Is fatigue mechanism implicated in intraoral fracture of narrow dental implants? A thorough retrieval analysis of two failed implant fixtures retrieved from a single patient.

Purpose: This study aimed to perform a thorough failure analysis of two fractured narrow dental implants after medium-term in vivo use. Materials and methods: The top parts of two fractured Narrow Dental Implant (NDI) fixtures were retrieved from two different locations at two different times from the same patient. The NDI-specimen-1 was 12-months in service while the NDI-specimen-2 was 17-months in service. In both cases, the top parts of the fractured NDI fixtures that were attached to prosthetic components were retrieved and subjected to thorough, non-destructive and destructive testing. Results: Light Microscopy (LM) and Scanning Electron Microscopy (SEM) revealed that both the retrieved fractured NDIs failed because of fatigue, characterized by beach and ratchet marks. Macroscopic examination revealed that fatigue cracks initiated at the internal thread surfaces of the implants and propagated around them until final fracture. Both samples fractured near the end of the retaining screw and followed the root of the internal thread. Optical and SEM analyses revealed a uniform distribution of irregularly shaped grains (diameter = 2 to 5 µm). X -ray Energy Dispersive Spectroscopy (EDS) analysis showed that the NDI-specimen-1 was made using Ti-14%Zr with a Vickers Hardens (HV) of 288 ± 5. Conclusion: Since the fracture occurred by a fatigue; thus, an increase in fatigue resistance will be beneficial for the longevity of NDI.

Failure analysis of eleven Gates Glidden drills that fractured intraorally during post space preparation. A retrieval analysis study.

The purpose of this study was to determine the failure mechanism of clinically failed Gates Glidden (GG) drills. Eleven retrieved GG drills (sizes #1 to #3) which fractured during root canal preparation were collected and the fracture location was recorded based on macroscopic observation. All fracture surfaces were investigated by a SEM. Then the fractured parts were embedded in acrylic resin and after metallographic preparation, the microstructure and elemental composition was evaluated by SEM and EDS. The Vickers hardness (HV) of all specimens was also determined. Macroscopic examination and SEM analysis showed that the drills failed near the hand piece end by torsional fatigue with fatigue cracks initiating at several locations around the circumference and propagating toward the center. Final fracture followed by a tensile overloading at the central region of cross section. Microstructural analysis, hardness measurements and EDS show that the drills are made of a martensitic stainless steel like AISI 440C. Based on the findings of this study, clinicians should expect fatigue fracture of GG drills that have small size during root canal preparation. Selection of a more fatigue resistant stainless steel alloy and enhancing the instrument design might reduce the incidence of quasi-cleavage fracture on GG drills.

Failure analysis of ParaPost drills that fractured in service: a retrieval analysis study.

The aim was to determine the fracture mechanism of two clinically failed ParaPost drills. First, the fracture planes were analyzed by scanning electron microscopy (SEM). The drill end of one of the fractured pieces of each drill was then embedded in resin and after being metallographically ground and polished, was chemically etched. The microstructure and elemental composition were then examined by SEM/EDS analysis while hardness was determined with a Vickers testing device. Fractographic analysis revealed that both drills failed in a brittle manner and showed a pattern characteristic of a quasi-cleavage fracture mode. SEM and EDS analysis revealed a random distribution of a second phase enriched in Mo, W, and V, probably appended to (Mo, W, V)×C carbides, while the alloy composition is similar to M3 tool steel, a high-speed molybdenum tool steel. The microhardness of a ParaPost Drill #1 was found to be HV 862±29 and that for a Drill #2 was 846±16, with no significant differences (p>0.05). In both cases, fracture originated from surface points acting as stress concentrators and facilitating brittle fracture in the quasi-cleavage mode indicating that failure rate might be further minimized by a better instrument design.

Effect of Nd:YAG laser parameters on the penetration depth of a representative Ni-Cr dental casting alloy.

The effects of voltage and laser beam (spot) diameter on the penetration depth during laser beam welding in a representative nickel-chromium (Ni-Cr) dental alloy were the subject of this study. The cast alloy specimens were butted against each other and laser welded at their interface using various voltages (160-390 V) and spot diameters (0.2-1.8 mm) and a constant pulse duration of 10 ms. After welding, the laser beam penetration depths in the alloy were measured. The results were plotted and were statistically analyzed with a two-way ANOVA, employing voltage and spot diameter as the discriminating variables and using Holm-Sidak post hoc method (a = 0.05). The maximum penetration depth was 4.7 mm. The penetration depth increased as the spot diameter decreased at a fixed voltage and increased as the voltage increased at a fixed spot diameter. Varying the parameters of voltage and laser spot diameter significantly affected the depth of penetration of the dental cast Ni-Cr alloy. The penetration depth of laser-welded Ni-Cr dental alloys can be accurately adjusted based on the aforementioned results, leading to successfully joined/repaired dental restorations, saving manufacturing time, reducing final cost, and enhancing the longevity of dental prostheses.

Biotribological behavior of two retrieved implant abutment screws after long-term use in vivo.

PURPOSE: To evaluate the effect of functional cyclic loading on two retrieved abutment screws used with single implant supporting cement-retained porcelain-fused-to-metal crowns by characterizing the structure, properties, and biotribologic behavior after long-term use in vivo. MATERIALS AND METHODS: Two abutment screws were retrieved from the same patient and same implant at two different times. An external hex implant was used to replace a missing central incisor. A traumatic incident occurred 9 months after insertion and displaced the implant palatally by bending the screw. A second, similar incident occurred 31 months after insertion. In both cases, the abutment screws were retrieved and subjected to thorough nondestructive and destructive testing. RESULTS: Light and scanning electron microscopic examinations revealed very minimal surface deterioration of the thread profile for the first screw (in service for 9 months) and demonstrated advanced metal adhesive wear in the form of galling for the second screw (in service for 31 months). The galling led to moderate thinning of the thread profile. Both screws were composed of Au-based alloy, where the microstructure of the matrix consisted of homogeneous equiaxed fine grains with two different second phase particles. CONCLUSION: It appears that the occurrence of adhesive wear on abutment screws in the form of galling is highly related to the length of in-service time in the mouth. This biotribologic behavior was inevitable and considered to be a normal consequence of long-term use in vivo. Metallurgic analysis indicated that both screws were identical in terms of composition and microstructure.

Mechanical behavior and failure analysis of prosthetic retaining screws after long-term use in vivo. Part 4: Failure analysis of 10 fractured retaining screws retrieved from three patients.

PURPOSE: The aim of this study was to perform a failure analysis on fractured prosthetic retaining screws after long-term use in vivo. Additionally, the study addresses the commonly asked question regarding whether complex repeated functional occlusal forces initiate fatigue-type cracks in prosthetic retaining screws. MATERIALS AND METHODS: Ten fractured prosthetic retaining screws retrieved from three patients treated with fixed detachable hybrid prostheses were subjected to a failure analysis. In patients 1 and 2, the middle three retaining screws of the prostheses were found fractured at retrieval time after they had been in service for 20 and 19 months, respectively. In patient 3, the middle three and one of the posterior retaining screws were found to be fractured at retrieval after they had been in service for 18 months. Low power stereomicroscopy and high-power scanning electron microscopy (SEM) were performed to analyze the fractured surfaces of the retaining screws examining fatigue cracks in greater detail. RESULTS: Typical fatigue failure characterized by ratchet mark formation was revealed by light microscopy and SEM for all examined screws. Using low magnification light microscopy, ratchet marks were visible on the fracture surfaces of only two screws. SEM examination revealed all three classical stages of fatigue failure, and it was possible to see the ratchet marks on the fracture surfaces of all specimens, indicating a fatigue zone. The final catastrophic overload fracture appeared fibrous, indicating ductile fracture. The final overload ductile fracture surfaces showed equiaxed dimples, suggesting tensile overload in all examined screws except in two specimens that showed an elongated dimple pattern indicating shear/tearing overload forces. CONCLUSIONS: Fracture of prosthetic retaining screws in hybrid prostheses occurs mainly through a typical fatigue mode involving mostly the middle anterior three screws. Fatigue cracks can grow in more than one prosthetic retaining screw, leading to fracture before the patient or clinician determines that any problem exists.

Mechanical behavior and failure analysis of prosthetic retaining screws after long-term use in vivo. Part 3: Preload and tensile fracture load testing.

PURPOSE: The aim of this study was to determine the preload and tensile fracture load values of prosthetic retaining screws after long-term use in vivo compared to unused screws (controls). Additionally, the investigation addressed whether the preload and fracture load values of prosthetic retaining screws reported by the manufacturer become altered after long-term use in vivo. MATERIALS AND METHODS: For preload testing, 10 new screws (controls) from Nobel Biocare (NB) and 73 used retaining screws [58 from NB and 15 from Sterngold (SG)] were subjected to preload testing. For tensile testing, eight controls from NB and 58 used retaining screws (46 from NB and 12 from SG) were subjected to tensile testing. Used screws for both tests were in service for 18-120 months. A custom load frame, load cell, and torque wrench setup were used for preload testing. All 83 prosthetic screws were torqued once to 10 Ncm, and the produced preload value was recorded (N) using an X-Y plotter. Tensile testing was performed on a universal testing machine and the resulting tensile fracture load value was recorded (N). Preload and tensile fracture load values were analyzed with 2-way ANOVA and Tukey post-hoc tests. RESULTS: There was a significant difference between preload values for screws from NB and screws from SG (p < 0.001). The preload values for gold alloy screws from NB decreased as the number of years in service increased. There was a significant difference between tensile fracture values for the three groups (gold alloy screws from NB and SG and palladium alloy screws from NB) at p < 0.001. The tensile fracture values for gold alloy screws from NB and SG decreased as the number of years in service increased. CONCLUSIONS: In fixed detachable hybrid prostheses, perhaps as a result of galling, the intended preload values of prosthetic retaining screws may decrease with increased in-service time. The reduction of the fracture load value may be related to the increase of in-service time; however, the actual determination of this relationship is not possible from this study alone.

Mechanical behavior and failure analysis of prosthetic retaining screws after long-term use in vivo. Part 2: Metallurgical and microhardness analysis.

PURPOSE: This study involved testing and analyzing multiple retrieved prosthetic retaining screws after long-term use in vivo to: (1) detect manufacturing defects that could affect in-service behavior; (2) characterize the microstructure and alloy composition; and (3) further characterize the wear mechanism of the screw threads. MATERIALS AND METHODS: Two new (control) screws from Nobel Biocare (NB) and 18 used (in service 18-120 months) retaining screws [12 from NB and 6 from Sterngold (SG)] were: (1) metallographically examined by light microscopy and scanning electron microscopy (SEM) to determine the microstructure; (2) analyzed by energy dispersive X-ray (EDX) microanalysis to determine the qualitative and semiquantitative average alloy and individual phase compositions; and (3) tested for Vickers microhardness. RESULTS: Examination of polished longitudinal sections of the screws using light microscopy revealed a significant defect in only one Group 4 screw. No significant defects in any other screws were observed. The defect was considered a "seam" originating as a "hot tear" during original casting solidification of the alloy. Additionally, the examination of longitudinal sections of the screws revealed a uniform homogeneous microstructure in some groups, while in other groups the sections exhibited rows of second phase particles. The screws for some groups demonstrated severe deformation of the lower threads and the bottom part of the screw leading to the formation of crevices and grooves. Some NB screws were comprised of Au-based alloy with Pt, Cu, and Ag as alloy elements, while others (Groups 4 and 19) were Pd-based with Ga, Cu, and Au alloy elements. The microstructure was homogeneous with fine or equiaxed grains for all groups except Group 4, which appeared inhomogeneous with anomalous grains. SG screws demonstrated a typical dendritic structure and were Au-based alloy with Cu and Ag alloy elements. There were differences in the microhardness of gold alloy screws from NB and SG as well as palladium alloy screws from NB. CONCLUSIONS: Significant differences within NB retaining screws and between NB and SG screws were found for microstructure, major alloy constituents, and microhardness.

Mechanical behavior and failure analysis of prosthetic retaining screws after long-term use in vivo. Part 1: Characterization of adhesive wear and structure of retaining screws.

PURPOSE: The general aim of this study and those presented in Parts 2-4 of this series was to characterize the structure, properties, wear, and fracture of prosthetic retaining screws in fixed detachable hybrid prostheses after long-term use in vivo. This part of the overall investigation addresses whether there are differences in thread wear between the screws closest to the fulcrum and those that are farthest from the fulcrum in fixed detachable hybrid prostheses. MATERIALS AND METHODS: The total number of prosthetic retaining screws used in this study was 100 (10 new and 90 used). New screws (controls) from Nobel Biocare (NB) were divided into Group 1 (slotted) and Group 2 (hexed). Ninety used screws (in service 18-120 months) were retrieved from fixed detachable hybrid prostheses in 18 patients (5 screws from each patient, 60 from NB and 30 from Sterngold). The used screws were divided into 18 groups. Additionally, each group was subdivided into A and B categories. Category A contained the middle three prosthetic screws, which were considered the farthest screws from the fulcrum line. Category B contained the most posterior two screws, which were considered the screws closest to the fulcrum line. All 100 screws were subjected to thorough, nondestructive testing. RESULTS: Light and scanning electron microscopic examination of all used screws for each group revealed surface deterioration of the active profile of the screw threads consistent with adhesive wear. The observed thread profile deterioration ranged from mild to severe. The wear was aggressive enough to cause galling, which led to thinning of the threads and, in severe cases, to knife-edges at thread crests. In ten groups, the most anterior three screws exhibited more wear than the most posterior two screws. In addition to thread wear, severe plastic deformation was detected on the bottom part of each screw for three groups, and a long external longitudinal crack was detected in one screw of Group 2. CONCLUSIONS: The findings of this study and those presented in Parts 2-4 demonstrate that different retaining screws from the same manufacturer and/or from different manufacturers have different geometrical design, microstructures, major alloy constituents, and microhardness, and that these differences influence their preload and fractured load values. In this part of the overall investigation, the occurrence of galling as a result of wear involving prosthetic retaining screws appears to be an inevitable and unavoidable consequence of long-term use in vivo in fixed detachable hybrid prostheses regardless of the intended/original preload value. The galling rate is greater on the middle three screws compared to the most posterior two screws in fixed detachable hybrid prostheses. The wear pattern is consistent with an adhesive wear mechanism; however, this study does not provide enough data to support a definitive analysis.

The effect of repeated torque and salivary contamination on the preload of slotted gold implant prosthetic screws.

STATEMENT OF PROBLEM: Screw loosening and/or fracture is common and has been attributed to many factors, including improper torque and preload. Purpose. This study evaluated the effect of repeated preload torque and salivary contamination on the preload of the slotted gold implant prosthetic screw. MATERIAL AND METHODS: Fifteen slotted gold prosthetic screws were tested in an unused stack of components consisting of an implant, an abutment cylinder, and a screw and gold cylinder. A custom load frame and load cell and associated electronics were used. The component stack was lubricated with human saliva. Each screw was tightened to 10 Ncm with a hand-held torque wrench fitted with strain gauge electronics and then removed 10 times. Preload values at the first (Group 1X), fifth (Group 5X), and tenth (Group 10X) repetition were measured. Repeated-measures 1-way analysis of variance (P<.05) and paired comparisons were used to analyze the data. RESULTS: The mean preload value increased from Group 1X (184.3 N +/- 28.9) to Group 5X (202.5 N +/- 27.7) to Group 10X (220.2 N +/- 29.0). The differences among these groups were significant (P=.004). CONCLUSION: Within the limitations of this study, higher preload was achieved after the repeated use of a saliva-lubricated gold prosthetic retaining screw.

The effect of repeated torque on the ultimate tensile strength of slotted gold prosthetic screws.

STATEMENT OF PROBLEM: Repeated torque may alter the mechanical properties and fracture resistance of certain prosthetic retaining screws. PURPOSE: This study evaluated the effect of repeated torque and salivary contamination on the ultimate tensile strength of one type of slotted gold prosthetic retaining screw. MATERIAL AND METHODS: Forty-five slotted gold prosthetic screws from the same manufacturer (Implant Innovations) were divided randomly into 9 groups of 5 screws each, with group 0X being an unused control. Groups 1XL, 5XL, 10XL, and 20XL were lubricated with human saliva, tightened to 10 Ncm, and removed 1, 5, 10, and 20 times, respectively. This procedure was performed in an unused stack of implant components consisting of an implant, an abutment cylinder and screw, and a gold cylinder. Groups 1XN, 5XN, 10XN, and 20XN were not lubricated. A custom load cell, load frame, and strain gauge torque wrench with associated electronics were used to apply torque; ultimate tensile strength was measured on a universal testing machine. Fracture load values were analyzed against torque cycles with 1-way analysis of variance and a Tukey test (P<.05). Differences between the 2 conditions (lubricated and nonlubricated) were characterized with Student's t test (P<.05). RESULTS: Mean fracture load values ranged from 97.6 +/- 2.2 kg (group 0X) to 102.0 +/- 2.1 kg (group 5XN). Analysis of variance and Tukey's test revealed no significant differences between the lubricated and nonlubricated conditions for all groups and no significant reduction of tensile strength. For 2 of the nonlubricated groups, 5XN and 20XN, a significant increase in fracture load values was recorded (P=.02 and P=.03, respectively). CONCLUSION: Within the limitations of this study, the results suggest that the slotted gold prosthetic screw tested can be tightened and removed up to 20 times without any effect on its ultimate tensile strength. The use of human saliva as a lubricant during torquing also had no apparent effect on ultimate tensile strength.

Defects in hexed gold prosthetic screws: A metallographic and tensile analysis.

STATEMENT OF PROBLEM: Prosthetic gold screw fracture remains a clinical problem in implant prosthodontics. PURPOSE: This study examined hexed gold prosthetic screws for internal defects and determined the effect of these defects on tensile strength. The microstructure, microhardness, and major constituents of the alloys also were determined. MATERIAL AND METHODS: Four intact hexed gold prosthetic screws, 1 from each of 2 different lots from 2 manufacturers (Implant Innovations and Nobel Biocare), were examined with standard metallographic techniques for defects, microstructure, microhardness, and major alloy constituents. Thirty-six screws, 9 from each of the 2 different lots of both manufacturers, were subjected to tensile testing to determine fracture load values. Analysis of variance and Tukey tests were used to identify differences between manufacturers and lots (P<.05). The fracture sites were examined retrospectively with a scanning electron microscope to identify defects that could have contributed to failure. The mode of fracture was characterized. RESULTS: There were no significant defects in the screws tested, but differences were observed in the microstructure, microhardness, alloy composition, and fracture load values for both manufacturers. Screws from the 2 manufacturers demonstrated distinctly different metallurgical characteristics, which highlighted differences in the manufacturing processes. Fracture load values ranged from 850 +/- 20 N to 1093 +/- 64 N. A significant difference was noted for mean fracture load values for the different lots of Implant Innovations screws (P<.05). Ductile fracture was the mode of failure. CONCLUSION: The results of this study suggest that variability in the physical properties of similar hexed gold prosthetic screws made by different manufacturers, as well as different lots from the same manufacturer, may affect clinical success.