Implant overdenture has better quality in its retention and stability than complete denture, and its mastication function is much more effective as well.1,2,3,4 Especially in mandible, 2-implant overdenture has already been proven as the first choice for edentulous patients through the two conferences (McGill consensus (2002), York consensus (2009)).2,5

Various attachment systems such as ball, bar, and magnet attachment have been used to retain or support the overdenture. The selection of the attachment systems has been mainly depended on the practitioners' personal preference based on their experience and training. Many clinical trials and systematic reviews have tried to evaluate and compare the attachments in various aspects.6,7,8 However, previous studies used different criteria or method to evaluate treatment outcomes. In proceeding long-term comparative study, establishing standardized evaluation criteria will help in increasing reliability of the study result by minimizing bias.9 There were few reports to suggest a guideline to compare the various attachments and evaluate implant overdenture on edentulous patients.

This systematic review aimed to analyze the evaluation criteria on mandibular implant overdentures through a systematic review and suggest standardized evaluation criteria.

A systematic literature search was conducted using the combined MeSH terms "mandibular prosthesis" or "Denture, Overlay" and "dental implants" or "dental prosthesis, implant supported" and "clinical study" or "comparative study" or "outcome assessment" or "epidemiologic studies" or "intervention studies" or "patient satisfaction" and limited by "Human" and "English" in the data base, Medical Literature Analysis and Retrieval System Online (MEDLINE). This is to search for all the published data until December 31^th 2011 which is relevant to evaluation of mandibular implant overdenture. After searching the combined MeSH terms through internet, the next journals were directly found and the data were added. Clinical Implant Dentistry and Related Research, Clinical Oral Implants Research, Implant Dentistry, International Journal of Oral and Maxillo-facial Implants, International Journal of Oral and Maxillo-facial Surgery, International Journal of Periodontics & Restorative Dentistry, International Journal of Prosthetics, Journal of Clinical Periodontology, Journal of Dental Research, Journal of Oral Implantology, Journal of Oral and Maxillo-facial Surgery, Journal of Oral Rehabilitation, Journal of Periodontology, Journal of Prosthetics, Journal of Prosthetic Dentistry, and Periodontology 2000.

Among journals which included data on evaluation of implant overdenture until December 2011, randomized and comparative clinical trials between attachments with same number of implants were included. At least one or more evaluation outcomes on implant survival rate, peri-implant tissue, prosthesis, or patient satisfaction should be reported. And intra-osseous implant with root form was only taken into consideration. In addition, a maxillary complete denture was only considered as an opposing prosthesis. Conventional loading was only included. And the articles written in English were included.

However, case reports or technical reports were excluded, and immediate placement of implant after extraction was excluded as well. The case in which follow up duration after the start of function was less than 1 year was excluded. The case in which there were no evaluation criteria of implant treatment outcomes was also excluded. The type with rigid connection such as milled bar or combinations of attachment types, cantilevered applications of attachments was excluded. The case with no abstract was also excluded (Table 1).

Articles were selected by 2 different reviewers based on inclusion and exclusion criteria. 1,159 articles in total were searched and 53 of them were selected through titles and abstracts. And then 29 articles were finally selected through full text reading (Fig. 1). After evaluation items of implant overdenture were listed, they were classified into the following 4 groups. 1) Survival rate of implant, 2) peri-implant tissue evaluation, 3) prosthetic evaluation, 4) patient satisfaction. Each group was classified into subdivisions and evaluated respectively. The items to evaluate peri-implant tissue were classified into plaque index, calculus index, gingival index, bleeding index, probing depth, attachment gingival level, amount of keratinized mucosa and marginal bone loss. The items to evaluate prosthesis were classified into abutment screw loosening, abutment fracture, female detachment, loose matrix, denture fracture and occlusal adjustment. The methods measuring patient satisfaction were divided into atypical questionnaire, Visual Analogue Scales (VAS), Oral Health Impact Profile (OHIP). In addition, it was also evaluated whether each method include the following items; overall satisfaction, improved retention, improved stability, chewing ability, speaking ability, appearance satisfaction, food impaction, social function, feeling pain, denture hygiene, denture comfort, easy to getting used, healing satisfaction after surgery.

Twenty nine articles included 21 studies which presented implant survival rate and 17 studies which evaluated periodontal tissue around implant. Also, there were 18 studies which presented prosthetic maintenance and complication, and 14 studies which presented patient satisfaction (Table 2). There were no studies to report cumulative survival rate among any studies reporting implant failure. Seventeen studies evaluating peri-implant tissue status presented following items as evaluation criteria; marginal bone level (14), plaque Index (13), probing depth (8), bleeding index (8), attachment gingiva level (8), gingival index (6), amount of keratinized gingiva (1). In the 18 studies evaluating prosthetic maintenance and complication, the items such as loose matrix (17), female detachment (15), denture fracture (15), denture relining (14), abutment fracture (14), abutment screw loosening (11), and occlusal adjustment (9) were used. Atypical questionnaire (9), VAS (4), OHIP-14 (1) were also used as the format of criteria to evaluate patients satisfaction in 14 studies (Table 3). The each method to evaluate patient satisfaction included following items; Overall satisfaction (11), improved retention (9), chewing ability (9), speaking ability (8), improved stability (5), appearance satisfaction (4), food impaction (2), social function (4), feeling pain (2), denture hygiene (2), denture comfort (1), easy to getting used (1), healing satisfaction after surgery (1)(Fig. 2).

Except for these, there were other studies reporting implant mobility (3), denture retention (3), maintenance cost after treatment (2), and comparing operative time and time which took for prosthesis (1).

In evaluating implant overdenture, survival rate of implant, peri-implant aspect, prosthetic maintenance and complication have been commonly used as conventional criteria. However, the methods of evaluating mandible implant overdentures were lack of consistency which might cause researchers to mislead to analyze data from the different studies. To increase reliability, some researchers tried to score on each category by the developed method which was called Delphi method.16 However, the result was still dependent on their capability and subjectivity.10 It seems that there was not a common guideline to evaluate implant overdentures.

In this study, any studies reporting at least 1 implant failure did not present or describe cumulative implant survival rate. Censored data include patients who are lost to follow up, patients who dropped out, patients who expired from other causes etc.9 The more censored patients are, the more influenced the validity of sample size becomes, and also study design itself could possibly lose credibility due to bias.9,11 Therefore, cumulative survival rate of implant considering the censored data is significant in evaluation of survival rate of implant for reliability.

A variety of periodontal evaluation criteria were also applied to evaluate peri-implant tissue. Among them, plaque index was the most frequently used as the evaluation criteria of peri-implant tissue. Both bleeding index and probing depth were the second frequently used. On the other hand, calculus index, which was not reported at all, was a relatively less important evaluation criteria for periimplant tissue. Not only traditional periodontal index, but sulcus fluid analysis, exudation, and mobility of implant could also be used in examination of tissue around implants.12 In contrast to the soft tissue evaluation which was based on the traditional periodontal index, the evaluation of marginal bone loss was lack of consistency due to different reference points.13 Nevertheless, plaque index, bleeding index, probing depth, attachment level and marginal bone loss, which were commonly used, might be suggested as minimum criteria to evaluate peri-implant tissue.

Prosthetic complications of mandibular implant overdenture in edentulous patients were varied a lot as: loss of retention, denture relining, fracture of clip/attachment, fracture of denture, fracture of opposing prosthesis, fracture of acrylic resin base, abutment screw loosening, and fracture of abutment screw. The classification of prosthetic outcome which was called six fields table analysis has been already suggested.14,15 However, it seemed that a few reports cited the table and prosthetic terminologies such as female and matrix were still confusing. Nevertheless, loose matrix, female detachment, denture fracture, denture relining, abutment fracture, abutment screw loosening, occlusal adjustment were commonly used without large deviation. The prosthetic evaluation criteria might be relatively well-standardized compared to other criteria.

Patient satisfaction can be affected by their subjective recognition of their dentures or by individual difference such as age, gender, and personality.16 Oral health-related quality of life (OHRQoL) has been used as evaluation criteria for dental prosthesis which could improve mastication and pronunciation, self-esteem and satisfaction on their own appearances.17,18 The OHIP which was established as evaluation method for OHRQoL in 1994 has been modified and translated. The unique OHIP-EDENT which specialized to edentulous patients, was recently developed.19 OHIP-EDENT is composed of 19 questions, and it is divided into 7 sub-groups (functional limitation, physical pain, psychological discomfort, physical disability, psychological disability, social disability, and handicap). VAS could be used as another implement to quantify patient satisfaction with reliability and credibility.6 To objectively evaluate patient's subjective satisfaction, psychometric instrument such as a structured questionnaire is desirable to be standardized by OHIP or VAS. In addition, assessment items to evaluate patient satisfaction were suggested to include the following apsects; physiological function (chewing and speaking ability), psychological aspects (overall satisfaction, appearance, and improved retention and stability), and social function.

In summary, the data on cumulative survival rate of implant as evaluation criteria of implant overdenture still seem to be lack in general. Criteria of peri-implant tissue and prosthetic evaluation seem to be relatively well standardized. Also, it seems that the evaluation format to measure patient satisfaction objectively such as OHIP or VAS has not generally used yet.

For evaluation of implant overdenture, it is necessary to include cumulative survival rate for implant evaluation. It is suggested that peri-implant tissue evaluation criteria include marginal bone level, plaque index, bleeding index, probing depth, and attached gingiva level. It is also suggested that prosthetic evaluation criteria include loose matrix, female detachment, denture fracture, denture relining, abutment fracture, abutment screw loosening, and occlusal adjustment. Finally standardized criteria like OHIP-EDENT or VAS are required for patient satisfaction.