The Influence of Matrix Type on the Proximal Contact in Class II Resin Composite Restorations Open Access

The increased demand for esthetic restorations and the emphasis on minimally invasive restorative procedures has led to an expanded use of resin composites for posterior restorations. Consequently, new clinical and research issues have emerged due to the sensitivity of the restoration technique with resin composites, especially in cases of Class II restorations.

One of the challenges associated with the use of resin composite restorations is the reconstruction of an intact proximal surface, and particularly, a proper interproximal contact area.1–2 This objective pertains to reproducing the natural proximal contour of a contact that is tight enough to prevent food impaction, which is crucial for the healthy maintenance of underlying periodontal tissues.3–5 Increased gingival inflammation and attachment loss, apart from the presence of overhanging restorations,6–7 have also been attributed to plaque accumulation due to loose proximal contacts.5–6 However, alveolar bone loss is not directly attributed to open interproximal contacts but is strongly related to the overall periodontal status of the patient.8 Annoyance and discomfort have been reported by patients who experience food impaction4 at sites of open contact. On the other hand, the contact point must not be too strong, in order to avoid the shredding and impaction of dental floss fibers interproximally or periodontal trauma induced by excessive force applied during flossing.9 A tight proximal contact can be more easily achieved with amalgam restorations by taking advantage of the condensing forces that can be applied to the material and the matrix towards the adjacent tooth. However, the difficulties in placing a resin composite, owing to its viscoelastic properties10–12 and polymerization shrinkage, 13–14 preclude the reconstruction as a proper proximal contact in Class II posterior restorations.

Several measures that involve the use of composites with various consistencies, such as high-viscous materials called “packable”9,15 composites with pre-polymerized particles16 or glass ceramic inserts,17–18 have been proposed to cure this deficiency and increase the packing force capacity with varying success. Other steps to improve the performance of resin composites in achieving proper proximal contacts include multilayer techniques16,19 and photopolymerization protocols, 20 which aim to reduce polymerization shrinkage. In addition, means, such as special instruments for resin composite placement,15–16 wedges, separation rings21 and a variety of matrix systems,15–16,22–26 have been used in an attempt to achieve the optimum proximal contour.

Two primary issues are associated with establishment of the proximal surface: surface contour and contact tightness. A few in vitro15,22,24 and in vivo23,25–27 studies have reported the effectiveness of some matrix systems to the formation of contact tightness. However, commercial matrices are continuously developed for achieving good proximal contact. Pre- and non-contoured, straight, sectional and circumferential metal or plastic matrix bands, combined with separation rings with or without the use of retainers, have been introduced. However, no studies have investigated the role of the matrix systems in efficiently reconstructing the contact contour of the intact tooth in Class II resin composite restorations.

This in vitro study evaluated four different matrix systems in terms of reproducing the proximal contact contour and location of the contact point, as well as measuring the contact tightness achieved in Class II resin composite restorations. The null hypothesis was that no differences exist among the matrices examined in reforming the proximal contact area and contact tightness.

For purposes of the current study, one frasaco manikin model (model AG-3DA, frasaco Gmbh, Tettnang, Germany) with all the plastic teeth placed and screwed into their sockets was used. The second premolar (#45) was replaced by a metal cast (CoCr) duplicate. The cast premolar was permanently fixed into the socket of the model. A standardized mesioocclusal (MO) cavity was prepared in the artificial mandibular first molar of the typodont (#46). The MO cavity received standardized shape and dimensions. A removable metallic cast guide was used for a standardized preparation of the occlusal and proximal outline of the dimensions of the cavity (Figure 1). The width and depth of the cavity were obtained by two different types of diamond burs in a high-speed handpiece (Brasseler USA, Savannah, GA, USA–#830 314 014 for the occlusal part and #835 314 014 for the proximal box). The dimensions of the proximal box of the prepared cavities were 1.5 mm mesio-distally, 4 mm occluso-gingivally and 4 mm bucco-lingually. The occlusal part was extended 2.5 mm mesio-distally, 2 mm occluso-pulpally and 2 mm bucco-lingually.

In total, 40 molars were used in the current study. After cavity completion, each molar was inserted and screwed firmly into the corresponding socket in order to be filled with resin composite (Spectrum THP-3, Dentsply DeTrey GMBH, Kostanz, Germany, compule, shade A1, Batch #0804003336). The molars were divided into four groups of 10 specimens each and the following matrices were used: i) circumferential straight metal matrix (Matrix Band, E Hahnenkratt GmbH, Königsbach-Stein, Germany, thickness 0.040 mm, Batch #11090) with Tofflemire retainer–A, ii) circumferential pre-contoured metal matrix system (Supermat Adapt SuperCap Matrix, KerrHawe SA, Bioggio, Switzerland, thickness 0.038 mm, Batch #3013365)–B, iii) circumferential pre-contoured transparent matrix system (Supermat Adapt Super cap Matrix, KerrHawe SA, thickness 0.050 mm, Batch #2964266)–C and iv) sectional pre-contoured metal matrix band combined with a separation ring (Palodent system standard matrices, Dentsply DeTrey GMBH, thickness 0.050 mm, Batch #080122)–D. In addition, there was a fifth group (REF) that consisted of intact frasaco mandibular first molars (n=10) used as a reference group.

The metal matrices (A, B and D) were secured interdentally with wooden wedges (Batch #26070700), while the transparent matrix (C) was held with transparent wedges (Batch #19070701) (Polydentia SA, Mezzovico, Switzerland). All the wedges were inserted fromthe lingual side. The resin composite was inserted into the cavity in three layers. The first layer was placed and pushed towards the matrix to form the proximal surface of the restoration with a mesio-distal thickness of 1 mm. The rest of the proximal box was completed, up to the level of the pulpal wall, at a cervico-pulpal thickness of 2mm. Finally, the remainder of the occlusal portion of the cavity was filled at an occluso-pulpal depth of 2mm. Each layer was cured occlusally for 20 seconds with an LED light-curing unit (Cure TC-01 220V, Spring Health Products, Inc, Norristown, PA, USA, 800 mW/cm²). After removal of the matrix and wedge, the restorations received additional photopolymerization from the buccal and lingual sides of the box for 20 seconds each. The restorations were not finished or polished. All the restorations were performed by one operator (DK).

To evaluate certain characteristics in the proximal surface of the restorations that were placed, the manikin model was fixed by the authors of the current study onto a testing machine (Tensometer 10, Alpha Technologies UK, Swindon, United Kingdom) with use of a custom-made setup that allowed the forces to be applied vertically at the interdental area of interest (Figure 2). An orthodontic wire (0.30 mm, GAC Orthodontic Laboratory, Racine, WI, USA) was then inserted under the contact area in a bucco-lingual direction. The wire was mounted on a custom-made retainer designed to fit on the other end of the tensionmeter; it helped to sustain the wire at a horizontal position throughout the movement (Figure 2). The wire was moved at a speed of 5 mm/minute in the occlusal direction and the resistance force was recorded during the movement. Each proximal contact was measured once and each wire was used to conduct only three consecutive measurements.

The obtained data were then processed to assess: a) the maximum force (F_max) recorded, which expressed the contact tightness (CT), b) the length of the contact arc (LCA) calculated as the distance in the x-axis between the first force recording and the wire release positions (Figure 3), c) the % ratio of the position of the maximum force (F_max) in the x-axis and d) the LCA (PCP), which represented the location of the contact point (Figure 4).

The values were subjected to statistical analysis by: i) one-way ANOVA followed by the post-hoc Tukey test to define differences among the four matrices per parameter assessed (α=0.05) and ii) the unpaired t-test to compare separately each of the matrices with the reference group for the LCA and PCP parameters (α=0.05).

Representative curves recorded during the experimental procedure from all groups of restorations performed by the four matrix systems are showed in Figure 4. The mean and standard deviation values for contact tightness (CT), length of contact arc (LCA) and % ratio of the position of the maximum force to LCA (PCP) values are summarized in Table 1. The one-way ANOVA analysis and the unpaired t-test results are presented in Tables 2 and 3, accordingly.

The sectional pre-contoured matrix system (D) presented the highest contact tightness; whereas no statistically significant differences were found among the other three matrices.

Regarding the LC, the teeth restored with the sectional pre-contoured matrix system (D) were found to have the longest LCA compared with all the other matrices. Among the remainder of the matrices evaluated, the LCA values were significantly higher in the case of the circumferential metallic pre-contoured matrix (B) and no differences were observed between matrices A and C. Likewise, no statistically significant differences were found for the LCA of teeth restored with the transparent matrix (C) and straight circumferential matrix (A) relative to the intact tooth values (reference). However, use of the circumferential metallic pre-contoured matrix (B) and sectional pre-contoured matrix system (D) was associated with higher LCA than the intact molars.

Regarding PCP values, matrix systems A and B presented the highest values, with no statistically significant difference between them, followed by group C. The lowest values of all groups were observed in matrix D. Moreover, the PCP values of restorations in all the matrix groups were statistically significantly different from the PCPs of the intact tooth.

The tightness and position of the point of contact between adjacent teeth and the contour of the proximal surfaces are critical factors to the health of periodontal tissues. 3–5 ,In vivo, the contact strength is influenced by several factors, including tooth type, location of the tooth, time of day,3 postural change,28 periodontal condition of the tooth29 and showing a high individual variability. The foregoing parameters are difficult to simulate in laboratory conditions and, at the same time, their impact cannot be easily assessed in vivo. The problems of simulating the clinical analogue and relevance of the results of the current study to a clinical situation relate to the fact that the teeth are fixed firmly in their stiff sockets on the manikin model, thus precluding the presence of physiologic tooth movement.30 On the other hand, the stiffness of the manikin model system may be advantageous for purposes of this investigation, because the teeth that were examined allowed for the evaluation of each matrix under identical conditions, which greatly affects the comparability of the data obtained from the different matrices.

The contact point between posterior teeth is located at the transition of the middle to the occlusal third of the proximal surface cervico-occlusally,15 and the morphology of the proximal surface shows considerable variability. Therefore, matrix bands with predefined contour, convexity and height cannot be properly fitted to different tooth types. Practitioners tackle this challenge by combining the matrices with wedges or separation rings. In the current study, all the matrices were secured with one wedge each and it was always inserted from the lingual side, because the lingual triangular embrasure was wider than the buccal one.31 The single wedge was considered to offer adequate support and satisfactory adaptation of the matrix towards the restored tooth. The straight circumferential matrix lacks a contour but is more easily adjustable; whereas the pre-contoured matrices are believed to create more anatomically correct surfaces, 15 and the transparent matrix with reflective wedges might allow for better light transmission and allegedly more efficient polymerization of the underlying resin composite.32–33 

Due to viscous flow of the restorative materials before photoactivation, the matrix band, which was initially deformed during application of the first layer of composite, was observed to quickly return to its original shape, causing some loosening of contact with the adjacent tooth. This drawback may be eliminated by displacing the matrix towards the adjacent tooth9 prior to the restoration. However, in the current study, the matrices were not manipulated in such way, because this procedure is not easily reproducible and would initiate a confounding factor.

Occasionally, resin composites of several consistencies and different application protocols have been studied in terms of achieving the tightest contact point. The use of highly viscous materials9,15–17 and special instruments15,17 tends to be more efficient than materials of medium viscosity; whereas, the multi-layer technique showed better results.16–17,19–20 In the current study, the material of choice was a resin composite of medium viscosity due to the higher polymerization shrinkage of low viscosity composites22 and the lack of specific advantages of high viscosity resin composites.15 Regarding the experimental procedure, the resin composite was shaped properly and all excess was removed carefully before polymerization in order to avoid finishing with burs, leaving the proximal surface intact.

The measurements for evaluation of the proximal contact characteristics were conducted on a tensometer using a 0.30 mm-wide orthodontic wire, which was pulled interproximally in a cervico-occlusal direction. The custom-made setup assured that no horizontal forces were applied during the procedure, which is not achieved in vivo23–26 and could lead to false results. Studies on this field have employed metal blades22–24,27 or dental floss16–17 to semi-quantitatively assess the contact point strength. The use of this type and size of wire allowed for a more comprehensive evaluation of the proximal surface, relative to metal blades.22–24,27 This is because the amount of contact strength is the only characteristic that can be measured with a metal blade; whereas, with the use of a wire, a wider area of the proximal surface can be examined and the contour of the reconstructed proximal surface and a proportional cervico-occlusal position of the contact point can be assessed.

Each restoration was measured once because, during the pilot phase, it was observed that its proximal surface underwent slight attrition during each passthrough of the wire after multiple measurements. In addition, it was decided to perform three measurements with the same piece of wire, because, if used more often, usage caused rough graphs due to deformation of the active surface of the wire. Furthermore, replacement of the adjacent second premolar (#45) by a wear-resistant metal (CoCr) cast duplicate assured that its distal proximal surface would only sustain negligible wear during the restorative and measurement procedures.

The contact tightness (CT) measurement was expressed in N units as the maximum force recorded, while drawing the wire interproximally in a cervicoocclusal direction. The Palodent matrix system created, by far, the tightest contact between the restoration and adjacent tooth; this is confirmed by nearly every other previous study15,22–24,27 and guarantees longevity of a tight enough proximal contact point.27 There were not any statistically significant differences among all the other matrices, which created a contact point of inferior strength, irrespective of their unique characteristics.

The results of the current study suggest that the width of the matrix plays no particular role in CT, since the sectional matrix is as wide as the transparent one and is even wider than the straight and precontoured circumferential matrices. Additionally, the material of the matrix cannot be considered a contributing factor, since there were no statistically significant differences between matrices B and C. Similarly, the convexity of the matrices does not appear to have a predominant effect on the CT, since no difference was observed between matrices A and B. Apparently, use of the separation ring in addition to the wedge seems to be the sole significant factor that could explain the variation in CT data. Loomans and others, in an in vivo study for indirect assessment of the CT, reported that separation rings, either combined or not combined with wedges, induced better separation of teeth than merely using wedges.21 

It would have been very beneficial to compare the CT obtained with the chromium-cobalt premolar and restored teeth in each matrix group, with the CT between the CoCr premolar and intact teeth. However, this was considered unfeasible, because the following results were partially unsustainable. The actual measured forces were not tensile, but frictional forces developed between the metal wire and the two proximal surfaces. Because the acrylic resin surface of the intact first molars and the resin composite surface of the restored first molars constituted two distinct materials, no conclusions should be made based on the higher contact strength they provide. The reason is that the other matrices may create adequate CT relative to that of intact teeth, though lower than the one achieved by sectional matrices. In addition, there is evidence26–27 that tighter contacts tend to loosen after a period of time, probably due to proximal wear of the restorative materials or “adaptation mechanisms” of periodontium to compensate for tighter than necessary contact strength.

Apart from the contact point strength, the formation of an anatomically correct proximal contour reproducing the original surface is another prerequisite for a successful restoration. The convexity of the proximal surface is determined through the combination of LCA and PCP values. The means of LCA and PCP are independent of the material of the proximal surfaces. Therefore, unlike the values of CT, there is the opportunity to compare the values recorded on intact teeth, which allow better scientifically grounded conclusions.

High LCA values, such as in the case of the sectional pre-contoured matrix, might imply the presence of an over-contoured surface, and a small LCA is acquired from a rather flattened proximal plate, both of which are detrimental to the health of periodontal tissues. When LCA is combined with CT, characteristically, this drawback of the sectional pre-contoured matrix becomes clearer. The combination of high CT and high LCA values indicates a profound convexity of the proximal surface that strangles the interdental periodontal tissues. This phenomenon is also observed in the circumferential pre-contoured matrix, albeit to a lesser extent. The remaining two types of matrices provided less convexity than the former matrices, but they were statistically similar to that of intact teeth. This was not a surprise for the straight circumferential matrix, which structurally lacks any contour, but it was a surprise for the transparent matrix, which was pre-contoured. This behavior of the transparent matrix might be derived from the following facts: a transparent wedge was used, which had a slightly different morphology than the wooden wedges used for all the other metal matrices; polymerization shrinkage might have played its part, since better polymerization is achieved through transparent matrices and reflective wedges32–33 and finally, the nature of the material itself and its thickness (thicker than A and B matrices) might make this type of matrix easier to fit the tooth morphology than its metal analogue but less vulnerable to pressure.

The values of PCP display the position where the maximum force was recorded, which represents the contact point, in relation to the whole LCA that was measured. The higher the PCP, the more occlusally the contact point is located, and conversely, the lower the PCP, the more cervically the strongest contact is placed by the matrix. It is worth mentioning that, in the current study, none of the four types of matrices managed to recreate the contact point at a height similar to where it exists among intact teeth. More specifically, the teeth restored with a sectional pre-contoured matrix were the only ones that obtained a more cervical contact point than that of intact teeth.

This methodology cannot be linked directly to clinical practice, due to the fact that a rather thick orthodontic wire was used, and because of the artificial manikin model, where the teeth had no opportunity for micro-movement similar to that allowed by the periodontal socket. However, this methodology offers significant indications about the abilities and limitations of the most commonly used matrices. The relatively greater cervical contact point of the sectional pre-contoured matrix, combined with its high CT and LCA, amplifies the fact that the sectional pre-contoured matrix recreates an over-contoured proximal surface. Among the circumferential matrices, the one that shows the best adaptation is the transparent precontoured matrix, although its contact point is placed more occlusally than that of intact teeth at a statistically significant degree. Finally, the two metal circumferential matrices presented similar PCP, even statistically significantly higher than that of the transparent matrix. This finding was partly expected for the straight matrix. Due to the lack of contour, the contact point of the matrix with the adjacent tooth is transferred fairly occlusally on the proximal surface, but the metal pre-contoured matrix was presumed to resemble at least the transparent one. However, given that the straight circumferential matrix also showed similar CT, but more suitable contour than the precontoured one, the initial matrix was considered to be more efficient than the latter.

The three circumferential matrices established similar contact tightness, regardless of their individual characteristics. However, it was not possible to compare them with the contact tightness among intact teeth in order to decide whether it was adequate. The sectional pre-contoured matrix, although establishing the tightest contact of all systems assessed, reconstructed the worst proximal contour relative to that of the intact tooth in terms of preservation of the health of periodontal tissues. All four types of matrices evaluated in the current study failed to place the contact point at a position similar to that found in the intact tooth.

The authors thank Biomaterials Laboratory, School of Dentistry, University of Athens, Greece, for providing the equipment to conduct the measurements.

The study was presented as a poster under the title “The influence of matrix type on the proximal contact in Class II resin composite restorations. In vitro study” during the 4th ConsEuro 2009 Meeting on Prevention, Restoration and Aesthetics, which took place in Seville, Spain on 12–14 March 2009.

The study was funded by the project “Kapodistrias,” ELKE, University of Athens, Greece.