Access Cavity Preparation & Working Length Determination INTRODUCTION Most of the endodontic literature published, specially the text books have a tendency to concentrate or rather emphasize on the preparation of access cavities in teeth with ideal anatomic crowns and root canal systems. It is however, equally important for the clinician to be well versed with the practical world of the canal morphologies and their complexities that exist. Right from the early work of HESS to the recent studies which demonstrate the anatomic complexities of the root canal system, it has been shown that a root with gracefully tapering canal and a single apical foramen is more of an exception rather than a rule. Investigators have shown multiple foramina, deltas, accessory canals etc. in most of the teeth. The success of root canal therapy, thus has been equally attributed to a careful access cavity preparation as much as to the obturation. As Ingle rightly states, the apical–moisture–proof seal, which is so very essential for the success of non – surgical endodontic therapy is not possible unless the space to be filled is carefully prepared to receive the restoration. If mentioned as in relation to Operative Dentistry “The final restoration is rarely better than the initial cavity preparation”.
Divisions of Cavity Preparation For sake of descriptive convenience Ingle has divided endodontic cavity preparation into A. Coronal Preparation 2. Radicular Preparation
Principles of Endodontic Cavity Preparation Any discussion of cavity preparation must ultimately revert back to the basic principles of cavity preparation established by Dr. G.V. Black. -
By slightly altering these principles, a list of principles of Endodontic cavity preparation is established. Thus as the preparation of coronal and radicular portions was divided, similarly the principles, therefore we have :
Endodontic Coronal Cavity Preparation: - Principles I. Outline Form II. Convenience Form III. Removal of Remaining Carious Dentin (and defective restoration) IV.Toilet of the Cavity I. Outline Form In order to obtain complete access for instrumentation from the cavity margins to the apical foramen the outline form of the endodontic cavity must be correctly shaped and positioned. Moreso, the external outline form evolves from the internal anatomy of the tooth established by the pulp. Due these
internal external relationships endodontic preparations are through necessity done in a reverse manner i.e. from the inside of a tooth to the outside. Thus to achieve an optional preparation with an ideal outline form, three factors of internal anatomy must be considered. i. The size of the pulp chamber ii. Shape of the pulp chamber iii. Number of individual root canals, their curvature and their position. i. Size of the pulp chamber ďƒ in young patients the endodontic cavity preparation is surely more extensive than in an older patient in whom the pulp has receded and the chamber is small in all 3 dimensions. ii. Shape of the pulp chamber ďƒ for an ideal outline form, the finished outline form should accurately reflect the shape of the pulp chamber. Eg., floor of pulp chamber in molars is triangular. iii. Number, position and curvature of root canals : As far as the outline form is concerned. In order to accommodate extra canals (when present) into the prepared cavity and to instrument each canal efficiently without interference, the cavity walls have to be extended to allow an unstrained instrument approach to the apical foramen. This could be regarded as a change for convenience in preparation.Thus we could say that the convenience form partly regulates the outline form.
II. Convenience Form: -
In operative dentistry the term convenience form was concieved by G.V. Black as a modification of the cavity outline form to establish greater convenience in the placement of intra coronal restorations. In endodontic therapy however, convenience form makes more convenient (and accurate) the preparation as well as filling of the root canal.
Four important advantages / benefits are gained through this convenience form : i.
Unobstructed access to canal orifice
ii. Direct access to apical foramen iii. Cavity expansion to accommodate various filling techniques iv. Complete authority even enlarging instrument Thus it must remembered that a failure to properly modified the access cavity outline by extending the convenience form will ultimately lead to failure by either root perforation, “ledge” or “shelf” formation within the canal, instrument breakage or the incorrect shape of the completed canal preparation often as “zipping” or “apical transportation”. III.
Removal of Remaining Carious Dentin:
This, according to Ingle must be done for 3 reasons viz. a. To eliminate mechanically as many bacteria as possible from the interior of the tooth
b. To eliminate discolored tooth structure with may eventually cause staining of the crown c. To eliminate any possible bacteria laden saliva leaking into the prepared cavity -
While performing this step if a perforation results, sealing the perforation with a cement / adhesive composite from inside the cavity, preferably has been stated.
Another thought expressed by Ingle is that during removal of the defective restoration or common teeth structure if the tooth is sufficiently weakened and access cavity exposed to salivary contamination, restoration of the missing walls could be postponed until completion of the radicular cavity preparation, as this would allow greater access for instrumentation. The cavity should however be isolated with proper radicular placement.
IV.Toilet of the Cavity: This pertains to the removal of all of the caries, debris and necrotic material from the chamber before the preparation is begun. -
Calcified or metallic debris carried into the canal may act as obstruction during the canal enlargement. On the other hand, soft debris carried in from the chamber would increase bacterial population in the canal. Thus is it imperative to removal all debris.
Instruments used for this purpose include, a slow speed round bur, long blade endodontic spoon excavator and of course another good method is copious irrigation with Sodium hypochlorite. 5
Access Cavity Preparation – For Individual Teeth. For sake of simplicity we have grouped the teeth for access opening as follows: a. Maxillary anterior. Teeth
d. Mandibular anterior
b. Maxillary Pre-molars
e. Mandibular Pre-molars
c. Maxillary molars
f. Mandibular molars
In addition we would be dealing with access openings along with the following subheadings viz: 1. Average tooth length 2. Pulp chamber & Root Canal anatomies 3. Anatomic relations in situ 4. Access openings 5. Errors / Complecations
Access cavity preparation for maxillary anterior teeth: General features: (for all anterior maxillary teeth) 1. The entrance is always made through the lingual surface in the middle 1/3 region. 2. Initial entrance / penetration is made with a round ended tapered fissure bur. Only enamel is penetrated. The bur should not be forced but allowed to cut it’s own way, because if it is forced it will act as a wedge and causes enamel to “check” or “craze” and would therefore weaken the tooth.
3. Along with initial penetration, comes the convenience extension where the same bur with itâ€™s tip in the middle is turned incisally so that the bur parallels the long axis of the tooth and the enamel and dentin are beveled incisally. 4. So the preliminary outline form which is triangular is ready with a short incisal bevel and then the dentin â€œnestâ€? which would receive the no. 2 or 4 round bur to penetrate into the pulp chamber. 5. Slow speed, contra-angle no. 2 or 4 round bur then used to penetrate into the pulp chamber. -
High speed instruments, according to Ingle, must be avoided due to a lack of tactile sensation with these.
6. Once penetrated, the round bur should be worked from inside to outside to remove the lingual and labial walls of the pulp chamber. 7. Then a surgical length bur or a long tapering diamond point (accessory to Ingle) or Gates Glidden drills (of size 4 usually) are used to eliminate the lingual shoulder. 8. The no. 1 or 2 round bur maybe used laterally and incisally to eliminate pulpal horn debris and bacteria. This step aids in preventing future discoloration. 9. So the final preparation we have is mostly a triangular shaped opening, which funnels down to the canal orifice.
A short note on: Location of canal orifices (Messing & Stock) Sometimes it is difficult to locate a canal orifice in the pulp chamber floor, particularly in the posteriors. Therefore, for one, a thorough knowledge of the number of canals likely to be present and their location is essential. Usually, a good pre-operative radiograph or two from different angles is useful. Other methods used to locate canals include : a. a good access opening which allows a complete view of the pulp chamber and the canal orifices. b. (DG 16) canal explorer c. Binocular loops â€“ with a magnification of X2. These could be fitted onto spectacles. d. For the posteriors, one canal maybe located but it is not possible to decide which one. Then an instrument is placed in the canal and radiograph taken. Identification is made using the buccal object rule. e. Transillumination of the tooth using a fiber optic, keeping the light at a gingival level may reveal position of the canal orifices. f. Dyes such as iodine maybe used which show the canal orifice as a darker area. g. As a last resort a bur maybe used.
A hole 2mm deep is cut where the canal orifice is expected and parallel to the long axis of the tooth. If the canal is not located then the bur should be removed from the handpiece placed in the prepared hole and retained by soft wax. A pencil line is drawn on the buccal surface of the tooth and a radiograph taken. A, 2-D picture is obtained with helps to provide correct allignment of the bur. Then further penetration into the root maybe carried out.
I. Access cavity preparation for maxillary central incisor: Before going in to the detail of the access opening, a short update on the tooth anatomy and it’s relations are necessary. A. Average tooth length 21. 8mm. B. Pulp chamber - located in the centre of the crown - broad mesio – distally - broadest - incisally - has 3 pulp horns – with correspond to the developmental mamelons in a young tooth. 3. Root and Root canal –
Root canal – broad labio-lingually
Ovoid in cross - section mesio – distally
Conical in shape
Statistics reveal that majority of roots are straight (75%), some curve distally (8%), mesially (4%) palatally (4%) or labially (9%).
4. anatomic relations in situ: Labially – labial cortical plate maybe fused with the root, because of this proximity fenestration’s and dehiscence maybe present and also abscesses may perforate the cortical plates. -
Apex of the tooth is in relation with the floor of the nasal fossa.
E. Access opening: -
Is similar as mentioned before in general features.
Initial penetration – 2 schools of thought Grossman prefers a no. 4 round bur with high speed and coolant, while Ingle suggests a round ended tapering bur.
Enamel is penetrated the middle 1/3 of lingual surface
Then “drop” into chamber with a slow speed no. 4 carbide bur.
Remove all debris
Remove lingual shoulder using gates gladden drill, working inside out with light strokes.
Lingual shoulder is not an anatomic entity but a prominence of dentin created when the lingual roof is removed.
Therefore on removal of this shoulder and lingual root one gains direct access to the apical area of the root canal.
Thus the access cavity prepared in the maxillary central has a angular shape, with it’s apex towards the cervical zone.
II. Access cavity preparation for maxillary lateral incisor: A. Average tooth length
B. Pulp chamber
– similar to central – has only two pulp hours
C. Root and Root canal
– conical in shape – has a finer diameter – lateral canals are frequent (26%)
Majority of the roots curve distally, and therefore the root tips maybe in the centre of the cancellous bone pointing distally, thus giving an indication why abscesses arising from the laterals usually drain palatally.
C. Access opening – similar to central, but is smaller and more ovoid in shape. -
Technique for entry is same except that a smaller i.e. a no. 2 round bur may be used instead of no. 4 as for central.
Access cavity preparation in maxillary lateral need to be modified in certain case such as in anomalies such as dens invaginatus, peg laterals and talon cusps.
Access Cavity Preparation for Maxillary canines:
A. Average tooth length – 26mm B. Pulp chamber – Largest amongst single rooted teeth 11
Flame shaped – mesio – distally
Only one pulp horn present
C. Root canal – oval in shape, wider in labio palatal direction D. Anatomic relations in situ – the canine root often called as the “canine pillar” is positioned in the cancellous portion of the maxilla between the nasal cavity and maxillary sinus. -
Because of it’s great size it causes the most prominent bulge in the maxilla called alveolar or canine eminence.
Abscesses from the maxillary canine usually perforate the labial cortical plates below the insertion of the levator muscles of the upper lip and drains into the buccal vestibule. (If perforation is below this insertion the abscess drains into the canine pace and causes cellulitis).
E. Access Opening: -
Shape of the opening is ovoid, as dictated by the pulp chamber anatomy.
Maxillary Anterior Teeth: Errors in Cavity Preparation 1. Perforation – usually at the labratervical level caused by failure to complete convenience extension at the incisal. 2. Gouging – of labial and distal walls due to failure to recognize 29° lingual and 16° mesial inclination of teeth respectively.
3. Pear Shaped preparation - due to failure to provide convenience extensions – causes inadequate debridement and obturation – thus leads to failure. 4. Discoloration – of crown caused by a failure to remove pulpal debris 5. Ledge Formation and Perforation – caused by a very small cavity preparation and thus reduced access in apically curved canals. Access Cavity Preparation for Mandibular Anterior Teeth General Features: The basic principles followed in access preparation in mandibular anterior are similar to those of maxillary anterior teeth. They are as follows: 1. The entrance / initial penetration is always begun at the middle 1/3 zone. A common error here is to begin far too gingivally. 2. Initial penetration – of enamel only – is done with high speed tapered fissure bur (701 U) with an air water coolant. Do not force the bur. 3. Convenience extension towards incisal continuous with the initial penetration. Maintain point of bur in central cavity and rotate handpiece towards incisal and mesio distal so that bur parallels long axis of the tooth. Enamel and dentin are beveled toward incisal. 4. The preliminary cavity outline is thus formed with is roughly triangular in shape with apex cervically. 5. Then penetrate into pulp chamber with slow speed no. 2 round bur.
6. Once into the chamber, working from inside chamber to outside suing the round bur remove lingual and labial walls of pulp chamber. 7. Then use a long surgical bur or Gates Glidden drill (No. 2) to remove the lingual shoulder. 8. Occasionally a no.1 round bur maybe used laterally and incisally to eliminate pulpal horn bebris and bacteria. This also prevents future discoloration. 9. Final preparation shows a angular preparation which funnels down to the orifice of the canal.
Mandibular central Incisor 1. Average tooth length 20.8 mm. 2. Pulp chamber
smallest in the arch. pulp chamber is flat mesio-distally, ovoid labiolingually has 3 pulp horns when recently erupted which calcify and disappear early because of constant masticatory stress.
3. Root and Root canal – flat – mesio - distally – wide – labio – lingually
– has a ribbon – shaped configuration in middle 1/3 where bifurcation usually occur and perforates usually can occur here. 4. Access Opening – same as mentioned with greatest dimension oriented incisogingivally. Mandibular Lateral Incisor Average Tooth Length – 22.6 Pulp chamber
same, but lateral tooth has larger dimensions
maybe fused to labial cortical plate
same as central
Mandibular canine Average tooth length – 25mm more wide labiolingually
single cusp Anatomic Relation same as mandibular incisors same as maxillary cuspid with anatomy variations.
Access Cavity Preparation in Maxillary Premolars General features: 1. As we know entrance to the pulp chamber in all posterior teeth always gained
access/penetration is made II’l to the long axis of the tooth in exactly the centre of the central groove. For this the 701U tapered fissure bur with high
speed and air-water coolant is recommended for use. This initial penetration must be restricted to only the enamel surface. 2. Further penetration then into the pulp chamber is done using a slow speed round bur until the characteristic “drop” of the bur is felt. As Ingle states, if the chamber happens to be calcified and the drop is not felt then the vertical penetration is made until the contrangle rests against the occlusal surface. This depth is approx 9mm, the position of the floor of the pulp chamber that lies at the cervical level. Then while removing the bur the orifice is widened – buccolingually to twice the width of the bur to allow exploration of the canals orifices. 3. The endodontic explorer can be used to locate the canal orifices. 4. Following this, the no. 2 or 4 round bur is used at low speed, working from inside the pulp chamber to outside, to extent the cavity bucco-lingually by removing the roof of the pulp chamber. 5. Then the bucco-lingual extension and final finishing of the access cavity is accomplished using the 701U fissure bur at high speed. 6. The final bucco-lingual ovoid preparation reflects the anatomy of the pulp chamber and position of buccal and lingual orifices. Maxillary 1st Pre Molar Average tooth length – 21.5mm Pulp chamber
– narrow – mesio – distally
Wide – bucco- palatally – 1 pulp horn under each cusp – Roof of the pulp chamber is coronal to cervical line – Floor of pulp chamber is usually convex lies deep in the coronal third of the root below the cervical line. 3. Roots and Root canals – – usually has 2 roots – when roots are fused, a groove running in an occlusoapical direction divides the root into buccal and palatal portions. -
The palatal canal is generally the larger of the two and is directly under the palatal cusp and it’s orifice can be traced by following palatal wall of the pulp chamber
The buccal canal is directly under the buccal cusp and it’s orifice can be penetrated by following the buccal wall of the pulp chamber.
4.Anatomic relation – the maxillary first premolar lies below the maxillary sinus and is separated from it by a thin layer of spongy and compact bone. 5. Access opening – additional points are to be noted are:
a) The walls of the access cavity are smoothened and sloped slightly towards the occlusal surface. This occlusal divergence creates a positive seat for the temp filling. b) The borders of this ovoid access cavity should not extend beyond half the lingual incline of the facial cusp and half the facial incline of the palatal cusp. The access cavity preparation for endodontic treatment of a premolar differs from Black’s cavity preparation for an occlusal restoration (Class I). In Black’s preparation the ovoid shape runs mesiodistally and ecompasses all pits and fissures whereas endodontic preparation runs ovoid in a bucco-lingual direction and permits direct access to the root canal. Maxillary 2nd premolar Average tooth length – 21.6mm Pulp chamber – similar to 1st premolar, may have single orifice, but with 2 pulp horns. Root and Root canals – Usually single rooted (90.3%) -
(Grossman) roots maybe straight (37.4%), distal curve (33.9%) buccal curve (15.6%), ‘S’ or bayonet curve (13%).
Canals are usually ovoid in bucco-lingual direction
Anatomic Relation: roots are closer to the max sinus.
The sinus may dip down and surround the tip of the root or roots forming prominences in the sinus floor.
The roots and sinus maybe separated by a thin layer of bone, or bone maybe totally absent thus leaving only the periodontal membrane and the schneiderian membrane of the sinus.
Access Opening – Same as Ist Premolar
Errors in Cavity Preparation in Maxillary premolars (Ingle). 1. Under extended preparation – exposing only pulp horns 2. Overextended preparation – undermining of enamel walls while searching for a receded pulp. 3. Perforation – when distoaxial inclination of tooth (10°) is neglected by clinician. 4. Broken Instruments – seen when internal cavity preparation has not completely exposed orifices.
Access Cavity Preparation for Mandibular P.M. General features: 1. As in all posterior – initial penetration is performed through the occlusal surface, more precisely at the centre of the central groove. -
This is done using a high speed, air-water coolant assisted no. 702U taper fissure bur.
At this juncture only the enamel is penetrated.
2. Once the enamel thickness has been penetrated a slow speed, no. 4 round bur is used then to open the pulp chamber. If chamber is calcified penetration is continued till contrangle rests on the occlusal surface. Then while removing the bur, the occlusal opening is widened buccolingual to twice width of the bur to allow room for exploration location of the orifices. The cavity should be cut more on the buccal than on the lingual cusps. 3. Using the endodontic explorer locate the canal(s). 4. Remove the roof of the chamber with the no. 2 or 4 round bur, working inside out. 5. Finish the buccolingual extension and cavity walls with the 702U fissure bur. 6. The finished bucco lingual ovoid outline reflects anatomy of the pulp chamber and position of the centrally located canal. Mandibular 1st Pre Molar 1. Average Tooth Length â€“ 21.9mm 2. Pulp chamber â€“ The mandibular first premolar is the transitional tooth between anterior and posterior teeth, and in anatomic structure resembles both.
It has a prominent buccal pulp horn.
The prominent buccal cusp and smaller lingual cusp give the crown a 30° lingual tilt.
3. Root and Root canal – usually single rooted with a single canal which is cone shaped and simple in outline. Narrow mesiodstally and broad buccolingually. 4. Anatomic Relation – closely related to alveolar plates and sometimes also to mental canal and foramen with maybe misdiagnosed for a peri apical pathosis. 5. Access Opening – in addition to mentioned : -
To compensate for the 30° lingual tilt and to prevent perforations the enamel is penetrated at the upper 1/3 of the lingual incline of the facial cusp.
Mandibular 2nd Pre Molar Average tooth length – 22.3mm Pulp chamber – lingual horn more prominent. Roots and Root canal
– usually single rooted – greater overall girth of root canal
– closer to mental foramen
ovoid opening is widen mesio-distally to the
wider pulp chamber.
Access cavity preparation for Maxillary Molar Teeth: General Features: 1. As in all postures, the entrance to the pulp chamber is always gained through the occlusal surface. -
The initial penetration is made at the exact centre of the mesial pit with the bur directed slightly in a lingual direction .
For this step, according to Ingle a high speed contrangle the 702 U tapering fissure bur with a rounded end is ideal for penetrating enamel uptil the dentin or even perforating cast gold restorations.
Amalgam restorations maybe penetrated with a no. 4 or 6 round bur.
2. Once the enamel is penetrated, the no. 4 round bur is used to open up the pulp chamber important to remember here is that the bur should be directed towards the orifice of the palatal or mesiobuccal canal orifice, where the greatest space in the chamber exists. -
The chamber â€œdropâ€? is felt when a proper chamber exists or else if calcified penetrate till c. angle restoration occlusal surface.
3. Then work the round bur inside out to remove roof of the pulp chamber and allow location of orifices with endodontic explorer.
The canals usually are 3 viz: mesiobuccal, distobuccal and palatal. Occassionally a fourth canal i.e. a second mesiobuccal canal maybe present in the maxillary 1st molars. 4. Once canals are well located, use the round bur working inside out again to remove remaining roof (if any) -
The amount of extension required in the access cavity can be guessed by the tension of the endodontic explorer against the walls of the endodontic cavity prepared.
5. The final finish and funneling of the cavity is then completed using the 702 U fissure bur or tapered diamond points at accelerated speed. 6. The final outline form is angular in shape with the base of the angle directed buccally. Maxillary 1st Molar: Average Tooth Length: 21.3mm Pulp chamber
largest in the dental arch
4 pulp horns â€“ mesiobuccal, distobuccal mesiopalatal, distopalatal.
Therefore pulpal roof has a rhomboidal appearance
However floor is angular in shape with apex at the palatal orifice.
Anatomic dark lines, mentioned in Orbans as the â€œDentinal Mapâ€? connect the orifices.
The palatal orifice is the longest, round or oval and
easily accessible. MB - Mesiobuccal
- The MB orifice lies below the MB cusp, is long buccopalatally and may have a depression at the palatal end where 4th orifice i.e. second MB orifice maybe present. -
The distobuccal orifice is located slightly distal and palatal to the MB orifice.
Root and Root Canals -
3 roots – 3 canals – mesiobuccal mesiolingual Palatal
Mesiobuccal root -
is broad in a bucco-lingual direction usually MB roots have a distal curve but may also be straight or ‘S’ (bayonet) shaped.
Lateral canals (1%) or two separate canals (14%) maybe present.
Small and more less round in shape
Usually straight (54%) but maybe distally curved or S – shaped.
Single canal, lateral canals are occasionally present (36%) i.e. more than MB root.
Longest of the 3
May sometimes curve buccally in the apical zone â€“ important to pre-curve instruction or else would lead to perforation.
Lateral canals are present not only in roots (45%) but may also be in the trifurcation areas (18%). Anatomic Relations of maxillary 1st molar -
Lies under the maxillary sinus
So the alveolar socket may protrude into the sinus thus a bony prominence maybe produced in the sinus.
The roots maybe separated from the sinus by a periodontal ligament and mucopenosteal lining of the sinus.
This close relation thus produces soreness in maxillary posteriors due to sinusitis or infection of sinus due to pulpitis.
The divergence of the roots may : 1. Permit sinus to drop into trifurcation 2. Place root surfaces in close proximity to conrtical plates and palatal root close to lateral area of the nasal floor. Access Opening:
The access opening as I’ve mentioned is angular with the round corners extending towards, but not including the mesiobuccal cusp tip, marginal ridge and oblique ridge.
The angular permits direct access to the root canal orifices.
Loose debris during the mentioned preparation can be removed with 5.25% NaOCl.
Maxillary Second Molar: 1. Average Tooth Length – 21.7mm 2. Pulp Chamber
- similar to first molar but a. Narrower mesiodistally b. Roof is more rhomboidal in appearance c. Floor is obtuse L’ed angle d. MB & DB canal orifices are very close and may appear to have a common opening.
3. Root and Root Canals – 3 roots – closely grouped, maybe fused to form a single conical root (46%) 4. Anatomic relations
- More close to maxillary sinus than 1st molar
5. Access opening
- Same as for maxillary first molar with variations
as anatomy dictates. Errors in Access Preparation for maxillary molars.
1. Under extension – Only pulp horns are exposed 2. Over extension – gouging and weakening of tooth 3. Perforation – in function – by failing to realise depth of pulp chamber had been reached. 4. Inadequate preparation – by not understanding the inclination of the tooth (specially unopposed teeth) Access Cavity preparation for Mandibular Molars. Same as for maxillary molars. 5. When penetrating into pulp chamber with a no. 4 or 6 round bur, bur should be directed towards orifice of mandibular or distal canal. 6. Final cavity is triangular or trapezoidal or rectangular in shape with base directed mesially. Mandibular 1st Molar: Average Tooth Length : 21.9mm Pulp chamber
: Has 4 pulp horns – mesiobuccal, mesiolingual, distobuccal, distolingual. : Roof is often rectangular in shape : Floor is rhomboidal : 3 orifices – MB. ML. Distal
MB orifice – Is under the mesiobuccal cusp tip
(Grossman advocates use of Starlite D-11 explorer to locate MB orifice.
ML orifice - Located in a depression formed by the mesial and lingual walls Distal orifice - Widest buccolingually, oval in shape - additional distal orifices maybe present
Anatomic Relation in situ -
Mesial root is in close proximity to buccal cortical plate while distal is centrally located.
Apex of roots maybe close to mandibular canal depending on length of roots and height and body of mandible.
Access opening for mandibular 1st molar: -
In additional to the mentioned features, the access opening should extend towards the mesiobuccal cusp to expose the MB orifice, lingually slightly beyond the centrol groove and distally slightly beyond the buccal groove.
Mandibular 2nd Molar Average Tooth Length â€“ 22.4mm Pulp chamber â€“ Came as first molar but smaller in size and root canal orifices are smaller and closer. Relation in sites â€“ same as 1 st molar except that mesial root is centrally located and distal root is close to lingual cortical plate. 28
May be more closer to mandibular canal.
Access Opening â€“ same as mandibular first molar but smaller, greater amount of MB cusp may have to be removed because of bucco-axial inclination of tooth and thus access to MB canal.
Working Length Determination Successful root canal treatment has been stated to be the performance of a meticulous art. If clinicians are to provide endodontic therapeutic procedures with a high degree of success they must possess a complete understanding of the basic principles of endodontics which include 1. Knowledge of internal anatomy of teeth and 2. Mechanisms of determining the tooth length and working length of root canals. The important of correct working length determination lies in the facts that, an inaccurately determined root canal length may either lead to I. Apical perforation and overfilling of the root canal with increased incidence of post operative pain. II. Incomplete instrumentation and under filling with subsequent problems, among which notable would be persistent pain and discomfort from inflamed shreds of retained pulp tissues.
Thus to determine the precise working lengths many methods have been proposed over the years.
But lets first take a look at the requirements of these methods as stated by Ingle : These are : 1. Method must be accurate 2. Should be easily and readily performed and 3. Easily confused.
Methods â€“ Many methods have been proposed to determine the working length of root canals and also the length of teeth. I. Grossmanâ€™s Method -
An instrument is placed into the canal extending to the apical construction (as act by tactile sensation) and then a radiograph is taken
A stopper is also placed at the incisal / occlusal margin to help know the actual instruction length.
The radiographic lengths of both the tooth and instrument are then measured and also is the actual length of the instruments
The actual length of the tooth is then measured using the mathematical formula
Actual Length Of tooth
Actual length of instrument
Radiographic length of tooth
Radiographic length of instrment
II. Ingle’s Method 1. Measure the tooth on the pre-operative radiograph. 2. Substract at least 1.0mm for “safety allowance” such as for image distortion or magnification. 3. Set the endo scale at this tentative W.L. and adjust the stop on the instrument at that level. 4. Then place the instrument in the canal until the stop reaches point of reference. 5. Expose and develop the radiograph. 6. On the radiograph measure the difference between end of the instrument and end of the root (add or substract) this amount to the original measured length. 7. Finally from this adjusted length subtract 1.0mm “safety factor” to confirm the apical termination of the root canal at the C.D.J. Coming now to a chronological order of how generally methods to this day were developed or determination of the wave length :
1. 1950 – Bregmen used 25mm length feat probes with steel blades fixed with acrylic resins as a stop leaving a free 10mm for placement into the canal. This probe is placed into the tooth until the metabolic end touches the reference plane. The radiograph is then taken and following measured : CAD -
Apparent tooth length (as seen in the radiograph)
Real instrument length.
Apparent instrument length.
Real tooth length is measured using the formula. CRD = CRI X CAD CAI
2. 1960 – Best – determined the tooth length by fixing a steel pin – 10mm long to the labial surface of a tooth with utility wax keeping the pin parallel to the long axis of the tooth and a radiograph is obtained. -
This radiographs is then carried to a (BW) gauge which would indicate the tooth length.
Ortho wires also used
3. 1963 – Everett and Finot – designed a diagnostic X-ray grid system for determine tooth length. -
This diagnostic x-ray grid system consists of lines 1mm apart running lengthwise and crosswise.
Every 5th mm has a heavier line for easier reading.
Actually, enamelled copper-wires were placed in a plexiglass and fixed to the regular p.a. film. The grid has been of use to accurately determined tooth length.
Coming next to recent advances : 1. Xeroradiography : The new-radiography technique has potential use in endodontics. The word Xeros is from a greek word meaning dry which differentiates â€“ Xeroradiography from the conventional photochem system in that it does not need wet chemical processing or a dark room. -
Xeroradiography uses a rigid aluminium photoreception plate. This plate is electrically charged, placed in a light proof plastic cassette positioned in the mouth and exposed to x-rays.
When exposed to x-rays, the charge on the photoreceptor is dissipated according to the tissue density and a latent electrostatic image is formed.
This latent image is transformed to a visible image by the deposition of specially pigmented particles attracted to the photoreceptor plate.
This Xeroradiography may be viewed by either reflected or transilluminated light.
It has a property of edge enhancement and endodontic studies concluded that Xeroradiography provided better visualization of metallic instruments tips and root apices allowing a more accurate length measurement.
The radiation levels for Xeroradiography are also much lesser.
Therefore it is valuable addition to the endodontists armamentarium. Further developments in working length measurements led to develop of
some non-radiographic methods. 1982-Negm â€“ Introduced a novel non-radiographic method -
The new instrument (Apex finder) is used to locate the apex as well as measure the root length.
The method is based on the insertion of a plastic fine tapered barb shaft through a bevelled tube into the root canal.
When resistance to withdrawal is felt with indicates that some barbs have engaged the apical margin.
Electrical Root Length Determination: -
Was begun as early as 1962 when it was first demonstrated by Sunada.
Sunada has found the resistance to passage of an electric current, when an instrument introduced into the root canal, reaches the apical foramen to be 40 M.A.
In this one electrode is attached to the patients cheek and the other gently introduced into the root canal until the micro-ammeter indicates 40MA. This length corresponds to the tooth length.
Sunada’s ideal was the followed by Inove and Saito and then these systems were marketed : Eg : FORMATRON IV – Formatron – Parkell USA. -
Evident etc. Also we have Endometer / Audiometer which indicate reaching apex by
needle deflection / beep 53-83% success. Ushjama (1983) The modern apex locations mark by sending a mild A.C. into the canals and then monitoring the p gradient along the canal with needle type electrodes. -
The density of the constant current will be highest at the narrowest point and the electric field will change at the apical foramen.
Some of the early apex locations used DC which had to be used in canals without vital pulp tissues, blood, tissue fluids or pus, electrocytes and NaOcl, saline, EDTA or metallic restorations. However todays AC locators only electrocytes and metal restorations
may show false readings. -
However of late SAITO has redesigned these AC-EAL which work even in the presence of NaOcl – Eg: Endex Unit, Endo Hygiene Cater
Conclusion : Thus to conclude these electronic units, the new generation electronic apex locators have characterized notably improved clinical accuracy and ease of operation which thus promise their acceptance for the future in endodontic practice.