Written by E. Steve Senia, MS, DDS, and William L. Wildey, DDS Saturday, 31 March 2007 19:00
In Part 1 of this article we introduced the new LightSpeed LSX system (Discus Dental Endodontics). In Part 2 we present the recommended clinical technique and clinical cases illustrative of its efficacy.
Although access is abbreviated because of space limitations, it should never be underestimated. Poor access leads to missed canals, strip perforations, broken instruments, and lost time. Poor access is frequently the cause of an endodontic failure. The objective of a good access preparation is to make a straight-line glide path from the occlusal surface to about mid root.
Correct access is easily confirmed by the existence of a path guiding instruments into canal orifices without looking or using a mirror. (See “Straight-Line Access—A Must for Faster and Better Endodontics,” Dentistry Today. January 2003;22:40-45; also available online at dentistrytoday.com). After access, the coronal 4 to 5 mm of the canal is flared in a crown-down fashion—not with the LSX—but with an instrument of your choice. Explore the canal, determine and secure patency to working length with at least a size No. 15 K-file, and begin instrumentation with the technique described below.
Step 1. Instrument the Apical Third
Figure 1. LSX: the short blade and no taper give accurate tactile feedback.
Start with the LSX No. 20 rotating in Discus Dental Endodontics’ EndoPAL cordless handpiece (2,500 RPM), enter the canal, and slowly advance it to working length (WL). Immediately remove it from the canal (still rotating). Continue with sequentially larger sizes to WL. You will feel a resistance to apical advancement only when the blade is cutting. At resistance—pause briefly—and then in a slow, continuous, and gentle manner go to WL. Irrigate and continue with larger sizes until reaching the final apical size (FAS). FAS is the instrument size that meets resistance 4 to 5 mm from WL and requires a firm push to advance it from there to WL (more about this later).
Note: The smaller LSX instruments may not meet any resistance (not cutting) because the canal diameter may be larger. The LSX blade is very short. Above it is a nontapered, noncutting shaft that is very flexible and doesn’t cut (Figure 1). LSX is unlike a typical tapered instrument with a long blade that would be cutting somewhere along its length.
Step 2. Complete the Apical Preparation
Figure 2. Apical 5 mm preparation for obturating with SimpliFill Plug.
Figure 3. SimpliFill Apical Plug (Gutta-Percha or Resilon). Carrier removal facilitates backfilling, retreatment, and post placement.
Step 1 prepared the apical canal with a parallel shape. Complete the apical preparation by funneling the top of the parallel shape with the next larger instrument (than the FAS). Set the rubber stop at WL minus 4 mm and instrument to this length (Figure 2). The apical canal is now shaped to fit a 5 mm SimpliFill Gutta-Percha (Discus Dental Endodontics) or Resilon (Pentron Clinical Technologies) Plug (Figure 3).
Step 3. Instrument Mid Root
Recall that the coronal 4 to 5 mm of the canal was prepared first, followed by the preparation of the apical 5 mm. The remaining 4 to 5 mm of mid root is cleaned and shaped, blending the middle third with the coronal and apical thirds.
Start with the next larger LSX (than the size used 4 mm short of WL). Go to resistance, pause, then push 2 to 3 mm (very long or very short roots might require more or less than 2 to 3 mm). Do not advance any mid root instrument into the apical 5 mm. Doing so will result in a loose-fitting SimpliFill Plug. Continue with sequentially larger LSXs as just described (to resistance—pause—push 2 to 3 mm). The mid root preparation usually takes 2 or 3 instruments.
Step 4. Recapitulate
The purpose of this step is 3-fold: (1) to confirm that no debris remains in the canal; (2) to check that working length was maintained, and (3) to check the apical stop. When an apical stop is present, the FAS instrument should not move (past WL) when pushed (by hand, not spinning in handpiece) with reasonable force. Movement past working length indicates the absence of an apical stop. Caution must be used to avoid pushing obturating material out the end of the tooth.
Obturation and Irrigation
SimpliFill Apical Plugs are so accurately sized they can be “press-fit” into the apical canal. The hydraulic forces developed by the tight fit forces sealer into open spaces. The science is clear—it is sealer, not gutta-percha, that creates the seal. Gutta-percha by itself seals poorly. We know of no obturation system incompatible with LSX instrumentation. However, other obturating systems may require deviation from our instrumentation technique.
There is no such thing as too much irrigation. The chamber and canal should always be flooded during instrumentation. After each cutting instrument, irrigate with NaOCL or irrigant of your choice until the solution is clear. Then suction out the irrigant (suction greatly enhances debris removal) and flood the canal and chamber with liquid (not paste) EDTA.
Using LSX Correctly
The key to using the LSX is always advancing it slowly so that you can feel when resistance is first encountered (the blade is starting to cut). Upon feeling resistance, pause, and in a slow, continuous, and gentle manner push the LSX to WL. Never push so hard or so fast that the shaft bows or buckles.
We are often asked, “When is the correct final apical size reached?” The explanation lies with the concept of working width, a term first introduced by Dr. Yi-Tai Jou1 at the University of Pennsylva-nia. It is historically important because a name has finally been given to a dimension that must be considered during canal instrumentation.2 Canals are 3-dimensional and require that attention be given to both working length and working width (diameter). The concept of 3-dimensions is not new in endodontics, but we are noticing a trend toward the greater importance of apical preparation sizes.
Working width is best understood by studying cross-sections of apical canals. If the greater diameter of the original canal is measured, the correct working width (WW) is an instrument size slightly larger than that dimension. This size removes the most infected dentin and pulpal remnants. This is best shown in a LSX Technique Video available at LightSpeedEndo.com.
Realistically, we should only expect to clean the main canal. We simply cannot expect an instrument to be able to touch all aspects of the canal system (lateral canals, apical deltas, fins, culde-sacs, and isthmuses). There are also apical canals that are so oval that making them round is not possible—or even wise. Fortunately, these cases are rare.
Working Width: Where Is It Applied?
Figure 4. Apical canal showing location of working width (blue arrows) coronal to the constriction.
Before clarifying the location of WW, let’s review apical anatomy. The apical constriction is the narrowest part of the canal, not the foramen. Instrumenting to the foramen (with a large instrument) destroys the constriction and the possibility of developing an apical stop. This is not a good idea, and is one of the reasons (among many) why the constriction is our recommended target working length.
It is not correct that the final apical instrument size (master apical file or MAF) is determined by the size of the constriction. The goal of instrumentation is not to match the final instrument size with the size of the constriction. Doing so would ignore the essence of instrumentation, which is to clean all walls of the main canal and not just those of the constriction. This concept ignores the fact that most canals enlarge so significantly coronal to the constriction (Figure 4) that standard tapered instruments (.02, .04, .06, .08) are, for the most part, inadequate in this region, resulting in underprepared apical canals.
Just as we do not want to prepare canals too short or too long—we also do not want to prepare them too small or too large. The ideal working width is not always attainable. However, even in such cases we should try to clean as best as we can.
HOW DO WE KNOW WHEN THE FINAL APICAL INSTRUMENT SIZE IS REACHED?
|Figure 5. Apical cross-section of a correctly prepared canal. Notice there is plenty of dentin remaining around this canal.|
Now that we understand working width, let’s answer this question. By instrumenting and then cross-sectioning many teeth we learned that the LSX instrument that meets resistance 4 to 5 mm short of WL and then requires a firm push to reach WL closely approximates the correct WW (Figure 5). While no technique is perfect, and a correct WW is a clinical judgment, the LSX goes a long way toward making it as accurate as possible. We call the instrument that approximates the correct WW the Final Apical Size (FAS).
A CONTROVERSY IN PROGRESS
Figure 6a. Left: apical cross-section of mesial root before instrumentation.
Figure 6b. Right: after instrumentation with No. 45 LightSpeed versus No. 45 Ni-Ti tapered file. Transporta-tion with the Ni-Ti tapered file enlarged the canal more than necessary. The LightSpeed preparation is conservative, yet ideal.
Figures 7a and 7b. LightSpeed’s suggested apical preparation sizes.
Unfortunately, LightSpeed’s ability to prepare canals to larger sizes (when required) has been distorted. Some say LightSpeed advocates over-preparing the canal, a criticism that is totally unfair and inaccurate. We have always taken a conservative approach to instrumentation, including resisting excessive coronal flaring. Our goal has always been clear: remove only a minimum amount of dentin from all canal walls,3,4 while minimizing under- and over-preparation (Figures 6a and 6b).
Research supports our belief that apical canal diameters require larger apical instrument sizes than are currently in vogue. Before Ni-Ti, rigid stainless steel instruments gave us 2 choices: (1) keep the apical preparations small or (2) suffer the mishaps of ledges, zips, perforations, etc. This forced compromise made small apical sizes the norm. But times have changed, and now Light-Speed’s “suggested” apical preparation sizes may be more normal than once thought (Figures 7a and 7b). As additional evidence accumulates, more and more dentists are realizing that traditional apical preparation sizes are much too small in most cases.
It is vitally important to bear in mind that canals can be very small or very large, and that there is a wide variety of canal diameters between this range. Canal size is influenced by tooth type, patient age, caries, trauma, etc. As succinctly stated by Larz Spangberg,5 one size [instrument] does not fit all. A “one-size-fits-all” mindset utilizing only a few instruments is not supported by science.
LSX AND SIMPLIFILL RESILON CASES
Figure 8. Case 1 involves tooth No. 30. Left: pretreatment radiograph. Center: post-treatment radiograph. Right: 6-month recall. MB=No. 55, ML=No. 55, D=No. 60. Courtesy of Dr. William L. Wildey.
Figure 9. Case 2 involves tooth No. 30. Left: pretreatment radiograph. Center: post-treatment radiograph. Right: 5-month recall. MB=No. 55, ML=No. 55, DB=No. 70, DL=No.70. Courtesy of Dr. William L. Wildey.
Figure 10. Case 3 involves tooth No. 19. Left: pretreatment radiograph. Center: post-treatment radiograph. Right: 6-month recall. MB=No. 55, ML=No. 55, D=No. 65. Courtesy of Dr. William L. Wildey.
Figure 11. Case 4 involves tooth No. 30. Left: pretreatment radiograph. Center: post-treatment radiograph. Right: 14-month recall. MB=No. 50, ML=No. 50, D=No. 50. Courtesy of Dr. Kendel Garretson.
The cases shown in Figures 8 to 11 demonstrate the variability of apical diameters. A full range of instrument sizes must be available. Trying to get by with just a few instrument sizes ignores the variability of canal diameters. All cases were single appointments and treated as follows: after access was made using the LightSpeed StraightLine Access Kit (Discus Dental Endodontics), the canals were instrumented with LSX and obturated with SimpliFill Resilon Plugs using Simpli-Fill Sealer.
The authors thank Steven Senia, BSIE, MBA, for his contribution to this article.
1. Jou YT, Karabucak B, Levin J, et al. Endodontic working width: current concepts and techniques. Dent Clin North Am. 2004;48:323-335.
2. Senia ES. Canal diameter: the forgotten dimension. Dent Today. May 2001;20:58-62.
3. Zuckerman O, Katz A, Pilo R, et al. Residual dentin thickness in mesial roots of mandibular molars prepared with LightSpeed rotary instruments and Gates-Glidden reamers. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003;96:351-355.
4. Weller PJ, Svec TA, Powers JM, et al. Remaining dentin thickness in the apical 4 mm following four cleaning and shaping techniques. J Endod. 2005;31:464-467.
5. Spangberg L. The wonderful world of rotary root canal preparation. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2001;92:479.
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