Figure 7. Rotational joint. Thompson dowel-rest system is a custom made attachment in which a retentive dimple (a) and box (b) are placed in cast crown on abutment tooth. Removable partial denture has a flexible cast arm (c) that engages dimple on lingual surface of crown, and a metallic bloc (d) that seats in the box. The Dalbo attachment is a good example of a ball and socket joint in which the ball is cantilevered off the abutment tooth and the socket is attached to the prosthesis. The wall of the metal socket has several small slits to provide a resilient entrance to the socket and offers some direct retention to the attachment when the socket engages the ball over its height of contour.
Figure 8. Rotational joint. Dalbo hinge joint has a ball (a) at end of a parallelsided metal block soldered to an abutment cast crown. This serves as a retentive rest seat for socket (b) that fits over and engages circumference of ball. Socket is connected to prosthesis and can rotate around ball and away from metal block.
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