Creating spacing under temporary crowns

The issue with directly made temporary crowns are that the bisacryl materials are in general too accurate and once cemented, the lack of space under the crowns means that the temporary will always be seated high due to the hydraulic pressure on the cement. This is why when you are checking the occlusion with the temporary crown dry, everything may seem okay but once it is cemented there is usually a fair bit of occlusal adjustment required. The problem is amplified with thicker temporary cements such as eugenol based cements e.g Tempbond as the film thickness is extremely thick and it will be difficult to flow out as you seat the crown down. You may find that excessive occlusal adjustment leads to thinning of the crown and with conservative preps you may perforate or have frequent crown breakages during function.

One strategy to overcome this is to use thinner cements e.g Durelon polycarboxylate cement which is technically a permanent cement. This increases the need to section off your temporaries at the insert stage as the cement layer will be harder to debond with pressure on the crown. As there is some adhesion to tooth structure, you will need to try harder to remove it off the tooth prior to cementing and may need to ultrasonic and air abrade the tooth.

Another method I have been testing out lately is to take a bur and hollow out the internal surface of the crown slightly. This is kind of like directly creating die spacing for cement to sit. This will potentially reduce the hydraulic pressure on the cement and account for the film thickness of the cement layer. I have tried using cylindrical burs but don't like the lack of versatility as it only allows you to prepare the whole length of the wall at any one time. A small football bur can allow you to focus on certain sections of the wall at a time and I have tended to leave the margin untouched. In my head this will seal the preparation better especially when the margin is quite fine and it will keep a positive seat for the crown. In reality, the occlusal surface will act as a positive stop but the film thickness of the cement will be represented here as well so you may wish to reduce the internal surface at the occlusal as well. However reducing the temporary crown here may be less predictable than adjusting the occlusion post insertion.

Crown preparation is represented in yellow. Left is a trimmed temporary crown. The lack of cement space causes the crown to be high in occlusion. Middle has had the temporary crown hollowed throughout except for the margin. The margin is acting as a positive stop and sealing the margins. However the internal surface at the occlusal aspect is thinner which may weaken the crown under force. Right has had the axial walls reduced which leaves the occlusal thickness intact but the crown will still have resistance to seating with cement at the occlusal aspect. The margins have been slightly thinned which can lead to chipping at the margin and a poor seal