Sectional matrices and separating rings: the contact point is created by separation, not by thickness

The interproximal contact point is the detail patients check first, by passing floss. An absent contact causes food impaction; an overly tight one prevents floss passing and irritates the papilla.

The commonest conceptual error is believing the contact is achieved by adding material. It is achieved instead by separating the teeth during the restoration and letting them return to their original position once the matrix is removed.

Separation is possible because the periodontal ligament permits a few tenths of a millimetre of physiological movement. That space, exploited during the restoration, becomes the thickness of the contact.

Circumferential matrices encircle the whole tooth and were for decades the standard tool. Their limitation is geometric: they produce a flat proximal surface, whereas the natural surface is convex with the point of maximum contact near the occlusal margin.

A flat surface creates a contact extended vertically rather than punctate, and floss passes with difficulty over its whole height.

Sectional matrices cover only the sector concerned and have a pre-curved convexity reproducing the anatomy. Combined with a separating ring, they are today the reference for Class II restorations.

The ring performs two functions at once: it separates the teeth and adapts the matrix to the cervical margin with its tines. The second function is the less noticed one, and it prevents excess material at the neck.

Rings have calibrated force, and using an unsuitable one produces opposite results. Too weak a ring does not separate; too strong a one distorts the matrix and may collapse its ends into the undercut.

MOD restorations require two rings, and this demands models with compatible geometry: two standard rings on the same tooth interfere with each other.

Band thickness bears directly on the contact. Bands of 50 micrometres are standard; those of 38 leave a tighter contact and suit cases where space is already reduced.

The difference between the two is of the order of a hundredth of a millimetre, and seems irrelevant until one notes that this is exactly the order of magnitude of a contact.

The wedge completes the system and performs a task the ring does not: sealing the cervical margin against the gingiva. An anatomical wedge, its concavity facing the papilla, adapts the matrix without crushing the tissue.

In summary: pre-curved sectional matrix for anatomy, separating ring to create the space that becomes contact, anatomical wedge to seal the cervical. Adding composite to strengthen a weak contact produces an overcontoured restoration, not a better contact.