## 4 Discussion  
The objective of this study was to evaluate the retentive forces and their change over time of clasps with different designs that were digitally manufactured. The prosthetic clasps are subject to daily movement during insertion and removal of the RPD by the patient. Several studies have examined the retentive force over multiple cycles of insertion and removal: 15,000 [8], 16,000 [15], 25,000 [34] and others [4; 16; 18; 21; 36; 37]. In the present study, 20,000 cycles were performed, corresponding to a lifetime of 13 years, since changes in retention were observed only around this number of cycles. The clasp retentive force is the force required to remove it from the tooth. It has been suggested that the retentive force for proper denture function is 5 N [12; 21]. In the present study, it was shown that the initial retentive force of the back-action clasp was 10.58 N on average, and for the reverse back-action clasp was 11.10 N. This is consistent with the study of [PERSON], whose initial retentive force of Co-Cr clasps was 13 \(\pm\) 4 N [38]. When evaluating the initial retentive forces versus the final retentive forces per tooth, the back-action clasp had lower initial and final retentive forces at tooth 1 compared to the reverse back-action clasp, but at tooth 2, the final retentive forces were higher for back-action forces. At tooth 3, the initial force of the back-action clasp was higher than that of the reverse back-action clasp, but the final force was lower. These discrepancies may be caused by differences in the tooth crown morphology. To address these discrepancies, the change in retention was calculated using Equation (1) to determine the variations in retention forces across cycles and to account for the initial retention force of each clasp. It is also important to highlight the fact that of the three teeth used, tooth 2 had a less retentive morphology and therefore had more constant values across cycles. Both types of clasps have been shown to have some loss of retention over the loading cycles. However, the clasp with the back-action clasp was the one that showed the greatest change in retention at the end of the tests. At the beginning of the experiments, both clasps showed a negative change in force, i.e., the retention values increased compared to the initial evaluation. This phenomenon was also found in other studies [4; 8; 36; 39]. This can be justified because the tests were performed under cold conditions, and the insertion axis given by the testing machine was parallel to the longitudinal axis of the tooth, which is not the only axis that patients use when removing the removable framework [36]. The initial increase in retention forces can also be explained by the wear of the tooth and the inner surface of the clasp, which could have led to an increase in roughness at the beginning of the tests [8]. In Figures 13 and 14 the back-action clasp shows a tendency to decrease retention, with discrepancies between the values of the individual teeth. The reverse back-action clasp generally tends to maintain the values relatively constant. The fact that the reverse  
Figure 17: Boxplot diagrams of the change in retention of the two clasp types at 5000 cycles. Change in retention in (N). back-action clasp has a longer arm may mean that it is more flexible, which may result in more constant values, while the back-action clasp with its shorter retention arm loses more retention over time [15]. In a study by [PERSON] et al. the comparison of the retentive forces of two types of claps, namely the back-action clap and the conventional Akers clasp, showed a statistically significant difference in retentive forces at 4000 cycles [15]. This is consistent with the results of the present study, which showed that design affected the change in retention at 4000 cycles (\(p=0.049\)) and 5000 cycles (\(p=0.049\)). However, there was no effect on the change in retention in the remaining cycles, which contradicts the results of [PERSON] et al., [PERSON] et al. and [PERSON] et al.