The cervical margin of a matrix band should extend approximately 0.5 mm below (apical to) the gingival cavosurface margin of the cavity preparation. This positions the band just past the floor of the box, creating a sealed wall that contains restorative material and prevents excess from flowing into the gingival tissues. The band must sit tightly enough against the tooth that an explorer tip cannot pass between the band and the cavosurface margin.
Why 0.5 mm Below the Cavity Floor
The gingival cavosurface margin is the junction where the prepared cavity meets intact tooth structure at the floor of a proximal box. Seating the matrix band 0.5 mm past this point serves two purposes: it ensures the restorative material fully covers the prepared margin without falling short, and it creates a physical barrier that blocks gingival crevicular fluid and blood from contaminating the restoration during placement. Even a small gap at the cervical margin can allow moisture to reach the bonding interface, which is especially damaging for adhesive materials like composite resin.
If the band sits too high, above the cavity floor, the restorative material has no wall to pack against in that area. The result is an incomplete or poorly adapted margin. If the band extends too far below the margin, excess material gets packed into the sulcus, creating an overhang that is difficult to detect on the proximal surface and even harder to remove after the restoration is finished.
What Happens When the Seal Is Wrong
A matrix band that is poorly adapted at the cervical margin frequently produces overhanging restorations. These overhangs act as sheltered ledges where bacterial plaque accumulates. Research has consistently linked overhanging restorations to localized gingivitis and periodontal disease. The problem is not mechanical irritation from the overhang itself but the plaque buildup it encourages in a spot the patient cannot clean effectively. A rough or bulky overhang at the proximal surface also raises the risk of secondary caries forming at the restoration margin.
Detecting an interproximal overhang clinically is difficult because it sits between the teeth, hidden from direct view. Removing one after the material has set is equally challenging, particularly with tooth-colored composites. Getting the cervical seal right before placing material is far easier than correcting the problem afterward.
How Wedges Affect Cervical Adaptation
A wedge placed from the lingual or buccal embrasure pushes the matrix band firmly against the tooth at the gingival margin, closing any gap between the band and the cavity floor. The direction of wedge insertion matters. Inserting from the buccal versus the palatal side produces different effects on how tightly the band seals, because the interproximal space is rarely symmetric. In some cases, placing wedges from both sides of the cavity provides a more complete cervical seal.
Ideally, the wedge engages the tooth right at the level of the cavity margin. Most plastic wedges are contoured and flexible, designed to wrap around the interproximal curvature of the tooth. When a wedge alone cannot fully close the cervical margin, PTFE tape (plumber’s tape) can be layered beneath or alongside the wedge to plug remaining gaps. This is a common supplementary technique when the anatomy of the embrasure makes a perfect wedge fit difficult.
Circumferential vs. Sectional Matrix Bands
The type of matrix system you use influences how well the cervical margin adapts. Circumferential bands, like the Tofflemire system, wrap entirely around the tooth. They are versatile but tend to produce more overhanging proximal borders and weaker contact points compared to sectional systems. The flat profile of a circumferential band does not naturally conform to the curved anatomy of the proximal surface, making tight cervical adaptation harder to achieve.
Sectional matrix systems use a pre-contoured band that covers only the proximal surface being restored. Combined with a separation ring that applies outward pressure, these systems generally produce tighter interproximal contacts and better anatomical contour. The ring also helps stabilize the band at the cervical margin by pressing it against the wedge and tooth simultaneously. For routine Class II composite restorations, sectional systems typically offer a more predictable cervical seal, though the choice depends on the clinical situation and the depth of the preparation.
Verifying Correct Placement
Before placing any restorative material, confirm the cervical seal by running an explorer along the junction between the band and the gingival cavosurface margin. You should not be able to slip the explorer tip between the two surfaces. If you can, the band needs to be repositioned or the wedge adjusted. After burnishing the contact area of the band with a ball burnisher to improve its contour against the adjacent tooth, check the cervical margin again, since burnishing can shift the band slightly.
For circumferential bands, visual inspection from the occlusal helps confirm that the band extends just past the gingival floor without excessive material pooling beneath it. With sectional bands, resistance felt when seating the ring is a tactile indicator that the system is stable, but it does not replace direct verification of the cervical margin with an explorer.
When the Margin Extends Deep Below the Gingiva
Standard matrix placement assumes the gingival margin of the cavity is at or near the free gingival margin of the tissue. When decay or fracture extends well below the gumline, achieving isolation and matrix adaptation becomes significantly more difficult. Crevicular fluid and bleeding compromise the field, and a conventional wedge may not reach deep enough to seal the band against the tooth.
One approach for these deep lesions is trimming a Tofflemire band down to roughly 2 mm in height and seating it into the gingival sulcus until the tissue blanches slightly. That blanching confirms the band is pressing against the sulcular tissue and creating a temporary seal. This modified technique has been used successfully to isolate subgingival root caries for adhesive restorations without surgical crown lengthening.
Another strategy is deep margin elevation, a technique where composite resin is first used to build the subgingival margin up to a supragingival position. Once the deep margin has been relocated above the gumline, a standard matrix band and wedge can be placed in the usual way for the final restoration. This two-step approach avoids the contamination risks of trying to restore a deep subgingival margin in a single visit and improves the long-term seal of the final restoration.