Why do PFM crowns show a metal line?

PFM (porcelain-fused-to-metal) crowns rely on a cast metal coping for strength, then porcelain layered over it for esthetics. The coping must be masked with opaque porcelain before the body layers go on. When the opaque layer is too thin, or when the margin is prepared as a chamfer rather than a shoulder, the metal reads through the translucent porcelain and produces the characteristic dark line. Gingival recession compounds it: even a well-masked PFM will expose the unporcelained metal collar over time.

For general dentists and prosthodontists managing anterior restorations, the metal line is often the reason a patient returns requesting replacement. The solution at the lab level is a collarless or all-ceramic margin design, sufficient shoulder width (typically 1.2-1.5 mm), and full opaque coverage carried to the finish line. Without those three conditions present in the prescription, the technician is working against the geometry.

All-ceramic options, particularly full-contour or cutback zirconia, eliminate the substructure opacity problem entirely. Lithium disilicate pressed to a zirconia coping also avoids the grey halo. If your practice still places PFMs in anterior positions, the lab conversation should happen before preparation, not after the case arrives. Margin design is a shared decision.