Proximal contact tightness for Class II direct composite resin restorations: a literature review

Matrix system type

The two most common types of contemporary matrix systems available for posterior composites are circumferential and sectional. There is a now wide range of matrix systems on the market specifically designed for use with composite resin, some of which are listed in Table 1.

Mackenzie et al summarized what the ideal matrix system should achieve in Table 2.³

Contemporary sectional matrix systems with separating rings are designed to meet the aims in Table 2 and have been shown to achieve greater PCT than circumferential matrix bands.^{5,6,7,9,10,12}

A randomized controlled trial (RCT) was carried out in a general practice setting by Loomans et al to investigate the clinical changes of PCT when restoring Class II cavities.⁷ In this study, three different matrix systems, listed in Table 3, were assigned randomly to a moderate sample size of 71 pre-wedged cavities using a computer programme. PCT was assessed using a tooth pressure meter (TPM), which is a method widely regarded as a reliable technique and has been used in many laboratory and clinical studies.^{67,8,9,10,12,13,16} Using an incremental build-up technique with Clearfil AP-X micro-hybrid composite (Kuraray, Germany) for all groups, the results showed that the PCT decreased for the circumferential matrix group and increased for both sectional matrix systems compared to the pre-treatment values, and the differences were statistically significant. There was no statistical significance in PCT between the restorations placed with the two sectional matrix systems despite difference in operator experience with the systems.⁷

A more recent RCT by Wirsching et al was conducted in a secondary care setting with students of the clinic placing two- and three-surface posterior composite restorations in 85 patients.¹⁰ There were only two matrix system groups in this case: sectional (Palodent) and circumferential (Tofflemire pre-contoured metal band; Hawe, UK) and all the measurements were conducted by one investigator using the TPM.¹⁰ The sectional matrix system increased, whereas the circumferential matrix decreased the PCT compared to the pre-operative readings for both the two- and three-surface cavities.¹⁰

One of the main findings from reviewing the available literature is that using sectional matrices with separation rings creates tighter contacts than circumferential matrix systems. This is a consistent finding in all in vitro and in vivo studies that compared different matrix types including the two RCTs described above.^{5,7,9,10,12,16,18}

Despite the evidence that sectional matrices with the use of separating rings are superior in achieving tighter contacts, their use has not been as high as one would expect. Only 10% of respondents in the survey-based study by Gilmour et al used sectional metal matrix systems to restore occlusoproximal cavities, whereas 61% said they used circumferential matrix systems.¹⁹ The reasons for this were suggested to be the increased cost of the sectional matrix systems, the increased time required and the lack of training and confidence in using them.

Different types of separation and wedges

The RCT by Loomans et al concluded that the use of the separation ring device was the most influential factor in achieving good PCT.⁷ Saber et al also supported this view because it showed that the use of separating rings, irrespective of the type of matrix, increases the PCT and furthermore suggested that the rings are superior to the use of wedges as they produce constant separation at the height of the proximal contact, rather than temporary rotation or elongation at the point where the wedge is inserted.¹²

This concept was investigated further in a clinical study by Loomans et al where 27 healthy patients with sound dentitions were selected and randomly assigned to one of two separation approaches to assess the effect of wedges and separation rings on PCT.¹³ The methodology used a split-mouth approach with two independent observers performing the measurements with the TPM. One contact in the fourth quadrant had either a wooden wedge or a separating ring placed, and the contralateral contact in the third quadrant had both a wooden wedge and a separating ring placed.¹³ The results further supported the finding that separation rings are more influential in regard to PCT as they produced a prolonged effect on adjacent teeth compared with the shorter effect observed with wedge placement.¹³ This agreed with previous evidence that contact tightness recovers considerably over a short period of time after the initial insertion of a wedge.²⁰

Regarding wedges, most associated studies, including Loomans et al, reference wooden wedges that were observed to absorb water and become soft during use, affecting their ability to separate the teeth.^7,9,12,13,20 Newer products, such as Wedge Wands (Optident, Ilkley, Yorkshire), are now available that are made of more rigid materials and have aids, such as handles, to make them easier to use, which can either be bent and left in situ or twisted and removed. Another example is FlexiWedges (Common Sense Dental Products Inc, Springlake, MI, USA) (Figure 3). They also have shapes designed to encourage separation, such as greater taper and ridges that help to compress the interdental papillae, but do not deform the matrices.³ However, there is currently a lack of evidence to confirm whether these have a positive effect on PCT.

Material selection of matrix bands

Clinically, metal matrices are favoured over transparent ones because they can be burnished, are more rigid and tend to be thinner, and easier to place. Clear matrices usually require the contact to be cleared horizontally and vertically, which may result in a more destructive preparation than is necessary.¹¹ However, Kampouropoulos et al showed no significant difference between clear and metal matrix bands in relation to PCT.⁵

Another RCT that reviewed restorative criteria of Class II direct composite restorations over a 2-year period, demonstrated that the PCT significantly decreased over time for restorations placed with transparent matrices compared to metal matrices.²¹ However, the reliability of the results from this study could be questionable owing to the more subjective assessment approach.

Width of the matrix band

Another study by Loomans et al suggested that the thickness of band may have influenced the PCT because the thinner sectional bands performed better than the thicker circumferential bands.⁶ However, in various comparable studies, ultrathin bands of 0.035–0.04 mm did not always produce a lower PCT than wider 0.05-mm bands. Also, bands of the same width produce significantly different PCT.^5,10,12,16 Furthermore, in one study the 0.05-mm Palodent sectional matrix created a significantly greater PCT than the 0.038mm and 0.04-mm circumferential bands.⁵

Contoured versus straight matrix bands

Two in vitro studies were found comparing both straight and contoured variants of the same type of matrix. In these cases, the convexity of the circumferential matrices tested had no significant effect on the PCT.^5,6

Displacement of matrix towards the adjacent tooth

It is widely recommended that matrices must be burnished or held against the adjacent tooth to ensure good embrasure anatomy and therefore ensure the contact can be cleaned effectively.³ The displacement of the matrix band towards the adjacent tooth contact has evidence both for and against improving PCT.^{5,7,12,17,18,22}

Saber et al demonstrated no significant difference in the use of circumferential bands with or without using hand-instruments to hold the matrix to the adjacent tooth during setting.¹²

Other techniques have been developed where the operator uses instruments to apply pressure to the contact area during light curing. Some of these techniques were compared in a study by El-Mowafy et al (Table 4).²² The study showed that using the Beta Quartz glass-ceramic inserts held in place with a hand instrument, improved the number of ‘clinically acceptable’ contacts compared to the control group. However, the subjective assessment approach and the fact that the ceramic inserts were encased in the composite without subsequent evaluation of its impact, means this approach requires further research.²²

Table 4. Results of study to assess four different contact forming techniques.22

Type of restoration	Open	Not tight enough	Ideal	Too tight	Clinically acceptable*
Amalgam (control)	0	5	20	5	25
Heavy wedging only (Group 1)	25	5	0	0	0
Light-up attachment (Group 2)	3	13	14	0	14
Contact Pro (Group 3)	0	11	19	0	19
Glass-ceramic insert (Group 4)	0	3	27	0	27

* Contacts that were ideal or too tight were considered clinically acceptable; those that were open or not tight enough were considered clinically unacceptable.

Other contact forming instruments exist, such as the LM-Contact Former (LMDental, Pargas, Finland), the Dr Belvedere Composite Contact Former (American Eagle, Young Innovations Europe GmbH, Heidelberg, Germany) metal instrument and those shown in Figures 4 and 5; however, the available evidence for their effectiveness is limited.^17,18,23

These contact forming instruments work by placing the instrument into the uncured composite and applying pressure on the inside of the matrix band towards the adjacent tooth (Figure 6) and then curing through the transparent material. The void left by the instrument tip is then filled with the final increment of composite.

In summary, there is limited and conflicting evidence as to whether displacing the matrix towards the adjacent tooth increases the PCT and how different instruments compare against each other.

Viscosity of composite material

Due to the satisfactory PCT achieved when condensing amalgam against matrix bands it has been suggested that more ‘packable’ or ‘condensable’ composites may achieve tighter contacts compared to more flowable varieties.²⁴ Some studies have assessed whether the viscosity or type of composite materials has an influence on the PCT.^16,18,25

One in vitro study looked at a variety of different materials including flowable, bulk-fill and Sonicfill types that were all assessed with two types of sectional matrices and one circumferential matrix.¹⁶ The methodology was sound and used the TPM in 105 plastic teeth with an independent measurer who was blinded to the type of material and matrix used. The main finding of the study was that, despite the fact that Sonicfill contains modifiers claimed to reduce the viscosity by 87% during placement, this material produced similar contact tightness results as the much more viscous bulk-fill (Tetric EvoCeram Bulk-fill, Ivoclar Vivadent, New York, USA) and nano-hybrid (Tetric EvoCeram) composites.¹⁶. In comparison, the flowable materials, G-ænial Universal Flo (GC America Inc, Alsip, Illinois, USA) and SDR (Dentsply), created contacts with significantly reduced PCT.¹⁶

In another in vitro study, Loomans et al reported a stronger statistical significance for PCT when using medium- or high-viscosity composite.²⁵ Peumans et al evaluated the influence of viscosity of composite on the PCT using a different type of proximal contact assessment where different sizes of blades were passed through the contacts and assessed by someone other than the operator.¹⁸ They concluded that the type of composite did not play a significant role in achieving tighter contacts when the sectional matrix was used, but that the most condensable composite did create a slightly better PCT value when the circumferential matrix was used.¹⁸ A strength of this study was that they assessed the differences in results produced by the two operators and showed that the more experienced clinician achieved higher PCT values, which may be another factor for further research.¹⁸

The claim that using a more viscous material to increase PCT has evidence to both support and refute it. It has been suggested that the more likely influencing factor regarding material selection is the polymerization shrinkage, in that materials that shrink less on setting should create tighter contacts.¹⁶

Polymerization shrinkage

The impact of polymerization shrinkage is shrinkage stress, which is a well-documented concept. The consequences include marginal gap formation, sensitivity, marginal staining and secondary caries.¹⁵ An in vitro study with a moderate sample size, 60 cavities in plastic teeth, looked at the effect of polymerization shrinkage on PCT. Two types of composite were compared; one traditional methacrylate-based material with high shrinkage (Z100, 3M ESPE, location; 2.26–2.61%) and one low-shrinkage silorane-containing material (Filtek Silorane, 3M ESPE; <1%).¹⁵ The methodology was consistent with similar studies discussed, and the low shrinkage composite achieved significantly higher PCT values compared with the high shrinkage materials.^7,10,13,15 The results reported by El-Shamy et al supported this finding with low-viscosity, high-shrinkage materials creating looser contacts.¹⁶

Another factor that may also have an influence on PCT is composite placement technique, for example bulk-fill versus incremental techniques, although it is expected that this factor is more likely to be associated with the extent of shrinkage stress seen with different materials, and requires more research.

Changes to contact tightness over time

One of the suggestions in Loomans et al was that PCT may change over time.⁷ Subsequent studies have shown that contacts can loosen over time, probably due to wear of the material or adaptation of the periodontium.^8,21 Furthermore, reduced PCT values after treatment have been shown to improve over time, although they remain significantly weaker.⁸ It is worth mentioning that if the operator has produced a tight contact immediately after composite placement, then conventional tweezers may not grip the matrix firmly enough to remove it, hence artery forceps are an essential part of the clinician's armamentarium when placing composite restorations in posterior teeth.