# Digital Design of Minimally Invasive Endodontic Access Cavity
## 4 Discussion  
The initial aim was to identify and classify different PEACs, based on a rational approach: to minimize interferences during endodontic instrumentation by allowing a more direct access and to minimize reduction of teeth's strength, by avoiding unnecessary weakening of tooth structure on the occlusal entry point. Each access cavity was designed taking into consideration the concept of \"straight line access\" (SLA), aiming at reducing interferences causing instrumentation stress on the rotary instruments. Differently than any other previous study, these SLAs were designed in three dimensions, differing in two parameters. First one is the view since the canals can be viewed both from a bucco-palatal direction and a meso-distal one. The different views clearly show different original canal trajectories. Consequently, the starting coronal points (SCP) are different in all the cases, being located in different parts of the occlusal surface, to ensure the desired straight-line access. The second aim was to analyze if DNS was able to precisely plan and execute these cavities in vitro. Three-dimensional tooth replicas were selected to ensure that no differences in the anatomy of canals and crowns could alter the comparison between the different PEACs. The results of the present study showed the DNS software, which is clinically used to plan and execute placement of dental implants according to different axis, proved to be very simple to use and provided clear 3D images of treatment planning. The DNS (Table 1) allowed to easily perform PEACs with an overall good accuracy, with statistically significant differences between the cavities; high precision is obviously recommended to avoid unnecessary removal of dentine, but in plastic teeth it is not possible to establish how these variations of the angulation could result in a reduction of the tooth strength. Same limitations, however, could also be present in extracted teeth due to the specific influence of occlusion forces in each patient and the differences in the dimensions of the crowns. There is no consensus if in mesial canals is better to preserve, partially remove or eliminate any given triangle of dentin, to ensure a more direct straight-line access to the apical third [20; 21]. When eliminating these triangles of dentin, canal opening is intentionally relocated slightly away from an external root concavity and toward the greatest bulk of dentin. In a PEAC (Figure 5), this goal can be directly achieved by choosing an end point a few mm more apically than the orifice, intentionally relocating the end point as previously described; in the classical bucco-palatal view the end point is relocated more mesially, while in the hidden meso-distal view it is relocated more bucucally. Figure 2 shows the location of the SCPs on the occlusal surfaces: the access cavity X1 is located on the occlusal face between the MB and the DB cuspids, in an area between the facial sulcus and the central fossa. This type of access cavity could probably lead to a weakening of the marginal ridge, a structure that plays an important role in tooth's strength. The access cavity Y1 is located on the occlusal face at the center of the MB cuspid, which is fundamental during chewing; it is a mold for the antagonist element. Theoretically, its involvement and consequent weakening could be a negative factor affecting tooth's strength. Access cavity X2 is located on the occlusal surface, nearby the MB cusp, but away from the marginal ridge. This access cavity can theoretically provide valid instrumentation and avoid involvement of important tooth structure. The access cavity Y2 is located on the occlusal face, nearby the MB cusp, but more mesial than the X2 cavity; it can be considered similar to X2, theoretically allowing a proper access to canal.