Endodontic Management of the Mandibular Second Molar

Our goal in endodontic therapy is to clean, shape, and fill the root canal system. Complex root canal anatomy is often the culprit that results in endodontic failures. We all have mental maps of root canal anatomy in the teeth we treat. The problem is that our mental map may be wrong, as we tend to see what we want to see. This leads to some of the most common problems of endodontic failure: missed canals and incomplete obturation. Dr. L Stephen Buchanan has posted some amazing 3-D images of teeth on his Web site (endobuchanan.com), and he has developed these images to try and get us to think about the anatomy of the root canal space. Teeth are not the flat drawing we saw in our dental school endodontic text. We may soon have the ability to see this root canal anatomy clinically in 3-D, but until then we need to learn to do this mentally.
My goal here is to take a commonly treated tooth, the mandibular second molar, and help you develop a mental map. Then I’ll show you how I use my mental map to treat a mandibular second molar.


Figure 1. How many canals are in tooth No. 13? Figure 2. Is this a surprise to you?
Figure 3. Mandibular second molar with 2 roots.

ROOT CANAL ANATOMY: A BACKGROUND
 
There has been a great deal of research focused on root canal anatomy (I’m sure you remember trying to stay awake in dental anatomy class) but the results from various studies don’t always agree with each other. One study will say that the upper first molar has an MB2 canal 50% of the time, and the next study 95%. These discrepancies are a result of the study design. Some studies simply have too few numbers to give an accurate statistic. Older studies relied on grinding or sectioning the teeth, which destroyed the anatomy they were trying to see. Many of the older studies didn’t involve microscopes, so some canals might have been missed. Don’t let the differences in the research confuse you. It is important to look at the numbers and try to understand the anatomy of the root canal space in each tooth. Averaging the numbers from the studies may give the best idea of canal anatomy. Recent studies use micro-CT scanning, which can be assembled into 3-D images. This is the process that Dr. Buchanan uses to produce the 3-D images on this Web site. Brown and Herbranson (3-D Tooth Atlas) have also produced an amazing compilation of canal anatomy using tomography.
You should know the anatomy of every tooth you intend to treat, and develop a mental map. For example, in Figure 1, do you see 1, 2, or more canals in tooth No. 13? Knowing the possible canal configurations in every tooth can immediately improve your endodontics (Figure 2). Root canal anatomy increases in complexity as you go distal from the midline.
Multiple roots and canals occur with increasing frequency in posterior teeth. Mandibular second molars (not counting third molars) have the most variations of canal anatomy of any teeth. Thankfully, the majority of mandibular second molars have similar anatomy, but the ones that vary are really wild.
Manning1 looked at mandibular second molars and found that 22% had one root, 76% had 2 roots, and 2% had 3 roots. In the 2-root group, most of the teeth had one or 2 canals in the mesial root that joined before the apex (Weine Type II) and 1 canal in the distal root. About 25% of the mesial roots had 2 separate canals from pulp chamber to apex (Weine Type III). These teeth have the typical triangular shape on the pulp chamber floor. Look at the 3-D mandibular molar media clips posted on endobuchanan.com (Media Showcase section). These will help you develop a mental map.
It’s easy to identify these teeth with a radiograph because they have 2 separate roots. Trabeculated bone with a lamina dura can be seen between the roots (Figure 3). The good news is that 3 out of 4 mandibular second molars have this configuration, so the difficulty and case selection is dependent on other factors (ie, calcified canals, curvature, canal length, etc).

Figure 4. Mandibular second molars with a fused root.

Now let’s look at the wild teeth. The mandibular second molars often don’t have 2 roots but a single fused root (Figure 4a and 4b). The canal configurations of these teeth vary from one canal to 4 canals and commonly contain multiple isthmuses and anastomosis. One, 2, and 3 canal-fused root molars are common.


Figure 5. Endodontic treatment of a mandibular second molar.

DEVELOPING A MENTAL PICTURE

The most important step in treating mandibular second molars is to recognize the basic root configuration. Does the tooth have 2 roots or one? Off-angled radiographs assist in developing a mental map of the canal configuration. Single rooted mandibular molars are more difficult to analyze radiographically because the anatomy varies so much. Some day we may all have some form of computed tomography in our office and be able to accurately map teeth in advance.
Take a straight on and a mesial-angled (20° to 30°) radiograph. Distal angle radiographs tend to superimpose the thicker part of the mandibular bone over the tooth and may not be useful in second molar cases. If the mesial canals are close together, the off angle radiograph may produce no useful information. After establishing the basic pulp chamber space and position, I access the tooth and begin exploring the canals under the operating microscope. The initial exploration begins with an explorer and small stiff endodontic files (No. 10, 15 C files). Until the number and position of the canals are determined, it is usually best not to use any engine-driven instruments on the pulp chamber floor. A small ultrasonic tip (BUC1 [Obtura Spartan]) may be used to remove pulp stones. I take a working radiograph with files in the canals to show the canal anatomy. These used to be called working-length radiographs. Since electronic apex locators are far more accurate in determining canal lengths, I use these images to develop a mental map of the canal anatomy and get the approximate location of any potential difficulties. Severe curves and junctions of canals can produce instrument fractures.
In the case seen in Figures 5a, 5b, and 5c, the working film shows the 2 mesial canals joining with the distal canal at the apex. This information is not obtainable from the preoperative radiograph.
Sometimes we are simply given a “gift.” This happens when we get a single round or oval canal. These can be some of the most straightforward cases that one can treat. These teeth have a large and often straight canal that can be easily treated (Figure 6).

Figure 6. Single canal mandibular second molar.
Figure 7. Complex mandibular second molar with 2 canals.
Figure 8. Joined canals in mandibular second molar.
Figure 9. Failing endo in mandibular second molars.

The next step up in complexity is the second molar with 2 canals in a fused root. Again, the main complicating factor may be the anatomy of the root canal space (Figures 7a and 7b). These teeth can have 2 canals that exit in 2 separate foramens, or they can begin as 2 canals and join into one before they exit (Figure 8).
Endodontic failures often occur in these teeth. Many times it is assumed that the fused root only has one canal, and the additional anatomy is not treated. Pulp tissue may also be pushed from one canal into the adjoining canal causing a blockage. Careful mapping of the canals with working radiographs is needed before larger or rotary instruments are used. Remove as much coronal pulp tissue as possible with hand instruments before progressing down the canal. Follow Dr. Buchanan’s recommendations on lubrication and irrigation outlined on his Web site. This will minimize failure due to the blocking of canals by tissue compaction and filling the case short (Figure 9).


Figure 10. C-shaped pulp chamber (left). Figure 11. C-shaped immature apex (right).

C-SHAPED MOLARS
 
One of the most interesting configurations is known as a C-shaped mandibular second molar. The C refers to distinct pulp chamber anatomy that contains one or multiple canals connected by a slit or fin (Figure 10).
This C commonly extends from the mesial lingual extent of the pulp chamber to the distal lingual, but can be the opposite (mesial buccal to distal buccal with the C inverted). The opening of the C is accompanied by a groove on the lingual (or buccal for inverted case) side of the root. This C-shaped pulp chamber may continue into the canal space as a C, or divide into a number of different configurations. The C-shaped canals are formed when Hertwig’s epithelial root sheath fails to fuse on the buccal or lingual. Figure 11 shows an immature apex that has a C configuration; one can imagine that the root sheath could pinch off at the apex in almost any configuration as apical development continues.
Cooke and Cox2 published the first literature on C-shaped teeth. It was assumed for many years that the incidence was lower because of the difficulties in recognizing the C-shaped tooth in radiographs. Most of the early studies were case reports and studies showing the incidence in different racial populations. The incidence of C-shaped mandibular second molars ranges from 2.7% to 30%. This varies in different racial groups, with the Asians exceeding 30%. The incidence in the US population has been estimated to be around 8%. Interestingly, a C-shaped molar socket was found in a European Neanderthal jawbone.3 This might explain my problem patients.

Figure 12. Radiographs of teeth with potentially C-shaped pulp chamber.

It is important to identify these potential troublemakers early in treatment planning. All C-shaped teeth have a fused root. Mandibular second molars that have 2 roots don’t have the C-shaped configuration. C-shaped molars have a longitudinal groove on the lingual or buccal surface that looks like separate roots on a radiograph, but upon careful inspection you can see that the roots are fused (Figures 12a and 12b). The roots can have a conical or blunted apex. They also seem to have a pulp chamber floor that is located more apical than the first molar.
If a C-shaped tooth is suspected, it’s probably the time to assess your skills. These cases only appear 8% of the time in mandibular second molars in the US population. If you have a large number of asian patients, you may see this configuration approaching one in 3. Let’s look at why these cases may present a number of treatment challenges.
In 1991, Melton, et al4 produced the first study that examined the anatomy and histology of C-shaped canals. They classified the C shape into 3 types; continuous, semicolon, and separate. The best research on C-shaped teeth is coming out of China. With a population approaching 1.5 billion, and an incidence of one in 3 Asians having mandibular second molars with C-shaped canals, they have the number of cases needed to produce good studies. Recently, Fan, et al5,6 recognized additional configurations and reclassified Melton’s canal types.4 The canals are classified into 5 categories. C1 is a continuous C shape from the pulp chamber to the apex. C2 is a semicolon shaped canal. The comma can be on the mesial or the distal. The C3 configuration can be a single mesial and single distal canal or a mesial lingual, mesial buccal and a distal canal. C4 is a single round or oval canal. C5 is a section of the apex after the canal has exited (Figure 13a).5,6

Figure 13. Pulpal anatomy in C-shaped teeth. (Courtesy of Dr. Bing Fan.)

Fan5,6 uses micro-CT scans to section the C-shaped roots from the pulp chamber to the apex and he has graciously provided me with some of his micro-CT scans. The scan in Figure 13b shows serial sections from the pulp chamber (a) to the apex (f). The canal anatomy changes as you progress down the canal. This tooth starts out as a C1, changes into a C2, and then a C3. It goes back to a C1, C2, and finally a C3. When you see these scans, it’s easier to understand the challenges these teeth present (Figure 13b).
When a tooth has a C3-D configuration it usually has a single mesial and distal canal. The canals are located at the ends of the C in the mesial lingual and distal lingual corners of the pulp chamber. The isthmus between the canals should be instrumented with small files. There is a risk of perforation into the groove on the lingual (or buccal if inverted) if instrumentation is too big or aggressive.

Figure 14. Micro-CT scan of a C3c root and radiograph of potential problems. (Courtesy of Dr. Bing Fan.)
Figure 15. Failing C-shaped endodontic therapy.

You can see from this micro-CT scan of a C3c root (Figure 14a), the dentin can be very thin on the lingual. The mesial buccal and mesial lingual canals have been prepared with a 30-.06 rotary file. Even with this limited preparation, you can see how little dentin is left between the canal and the lingual groove. Landed rotary endodontic files should be used because they remain centered in the canal.7 GT and GTX files (DENTSPLY Tulsa Dental Specialties) are indicated for these cases (Figure 14a). C-shaped anatomy also precludes the use of Gates Gliddens drills and posts. Bonded composite or amalgam cores are the treatment of choice for buildups. I see perforations and post failures because of this anatomy (Figure 14b).
The micro-CT scan (shown in Figure 14) also helps us understand why thermoplasticized filling techniques must be used in order to fill the isthmuses and anastomosis between the canals. The optimal filling technique is the Continuous Wave Condensation Technique. Lateral condensation and silver points can’t effectively obturate this anatomy (Figure 15 and 15b; Failing C-shaped roots).

CASE REPORT

Now that you have a brief overview of the mandibular second molar, let’s review a clinical case and discuss how I managed the endodontic treatment.

Figure 16. Preoperative radiograph of tooth No. 18. Figure 17. Danger area for perforation in C-shaped teeth.
Figure 18. Working radiograph tooth No. 19. Figure 19. Micro-CT scan section of similar root.
Figure 20. Postoperative radiograph No. 19.

The patient is a 58-year-old female (non-Neanderthal) with an unremarkable medical history. She went to her dentist complaining of temperature sensitivity to cold. Her dentist diagnosed irreversible pulpitis in tooth No. 18 and prepared an access opening. Upon entering the pulp chamber, the dentist could localize only one canal in the mesial lingual. The patient was then referred to my office. The patient’s chief complaint of cold sensitivity had improved, but the tooth became tender to bite on and ached spontaneously. Preoperative radiographs were taken (Figure 16).
Tooth No. 19 had been extracted and tooth No. 18 was mesially inclined. The tooth appeared to have 2 separate roots with 2 visible canals. After anesthesia, a rubber dam was placed and the temporary removed under magnification with a dental operating microscope. The pulp chamber was full of pulp stones and these were removed with an ultrasonic BUC 1 tip. (The pulp chamber in C-shaped canals can be calcified and contain pulp stones.) It is important to not to over-prepare the C-shaped orifice on the floor of the pulp chamber. It is easy to perforate the pulp chamber into the lingual groove if too much tooth structure is removed. The area outlined in red in Figure 17 is the tooth structure above the lingual groove (or buccal groove). It looks like a pulp stone, but there is no canal anatomy below this structure (Figure 17).
The pulp chamber anatomy was a C, running from the mesial lingual to the distal lingual. The mesial lingual canal orifice had been instrumented. There was no vital tissue visible in the pulp chamber. The bone visible between the roots in the pre-op radiograph is probably due to a very deep lingual groove or dense bone on the buccal. (You’ll see that this changes in the working radiograph.)
The pre-op radiograph shows the limits of conventional radiography. Without the microscope, I would have had the wrong mental map. The tooth looked like a 2-rooted 2-canal molar. This may lead to procedural errors. ProLube (DENTSPLY Tulsa Dental Specialties) was placed in the pulp chamber, and the chamber explored along the C with a No. 10 C file. I located a mesial buccal, mesial lingual, and a distal canal. This configuration is the categorized as C3 based on Melton’s classification.4 I started progressing down the canals alternating ProLube and EDTA. To improve the visibility of the canal anatomy, I rinsed the EDTA out with alcohol and dried the pulp chamber floor. I could get a small catch with an explorer in the distal buccal of the C. I assumed that this was an isthmus of the distal canal, consistent with the C3 configuration. The No. 10 C files were placed in the canals and the following working radiograph was taken (Figure 18).
It appeared that there was a perforation in the furca of the tooth. This was not a perforation. In a C-shaped tooth, the canals are located in a 180° arc. The mesial buccal canal is much more distal than you see in the more common 2 rooted mandibular second molar. The fact that I knew that the pulp chamber is C-shaped and I had a mental map of the anatomy helped to interpret this radiograph. The interesting feature is the 2 distal canals. They appear to merge near the apex. The micro-CT scan shown in Figure 19 shows a similar root. This helps to visualize what’s going on. There are files in 3 canals and you can see a unlocated canal distal to the mesial buccal canal (Figure 19).
With the map of the tooth complete, I began the instrumentation. Glide paths to size No. 15 were completed in each canal. I established apical patency. The working lengths were measured with a Root ZX (J. Morita USA). I then irrigated with full strength heated sodium hypochlorite using a 30-gauge side-vented needle. One of my main concerns was to have the hypochlorite in the canals as long as possible to help dissolve the tissue out of the isthmuses and anastomosis that could not be instrumented. Ultrasonic activation of the hypochlorite was re-peated throughout the instrumentation. The rotary instrumentation began with a GTX 20-.06. I was able to get to the working length in the mesial canals with this file. A GTX 20-.04 was the first instrument to length in the distal canals. The apices were gauged and the final instruments used were a 30-06 in the mesials and a 20-06 in the distal canals. I have used 3 GTX rotary files. Since the exploration and instrumentation took less than 30 minutes, I rinsed the case with EDTA for 1 minute and then left hypochlorite to soak in the canals. I like to have the hypochlorite in the canals as long as possible to clean the canal system. This is a case where negative pressure irrigation may have been beneficial.8
The mesial canals appeared to have separate exits so master cone fitting was not complicated. The distal canals appeared to join apically. I always confirm this with files or paper points. If they join, I want to figure out the sequence of master point placement, because one of the 2 canals may have a less abrupt apical path and will be easier to place the first master point. After the master points were fitted and the point placement was checked radiographically. Occasionally, an irregularity in the canal will fold or bend the gutta-percha so I wanted to make sure that the insertion path was safe. The canals were then dried and the lengths checked with paper points. Length determination with paper points is a concept developed by Dr. David Rosenberg (Vero Beach, Fla). I often find that it detects errors in length produced by electronic apex locators.
When filling C-shaped teeth with isthmuses and anastomosis, the Continuous Wave of Condensation filling technique (developed by Dr. L Stephen Buchanan) has a distinct advantage. Carrier-based techniques and injection techniques are completed one canal at a time. There is a chance that gutta-percha and sealer from the canal being filled will be forced through an anatomoses and block the adjacent canal. With the Continuous Wave of Condensation technique, all the master points are placed and seared off at the orifice. The master point in adjacent canals can be held firmly with a hand plugger while the nearby canal is condensed. This may increase the hydraulic pressure and force softened gutta-percha into more anatomy. The canals were back filled with an Elements Obturation Unit (SybronEndo) and the access closed. (The final radiograph is shown in Figure 20.)

CLOSING REMARKS

Endodontic treatment in mandibular second molars can be challenging and fun. You may not want to treat complicated endodontic anatomy like the second mandibular molar presented in this article, but understanding the canal anatomy and producing a good mental map is a key to successful endodontic therapy in any tooth. Best wishes on your journey!


References

    1. Manning SA. Root canal anatomy of mandibular second molars. Part 1. Int Endod J. 1990;23:34-39.
    2. Cooke HG III, Cox FL. C-shaped canal configurations in mandibular molars. J Am Dent Assoc. 1979;99:836-839.
    3. Keith A, Knowles FH. A description of teeth of Palaeolithic man from Jersey. J Anat Physiol. 1911;46:12-27.
    4. Melton DC, Krell KV, Fuller MW. Anatomical and histological features of C-shaped canals in mandibular second molars. J Endod. 1991;17:384-388.
    5. Fan B, Cheung GS, Fan M, et al. C-shaped canal system in mandibular second molars: Part I—Anatomical features. J Endod. 2004;30:899-903.
    6. Fan B, Cheung GS, Fan M, et al. C-shaped canal system in mandibular second molars: Part II–Radiographic features. J Endod. 2004;30:904-908.
    7. Gluskin AH, Brown DC, Buchanan LS. A reconstructed computerized tomographic comparison of Ni-Ti rotary GT files versus traditional instruments in canals shaped by novice operators. Int Endod J. 2001;34:476-484.
    8. Nielsen BA, Baumgartner JC. Comparison of the EndoVac system to needle irrigation of root canals. J Endod. 2007;33:611-615.

Suggested Readings

Gao Y, Fan B, Cheung G, et al. C-shaped canal system in mandibular second molars. Part IV: 3-D morphological analysis and transverse measurement. J Endod. 2006;32:1062-1065.

Jafarzadeh H, Wu YN. The C-shaped root canal configuration: a review. J Endod. 2007;33:517-523.

Min Y, Fan B, Cheung GS, et al. C-shaped canal system in mandibular second molars. Part III: The morphology of the pulp chamber floor. J Endod. 2006;32:1155-1159.

Walker RT. Root form and canal anatomy of mandibular second molars in a southern Chinese population. J Endod. 1988;14;325-329.

Weine FS and Members of the Arizona Endodontic Association. The C-shaped mandibular second molar: incidence and other considerations. J Endod. 1998;24:372-375.


Acknowledgement

I would like to thank Dr. Bing Fan from the Endodontic Center, School and Hospital of Stomatology, Wuhan University, Wuhan, China for the micro-CT scans used in this article.


Dr. Warren received his DDS from Northwestern University Dental School in 1981. He practiced for 5 years as a GP before returning to Portland, Oregon, to begin a residency in endodontics. He received certificate in endodontics in 1988 from Oregon Health Sciences University. He has practiced endodontics for 20 years at Cotton-wood Endodontics in Salt Lake City, Utah. Dr. Warren can be reached by e-mail at rkwarrendds@hotmail.com.

Disclosure: Dr. Warren did not report and disclosures.