Dr Raelene Sambrook Melbourne Dental School The University of Melbourne 720 Swanston Street Carlton VIC 3053 Email:firstname.lastname@example.org
Background: Implant supported restorations (ISRs) for the single implant may be cement retained or screw retained. Limited scientific evidence exists to support the superiority of a retention type for either implant or prosthetic success. The aim of this study was to assess preferences of Australian prosthodontists when restoring single implants. In particular, clinical practices for cross-pin retained implant supported restorations for a single implant were investigated.
Methods: A written questionnaire comprised of seven questions, some of which had multiple parts and of both open- and closed-format, was sent to 124 Australian prosthodontists. The questionnaire asked recipients to identify: (1) their preference for retention choice when restoring a single implant; (2) the frequency of use; and (3) clinical practice when restoring a cross-pin retained restoration.
Results: Seventy-seven per cent of respondents indicated that direct to fixture (DTF) retention was their first preference. DTF retention was also the most frequently employed restoration for single implants. Respondents indicated that cross-pinned ISRs are employed to maintain retrievability or when DTF is not possible. The majority of respondents indicated they always or sometimes use a gasket with cross-pin retained restorations, though gasket type varied. Thirty-eight respondents (31%) indicated that they would never use a cross-pinned retained restoration for a single ISR.
Conclusions: Australian prosthodontists prefer, and more frequently restore single implants, using DTF retention. Queensland prosthodontists prefer cement retained ISRs. In comparison, cross-pinned restorations tend to be the least favoured and least used retention type. In addition, variation in opinion exists regarding the need for a gasket and type of gasket to be placed.
Since the application of implants for use in the partially edentulous jaw and the development of prosthetic techniques for restoring single implants, differences in opinion have existed regarding the most appropriate retention type. Retention of the implant supported restoration (ISR) is often described as being screw or cement retained. Screw retention may use a single abutment screw, retaining the restoration directly onto the implant, commonly called direct to fixture (DTF) retention, or an accessory screw which retains the restoration onto the screw retained abutment. Accessory screws may be vertically positioned, as with an occlusal or prosthetic screw, or horizontally positioned, such as a transverse or cross-pin screw. Cement retention utilizes a screw retained abutment and the restoration is retained on the abutment with provisional or definitive cement.
Cement retained restorations were introduced to overcome the clinical limitations of implant angulation experienced with DTF retention. Though consequently, cement retention challenged the philosophy of predictable prosthesis retrieval. Cross-pin retained restorations were also introduced to provide an effective restorative option for a poorly angulated implant yet maintained predictable retrieval.1
Despite single implants being accepted as a predictable treatment option for replacement of a missing tooth, there is a lack of scientific evidence supporting the superiority of cement or screw retention as it pertains to either implant or prosthesis success or survival.2 To date, few in vivo studies comparing cement and screw retained approaches have been conducted.2–4 In addition, use of available evidence to support retention choice is limited by the accuracy of complication recording during the maintenance phase of implant treatment,5,6 the market driven approach to implant research7 and the influence of industry sponsorship on clinical trials and outcomes.8 Regardless of this, a number of opinion papers have been written to facilitate clinical decision making for retention type.9–11 Unfortunately, the validity of the factors used to compare cement and screw retention, such as passivity, aesthetics, simplicity and cost, is questionable and the supporting evidence is weak. More recently, a review on the prosthetic restoration of implants compared screw and cement retention against biologic, technical, aesthetic and patient-related outcomes.12 Generally though, the scientific rigour in comparison of retention choice for ISRs has been lacking.
Despite the lack of evidence, it has been suggested that cement retention is currently the most preferred retention type.9,10 In addition, a number of recent systematic reviews have indicated that the majority of single ISRs in their analyses were cement retained.13–15 This paradigm shift towards cement retention of crowns for ISRs has evolved with limited scientific documentation.10,16,17
To date, no study has assessed the preferences for retention type of practising clinicians. The aim of this study was to assess preferences of Australian prosthodontists when restoring single implants and in particular, clinical practices when using a cross-pin retained ISR.
Materials and Methods
A voluntary, anonymous questionnaire was mailed to Australian prosthodontists. The questionnaire comprising of seven questions, some of which had multiple parts, and of both open- and closed-format. All Australian prosthodontists whose addresses were publically listed, either from the Australian and New Zealand Academy of Prosthodontists website or state dental board websites were included in this survey (n = 124). From the available addresses, no prosthodontists were identified in the Northern Territory or Tasmania and a single prosthodontist from the Australian Capital Territory was allocated to New South Wales.
The questionnaire was accompanied by a letter explaining the objectives of the study and requesting participation. A numbered, reply paid envelope was included but there were no identifying marks on the questionnaire itself. All questionnaires were de-identified and coded to ensure confidentiality. This also allowed two follow-up questionnaires to be mailed to non-responders. Human Ethics approval was obtained from The University of Melbourne Human Research Ethics Committee (HREC number 0931362).
The questionnaire was designed to assess the prosthodontists’ opinions regarding: (1) preference and frequency for restoring single implants; and (2) common practices (or lack thereof) for using cross-pin retention for single implants. The data collection period extended from July 2009 to January 2010.
The data were entered manually into a Microsoft® Excel spreadsheet (Microsoft Corporation, WA, USA). Each possible response to each question was allocated its own column and any non-numerical data were numerically coded for ease of data manipulation. The spreadsheet was imported into SPSS™ (SPSS Inc., Chicago, IL, USA) statistical software package. Frequency of responses to each question was used to analyse and present the data.
From the 124 questionnaires sent out, a total of 107 questionnaires were returned (86%). Of the questionnaires returned, 10 were not completed due to the prosthodontist being retired, not providing implant treatment or an incorrect mailing address. The number of completed questionnaires used for data analysis was 97 (or 78%) of the total number of questionnaires sent. Table 1 compares the regional spread of the respondents.
Table 1. Geographic characteristics of the respondents†
†Percentage of total respondents in parenthesis.
Preference for retention type and frequency of use
Prosthodontists were asked to indicate their preference for retention when restoring a single implant by ranking cement retained, DTF, or cross-pin retention from 1 to 3 (1 being the most preferred option). DTF retention was indicated as the first preference for 77% of the respondents, cement retained for 18%, and cross-pin retention was the first preference for only 5% of respondents. Figure 1 shows the regional differences in retention choice for a single ISR. For all states, except Queensland, DTF was preferred for retaining a single ISR. In comparison, prosthodontists practising in Queensland indicated a slightly higher preference for cement retained restorations.
The respondents were asked to indicate the frequency with which they restore single implants, again by ranking cement, DTF, or cross-pinned restorations from 1 to 3. Across Australia, DTF appears to be the most frequently employed technique for retaining single ISRs (62%), while cement retained and cross-pin retained restorations accounted for 23% and 15% respectively. Figure 2 shows the regional differences for the most frequent retention method used for a single ISR.
Use of cross-pinned restorations
Regardless of preference or frequency for a particular retention type, prosthodontists were asked the following open-ended question: ‘Why would you use a cross-pinned restoration for a single implant?’. Multiple responses were provided (n = 122) with the most frequent being retrievability (32%) and DTF not possible (25%). Conversely, 38 respondents (or 31% of the responses) indicated that they would never use a cross-pin to retain a single ISR (Fig. 3).
Prosthodontists were asked to indicate the occurrence (yes or no) of gingival recession, peri-implantitis, fistula formation or malodour as seen in their practice with cross-pinned restorations. Their responses suggest that malodour is the most common of these biologic complications to be seen clinically (Fig. 4).
Respondents were asked if they always, sometimes or never place a gasket when restoring an implant with a cross-pinned restoration. Of the 56 respondents, 48% indicated they would always use a gasket when restoring with a cross-pinned restoration. Twenty-nine per cent only sometimes use a gasket and 23% never place a gasket.
If respondents answered ‘always’ (n = 27) they were asked to provide a reason why they place a gasket. Multiple answers were provided (n = 37). It was indicated that a gasket was used to fill a void space (27%), provide a seal (19%), reduce malodour (16%), and reduce bacterial load (16%). Less commonly, a gasket was used to aid retention (8%), reduce soft tissue inflammation (8%) and ‘other’ accounted for 5% of responses. If respondents answered ‘sometimes’ to gasket placement (n = 16), it was indicated that a gasket was placed when there was a need to control malodour, provide plaque control and aid retention (Fig. 5).
To determine gasket types currently used, the questionnaire provided combinations of materials and methods and space was also provided for respondents to indicate alternative materials and techniques. Interestingly, some of the 43 respondents indicated the use of more than one type of gasket (n = 59) and in one case, three different gaskets were suggested by a single respondent. As described in Table 2, most commonly, TempBond® (Kerr, Michigan, USA) cement alone was used on the inner surface of the crown (22%). A mixture of TempBond® cement and Vaseline® (Chesebrough-Ponds USA Co., Minnesota, USA) was also used as a gasket (20%). Less used was a silicone material placed over the abutment screw (14%).
Table 2. Gasket types used when restoring with a cross-pinned ISR†
†Percentage of total responses in parenthesis.
‡Temp Bond® (Kerr, Michigan, USA).
§Cavit™ (3M ESPE, Minnesota, USA).
¶Vaseline® (Chesebrough-Ponds USA Co., Connecticut, USA).
TempBond®‡ cement alone, placed on the inner surface of crown to seal crown
TempBond® cement and Vaseline®¶ admixture, placed on inner surface of crown to seal crown
Silicone material placed over abutment screw
Cavit™§ placed over abutment screw
Cavit™ placed over abutment screw and TempBond® cement to seal crown
Wax placed over abutment screw and TempBond® cement to seal crown
Wax placed over the abutment screw and Vaseline®to seal crown
Figure 6 shows the responses to the questions ‘When do you remove the cross-pin?’ and ‘When do you replace the gasket?’. Of the responses (n = 55), 87% remove the cross-pin and crown only as required and 75% only replace the gasket as required.
The final question asked was ‘Why do you replace the gasket?’. Multiple answers were provided (n = 57) by the respondents (n = 41) and are displayed in Fig. 7. Most commonly, the gasket was only replaced when the crown needed to be removed or replaced (30%), whilst prosthodontic complications (including tightening of screws) accounted for an additional 19%. Gasket replacement was also done to reduce malodour (18%) and respondents indicated that soluble gaskets needed to be replaced (18%).
One aim of this survey was to gain insight into the current practices of Australian prosthodontists when restoring single implants and, in particular, clinical practices as they relate to the use of cross-pin retained restorations. Currently, opinion papers9,10 and systematic reviews13,15 suggest that cement retained ISRs are the preferred retention type.
Confidence for using cement retained ISRs has evolved based on clinical studies that have demonstrated a reduction in screw loosening with modifications to the components for single crowns.18 Cementing the crown not only accommodates a poorly aligned implant, but also replaces the need for accessory screws and any associated screw-based complications. Even though cement retention is regarded as being simpler, has greater passivity than screw retention, has better aesthetics and can be retrievable,9,10 the results of this study suggest that Australian prosthodontists prefer and more frequently restore single implants using DTF retention. At a state level, it was interesting to note that prosthodontists from Queensland not only prefer but also, more frequently, use cement retention for single ISRs.
Cross-pin retention was indicated as the most frequent method for retaining an ISR by 15% of the respondents. The cited advantages of cross-pin retention being predictable retrieval and retention, improved aesthetics and occlusal morphology.19–21 The results suggest cross-pin retention has been utilized instead of cement retention when DTF is not possible. In comparison, 38 of the 97 respondents (39%) indicated they would never use a cross-pinned restoration for a single ISR. The disadvantages of a cross-pinned ISR include increased cost for restoration construction,1 increased technical complexity (at a laboratory level),1,19,21 and biologic complications related to leakage.21
The clinical significance of leakage and the internal void of a cross-pinned restoration has not yet been determined. Anecdotally, this dead space has been associated with both malodour and fistula formation. Although gasket placement has been recommended to limit leakage at the crown-abutment interface,21 not all respondents place a gasket when using a cross-pinned restoration. Interestingly, the reasoning for gasket placement differed between the ‘always’ and ‘sometimes’ group. In addition, this study demonstrated a lack of consistency in gasket type for cross-pinned restorations. Gaskets differed in both material choice and placement technique. Theoretically, all of these gaskets should permit retrieval of the restoration.
The use of definitive or provisional cementation has been a topic of discussion for cement retained ISR. Although there is very little evidence to support the selection of one cement over another when ‘permanent cementation’ versus ‘retrievability’ is the objective,22 provisional cements are recommended when there is a desire to maintain retrievability. Use of provisional cements for cement retained ISR has been associated clinically with cement washout and crown dislodgement.23 For a cross-pinned restoration, even though the crown is predictably retained by the cross-pin screw, cement wash-out may compromise the ability of the gasket to prevent leakage. Therefore, the effectiveness of a gasket of provisional cement may be time dependent. Despite the unknown efficacy of a gasket long term, ‘replacing the soluble gasket’ accounted for only 18% of reasons for replacing the gasket whilst more commonly, the gasket was replaced when there was the need to remove or replace the crown (30%) or due to technical complications (20%).
This study was conducted to identify current practices of Australian prosthodontists when restoring a single implant and in particular clinical practices when utilizing cross-pin retention. Across Australia, the use of DTF retention is the preferred method of retaining an ISR. At a state level, Queensland prosthodontists prefer to use cement retention for an ISR. Despite cross-pinned restorations being the least preferred and less frequently used restorative type, they are used to maintain retrievability or when DTF is not possible. The majority of respondents indicated they always or sometimes use a gasket with cross-pinned ISRs though reasons for gasket use, gasket material and gasket placement varied.
This study highlights the differences in opinion which exist regarding retention type for ISRs. In addition, the diversity of responses with respect to gaskets and cross-pinned ISRs reflects the anecdotal approach by clinicians to gasket placement.
I would like to recognize the following for their financial contribution to this project: the Australian Dental Research Foundation (ADRF); the Australian Prosthodontic Society (APS); and the University of Melbourne Postgraduate Research Fund (PGRF).