Access in Endodontics - Why is it so important?
(STILL UNDER CONSTRUCTION)
Endodontics can be difficult. In many cases, clinicians make it more difficult than necessary by not creating a proper access that allows them straight line approach to the canals. This is especially true for Mesiobuccal canals in both Mandibular and Maxillary molars. Before dealing specifically with molars, lets examine a few access concepts that will allow us to reach the apex more successfully.
Concept # 1 - Files can only "do" two curves at a time
Although the development of NiTi metals has improved instrument flexibility,it is important to understand one basic fact about endodontic instruments: In vivo, they can only be expected to consistently negotiate two curves at a time, especially files greater than a size # 15 or 20. Because canals are curved both in a mesio-distal and bucco-lingual directions, we must initially provide as straight line access as possible in order to allow maximum freedom for the instrument in the more confined, difficult apical sections. Most endodontic cases that are ledged occur because the clinician is attempting to guide the instrument through too many curves. The instrument is unable to negotiate the final curve, resists, and then a ledge is made. Limited purposeful straightening of the cervical portion of the canal is the key to negotiating the final apical curves. This is the reason for the popularity of the "Crown-Down" technique.
(Astute clinicians will understand that the "Crown-Down" technique is merely the extension of the principles of straight line endodontic access to the deeper areas of the canal space. In the same way that we all have been taught to remove "Triangles 1 and 2" when accessing an anterior tooth, it all begins with how the instrument is initially guided into the canal orifice. If the instrument must negotiate the first curve at the access or orifice, it is unlikely it will be able handle the apical curves.)
Concept #2 - The access is complete when a DG 16 explorer can "stand" unaided in the canal.
How do we know when access is sufficient? I have taught students to literally place a DG16 explorer in the canal and have it "stand up" (balanced in the canal orifice) without being held. A conscious effort should me made to ensure that the explorer does not touch any other part of the access. When it "stands" on its own, access is sufficient.(Show DG16 stand up picture)
Concept #3. Buccal extension of the access
Buccal extension of the access is now the rule. By extending the access opening to "nontraditional areas" we allow for better direct line access to the deeper portions of the canal. While there is no need for unjustified removal of sound coronal tooth structure, those clinicians who criticize these accesses for being "too big" are most often NOT the clinicians who are performing the endodontics. (See Example Cases below for specific modifications of different teeth)
Concept #4. Creation of the "Glidepath"
General Dentists are often amazed at how quickly Endodontists treat their cases. Speed of treatment has very much to do with how easily and quickly instruments can consistently be placed in the canals. Dr. Ken Serota coined the term "Glidepath" with reference to how instruments are inserted into the canal orifice. The term describes the way that endodontic access is designed to
This dramatically decreases treatment time by :
- (a) allow for straight line access to the canal and
- (b) allow the clinician to place instruments in the general vicinity of the orifice and have them passively led into the canal space as they are inserted.
- i.e./ If the access is designed with the proper Glidepath, placing an instrument in "the vicinity" of the DB part of the access leads it directly into the DB orifice with no effort.
Concept #5. Coronal tooth structure is not sacred
- (a) limiting the need for mirrors
- (b) eliminates the need to "search" for the orifice with the file
- (c) reduces the chance of acutely bent instruments that must be discarded
Endodontists who perform these type of accesses are occasionally criticized by Prosthodontists and Cosmetic dentists for being too aggressive in removal of sound coronal tooth structure. It is sometimes necessary to remind these clinicians that their restorative treatment depends on sound periapical health. ( Both Endodontic and Periodontal) The best Prosthodontics is of absolutely no value in cases where the endodontic or periodontal treatment results are less than optimal. Straight line access is integral to successful endodontics.
Example Case 1- The Maxillary Lateral Incisor
Higher failure rates are associated with treatment of the maxillary lateral incisor. We know that the apical section of this root curves both distally and palatally in most cases. This is not a particularly difficult tooth to treat when proper access is made. However, when access is small and poorly designed, the final palatal curve is almost impossible to negotiate. The result is that these cases are often ledged, filled short or perforated at the final curve. (Diagram)
Example Case 2- The Mandibular Incisor with 2 Canals
Buccal extension of the access is not a new concept. Some clinicians have even suggested that anterior teeth with two canals ( such and mandibular incisors and Cuspids) should actually be accessed from the buccal surface! They claim that this produces straighter line access to the often missed lingual canals and they are correct. These lingual canals are missed because most incisor accesses are made at the cingulum. The instrument must first be placed toward the buccal as it is negotiated into the buccal canal orifice. But with the lingual canal, It must immediately make a sharp transition toward the lingual to get into the lingual orifice. The classic endodontic access in the mandibular incisor often does not take this into account. That is why when an Endodontist opens a suspected two canal lower anterior tooth, you will often see the access is moved very far labially, In many cases it incorporates the lingual dentin and enamel all the way to the incisal edge. We may be criticized for the aesthetics of this type of access but it is the only way that these cases can be treated successfully by non-surgical means.
Example Case 3- MB canals of Mandibular Molars
We can also extend these principles to the molars. Mesiobuccal canals of mandibular molars have a particular characteristic that often is not noticed by clinicians performing endodontics. Whereas the the Mesiolingual orifice is centered under the mesiolingual cusp, the mesiobuccal canal orifice is often located much further buccally than anticipated. The "Classic" trapezoidal endodontic access preparation does not take this into consideration. (Diagram)
Unless accommodation is made, the file shaft often is quite close to the MB cavosurface margin of the access or orifice and may be deflected by it. The file must first "negotiate" this curve as it enters the orifice. This happens even before it again must curve toward the lingual aspect of the tooth where it sometimes joins the mesiolingual canal on its way to the apical foramen. In most cases the file is able to negotiate two of these turns but as the final curve occurs either mesially or distally, there is so much tension on the instrument that, as in the maxillary lateral incisor, the canal often ledges. The clinician becomes frustrated and the case is filled short of the apical foramen.
The best way to prevent this is by modification of the access by buccal extension. This is done by again extending the principles of "Triangle1 and 2" by removing the cusp tip of the mesiobuccal cusp. In some cases where CL V buccal restorations have been placed, the orifice may be even further down the root than anticipated. In that case a "slot" is actually cut in the MB cusp that allows for straight line placement of the file into the orifice. You will notice immediately that the "first curve" that you negotiate is no longer high up in the area of the orifice,it is further down the canal. This allows you instrument to negotiate the two turns that are required to reach the terminus, the lingual curve and the M-D curve ( depending on the root curvature).
Example Case 4 - MB1 canals of Maxillary Molars
Good direct visibility is paramount. This may include removal of the ENTIRE MB cusp in molars. The Buccal canals are often tucked far to the buccal and you will NEVER see them if you don't do this. They also often have multiple curves ( bucco-lingual and Mesio distal) You'll never negotiate those unless you get as straight line access to the orifice as you can. I sometimes even use a "slot" type access that moves the opening very far buccally. Cameras or scopes won't help you if this is not done correctly. (2) The creation of a "Glide Path" is essential to smooth shaping. You really should be able to place instruments into the canals WITHOUT LOOKING. If you are constantly picking up your mirror to get an instrument into the canal, you are wasting time and causing much unnecessary stress. The problem in that case is that the access is gouged or ledged and the instruments to not naturally glide down the prepared access walls right into the canal. i.e./ place the instrument "somewhere" in the MB of the orifice, and it goes directly down the canal. Same for DB, palatal etc. This may seem like an obvious concept but the access cavity is much like a crown prep in that it has to be refined and contoured properly t be effective. (Some of the aesthetic dentistry hot shots even criticize Endodontists for accesses that are " too big". In a lecture I saw a couple of years back I recall Faigenbaum saying that he would "ream out" (no pun intended!) an Endodontist for making too big an access. My response: Listen hot shot - If I don't clean, shape and fill the canals properly, the endo will fail, the patient won't be comfortable and it won't matter what kind of fancy restoration you put on there.)
Example Case - MB2 canals of Maxillary Molars
The MB2 of maxillary molars is most often very small. Recent studies have shown that as many as 90 % of first molars have these canals. Fortunately almost all but 4-6 % of them are joined to MB1. In those cases it may be possible to just treat MB1 without even knowing about MB2 because sealing the common formen is often enough.
Location of MB2 is a whole topic in itself and I will address that in another area of the web site. Having found MB2, it is important to get direct line access in this canal because of the minimal diameter of the canal and the tendency for their to be a "proximal lip" that covers the canal from the mesial aspect. If this mesial "lip" is not removed, the file will be deflected distal upon initial insertion. Just like the other canals mentioned above, this initial bend will make it difficult to negotiate the file as it moves buccally to join MB1 and then distally as the root curves. Creation of a proper Glidepath for this canal also solves the problem of repeated inadvertent entrance to MB1 when what you to do is insert the instrument into MB2.
Exceptional or Unusual Anatomy
In the case of bayonet or dilacerated canals, there may be another curve that can be very challenging even with adherence to the above principles. I believe that these cases should be radiographically anticipated before treatment and referred before the clinician ledges the case or breaks an instrument.