Multi-unit implant impression accuracy: A review of the literature.

Q U I N T E S S E N C E I N T E R N AT I O N A L

PROSTHODONTICS

Mirza Rustum Baig

Multi-unit implant impression accuracy: A review of the literature Mirza Rustum Baig, BDS, MDS1 Background: Accurate implant impressions are critical for the achievement of passive fit of an implant prosthesis which in turn contributes to the long-term success of the implant treatment. There is inconclusive evidence on the techniques and materials used for making multi-unit implant impressions. Objective: To evaluate the scientific data related to different aspects of multi-unit implant impression accuracy and draw useful conclusions from the review for application in clinical practice. Method and Materials: Studies from 1990 to November 2012 were evaluated. Papers examining implant impression accuracy for two or more implants were selected for review. Case reports, technique articles, and incomplete studies were excluded. Fifty-nine studies were selected in total for evaluation, three among them clinical and the rest in vitro. Results: Fifteen studies compared PVS (polyvinyl siloxane) and PE (polyether), 11 found no differences between the two materials in terms of impression accuracy. Thirty studies ana-

lyzed the splint effect, 13 found splinting better and 13 others elicited no differences between splinting and non-splinting. Among the 25 studies examining pickup and transfer impression techniques, 12 favored pickup over transfer and 11 found no differences between the two. Twelve studies assessed implant angulation effects and found significant differences in accuracy for 20- to 25-degree angulation, and no differences for 5- to 15-degree angulation for most studies, except two. Conclusion: PVS and PE were the preferred impression materials for multi-unit implant impressions. The evidence on splinting was inconclusive and the data supporting splint to non-splint were neutral. Pickup was the better performing technique than transfer, especially with increased number of implants. Implant angulation of 20 to 25 degrees negatively affected the multi-unit implant impression accuracy. (Quintessence Int 2014;45:39–51; doi: 10.3290/j.qi.a30769)

Key words: accuracy, angulated implants, digital impressions, impression material, misfit, multi-unit implant impressions, passive fit, splinting

Accurate multi-unit implant impression making is a challenging procedure in implant dentistry due to several reasons. The first and foremost is the complexity 1

Senior Lecturer, Restorative Dentistry (Prosthodontics), School of Dentistry & Health Sciences, Charles Sturt University, Orange NSW 2800, Australia; and Adjunct Senior Lecturer, Department of Prosthetic Dentistry, Faculty of Dentistry, University of Malaya, Kuala Lumpur, Malaysia; and Visiting Assistant Professor, Department of Prosthodontics, Ragas Dental College & Hospital, Chennai, India.

Correspondence: Dr Mirza Rustum Baig, Restorative Dentistry – Prosthodontics, School of Dentistry & Health Sciences, Charles Sturt University, Orange NSW 2800, Australia. Email: [email protected]; mbaig@ csu.edu.au

VOLUME 45 • NUMBER 1 • JANUARY 2014

surrounding the attainment of “passive fit” of the implant prostheses, which is directly related to the accurate three-dimensional (3D) transfer of the implant positions to the working cast, among other factors.1-4 Secondly, there is a substantial amount of data examining the various factors, mainly clinical (such as impression technique, material used, effects of splinting, implant angulation, and depth), influencing the accuracy of impressions, and the results are conflicting or inconclusive with regards to the various recommendations.4-6 Thirdly, there is a lack of homogeneity of

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Q U I N T E S S E N C E I N T E R N AT I O N A L Baig

research conducted in the area, as it is with most dental implant-related studies; the issue is further compounded due to the various combinations of implant systems, prosthetic components, impression/splinting/ tray materials, accuracy evaluation methods, distortion types (absolute or relative), connection levels (implant or abutment level impressions), connection types (internal or external), and techniques employed by different studies.7-17 Additionally, most of the current available evidence is in vitro,4,5,8-17 and there are very few clinical studies18-20 in the area, to date. The primary goal of implant impressions is to obtain an accurate working cast to improve the chances of production of a passively fitting implant prosthesis.4,6 Absolute passive fit is impossible to achieve clinically, as endorsed by various researchers working in the field,3,4,18 but question remains about the permissible amount of clinical misfit that will not cause potential complications.21,22 “Machining tolerance” has been suggested by some authors,23,24 and a range of threshold values for tolerable clinical misfit have been proposed25,26 over the years to address this issue, with the evidence still evolving. Although it is acknowledged and agreed unanimously that “passive fit” as a factor contributes to the long-term success of the multi-unit implant restorations,2,27 it has been difficult to pinpoint “inadequate fit” as a direct cause for dental implantrelated complications and failures in the clinical studies. However, it is in the best interests of the clinician to aim for a restoration with the least misfit, following the best known methods of fabrication and evaluation of the prosthesis before delivery. The fit evaluation tests3,21,28 can be performed on the laboratory working casts and in situ (Fig 1). Though numerous research reports evaluating the factors affecting implant impression accuracy have been published in recent decades, extensive reviews on the subject have generally been rare.4,6 One of the few papers provided a detailed review, examined the different clinical factors influencing the accuracy of implant impressions in general, and made useful recommendations to that effect.4 Another recent review paper focused on the role of a specific factor that might affect

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implant impression accuracy, namely implant angulation, and made some conclusions in that regard.5 Since the publication of these articles4,6 that evaluated data up to mid-2008, fresh evidence5,8,9,11-15,17-20,29-31 has emerged in the area, including the use of new hybrid impression materials,19 splinting materials,14 and digital impression techniques.29-31 Importantly, a few clinical studies18-20 supplement the predominant laboratory-based investigations conducted previously. The latest works need close examination for assessment of their value and implications on the current clinical practice. An updated review is therefore warranted to factor in the current developments, reflect the most recent findings relevant to the topic, and reconstitute the recommendations, if necessary. The objective of this article is to review the evidence-based literature on implant impression accuracy and establish the current status on the best practice with regards to multi-unit implant impressions, based on the data.

METHOD AND MATERIALS A data search was performed on PubMed’s electronic databases for clinical studies, in-vitro studies, dental reports, and review articles, using the following search terms in various combinations: “implant impressions”,

Fig 1 Periapical radiograph showing vertical marginal gaps between the splinted implant-supported fixed prostheses and the terminal abutments.



“accuracy”, “passive fit”, “digital impressions”, “splinting”, “angulated implants”, “deeply placed implants”, ”intra-oral scanning”, and “multi-unit implants”. The years searched were 1990 to November 2012. To select the studies, all obtained reports were reviewed. Titles and abstracts were screened for relevance. The full texts of suitable abstracts were obtained and assessed using the following inclusion and exclusion criteria. Inclusion criteria: • studies dealing with implant impression accuracy • studies published since 1990 (inclusive). Exclusion criteria: • case reports or technique articles

Table 1

•

only abstracts published in journals or conference proceedings • studies evaluating single-tooth implant impression accuracy • review articles • studies in languages other than English. References from selected studies were additionally used to identify pertinent literature. The initial literature search yielded 735 articles. Based on the first screening of the titles and abstracts, 63 studies were found eligible. Seven studies32-38 were excluded because the inclusion criteria were not satisfied (Table 1). Full-text assessment of the remaining studies led to the selection of 59 articles7-20,39-83 for final analysis (Fig 2): 56 were in-vitro studies7-17,39-83 and three were clinical.18-20

Excluded studies

Study

Reasons for exclusion

Daoudi et al32; Lee & Gallucci33; Ramsey & Ritter34; Linkevicius et al35; Vigolo et al36; Daoudi et al37; Vigolo et al38

Single-tooth implant impression evaluation

Ramsey & Ritter34

Case report: clinical digital impression using coded healing abutment

Total studies identified from electronic search (n = 735)

Studies excluded as titles and/or abstracts did not comply with the inclusion criteria (n = 672)

Studies identified for screening of full text evaluation (n = 63)

Studies included due to the following reasons: (n = 3) • Examination of references of included articles

Studies excluded due to the following reasons: (n = 7) • Case reports • Single-tooth implant impression evaluation

Studies included in the final review (n = 59)

Fig 2

Search strategy for studies related to multi-unit implant impression accuracy.


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Fig 3 Pickup implant impression made with polyether impression material.

RESULTS Nearly 95% of the articles selected for analysis were invitro studies. The majority of the studies on multi-unit implant impression accuracy had been published in the last 12 years, with more than half of those papers in the last 3 years, including the three clinical studies.

angulation (Fig 3). Among the 17 studies7-9,12,13,19,39-49 that investigated the impression material differences, 15 of them compared polyvinyl siloxane (PVS) and PE impression materials,7-9,12,13,39-48 one compared vinyl siloxanether (VSE) with PE,19 and another compared PVS with irreversible hydrocolloid49 (Table 2). The majority of the studies12,13,39-48 have shown comparable accuracy, with insignificant differences between PVS and PE for multi-implant impressions, but a few others7,8 have reported greater accuracy with PVS in comparison to PE, albeit in specific situations when the implants were placed deeply subgingivally7 or were nonparallel with variable impression coping connection lengths.8 On the other hand, PE has been found to produce better results, in terms of implant cast accuracy8 and abutment-framework interface gaps,9 than PVS by some authors. A recent clinical study examined the new impression material VSE and found it to be equivalent or superior to PE.19 Irreversible hydrocolloid with splinting was also tested and found to be as accurate as PVS with non-splinting in another recent investigation.49

Splinting vs non-splinting Impression material Very few studies have examined impression material as a factor affecting multi-unit impression accuracy. Most authors had employed polyether14,16-18,20,46,50,54,57,65,74-76 (PE) impression material as a stock standard for performing the accuracy evaluation, involving other variables such as impression technique and implant

Table 2

Among all the studies examining the multi-unit implant impression accuracy, the “effects of splinting vs nonsplinting” was the most investigated area, and also the most controversial. Of the 30 studies analyzed,10-18,46,49,50-68 13 supported the splint technique,10,11,15,16,18,46,50-56 five supported non-splinting over splinting,58,65-68 and 13 found no significant differences

Studies comparing the multi-unit implant impression accuracy: impression material

Study

Study outcome 8

42

7

Sorrentino et al ; Lee et al

PVS more accurate than PE

Del’Acqua et al9; Sorrentino et al8

PE more accurate than PVS

Rutkunas et al39; Chang et al12; Mostafa et al13; Aguilar et al40; Wenz & Hertrampf41; Wostmann et al42; Holst et al43; Cehreli & Akça44; Akça & Cehreli45; Assuncao et al46; Wee47; Lorenzoni et al48

No significant differences between PVS and PE

Enkling et al19 (clinical study)

VSE more accurate than PE

Ferreira et al49

No significant differences between PVS and irreversible hydrocolloid



between the splinting and non-splinting techniques with multi-unit implant impressions.12-14,17,49,57-64 It was found that the evidence advocating splinting and that demonstrating non-effect is equally distributed (Table 3). In addition to the simple comparative analysis, no relation was found between the number of implants and the splinting technique (splint and nonsplint) for edentulous arch multi-unit impression accuracy. Among studies evaluating four to six implants, 11 studies10,11,15,16,18,46,50,51,54-56 favored splinting compared to non-splinting and 11 showed no differences12-14,17,49,58,60-64 between the two approaches. Most studies reported in the literature utilized autopolymerizing acrylic resin as the splinting material, usually in combination with dental floss.11,14,15,18,46,52,5759,60,65 Preformed bars of autopolymerizing acrylic resin were also often used, in conjunction with freshly mixed self-cure resin for splinting purposes. The acrylic resin bars were sectioned to appropriate lengths to span the spaces between the impression copings and then luted to the impression copings with acrylic resin using the bead-brushing technique, with the aim of minimizing polymerization shrinkage.12,13,16,17,50,52,54,80 Light-polymerized composite resin with preformed composite resin bars,50,51 bite registration silicone,14 bite registration PE,14 and metal bars with autopolymerizing acrylic resin10,80 had also been used as splinting materials, but sparsely. Some of the studies that advocated the splinting technique over the non-splinted technique have shown further that splinting with autopolymerized acrylic

Table 3

resin, sectioned post-setting, and rejoined, yielded the best results within the various splinting group combinations.11,53 However, this finding was not applicable to all studies that examined the effect.12,50,52,60 One study80 compared self-cure acrylic resin splinting and metal splinting and found that metal splinting was significantly more accurate for a complete arch edentulous situation with four implants. Another study14 compared bite registration silicone and PE with self-cure acrylic resin splint and found no differences between the three materials, with a slight advantage for bite registration PE splint, although it was statistically insignificant. As for the two studies that examined the composite resin splints, the results were mixed. One study found better accuracy with acrylic resin splints compared to the composite resin,50 and another found the composite resin splint demonstrated better accuracy than the non-splinted technique.51

Transfer vs pickup Twenty-five studies12,13,17,20,41,42,45,46,54,57,58,61,63,64,67,69-78 compared the differences between pickup and transfer impression techniques in terms of accuracy (Table 4). Of the 25, 12 studies13,17,42,46,57,58,63,67,69-72 concluded that pickup impressions were significantly more accurate than transfer, and 1112,20,41,45,53,61,74-78 showed no statistically significant differences between the two techniques. The evidence base supporting transfer over the pickup technique was negligible.64,73 Comparing studies based on the relationship between the impression technique (pickup and trans-

Studies comparing the multi-unit implant impression accuracy: splinting vs nonsplinting techniques

Study

Study outcome

Ongul et al50; Al-Quran et al11; Papaspyridakos et al18 (clinical study); De Faria et al15; Del’Acqua et al51; Filho et al52; Cabral & Guedes53; Vigolo et al16; Naconecy et al10; Assuncao et al46; Vigolo et al54; Assif et al55; Assif et al56

Splinting significantly better than nonsplinting

Ferreira et al49; Chang et al12; Stimmelmayr et al17; Hariharan et al14; Mostafa et al13; Lee et al57; Del’Acqua et al58; Choi et al59; Kim et al60; Herbst et al61; Hsu et al62; Barrett et al63; Humphries et al64

No significant differences between splinting and non-splinting

Assuncao et al65; Del’Acqua et al58; Burawi et al66; Philips et al67; Inturregui et al68

Non-splinting significantly better than splinting


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fer) and the number of implants, 11 studies20,53,57,69,70-72,75-78 were identified as having used two or three implants. Among them, five studies57,69,70-72 showed pickup was better than transfer, and a marginally higher number 20,53,75-78elicited no differences between the two techniques. On the other hand, 12 studies12,13,17,41,42,45,46,58,61,63,67,75 estimated accuracy with four to six implants; seven favored pickup over transfer13,17,42,46,58,63,67 and five showed no differences.12,41,45,61,75 A definite trend was observed, in that the pickup seemed to be better than transfer when there were higher numbers of implants involved. With regards to the use of impression copings versus abutments, with direct or indirect impressions, the results were mixed. Two studies70,76 supported multiunit implant level impressions, both open70 and closed tray76 with “impression copings”, over the direct abutment impressions “without impression copings”, while three other studies12,45,75 elicited no significant differences between the two techniques. Four12,45,75,76 of the five studies mentioned above, using abutment transfer

Table 4

techniques, had engaged plastic caps with positioning cylinders fitted on to the solid abutments for impression making. The one remaining study examined impressions made with the cementable abutments directly, with no transfer components.70

Digital vs conventional impressions Only three studies29-31 compared digital multi-unit implant impression accuracy with conventional impressions. These studies have all been published in the last 24 months. Each of the investigations differed from the other in some respect (Table 5). The only similarities were that all the experiments were performed in vitro. Two studies simulated partially edentulous situations,29,30 and one study31 was designed to assess a completely edentulous arch. The number of implants used in the studies varied between two and four. Overall, only one study29 had favorable results with digital impressions compared to conventional, but the study had performed digital impressions of the abutments directly on the master cast and not at the implant level.

Studies comparing the implant impression accuracy: transfer vs pickup impression techniques

Study

Study outcome 69

70

17

71

13

Tarib et al ; Kwon et al ; Stimmelmayr et al ; Jo et al ; Mostafa et al ; Lee et al57; Wostmann et al42; Del’Acqua et al58; Assuncao et al46; Philips et al67; Barrett et al63; Carr72

Pickup (open tray, direct) more accurate than transfer (closed tray, indirect)

De La Cruz et al73; Humphries et al64

Transfer (closed tray, indirect) more accurate than pickup (open tray, direct)

Rashidan et al74; Chang et al12; Alikhasi et al75; Gallucci et al20 (clinical study); No significant differences between pickup and Walker et al76; Wenz & Hertrampf41; Conrad et al77; Cabral & Guedes53; transfer Akça & Cehreli45; Herbst et al61; Carr78

Table 5

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Studies comparing implant impression accuracy: digital vs conventional impression techniques

Study

Study outcome

Karl et al29

Digital abutment impressions (equivalent to clinical intraoral scan): better or equal to conventional impressions

Stimmelmayr et al31

Conventional impressions and scanning of ‘scanbodies’ on lab analogs/casts: significantly better than direct scanning of ‘scan bodies’ on original implants (ie, equivalent to clinical intraoral scan)

Eliasson & Ortorp30

Conventional impressions with impression copings and normal cast fabrication were significantly better than the conventional impressions with coded healing abutments (without copings) and scanning in lab to produce casts with robotically placed implant analogs



The other two studies30,31 attempted to record the implant positions using either scan bodies31 or digital coded healing abutments30 in the place of impression copings, and found less accuracy with the digital impressions in comparison with the conventional. Apart from these three studies, one other paper17 adopted the digital approach indirectly as an accuracy evaluation method of impression techniques (transfer, pickup, and splint pickup). Casts poured from impressions using conventional impression techniques with

Table 6

impression copings were digitally scanned. Scan bodies were used for recording a second impression of the experimental and master casts and the digital data obtained were analyzed for determining distortional differences between the different test groups and the control model.

Straight vs angulated implants Most studies investigating the effects of implant angulation on the multi-unit implant impression accuracy

Studies comparing implant impression accuracy: angulated implants

Study

Implant connection type

Number of Implant specimens number

Rutkunas et al39

Implant level/inter- 1 nal connection

Jang et al79

Implant level, internal connection

Implant angulations (in degrees)

Study outcome

2

5 and 25

Significant differences in inaccuracy for 25-degree angulation; increased angulation decreased the accuracy

10

2

0, 5, 10,15, and 20

Significant inaccuracy (gap) when divergent angle was 20 degrees; generally, more the divergence greater the gap between framework and the abutments

Gallucci et al20 (clinical study)

11 Multi-base abutment level/internal connection

2

≤10

No differences in accuracy for 10-degree angulation

Sorrentino et al8


4

10 (with alveolar bone undercuts)

Significant differences in accuracy between parallel and non-parallel implants, with 10-degree angulation

Jo et al71

Internal connection

10

3

0 and 10

No differences between parallel and non-parallel implants for 10-degree divergence

Filho et al52

Implant level/ external connection

6

2

0 and 25

Significant differences in accuracy with 25-degree implant divergence; inclined implants caused more inaccuracy

Lee et al57


2

0 and 10

Significant differences in accuracy with closed tray; no differences with open tray

10

2

0 and 25

Significant differences in accuracy with 25-degree implant divergence; inclined implants caused more inaccuracy

Choi et al59


2

0 and 8

No difference in accuracy between 0and 8-degree angulated implants

Conrad et al77

Implant level/ external connection

5

3

0, 5, 10, and 15

No significant differences in accuracy for all the three angulations. No differences between closed tray and open tray.

Carr78

Multi-base abutment level/external connection

10

2

0 and 15

No significant differences in accuracy

Carr72

Multi-base abutment level/external connection

7

5

≤ 15

No significant differences with direct technique; indirect technique showed inaccuracy for 15-degree angulation

Assuncao et Implant level/ al65 external connection


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Fig 4 Intraoral view of severely angulated implants (indicated by abutment level open tray impression copings in place), which may lead to impression inaccuracy.

Fig 5 Intraoral frontal view of the pickup impression copings fitted onto slightly angulated implants in the edentulous mandibular arch prior to impression making.

had been performed with two implants,20,39,52,57,59,65,78,79 except for a few papers which studied the effect with three to five implants.8,71,72,77 Of the 12 studies analyzed in this review, one was a clinical pilot study,20 and the rest had been conducted in vitro8,39,52,57,59,65,71,72,77,78,79 (Table 6). Seven studies8,20,39,57,59,71,79 estimated impression accuracy with internal connection implants, and five52,65,72,77,78 with external connection. Impressions were made at the implant level with impression copings in most of the studies, except a few20,72,78 where intervening multi-base abutments were used and abutment level impressions were made with the impression copings fitted onto the abutments. Three studies investigated the effects of 25-degree angulation39,52,65 and one study up to 20 degrees,79 and all found significant differences in accuracy between experimental casts and master casts. All the four studies that investigated the increased divergence (20 to 25 degrees) experimented with a two-implant model.39,52,65,79 Splinted direct technique was found to be the most accurate for multi-unit situations with two highly unparallel (20- to 25-degree divergence) implants.39,52 This finding was, however, in disagreement with a few other studies that showed no correlation between direct splint and non-splint,57,59,65 although it is worth noting that two studies among them evaluated implant divergence of 8 to 10 degrees.57,59

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Fig 6 Intraoral occlusal view of the pickup impression copings fitted onto severely angulated implants.

Ten studies8,20,39,57,59,71,72,77-79 assessed the effect of 5to 15-degree implant divergence. All studies, except two, found no significant differences in accuracy up to angulations of 15 degrees compared to parallel implants. Both studies that found significant differences at 10 and 15 degrees divergence8,72 had been designed to assess four to five implants, whereas all others that elicited no differences had either evaluated with two20,39,57,59,78,79 or three71,77 implants. Hence, a correlation was seen between increased number of implants and impression inaccuracy even for minor angulations (Fig 4). Five8,52,59,65,79 of the 12 studies had employed only pickup technique and the rest20,39,57,71,72,77,78 compared pickup with transfer. Among those that compared the techniques, four studies recommended pickup,39,57,71,72 and three showed no differences.20,77,78 Therefore, overall, a general lean towards pickup impressions being better with angulated implants was observed from the data (Figs 5 and 6).

Other factors affecting accuracy With regards to the tray type and material, rigid custom trays81 or modified metal stock trays82 produced more accurate impressions in comparison with the polycarbonate (plastic) stock trays. On the reuse of impression copings, it has been shown recently that the impression accuracy is unaffected when copings were reused up to ten times.83



Some non-splint techniques have shown improved accuracy in comparison to splinted methods, achieved through impression coping modification prior to impression making.7,51,54 Several types of modifications have been attempted, by extending the length of the impression coping by a few millimeters using an additional piece joined to the original coping using selfcure resin,7 providing additional extensions to the retentive part of the copings or thickening the body of the coping using self-cure acrylic resin,51 and air abrading and coating the impression copings with adhesive prior to impressions.9,54

DISCUSSION The present review paper selected and analyzed the relevant scientific works investigating the clinical factors affecting multi-unit implant impression accuracy. Most of the assessed studies were in vitro and mainly examined four clinical factors affecting implant impression making, namely impression material, splint or nonsplint, impression technique (open tray or closed tray), and implant angulation. On analysis of the data, a recent increase in the publication of articles related to implant impression accuracy was noted. The finding may be indicative of the fact that there is great interest in this area of research, and, additionally, with the advent of new impression techniques and materials, more in vitro and clinical studies are being conducted.

Impression materials PVS and PE continue to be the impression materials of choice, although PE has been the preferred material with many authors, especially those assessing multiunit implant impression accuracy in edentulous arches.10,11,14,16-18,46,50,54,74 New hybrid materials are being tested,19 but there is inadequate evidence at this point to make meaningful comparisons with the traditional materials.

Splint vs non-splint The findings of this study disagreed with an earlier review paper4 that found more studies favoring splint-


ing of impression copings to produce accurate impressions. In the present paper, there were no differences found between splint and non-splint, based on the number of studies that examined the effect and supporting either technique. The present evidence on implant impression accuracy with splint is mainly based on autopolymerizing resin as a splint material utilized in different ways.1118,46,52,54,57-59,60,65,80 The other materials such as composite resins,50,51 bite-registration silicones, and PEs14 have only been employed recently for splinting, in multiimplant impressions. In the limited studies performed with these materials,14,50 the results have been found to be promising. More research is required to substantiate use of these materials on a regular basis. Preformed acrylic resin bars connected to the copings using acrylic resin with or without sectioning and rejoining has been increasingly employed as the preferred splinting technique by many authors in the recent literature.12,13,16,17,50,52,54,80 Studies comparing different splinting techniques and materials in terms of impression accuracy are few in number,14,50,51,80 and more studies are required in the area.

Pickup vs transfer The results of this review paper were in accordance with the findings of an another article on the subject,4 in that a correlation was found between the number of implants and the accuracy of the impression technique (pickup or transfer). Overall, evaluating the entire data, it seemed that no technique had a distinct advantage over the other, although pickup was the preferred technique with greater number of implants.12,13,16,17,50,52,54,80 The data on impression techniques with regards to direct or indirect methods reported some papers presenting a combined approach.12,45,70,75,76 These studies evaluated abutment level impressions with snap-ons or plastic coping cylinders. Technically, the impressions were pickups as the components were transferred into the set material of the impressions; however, as these were performed with a closed tray that still required abutments to be transferred and repositioned into the snap-ons or plastic copings without direct contact with

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Fig 7 Intraoral maxillary occlusal view showing multi-base abutments on selected implants to counter implant angulation and depth issues prior to open-tray impression making.

the impression material, they were considered and analyzed as indirect or closed tray techniques in this review.

Digital vs conventional impressions The scientific research on the combined use of digital impressions with CAD/CAM (computer aided design/ computer-assisted manufacture) techniques for the fabrication of multi-unit implant restorations is at a preliminary stage. Very few accuracy studies17,29,30,31 have been reported on the use of intraoral scanners and laboratory scanners to record 3D dental implant positions using scan bodies and/or digital coded healing abutments. More in-vitro and clinical studies need to be performed comparing the accuracy of intraoral digital impressions vs conventional impressions, intraoral scan vs laboratory scan, scan bodies vs digital coded healing abutments, intraoral digital impressions abutment level vs implant level, and complete arch intraoral scans covering greater implant number to test the reliability and accuracy of these techniques before their clinical application.

Implant angulation Most of the studies evaluating the effects of implant angulation had done so with two implants,20,39,52,57,59,65,78,79 with just three studies investigating the effect on three to five implants71,72,77 (Table 6). All the studies, except one,72 were designed to examine only partially edentu-

48

lous situations and so the overall results may not be relevant for impressions made with a higher number of implants and complete edentulous situations. More research is needed with increased numbers of implants to lend further clarity to the subject. Presently, there is mixed and/or inadequate evidence with regards to the best impression technique (pickup or transfer; splint or non-splint)20,39,52,57,59,65,71,77,78 with angulated implants, and it needs to be explored further. Evidence has shown that the length of the connection part of the impression coping in internal connection implants could influence the accuracy of the multiunit implant-level impression,8 as the shortened length offsets the higher removal stress induced by the implant divergence. This possible phenomenon needs to be thoroughly assessed. The implant angulations can be further managed through the use of multi-base abutments and abutment level impressions with impression copings.12,20,78 The advantage is that the abutments aid in eliminating the increased contact area between the impression copings and the internal connection implants, which in turn reduces the deformation of the impression material and movement of the copings during removal and transfer (Fig 7).

Other factors affecting accuracy The majority of the studies14,30,42,43,50,70,71,74,75,77 investigating accuracy had performed measurements in the three axes, x, y, and z, on the sample casts and compared them with the standard readings on the master model to determine the distortion produced. These distortion values were then statistically evaluated for significance between different test groups. It was often found that differences significant in one dimension were not necessarily reproduced in the other. An additional observation was that if a particular test group was favored in one dimension, it was not always better in terms of least discrepancy in other dimensions too. Hence, within these experiments, the findings were mixed at times, giving rise to study results in “parts” and an inconclusive overall significance.14,58 Almost all the studies have been cautious with their recommendations in such scenarios and have addressed the situa-



tion by discussing the differences in each dimension and test group separately. Therefore, even though the study conclusion may project insignificance statistically from the hypothesis standpoint, the results need to be carefully interpreted keeping in mind the applied evaluation method and other fine details. A few studies30,42,74 have taken it one step further by calculating the consolidated 3D differences obtained from the individual dimensional discrepancies in the x, y, and z axes. This then enabled them to draw conclusions based on the statistical differences between the 3D values of test groups, rather than individual dimensions. As there are no clear quantitative guidelines on the maximum allowed deviations or discrepancies for the implant impressions to be treated as unacceptable, it is difficult to make definite recommendations based on the discrepancy values measured in the studies. Therefore, the current investigations only drew comparisons between test and control groups and matched the data against previous studies. The linear discrepancy between the master cast and the sample casts at the connection level has generally been reported to be in the range of 0.6 to 136 μm, based on the findings from a number of studies.4,7-9,12-14,16,54,60,62,67,73 A recent study described the range of acceptable 3D clinical misfit for one-piece implant prosthesis as 56 to 72 μm.26 Jemt25 suggested that prosthesis misfits smaller than 150 μm were acceptable. Therefore, the distortions of multiunit implant impressions measured in studies, under controlled experimental conditions, may not actually cause misfit of the prostheses on their own, but the combined effect of several factors, clinical and laboratory, may lead to unsatisfactory fit in the final prosthesis. Hence, it is imperative to make impressions and casts as accurate as possible to minimize the total distortion effect. Much of the current available evidence in the area is in vitro in nature and clinical studies are required to validate the results obtained from the laboratory studies. A meta-analysis was not performed due to the lack of homogeneity between studies, and this also limited the relevance of the conclusions drawn.


CONCLUSION Within the limitations of this study, the conclusions based on the literature review are: • PVS and PE seem to be the impression materials of choice for multi-unit implant impressions. • Pickup impression technique is marginally better than the transfer technique for producing accurate impressions, especially with increased numbers of implants. • Splinting of impression copings did not seem to affect the accuracy of the multi-unit implant impressions. • For internal and external implant connections, implant angulation seems to have a negative effect on the multi-unit implant impression accuracy, more so with increased divergence. • Digital impressions can’t be recommended for routine clinical use, based on the current evidence.

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Accuracy Comparison of Implant Impression Techniques: A Systematic Review.

Smoke-free multiunit housing: a review of the scientific literature.

Comparative evaluation of the effects of implant position, impression material, and tray type on implant impression accuracy.

Critical appraisal of implant impression accuracies: A systematic review.

Examination of the Position Accuracy of Implant Abutments Reproduced by Intra-Oral Optical Impression.

In Vitro Implant Impression Accuracy Using a New Photopolymerizing SDR Splinting Material.

Comparison of implant cast accuracy of multiple implant impression technique with different splinting materials: An in vitro study.

Three-dimensional accuracy of plastic transfer impression copings for three implant systems.

Accuracy of Implant Position Transfer and Surface Detail Reproduction with Different Impression Materials and Techniques.

Ridge preservation for implant therapy: a review of the literature.

Radiolucent inflammatory implant periapical lesions: a review of the literature.

Accuracy of interchangeable implant impression systems: an in vitro pilot study.

A Comparative Evaluation of the Linear Dimensional Accuracy of Four Impression Techniques using Polyether Impression Material.

In Vitro Comparative Evaluation of Different Types of Impression Trays and Impression Materials on the Accuracy of Open Tray Implant Impressions: A Pilot Study.

A technique to splint and verify the accuracy of implant impression copings with light-polymerizing acrylic resin.

The elastomers for complete denture impression: A review of the literature.

Diagnostic accuracy of impression-free digital models.

Effect of 2 impression techniques on the dimensional accuracy of working implant prosthesis models: an in vitro study.

Evaluation of the three-dimensional accuracy of implant impression techniques in two simulated clinical conditions by optical scanning.

Risk indicators for peri-implant mucositis: a systematic literature review.

Accuracy of impression materials measured with a vertical height gauge.

Single implant supported mandibular overdenture: A literature review.

Titanium endosseous implant-soft tissue interface: a literature review.

Implant overdenture impressions using a dynamic impression concept.