¹Associate Professor, Department of Fix Prostodontics, Dental Branch, Islamic Azad University, Tehran, Iran

²Faculty member, Department of Fix Prostodontics, Dental Branch, Islamic Azad University, Tehran, Iran

³Assistant Professor, Department of Periodontology, School of Dentistry, Alborz University of Medical Sciences, Tehran, Iran

⁴Assistant Professor, Department of Fix Prostodontics, Dental Branch, Islamic Azad University, Tehran, Iran

⁵Assistant Professor, Department of Periodontology, Dental Branch, Islamic Azad University, Tehran, Iran

⁶Dentist, Private Office, Iran

Article Publishing History

Introduction

A variety of impression techniques for the fabrication of implant supported prosthesis have evolved in the past decade. Selection of a specific technique depends on the evaluation of a particular patient and the clinical situation present. In the fabrication of implants, the primary objective of impression making is to record and transfer the relationship between the non-yielding, osseointegrated fixture abutments and reproduce the relationship in the master cast (Nayar et al. 2014). Prosthesis misfit may lead to mechanical and biological problems in supporting implants. Mechanical complications that might arise from prosthesis misfit include screw loosening, abutment or implant screw fracture and occlusal inaccuracy (Ebadian et al. 2015).

The first step to ensure the passive fit of the implant-supported framework is accurate recording of the implants’ positions and distances through the impression procedure (Conrad et al. 2007). There are many potential factors which influence the accuracy of implant-supported superstructures such as mandibular flexure, impression technique, impression material and fit tolerance between intra-oral abutments using the impression copings (Assuncao et al. 2004). Various techniques have been suggested to achieve an accurate master cast (Assuncao et al. 2004). Dental impression is used to produce a positive replica of the structure for use as a permanent record or in the production of a dental restoration or prosthesis (Alikhasi et al. 2013). Most of implant impression techniques, such as, pick-up, and transfer techniques and splint and non-splint techniques, have been introduced, in search of the most accurate technique. In certain clinical situation, some of the factor such as the angulations or depth of implants, may affect the accuracy of the implant impressions (Prakash and Chowdhary, 2016).

Figure 1: The positioning of the interspaces

An inaccurate impression may result in prosthesis misfit, which may cause biological and mechanical complications. Various mechanical complications such as loosening of screw, fracture of screw or implant and occlusal inaccuracy may have been arisen from prosthesis misfit (Prithviraj et al. 2011). Along with the evolution of acrylic resin metal implant supported fixed complete denture for an edentulous jaw, the splint technique for an implant impression was introduced (Lee et al. 2011). It is reported the splint technique have greater accuracy as compared to the non-splint technique (Prakash and Chowdhary, 2016).

Some of problems faced with the splint technique are fracture of the connection between the splint material and the impression copings and distortion of the splint materials. The metal-splinted direct technique produced the most accurate casts, then the acrylic resin-splinted direct, indirect and unsplinted direct techniques (Papaspyridakos et al. 2011).Despite researches were done on accuracy of Splinted and non-splint in open-tray impression techniques, scarce information exist on accuracy of these techniques on angled implants. Due to the effect of various impression techniques on accuracy of final cast dimensions and controversies about the best techniques, this study was performed to compare the Splinted and non-splint in open-tray impression techniques on 15 and 25º angled implants.

Material and Methods

At first steel model in 8 cm diameter and 3 cm height were made with 4 holes to stabilize 4 implants. Two central implants had 12 cm interspace from each other and 17 mm from angled implants. Central implants were perpendicular and the other implants had the divergence of 15 and 25º. The implants and Teries were fabricated using acrylate and polymeric acryl. A total of 30 tery were fabricated (n= 15 in each group). In group A, Open tray with splinted impression copings and group B with splinted impression copings non-splinted. Polyether impression material (3m ESPE Impregum, USA) with 4 mm thickness was used to make the impression. In order to make the main casts, stone plaster type IV were used.

The Positioning Of The Interspaces

A₁: distance between anteroposterior analogous

A₂: distance between distal-lateral proximal left and right analogous

A₃: distance between mediolateral distal left and right analogous

A₄: distance between mediolateral distal left and right analogous

Then, Splinted and non-splint in open-tray impression techniques were evaluated. Each casts were measured by coordinated measuring machine (CMM) device for implant position. (Zeiss, Industrial Mess Technique, Oberkochen, Germany). The accuracy of CMM for the x, y and z axes was <0.0001 mm. The same operator used probe head and single probe in all measurements. Umess software (SW, Umess UNIX/ LINUX, Zeiss, Oberkochen, Germany) was used for geometric transformation and data collection. The content of dimensional changes in transfer of implant positions was reported in for the four interspaces (A₁, A₂, A₃ and A₄).

Statistical Analysis

The operator was blind about test groups. Multivariate two-way analysis of variance (ANOVA) was undertaken to determine whether significant differences existed between groups and one sample t-test was used to compare the test groups with master model (P<0.05). Results

According to the results (table 1), significant difference detected in interspace of perpendicular and angled implant during casting by Splinted (19.014±0.04) and non-Splinted (18.896±0.05) impression techniques in A₁ position (P=0.0001).

Table 1: interspace of perpendicular and angled   implant during casting by Splinted and non-Splinted impression techniques in A1 position
	Mean ± SD (mm)	P. Value
splinted	19.014±0.04	0.0001
Non-splinted	18.896±0.05

As seen in table 2, no significant difference detected on interspace of perpendicular implant during casting by Splinted (15.763±0.01) and non-Splinted (15.772±0.01) impression techniques in A₂ position (P=0.143).

Table 2: interspace of perpendicular implant during casting by Splinted and non-Splinted impression techniques in A2 position
	Mean ± SD (mm)	P. Value
splinted	15.763±0.01	0.143
Non-splinted	15.772±0.01

Based on the results (table 3), interspace of 15 and 25º angled implant during casting by Splinted and non-Splinted impression techniques in A₃ position were 62.619±0.05 and 62.664±0.02, respectively (P=0.005).

Table 3: interspace of 15 and 25º angled implant during casting by Splinted and non-Splinted impression techniques in A3 position
	Mean ± SD (mm)	P. Value
splinted	62.619±0.05	0.005
Non-splinted	62.664±0.02

A significant difference detected for interspace of 15 and 25º angled implant during casting by Splinted (54.019±0.05) and non-Splinted impression (54.063±0.02) techniques in A₄ position (P=0.005) (table 4).

Table 4: interspace of 15 and 25º angled implant during casting by Splinted and non-Splinted impression techniques in A4 position
	Mean ± SD (mm)	P. Value
splinted	54.019±0.05	0.005
Non-splinted	54.063±0.02

The results (table 5) of the changes in interspace of perpendicular and angled implant using Splinted and non-Splinted impression techniques in A₁ position is presented in table 5. No significant difference detected using splinted (0.01 mm) impression technique in A₁ position (P=0.13) while significant change observed using non-Splinted (0.999 mm) impression (P=0.0001).

Table 5: the changes in interspace of perpendicular and angled implant using Splinted and non-Splinted impression techniques in A1 position
	Difference (mm)	P. Value
splinted	0.01	0.13
Non-splinted	0.999	0.0001

As seen in table 6, no significant difference detected on implant position using splinted (0.2 mm) and non-Splinted impression (0.3 mm) techniques in A₂ position (P=0.45).

Table 6: the changes in interspace of perpendicular implant using Splinted and non-Splinted impression techniques in A2 position
	Difference (mm)	P. Value
splinted	0.2	0.45
Non-splinted	0.3	0.45

Furthermore, no significant difference detected on changes in interspace of 15 and 25º angled implant using splinted (0.02 mm) and non-Splinted (0.03 mm) impression techniques in A₃ position

No significant differences detected on interspace changes of 15 and 25º angled implant using Splinted (0.01 mm) impression (P=0.45) while significant difference detected using non-Splinted impression (0.054 mm) (P=0.0001).

Discussion

Based on the results, it seems, splinted impression technique is recommended for angulated implants. Many clinical studies emphasize the passive fit of implant-supported superstructures for the long-term success of treatment. The mean change in distances between analogues in samples in the anteroposterior direction was more than mediolateral direction compared with the original model (Lee et al. 2008). Some studies reported a higher accuracy for open impression technique than closed impression technique when impression was made from 4 or more implants (Papaspyridakos et al. 2012). Many studies have recommended splinting of implants to increase the accuracy of the impression, although the success of this technique is questionable (Lee et al. 2008). Splinting can be done with different materials such as autopolymerized acrylic resins, light-polymerized acrylic resins or dental stones (Holst et al. 2007). Splinting with acrylic resin may be difficult and time-consuming and distortion of this material may be a problem (Holst et al. 2007).

Table 7: the changes in interspace of 15 and 25º angled implant using Splinted and non-Splinted impression techniques in A3 position
	Difference (mm)	P. Value
splinted	0.02	0.45
Non-splinted	0.03	0.45

In a study, Balouch (2013) in a study, based on their report dimensional changes were 129 ± 37 and 143.5 ± 43.67μ in closed tray and open tray, while coefficient of variation in closed-tray and open tray were reported to be 27.2 and 30.4%, respectively. Closed impression technique had less dimensional changes in comparison with open tray method, so this study suggests that closed tray impression technique is more accurate (Balouch 2013).

Table 8: the changes in interspace of 15 and 25º angled implant using Splinted and non-Splinted impression techniques in A4 position
	Difference (mm)	P. Value
splinted	0.01	0.45
Non-splinted	0.054	0.0001

Among the impression making methods presented in the literature, the splinted technique has gained popularity and has proven to be the most accurate (Assunção et al. 2008). The splinted direct techniques use square transfer copings, connected to each other with a rigid material, in a customized open impression tray. Although different materials have been tested to splint impression copings, such as composite resin, impression plaster, and stainless steel pins; acrylic resin, alone or in combination with dental floss, is the material used most often to prevent individual coping movements during the impression-making procedure (Del Acqua et al. 2010). Even though there was no consistent result for higher accuracy with any one technique as opposed to the other, splint or non-splint, more number of studies has reported increased accurate implant impressions with the splint technique than with the non-splint technique (Vigolo et al. 2005).

The accuracy of a splinted impression technique depends upon its resistance to deformation under the forces of impression material. Thus, theoretically, a technique that uses a more rigid splint material would produce a more accurate master cast. Therefore, the rigidity and dimensional stability of a metal framework in combination with impression plaster might make it a good choice for splinting the impression copings (Lee et al. 2008). The splinting technique using light cured acrylic resin was significantly less accurate than by using autopolymerizing resin or by impression plaster. This may be caused by the incomplete polymerization of the light cured acrylic resin; another reason may be that the shrinkage during polymerization of the light cured acrylic resin creates stresses at the impression coping acrylic resin interface (Assunção et al. 2008). Also, Daoudi et al. (2004) compared the closed tray technique at the implant level with the open tray technique at the abutment level for single tooth implants and found the open tray technique to be superior and more predictable. Furthermore, Carr (1992) compared the open and closed tray techniques with a 5 implant mandibular cast where the interabutment divergence angles were all less than 15 degrees. The open tray technique was found to be superior as it provided the most accurate working cast.

Some of the studies that advocated the splinting technique over the non-splinted technique have shown further that splinting with autopolymerized acrylic resin, sectioned post-setting, and rejoined, yielded the best results within the various splinting group combinations. However, this finding was not applicable to all studies that examined the effect (Rustum Baig, 2014). Splinted direct technique was found to be the most accurate for multi-unit situations with two highly unparallel (20- to 25-degree divergence) implants. This finding was, however, in disagreement with a few other studies that showed no correlation between direct splint and non-splint (Filho et al. 2009). 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 (Alikhasi et al. 2013). Some non-splint techniques have shown improved accuracy in comparison to splinted methods, achieved through impression coping modification prior to impression making (Lee et al. 2008). In conclusion, these results suggested splinted impression technique is recommended for angulated implants.

References

1. Alikhasi M, Bassir S, Naini R.(2013) Effect of multiple use of impression copings on the accuracy of implant transfer. Int J Oral Maxillofac Implants ;28: 408–414.
2. Alikhasi M, Siadat H, Rahimian S.(2013) The Effect of Implant Angulation on the Transfer Accuracy of External-Connection Implants. Clin Implant Dent Relat Res. Dec 3. doi: 10.1111/cid.12185
3. Assunção WG, Cardoso A, Gomes EA, Tabata LF, dos Santos PH. (2008) Accuracy of impression techniques for implants. Part 1 – influence of transfer copings surface abrasion. J Prosthodont. 17: 641-7.
4. Assuncao WG, Filho HG, Zaniquelli O. (2004) Evaluation of transfer impressions for osseointegrated implants at various angulations. Implant Dent 13:358-66.
5. Balouch F., Jalalian E., Nikkheslat M., Ghavamian R., Toopchi Sh., Jallalian F., Jalalian S.(2013) Comparison of Dimensional Accuracy between Open-Tray and Closed- Tray Implant Impression Technique for 15° Angle Implants. J Dent Shiraz Univ Med Sci, Sept. 14(3): 96-102.
6. Carr AB. (1992) Comparison of impression techniques for a two-implant15-degree divergent model. Int J Oral Maxillofac Implants. 7:468-75.
7. Conrad HJ, Pesun IJ, DeLong R, Hodges JS. (2007) Accuracy of two impression techniques with angulated implants. J Prosthet Dent 97:349-56.
8. Daoudi MF, Setchell DJ, Searson LJ. (2004) An evaluation of three implant level impression techniques for single tooth implant. Eur J Prosthodont Restor Dent. 12:9-14.
9. Ebadian B, Rismanchian M, Dastgheib B, Bajoghli F. (2015). Effect of different impression materials and techniques on the dimensional accuracy of implant defi nitive casts. Dental Research Journal 12(2): 136-143.
10. Filho HG, Mazaro J, Vedovatto, E, Assuncao W, Santos P.(2009) Accuracy of impression techniques for implants. Part 2: Comparison of splinting techniques. J Prosthodontics 18:172–176.
11. Holst S, Blatz MB, Bergler M, Goellner M, Wichmann M.(2007) Infl uence of impression material and time on the 3-dimensional accuracy of implant impressions. Quintessence Int 38: 67-73.
12. Lee H, Ercoli C, Funkenbusch PD, Feng C.(2008) Effect of subgingival depth of implant placement on the dimensional accuracy of the implant impression: an in vitro study. J Prosthet Dent 99:107–113.
13. Lee H, So JS, Hochstedler JL, Ercoli C.(2008) The accuracy of implant impressions: A systematic review. J Prosthet Dent 100:285-91.
14. Lee YJ, Heo SJ, Koak JY, Kim SK. (2009) Accuracy of different impression techniques for internalconnection implants. Int J Oral Maxillofac Implants. 24(5):823-30.
15. Mirza Rustum Baig. (2014) Multi-unit implant impression accuracy: A review of the literature. Quintessence Int 45:39–51
16. Nayar S, Rathinavel PM, Bhuminathan S, Raghavendra jayesh S, Vidhya J. (2015) Impression technique in implantology: A Review. RJPBCS. 5(2): 1934- 1940.
17. Papaspyridakos P, Benic GI, Hogsett VL, White GS, Lal K, Gallucci GO. (2012)Accuracy of implant casts generated with splinted and non-splinted impression techniques for edentulous patients: An optical scanning study. Clin Oral Implants Res 23:676-81.
18. Papaspyridakos P, Lal K, White GS, Weber HP, Gallucci GO. (2011) Effect of splinted and nonsplinted impression techniques on the accuracy of fit of fixed implant prostheses in edentulous patients: a comparative study. Int J Oral Maxillofac Implants. 26(6):1267-72.
19. Prakash S, Chowdhary R. (2016). Impression techniques and impression materials in dental implant supported restorations- a systematic review. International Journal of Recent Scientific Research. 7(4): 10285-10295.
20. Prithviraj DR, Pujari LM, Garg P, Shruthi DP (2011) Accuracy of the implant impression obtained from different impression materials and techniques: review. J Clin Exp Dent 3(2):e106–e1118.
21. Vigolo P, Fonzi F, Majzoub Z, Cordioli G.(2005) Master cast accuracy in single-tooth implant replacement cases: an in vitro comparison. A technical note. Int J Oral Maxillofac Implants. 20:455-60.
22. Del Acqua MA, Chavez AM, Castanharo SM (2010) The effect of splint material rigidity in implant impression techniques. Int J Oral Maxillofac Implants. 25:1153-1158.