## 1 Introduction  
Conventional endodontic treatment is based on the mechanical removal of the infected pulp tissue, chemical disinfection, and root canal filling. The success rate can reach 98% according to the literature [1]. However, the success rate of orthograde treatment is highly determined by the anatomical knowledge [2, 3], and even this cannot provide complete success on its own. Complex apical anatomy, e.g., ramifications, \({}^{1}32^{1-2-1}\)[4], or Type VII [5, 6] configurations, can make sufficient orthograde cleaning impossible. In most of these cases, the residual periapical inflammation cannot be resolved by re-treatment either. So, in these cases, the retrograde approach, apicoectomy, is the proper treatment of choice [7]. Apicectomy is the surgical removal of the apical portion of the root to eliminate any remaining reservoir of infection (lateral canals or ramifications that cannot be instrumented from the orthograde direction). To effectively remove these structures, it is advisable to excise the apical 2 to 3 mm of the final third of the apex [8; 9]. According to the modern principles, the section must be performed perpendicularly to the root axis to minimize the chance of bacterial leakage through the dentinal tubuli [10]. Following these principles, the elimination rates may be as high as 93 to 98% [11]. The perpendicular technique was a great innovation compared to the previously applied 45\({}^{\circ}\) technique with 60 to 70% elimination rates [12]. However, as the operation is conventionally carried out with burs or a piezoelectric device [8; 12] and without any physical guidance, it is still an invasive procedure with prolonged and often painful healing. Therefore, these procedures should tend towards minimal invasiveness, which appears to be addressed most efficiently by increasing the accuracy [13; 14]. The first evidence that CAD/CAM guides can be used to increase the accuracy of apicoectomy was published by [PERSON] and colleagues in 2007 [15]. The in vivo application of this idea only appeared 10 years later, due to the steep development of stereolithographic techniques. At first, only the localization of the apex was guided to help the osteotomy performed by [PERSON] et al. [16]. Another approach was to use a 3D-printed soft tissue retractor during the apicoectomy procedure [17]. The implementation of 3D implant planning software into endodontic surgical procedures made the optimalization of depth, angle, and localization possible [18; 19; 20]. The very first steps of the evolution of this technique were focusing on a non-guiding template, only to be an optical reference prior to or during the free-hand apicoectomy. [PERSON] and co-workers [21] were the first to use a trephine bur to perform a fully guided osteotomy, apicoectomy, and biopsy at the same time. Since then, this method has been discussed in various studies [22; 23; 24; 25; 26; 27; 28; 29; 30], mainly case presentations. The accuracy and efficacy of this method have not been examined so far. The trephine creates a semicircular section [23; 31]. Given that the resected surface does not form a perpendicular flat line with the root axis, the remaining concave surface may harbor lateral canals or other potential bacterial reservoirs (Figure 1). Consequently, a situation arises where, despite the potential superior accuracy and precision of a guided trephine in comparison to conventional approaches, clinical effectiveness might be compromised due to the aforementioned concern. As this method gains popularity, there is a pressing need for dedicated preclinical investigations to elucidate and address this potential issue. Figure 1: Clinical (periapical X-ray) images of guided endodontic surgery performed with a trephine bur. (**a**) Right after surgery; (**b**) at the six-month follow-up; (**c**) at the three-year follow-up.