###### Abstract  
The purpose of this study was to evaluate the retentive forces over time of removable partial denture claps fabricated by the digital method. Occlusal rest seats were fabricated on three premolar teeth fixed in acrylic blocks (9 \(\times\) 20 \(\times\) 40 mm). Digitization of the teeth was performed using a laboratory scanner (Zirkonzahn Scanner 8600 GmbH, Gais, Italy). After the analysis and determination of the insertion axis, two types of claps with mesial occlusal rests were designed per tooth: the back-action and the reverse back-action claps, using the Partial Planner Zirkonzahn program. The file was sent for fabrication of six metal claps from a cobalt-chromium SP2 alloy in the EOSINT M270 system by a direct laser sintering process. The Instron 5544 universal testing machine was used to perform 20,000 cycles of clasp insertion and removal in the corresponding tooth with a load cell of 100 N and a speed of 2.5 mm/s. The retentive force was recorded for each of the 1000 cycles, and the change in retention over time was calculated. Statistical analysis was performed using the nonparametric Mann-Whitney test and a significance level of 5%. At 16,000 cycles, a maximum change in retention of 3.74 N was recorded for the back-action claps and a minimum of \(-\)24.28 N at 1000 cycles for the reverse back-action claps. The reverse back-action claps exhibited statistically significant lower change in retention than the reverse-action claps at 4000 and 5000 cycles. No differences were observed in the remaining cycles. During the 20,000 cycles, the change in retention was low regardless of the type of clasp. For most cycles, there were no differences in the change in retention between the two types of claps. removable partial denture metal framework; direct laser sintering; back-action clasp; reverse back-action clasp; retentive force; experiments; additive manufacturing; CAD-CAM +
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Fixed rehabilitation with implants has high success rates and has become increasingly popular in recent years. However, not all patients are good candidates for this type of treatment due to health, financial, anatomical, or psychological circumstances [7]. One alternative is the rehabilitation of dedentulous areas with removable prostheses, which is still widely used in clinical practice [5; 8]. Removable partial dentures (RPD) are prostheses that restore dedentulous areas in partial dentulism, and can be performed rapidly with esthetic and functional benefits [5; 9; 10; 11]. The development of this type of prosthesis remains a challenge. Proper planning and selection of RPD materials is key to reducing the incidence of complications. When designing RPD, it is important that the framework has stability and a correct fit which is mostly provided by the major connector and clasps [12]. It should also provide support and retention, and not interfere with abutment teeth and other supporting structures when masticatory forces are applied [13; 14].