INCORPORATING CUSP PROTECTION A diagram showing the basic principles of cusp protection as required in a Size 3 lesion. Note that the red line demonstrates the conventional out line of the cavity design. The modification shown in green suggests that the bucco-lingual width of the cavity remains unchanged at the gingival margin but the buccal or lingual cavity wall be leaned out towards the tip of the cusp in a straight line to allow strength in the restorative material which in turn will offer protection to the cusp. The cusp that is to be under load – upper = palatal, lower = buccal – requires greater support and is therefore reduced by about 1.5mm in height. The non-working cusp needs little protection so will be reduced by 0.5mm only. The red line shows the conventional cavity outline but it leaves the medially facing cuspal inclines subject to occlusal load. The green lines show the suggested cavity outline following preparation