Grado de Viabilidad

This requirement is rarely mentioned but is critical to the success of any endodontic procedure. Omission of this requirement predisposes teeth to a variety of procedural errors such as perforation and

transporta-tion and encourages the opportunity for root fracture.

Maintenance of the integrity of the apex may also fa-cilitate filling.

Although adequate amounts of coronal tooth struc-ture must be removed to provide access to the middle and apical thirds of the preparation, this tooth struc-ture should be removed judiciously. Generally, a lar-ger bulk of dentin exists in the upper and on the outer portions of roots. Thus, the removal of larger amounts of tooth structure coronally and peripherally is encou-raged. Maintenance of the furcation area of teeth is critical. Therefore, emphasis should be also be remo-val of tooth at the expense of the outer walls is po-sitively important curves. When this process is cared out carefully, a preparation can be created that provi-des almost unlimited access to all areas of the root ca-nal system eliminating the opportunity for the proce-dural errors removal of tooth structure and root frac-ture (Figs. 16.4 A, B).

Root fracture, when removal of tooth structure is sustained, there is usually a discovery several months or several years after initial endodontic treatment.

These fractures may have been present prior to treat-ment, but are usually the result of failure of the root to withstand masticatory stress after treatment or as a direct result of excessive condensation pressure du-ring compaction of filling materials. These fractures will occur in the weakest part of the root and often correspond to areas of the preparation where over enlargement has occurred. It is not surprising, there-fore, to find many fractures emanating from the fur-cation areas of teeth where excessive thinning has

A B

Fig. 16.4. A, B. Computer graphic schematics of the mesial root lower human adult molar demonstrating the close proximity of root canal sy-stems to the furcation areas. Note that the bulk of tooth structure exists on the outer walls of the curve and on the periphery of the root. These findings have significant implications during endodontic cavity preparation (Berutti, 1992).

occurred or opposite posts that have been placed in-discriminately.

Finally, conservation of tooth structure should not only lead to the maintenance of the integrity of the root complex but should enhance filling. When the apical foramen is kept small, the extrusion of sealer is minimized. Further, a foramen that corresponds to a 20 or 25 file will completely impede the flow of war-med gutta-percha during compaction (Fig. 16.5).

R-type instruments are historically the oldest hand instruments. They were the first instruments to appear, introduced by Maynard (1838) and fashioned from pia-no wire. The most typical R-type instrument used to-day is the barbed broach. It is manufactured from soft iron wire that is tapered and notched to form barbs or rasps along its surface. These instruments are general-ly used in the gross removal of pulpal tissue or debris from the root canal system. Another R-type File is a rat-tail file, which is also of historical interest but sel-dom used today.

In the early part of the twentieth century, the Kerr Manufacturing Company developed the K-type instru-ment. K-type instruments that remain in current usa-ge are reamers and K-files. They are available in either carbon steel, stainless steel and, more recently, nickel-titanium. Round wire of varying diameters is ground into three or four-sided pyramidal blanks and then ro-tated or twisted into the appropriate shapes. These shapes are strictly controlled by the American National Standards Institute (ANSI) and the International Stan-dards Organization (ISO). The manufacturing process for reamers and files is identical, however, files have a greater number of cutting flutes per unit length than reamers. Reamers and files are used to clean and sha-pe the root canal system. Reamers are used in a rota-tional direction only, whereas files can be used in a rotational or push-pull fashion. Files made from three-sided or triangular blanks have smaller cross sectional areas. Thus, these instruments are more flexible and less likely to fracture. They also display larger clearan-ce angles and are more efficient during debridement.

Triangular Files are, therefore, generally considered more desirable.

H-type files are manufactured by grinding flutes to tapered round metal blanks to form a series of in-tersecting cones. Cutting occurs in the pull direction only. Hedstrom files are extremely efficient cutting in-struments. All hand instruments are available in 21, 25 or 31 mm lengths.

Newer designs have also been studied. Schäfer ⁴⁶ compared traditional instruments with square and triangular cross-sections (K-type) and instruments with round cross-sections (H-type) to experimental in-struments with rhomboidal cross-sections. The expe-rimental instruments cut more efficiently than any of the traditional instruments used in either a push-pull (linear) or rotary modality. He also compared in-struments with 16, 24 or 32 cutting flutes along the instruments working surface. Instruments with 24 cut-ting flutes were the most efficient.

Fig. 16.5. An endodontically treated lower second molar displaying multiple portals of exit. When the apical foramen is kept small, obturation of fine rami-fications is maximized and the extrusion of sealer is minimized. Further, a fora-men that is as small or smaller than a 20 or 25 file will completely impede the flow of warmed gutta-percha during compaction (Scianamblo 1992).

INSTRUMENTS & MATERIALS

Instruments

The creation of an endodontic cavity preparation that is functional and predictable in design is an arti-stic rendering that requires a thorough knowledge of available armamentarium. This armamentarium inclu-des a combination of hand and engine driven instru-ments.

Hand instruments are all manufactured from metal wire of varying sizes. The metallurgical properties of these wires have been engineered to produce a wide range of physical properties. These wires are then ground or cut to produce specific shapes and styles.

They have been designated either R-type, K-type or H-type instruments according their varying characte-ristics.

The most commonly used engine driven instruments for endodontic cavity preparations are G-type reamers or Gates Glidden drills. They are extremely efficient and safe. The G-type drills are available in carbon or stainless steel. It has a short flame-shaped head atta-ched to a long shank. The flutes of the head are spira-led with a wide rake-angle. The tip usually has a non-cutting surface to prevent perforation. The instrument is used as a side-cutting instrument only, and is rela-tively rigid; therefore it can only be used in a straight line.

The G-type drill is available in 14, 18 and 25 mm lengths measured from the tip to the shank where it inserts into a standard slow-speed handpiece. They are available in varying diameters of 0.30 mm to 1.5 mm from sizes 1 through 6 respectively.

Walia ⁵⁵ first discussed the use of nickel-titanium ro-tary instruments in endodontics. Although he is credi-ted with the introduction of this material in dentistry, alloys of nickel-titanium and its various usages have been discussed for several decades. The instruments can be found in a myriad of shapes and sizes. They are all cut from round nickel-titanium blanks. In cross-sec-tion the earliest instruments had two cutting surfaces (McSpadden,³³) with radial lands providing the instru-ment with a so-called counter-balancing characteristic.

This system, also known as the Quantec System, is still available and features a multiplicity of instruments with various tip sizes and tapers. Subsequent instru-ments had three or more cutting surfaces. The design of these instruments is credited to Arpaio,³ Heath ¹⁹ and Buchanan.⁸ All of these instruments, however, were designed with constant or uniform helix angles with a right-hand or threading helix, which is proble-matic. Constant helix angles with right-handed cut and a right-hand helix cause the instruments to thread in-to the canal like a screw. As the instrument twists in the canal it becomes bound precipitating spontaneous breakage. The addition of radial lands did mitigate the threading problem. However, the instruments become less efficient cutting devices. In addition, an increase in torque is required to drive the instrument, causing fatigue and premature breakage. A new design elimi-nating the radial lands and altering the helix angles has been introduced by the Brasseler and is called the RaCe System. Maillefer ³⁰ in cooperation with Tulsa Dental products also introduced an instrument with a progressive taper along the working surface called the Protaper. Breakage may still occur with these instru-ments due to the fact that they still display right handed-helix with a right-handed cut, which bind in the canal.

Until recently, the work of Mizarhi et al.³⁶ Hülsmann el al.²² and others indicated that the most effective methods of endodontic cavity preparation were ac-complished by hand instrumentation. However, the advent of super-elastic nickel-titanium rotary instru-ments has altered our perspective. Super-elastic in-struments, such as these, can be carried around curves without exceeding their bending or tortional moments of failure. Stainless steel instruments, of course, are hi-ghly predisposed to both, bending and tortional fai-lure. Several investigators, including Glosson et al.,¹⁷ Esposito et al.,¹⁴ Schafer and Florek ⁴⁶ and others, ha-ve suggested that nickel-titanium rotary instruments were superior to hand instrumentation, in maintaining the original anatomy. However, Schafer and Zapke,⁴⁹ Ahlquist et al.,² and Schafer and Schlingermann ⁴⁸ found that canals prepared manually were cleaner than those prepared using rotary Ni-Ti instruments.

Indeed, Peters et al.⁴⁰ demonstrated that the rotary ni-ckel-titanium systems that they studied left 35% of the canal surface area unchanged.

In addition, Schafer ⁴⁶ found that instruments with triangular and square cross-sections left all curved ca-nals poorly cleaned and shaped, whereby tooth struc-ture was removed almost exclusively from the outer-wall of the curve. It is the observation of many clini-cians, as well as the author, that the greatest failing these instruments is the continued predisposition to tortional failure and breakage, the findings, of which, have been verified by Kum et al.,²⁸ Calberson et al.,⁹ Schafer and Florek ⁴⁷ and others.

The endorsement of the safety and efficiency of all of these instruments is probably over-stated, particu-larly by the manufacturers. Further, some researchers indicate that the effectiveness of any one instrument, over another, is not statistically significant (Kum et al.,²⁸ Peters et al.⁴⁰ and Ahlquist et al.² Notwithstanding the criticisms above, most clinicians find that nickel-ti-tanium instrument systems facilitate their treatment. In addition, rotary instrumentation facilitates the coronal removal of debris, which is desirable. And although nickel-titanium instruments do break, the use of tor-que controlled hand pieces can, to some extent, con-trol the tortional limits placed of the instruments. In addition, the use of the instruments at slower speeds or speeds in the range of 150 rpms seem to reduce fa-tigue.¹³ In addition, instruments with low or negati-ve rake angles are safer, but less efficient than instru-ments with neutral or positive rake angles. And tapers of 0.4 mm or less are safer than larger tapers, but are less efficient. Finally, fatigue and intimate failure can

be mitigated by frequent instrument replacement and moderate usage. ¹⁶

Thus far, all of the instruments that have been di-scussed have a right-handed helix and right-handed cut. Unfortunately, all instruments with this design are pre-disposed to binding or “taper-lock” and failure.

The newest approach to root canal preparation is found in the use of instruments that display a left-han-ded or reverse helix with a right-hanleft-han-ded cut. These in-struments are unusual in that they are the first instru-ments that will cut while rotating in a clock-wise di-rection, but do not thread or bind in the canal space.

The instruments feature a sharp (positive) rake angle without radial lands. They also center extremely well as the result of the compensating forces produced by the reverse helix, when pitted against right-hand cut/

rotation. For further safety, they also feature tip sizes and tapers that increase egually as a function of the area of a circle. This distribution is derived, by divi-ding the total surface area of the preparation by the number of desired instruments.

Remembering the formula for the area of a circle is A = πr², this distribution is exponential as oppo-sed the conventional instruments that increase in si-ze linearly. They can be used sequentially or

alterna-tively with convention instruments having right-han-ded helix and right-hanright-han-ded cut. They are currently being manufactured under the name CPT or Critical Path Technology.

Current research does not completely endorse hand instrumentation or rotary instrumentation exclusively.

Endodontic preparations, however, will be done using both hand instruments and rotary instruments due to the facility and safety of these instruments, when used in combination, regardless of their short-comings.

Thus, all of these systems require further evaluation and modification.

Irrigating agents

The complexity of the root canal system was deli-neated by Hess,²¹ Okumura,³⁸ Pucci,⁴¹ and others, whi-le endodontics was in its infancy. Regrettably, many of these early observers concluded, that the root ca-nal system was so complex it could not be adequately cleaned or obturated (Figs. 16.6 A, B).

As stated earlier, these conclusions, in conjunction with the focal infection theory, accounted for the

A B

Fig. 16.6. A, B. Vulcanite perfusions of the root canal systems of lower adult human molars demonstra-ting the anastomoses, fins, cul-de-sacs, and secondary and tertiary ramifications common to the human dentition. It was recognition of these complexities that hindered the advancement of clinical endodon-tics in the early 1900’s (Hess, 1916).

wholesale extraction of pulpitic and necrotic denti-tion in the early 1900’s. It was also the impetus for the rapid development of prosthetic dentistry during that time.

Although complete debridement of the root canal system was deemed impossible, it was believed that these systems could be sterilized using a broad range of sterilizing or pharmacological agents. These agents included bases, acids, aldehydes, phenols, antibiotics and steroids.

Subsequent research, including that by Grey,¹⁸

Ruddle,⁴⁵ Klinghofer ²⁵ and others has demonstrated that the root canal system can be thoroughly debri-ded and cleaned safely, using nothing more than a di-lute solution of sodium hypochlorite, and the proper enlargement or shaping of the root canal system itself (Figs. 16.7 A-F).

To this extent, it is now recommended that all root canals be irrigated with copious amounts of sodium hypochlorite in at least a 2.5% solution. In addition, this solution should remain in the system throughout the cleansing and shaping procedure and be replenished

E F

Fig. 16.7. A, B. Hemotoxylin and eosin sections of an upper first bicuspid cleansed and shaped using 2.5% sodium hypochlorite as an irrigant. The primary canal and a significant lateral canal have been totally cleansed demonstrating the efficacy of sodium hypochlorite as an irrigan (Grey 1970). C, D. Hemotoxylin and eosin sections of an upper bicuspid cleansed and shaped using nor-mal saline as an irrigant. A large amount of undigested debris remains in the isthmus area between the two primary canals (Grey 1970). E, F. Scanning electron micrographs (sectionl and plane views) of dentinal tubules exposed following endodontic instru-mentation using 2.5% sodium hypochlorite alone as an irrigant (Ruddle, 1976).

C D

A B

frequently. Thus, a reservoir of irrigant should be main-tained at all times during the root canal preparation.

This reservoir can be accomplished with the help of pre-endodontic restorations or banding, if necessary.

Numerous other irrigating solutions have also been suggested for use. These include saline, hydrogen pe-roxide, and a variety of acids, bases and other reagents.

However, Baumgartner and Mader ⁵ and others have demonstrated that nothing is more effective than so-dium hypochlorite in at least a 2.5% solution either alone or in combination with other media.

Lubricating and chelating agents

The most common agents advocated for these purposes are ethylenediaminetetracetate or EDTA, EDTA and Cetrimide or EDTAC, urea peroxide, EDTA and urea peroxide or RC-Prep, urea peroxide in an anhydrous glycerin base or Gly-oxide. These substan-ces have been advocated to aid in the cleansing and enlargement of canals, particularly those that are scle-rosed or calcified.

Nygaard-Ostby ³⁷ was the first to demonstrate that EDTA was an effective chelating agent forming a solu-ble calcium- chelate when exposed to dentin. He also investigated Cetrimide, a quaternary ammonium bro-mide, which theoretically acts to reduce surface ten-sion and increases penetrability, when combined with EDTA. Yamada et al.⁵⁷ have also demonstrated that EDTA acts to remove the deposits, or the smear layer, that is left after dentinal surfaces are ground or pla-ned. Most recently, Stewart ⁵⁴ demonstrated that RC-Prep with 15% carbamide in combination with sodium hypochlorite was the most effective method of remo-ving the smear layer. Koskinen et al.,²⁷ however, de-monstrated that most these materials are ineffective in digesting the organic component of dentin or pulp tis-sue and Michelich et al.³⁵ have demonstrated that the smear layer inhibits the percolation of bacteria and inhibits leakage after obturation. It has also been ob-served by Patterson ³⁹ that these materials have an ex-tended half-life and that they continue to react with dentin after filling has taken place. Clinicians have al-so observed that the root canal system is predisposed to ledge formation, transportation, and perforation when EDTA is employed. In light of this information, the use of EDTA is considered to be controversial.

In addition, materials containing glycerol or glyce-rin can quickly form emulsions duglyce-ring root canal pre-paration. These emulsions can obstruct the secondary

anatomy, i.e., lateral or accessory canals that the ope-rator is attempting to address. Therefore, the use of this material may be contraindicated.

Intracanal medicaments

As was previously discussed, intracanal medica-ments were an outgrowth of numerous misconcep-tions that existed during the inception of endodontic therapy. Specifically, it was assumed that root canal systems were so complex that they could not be clea-ned or filled. Thus, a pharmacological approach to endodontics was undertaken in an attempt to sterilize or fix the organic component of the root canal system that, as it was assumed, could not be removed.

Research now exists demonstrating that the endo-dontic cavity can be completely cleaned and sealed.

Thus, the use of intracanal medicaments are deemed to be unnecessary. In addition, many of these reagents are toxic, as well as allergenic. In the event that an in-terim appointment is required, leaving some residual sodium hypochlorite in the root canal space appears to be quite safe and may also serve as an effective

Thus, the use of intracanal medicaments are deemed to be unnecessary. In addition, many of these reagents are toxic, as well as allergenic. In the event that an in-terim appointment is required, leaving some residual sodium hypochlorite in the root canal space appears to be quite safe and may also serve as an effective