When might a dentist choose to use a custom tray instead of a stock tray for impressions?

A dentist may choose to use a custom tray instead of a stock tray primarily to ensure a tighter fit during the impression-taking process. Custom trays are specifically designed to fit the unique anatomy of a patient's mouth, which provides greater accuracy in capturing dental impressions. This tight fit minimizes the risk of distortion and allows for better reproduction of oral structures, ensuring that the resulting model is precise for various restorative or orthodontic procedures.

In contrast, stock trays are pre-manufactured and may not provide the optimal fit for every patient, potentially leading to errors in the impression. A custom tray helps in achieving a more reliable impression by maintaining the impression material in the desired position, thereby improving the quality of the dental work that follows.

While other factors like the patient's special needs or the availability of materials may influence the choice of tray, the critical advantage of a custom tray lies in its capacity to deliver a superior fit for the purpose of obtaining accurate impressions.