated under categories 10 and 11, respectively. The last category is related to periabutment or interabutment mucosal enlargement, an accepted nuisance factor for both maxillary5,9,10,17 and mandibular implant overdentures.35

Table 2 Characteristics and Definitions of the Six-Field Protocol for Implant Overdentures
Field Definition Category
Success Review of patient records during the study period reveals no evidence of retreatment except for accepted maintenance.* No. of implants, support differentiation,± and status of the   opposing arch§ are identified. 1–8, 10, 12
Survival Patient cannot be examined directly, but the patient or another clinician confirms no evidence of retreatment except that described for a successful outcome. No. of implants, support   differentiation,± and status of the opposing arch§ are identified. 1–8, 10, 12
Unknown
(lost to
follow-up)
Patient cannot be traced; surviving or successful implant overdenture removed to allow provision of a new overdenture, eg, conversion to another overdenture design with additional implants or a fixed implant prosthesis using the same or additional implants. --
Dead Patient died during the study period regardless of whether successful or surviving criteria were experienced before death. --
Retreatment
(repair)
Treatment of implant overdenture and/or mucosa where marginal integrity and associated patrices/ matrices are maintained irrespective of modifications as long as it continues as an implant overdenture. More than two replacements of either patrix or matrix in the first year or more than  five replacements in the first 5 years. Includes replacement of worn or fractured overdenture teeth/  fractured overdentures, relining of overdenture more than once in 5 years, or excision of patrix-  associated mucosal enlargement as a result of infringement on the shoulder/undersurface of the  patrix. No. of implants, support differentiation,± and status of the opposing arch§ are identified. 1–10, 12
Retreatment
(replace)
Part or all of implant overdenture is no longer serviceable because of either loss of implants or irreparable mechanical breakdown. A replacement prosthesis is indicated. No. of implants,support differentiation,± and status of the opposing arch§ are identified. 11
*Includes patrix activation/repair/replacement, matrix activation/repair/replacement, and asymptomatic periimplant/interabutment mucosal enlargement not requiring excision. There is a limit of two replacements of either patrix or matrix in the first year and five replacements in 5 years, and one reline of the overdenture base in 5 years.
Designated as one, two, three, four, five, or more than five. Horseshoe designs for maxillary overdentures are identified.
±
Designated as implant only or implant and mucosa.
§Designated as dentate, removable partial denture, complete denture, fixed implant prosthesis of any form, or implant overdenture. A combination of these designations is possible.

Relining, although subjective in clinical assessment, has been considered part of the long-term clinical service for implant overdentures.5,7,13,17,19,25,27–34 The following criteria are proposed to facilitate an objective assessment. A reline is considered necessary when: (1) firm pressure on the occlusal surface reveals lack of vertical space between the overdenture base and the retentive component(s) (matrix or patrix) attached to the implant(s); (2) rocking of the overdenture occurs on the abutment(s), indicating a lack of anteroposterior stability; (3) there are patient complaints of increasing food accumulation underneath the overdenture; and (4) a wash impression on the undersurface of the overdenture with a nonviscous impression material reveals a thickness of greater than 1 mm (measured with a graduated periodontal probe). The criteria for remaking implant overdentures are subject to professional judgment in individual patients, and they should correspond where possible to similar objective measures developed for assessing conventional complete denture treatment.36

Grouping of Categories into Fields

The aforementioned 12 categories can then be subsequently assigned to one of six objectively defined outcome fields (Table 2). Specific patrix and matrix replacements, especially in the first year of service, do not necessarily preclude a "success" outcome. Therefore, procedures grouped in categories 1 to 8 should be classified as maintenance or repairs27,28,32 depending on the frequency of their occurrence. A limit of two replacements (patrix or matrix) in the first year, with a maximum of five replacements, and one reline of the overdenture base (category 10) in a 5-year period is proposed as constituting acceptable maintenance. Further replacements would be considered excessive and would therefore change the outcome classification from success to retreatment. Mucosal enlargement (category 12) that is asymptomatic and does not infringe on the shoulder of the abutment or interfere with oral hygiene underneath the bar is included under success. These categories (1 to 8, 10, and 12) are then also applicable if the implant overdenture is deemed "surviving" (the second field). Here, the implant overdenture is still in service but has not met the success criteria, which include direct examination of the patient. The third and fourth fields of "unknown" (lost to follow-up) and "dead" are self-explanatory and contain similar details to the protocol for fixed implant prostheses.11 The fifth field of "retreatment" involving repair would involve any repairable fractures (category 9) and replacements of the patrix or matrix (categories 1 to 8) or relining (category 10) of the overdenture base that does not constitute acceptable maintenance. In addition, any periimplant or interabutment mucosal enlargement (category 12) needing excision as a result of infringement on the shoulder or the undersurface of the patrix is included in this field. Finally, the sixth field of "retreatment" involving failure of the prostheses will only involve new implant overdenture construction (category 11).

Number of Implants and the Opposing Arch/Prosthesis

In view of the varied amount of support provided by different numbers of implants supporting overdentures, the actual number of implants and differentiation between implant-mucosa or implant-only support should be noted as well as the status of the opposing arch. These aspects should therefore be included alongside the respective fields mentioned.

Cost Effectiveness of the Prosthodontic Treatment over Time

Within a time frame of 5 years, two additional fields are proposed separately to identify a ratio of (1) "implant overdenture fabrication costs" ($US), determined from initial fabrication costs of professional time-related fees, laboratory fees, and implant component cost; and (2) "prosthodontic maintenance in the first 5 years" ($US), determined from the same costs, as related to prosthodontic maintenance. These cost ratios will make the proposal more acceptable to "real world" dentists and prosthodontists.

Discussion

Future research on evaluation of varying treatment modalities for edentulous or partially edentulous patients must include both outcome measures of prosthesis longevity and evaluation of economic cost effectiveness.2,13,37 The protocol proposed in this article would assist clinicians to accurately compare both prosthesis longevity and economic cost effectiveness of different implant systems and overdenture designs. However, assessment of the totality of prosthodontic treatment involves aspects of the functional/physical domain, together with the psychologic domain, and is separate from aspects of the survival/longevity domain.1 Thus, this proposed classification protocol should be used in conjunction with a standardized categorization of the physiologic23,24 and psychosocial13 impact of the implant overdenture treatment on the rehabilitated patient. The presentation of implant-related outcome information in defined field tables has been viewed more favorably than outcomes presented in life tables.22

From an academic standpoint, this proposal can also be of assistance when future clinical trials are conducted or existing clinical trial results are presented. For this protocol to have maximum relevance, only data from prospective, controlled, randomized trials should be used. While this may be at variance with recent recommendations on prosthodontic treatment outcome studies,38 it could be difficult to apply the proposed criteria post hoc if they were not taken into account when the study was conducted.

Conclusion

A protocol for evaluating and interpreting the prosthodontic outcome of implant overdentures has been presented. It is an adaptation of criteria proposed for implant-fixed prostheses and involves a modification of six well-defined fields based on objective standards. The proposed protocol would, in both private practice and academic environments, allow a detailed comparative evaluation of overdenture designs and implant systems in terms of prosthodontic maintenance requirements, time to retreatment, and relative costs.

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