Which margin forms a 90-degree angle with the unprepared tooth surface?

The shoulder margin is designed to form a 90-degree angle with the unprepared tooth surface. This shape provides a strong, well-defined edge that is crucial for the retention and stability of crowns and other dental restorations. Its perpendicular nature allows for excellent adaptation to the surrounding tooth structure, which enhances the sealing ability of the restoration and reduces the risk of micro-leakage.

In contrast, other margins like the chamfer exhibit a rounded profile rather than a distinct angle, which is more suitable for certain types of restorations but does not provide the same level of support as the shoulder margin. The cylindrical margin does not apply in this context as it does not describe a common margin configuration used in prosthetic dentistry. The bevelled margin is characterized by an angle that is less than 90 degrees, allowing for a smoother transition between the restoration and the tooth preparation, but again, does not form the certain sharp angle needed for optimal retention in all cases.

Thus, the shoulder margin's unique 90-degree angle with the unprepared tooth surface makes it particularly advantageous for achieving strong connections in various prosthetic applications.