## 4 Discussion
In terms of resistance to occlusal loads, PEKK based full crowns show a fatigue limit above 720 N, a value considered sufficiently high to resist fracture in clinical conditions [13]. Therefore, it is reported that PEKK crowns offer a stable treatment option for patients, in particular for those that suffer from metal allergy [13]. These findings complement the outcomes of the present study, as high resistance to occlusal loads and low-stress magnitude on compressive loading support the use of PEKK as a suitable telescopic attachment system for over-dentures. A previous study investigated the retention forces between primary and secondary telescopic crowns milled from various materials [1]. According to the authors, the material combination in telescopic attachments affected the retention forces and wear, with the combination of Titanium and PEKK presenting a promising result [1]. The present study supports their finding, suggesting that the combination between titanium and PEKK is a suitable option since this model showed an intermediate behavior. However, if a metal-free treatment is planned, the use of Zirconia or PEKK primary crown is a suitable option. Interestingly, the study by [PERSON] et al. evaluated the performance of telescopic crown material combination, including PEKK-PEKK, Zr-PEKK, and CoCr-PEKK [2]. They observed that the maximum retention force was exhibited by PEKK-PEKK crown combination, in comparison to the other groups. In addition, the authors reported that the long-term friction force remained constant during the evaluation [2]. These findings are in line with the present study, which showed the combination of PEKK-PEKK crowns to be most promising for telescopic crown retained overdentures. Previously, in vitro investigations compared retention forces of secondary telescopic crowns made of PEKK in combination with primary crowns made of four different dental materials [25]. According to the authors, all tested primary crown materials (PEKK, Zirconia, Gold, and Non-precious metal alloy) with a high-performance polymer PEKK as the secondary crown reached acceptable forces for overdenture retention over a period equivalent to 10 years of use [26]. Therefore, the materials combination affects the retention of telescopic crowns, as reported before [6]. In a study by [PERSON] et al. [6], telescopic crowns made from PEEK showed stable retention load values [6]. The present study showed similar findings, showing low-stress concentration on PEKK material, which is a result of its low elastic modulus and high strength properties. Previous studies have compared the stress induced in the mandible around implants, using two different attachment systems, locator, and telescopic crowns [35]. According to the authors, the locator attachment may demonstrate superior clinical performance compared to telescopic in terms of stress concentration [34; 35]. However, for tooth-supported retention systems, the locator system is less suitable. In addition, comparing the stress distribution in mandibular implant-supported overdentures and tooth-supported overdentures with telescopic crowns, it is reported that the implant group presented more stress concentration in the bone compared to the teeth-supported model [36]. This is attributed to the presence of periodontal ligament and more rotational freedom to dissipate the loads in the lever arm [36]. However, these studies did not assess stress concentrations and complications around PEKK abutments. In a study by [PERSON] et al. [37], strain gauge analysis was performed around implants with telescopic attachments of different materials combinations (Zr-Zr, PEKK-PEKK, and Zr-PEKK). Zr-PEKK combination telescopic attachments showed the least strain transmitted to the implants. It is pertinent to mention that implant overdentures were not assessed in the present study.