Root Canal Treatment (RCT) is absolutely packed with tiny details, and every single one of them has a massive impact on the overall success of the case. Among these crucial details are manual files, or hand files. Even though rotary systems have pretty much become the standard, those small hand files remain the foundation upon which we build all our work.
Perhaps the two files we handle most often are File 10 and File 15. Many practitioners tend to see them as just sequential numbers, but the truth is, each has a distinctly different and critical function. If you use one in place of the other, you could very well be opening the door to a host of problems you’d rather avoid.
In this article, we’re going to break down the exact core difference between these two files, when to use each one, and how to get the maximum benefit from them to ensure a correct and successful start to any endodontic case.
File 10: The Precise Canal Explorer (The Patency File)
Think of File 10 as that small flashlight you shine into a dark place to make sure the path is clear. It’s an extremely thin and flexible file, which makes it perfect for one specific and incredibly important job: ensuring Apical Patency.
So, What Exactly is Apical Patency?
Simply put, it means confirming that the root canal is open and completely clear all the way to its end at the Apical Foramen, with no obstructions from dentin mud or tissue debris. File 10, with its remarkable flexibility, can “glide” into the tightest spaces, making it the absolute best tool for this task. You gently advance it until you feel it has just barely passed beyond the canal’s terminus with a slight movement.
The Fatal Mistake: Determining Working Length with File 10
Please, never, ever use File 10 to determine your Working Length. Why not?
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It’s Too Thin and Flexible: This extreme flexibility makes it incredibly easy for it to pass beyond the Apical Foramen and penetrate too far into the surrounding periapical tissues without you feeling clear resistance.
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The Result: You’ll end up with an over-estimated length, leading to over-instrumentation, which can potentially cause pain and inflammation for your patient post-procedure.
The Bottom Line: File 10’s job is to tell you, “The path is open,” not “The path is this long.” It’s merely a scout file, not a surveyor.
File 15: The Path Engineer and Glide Path Champion
File 15, now this is the engineer who truly paves the way. It’s the first file that’s sturdy enough to do some real work, and it has two fundamental and crucial functions:
1. Determining the Working Length
After File 10 has reassured you that the canal is patent, it’s File 15’s turn. It’s the first file you can reliably trust to get an accurate Working Length reading using an Apex Locator. Its relative stiffness allows it to stop precisely at the Apical constriction, giving a correct reading without easily over-extending like File 10 (1).
2. Creating the Glide Path
This is perhaps File 15’s most important role in the era of rotary instrumentation. The Glide Path is like the “asphalt road” you prepare for your rotary files to travel smoothly and safely. If you try to introduce a rotary file into a narrow canal without properly paving its way, you’re essentially inviting it to fracture.
How to Create a Proper Glide Path with File 15:
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Advance the file to the full Working Length.
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Perform a “Watch Winding Motion”—gently turn the file a quarter turn clockwise and a quarter turn counter-clockwise while keeping it in place. This motion carefully widens the canal, especially at the apex.
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You can then withdraw the file 1-2 mm coronally and repeat the same motion to widen the section above.
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Continue this process: perform some watch winding, then irrigate thoroughly, and repeat until you feel File 15 is “loose” within the canal at the full Working Length. When it reaches this stage, you’ve successfully created a robust Glide Path, ready to receive your rotary files.
Building an excellent Glide Path with File 15 is the primary guarantee to prevent Rotary File Binding and subsequent File Fracture inside the canal (2).
The Art of Working Length Determination: Mastering Your Apex Locator
Determining working length with an Apex Locator is an art, and to ensure a 100% accurate reading, there are a few essential rules you must adhere to:
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Confirm Patency First: Before connecting the device, you must ensure File 10 passes through the apex. If the canal is blocked, the device will give you an incorrect reading.
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Proper Isolation: You need excellent isolation, ideally with a rubber dam. Any contact between the file and the patient’s lip or gingiva will create a short circuit, causing the device to beep instantly.
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Moist Canals: Modern Apex Locators often perform best when the canal is slightly moist, typically with Sodium Hypochlorite. A completely dry canal or one that is overfilled can lead to inaccurate readings (3).
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Dry Pulp Chamber: Contrary to the canals, the pulp chamber must be completely dry. Any collection of irrigant there will create electrical conduction with the gingiva, skewing your reading.
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Remove Metal Restorations: If the tooth has an amalgam restoration that is in contact with the gingiva, you absolutely must remove it before measuring the length. It will cause contact with the file clip and give an erroneous reading.
Conclusion: Every File Has Its Proper Role
Don’t treat these small files as mere numbers. Understand the specific function of each one:
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File 10: This is your “explorer.” Its job is to confirm “The path is patent.”
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File 15: This is your “engineer.” Its role is to tell you “The path is this long” (Working Length) and to prepare the “asphalt road” (Glide Path).
By allowing each file to perform its designated role, you’re not just working correctly; you’re also making your work faster, safer, and significantly more successful in the long run.
