17. Which one is the disadvantage of crown down technique? a. Permit straighter access to the apical region b. Eliminates coronal interferences which allows better determination of apical canal sizes c. Removes bulk of tissue and microorganisms before apical shaping d. More time consuming than step back technique. e. Working length less likely to change.

The correct answer is d. More time consuming than step back technique.<br /><br />The crown down technique is a method used in endodontics to shape the root canal. It involves removing the coronal portion of the tooth and then gradually shaping the canal from the crown down to the apex. While this technique has several advantages, such as allowing straighter access to the apical region, eliminating coronal interferences, removing bulk of tissue and microorganisms before apical shaping, and making the working length less likely to change, it is more time consuming compared to the step back technique. The step back technique involves removing a small amount of dentin at each step and shaping the canal as you go, which can be more efficient in terms of time.