Before trimming teeth on a stone cast for an immediate denture, what should be done?

Marking a line 3mm above the free gingival margin on a stone cast is essential before trimming teeth for an immediate denture because it serves as a guide for the clinician to ensure that the appropriate amount of tooth structure is preserved. This step is particularly important to facilitate the proper fitting of the denture and to maintain aesthetics and function in the final restoration.

When this guideline is marked, it allows the clinician to achieve a consistent and desired contour that mimics the natural gingival margin. This is especially critical in immediate dentures, where the preservation of the tooth position and surrounding structures is key to obtaining a harmonious and natural appearance once the denture is in place.

Other options present procedures that may not be as pivotal before the actual trimming of the cast. For instance, soaking the cast in water serves to keep the stone hydrated but does not directly impact the trimming process. Polishing the cast prior to trimming might enhance the surface finish afterward, but it is not a necessary preparatory step. Measuring all teeth for uniformity is useful in assessing overall alignment and occlusion but is not a pre-trimming requirement. Thus, marking the line above the free gingival margin stands out as the most critical step in this process.