## 4 Discussion
In the current study, seating using finger pressure was used. This may lead to different cement film thicknesses compared to other studies and may lead to possible differences from studies using other methods. Since finger pressure was found to differ from person to person [72], all endocrowns in the current study were seated by one investigator to reduce the chance of pressure variability. This procedure was adopted in several previous studies in the literature [73, 74, 75]. Although the total mean fracture resistance showed no differences between the two tested endocrown designs, the failure mode variations seemed intriguing to investigate further. Therefore, a detailed view of the fracture resistance sorted by the failure modes was initiated. It showed how different failure modes relate to the fracture resistance in Table 3. This initial view of the variability of patterns of failure modes and extent may further be investigated or observed by future similar studies of endocrowns and various design characteristics, either in vitro or in vivo. Additionally, the load was applied in a single axial direction in the current study. Application of load from two directions could produce different fracture modes and/or strength values. Previous studies showed that axial loading produced different fracture resistance values compared to lateral loading of lithium disilicate endocrowns [40; 50], which could be attributed to the material's flexural properties, including a high elastic modulus and rigidity [44; 50]. One of the limitations of this study is that the detailed effects of marginal and internal discrepancy of the endocrowns extension box design were not thoroughly studied or evaluated in relation to the fracture resistance or failure modes of endocrowns. First, because of the extension of the restoration into the pulp chamber, marginal and internal fit are usually influenced by the cavity depth [76; 77]. Second, most of the measurements were taken using a digital caliper (buccal wall width, lingual wall width, mesial box depth, distal box depth, mesial box width, and distal box width) and using standardized restoration through digital planning (occlusional thickness, cusp angle, and height). However, the surface area of the pulp chamber was not measured, which might influence the fracture resistance and the failure mode [23]. Although the current study is an in vitro lab study, and in one part it might not be duplicated clinically, the examination of the roots visually gives an indication of how difficult, complex, and complicated the detection of failures and cracks are in clinical situations. The differences between the evaluators of tooth fracture can be very extreme due to many factors which may affect the decision of classifying the tooth into restorable or non-restorable based on the catastrophic nature of the fracture line extension. Among these factors is the visualization of the fracture line and tracing their ends apically by direct vision or other diagnostic tools. Extra care and enough time should be taken into consideration during the examination of the occurrences of possible fractures on teeth, especially endocrowns, which were examined in this study, to ensure thorough detection and decision on teeth restorability. It should not be taken lightly when the treatment option of endocrowns encounters failure of the restoration. Therefore, the authors suggest further studies to evaluate additional proximal boxes to endocrown design to further apply what is observed and known from in vitro studies before the general, imprudent, or injudicious use or application of various endocrown designs in any clinical situation.