In vitro evaluation of the shoulder stop in a self-threading pin

S chaefer and Reisbick’ studied the Link Series selfthreading pin (Whaledent International, New York, N.Y.). This was an improved two-in-one pin with two shear points instead of one. They found no significant differences in seating depths of either the first or second pin after testing Minim (0.024 inch/O.610 mm) and regular size (0.031 inch/O.775 mm) pins. Whaledent improved the Link Series pin technique by developing the Link Plus system with a shoulder stop and a tapered lead tip on each pin. According to the manufacturer, the shoulder stop provides “stress-free shearing in both stages, minimum apical stress at bottom of channel and more consistent and full pin depth placement.” The tapered lead tip permits easy access into the pin channel. Kelsey et a1.2 compared the Link Series and the Link Plus two-in-one pins. They discovered that the pins could be seated completely with the exception of the second pin of the Link Plus system, which required an Auto Klutch (Whaledent International) reduction gear handpiece for complete seating. However, Whaledent stated that Link Plus pins can be installed easily with a conventional latchtype handpiece, but the gear reduction handpiece facilitates channel preparation and pin placement. They also stated that the pin “will shear when seated at the shoulder stop.” They presented a schematic drawing (Whaledent International, printed in U.S.A., Oct. 1982) depicting the shoulder stop as a safeguard even with the dentinal surface. An effective shoulder stop would have to be limited by the dentinal surface to fulfill the third provision, “minimum apical stress at bottom of channel.” The purpose of this study was to determine whether the shoulder stop is effective and whether there is minimum apical stress when the pins are fully seated. The ability of the conventional speed air-driven contraangle handpiece to prepare the channel and place pins was also evaluated.