Q. Would you discuss the causes of blocked and ledged canals?
A. Blocks and ledges are attributable to working short, apical preparation first schemes, the types of instruments employed and their methods of use.
1) Working Short: Generations of dentists have been trained to "work short" of the radiographic terminus (RT). Every dentist who has worked short has experienced the frustration of apical "blockage", ending up even shorter than was their intent. Apically blocked canals frequently hold combinations of pulp, bacteria, related irritants and dentine mud in their terminal extents. The ledge is a progression of the block. Aggressively trying to place larger files to full working length in a blocked and curved canal reduces the possibility for the file to "follow" the true canal. Files begin to dig into the outer wall and the block becomes a ledge.
2) Apical Preparation First: With "apical preparation first", the approach to canal preparation is to negotiate and prepare the apical one-third of the root canal first followed by a coronal flaring technique to facilitate obturation. The breakdown is the failure to recognize that frequently the rate of taper of the instrument exceeds the rate of taper of the canal which prevents the file’s apical movement. When an instrument binds on its more shank-side cutting blades, the clinician looses apical file control. Canals typically exhibit their greatest curvatures and deep divisions in their apical extents. Specifically, passing a precurved negotiating file through a coronally tight and underprepared canal straightens the instrument. Unknowingly attempting to work straighter files to length in curved canals first invites the block then predisposes to the formation of a ledge. Further contributing to breakdowns in the apical preparation first sequence is the fact that non-flared canals hold a small or non-existent volume of irrigation which in turn invites the accumulation of dentine mud. Working short, in conjunction with attempting to prepare the apical one-third first, has contributed to blocked foramens or in other instances, has led to canals that have been ledged, externally transported or apically perforated.
3) Instruments Employed: The types of "instruments employed" to prepare canals has traditionally been files, reamers and hedstroems in conjunction with rotary driven instruments such as gates glidden drills. In practice, increasing stiffness was clinically noted when progressing through any type of instrument series. In fact, the stiffness problem was compounded by the mathematical fact that there is a nonlinear increase in apical diameters between successively larger ISO files. Specifically, there is a significant "percentage change" among the most terminal diameters of the smaller instruments.
4) Method of Use: Beyond imperfect file designs, a major contributor to cleaning and shaping breakdowns has been, and continues to be, "method of use". Techniques advocating getting to length early have encouraged aggressive cutting action. Screwing larger, less flexible files into canals is the primary reason for blocks, ledges, transportations, perforations, interappointment flare-ups, short-term failure, surgeries and extractions. Preparation breakdowns continue to drive the growing field of endodontic retreatment.