Capturing an accurate impression is a key step in restoration success. It dictates the ability of the lab to fabricate a well fitting restoration and create adequate marginal seal. When we think about accuracy at the margins we need to strive to take impressions with adequate flash beyond the edge of the prep. Onlay preparations have the unique characteristic of having both horizontal and vertical margins we need to capture. Most often where I see impressions missed for onlays is at the transition between the gingival margin and the buccal and lingual margins in a proximal box.
One of the contributing factors for accuracy when taking final impressions is handling of the light body material. Any place we release the pressure on the gun, change direction or pull the tip we are likely to introduce a void and miss a small area of the impression. Smooth consistent delivery of the injectable material is critical, and this can be challenging with onlay preparations. If we think about a full coverage crown the margin line stays all in one plane. We place the tip of the light body material just above the margin so that half the material is flowing over the prep and the other down into the retraction space we created. We slowly and smoothly move the tip around the 360 degree circumference of the margin without changing direction. We return to the beginning and go beyond our original starting point and continue to inject material up over the wall of the prep keeping continuous pressure until we are off the tooth.
When I take an impression for an onlay I follow a modification of this same technique, and inject 360 degrees around the tooth first, then with continuous pressure return to the proximal boxes and fill them before finishing. I have found that if I place the tip into the proximal box and inject the margins all from within, I get a void at the transition of the box to the unprepared tooth surface on the buccal or lingual. If I move around by injecting one area, stopping, moving the tip and injecting again, I always introduce air bubbles and have to take the impression again.