Ask the Expert: An Interview with Nate Lawson, DMD, PhD on Sectional Matrix Systems

A circumferential matrix placed in a Tofflemire retainer is necessary for placing an amalgam restoration. The reason is that amalgam is forcefully condensed against the matrix band and it needs to maintain a tight seal against the tooth. With direct composite restorations, the composite material is no longer forcibly condensed against the matrix. Rather it is flowed or packed into the tooth preparation. There are several advantages of using sectional matrices. First, sectional matrix bands provide more incisal-gingival contour than flat circumferential matrix bands. Second, a sectional matrix is often easier to place on a two-surface preparation than a circumferential band because the circumferential band must pass through the unbroken contact on the opposite side of the tooth. Third, the use of a separating ring with a sectional matrix leads to a stronger, consistent contact than the use of a circumferential band and a wooden wedge.

The DualForce Ultra-Wrap Matrix Bandhas an anatomical contour that differentiates it from earlier bean-shaped sectional matrices. The matrix has a lip on the occlusal edge of the band that helps to form the occlusal embrasure. Additionally, the matrix has a gingival apron (that can be trimmed) that extends gingival beyond the bottom of the box to help adapt the matrix to the tooth while still forming a cleansable emergence profile. The matrix has a U-shaped contour in the buccal-lingual direction that helps to produce a wide buccal-lingual contact. The thickness of the matrix is about 35 microns, however, it is not dead soft. The firmness of the matrix helps to prevent crimping when inserting the matrix and deformation while adjusting the matrix.

If the matrix is too big or small, it will affect the incisal-gingival location of the contact point and complicate sculpting of the uncured composite. The matrix should extend slightly gingival to the bottom of the box and about 0.5 mm occlusal of the adjacent marginal ridge. Therefore, the clinician may measure the distance from the bottom of the box to the height of the adjacent marginal ridge with a periodontal probe, and a matrix should be chosen to be about 1 mm taller than that distance. This will allow the matrix to extend beyond the bottom of the box and slightly above the adjacent marginal ridge. 

With two pounds of separating force provided by a separating ring, it is no longer necessary for a wedge to provide the primary source of separation. The major function of a wedge is to seal the gingival box. TheDualForce Active-Wedge is an anatomically contoured wedge. Unlike a traditional wooden wedge, the DualForce Active-Wedge is wider in the mesial-distal dimension on either side of the contact point. It is hollow so that its tip can be passed through the contact point and then automatically expands on the other side. Also the wedge is shorter incisal-gingival at the contact point than on either side of the contact point. Therefore, this geometry prevents the wedge from encroaching into the contact area.

The wedge should be placed from the direction of the larger embrasure. Typically this will be the lingual embrasure; however, it may be more beneficial to place the wedge from the side of the tooth (relative to the contact point) that has received more preparation. If more of the buccal aspect of the tooth was prepared than the lingual aspect, the wedge would serve more benefit by adapting the matrix to the buccal aspect.

We have compared the separating force produced by several popular separating rings. The rings were tested by connecting the opposite ends of two adjacent typodont teeth (mesial of 1st premolar and distal of 2nd premolar) to a load sensor. Different rings were placed on the premolars and the force placed on the load sensor was recorded. The DualForce Ring recorded the highest separating force of any ring tested. This separating force may be attributed to the two independent rings. Additionally the contour of the feet allow tight adaptation of the matrix to the buccal and lingual walls of the preparation to prevent excess composite flash.

Side-by-side Class II preparations present an additional clinical challenge as the space between the two teeth must be properly allocated and if restored at the same time, the thickness of two matrix bands must be compensated for. A predictable but less efficient method is to restore the teeth individually. This offers the advantage of allowing adjustment of the first restoration and only using one band for the final restoration. If both restorations are attempted at the same time, the powerful separation force of the DualForce Ring may help compensate for the thickness of two bands.