At what angle should class II preparation for amalgam be finished in relation to the external surface of proximal cavity walls?

The class II preparation for an amalgam restoration requires that the external walls of the proximal cavity be finished at a right angle relative to those walls. This is crucial in order to create a proper bond between the amalgam and the tooth structure. A right-angle preparation minimizes the potential for stress concentrations at the margins, which can lead to failure of the restoration over time. It also helps in ensuring that the amalgam can adequately fill the cavity preparation providing both structural support and retention.

Furthermore, finishing the walls at a right angle allows for better adaptation of the amalgam material, promoting a more definitive seal against micro-leakage and decay, which is vital for the longevity of the restoration.