Decrease of Retentive Force by Repeated Attachment and Detachment of Bar Attachment

Abstract

The retentive force of a clip was found to be proportional to its length in experimental results using 10 new CM bars®. The attachment and detachment cycle of the clip repeated 10,000 times resulted in about a 45.9% decrease in the retentive force with no effect of its length. The gradient of S-N (strength vs. number of cycles) curve for the clip of 2-mm length decreased steeply at about 900 repeated cycles and those for clips of 3-mm, 4-mm and 5-mm lengths decreased similarly at about 400 repeated cycles. The spring action of the clip was reduced as a result of these repeated cycles in addition to the normal strength-loss due to metal fatigue. The ones over 3 mm in length still showed a retentive force stronger than 4.90 (N) which was required for the bar attachment after 10,000 repeated cycles. Consequently, the retentive force of the 2-mm length clip seemed to be weak for clinical use, and we should employ a clip of at least 3 mm or longer.