A2 ENDO TRIBUNE Dental Tribune Middle East & Africa Edition | 3/2019 3-D endodontic instrumentation: Revision of a historical protocol By Dr Kenneth S. Serota, USA The past The goal of the instrumentation phase of root canal therapy is to debride, disinfect and shape the root canal space prior to root ﬁlling while retaining an optimal amount of tooth structure. This is of para- mount importance in the regions of peri-cervical dentine and isthmus/ furcal anatomy.1 Historically, the signiﬁcant ﬂaws of stainless-steel ﬁles and reamers were their cutting geometry and rigidity. The techni- cal protocol for these instruments, even Dr Schilder’s innovative en- velope of motion,2 failed to correct debridement inadequacies. The root canal does not natively present in the round; Dr Schilder’s approach, while an improvement, failed to address the instrument design and technique changes required to opti- mise shaping and cleaning of the ca- nal space (Figs. 1 & 2). The root shape mimics the canal shape.3 Therefore, it is impossible to adequately sculpt the interfacial dentine of the canal unless the ﬁle chosen corresponds to the largest diameter of the non- round canal (Fig. 3), which can lead to weakening or perforation of the root structure. Studies assess- ing the planes of geometry of the root canal repeatedly demonstrate that the buccolingual diameter is greater than the mesiodistal diam- eter—canals are predominantly ovoid throughout the dentition, not round.4 Until recently, our reliance upon ﬂat ﬁlm radiography to assess the spatial dimensions of root ﬁlling furthered the lack of appreciation for ﬁle taper sizes and ﬂexibility fundamentals. The z-axis was hidden from view in ﬂat ﬁlm periapical radiographs; only the narrower mesial–distal dimen- sions of the root canal space were evidenced (Fig. 4). Faux 3-D imagery could be produced in theory by combining of angled mesial, distal and central ray radiographic pro- jections. In 2-D, cleaning to the nar- rowest diameter appears adequate in post-treatment radiographs. The introduction of microcomputed tomography (µCT) and cone beam computed tomography (CBCT) has changed our understanding of the planes of geometry produced by our current treatment protocols. Mapping of the root canal space by µCT after instrumentation demon- strates that barely 50% of the canal is cleaned (Fig. 5).5, 6 The idiom, “you can’t put a square peg into a round hole” suggests an endodontic idiom: you can’t put a round ﬁle into an ovoid canal and achieve the desired result. The most under-appreciated sequela of round ﬁles is the creation of sig- niﬁcant amounts of dentinal debris. Traditionally, the focus has been on the debris pushed through the apex during instrumentation to avoid posttreatment pain caused by peria- pical inﬂammation. The assumption that residual debris moves coronally and is ﬂushed from the canal by irri- gants is questionable. In fact, debris is pushed into the non-round parts of the canal, blocking these areas from further cleaning and disinfection by irrigation solutions and adjunctive technologies.7, 8 Fig. 1: The envelope of motion, as described by Dr Schilder, is generated by pre-curving a reamer and rotating and withdrawing the instrument during the working cycle. All the work is done on the outstroke, obviating the potential for ledge creation. Fig. 2: An axial view (cross section) of the mesial root of a mandibular molar demonstrates that the geometry of the canal space is irregular, elliptic/ovoid, but not round. (Unknown source) Fig. 3: The root shape mimics the canal shape. As such, making a round shape using the largest diameter ﬁle is clinically impractical. Using a preset taper greater than 0.04 jeopardises the integrity of the root structure. Fig. 4: CBCT provides a z-axis image that demonstrates the number of canals the second mesiobuccal canal could readily have been compromised with a relied upon. (Courtesy of Dr Martin Trope) Fig. 5: Micro-CT shows green (untreated canal) and red (treated portion of the canal after the use of a round ﬁle of minimum diam- eter). Less than 50% of the interfacial dentine was touched and debrided. (Courtesy of Dr Frank Paqué) thus preventing wall weakening or strip perforation. However, each gen- eration of NiTi ﬁles, whether ground, twisted or heat-treated, shaped and cleaned far less debris than expected from the root canal space. Unfortu- nately, while a few systems included 0.04 tapers, the vast majority of sin- gle- or multi-tapered ﬁles have 0.06, 0.07 and 0.08 tapers. Some of the lat- est systems use asymmetrical rotary motion, conforming S-shaping and reciprocal motion. Unfortunately, separation of an NiTi instrument due to taper lock, cyclic fatigue and torsional resistance remains an om- nipresent concern. The advantages of super-elasticity and self-centring were incalculable; however, the im- provements were compromised by the persistence of round-core man- ufacturing (Figs. 7 & 8). The ﬂaw in every iteration of NiTi ﬁles remains the same: the cutting geometry pro- duces a round shape. Inevitability of bio-minimal adaptive shaping A new generation of adaptive/vir- tual core ﬁles, the XP-3D system, has dramatically changed the landscape Fig. 6: An irregular canal space is shown after instrumentation with a ﬁle (round core). Note the existing debris accumulation in the canal irregularities resultant from instru- mentation. (Courtesy of Dr Gustavo De-Deus) Additionally, when irregularities are compacted with detritus, increased pressure is exerted within the canal space with the attendant possibility of microfractures (Fig. 6). This is of critical concern with the new gen- eration of nickel-titanium (NiTi) ﬁles, but not a factor with use of the XP-3D Shaper (Brasseler USA).9 The trend towards fewer ﬁles and larger tapers exacerbates this potential fracture problem. Cognitive dissonance The introduction of NiTi ﬁles fos- tered a transition to instruments that would potentially obviate the ﬂaws inherent in the use of carbon and stainless-steel ﬁles. NiTi ﬁles are super-elastic and self-centring, and avoid ellipticisation of the apical terminus. With appropriate taper selection, NiTi instruments should prevent thinning of the coronal and middle thirds of the root minimising of endodontic instrumentation. The XP-3D Shaper was designed to adapt to the anatomical shape of the canal while respecting the native frame- work of the root canal space without packing debris into untouched areas. The XP-3D Finisher (Brasseler USA) has a reach of at least 3mm, thereby touching even the widest canal di- ameters while not changing the orig- inal shape of the canal.10 Booster Tip The Booster Tip (BT) lead section ﬁts into the preestablished glide path, ensuring precise guidance and cen- tring of the instrument. A traditional glide path instrument produces a 15.02 or 10.04 size/taper. There are no cutting ﬂutes on the lead section of the BT, ensuring precise guidance and centring of the instrument. The XP-3D Shaper has a BT, which ena- bles the instrument to follow the glide path into the apical component to a depth of 0.25 mm. The next 0.25 mm section of the BT is conﬁgured with six cutting ﬂutes. Rotation of these ﬂutes sizes the next 0.25mm of the canal space from a 15.02 to a 30.02 (size/taper) instrument; thus, the apical size chosen for the XP-3D Shaper is size 30 (Fig. 9). XP-3D Shaper To better explain the unique prop- erties of the ﬁle, the physical char- acteristics of the MaxWire technol- ogy must be understood. At room temperature, the XP-3D Shaper is in the martensitic phase, enabling it to be bent and more readily placed in the canal. No more than three to ﬁve easy up-and-down strokes (swaths) of the serpentine XP-3D Shaper with the BT should result in an api- cal terminus shaped to a size 30 ﬁle and a canal taper of 0.02 (Figs. 10 & 11). The choice of a 0.3mm diameter enables a 31-gauge irrigating needle to approximate the working length, preventing vapour lock. Maximum irrigation efﬁciency is ensured. Addi- tionally, a shelf for seating the gutta- percha point prior to root ﬁlling is created. With an increasing number of strokes, the ﬁle has the capac- ity to expand from tapers of 0.01 to 0.02/0.04/0.06/0.08 while main- taining the ﬂexibility of the original 0.01 taper. At body temperatures, the ﬁle attains its austenitic charac- teristics and attempts to achieve its potential of an 0.08 taper, a maxi- mum that is needed in only the most unique cases. ÿPage A3 Fig. 7: The majority of the root canal space is ovoid. As demonstrated by the canal shape at successive levels from the apex, round ﬁles, in spite of self- centring, can weaken the root structure with a typical 0.06-tapered instrument and will NOT debride the canal in its entirety. (Courtesy of Dr Gustavo De-Deus) Fig. 8: There are approximately 157 ﬁle systems available globally. Most are made from round blanks; canals, however, are not “made” in the round. Fig. 9: The Booster Tip has no cutting ﬂutes on the ﬁrst 0.25mm. The next 0.25mm section has six cutting ﬂutes, which alters the apical extent of the canal to a size 30.02 (size/ taper) instrument. The tip design of traditional NiTi instruments enables the instrument to follow the glide path rather than actively cutting and risk ledging or torsional failure if the tip inadvertently catches in an irregularity in the canal wall. (Courtesy of Dr Sebastián Ortolani Seltenerich)
Dental Tribune Middle East & Africa Edition | 3/2019 ◊Page A2 ENDO TRIBUNE A3 As much healthy tissue as possible must be maintained; therefore, it is recommended that when the work- ing length has been achieved in the ﬁrst three to ﬁve strokes, an addi- tional ten long strokes will achieve a 0.04 taper, which is sufﬁcient to adequately disinfect the root canal space in very tight canals. In larger canals, the ﬁle will easily create larger tapers, as lesser dentinal resistance is met. As a function of its serpentine shape, light brushing and up to 30 long strokes will result in over 90% of the walls being touched in these larger non-complex canals (Figs. 12 & 13). To summarise: the ﬁle is adaptive to the original shape of the canal; thus, the tooth shapes the canal space, in contrast to round NiTi ﬁles, where the ﬁle shapes the tooth. As shown in Figure 10, the ﬁle has a sinusoidal/ serpentine shape. The space avail- able for this shape in motion enables a light brushing technique to adapt and debride 90% or more of the walls in larger non-complex canals, which contrasts dramatically with the debris removal with round NiTi ﬁles. As previously discussed, round ﬁles will pack debris into the canal irregularities, a major drawback in sufﬁciently cleaning a canal. The ser- pentine shape, virtual core and 0.01 taper of the XP-3D Shaper enable it to adapt to the canals and ensure that debris remains in turbulent solution, ensuring its optimal removal from the canal (Fig. 14). This enables the ir- rigants to work maximally as the ca- nal is shaped. Tests using photoelas- tic models have shown that apical pressure is not built up using the XP-3D Shaper, obviating concerns regarding microcracks. Round-core ﬁles should signiﬁcant generation of apical pressure (Fig. 15). Recently, new and costly irrigation devices have been introduced in the endodontic armamentarium as adjuncts to the traditional side- vented needle and passive ultra- sonic irrigation. The EndoActivator (Dentsply Sirona), the EndoSafe Plus (Vista Dental), the Endovac Pure (api- cal negative pressure irrigation; Kerr) and the GentleWave (Sonendo) are all relatively new.11 The GentleWave system claims to be capable of re- moving residual tissue, the smear layer, bioﬁlm and bacteria from the tubules.12 Further scientiﬁc assess- ment of this device remains to be done. XP-3D Finisher The XP-3D Finisher is used adjunc- tively to the XP-3D Shaper. The Fin- isher’s design allows it to access and scrape untouched components of the canal walls without altering the canal shape created by the XP-3D Shaper. The ﬁle has a tip diameter of 0.25 mm with an 0.00 taper. It is extremely ﬂexible and thus has tre- mendous resistance to cyclic fatigue. For more information contact: Dr Kenneth Serota graduated with a DDS from the University of Toronto Faculty of Dentistry in Canada in 1973 and received his Certiﬁcate in En- dodontics and Master of Medical Sciences from the Harvard–Forsyth Dental Center in Boston in Massachusetts in the US. Active in online education since 1998, he is the founder of the ROOTS endodontic forum and the NEXUS interdisciplinary forum. Dr Serota is an adjunct clinical instructor in the University of Toronto postdoctoral endodontics department. Fig. 12: In small (mesial) canals, the XP-3D Shaper ﬁle will ﬁrst reach a 0.3mm diameter and in time increase the canal taper subject to the resistance of the dentine. The virtual/ adaptive core prevents packing of debris in irregularities. Fig. 13: The µCT image to the left shows the packing of the debris into the isthmus by a reciprocating ﬁle. The image on the right shows the canal after preparation with the XP-3D Shaper. Increased resistance due to packing of debris is a common ﬂaw in round NiTi ﬁles and can result in fracture. Fig. 14: The image shows the comparison of the mechanism of cutting by a ﬁle made from a round blank and by the XP-3D Shaper. No matter how much relief is provided by reducing the taper along a ﬁle with an apical third taper of 0.06/0.07/0.08, enhanced resistance is created and irrigation turbulence is not enhanced. The OPPOSITE is true of the Shaper. Fig. 15: Photoelastic stress analysis using a monochromatic light source and plastic models demonstrates that a reciprocating ﬁle (A) creates high stress in the apical third, a rota- tional ﬁle (B) shows strong stress in the apical third and the XP-3D Shaper ﬁle (C) shows no stress in the apical third. Fig. 16: The dimensions of the XP-3D Finisher are shown in the martensitic and austenitic phases. At body temperature, the last 10 mm of the instrument during rotation achieves a sickle shape with a diameter of 3 mm. Pressure on the bulb can further enhance the tip diameter. Fig. 17: The anatomy of the canal will cause the XP-3D Finisher to expand or contract and enter small irregularities in the canal walls with an up-and-down motion. No other ﬁle can reach these indentations. Fig. 18: The natural expansion and contraction of the XP-3D Finisher contacts the irregularities of the canal walls. It is insufﬁciently sturdy to alter the original shape created by the XP-3D Shaper. Fig. 19: The XP-3D Finisher creates a robust turbulence within the irrigating solutions. Studies have shown it to remove microﬂora to a depth of 40 µ. The spoon-shaped design of this ﬁle is created in a mould in the austen- itic phase. At room temperature, the martensitic phase can be manipulat- ed to any shape. Upon insertion into the canal, the ﬁle is heated to body temperature (35°C), and the mate- rial seeks to revert to the austenitic phase (Fig. 17). In the austenitic phase, it forms a uniquely shaped cleaning instrument. At body temperature, the apical 10.0 mm of the ﬁle trans- forms into a bulb/sickle shape, while retaining a depth of 1.5mm. Without squeezing the bulb, rotation of the ﬁle produces a tip size of 3 mm. How- ever, if the bulb is squeezed, the tip will expand to a maximum of 6 mm. The instrument cannot cut; thus, its only impact is scraping, which re- moves microbes up to 40µ up the tubules (commensurate with root planing in periodontal therapy).13, 14 As it is moved up and down in the canal, a vigorous agitation of the ir- rigants (sodium hypochlorite and EDTA) occurs, which adds to an en- hanced inhibition or eradication of microﬂora presence from the root canal space (Figs. 18 & 19). Retreatment The XP-3D Finisher ﬁle has been modiﬁed for retreatment. The core is 0.03 mm in diameter with an 0.00 taper. This provides a more robust adaptation to the interfacial dentine, enhancing the removal of residual gutta-percha and debris from the ir- regularities (Fig. 20). Conclusion Preliminary studies of XP-3D ﬁles have shown remarkable removal of soft tissue, fewer residual dentinal chips in an isthmus, and bio-min- imalistic shapes of the root canal space (optimal taper of 0.04), result- ing in lower dentinal stress (fewer microcracks). An efﬁcient debride- ment and disinfection of the apical third area is achieved by the BT and the serpentine design of the Shaper. Have we achieved the ideal fusion of technology and biology for long- Fig. 20: A study by Alves et al. demonstrated that the reduction of residual debris in the canal space using the XP-3D retreatment Finisher was 69% greater by comparison to standard round ﬁles.15 term positive patient-centred treat- ment outcomes? Perhaps. What has been achieved is a redress of a design ﬂaw that has persisted for much too long. This design change will bring endodontics closer to the desired ob- jective of bio-minimal shaping that is tooth-directed. This will protect the native anatomy of the root, mini- mising functional stress and fracture potential. Editorial note: A list of references can be obtained from the publisher. This article was originally published in roots international magazine of endodontics, Issue 4/2018.
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