Loss of Working Length

Loss of working length during cleaning and shaping is a common and frustrating procedural error. The problem is often only noted on the master-cone radiograph or when the master apical file (MAF) is short of the intended or initial working length. Even worse, it is often not noted until the canals are filled and a crown is placed. Too many times it is never noted (Fig. 10-1). Assuming that a clean, dry canal with proper shape has been developed, reestablishing canal length becomes time consuming, tedious, and often hopeless. Loss of working length may be secondary to other procedural errors (e.g., canal blockages, ledges, perforations, fractured instruments) which occur during the canal enlarging and shaping process and can be identified early in the procedures (Fig. 10-2).^21,27,29 In most instances, however, the loss of working length is due to the packing of dentin chips in the apical third of the canal. The occurrence of this problem, with some subtle exceptions, is almost exclusive to the use of hand stainless steel instruments in the root canal. The main shortcomings that promote loss of working length are listed below, and logic would support preventing the problem by eliminating these shortcomings.

FIGURE 10-1 A 39-year-old presented with pain to biting on the mandibular first molar. He related that he could not understand why he was having pain; root canal treatment and a crown had been completed only 2 weeks prior to this painful episode. Note the incomplete root canal treatment with possible ledges in the mesial canals.

FIGURE 10-2 A, Preoperative radiograph showing significant curvature of the mesial canals, which should warn the clinician that ledging is more likely to occur if instrumentation is not performed cautiously. B, Master apical file radiograph shows that canals were instrumented larger than appropriate for size and curvature of mesial roots. Notice the loss of canal length and ledge formation. C, Mesial canals are fully ledged, and distal canal is partially ledged.

FIGURE 10-3 Packed amalgam particles in the apical extent of the canal block the apical 3 mm of the root canal.

FIGURE 10-4 A, Improperly placed stop on a file can significantly change the accuracy of the working length and maintenance of canal length during enlarging and shaping. B, Cross-section of extracted tooth with files and stops in place. Stops are positioned at a proper angle with respect to the shaft of the file and the reference point on the tooth surface.

Each one of these errors is easily avoided with (1) attention to detail during instrument application and (2) generous use of irrigants during enlarging and shaping.

If nickel-titanium (NiTi) rotary instruments are used properly during canal enlarging and shaping, the same shortcomings may enter into the process, but loss of working length occurs infrequently. The major cause of loss of working length with NiTi rotary files appears to be excessive use of these instruments, retaining them in the canal for longer than 1 to 3 seconds while they are achieving their goals apically. It is not the packing of chips that occurs, but rather there is a tendency of these instruments to move laterally in the canal, especially in larger sizes and when placed apically over and over during the enlarging and shaping process.

Should loss of working length occur that is due to packed dentinal and tissue debris (Fig. 10-5, A) the clinician should choose a small but stiff instrument (e.g., No. 15 K-file) to bore through the obstruction. If the obstruction is caused by particles of temporary or permanent restorative materials, an instrument smaller than the last instrument placed into the canal system is used (e.g., No. 8, 10, or 15 K-file or C+ file [Dentsply Maillefer, Ballaigues, Switzerland]). A 45-degree curve is placed at the apical 3 to 4 mm of the instrument (see Fig. 10-5, B), and the file is inserted into the canal and slowly rotated circumferentially to detect a catch (the space between particles and canal wall; see Fig. 10-5, C).³¹ Once the catch is felt, the file is carefully rotated in a stem-winding fashion along with a slight in-and-out motion until the tip of the instrument bypasses the obstruction and negotiates the canal to length. Placing a relatively straight instrument into a milieu of metallic filings or dentin debris may push the particles more apically into the canal or periapical tissues, making them irretrievable. All recovery procedures should be performed with a lubricant in the canal.

FIGURE 10-5 A, Diagram showing dentinal chips and tissue debris packed in the apical portion of the canal. B, A stiff file is curved in the apical 1 to 3 mm at a 30- to 45-degree bend and placed to the level of the blockage. C, The file is rotated slowly until it catches in the debris then advanced slowly in millimeter strokes to remove the blocking debris. D, Once the catch is felt, the file is carefully rotated in a stem-winding fashion along with a slight in-and-out motion until the tip of the instrument bypasses the obstruction and negotiates the canal to length.

Once the instrument begins to advance further into the canal, or if it expediently reaches the estimated working length, a radiograph is obtained to verify the position of the file. The instrument should not be removed until it can be used in small-amplitude strokes, moving circumferentially to dislodge the packed debris. When sufficient space has been developed through or along the side of the blockage, a smaller-sized Hedström file can be placed to length. Moving this instrument on the outstroke will eliminate the debris. To bore through dense blockage with dentinal chips, chelating agents such as RC-Prep, REDTAC, or liquid ethylenediamine tetraacetic acid (EDTA; see Chapter 11) may be used to soften the plug to facilitate penetration. If the plug has not been penetrated, the file may be creating a false canal in the dentinal wall.

Whenever there is failure to penetrate or bypass blockage, complete enlarging and shaping at a new working length coronal to the blockage (Fig. 10-6) are recommended.³³ Variations in obturation techniques such as diffusion, thermoplasticized gutta-percha, warm vertical compaction, or core-carrier procedures can be used to enhance penetration of the gutta-percha and sealer around or through the blockage (see Chapter 12). Periodic reassessment is necessary, and if further treatment is deemed appropriate, surgical intervention to correct the problem may be warranted (Fig. 10-7).⁹ If surgery is not feasible, intentional replantation^15,57 or extraction and replacement with an artificial prosthesis such as an implant may be indicated if symptoms persist.

FIGURE 10-6 A, Note the block (most likely a ledge) in the mesial canals of a mandibular molar. B, After placement of a 45-degree bend, one of the canals was penetrated and negotiated to the working length. The other canal could not be penetrated, or possibly it joined the patent canal. C, Obturation with a softened gutta-percha and sealer technique.

FIGURE 10-7 A, Maxillary molar with a separated instrument segment around the curve in the mesial buccal canal. The tooth was symptomatic. B, Following apical surgery and a root-end filling in the mesial buccal canal; 3-month reevaluation.

Many authors and clinicians would also list separation or breakage of intracanal instruments, especially in curved canals, as a major cause of canal blockage or loss of working length (Fig. 10-8).²⁸ This problem has been addressed in Chapters 5 and 14. However, during the application of NiTi rotary instruments, there are unique circumstances that tend to predispose to instrument separation and canal blockage (Fig. 10-9). In many respects, the concern for instrument breakage has prevented many clinicians from adopting the use of rotary NiTi instruments. This is unfortunate because the minimal potential for instrument separation does not have to be a deterrent to usage if some basic principles are followed. First, these instruments are always to be used in a crown-down technique after a tapered pathway for the instruments has been developed. The pathway will ensure ease of movement with minimal pressure, binding, and torque on the instrument during canal penetration. The pathway can be created with stainless steel hand K-files (No. 06 to 20) intended for this purpose⁴ or with NiTi rotary PathFiles (Dentsply Maillefer, Ballaigues, Switzerland). The pathway that is developed favors variably tapered NiTi instruments and small constant-tapered instruments (0.04 to 0.06, Nos. 15 and 20). When time is taken to develop the pathway, additional important information is identified by the clinician, such as canal curves, the joining of canals, rapid deviations, and multiple canals. In light of these findings, most causes for loss of working length can be prevented.

FIGURE 10-8 Broken instrument segment around the curve at the junction of the mesial buccal and distal canals—a common canal blockage.

FIGURE 10-9 Diagrammatic representations of the four most common reasons for breakage of any endodontic intracanal instrument: canal convergence (A), S-shaped canals with multiple curves (B), abrupt canal deviations (C), and severe and rapid apical curvatures (D). These anatomic complexities are especially important in the application of NiTi rotary instruments. E, Histologic evidence for t/>

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