Influence of apical preparation size and irrigation technique on root canal debridement: a histological analysis of round and oval root canals.

AIM: To examine the debridement of round and oval root canals prepared to two apical sizes with and without ultrasonically activated irrigation. METHODOLOGY: Mandibular premolars with round (n = 48) and oval (n = 48) root canals were pair-matched after microCT scanning and randomly divided into two experimental groups (n = 20): group 1, rotary NiTi to size 20, .04 taper; group 2, rotary NiTi to size 40, .04 taper. Specimens were subdivided into two subgroups (n = 10): subgroup A, syringe and needle (SNI); subgroup B, ultrasonically activated irrigation (UAI). Untreated canals (eight oval and eight round) served as controls. Specimens were processed for histological evaluation for measurement of the remaining pulp tissue and debris (RPT), and the perimeter percentage of root canal area untouched by the instruments (PRAU). Following assessment of normality, multiple-way anova models were used to study the effects of preparation size, irrigation technique and canal cross-sectional shape, and their interactions on the RPT and PRAU (α = 0.05). RESULTS: All experimental groups had significantly less RPT than the control (P < 0.05). Both the preparation size (20 vs. 40) and the irrigation technique (SNI vs. UAI) had a significant effect on RPT (P = 0.006 and P < 0.001, respectively). Groups irrigated with SNI always had significantly greater RPT than those irrigated with UAI, irrespective of the preparation size (P < 0.001). Canals prepared to size 20 had significantly greater RPT than those with size 40 in the SNI subgroup (P < 0.001), but there was no significant difference in the UAI subgroup (P = 0.481). CONCLUSIONS: Root canals prepared to a larger size (40) were cleaner than those prepared to a smaller size (20), when irrigation was performed with a syringe and needle. When the irrigant was ultrasonically activated, smaller preparations resulted in canals that were as clean as larger preparations. This finding was common to both round and oval canals of freshly extracted premolars with vital pulp tissue.

Effect of root canal irrigation protocols on the dislocation resistance of mineral trioxide aggregate-based materials: A systematic review of laboratory studies.

The aim of this systematic review was to address the question: Do different irrigating protocols have an impact on the dislocation resistance of mineral trioxide aggregate (MTA)-based materials? The review was performed using a well-defined search strategy in three databases (PubMed, Scopus, Web of Science) to include laboratory studies performed between January 1995 and May 2017, in accordance with PRISMA guidelines. Two reviewers analysed the papers, assessed the risk of bias and extracted data on teeth used, sample size, size of root canal preparation, type of MTA-based material, irrigants, canal filling method, storage method and duration, region of roots and the parameters of push-out testing (slice thickness, plunger dimensions and plunger loading direction), the main results and dislocation resistance values (in MPa). From 255 studies, 27 were included for full-text analysis. Eight papers that met the inclusion criteria were included in this review. There was a wide variation in dislocation resistance due to differences in irrigation sequence, time and concentration of irrigants, storage method and duration, and the parameters of push-out bond strength testing. A meta-analysis was not done but qualitative synthesis of the included studies was performed. No definitive conclusion could be drawn to evaluate the effect of irrigation protocols on dislocation resistance of MTA-based materials. Recommendations have been provided for standardized testing methods and reporting of future studies, so as to obtain clinically relevant information and to understand the effects of irrigating protocols on root canal sealers and their interactions with the dentine walls of root canals.

Correction: The nanotipped hairs of gecko skin and biotemplated replicas impair and/or kill pathogenic bacteria with high efficiency.

Correction for 'The nanotipped hairs of gecko skin and biotemplated replicas impair and/or kill pathogenic bacteria with high efficiency' by X. Li, et al., Nanoscale, 2016, 8, 18860-18869.

The nanotipped hairs of gecko skin and biotemplated replicas impair and/or kill pathogenic bacteria with high efficiency.

We show that gecko microspinules (hairs) and their equivalent replicas, bearing nanoscale tips, can kill or impair surface associating oral pathogenic bacteria with high efficiency even after 7 days of repeated attacks. Scanning Electron Microscopy suggests that there is more than one mechanism contributing to cell death which appears to be related to the scaling of the bacteria type with the hair arrays and accessibility to the underlying nano-topography of the hierarchical surfaces.

Long-Term Follow-Up of a Revascularized Immature Necrotic Tooth Evaluated by CBCT.

This case study reports the successful treatment of an immature upper premolar with periapical pathosis and sinus tract using revascularization technique. Clinical and radiographic examination demonstrated the recovery of vitality, continued root development, and periapical healing at the 7-month follow-up. In addition, severe calcification of the canal was noted at the 36-month follow-up. At the 66-month follow-up, cone-beam computed tomography (CBCT) revealed complete periapical healing, apical closure, increase in root length and thickness of dentin, and severe calcification of the root canal. Even though the nature of tissue within the root canal is unknown, revascularization appears to give good clinical and radiographic success. This case report highlights that severe calcification of the canal is one of the long-term outcomes of revascularized root canals.

'Screw-in' tendency of rotary nickel-titanium files due to design geometry.

AIM: To examine the effect of several standard geometric characteristics of rotary instruments on the 'screw-in' forces and stresses generated on root dentine using 3D finite element analysis (FEA). METHODOLOGY: Four cross-sectional designs (triangular, slender-rectangular, rectangular and square) were evaluated. The area of the triangular cross-section and of the slender-rectangular model were the same. Another rectangular model had the same centre-core diameter as the triangular one. Each design was twisted into a file model with 5, 10 or 15 threads over its 16-mm-long working section. Three curved root canals were simulated as rigid surface models: θ = 15 degrees/R = 36 mm radius; θ = 30/R = 18; and θ = 45/R = 12. A commercial FEA package was used to simulate the file rotating in the canal to determine the 'screw-in' force and reaction torque on the instrument. RESULTS: Instruments of a square cross-section had the highest 'screw-in' force and reaction torsional stresses followed by the rectangle, the triangle design and the slender-rectangle design, respectively. The file with closer pitch generated lower stresses, compared with that with longer pitch. The greater the root canal curvature, the higher the 'screw-in' force and reaction torque generated. CONCLUSION: This study demonstrated that the 'screw-in' tendency depends on both the instrument geometry and canal curvature. Clinicians should be aware that certain instrument designs are prone to develop high 'screw-in' forces, requiring the operator to maintain control of the handpiece or to use a brushing action to prevent instruments being pulled into the canal.

Regeneration of dental pulp tissue in immature teeth with apical periodontitis using platelet-rich plasma and dental pulp cells.

AIM: To investigate the new tissues growing into the pulp space of immature dog teeth that were infected, disinfected and filled with blood clot (BC), dental pulp cells (DPCs), platelet-rich plasma (PRP) or a combination of DPCs and PRP in immature dog teeth with apical periodontitis. METHODOLOGY: Fifty-six immature roots from mandibular premolars of four beagles were divided into four experimental groups (n = 40) and two control groups. After the induction of apical periodontitis, the root canals of experimental groups were disinfected with NaOCl irrigation and a tri-antibiotic paste medication. The canals were then filled with different materials according to the experimental group: BC group, DPCs group, PRP group or DPCs + PRP group. Access cavities were sealed with MTA and composite. Radiographs were taken after 90 days, and the jaws including the teeth were processed for histologic analysis. The data were statistically analysed using chi-square evaluation and Student's t-test. RESULTS: Radiographic analyses demonstrated no significant difference between experimental groups in periradicular bone healing (P > 0.05), whilst those groups that used DPCs produced a significantly greater root thickening (P < 0.01). The histologic evaluation showed that the groups with PRP formed more tissues in the canals (P = 0.01). The groups with DPCs had substantially more mineralized tissue formation in the canal than those without DPCs, especially in the apical third. In DPCs + PRP group, bone-like tissue grew into the canal space from the periapical tissue. CONCLUSIONS: A combination of DPCs + PRP increased vital tissue regeneration within the root canals of immature teeth associated with apical periodontitis.

Agreement between periapical radiographs and cone-beam computed tomography for assessment of periapical status of root filled molar teeth.

AIM: To assess the agreement between periapical radiograph (PA) and cone-beam computed tomography (CBCT) for periapical assessment of root filled maxillary and mandibular molars. METHODOLOGY: Periapical radiograph and CBCT (iCat) images of 60 previously root filled molars (30 maxillary and 30 mandibular) were obtained at a review clinic. Agreement between PA and CBCT assessments of (i) number of canals per tooth, (ii) number of lesions per tooth, (iii) mesial-distal dimension of lesions, (iv) coronal-apical dimension of lesions and (v) presence of 'J'-shaped lesions were determined in comparison analyses and correlation analysis. RESULTS: There were significant differences between PA and CBCT assessment for the mean number of canals (P < 0.001) and periapical lesions (P < 0.001), mean mesial-distal (P < 0.001) and coronal-apical dimension of the lesion (if present; P < 0.001) and the mean number of 'J'-shaped lesions (P < 0.05). The magnitude of the statistical differences (or bias) was greater for maxillary than mandibular molars regarding the number and size of the lesions identified. Correlation values were weaker between PA and CBCT assessments of maxillary molars than for mandibular molars in all parameters assessed. CONCLUSION: There were substantial disagreements between PA and CBCT for assessing the periapical status of molar teeth, especially for the maxillary arch. The findings have implications in periapical diagnosis and for evaluating the outcome of endodontic care.

Quality of life and psychological well-being among endodontic patients: a case-control study.

BACKGROUND: Much is already known about the consequences of endodontic disease from clinicians' perspectives; a significant omission is an understanding from patients' perspectives. This study aimed to determine oral health-related quality of life (OHQoL) and psychological distress among subjects referred for endodontic care compared to patients in periodontal maintenance. METHODS: This was a case-control study involving 200 patients; 100 patients requiring endodontic treatment and 100 control subjects (periodontal maintenance patients). OHQoL was assessed using the short form Oral Health Impact Profile measure (OHIP-14) and psychological well-being using the short form of the General Health Questionnaire (GHQ-12). Variations in OHIP-14 and GHQ-12 scores between the 'case' and 'control' group were determined, and the magnitude of such differences through effect size (ES) calculations. RESULTS: There were significant differences in OHIP-14 summary scores between the case and control groups (p < 0.001) and significant differences across all seven domain scores (p < 0.05). The ES was moderate (0.63) with respect to summary OHIP-14 scores. There were also significant differences in GHQ-12 scores between the case and control groups (p < 0.05), but the ES was small (0.36). CONCLUSIONS: OHQoL and psychological well-being is compromised among patients seeking endodontic treatment, and to a greater magnitude than patients in periodontal maintenance.

Long-term outcome of primary non-surgical root canal treatment.

AIM: The aim of this study is to examine the survival distributions of primary root canal treatment using interval-censored data and to assess the factors affecting the outcome of primary root canal treatment, in terms of periapical healing and tooth survival. MATERIALS AND METHODS: About one tenth of primary root canal treatment performed between January 1981 and December 1994 in a dental teaching hospital were systematically sampled for inclusion in this study. Information about the patients' personal particulars, medical history, pre-operative status, treatment details, and previous review status of the treated teeth, were obtained from dental records. Patients were recalled for examination clinically and radiographically. Treatment outcomes were categorized according to the status for periapical healing and tooth survival. The event time was interval-censored and subjected to survival analysis using the Weibull accelerated failure time model. RESULTS: A total of 889 teeth were suitable for analysis. Survival curves of both outcome measures (periapical healing and tooth survival) declined in a non-linear fashion with time. Median survival of the treated teeth was 119 months (periapical healing) and 252 months (tooth survival). Age, tooth type, pre-operative periapical status, occlusion, type of final restoration, and condition of the tooth/restoration margin were significant factors affecting both periapical healing and tooth survival. Apical extent and homogeneity of root canal fillings had a significant impact towards periapical healing (p < 0.05), but not tooth survival. CONCLUSION: The longevity of treated teeth based on tooth survival was considerably greater than that of periapical healing. Both outcome measures were affected by a number of socio-demographic, pre-, intra-, and post-operative factors. CLINICAL RELEVANCE: Root canal-treated teeth may continue to function for a considerable period of time even though there may be radiographic periapical lesion present. Decision for extraction may be due to reasons other than a failure of the periapical tissues to heal.

A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments.

AIM: To evaluate the bending fatigue lifetime of nickel-titanium alloy (NiTi) and stainless steel (SS) endodontic files using finite element analysis. METHODOLOGY: The strain-life approach was adopted and two theoretical geometry profiles, the triangular (TR) and the square cross-sections, were considered. Both low-cycle fatigue (LCF) lifetime and high-cycle fatigue (HCF) lifetime were evaluated. RESULTS: The bending fatigue behaviour was affected by the material property and the cross-sectional configuration of the instrument. Both the cross-section factor and material property had a substantial impact on fatigue lifetime. The NiTi material and TR geometry profiles were associated with better fatigue resistance than that of SS and square cross-sections. CONCLUSIONS: Within the limitations of this study, finite element models were established for endodontic files to prejudge their fatigue lifetime, a tool that would be useful for dentist to prevent premature fatigue fracture of endodontic files.

A mathematical model for describing the mechanical behaviour of root canal instruments.

AIM: The purpose of this study was to establish a general mathematical model for describing the mechanical behaviour of root canal instruments by combining a theoretical analytical approach with a numerical finite-element method. METHOD: Mathematical formulas representing the longitudinal (taper, helical angle and pitch) and cross-sectional configurations and area, the bending and torsional inertia, the curvature of the boundary point and the (geometry of) loading condition were derived. Torsional and bending stresses and the resultant deformation were expressed mathematically as a function of these geometric parameters, modulus of elasticity of the material and the applied load. As illustrations, three brands of NiTi endodontic files of different cross-sectional configurations (ProTaper, Hero 642, and Mani NRT) were analysed under pure torsion and pure bending situation by entering the model into a finite-element analysis package (ANSYS). RESULTS: Numerical results confirmed that mathematical models were a feasible method to analyse the mechanical properties and predict the stress and deformation for root canal instruments during root canal preparation. CONCLUSIONS: Mathematical and numerical model can be a suitable way to examine mechanical behaviours as a criterion of the instrument design and to predict the stress and strain experienced by the endodontic instruments during root canal preparation.

Stress distribution of three NiTi rotary files under bending and torsional conditions using a mathematic analysis.

AIM: To compare and evaluate the stress distribution of three NiTi instruments of various cross-sectional configurations under bending or torsional condition using a finite-element analysis model. METHODOLOGY: Three NiTi files (ProFile, ProTaper and ProTaper Universal) were scanned using Micro-CT to produce a three-dimensional digital model. The behaviour of the instrument under bending or torsional loads was analysed mathematically in software (ABAQUS V6.5-1), taking into consideration the nonlinear mechanical characteristic of NiTi material. RESULTS: ProFile showed the greatest flexibility, followed by ProTaper Universal and ProTaper. The highest stress was observed at the surface near the cutting edge and the base of (opposing) flutes during cantilever bending. Concentration of stresses was observed at the bottom of the flutes in ProFile and ProTaper Universal instruments in torsion. The stress was more evenly distributed over the surface of ProTaper initially, which then concentrated at the middle of the convex sides when the amount of angular deflection was increased. CONCLUSION: Incorporating a U-shaped groove in the middle of each side of the convex-triangular design lowers the flexural rigidity of the origin ProTaper design. Bending leads to the highest surface stress at or near the cutting edge of the instrument. Stress concentration occurs at the bottom of the flute when the instrument is subjected to torsion.

Fatigue testing of a NiTi rotary instrument. Part 1: Strain-life relationship.

AIM: To examine the fatigue behaviour using a strain-life approach, and to determine the effect of water on the fatigue life of a NiTi rotary instrument. METHODOLOGY: Instruments of one brand of NiTi engine-file (size 25, ProFile 0.04 and 0.06) were subjected to rotational bending either in air or under water, the number of revolutions to fracture (N(f)) being recorded using an optical counter and an electronic break-detection circuit. The effective surface strain amplitude (epsilon(a)) for each specimen was determined from the curvature of the instrument (on a photograph) and the diameter of the fracture cross-section (from a scanning electron micrograph of the fracture surface). Strain was plotted against fatigue life and the low-cycle fatigue (LCF) region identified. Values were examined using two-way analysis of variance for difference between various instrument-environment combinations. RESULTS: A total of 212 instruments were tested. A strain-life relationship typical of metals was found. N(f) declined with an inverse power function dependence on epsilon(a). A fatigue limit was present at about 0.7% strain. The apparent fatigue-ductility exponent, a material constant for the LCF life of metals, was found to be between -0.45 and -0.55. There was a significant effect of the environmental condition on the LCF life, water being more detrimental than air. CONCLUSIONS: The fatigue behaviour of NiTi rotary instrument is typical of most metals, provided that the analysis is based on the surface strain amplitude, and showed a high-cycle and a LCF region. The LCF life is adversely affected by water.

Fatigue testing of a NiTi rotary instrument. Part 2: Fractographic analysis.

AIM: To examine the topographic features of the fracture surface of a NiTi instrument after fatigue failure, and to correlate the measurements of some features with the cyclic load. METHODOLOGY: A total of 212 ProFile rotary instruments were subjected to a rotational-bending test at various curvatures until broken. The fracture surface of all fragments was examined by SEM to identify the crack origins. The crack radius, i.e. extent of the fatigue-crack growth towards the centroid of the cross-section, was also measured, and correlated with the strain amplitude for each instrument. RESULTS: All fracture surfaces revealed the presence of one or more crack origins, a region occupied by microscopic striations, and an area with microscopic dimples. The number of specimens showing multiple crack origins was significantly greater in the group fatigued under water than in air (P < 0.05). A linear relationship between the reciprocal of the square root of the crack radius and the strain amplitude was discernible (P < 0.001), the slopes of which were not significantly different for instruments fatigued in air and water. CONCLUSIONS: The fractographic appearance of NiTi engine-files that had failed because of fatigue is typical of that for other metals. The fatigue behaviour of NiTi instruments is adversely affected by water, not only for the low-cycle fatigue life, but also the number of crack origins. There appears to be a critical extent of crack propagation for various strain amplitudes leading to final rupture (akin to the Griffith's criterion for brittle materials).

Low-cycle fatigue of NiTi rotary instruments of various cross-sectional shapes.

AIM: To compare the low-cycle fatigue (LCF) behaviour of some commercial NiTi instruments subjected to rotational bending, a deformation mode similar to an engine-file rotating in a curved root canal, using a strain-life analysis, in water. METHODOLOGY: A total of 286 NiTi rotary instruments from four manufacturers were constrained into a curvature by three rigid, stainless steel pins whilst rotating at a rate of 250 rpm in deionized water until broken. The number of revolutions was recorded using an optical counter and an electronic break-detection circuit. The surface strain amplitude, calculated from the curvature (from a photograph) and diameter of the fracture cross-section (from a scanning electron micrograph), was plotted against the number of cycles to fracture for each instrument. A regression line was fitted to the LCF lives for each brand; the value was compared with that of others using one-way analysis of variance (ANOVA). The number of crack origins observed on the fractographic view was examined with chi-square for differences amongst various groups. RESULTS: A linear strain-life relationship, on logarithmic scales, was obtained for the LCF region with an apparent fatigue-ductility exponent ranging from -0.40 to -0.56. The number of crack-initiation sites, as observed on the fracture cross-section, differed between brands (chi(2), P < 0.05), but not LCF life (one-way ANOVA, P > 0.05). CONCLUSIONS: The LCF life of NiTi instruments declines with an inverse power function dependence on surface strain amplitude, but is not affected by the cross-sectional shape of the instrument.

Morphometric study of the apical anatomy of C-shaped root canal systems in mandibular second molars.

AIM: To investigate the apical anatomy of C-shaped canal systems in mandibular second molars by micro-computerized tomography (microCT) and stereomicroscopy. METHODOLOGY: Forty-four permanent mandibular second molars with a C-shaped root canal systems from a native Chinese population were scanned at 100 microm intervals by microCT at a resolution of 30 x 30 microm. The apical 5 mm of each tooth was reconstructed three-dimensionally for visualization and classification of the canal configuration using Vertucci's criteria. The main and auxiliary (accessory) foramina were examined under a stereomicroscope. RESULTS: Type IV and VIII canal configurations were most often found in the apical 5 mm of these canal systems. The prevalence of accessory canals, lateral canals, inter-canal communications and apical delta were 41%, 25%, 27% and 11%, respectively. Approximately 80% of C-shaped canals had 1-3 apical foramina; the prevalence of accessory foramina was about 48%. The mean distance between the main foramen and the anatomic root apex was 0.84 mm, and that between the accessory foramen and the apex was 1.61 mm. The mean (shortest and longest) diameters of major and accessory foramina were 0.19 - 0.32 mm and 0.07 - 0.10 mm, with a mean form factor of 0.73 and 0.82, respectively. CONCLUSION: The apical anatomy of C-shaped root canal systems in mandibular second molars is extremely complex with many anatomical variations.

Comparison of defects in ProTaper hand-operated and engine-driven instruments after clinical use.

AIM: To compare the type of defects and mode of material failure of engine-driven and hand-operated ProTaper instruments after clinical use. METHODOLOGY: A total of 401 hand-operated and 325 engine-driven ProTaper instruments were discarded from an endodontic clinic over 17 months. Those that had fractured were examined for plastic deformation in lateral view and remounted for fractographical examination in scanning electron microscope. The mode of fracture was classified as 'fatigue' or 'shear' failure. The lengths of fractured segments in both instruments were recorded. Any distortion in hand instrument was noted. Data were analysed using chi-square, Fisher's exact or Student's t-test, where appropriate. RESULTS: Approximately 14% of all discarded hand-operated instruments and 14% of engine-driven instruments were fractured. About 62% of hand instruments failed because of shear fracture, compared with approximately 66% of engine-driven instruments as a result of fatigue (P < 0.05). Approximately 16% of hand instruments were affected by shear, and either remained intact or was fractured, compared with 5% of engine-driven instruments (P < 0.05). The length of the broken fragment was significantly shorter in hand versus engine-driven group (P < 0.05). Approximately 7% of hand instruments were discarded intact but distorted (rarely for engine-driven instruments); all were in the form of unscrewing of the flutes. The location of defects in hand Finishing instruments was significantly closer to the tip than that for Shaping instruments (P < 0.05). CONCLUSIONS: Under the conditions of this study (possibly high usage), the failure mode of ProTaper engine-driven and hand-operated instruments appeared to be different, with shear failure being more prevalent in the latter.

Root morphology--a study of the mandibular second molar of ethnic Chinese.

The C-shaped canal system often is a challenge to optimal cleaning and shaping. This present study was aimed to determine the incidence of fused root patterns, which indicates the presence of a C-shaped canal system, in mandibular second molars of ethnic Chinese in Hong Kong. One thousand clinical and radiographic records of patients in a dental teaching hospital were randomly selected and screened. From the panoramic radiographs, 399 patients with a total of 769 mandibular second molars were suitable for analysis. Of these selected patients, 370 (92%) presented with both 37 and 47, of which 108 (29%) pairs showed the same fused root pattern. Of all 769 second molars examined, 203 (26%) presented with a radiographic Type I morphology, and 68 (9%) and 54 (7%) with Type II and III morphology, respectively. It was concluded that approximately 42% of fused-root mandibular second molars of Hong Kong Chinese patients might be associated with a C-shaped root canal system.

Defects in ProTaper S1 instruments after clinical use: fractographic examination.

AIM: To investigate the mode of failure of a brand of nickel-titanium instruments separated during clinical use, by detailed examination of the fracture surface. METHODOLOGY: A total of 122 ProTaper S1 instruments were discarded from an endodontic clinic at a stomatological school in China over a period of 17 months; 28 had fractured. These fractured instruments were ultrasonically cleaned, autoclaved and then examined under a scanning electron microscope. From the lateral view the fracture was classified into 'torsional' or 'flexural'. The specimens were then re-mounted and the presence of characteristics of shear failure and fatigue striations was recorded under high-power view of the fracture surface. The difference in the mean lengths of fractured segment between the shear and fatigue groups was compared using Student's t-test. RESULTS: Twenty-seven separated instruments were available for analysis. Under low-power magnification, only two fell into the category of 'torsional' failure when examined laterally; the others appeared to be 'flexural'. Close examination of the fracture surface revealed the presence of fatigue striations in 18 specimens. Nine instruments (including the two putative 'torsional' failures above) fell into the shear fracture group, in which fatigue striations were absent or characteristics of shear failure of the material were found. The mean length of fractured segments resulting from fatigue failure (4.3+/-1.9 mm) was significantly greater than that for shear failure (2.5+/-0.8 mm) (P<0.001, two-sample t-test). CONCLUSIONS: Examination of the fracture surface at high magnification is essential to reveal features that may indicate the possible origin of cracks and the mode of material failure. Macroscopic or lateral examination of separated instruments would fail to reveal the true mechanism of failure. Fatigue seems to be an important reason for the separation of rotary instruments during clinical use.