Joseph E.A. Palamara

Defects in Rotary Nickel-Titanium Files After Clinical Use

The purpose of this study was to analyze the type and frequency of defects in nickel-titanium rotary endodontic files after routine clinical use, and to draw conclusions regarding the reasons for failure. All of the files (total: 378, Quantec Series 2000) discarded after normal use from a specialist endodontic practice over 6 months were analyzed. Almost 50% of the files showed some visible defect; 21% were fractured and 28% showed other defects without fracture. Fractured files could be divided into two groups according to the characteristics of the defects observed. Torsional fracture occurred in 55.7% of all fractured files, whereas flexural fatigue occurred in 44.3%. The results indicated that torsional failure, which may be caused by using too much apical force during instrumentation, occurred more frequently than flexural fatigue, which may result from use in curved canals.

Physicochemical Characterization of Casein Phosphopeptide-Amorphous Calcium Phosphate Nanocomplexes

Milk caseins stabilize calcium and phosphate ions and make them available to the neonate. Tryptic digestion of the caseins yields phosphopeptides from their polar N-terminal regions that contain clusters of phosphorylated seryl residues. These phosphoseryl clusters have been hypothesized to be responsible for the interaction between the caseins and calcium phosphate that lead to the formation of casein micelles. The casein phosphopeptides stabilize calcium and phosphate ions through the formation of complexes. The calcium phosphate in these complexes is biologically available for intestinal absorption and remineralization of subsurface lesions in tooth enamel. We have studied the structure of the complexes formed by the casein phosphopeptides with calcium phosphate using a range of physicochemical techniques including x-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and equilibrium binding analyses. The amorphous nature of the calcium phosphate phase was confirmed by two independent methods: x-ray powder diffraction and selected area diffraction. In solution, the ion activity product of a basic amorphous calcium phosphate phase was the only ion product that was a function of bound phosphate independent of pH, consistent with basic amorphous calcium phosphate being the phase stabilized by the casein phosphopeptides. Detailed investigations of calcium and calcium phosphate binding using a library of synthetic homologues and analogues of the casein phosphopeptides have revealed that although the fully phosphorylated seryl-cluster motif is pivotal for the interaction with calcium and phosphate, other factors are also important. In particular, calcium binding and calcium phosphate stabilization by the peptides was influenced by peptide net charge, length, and sequence.

Torque During Canal Instrumentation Using Rotary Nickel-Titanium Files

Nickel-titanium engine-driven rotary instruments are used increasingly in endodontic practice. One frequently mentioned problem is fracture of an instrument in the root canal. Very few studies have been conducted on torsional characteristics of these instruments, and none has been done under dynamic conditions. The purposes of this study were to measure the torque generated and the apical force applied during instrumentation with a commercial engine-driven nickel-titanium file system, and to relate torque generated during simulated clinical use to torsional failure of the instruments. Ten extracted human teeth (five with small-sized and five with medium-sized straight root canals) were instrumented with Quantec Series 2000 files, and the torque and apical force generated were measured. The applied apical force was generally low, not exceeding 150 g in either small or medium canals. The torque depended on the tip size and taper of each instrument, and on canal size. Instruments with 0.05 and 0.06 taper generated the highest torque, which was greater in small than in medium canals. The torque at failure was significantly (p < 0.001) higher than torque during instrumentation, but with considerable variation in the extent of the difference.

Load and strain during lateral condensation and vertical root fracture

Vertical loads and root surface strains in extracted teeth during lateral condensation using finger and hand spreaders were measured and compared with loads and strains at fracture. Six groups each of 10 teeth were tested: maxillary central incisor, premolar and molar; and mandibular incisor, premolar and molar. Root strains were measured using strain gauges mounted on the apical and middle third of the buccal root surface. Statistical analysis was performed at the 95% level of confidence. The maximum loads and strains generated by finger spreaders were significantly lower than those generated using a hand spreader (D11T). These loads and strains were also significantly lower than the values at fracture. Most fracture lines were in a buccolingual direction, but maxillary premolars with two separate roots and the mesiobuccal root of maxillary molars showed more variation in fracture site. The results suggest that lateral condensation alone should not be a direct cause of vertical root fracture. The use of finger spreaders, however, is associated with lower risk.

Effect of Hydrogen Peroxide and Sodium Perborate on Biomechanical Properties of Human Dentin

This study compared the ultimate tensile strength, micropunch shear strength, and microhardness of bleached and unbleached human dentin. Forty-four intact premolars were root canal treated and randomly divided into four groups. Bleaching agents were sealed in pulp chambers, as in clinical use. Group 1 (control) was treated with water, group 2 with 30% hydrogen peroxide, group 3 with sodium perborate mixed with water, and group 4 with sodium perborate mixed with 30% hydrogen peroxide. The teeth were stored in saline at 37 degrees C for 7 days. The teeth were then sectioned and biomechanical tests were carried out on dentin specimens that were obtained from all teeth. Intracoronal bleaching with 30% hydrogen peroxide and sodium perborate used either alone or in combination weakened dentin. Hydrogen peroxide alone tended to be more damaging than sodium perborate used alone or sodium perborate mixed with hydrogen peroxide.

Anisotropy of Tensile Strength of Root Dentin

An effect of dentinal tubule orientation on mechanical properties of dentin has been difficult to demonstrate. We have tested the hypothesis that ultimate tensile strength (UTS) of dentin is affected by tubule (and hence collagen fibril) orientation. The UTS of human root dentin was investigated by direct tensile and diametral testing of specimens of known orientation prepared from extracted teeth. Dumbbell-shaped samples were machined for direct tensile testing (load parallel or perpendicular to tubule direction) and cylinders for diametral testing (load at 0°, 45°, 67.5°, and 90°). Fractured surfaces were examined by SEM. UTS was lowest when the tensile force was parallel to tubule orientation, and greatest at 90° to tubule orientation (fracture parallel to tubule direction). SEM views of fractured surfaces suggested that microstructure contributes to fracture patterns. At least for properties involving disruption of the collagen matrix, root dentin shows a definite anisotropy.

Strain patterns in cervical enamel of teeth subjected to occlusal loading

OBJECTIVES This study was planned to investigate the variations in strains in enamel under different patterns of occlusal loading, using three-dimensional finite element analysis (3D FEA) and strain gage measurements in extracted teeth. METHODS A 3D FEA model of a mandibular second premolar was used to investigate effects of occlusal load on enamel surface strains, particularly in response to oblique directions of cuspal loading. Point loads of 100 N were applied axially and at 45 degrees from the vertical on the buccal or lingual incline of the buccal cusp, either in the bucco-lingual plane or at varying mesio-distal angulations (up to +/- 20 degrees). Patterns of strain observed in the FEA model were confirmed experimentally using strain gages on extracted premolars mounted in a servohydraulic testing machine. RESULTS Strains predicted from the FEA model were in excellent agreement with the strain gage measurements. Strains were concentrated near the cementoenamel junction (CEJ) regardless of load direction. A vertical load on the buccal cusp tip resulted in compressive strains on the buccal surface but small tensile strains in lingual cervical enamel. Strains resulting from oblique loads on buccal cusp inclines were complex and asymmetric, with either tension or compression occurring in any location depending on the site and angle of loading. SIGNIFICANCE The magnitude, direction and character of strains in cervical enamel are highly dependent on patterns of loading. The asymmetric pattern of strains in buccal cervical enamel in response to oblique occlusal forces is consistent with the common clinical picture of asymmetric non-carious cervical lesions.

Finite element analysis and strain-gauge studies of vertical root fracture.

Vertical root fracture seems to result from stresses generated within the root canal and typically occurs in a buccolingual direction through the thickest part of dentin. Because stresses in the canal wall are difficult to measure experimentally, we have attempted to correlate stress patterns derived from finite element models of maxillary and mandibular incisors with strain measurements on the root surfaces of extracted teeth. Finite element analysis indicated that circumferential tensile stresses were concentrated on the buccal and lingual surfaces of the canal wall, corresponding to areas of greatest canal wall curvature. Surface stresses were much lower and were consistently tensile on the proximal root surfaces but variable on the buccal and lingual surfaces. The measurement of root surface stresses does not provide a reliable picture of internal stresses in the root. Canal wall curvature is a major factor in stress concentration and hence in the pattern of fracture.

Comparison of electrode position in the human cochlea using various perimodiolar electrode arrays

OBJECTIVE This study was conducted to evaluate the insertion properties and intracochlear trajectories of three perimodiolar electrode array designs and to compare these designs with the standard Cochlear/Melbourne array. BACKGROUND Advantages to be expected of a perimodiolar electrode array include both a reduction in stimulus thresholds and an increase in dynamic range, resulting in a more localized stimulation pattern of the spiral ganglion cells, reduced power consumption, and, therefore, longer speech processor battery life. METHODS The test arrays were implanted into human temporal bones. Image analysis was performed on a radiograph taken after the insertion. The cochleas were then histologically processed with the electrode array in situ, and the resulting sections were subsequently assessed for position of the electrode array as well as insertion-related intracochlear damage. RESULTS All perimodiolar electrode arrays were inserted deeper and showed trajectories that were generally closer to the modiolus compared with the standard electrode array. However, although the precurved array designs did not show significant insertion trauma, the method of insertion needed improvement. After insertion of the straight electrode array with positioner, signs of severe insertion trauma in the majority of implanted cochleas were found. CONCLUSIONS Although it was possible to position the electrode arrays close to the modiolus, none of the three perimodiolar designs investigated fulfilled satisfactorily all three criteria of being easy, safe, and atraumatic to implant.

Effects of continuous-wave CO2 laser on the ultrastructure of human dental enamel

Laser-induced changes in plano-parallel sections were examined by light microscopy (LM) and scanning electron microscopy (SEM), and correlated with ultrastructural changes as observed by transmission electron microscopy (TEM). LM and SEM revealed two different changes--extensive crazing, and crazing and cratering. Rough exposed enamel was commonly found, resulting from lifting off and removal of the top layer of crazed, or crazed and cratered, enamel. The type of induced change was mainly dependent on the energy density used (range approximately 0.8 to approximately 200 J cm-2) and on enamel prism orientation. Lased enamel was also softer than unlased enamel. TEM of both crazed enamel and rough exposed enamel revealed that most crystals generally resembled those of unlased enamel in size and shape, but that inter- and intra-crystalline voids were present in some areas. The crazed and cratered enamel had significant ultrastructural changes: new homogeneous and inhomogeneous crystals of apatite with a different shape and larger size than those of the original, and a loss of prismatic structure. The lack of uniformity of the laser effect on crazed and cratered enamel was shown by variation in crystal packing (from good to poor), variations in crystal size from area to area, and the presence of pockets of poorly packed homogeneous crystals alongside pockets of well-packed inhomogeneous crystals. The crazing, crazing and cratering, rough exposed enamel and the greater number of voids, as well as the relative softness of lased enamel do not indicate an overall ultrastructural improvement. However, the larger apatite crystal size and loss of prismatic structure in crazed and cratered areas may partly explain previous observations of reduced rates of subsurface demineralization in lased enamel.