instruments of critical path technology (protaer next - part i
DESCRIPTION
Part one of overview on endodontic file with Critical Path Technology also known as Protaper Next from Dentsply MailleferTRANSCRIPT
1
Instruments of Critical Path April 10, 2013
Part I Michael J. Scianamblo, DDS
Abstract Instruments of Critical Path are a new set of endodontic instruments used to clean and enlarge the root canal space. Various prototypes that have been provided by Maiffeler (Dentsply International) are referred to nominally as X‐Files or Swaggering Files. The instruments feature symmetrical cross‐sections that are off‐set, placing the center of mass of the instrument at a specified distance away from the axis of rotation. The off‐set center of mass allows the instruments to generate precession, and form mechanical waves within the canal. Part I discusses the “critical path” of the root canal space, which is defined as a path that is equidistant circumferentially from the center of the canal forming an ideal endodontic cavity preparation. A theoretical or “critical set” of instruments is described, whereby the diameter of the files increase equally and is a function of the area of a circle πr^2, thereby expanding exponentially. Introduction The objective of a root canal preparation or ECP (endodontic cavity preparation) is to create a space that can be predictably cleaned or disinfected and filled to prevent further infection. These objectives were outlined by the earliest investigators Hunter (1911), Hess (1916), Rosenow (1917) and Stewart (1955). The first endodontic instruments were introduced by Maynard in 1838, who fashioned them from notched piano wire. They were called rat‐tail or R‐Type files and are still in use as the barbed broaches (Figure 1A). The Kerr Manufacturing Company developed K‐Type instruments (Kerr, 1919). Examples of K‐Type instruments are reamers and files (Fig 1B and C). They are available in carbon steel, stainless steel, and more recently, an alloy of nickel‐titanium. Traditional files and reamers were manufactured in 21, 25 and 30 mm lengths with a working or cutting surface that was extending from the tip (D0) to shank (D16) with a standardized 0.02mm per
2
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ure 2). H‐etal blankhe pull distrument
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‐type filess to formrection ots due pri
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ure 1
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(1976) co
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that the u
n produc
Weine, A
1974) wa
al prepara
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his taperin
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ylic blocks
es. Their c
utilization
and filing
usly tape
t coronal
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g a small
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ot canal s
use of file
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as the firs
ation refe
esign obj
ng space
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n of stand
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ring. The
to the ro
emory of
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es from th
ermediat
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more effic
ystem th
es serially
apering s
et al (198
Figu
st clinician
erring to t
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was acqu
an “envel
uate the e
ons were
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f instrum
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These cha
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o alleviate
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ged in a d
ide a deta
edure as c
cluded the
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ness of va
at discon
n either a
at were ir
he so call
aracterist
a predile
e this pro
of a pre‐cu
al Standa
e instrum
monstrat
oval of de
rations. T
ck modali
e the pro
discussion
ailed disc
cleaning a
e continu
rumentat
Weine et
arious inst
certing, d
a reaming
rregular i
led "elbow
tics were
ection to s
oblem, W
urved file
ards Orga
ment. Cof
ing that a
ebris from
hey also d
ity, was m
oblems de
n of anti‐c
cussion of
and shapi
uously tap
tion alter
al (1975)
trumenta
demonstr
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eine sugg
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ffae and B
a tapering
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escribed b
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Brilliant
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rated
ective
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e filing.
4
This method advocated the removal of conspicuous amounts of tooth
structure from the outer walls of the curve of a root canal system. This
provided a safer approach to the root apex, in addition to protecting the
furcation of multi‐rooted teeth. Marshall and Pappin (1980) advocated an
innovative approach to root canal preparation described as a crown‐down
technique. This method addresses the canal by expanding the preparation
coronally before an attempt is made to reach the apex. Pre‐curvature of
instruments was found to be unnecessary, however, the apical zip, as
described by Weine, could still be detected.
Roane et al (1985), described a technique for root canal preparation called “balanced force”. The technique was a variation of reaming, which included "back‐turning" the file in a counter‐clockwise direction, and purportedly maintained the contour of the canal without transporting the apical foramen. Theoretically, the restoring force or elastic memory of the file, as described by Weine, was be overcome when it is pit against dentinal resistance. However, Blum, Machtou and others (1997) found that these techniques, were a predisposing factors associated with unpredictable breakage. The advent of Nickel‐Titanium rotary instrumentation has changed the landscape of endodontic cavity preparation measurably. The earliest investigators including Glosson et al (1995) and Esposito et al (1995), suggested that Nickel‐Titanium rotary instruments were superior to hand instrumentation in maintaining the original anatomy requiring fewer instruments. However, Schafer (1999) found that Nickel‐Titanium instruments with traditional cross‐sections and sizes left all curved canals poorly cleaned and shaped, whereby tooth structure was removed almost exclusively from the outer‐wall of the curve. Kum et al (2000), Calberson et al (2002), and Schafer and Florek (2003) stated that the greatest failing of current NiTi designs is the continued predisposition to torsional and cyclic fatigue and breakage. Current research does not completely endorse hand instrumentation or rotary instrumentation exclusively. Endodontic cavity
5
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FIG. 3A
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A preparedisagreemcavity pregenerally (from 0.2calculate foramen abetter estReferring foramen wspace) ansegmentscalled fruto the ori
ed ECS (ements maeparationagreed t0‐0.30mmthe chanas the sptimate thto figurewere assund that lens, each sestums wifice. Thes
ndodontiay exist re, describehat the pm) and lage in surface is enle changee 4B, if theumed to ngth was egment coth the diase frustum
FIG. 4
ic cavity segarding ted here apreparatiorge in theface arealarged cires in resise preparebe 12mmsectioneould be viameter coms are no
4A
space) is sthe configas the ECPon shoulde orifice (s of the Ercumferentance thaed canal bms in lengd transveiewed seontinuallyominally r
FIG.
shown in guration P, or rootd be smal(from 1.0 ECP, betwntially (anat an instbetween gth (the aversely intoparately. y increasreferred t
4 FI
Figure 3Bof an idet canal prl in the a –1.2 mmween the nd equallrument uthe orificverage leo twelve These seing from to as D0 ‐
G. 4C
B. Althoual endodeparationpical foram). If we orifice any), we caundergoece and theength of tequal egments athe foram‐ D12.
ugh ontic n, it is amen
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are men
7
Referring back to figure 3A, assuming that the diameter of the ECS averages
0.20mm (initially) along its entire length, the surface area of each of a
transverse cross‐section of any part of the original canal would equal r2 or A = times (0.10)2, which equals 0.0314 mm2. Referring to figure 4A and 4B, if
the diameter of the foramen (D zero) would remain at 0.20mm and the
diameter at the orifice (D12) would increase nominally 1.0mms, the diameter
at the orifice would equal 1.2mm and the middle diameter (D6) would equal
to 0.70mm. The outline of this ideal cavity preparation in (figure 3B) is termed
the “critical path” of the endodontic cavity preparation. In calculating the
increases in surface areas of the various cross‐sections as the canal is
enlarged and using the formula for the area of a circle A = r2, the area at the orifice at D12 would increases from 0.0314 mm2 to 1.1131 mm2. This is an
increase in size of approximately 36 times. If the same calculation were made
at D6, the diameter of the ECS would increase from 0.0314 mm2 to 0.7854
mm2 and increase in size of 25 times. Thus, each of these areas is seen to
increase exponentially not linearly or geometrically. Almost all manufactures
of endodontics instruments use the ISO system to sequence files, which is
linear (see table 1). Thus, arriving at an ideal ECP efficiently and safely with a
small number of instruments is problematic useless an exponential system for
sizes is adopted.
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The cross‐sectional diameters of traditional instruments specified by the ISO System. Instruments or tip sizes 20‐70 with an invariable taper 0.02mm per
mm are shown. Expressed in 100’s of mm’s
20/02
25/02
30/02
35/02
40/02
45/02
50/02
55/02
60/02
D1 20 25 30 35 40 45 50 55 60
D2 22 27 32 37 42 47 52 57 62
D3 24 29 34 39 44 49 54 59 64D4 26 31 36 41 46 51 56 61 66
D5 28 33 38 43 48 53 58 63 68
D6 30 0%
35 17%*
40 14%
4513%
5011%
5510%
60 9%
65 8%
707%
D7 32 37 42 47 52 57 62 67 72
D8 34 39 44 49 54 59 64 69 74
D9 36 41 46 51 56 61 66 71 76
D10
38 43 48 53 58 63 68 73 78
D11
40 45 50 55 60 65 70 75 80
D12
42 47 52 57 62 67 72 77 82
*Represents the percentage increase in areal diameter from the previous cross‐sectional. The average of these changes from at D6 is 10%.
Table 1 Further evaluating the conventional ISO system, we again refer to figure 2 and the diameter at D1. The instruments are specified to increase in 0.05mm increments from as D1 beginning with the number 10, which is 0.10mm at D1, progressing to the number 15, which is 0.15mm, and continuing 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, up to 0.60. Then, the instruments increase in size via 0.10mm increments to 0.70, 0.80, 0.90, 1.00 etc. All instruments maintain a constant taper of 0.02mm per mm (with calibrations
9
in 100ths of mms) from D1 to D16. Thus, using an ISO sequence of instruments to increase the diameter at D6 from 0.20mm to 0.70mm would require the use of at least 9 different instruments that increase in diameter linearly and somewhat randomly. As seen in table 1, the change in diameter from a size 30 file to a size 70 file at D6 ranges from 17% to 8%. Thus, the change in diameter over this 9 instrument sequence is exactly 9% or a linear change of exactly 1% per instrument. While it may be reasonably safe it is not considered efficient, and is inconsistent with the rapid change in diameter that occurs during ECP. Other investigators have recognized the deficiencies of the ISO system. Schilder (1991) attempted to mitigate the problem by imposing a constant 29.17% increase size of the diameter of the instrument at D1, while maintaining a constant taper of 0.02mm/mm. This instrument set was called Series 29, whereby the ISO #10 (0.10mm at D1) was designated as the No. 1 and the ISO #60 (0.60mm at D1) the No.8. As described in the patent, literature this system could also be applied to a set of 6 instruments with a constant 43.1% increase at D1 or a set of 7 instruments with a constant 34.8% increase at D1. Although Schilder did not delineate this mathematical formula to determine how a set containing N number of instruments would have an equal percent increase, you would need N‐1 steps to get from instrument no. 1 (0.10mm at D1) to the number 8 (0.60mm at D1). You would then need to find the increase ratio, A, where A=6^(1/N‐1). For a series of 8 instruments A = 6^(1/(8‐1) or 6^(1/7). To find percent, we multiply the ratio by 100%, and then to find increase, we subtract 100%. A numerical sequence whereby each term (or number) is multiplied by the same factor to obtain the following term is a called geometric sequence. Thus, this set of instruments was more efficient, but was geometrical not exponential, which is more logical. With the introduction of nickel‐titanium alloys in endodontic drill manufacture, it became clear that large amounts of tooth structure could be removed rapidly in and around the greater curvature of the canal. This inspired the introduction of a broad range of new instruments with common
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tip sizes, but with tapers that were significantly greater than the 0.02 taper of the ISO conventional instruments (McSpadden 1998 and Johnson, 2000). These included instruments with 0.04, 0.6, 0.8, 1.0 and 1.2 mm per mm tapers. Some of the first nickel‐titanium instruments to become popular were Profile and the Profile GT manufactured by Tulsa Dental Products. The distribution of the diameters of the 0.06 tapered instruments is listed in table 2 below. Although fewer instruments are required to carry out the same work as the instruments designated by the ISO system, it can be seen that these instruments still increase in size linearly. With these deficiencies in mind, a new instrument sequence derived by subdividing the ECP, and particularly the most vulnerable area of the canal, into equal parts was undertaken. This set of instruments is termed the “critical set” which dictates equal subdivision of the "critical path" (see Figure 5A and 5B). The arrays of diameters using tip sizes 15, 20, 25, 30, 35, and 40 and 0.06
tapers of typical NiTi instruments are shown below.
Expressed in 100’s of mm’s
15/06 20/06 25/06 30/06 35/06 40/06D1 15 20 25 30 35 40
D2 21 26 31 36 41 46
D3 27 32 37 42 47 52
D4 33 38 43 48 53 58
D5 39 44 49 54 59 64
D6 45 7%*
5011%
5510%
609%
65 8%
70 7%
D7 51 56 61 66 71 76
D8 57 62 67 74 77 82
D9 63 68 73 80 83 88
D10 69 74 79 86 89 94
*Represents the percentage increase in areal diameter from the previous cross‐sectional. The average of these changes from at D6 is 7.5%.
Table 2
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Instruments of Critical Path‐Progressive and Equal Diameters Method Again, referring to Figure 4A, 4B and 4C, the critical path of the endodontic cavity preparation has been defined as that space which lies between the orifice area and the foramen of the tooth, whereby the diameter of the canal has been enlarged from of the foramen to the orifice and approximates 0.20mms to 1.2mms respectively and has a continuously tapering shape. Experienced clinicians are aware, that the instruments used in this enlargement procedure, particularly nickel‐titanium rotary instruments, are most vulnerable at the fulcrum or greatest curvature of the critical path. Indeed, instrument failure is rare in canals that are reasonably straight. Although the point of greatest curvature is somewhat variable, it is clear that the point, also known as the elbow of the curve, is most often equidistant between D0 and D12 or the mid‐point of the preparation. If the average length of the critical path is 12mm, the fulcrum would most often be found at D6 or that segment located approximately 6mm from the foramen. Thus, if six separate instruments are employed to accomplish the work of enlarging a curved canal, an ideal sequence of instruments could be constructed by calculating the changes that occur in the diameter of the ECS at D6 or the elbow of the curve. More specifically, if the canal is to be enlarged sequentially from 0.20 to 0.70mm at D6 and 0.20 to 1.2 mm at D12, an instrument set where the diameter of each instruments increases in
area equally can be devised using the formula of the area of a circle A = r2.
The specific calculations for these values can be found in table 3 and are derived in the following manner. With reference to D6 of Figure 4C and referring to Figures 5A and 5B, an outer circle and an inner circle form an annulus. If the total area of the inner circle is subtracted from the total area if the outer circle, the total area to be prepared or enlarged can be defined. Thus, the total area of the annulus at D6 can be subdivided into six separate areas each with its own diameter. These diameters may be nominally referred to as d0, d1, d2, d3, d4, d5, and d6 with corresponding radii r0, r1, r2, r3, r4, r5, and r6.
1
A
A
A Ioia
A
12
A outer circle
A of annu
A of annu
f the annor model nstrumenannulus a
A of annu 6
FIG.
e – A inner c
ulus at D6
ulus at D6
nulus werset of insnt would at D6 or
ulus at D6
5A
circle = (.3
= (.1225
= (.1125
e then distrumentincrease
= (.112 6
35)2 ‐ (.1
5) ‐ (.01
5)
vided equs, also cain diame
25) =
1)2
)
ually intolled the “eter by on
0.01875
o 6 segme“critical sene‐sixth th
FIG
ents to faset”, then he total a
G.5B
shion an each area of th
ideal
he
13
Thus, a formula to determine the various radii that emerge at D6 as the area of annuli at D6 are divided equally, whereas n is the number of the annuli to be prepared, is
rn2 = rn‐1 2 + 0.01875 = rn2 = rn‐1 2 + 0.01875 = rn2 = rn‐1 2 + 0.01875 Thus, the radius that occurs at r1 after instrument No. 1 is allowed to prepare the canal to its maximum diameter is r1
2= ro2 + 0.01875 r1
2= (0.1)2 + 0.01875 r1
2= 0.010 + 0.01875 r1
2= 0.0388 r1= 0.1696 If the diameter is twice the radius, then the inner diameter of D6 is d1, which is d1
= 0.339mm. Continuing to use the formula = rn2 = rn‐1 2 + 0.01875, values for r2, r3, r4, r5, and r6 can be determined and in term values for d2, d3, d4, d5, and d6 at D6 whereby the critical set of instrument can be derived. The cross‐sectional diameters, d1, d2, d3, d4, d5, and d6, that occur at D0, D6,
and D12 in the critical path of an endodontic cavity preparation. These diameters can be utilized to develop an ideal or preferred set of instruments, also called the critical set, and is, for the moment, theoretical.
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Result The total area of annulus at D6 can be subdivided into six separate areas each with its own diameter. These diameters may be nominally referred to
as d1, d2, d3, d4, d5, and d6 d0 d1 d2 d3 d4 d5 d6
D0 .20 .20 .20 .20 .20 .20 .20
D6 .20 .339 70%*
.43628%
.51515%
.58313%
.644 10%
.7008%
D12 .20 .522 .712 .86 .986 1.098 1.2*Represents the percentage increase in cross‐sectional area from the previous diameter or instrument.
Table 3 As mentioned above, constructing the critical instrument set will necessarily require tip sizes and tapers that cannot be garnered from conventional endodontic instruments classified by the ISO system, which vary linearly. A distributions of instrument diameters that would correspond to the critical path concept using novel tip sizes and tapers is offered below (see table 4). Interestingly, the average increase in diameter is 29%, however, this increase is achieved via an exponential progression not a geometric progression as described by Schilder (1991).
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Examples of arrays of diameters of instruments which conform to the “critical path” could be expressed as follows:
Expressed in 100’s of mm’s
20/00 15/05 20/06 24/07 27/08 29/09 30/10
D1 20 15 20 24 27 29 30
D2 20 20 26 31 35 38 40D3 20 25 32 38 43 47 50
D4 20 30 38 45 51 56 60
D5 20 35 44 52 59 65 70
D6 20 40 100%
5025%
5918%
6713%
74 10%
808%
Critical Set
20 0%
.339 70%*
.436 28%
.51515%
.58313%
.644 10%
.70008%
D7 20 45 56 66 75 81 90
D8 20 50 62 73 83 90 100
D9 20 55 68 80 91 99 110
D10 20 60 74 87 99 108 120
D11 20 65 80 94 107 117 130
D12 20 70 86 101 115 126 140Table 4
Discussion Providing a set of instruments that corresponds to the critical path concept, but has tip sizes and tapers that are recognizable to most clinician is challenging. The tip sizes and tapers provided by a novel set of instruments termed ProTaper Next, manufactured by Maillefer dental products accomplishes this objective (see table 5).
16
An array of five instruments in accordance with the "critical path" concept with common tips sizes and tapers
Expressed in 100’s of mm’s
20/00 17/04 25/06 30/07 40/06 50/06
D1 20 17 25 30 40 50
D2 20 21 31 37 46 56D3 20 25 37 44 52 62
D4 20 29 43 51 58 68
D5 20 33 49 58 64 68
D6 20 3785%
5532%
6515%
70 7%
756%
Critical Set
.33970%*
.43628%
.51515%
.583 13%
.64410%
Table 5 It can be further modified by applying variable tapers as defined by Maillefer (1998). When the volumetric expansion of the root canal space is calibrated by measuring the diameter of the X‐files and plotted from D0 (apex) to D10 (foramen), substantial uniformity in dentin removal is possible (figure 6). The X‐1 is the only instrument that deviates slightly from this uniformity, but may still be considered safe, because it would have the narrowest cross‐section at D6, and therefore be the most flexible instrument in the set.
1
F Wtf(
F
17
Figure 6
When thethe ProTafile seque(figure 7)
Figure 7
e same caaper Nextencing wa.
alibrationt, namelyas utilized
s and cal, ProTaped, a dispa
culationser Universrity in vol
s are madsal, in whlumetric
de for thehich the ISremoval
predeceSO systemis noted
ssor m of
18
Further testing was undertaken to demonstrate that this improvement in file design assists in endodontic cavity preparation in minimization of transportation, which is part of the next discussion, Instruments of Critical Path ‐ Dynamics of Precession ‐Mechanical Analysis. Conclusions The "critical path" of the endodontic cavity space is defined. The cross‐sectional area of that space is seen to expand exponentially and is a function of the area of a circle. A set of mathematical steps is outlined that would establish a "critical set" of instruments that could be used to enlarge the canal, dividing the work equally between each instrument. This concept and similar measurements were use to create a novel set of files called ProTaper Next. Calculations of the volumetric removal of dentin as compared to its predecessor ProTaper Universal were made demonstrating a more even distribution of removal.
19
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