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 »  Home  »  Endodontic Articles 5  »  Dynamic torque and apical forces of ProFile .04 rotary instruments during preparation of curved canals
Dynamic torque and apical forces of ProFile .04 rotary instruments during preparation of curved canals
Introduction- Materials and methods.

O. A. Peters & F. Barbakow
Department of Preventive Dentistry, Cariology and Periodontology, University of Zurich, Zurich, Switzerland.

Chemo-mechanical preparation is an integral part of conservative root canal treatment (Weine 1996, Schäfer 2000). Much information has been documented during the past 30 years on the mechanical properties of ISOnormed hand instruments (Camps & Pertot 1994a, 1994b). However, during the last decade several new types of continuously rotating instruments were introduced. The evolution from hand- to engine-driven techniques was facilitated by manufacturing rotary instruments from nickel–titanium with its array of special properties (Serene et  al. 1995, Thompson 2000). Initially, hand instruments were fabricated from nickel– titanium and their bending moments and degrees of deformation were tested (Camps & Pertot 1994c, Serene et  al. 1995).
Simultaneously, studies documented that enginedriven rotary techniques including Lightspeed (Lightspeed Inc, San Antonio, TX, USA), ProFile .04 and .06 (Dentsply Maillefer, Ballaigues, Switzerland) and GTFiles (Dentsply Maillefer) produced better centred preparations without procedural errors such as apical zipping or elbows (Glosson et  al. 1995, Portenier et  al. 1998, Peters et  al. 2001a). In addition, recent clinical research indicated that the outcome of endodontic therapy performed with nickel–titanium hand instruments was superior compared to teeth treated with stainless-steel counterparts (Pettiette et  al. 2001).

Nickel–titanium files, particularly the engine-driven types, are prone to fractures. This has been discussed in laboratory studies (Zuolo & Walton 1995, Thompson & Dummer 1997a, Bryant et  al. 1998a) and in a clinical survey (Barbakow & Lutz 1997). In that survey, 76% of the responding Lightspeed users reported at least one separated instrument after completing their one-day introductory courses. However, the underlying physical principles of rotary root canal instrumentation are not fully understood nor researched. Torque measurements are carried out using the guidelines set for the ISO 3630–1 test (International Organization for Standardization 1992), which cannot be extrapolated to rotary instrumentation. Likewise, there is no concise norm for cyclic fatigue tests.
Furthermore, the few documented studies on torsional moments and forces exerted during actual canal preparation were carried out using straight canals (Blum et  al. 1999a, Sattapan et  al. 2000a). The reason for this is apparently related to technical difficulties (Fig. 1). However, nickel–titanium instruments are particularly helpful for successful shaping of curved canals. Several studies using simulated canals in plastic blocks have repeatedly proved that canal anatomy influences the performance of instruments (Thompson & Dummer 1997a, 1997b, Bryant et  al. 1998a, 1998b). This fact has recently been confirmed using canals in extracted human teeth (Peters et  al. 2001a, 2001b).
To date, several torque-controlled low-speed motors have been introduced to help reduce the incidence of separation when using rotary instruments (TriAuto ZX, Morita, Dietzenbach, Switzerland; Endostepper, S.E.T., Germering, Germany; ART-Teknika, Dentsply Maillefer). The efficacy and clinical rationale for using these torquecontrolled motors has been described recently in a case report (Gambarini 2000).
The efficacy of torque-controlled motors can be improved by relying on data collected during canal preparation. Therefore, the aim of the current study was to characterize physical parameters whilst shaping canals using a newly designed and specially constructed torque-testing platform. Torque, apically directed forces and the number of revolutions were determined for a sequence of ProFile .04 rotary instruments in both straight and curved simulated canals in plastic blocks and in curved canals in extracted human teeth. For comparison, selected ProFile instruments were also analysed according to ISO 3630– 1 and other documented, established cyclic fatigue tests.

Materials and methods.

General principles.
Engine-driven rotary endodontic instruments can be tested in a number of ways including tests according to current ISO norms (i), cyclic fatigue tests  (ii) or whilst preparing canals in plastic blocks and in extracted teeth (iii). Our newly developed torque-testing platform (Fig. 2) allows all three tests to be carried out because of the way the platform and its accessories are constructed. A main criterion of the device was to place the torque sensor between the endodontic instrument and the motor so that errors introduced by incongruent sensor axes and canal trajectories were avoided (Figs 1, 2). Variables which can be evaluated include torque on the instruments’ shanks, apically directed force, instrumentation depth and the number of revolutions. These variables can be tested using either manual feed or programmed linear feed to exclude operator variability. Furthermore, additional modules such as tempered steel phantoms can be fitted to the device and enable cyclic fatigue and taper lock tests (Fig. 3).

Figure 1. Schematic diagram showing different experimental set-ups and the relation between sensor (S) position and torque scores displayed as graphs below. Measurement in straight canals (A) can be executed correctly, whilst measurements in curved canals (B, C) lead to errors because sensors (dashed lines) are positioned differently. However, sensors positioned between motor (M) and rotating instrument yield exact scores in curved canals (D).

Schematic diagram showing different experimental set-ups and the relation between sensor

Figure 2. Major components of the torque-testing platform used during rotary preparation of curved canals:
A, force transducer;
B, torque sensor;
C, motor;
D, feed unit.

Sajor components of the torque-testing platform used during rotary preparation of curved canals

Figure 3. Tempered steel phantom to test cyclic fatigue fitted on the torque-testing device.

Tempered steel phantom to test cyclic fatigue fitted on the torque-testing device

Construction of the platform.
Figure 2 details the components of the torque-testing platform. Extracted teeth or plastic blocks are mounted on SEM stubs (014001-T, Balzers Union AG, Balzers, Liechtenstein) and secured into a specimen holder attached to a strain gauge with preamplifier (A & D 30, Orientec, Tokyo, Japan). The holder was constructed to permit lateral movement so that the device can be aligned for varying positions of the canal orifices. The torque sensor (MTTRA 2, Microtest, Microtec Systems, Villingen, Germany) and the motor (Type ZSS, Phytron, Gröbenzell, Germany) are mounted on a stable metal plate, which can be moved along a low-friction guide rail for a width of 5 cm. A linear potentiometer (Lp-100, Midori, Osaka, Japan) is attached to the sliding platform to record linear movements.
These movements can be either executed manually or by a linear drive (P01-2380, LinMot, Zürich, Switzerland), which is, in turn, controlled by a computer program called ‘ENDOTEST’ which was specifically written for this purpose (Division of Endodontology, University of Zurich). The current version of this program was written in Pascal and runs on a Macintosh Power PC (Apple, Cupertino, CA, USA). Data for torque, force and insertion depth are acquired from the sensors using three analogue canals with a 12–bit interface (PCI-MIO-16XE, National Instruments, Austin, TX, USA). Real time collection of data is possible with sampling rates solely dependent on the amount of allocated RAM. For example, the storage need for a series of 10 measurements, recorded during 1 min using all three canals with a resolution of 100 measurements per s is approximately 1.44 mB. The sensors were regularly calibrated using precision-made levers and a set of brass weights of 1– 400 g used according to the manufacturer’s instructions. Variables recorded during each measurement were logged as Nmm or Ncm, N and in mm, respectively, for torque, force and distance of canal preparation and were stored for off-line analysis.
Parameters, which can be preselected in ‘ENDOTEST’, include (i) the number of analogue canals measured (1–3, torque, force and insertion depth) and (ii) the time allotted for the measurements at a resolution varying from 1 s –1 to 10 000 s –1 . All the measuring modes can be calibrated and amplified, within the range selected for each parameter (iii). The computer program also transmits the commands required to run rotary instruments at speeds varying from 1 r.p.m. to 2000 r.p.m. (iv). The linear feed can be programmed to execute complicated movements and up to 10 preselected movements can be stored in a pull-down menu (v).

Experiment a.
A selected range of ProFile .04 instruments was tested to establish comparative scores for load at fracture and cyclic fatigue. ProFile instruments, sizes 20, 35 and 60 ( n = 8, Batch nos 168046, 1702700 and 1670470, respectively) were tested according to the ISO 3630–1 test. A soft brass chuck was fitted into the specimen holder and the apical 3 mm of each instrument was held. Rotation was set at 2 r.p.m. and torque was recorded in relation to angular deflection with an accuracy of 0.5 .
ProFile instruments, sizes 15, 30 and 45 ( n = 12, Batch nos 1604770, 1634800, 1634820, respectively) were tested to ascertain the number of rotations leading to separation in a tempered steel phantom with a 5-mm radius and a 90 angle (Haikel et  al. 1999). All instruments in this study rotated at 250 r.p.m. Six instruments in each group were rotated without linear advancement until separation was recorded. The remaining six instruments in each group were programmed to have a linear feed and an oscillating movement of 2 mm at 0.5 Hz, mimicking the clinically used ‘pecking’ motion. The time to fracture was recorded using a stop watch and the numbers of rotations calculated to the nearest full number.

Experiment b.
A sequence of ProFile .04 rotary instruments was used to prepare 10 plastic blocks with curved and 10 blocks with straight simulated canals (Ref. A 0177, Dentsply Maillefer). Working lengths in all canals were 18.5 mm and the curved canals had a 50 curvature with a 6.5- mm radius (Schneider 1971). All plastic blocks were then mounted on SEM stubs. Ten single-rooted mandibular incisors and canines selected from the Department’s pool of extracted teeth were cleaned and also mounted on SEM stubs. The teeth, stored in 0.1% thymol, were decoronated so that their working lengths were similar to those of the plastic blocks. Digital radiographs (Digora, Soredex, Helsinki, Finland) were exposed initially and after the crown-down phase to determine curvature and final working lengths. Mean canal curvature of the extracted teeth was 13.2 5.3.
Briefly, the preparation sequence included preflaring with a size 4 Gates-Glidden bur (Dentsply Maillefer) followed by a crown-down sequence using ProFile .04 instruments sizes 60–15 until working length was reached (Schrader et  al. 1999). A size 40 apical stop was then prepared before completing with a step-back sequence using ProFiles sizes 45 and 60. Canals were copiously irrigated using tap water and syringe with a gauge 27 needle. All specimens were prepared by the same experienced operator (OP) using the manual linear feed of the torque-testing device.

Data analysis.
Runs were recorded and stored in the proprietary format for subsequent off-line analysis. Then, original records were exported into a spreadsheet format (HIQ 2.2.1 for Macintosh, National Instruments). Maximum scores for torque and force were detected automatically. Numbers of rotations were counted under the condition that a minimum torque of 0.8 Nmm was present to exclude rotations occurring outside the canals. Variables were expressed as means ( SD) and compared using one- and two-way anova ’s from a commercially available statistics package (StatView 4.02, Abacus Concepts, Berkeley, CA, USA). The level of significance was set at 95%.