Photo illustrates composite short of the cavosurface margin. Although this was a very infrequent error that I made when first developing the Custom Matrix Technique, due to my emphasis on this factor, attendees in my hands-on workshops do make this error. As dentists we have been trained to add more composite than we need and grind away the excess. The ability to quickly eyeball the correct amount of composite for a restoration is most important for Class I and II, since excess composite leads to high occlusion.
Matrix Band Not Retighten Adequately
Occlusal anatomy and cavosurface margin are perfect and need no finishing. The excess composite mesial to the marginal ridge is due to the Tofflemire matrix retainer not being retightened to the same degree as when the matrix was constructed prior to preparing the tooth. Consequently, when the custom matrix was reseated, excess composite was forced up between the custom matrix and the matrix band. The Tofflemire matrix retainer should be snug but not overly tight when placed on the tooth both before preparation and after.
Note that the surface of the composite is smooth and well formed, but the composite is not pushed to the margins on the lingual, leaving a gap (arrows) and is not thin in those areas where it extends beyond the margins (buccal and distal). Place 5 pounds of pressure on the custom matrix for 5 to 10 seconds after seating it to allow time for the composite to flow and conform to the shape of the matrix.