Dhanraj M, Hemalatha Ranganathan, Ashish R. Jain and Acu. Varma

Department of Prosthodontics, Saveetha Dental College and Hospitals, Saveetha University, Chennai, India.

Corresponding Author E-mail: dr.ashishjain_r@yahoo.com

DOI : https://dx.doi.org/10.13005/bpj/1187

Abstract

The role of abutment tooth preparations and their modifications influencing retention in complete veneer restorations needs to be investigated further in detail. To estimate and compare the effectiveness of conventional and surface modified tooth preparation designs in providing retention  in complete veneer restorations. An electronic search was conducted for scholarly articles discussing about retention in complete veneer restorations written in English or translated into English listed with pubmed ,Cochrane library, Science Direct, Wiley online library, Google scholar data bases, The Newyork Academy of Medicine Grey literature Report and Ingenta Connect till october 31st 2016 with suitable keywords. The database search yielded 57 articles out of which 42 articles were discarded after reading the abstract. Full texts were obtained for the remaining 15 articles. 9 articles were selected based on the inclusion criteria and were subjected to data extraction. The effect size was difference between the means of forces dislodging the crowns in the conventional and surface modified abutments. The meta analysis  exhibited overall effect size     z = 27.99 (p < 0.00001) at 95 % CI indicating a very high statistically significant difference between the conventional and surface modified tooth preparation for complete veneer crowns in the amount of retention provided. Based on the evidence provided by the literature ,Surface modified abutments for complete veneer restorations  exhibited a greater the amount of retention  than the conventionally prepared abutments.  Hence abutment surface modifications  during tooth preparation can be incorporated in complete veneer restorations  in  clinical situations demanding additional retention to ensure better clinical success.  

Keywords

Abutments surface modifications; complete veneer crowns; retention; tooth preparation

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Copy the following to cite this article: Dhanraj M, Ranganathan H, Jain A. R, Varma A. Effect of Tooth Surface Modifications Influencing Retention in Complete Veneer Restorations-A Systematic Review and Meta Analysis. Biomed Pharmacol J 2017;10(2).

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Introduction

In the management of partially edentulous situations the patient preference hinges towards the fixed prosthesis in many situations. Amongst the fixed restorations, the two therapeutic options available to accomplish  tooth replacement include tooth supported and implant retained fixed partial dentures. Implant retained prosthesis is the primary modality of management in replacement of lost natural teeth. However, when implant restorations are not feasible the only fixed restorative  option left will be tooth supported fixed partial dentures. Complete veneer fixed restorations are the only choice is restoring endodontically treated teeth also.¹

Complete veneer restorations involve significant amount of tooth surface reduction to support the prosthesis. The amount of tooth reduction performed is dependent  predominantly on the choice of restorative material used. Metallic restorations are the most conservative involving the tooth reduction of 1mm circumferentially with chamfer margin. Metal ceramic restorative materials involve a moderate amount of tooth reduction of 1.5 mm circumferentially with shoulder margins.² All ceramic restorative materials involve the maximum amount of tooth reduction of 2 mm with shoulder or heavy chamfer margin and this is considered to be the most aggressive tooth preparation.

The common factors influencing retention include the taper and geometry of the tooth preparation, surface area of tooth involved, magnitude of dislodging forces, occlusal stresses and luting agents. The taper of the tooth preparation is a very important factor influencing retention.² The established guidelines for the amount of taper for clinically optimal retention ranges from 6 to 20 degrees. There are numerous studies demonstrating a progressive decrease in the amount of retention  with increased taper angle.² There are many factors which alters the occluso cervical height of the abutment teeth. Regressive alterations of the tooth surfaces viz.attrition, abrasion and erosion of teeth results in marked noncarious  loss of tooth structure  which leads to the increased prevalence of abutments with short clinical crowns in the human dentition.In addition to this,extensive caries, fractures involving the enamel and dentin ,occlusal disharmony ,abfraction and developmental anomalies involving the enamel and dentin also severely compromises the morphological structure and height of  the existing clinical crowns . When such teeth are subjected to tooth preparation , establishment of retention and resistance forms will be severely challenging to the clinicians. The dislodgment of crowns due to lack of retention and resistance can compromise function or esthetics.  Walton et al have reported short clinical abutments exhibited dislodgement of crowns in almost two-thirds  of the clinical situations.

The mechanical sucess of tooth preparations have been attributed largely  to both  the retention and resistance forms.³ Retention form   denotes all the features of a tooth preparation which enhances   the stability of the  restorations and resistance to  dislodgment along the path of insertion.  Resistance form denotes    the features in tooth preparation that improves the stability of the prostheses or restoration  and resistance to displacement  along an axis apart  the path of insertion. Resistance to lateral  displacing  forces  is  the most important determinant for retention of  cemented restorations and  crowns.⁴

Several authors have proposed a minimal axial taper ranging from 2 to 6 degrees for tooth preparation ,however 6 to 12 degree total occlusal convergence(TOC) offered the maximum retention for crowns.  Despite  this recommendation for a minimal TOC, in routine clinical procedures the normal  TOC acheived by operators  ranged from 12 to 27 degrees.⁵ Dodge et al stated various factors  such as accessibility of the preparation site, morphology of the prepared teeth, proximal contacts between the prepared and unprepared teeth, and  caliber of the operator  played important  roles in establishing  the optimal TOC  in complete veneer preparations. They  concluded  16 degrees was the optimal and the most commonly achievable convergence angle in clinical situations. Shillingburg et al also have concluded  the TOC is frequently between 10 and 22 degrees in tooth preparations for full veneer crowns.⁶

The least  maximum height required to provide sufficient  resistance for  complete veneer crowns  in the anterior region was 3mm and the taper was in the range between 6 to 20 degrees as reported by  Goodacre et al.They proposed  4 mm  as the least optimal  height for prepared molars   ,as molar teeth  preparation is done with greater occlusal convergence than the anteriors  and  because of their  wider  diameter  are subjected to greater occlusal forces.⁷

To improve the resistance form in tooth preparations with excessive axial taper or in short clinical crowns, additional auxiliary retention features could be incorporated.Apart from the preparation design ,the retention can be influenced by the luting cements and casting techniques as well.  Woolsey and Matich concluded that the proximal grooves demonstrated complete resistance to  dislodging  forces in the faciolingual directions.  The grooves and boxes prepared over the axial surfaces when placed in prepared axial surfaces,  enhanced the  resistance form to a marked extent. An another method of improving  the resistance form of  a tooth preparation with an excessive taper was to  prepare the apical portion with a reduced TOC.⁴

Reisbick and Shillingburg  were the initial investigators studying  the essential  features of a crown preparation  to improve  the retention and  resistance forms. They reported that the  axial grooves and boxes increased  the resistance form of the tooth preparation.  They  concluded that the placement of boxes was more effective  in providing  resistance than the   grooves. With regards to the placement of grooves, the  grooves placed in an interproximal location  offered more resistance over grooves placed in a buccolingual location  when   the forces were applied in a buccolingual direction.⁸

Several studies have shown that TOC is not consistent among the operators in invivo conditions.   Location of the tooth to be prepared  is a crucial factor in accompolishing  adequate  retention and resistance form . The maxillary and mandibular posterior teeth  frequently associated with  the most inadequate total occlusal convergence during tooth preparation.However the incorporation of additional retentive features in overtapered preparations were rarely observed in clinical situations.   Occlusal surface  preparation can also affect the resistance form in complete crown preparation.  The placement of inclined planes and functional cuspal bevel on the occlusal surface of a crown preparation  can increase the resistance form than the flat occlusal surface preparation. The placement of  occlusal isthmus will also improve the resistance form in tooth preparations.⁹ Despite the various  existing opinions , the overall effectiveness and performance   of the  various auxiliary retention and resistance  features remains uncertain in the literature. Hence this systematic review and meta analysis was performed to obtain more clarity over the effect of mechanical methods of tooth surface modifications influencing retention in complete veneer crowns.

AIM

To estimate and compare  the effectiveness  with the amount of retention provided by conventional and surface modified tooth preparation designs  in complete veneer restorations.

Objectives

To evaluate the effect of abutment height influencing  amount of retention  in complete veneer restorations.

To evaluate the effect of surface modifications influencing amount of retention in complete veneer restorations

To evaluate the effect of surface roughness of abutments influencing amount of retention in complete veneer restorations

Pico Analysis

Population

Teeth subjected to complete veneer tooth preparation.

Intervention

Conventional tooth preparation.

Comparison

Tooth preparation technique with surface modification.

Outcome

Retention measured as the tensile bond strength between the prepared abutment teeth and the complete veneer restoration.

Resistance

Variables of Interest

Influence of the following factors on marginal discrepancy

Resistance

Type of finish line

Amount of tooth preparation

Height of tooth preparation

Axial taper

Film thickness of luting agents

Sand blasting

Modification of intaglio surface of the crowns

Marginal leakage

Bio compatibility.

surface area

magnitude of dislodging forces

direction of dislodging forces.

type of luting agents.

Materials and Methods

Sources used

An electronic search was conducted for articles written in English or translated into English listed with pubmed ,Cochranelibrary, ScienceDirect, Wiley online library,Google scholar data bases,TheNewyork Academy of Medicine Grey literature Report,Ingenta Connect till October 31st 2015 reporting retention of complete veneer restorations

Pico Analysis

Search Methodology

The search methodology applied was a combination of MESH terms and suitable keywords.The terms complete veneer crowns and tooth surface modifications and other relavent keywords were not enlisted in the MeSH database and hence discrete keyword search was initiated based on PICO cluster.

Population

Crowns, Veneer ,Complete veneer, Full crown, Full coverage restoration, Complete coverage restoration, Total coverage restoration, Full metal crown, All metal crown, All metal  restoration,Metal ceramic crown, Metal ceramic restoration, Porcelain fused to metal crown, All ceramic crown.tooth preparation , complete veneer retainers

Intervention

Sand blasting, Roughness in tooth preparation, Surface modification in tooth preparation, Slots, Grooves, Pins, Pin holes, Ledges, Pin ledges, Keys, Keyways, Linear grooves, Buccal grooves, Proximal grooves, Palatal grooves, Axio proximal grooves, Incisal grooves, Isthumus, Occlusal isthumus, Proximal box, Offset, acid etching

Comparision

Abutments, Abutment height, Adequate abutment height, Finished surfaces, Tooth preparation for complete crowns, Tooth preparation for full veneers, Tooth preparation for porcelain fused to metal crowns, Tooth preparation for porcelain fused to metal veneers, Tooth preparation for all ceramic crowns, Tooth preparation for all ceramic veneers, Natural teeth abutment angulation

Outcome

Resistance, Retention, Tensile bond strength, Shear bond strength, Pull out strength, Pull out test, Tensile testing, Shear testing, Frictional fit, Frictional resistance, Retention failure, Retention loss, Retention fracture, Retention grooves, Retentive ability.

Selection of Studies

The review process consisted of two phases.During  the initial  phase titles and abstract of the search were thoroughly  screened for relevance and the full text of relevant abstracts were obtained. The articles which were obtained after first step of review process using the following inclusion and exclusion criteria were screened in second phase and relevant and suitable articles were identified and  isolated for further processing and data extraction

Inclusion Criteria

The articles discussing the following parameters were included for the systematic review.

Randomized control trials reporting retention in complete veneer tooth preparations.

Controlled invitro trial reporting retention in complete veneer tooth preparations.

Experimental research reporting retention in complete veneer tooth preparations.

Experimental research reporting Mesiodistal grooves.

Experimental research reporting  Mesiodistal boxes.

Experimental research reporting  Buccal-lingual grooves.

Experimental research reporting  Occlusalinclinedplanes.

Experimental research reporting  Occlusal isthmus.

Experimental research reporting  Reduced total occlusal convergence.

Exclusion Criteria

Articles and manuscripts discussing the following parameters were excluded:

Case reports or case series

Tooth prepartion involving endodontically treated teeth with post and core.

Complete veneer restoration for implant supported prosthesis

Tooth and implant supported prosthesis

Tooth preparation in decidious teeth

Tooth preparation in periodontically compromised teeth.

Tooth preparation in teeth underwent radisection or hemisection

Tooth preparation of over denture abutments.

tooth preparation in implant crowns.

tooth preparation for abutments receiving precision attachments.

abutment fractures.

Results

The database search yielded 57 articles out of which 42 articles were discarded after assesing the abstract. Complete  texts were obtained for the remaining 15 articles. 9 articles were selected based on the inclusion criteria and 6 articles were excluded. The finally selected 9 articles were subjected to data extraction.The CEBM levels of evidence were affixed for the selected articles and all the relevant information were extracted and processed and studied further in detail and subjected to statistical analysis.

Data Extraction

The data from the finally included studied were tabulated and the following information were extracted

Study designed applied

The teeth subjected to tooth preparation

Axial wall taper

Height of abutment teeth

Surface area of abutment teeth

Modification of abutment teeth

Amount of dislodging forces

Amount of stress

Type of luting agent

Measurement of the outcome

Gadgets used for measurements

Statistical test involved

Flow Chart for Search Stratergy

Graph 1   Click here to View graph

Results

All the included invitro studies where categorized as 3B as per Oxford Centre for evidence  based medicine .Six studies were identified discussing retention based on abutment modification by mechanical methods  (Table 1)and two studies were reported variation in retention level based on abutment height(Table 2) and three studies evaluated the amount of retention based on surface roughness (Table 3)induced over the abutment surfaces. The data were extracted from the afore mentioned studies and a meta analysis was done to evaluate the influence of surface modification over conventional methods.

Table 1: Variables of interest and general information of the selected articles.   Click here to View table

 

Table 2:  Extraction of existing data regarding retention.   Click here to View table

 

Table 3:  Data extraction regarding retention based on abutment modification.   Click here to View table

The effect size was difference between the means of forces dislodging the crowns in the conventional and surface modified abutments Meta analysis showed  overall effect size  z = 27.99 (p < 0.00001) at 95%CI  indicating a very high statistically significant difference between the conventional and surface modified abutments(Fig 1).

Figure 1: Meta analysis   evaluating the difference   amount of retention offered by conventional and surface modified tooth preparation techniques.   Click here to View figure

Discussion

The results of the meta analysis  clearly established surface modifications induced in the abutment preparation significantly altered the amount of retention in the prepared abutments. All the studies included in this  review  were experimental invitro studies and there might be a slight amount of variation when the results of the study are extrapolated  to address similar clinical situations. The other parameters influencing retention include the amount of dislodging forces, the amount of stress transmission in the abutments, periodontal health of abutments, off directional forces transmitted due to over contoured restorations perforated restorations can influence the outcome.

Emara et al¹⁰ evaluated the effect of proximal grooves on the retention and resistance of cast resin-bonded retainers for mandibular and maxillary  second molar teeth. The grooves significantly  increased the  debonding forces for maxillary molars . The effect of  the grooves over the  mandibular second molars was not significant.

Table 4:  Data extraction regarding retention based on abutment height.   Click here to View table

Huang et al¹¹ studied the effects of proximal grooves and abutment height on the retention and  resistance forms . Proximal grooves improved the resistance  with  abutment height of 4 mm, whereas addition of  grooves had no significant differences in resistance in  abutment heights of 2 and 3 mm. The 2 mm groups exhibited the least performance than the other groups irrespective of the proximal grooves. An abutment height of 3 mm exhibited adequate resistance for single cast crowns when self adhesive modified resin cement was used. Preparation   of proximal grooves on abutments shorter than 4 mm height  had no significant influence on the resistance.

Table 5:  Data extraction regarding retention based on surface roughness.   Click here to View table

Li et al¹² studied the relationship between the surface roughness of prepared teeth and the internal adaptation and retention of complete veneer  crowns following  preparation with diamond rotary  instruments of various  grit sizes. The highest mean surface roughness (SD) (4.8 (0.4) μm) was observed in teeth  reduced  with the coarsest diamond. The  teeth reduced with the finest diamond  instruments showed the lowest mean cement thickness (0.5 (1.2) um. The finer  tooth surface preparations exhibited less microleakage (P =.03). However, no significant differences in retention were observed (P=.19) across the experimental groups. Teeth prepared with the finer grit instruments had  smoother  surfaces and crowns with better internal adaptation. The grit size of the diamond  instruments did not affect the dislodging  forces between the complete veneer crowns and the prepared teeth.

Proussaefs et al ⁹ studied the effectiveness of various auxiliary preparation features over the retention and resistance form of complete veneer  crowns with reduced axial heights and total occlusal convergence.  Proximal grooves,  boxes, buccolingual grooves, occlusal inclined planes, and occlusal isthmus were less  effective at improving  the   resistance of crown  to displacement when the tooth preparation lacked resistance form . The only crown modification that showed increased resistance   than the control group was the reduced TOC in the cervical half of the axial wall.   Placement of  auxiliary retentive features such as grooves and boxes into a compromised tooth preparation with 2.5 mm occlusocervical dimension and 20-degree TOC were not effective when these retentive features possessed the same 20-degree TOC as the prepared axial walls.

Roudsari et al⁴ evaluated the effect of various  auxiliary features on the resistance form of crowns with reduced height of the axial walls  and increased total occlusal convergence. They concluded  the crown preparation modifications improved and enhanced  the resistance form. Neverthless, the reduction in cervical TOC was more effective than the placement of  proximal grooves.

Tuntiprawon ¹³ investigated the effect of axial surface roughness on the marginal adaptation  and retention of silver-palladium cast crowns luted with    zinc phosphate,   resin  and glass ionomer cements.Coarse and fine grit diamond instruments  were used to create varying levels  surface roughness in premolars. Superior retention for complete cast crowns was observed for tooth preparations done  with coarse diamonds and luted with Panavia 21 resin cement. Variations in axial surface roughness of tooth preparations  had no effect over  the marginal adaptation of the complete  coverage  cast crowns.

Vinaya et al ¹⁴  studied  the retention of complete cast metal  crowns in teeth with adequate and inadequate crown heights and also evaluated  the effects of auxiliary retentive features over retention form  in the same. Complete cast metal  crowns on teeth with adequate crown height exhibited greater retention than the ones with inadequate crown height. Proximal grooves provided greater amount of retention when compared with proximal boxes There was no statistically significant difference among long conventional and short groove preparation.

Arora et al ¹⁵  observed in  complete veneer  tooth preparation on a clinically compromised teeth with reduced occluso cervical height and increased TOC, the crown preparation modification that offered the greatest resistance form was the reduction of TOC at the apical 1.5 mm of the axial wall. Incorporation of inter proximal grooves with 1 mm of width and depth in the gingival third  also significantly improved the retention but less compared to  the cervical  tooth preparation.

The effectiveness of retentive grooves   are also  dependent on the  occluso cervical height of the crown and the axial taper. When the clinical crown height is critically reduced,  further incorporation of interproximal grooves may not improve resistance.¹⁶

Wiskott et al¹⁰ stated the  resistance to lateral displacement is a function of the distribution of  complex compressive ,tensile and shear force vectors acting at the luting cement ,restoration and tooth interface.    The length of the edentulous span, the type of abutments chosen like, pier abutments or cantilever abutments or tilted abutments, type of luting agents used, their compressive tensile and shear loading, dissolution of luting cements in saliva could be other factors capable of altering the retention and resistance outcome.The mechanical modifications incorporated involve high degree of precision and technique sensitivity and specialized equipment may be needed to incorporate the modifications into the restorations. However advanced CADCAM¹⁷ designing will enable the incorporation of these modifications more user friendly and less technique sensitive.

All the studies included in this review were performed in an in vitro environment and evaluation of retention by mechanical methods may not be possible in an in vivo situation. In clinical situation retention can be evaluated by assessment of longevity of the restorations over varying periods of time, hence more longitudinal studies assessing the performance of surface modified abutments needs to be initiated further to ensure better understanding of the phenomenon.¹⁸ In clinical situations fracture of the abutments, fracture of restorations or perforation of the restoration can also lead to loss of retention.

These factors should be carefully analysed and interpreted with due consideration.Surface modification through mechanical methods can be extremely challenging in clinical situations, however success can be ensured by following meticulous care in clinical and laboratory procedures.¹⁹

The incorporation of   specialised impression materials and optical impression have faciliated this process to a greater extent.²⁰ The surface modification  increase the surface area and provides mechanical locks providing additional amounts of retention and resistance as well.Hence the mechanical methods  can greatly resist the sheer or torsional rotational forces acting in the oral cavity during function ,thus enhancing the clinical longevity and performance of complete venner crowns.

Conclusion

Based on the evidence provided by the literature ,Surface modified abutments exhibited a greater the amount of retention  than the conventionally prepared abutments . Hence  abutment surface modifications during  complete veneer tooth preparation can be incorporated  in all clinical situations demanding additional retention to ensure better clinical success.

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