Review of Endodontic Instrumentation via Different Access Cavities – from Randomized In-Vitro Studies to Clinical Applications

Abstract Aim: Micro-invasive therapeutic approaches in endodontics differentiate between different modified types of access cavities: occlusal via a smaller occlusal opening (15 ° access angle), medial (via the caries defect) for premolars and molars (30 ° access angle) and cervical with wedge-shaped defects or root caries on anterior teeth and single-rooted premolars (45 ° access angle). Therefore, the aim of this work was (i) to create an in-vitro model for the clinical simulation of these three different modified access types, (ii) to assess the biomechanics of the instrumentation and (iii) to measure the volumetric substance loss of four different preparation systems. Material and Method: Simulated S-shaped root canals with a high degree of obliteration (ISO 10) in acrylic polymer bodies with canal entrance angles of 15 °, 30 ° and 45 ° were shaped with NaOCl (3%) irrigation under clinical simulation at body temperature (37 ° C) in randomized blinded sequences (n = 7) according to manufacturer's instructions: 1. Conventional geometry, not heat treated, up to 35 / .04 (F360, Komet) 2. Conventional geometry, heat-treated, spark-eroded up to 40 / .04 (Hyflex EDM, Coltene Hyflex) 3. Spiral geometry, heat treated, up to 36 / .03 (TruNatomy, DentsplySirona) 4. Distinctive spiral geometry, heat-treated (XP-Endo Shaper, FKG) The vectorization (AutoCAD) of the root canals was performed before and after preparation in the apical, middle and coronal third of the root canal. The volumetric loss of substance in cubic millimeters was recorded and statistically evaluated using an independent two-sided t-test. Clinical demonstrations include three cases of micro-invasive access cavities as final proof of principle of fast and safe root canal treatment. Results The total loss of volume at 15 ° entry angles is significantly lowest with TruNatomy and XP Endo. With an access angle of 30 °, the total loss remains low and increases with F360 and Hyflex .. At a 45 ° access angle, the group differences remain: TruNatomy (6.5 mm3) and XP Endo (6.4 mm3) versus F360 (8.3 mm3) and Hyflex (8.4 mm3). This difference in loss of simulated root dentin is highly significant. Conclusions Conventional instruments achieve optimal shape of apical third of root canals at all entrance angles only with higher substance loss in coronal and middle thirds . In contrast, the spiral instrument geometry contributes to dentin protection and supports the concept of minimally invasive endodontics for lifelong tooth preservation as shown in four clinical cases.