Qualitative SEM analysis of fracture surfaces for dental ceramics and polymers broken by flexural strength testing and crown compression.

PURPOSE: To perform qualitative analysis using scanning electron microscopy (SEM) of fracture surfaces for ceramic and polymeric dental materials broken via standardized flexural and crunch-the-crown (CTC) tests. MATERIALS AND METHODS: Zirconia, glass-ceramic, and polymeric (Trilor; TRI, Juvora; JUV, Pekkton; PEK) materials were loaded using crowns for CTC tests, discs (zirconia and glass-ceramics) for piston-on-3 ball tests, bars (polymer) for 3-point bend tests, and bars (zirconia, glass-ceramics) for 4-point bend tests. SEM was used to characterize the fracture surfaces and identify fracture surface features (e.g., origin, mist, hackle, and the direction of crack propagation [DCP]). Electron dispersive spectroscopy was used to identify the local chemistry. RESULTS: Fracture surface features were found to be less visually apparent for glass-ceramics than zirconia. For zirconia bars, fractures originated roughly midway between the corner and center for processing defects related to sintering. Fractures originated at the bottom corners of glass-ceramic bars (void or surface flaw) and PEK bars (surface flaw). TRI bar failures exposed glassy fibers. Fracture features were generally less discernable for discs compared to bars for zirconia and glass-ceramics. Ceramic crowns fractured into 2 to 3 pieces, with fractures originating at the occlusal surface and clear evidence for the DCP. Failures of TRI and JUV specimens (bars and crowns) were less catastrophic than for the ceramics, with exposed fibers (TRI) and surface cracks (JUV). PEK crown and bar fractures presented dimple (ductile) features formed due to microvoid coalescence followed by brittle crack propagation. CONCLUSIONS: The critical flaws responsible for failure initiation were a function of material composition and test configuration. Fractographic analysis can reveal problems associated with the manufacturing of materials, their handling, grinding and finishing/polishing procedures, the structural design and choice of material, and the quality of the final laboratory-delivered restoration.

An experimental study of flexural strength and hardness of zirconia and their relation to crown failure loads.

STATEMENT OF PROBLEM: Dental zirconia is often marketed and selected for restorative use based upon the biaxial flexural strength of prefabricated specimens (disks) without considering other mechanical and physical properties. PURPOSE: The purpose of this in vitro study was to test whether 4-point flexural strength, biaxial flexural strength, and/or hardness may correlate with failure loads for crowns made of different zirconia materials. MATERIAL AND METHODS: Three brands of zirconia (BruxZir, Cercon, and Katana) were used to fabricate anatomically contoured crowns, rectangular bars, and circular disks. The sample size was n=15 specimens per zirconia brand and specimen shape. The bars were tested for 4-point flexural strength by using the 4-point bending (4PB) test and Vickers hardness (VH), while the disks were tested for biaxial flexural strength by using a piston-on-3 ball (POB) test. Crowns were attached to resin abutments and compressed with a steel spherical indenter through a polyethylene sheet to assess the failure loads by using the "crunch the crown" (CTC) test. One-way ANOVA (α=.05) was used to test for statistically significant differences between groups, and Weibull analysis was used to assess the variability of the measured flexural strengths, failure load, and hardness. RESULTS: Statistical differences (P<.001) were found in comparing Cercon, BruxZir, and Katana ([4260 N=4186 N]>3195 N, respectively) with the CTC test and (396 MPa>[281 MPa=275 MPa], respectively) the 4PB test. No statistical differences (P=.084) were found among the zirconia brands (Cercon [384 MPa]=Bruxzir [359 MPa]=Katana [416 MPa]) for the POB test. No significant correlations (P>.05) were found between the 4PB, POB, or VH tests and the corresponding CTC test. The Weibull modulus varied for the different specimen geometries. CONCLUSIONS: The piston-on-3 ball, 4-point bending, and Vickers hardness test results were not correlated with the corresponding crunch-the-crown test.

Effect of liner and porcelain application on zirconia surface structure and composition.

The purpose of this study was to determine if there is an effect of liner and porcelain application (layering and pressing techniques) on the surface of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP), which were exposed to permutations of liner, layered porcelain, and pressed porcelain. Scanning electron microscope (SEM)/energy dispersive spectroscope (EDS) was used to identify changes in composition and microstructure after removing liner and porcelain with hydrofluoric acid. Simulated aging was also conducted to determine the effect of liner and porcelain on low-temperature degradation. The control group had a typical equiaxed grain structure, referred to as unaffected. When covered with liner or porcelain, some areas changed in structure and composition and were termed affected. The frequency of affected structure decreased when liner was covered with either layered porcelain or pressed porcelain. There were statistical differences (P<0.05) in the composition between affected and unaffected for zirconium (layered porcelain with liner: affected=60% (0.8%) (m/m), unaffected=69% (4%), layered porcelain without liner: affected=59% (3%), unaffected=65% (3%)) and oxygen (layered porcelain with liner: affected=35% (2%), unaffected=26% (4%), layered porcelain without liner: affected=35% (3%), unaffected=30% (2%)). However, there were statistical differences (P<0.05) in the composition for zirconium and oxygen of the aged layered porcelain without liner only. The liner should not be used before porcelain application, especially when using the layering technique for zirconia restorations. Furthermore, pressing should be considered the technique of choice over layering.

Correlation of flexural strength of coupons versus strength of crowns fabricated with different zirconia materials with and without aging.

BACKGROUND: The purpose of this study was to determine the presence of a correlation between flexural strength and simulated crown strength; a correlation between crown strength and mode of fracture; an effect of aging on the flexural strength; and an effect of aging on the crown strength. METHODS: Two hundred forty zirconia specimens were fabricated with 2 different designs, fully contoured crown shape specimens (n = 120) and rectangular coupons (n = 120), to provide 10 specimens each of 6 brands of zirconia (Lava Plus High Translucency [3M ESPE], Argen HT [Argen Corp], Zirlux [Ardent], BruxZir [Glidewell Laboratories], ZenoStar [Wieland Dental], and DDBioZX(2) [Dental Direkt]). One-half of each sample type was given a severe, simulated low-temperature aging treatment. The coupons were tested by 3-point flexural strength, and crowns were tested after luting to metallic abutments using resin cement. Statistical significance was evaluated by 2-factor analysis of variance (P = .05). RESULTS: Aging increased the mean (standard deviation [SD]) flexural strength for the following groups: Argen HT (995 [140] megapascals versus 677 [121] MPa before aging), Zirlux (939 [101] MPa versus 826 [169] MPa before aging), and ZenoStar (954 [81] MPa versus 764 [77] MPa before aging). There were statistical differences for the mean (SD) crown strengths for the following aged crowns: DDBioZX(2) had higher magnitudes (9,755 [1,095] MPa) than ZenoStar (8,864 [976] MPa), whereas Lava Plus High Translucency crowns had higher magnitudes (9,871 [942] MPa) than ZenoStar (8,864 [976] MPa). There was no effect of aging on the crown strength. There were statistical differences in the mode of fracture for the zirconia crowns between the following groups: nonaged and aged BruxZir (P = .014), nonaged and aged ZenoStar (P = .0226), and nonaged and aged Lava Plus High Translucency (P < .0001). There was no correlation between flexural strength and crown strength. CONCLUSIONS: There was no direct correlation between ranking of flexural strength and crown strength in the range of properties exhibited by these dental zirconias. PRACTICAL IMPLICATIONS: Flexural strength does not predict simulated clinical strengths for crowns.

Effect of grain size on the monoclinic transformation, hardness, roughness, and modulus of aged partially stabilized zirconia.

OBJECTIVE: Low-temperature-degradation (LTD) has been reported to cause property changes in yttria-tetragonal zirconia polycrystals (Y-TZP). The current study measured monoclinic phase transformation of Y-TZP with different grain sizes and corresponding property changes due to artificial aging. NULL HYPOTHESIS: the grain size of aged Y-TZP will not influence its transformation, roughness, hardness or modulus of elasticity. METHODS: Four groups of Y-TZP were examined with differing grain sizes (n=5). The line intercept technique was used to determine grain sizes on SEM images (100,000×). Artificial aging was accomplished by autoclaving at 2 bar pressure for 5 h. X-ray diffraction (30 mA, 40 kV) was used to measure tetragonal to monoclinic transformation (t→m). Surface roughness analysis was performed using a non-contact surface-profilometer. Nano-hardness and modulus of elasticity were measured using nano-indentation. RESULTS: SEM analyses showed different grain sizes for each sample group (0.350 µm, 0.372 µm, 0.428 µm, and 0.574 µm). The fraction of t→m transformation increased as grain size increased; furthermore, aging of zirconia caused increased roughness. Modulus and hardness after aging displayed no significant correlation or interaction with grain size. SIGNIFICANCE: Smaller grains caused less transformation, and aging caused increased roughness, but grain size did not influence the amount of increased surface roughness. Future studies are needed to determine the effects of grain size on the wear and fracture properties of dental zirconia.

Evaluation of zirconia-porcelain interface using X-ray diffraction.

The aim of this study was to determine if accelerated aging of porcelain veneering had an effect on the surface properties specific to a tetragonal-to-monoclinic transformation (TMT) of zirconia restorations. Thirty-six zirconia samples were milled and sintered to simulate core fabrication followed by exposure to various combinations of surface treatments including as-received (control), hydrofluoric acid (HF), application of liner plus firings, application of porcelain by manual layering and pressing with firing, plus accelerated aging. The quantity of transformed tetragonal to monoclinic phases was analyzed utilized an X-ray diffractometer and one-way analysis of variance was used to analyze data. The control samples as provided from the dental laboratory after milling and sintering process had no TMT (Xm = 0). There was an effect on zirconia samples of HF application with TMT (Xm = 0.8%) and liner plus HF application with TMT (Xm = 8.7%). There was an effect of aging on zirconia samples (no veneering) with significant TMT (Xm = 70.25%). Both manual and pressing techniques of porcelain applications reduced the TMT (manual, Xm = 4.41%, pressing, Xm = 11.57%), although there was no statistical difference between them. It can be concluded that simulated applications of porcelain demonstrated the ability to protect zirconia from TMT after aging with no effect of a liner between different porcelain applications. The HF treatment also caused TMT.

Effect of imaging powder and CAD/CAM stone types on the marginal gap of zirconia crowns.

OBJECTIVE: To compare the marginal gap using different types of die stones and titanium dies with and without powders for imaging. METHODS: A melamine tooth was prepared and scanned using a laboratory 3-shape scanner to mill a polyurethane die, which was duplicated into different stones (Jade, Lean, CEREC) and titanium. Each die was sprayed with imaging powders (NP, IPS, Optispray, Vita) to form 15 groups. Ten of each combination of stone/titanium and imaging powders were used to mill crowns. A light-bodied impression material was injected into the intaglio surface of each crown and placed on the corresponding die. Each crown was removed, and the monophase material was injected to form a monophase die, which was cut into 8 sections. Digital images were captured using a stereomicroscope to measure marginal gap. Scanning electron microscopy was used to determine the particle size and shape of imaging powders and stones. RESULTS: Marginal gaps ranged from mean (standard deviation) 49.32 to 1.20 micrometers (3.97-42.41 µm). There was no statistical difference (P > .05) in the marginal gap by any combination of stone/titanium and imaging powders. All of the imaging powders had a similar size and rounded shape, whereas the surface of the stones showed different structures. CONCLUSIONS: When a laboratory 3-shape scanner is used, all imaging powders performed the same for scanning titanium abutments. However, there was no added value related to the use of imaging powder on die stone. It is recommended that the selection of stone for a master cast be based on the hysical properties. PRACTICAL IMPLICATIONS: When a laboratory 3-shape scanner is used, the imaging powder is not required for scanning die stone. Whenever scanning titanium implant abutments, select the least expensive imaging powder.

Phase transformation of dental zirconia following artificial aging.

AIMS: Low-temperature degradation (LTD) of yttria-stabilized zirconia can produce increased surface roughness with a concomitant decrease in strength. This study determined the effectiveness of artificial aging (prolonged boiling/autoclaving) to induce LTD of Y-TZP (yttria-tetragonal zirconia-polycrystals) and used artificial aging for transformation depth progression analyses. The null hypothesis is aging techniques tested produce the same amount of transformation, transformation is not time/temperature dependent and LTD causes a constant transformation throughout the Y-TZP samples. METHODS: Dental-grade Y-TZP samples were randomly divided into nine subgroups (n = 5): as received, 3.5 and 7 day boiling, 1 bar autoclave (1, 3, 5 h), and 2 bar autoclave (1, 3, 5 h). A 4-h boil treatment (n = 2) was performed post-experiment for completion of data. Transformation was measured using traditional X-ray diffraction and low-angle X-ray diffraction. RESULTS: The fraction of t → m transformation increased with aging time. The 3.5 day boil and 2 bar 5 h autoclave produced similar transformation results, while the 7 day boiling treatment revealed the greatest transformation. The surface layer of the aged specimen underwent the most transformation while all samples displayed decreasing transformation with depth. SIGNIFICANCE: Surface transformation was evident, which can lead to rougher surfaces and increased wear of opposing dentition/materials. Therefore, wear studies addressing LTD of Y-TZP are needed utilizing accelerated aging.

The failure load of CAD/CAM generated zirconia and glass-ceramic laminate veneers with different preparation designs.

STATEMENT OF PROBLEM: Fracture of feldspathic porcelain laminate veneers represents a significant mode of clinical failure. Therefore, ceramic materials that withstand a higher load to fracture, especially for patients with parafunctional habits, are needed. PURPOSE: The purpose of this study was to examine the correlation of material (zirconia, TZP, glass-ceramic, IEC, and feldspathic porcelain, FP) design (incisal overlapped preparation, IOP, and three-quarter preparation, TQP), and fracture mode to failure load for veneers supported by composite resin abutments. MATERIAL AND METHODS: A typodont tooth prepared with 2 designs (IOP, TQP) and the corresponding 2 definitive dies were used to fabricate the composite resin abutments (30 for IOP and 30 for TQP). Ten veneer specimens for each system (Y-TZP, IEC, and FP), were fabricated for each design. The veneers were cemented, invested, and tested in compression until failure by using a universal testing machine. Significant differences were evaluated by 2-factor ANOVA (α=.05). RESULTS: No statistical mean load difference was noted between the preparation designs for Y-TZP (IOP: 244 ±81 and TQP: 224 ±58 N), IEC (IOP: 306 ±101 and TQP: 263 ±77 N), and FP veneers (IOP: 161 ±93 and TQP: 246 ±45 N). No statistical difference in the mean load was found among the 3 veneer materials for each preparation design except between IEC (306 ±101 N) and FP (161 ±93 N) veneers for TQP. CONCLUSIONS: Preparation design did not influence the failure load of the veneer materials. Zirconia veneers were the least likely to fracture but the most likely to completely debond; feldspathic porcelain veneers exhibited the opposite characteristics.

Recent advances in the development of GTR/GBR membranes for periodontal regeneration--a materials perspective.

UNLABELLED: Periodontitis is a major chronic inflammatory disorder that can lead to the destruction of the periodontal tissues and, ultimately, tooth loss. To date, flap debridement and/or flap curettage and periodontal regenerative therapy with membranes and bone grafting materials have been employed with distinct levels of clinical success. Current resorbable and non-resorbable membranes act as a physical barrier to avoid connective and epithelial tissue down-growth into the defect, favoring the regeneration of periodontal tissues. These conventional membranes possess many structural, mechanical, and bio-functional limitations and the "ideal" membrane for use in periodontal regenerative therapy has yet to be developed. Based on a graded-biomaterials approach, we have hypothesized that the next-generation of guided tissue and guided bone regeneration (GTR/GBR) membranes for periodontal tissue engineering will be a biologically active, spatially designed and functionally graded nanofibrous biomaterial that closely mimics the native extra-cellular matrix (ECM). OBJECTIVE: This review is presented in three major parts, including (1) a brief overview of the periodontium and its pathological conditions, (2) currently employed therapeutics used to regenerate the distinct periodontal tissues, and (3) a review of commercially available GTR/GBR membranes as well as the recent advances on the processing and characterization of GTR/GBR membranes from a materials perspective. SIGNIFICANCE: Studies of spatially designed and functionally graded membranes (FGM) and in vitro antibacterial/cell-related research are addressed. Finally, as a future outlook, the use of hydrogels in combination with scaffold materials is highlighted as a promising approach for periodontal tissue engineering.

Evaluation of the optical properties of CAD-CAM generated yttria-stabilized zirconia and glass-ceramic laminate veneers.

STATEMENT OF PROBLEM: When feldspathic porcelain (FP) laminate veneers are used to mask tooth discoloration that extends into the dentin, significant tooth reduction is needed to provide space for the opaque layer and optimize the bonding of the restoration. PURPOSE: The purpose of this study was to investigate the color effect of trial insertion paste (TP), composite resin abutment (CRA), and veneer regions on the optical properties of feldspathic porcelain (FP), yttria-stabilized zirconia (Y-TZP), and IPS e.max CAD HT (IEC) veneers. MATERIAL AND METHODS: A melamine tooth was prepared for a laminate veneer on a model, and a definitive cast was made. The definitive die was scanned by using the TurboDent System (TDS), then 30 CRA were machined and 10 veneers were fabricated for each ceramic material (FP, Y-TZP, IEC). The optical properties of different veneer materials, CRA (A(1), A(2), A(3)) and TP (bleach XL, opaque white, transparent, and yellow) were evaluated in the cervical, body, and incisal regions with a spectrophotometer. Results were analyzed by using 1-way ANOVA (.05). RESULTS: The color difference for all the veneers was affected by TP and CRA colors in different regions. The mean values for the Y-TZP veneer color coordinates (L*: 74 ±0.34, a*: 0.09 ±0.20, and b*: 17.43 ±0.44) were significantly different (P<.001) from those of IEC veneers (L*: 70.15 ±0.23, a*: -0.69 ±0.073, and b*:11.48 ±0.30) and FP veneers (L*: 70.00 ±0.86, a*: - 0.28 ±0.203, and b*: 13.86 ±1.08). There was no difference between IEC for L* and FP. Significant difference was detected (P<.001) in color coordinates among the 3 veneer materials for a* and b*. CONCLUSIONS: The TP color affected the color difference for all veneer materials except the Y-TZP, while there was no effect on the CRA color. The magnitude of color coordinates changed as a function of TP color and veneer material.

Influence of low-temperature environmental exposure on the mechanical properties and structural stability of dental zirconia.

PURPOSE: The effect of dental fabrication procedures of zirconia monolithic restorations and changes in properties during low-temperature exposure in the oral environment is not completely understood. The purpose of this study was to investigate the effect of procedures for fabrication of dental restorations by low-temperature simulation and relative changes of flexural strength, nanoindentation hardness, Young's modulus, surface roughness, and structural stability of yttria-stabilized zirconia. MATERIALS AND METHODS: A total of 64 zirconia specimens were prepared to simulate dental practice. The specimens were divided into the control group and the accelerated aging group. The simulated group followed the same procedure as the control group except for the aging treatment. Atomic force microscopy was used to measure surface roughness. The degree of tetragonal-to-monoclinic transformation was determined using X-ray diffraction. Nanoindentation hardness and modulus measurements were carried out on the surface of the zirconia specimens using a nanoindenter XP/G200 system. The yttria levels for nonaged and aged specimens were measured using energy dispersive spectroscopy. Flexural strength was determined using the piston-on-three-ball test. The t-test was used to determine statistical significance. RESULTS: Means and standard deviations were calculated using all observations for each condition and evaluated using a group t-test (p < 0.05). The LTD treatment resulted in increased surface roughness (from 12.23 nm to 21.56 nm for Ra and 15.06 nm to 27.45 nm for RMS) and monoclinic phase fractions (from 2% to 21%), with a concomitant decrease in hardness (from 16.56 GPa to 15.14 GPa) and modulus (from 275.68 GPa to 256.56 GPa). Yttria content (from 4.43% to 4.46%) and flexural strength (from 586 MPa to 578 MPa) were not significantly altered, supporting longer term in vivo function without biomechanical fracture. CONCLUSION: The LTD treatment induced the tetragonal-to-monoclinic transformation with surface roughening in zirconia prepared using dental procedures.

A novel spatially designed and functionally graded electrospun membrane for periodontal regeneration.

A periodontal membrane with a graded structure allows tailoring of the layer properties to design a material system that will retain its physical, chemical and mechanical characteristics for a period long enough to optimize periodontal regeneration. In this work a novel functionally graded membrane (FGM) was designed and fabricated via sequential multilayer electrospinning. The FGM consists of a core layer (CL) and two functional surface layers (SLs) interfacing with bone (nano-hydroxyapatite, n-HAp) and epithelial (metronidazole, MET) tissues. The CL comprises a neat poly(DL-lactide-co-ε-caprolactone) (PLCL) layer surrounded by two composite layers composed of a protein/polymer ternary blend (PLCL:PLA:GEL). Electrospinning parameters involved in fabrication of the individual layers (i.e. neat PLCL, ternary blend, PLA:GEL+10%n-HAp and PLA:GEL+25%MET) were optimized to obtain fibrous layers free of beads. Morphology, structure and mechanical property studies were carried out on each electrospun layer. The individual fiber morphology and roughness of the functional SLs, which are the n-HAp containing and drug-incorporating layers were evaluated by atomic force microscopy. The CL structure demonstrated higher strength (8.7 MPa) and a more elastic behavior (strain at break 357%) compared with the FGM (3.5 MPa, 297%). Incorporation of n-HAp to enhance osteoconductive behavior and MET to combat periodontal pathogens led to a novel FGM that holds promise at solving the drawbacks of currently available membranes.

Acellular dermal matrix graft: synergistic effect of rehydration and natural crosslinking on mechanical properties.

This investigation studied how the incorporation of a natural crosslinking agent, genipin (Gp), into the AlloDerm® (AD) rehydration protocol affects the biomechanical properties and the stability of the collagenous matrix. AD is a minimally processed, noncrosslinked, freeze-dried collagen-based graft. Samples were immersed in a saline solution for 5 min and then randomly assigned for further rehydration (30 min) into three groups, according to the crosslinking agent: G1-control (saline), G2-1 wt % genipin, and G3-1 wt % glutaraldehyde. Gp crosslinking for a prolonged time of 6 h (G4) was also investigated. After washing (5 min), samples were mechanically tested wet in tension. G2 demonstrated a significantly higher ultimate tensile strength (UTS) and E relative to G1. However, G3 did not show a noteworthy increase in these properties. A significant enhancement in UTS was found when Gp exposure time was increased from 30 min to 6 h. FT-IR revealed a protein backbone with no significant peak shifting for all samples due to crosslinking. However, a considerable decrease in -NH(2) peak intensity occurred due to crosslinking reactions. Additionally, DSC analyses indicated an important shift in the denaturation temperature for crosslinked samples. SEM micrographs revealed no alterations in the native fibrous morphology after crosslinking. Simultaneous genipin incorporation during the rehydration protocol of AlloDerm significantly enhances its biomechanical properties.

Freeze-dried acellular dermal matrix graft: effects of rehydration on physical, chemical, and mechanical properties.

OBJECTIVES: To test the effect of rehydration time over the range prescribed in the manufacturer's protocol on (1) the biomechanical properties and on (2) the recovery and stabilization of the collagenous matrix of AlloDerm. METHODS: A sterile dish containing warm saline solution was prepared, and samples rehydrated for 5 min. Subsequently, three other dishes with the solution were prepared and samples assigned into three groups according to the total rehydration time: 10 min (G1), 20 min (G2), and 40 min (G3). Uni-axial tensile testing was used to assess the biomechanical properties of the different groups and the control (dry condition). Physico-chemical properties were examined by Fourier transform infrared spectroscopy (FT-IR), and differential scanning calorimetry (DSC) as a function of rehydration time. RESULTS: ANOVA revealed a significant change in tensile strength (p=0.0269) and in elastic modulus (p=0.0306) for AlloDerm following different rehydration times. The lowest tensile strength was in the dry condition, whereas the highest was achieved after a 40 min rehydration. The shortest rehydration periods did not result in a statistically significant (p>0.05) change in elastic modulus. However, after 40 min the elastic modulus increased significantly when compared to the shortest periods. FT-IR confirmed the protein backbone recovery of the graft matrix after rehydration. DSC scans of rehydrated samples showed visible shifts in the denaturation temperature to higher values compared to as-received sample (dry) suggesting stronger polymer-water bridge formation, supporting the increase in the biomechanical properties. SIGNIFICANCE: The current study suggests that there are major changes on the biomechanical properties of the collagenous graft as rehydration time increases, which were also structurally confirmed by the physico-chemical analyses. Clinicians must be aware that the rehydration times of the manufacturer's protocol result in a significant range in mechanical and physico-chemical properties. Therefore, a rehydration time of at least 20 min guarantees not only better handling and mechanical properties but, most importantly, supplies a material that closely resembles the natural tissue.