# Digital Design of Minimally Invasive Endodontic Access Cavity
## 1 Introduction  
In endodontics, the development of safer motor instruments and techniques of instrumentation associated with the support of three-dimensional assessment of dental anatomy is leading clinicians to operate in a more secure environments with enhanced predictability of outcomes: performing minimally invasive access cavities represents the goal of all these improvements [1; 2; 3; 4].  
Minimally invasive endodontics (MIE) is a clinical approach aiming at performing endodontic techniques and instrumentation, with minimal loss of tooth structure. The basic concept is to disinfect the pulp chamber, then properly clean, shape and fill the root canal systems--without sacrificing extensive occlusal enamel and dentin in the crown and roots. Several studies have shown that the long-term survival of endodontically treated teeth is mostly related to the quality of post-endodontic restorations and to the increased risk of crown and root fractures [5; 6; 7].  
Many studies evaluating the ability of a tooth to withstand occlusal and functional forces have shown that a tooth with a significant loss of enamel and dentin is distinctly weaker than an intacttooth [8; 9; 10]. Losing marginal ridges and cusps can significantly and negatively affect tooth strength. Similarly, endodontically treated teeth are usually weaker than untreated ones; hard tissue destruction caused not only by various lesions, but also excessive, unnecessary aggressive removal of coronal tooth structure during access cavity are considered to be responsible for this loss of strength [11; 12].  
During the last decades, traditional endodontic cavities (TEC) have been commonly performed without significant changes over time, aiming at allowing a valid access (ideally a straight-line) to the endodontic apex, by reducing coronal interferences [13; 14; 15]. More recently, new minimally invasive endodontic cavities have been described and proposed to preserve dentin (and enamel) through strategic access: the conservative endodontic cavities (CEC) and ultraconservative endodontic cavity (UEC) or better known as \"ninja access cavity\" or PEAC (point endodontic access cavity). These variously sized access cavity designs are both aiming at improving tooth preservation, but they are different in the amount of tissue removal. There is still no accepted classification for these cavities: their differences are based on the assumption that when the diameter of the access preparation is decreased by a half, the operator removes four times less volume of the tooth structure--consequently, a smaller cavity will result in a stronger tooth, with an enhanced resistance to fracture [16]. PEAC is considered the most conservative, minimally invasive approach, but has been criticized because it does not allow to properly clinically detect canal orifices and induces more stressful canal instrumentation [17]. There is no consensus how a PEAC contributes to tooth's strength greatly because there is simply no agreement on how PEAC should be performed. Some authors start from the central fossa, following an oblique projection towards the canal orifices; in other studies a perpendicular straight orifice-directed design is proposed--known as the \"truss\" access cavity--in which separated cavities are prepared to approach the different canals [18].  
The purpose of the present study is to describe and classify four different types of PEACs and to analyze if a dynamic navigation system could allow to planning and precisely executing these cavities in vitro.