Mode of failure during dislodgement of prosthesis cemented with zinc polycarboxylate cement is usually seen at the:

LUTING INTERFACE Luting cements are designed to fill the microscopic gaps between a prosthesis and a prepared tooth.  Each surface is microscopically rough, having peaks and valleys, and only point contacts are made by the peaks.  The areas that are not in contact must be filled by the cement to prevent oral fluid flow and bacterial invasion.  One can seal the space by placing a soft adhesive between the two surfaces, which can conform under pressure to the roughened surface features. Ref: Phillips Ed 12th P: 311