Monday, January 28, 2013
In Part I of this series I discussed the goals of canal preparation with regard to choosing the ideal taper and conceptually treatment planning to achieve that goal during canal preparation.
In this second part I will build on that article and address the considerations when assessing the risk of perforation. This article will provide strategies to avoid this and similar iatrogenic events.
Assessing perforation risk requires that the clinician:
- Have an adequate radiographic assessment of the case
- Determine the optimal final taper of the root canal preparation before starting treatment
- Determine the optimal orifice opener for the given anatomy present
When it comes to obtaining an adequate radiographic assessment, cone beam technology is the gold standard for determining the true three-dimensional anatomy of any given root. Without CBCT, the next best option is standard digital radiography, taking two or three radiographic angles.
Determining the optimal final taper of the root canal preparation, in essence means deciding whether the root should be prepared to a .04 taper, .06 taper or .08 taper throughout its length. This should be done before treatment begins.
Thin roots and roots with significant curvature are generally prepared to a smaller taper. Less complex roots with greater root wall thickness can be prepared to larger tapers.
Figure 1: Mani Gates Glidden Drills are available in 28, 32 and 38 mm.
Clinically, this means that the palatal canal of most maxillary molars can be prepared to .08 taper and the mesial buccal root of most maxillary first molars can generally be prepared safely to .06 taper.
When determining what will be the proper orifice opener for the present anatomy, the clinician must optimally weigh the risks between using Gates Glidden (GG) Drills versus rotary nickel titanium (RNT) files for enlarging the orifice.
Figure 2: The MounceFile Controlled Memory .08/25 rotary nickel titanium orifice opener (top) retains its shape once a curve is placed upon it. The MounceFile Standard NiTi (SNT) .08/25 rotary nickel titanium orifice opener (bottom) returns to its original shape upon being curved or stressed.
There are several considerations for and against each of these methods. GG drills are less expensive, and in the event of fracture, are easy to retrieve. GG drills require that the clinician stage their use from largest to smallest moving apically to prepare the correct and desired taper.
For example, the GG 1 drill in the average mesial buccal root of an upper molar would be taken to the junction of the middle and apical third, the GG 2 drill 1-2 mm coronal to this, GG 3, 1-2 mm coronal to the GG 2 and perhaps a GG 4 used only at the orifice. Taper is built into the preparation by blending each of these successive GG insertions (Figure 1).
Figure 3: Excessive taper, canal transportation, and a lack of canal negotiation before preparation are visible in the final result of this treatment of #19. Given the initial root length and width, the prepared taper in the mesial root is far too large in addition to the canal transportation and ledging present. The best possible chance for negotiation of the mesial and distal roots was to have spent adequate time with small hand K files negotiating these roots to the apex and possibly using a step back approach.
RNT orifice openers are more expensive than GG drills and a .08-tapered orifice opener can enlarge almost any orifice and coronal third. One such orifice opener eliminates the need for multiple GG drills.
If the clinician wishes to expand the orifice to a larger diameter they can use the .08 in a brushing motion against all the walls of the orifice or canal as indicated. As a result, having .12 and .10 tapered orifice openers is not essential for the vast majority of clinical cases encountered (Figure 2).
It is important to mention that initial orifice enlargement must be proportional to the final expected canal taper. Specifically, using a GG 5 or GG 6, 3-4 mm down the mesial root of a lower first molar removes excessive dentin and makes subsequent preparation of a tapered funnel almost impossible.
Figure 4: A final radiograph showing a clinical case treated with the MounceFile CM assorted pack (.08/25, .06/25, .04/25, .03/25, .02/25, .03/30) using the techniques discussed in this article..
Ideally, the clinician chooses an orifice opener that allows a continuously tapering preparation from orifice to apex. Using the mesial root of a lower first molar as an example, if the final prepared taper were expected to be .06, the largest orifice opener needed would be generally being .08 and nothing larger (Figure 3).
And finally, it is noteworthy that the final prepared taper should be large enough to facilitate three-dimensional obturation and activated irrigation. Any removal of dentin beyond this requirement is unnecessary and potentially dangerous risking both perforation and long-term vertical fracture (Figure 4).
Part II of this clinical article has specifically discussed preventing perforation through choosing the correct taper. Emphasis has been placed on determining the optimal final prepared taper before instrumentation commences and using the correct system and instruments to achieve these goals.
I welcome your feedback.