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)