What issue arises with the metal stock tray in RPD impressions?

The issue that arises with the metal stock tray in RPD impressions is that it can distort the vestibules. During the impression-taking process, if the stock tray is not adequately adapted or positioned, it may cause excessive compression of the soft tissue in the vestibule areas. This distortion can lead to inaccuracies in the impression, resulting in an unsatisfactory fit of the removable partial denture. Proper adaptation of the tray is crucial in preserving the natural anatomy of the soft tissues, ensuring that the impression captures the correct shape and contours necessary for effective denture design and function.

This focus on the vestibule highlights the importance of minimizing tissue distortion to achieve a precise impression that accurately reflects the oral cavity, particularly in areas that are critical for retention and stability of the RPD.