Fractographic analysis of 35 clinically fractured bi-layered and monolithic zirconia crowns.

OBJECTIVES: The aim of this retrieval study was to analyze the fracture features and identify the fracture origin of zirconia-based single crowns that failed during clinical use. METHODS: Thirty-five fractured single crowns were retrieved from dental practices (bi-layered, n = 15; monolithic, n = 20). These were analyzed according to fractographic procedures by optical and scanning electron microscopy to identify fracture patterns and fracture origins. The fracture origins were closely examined. The crown margin thickness and axial wall height were measured. RESULTS: Three types of failure modes were observed: total fractures, marginal semilunar fractures, and incisal chippings. Most of the crowns (23) had fracture origins at the crown margin and seven of them had defects in the fracture origin area. The exact fracture origin was not possible to identify due to missing parts in four crowns. The crown wall thickness was 20% thinner and wall height 30% shorter in the fracture origin area compared to the opposite side. CONCLUSIONS: The findings in this study show that fractography can reveal fracture origins and fracture modes of both monolithic and bi-layered dental zirconia. The findings indicate that the crown margin on the shortest axial wall is the most common fracture origin site. CLINICAL SIGNIFICANCE: Crown design factors such as material thickness at the margin, axial wall height and preparation type affects the risk of fracture. It is important to ensure that the crown margins are even and flawless.

The effect of preparation taper on the resistance to fracture of monolithic zirconia crowns.

OBJECTIVE: Monolithic zirconia crowns have become a viable alternative to conventional layered restorations. The aim of this study was to evaluate whether the taper, and thus wall thickness, of the abutment or pre-defined cement space affect the fracture resistance or fracture mode of monolithic zirconia crowns. METHODS: A model tooth was prepared with a taper of 15° and a shallow circumferential chamfer preparation (0.5 mm). Two additional models were made based on the master model with a taper of 10° and 30° using computer-aided design software. Twenty monolithic 3rd generation translucent zirconia crowns were produced for each model with pre-defined cement space set to either 30 µm or 60 µm (n = 60). The estimated cement thickness was assessed by the replica method. The cemented crowns were loaded centrally in the occlusal fossa at 0.5 mm/min until fracture. Fractographic analyses were performed on all fractured crowns. RESULTS: The load at fracture was statistically significant different between the groups (p < 0.05). The crowns with 30° taper fractured at lower loads than those with 10° and 15° taper, regardless of the cement space (p < 0.05). The fracture origin for 47/60 crowns (78%) was in the cervical area, close to the top of the curvature in the mesial or distal crown margin. The remaining fractures started at the internal surface of the occlusal area and propagated cervically. SIGNIFICANCE: The fracture resistance of the monolithic zirconia crowns was lower for crowns with very large taper compared to 10 and 15° taper even though the crown walls were thicker.

Mechanical properties of cobalt-chromium 3-unit fixed dental prostheses fabricated by casting, milling, and additive manufacturing.

STATEMENT OF PROBLEM: New additive manufacturing techniques for nonprecious alloys have made the fabrication of metal-ceramic fixed partial dentures (FPDs) less expensive and less time-consuming. However, whether the mechanical properties produced by these techniques are comparable is unclear. PURPOSE: The purpose of this in vitro study was to evaluate the mechanical properties of cobalt-chromium frameworks for FPDs fabricated by 3 different techniques. MATERIAL AND METHODS: Thirty frameworks for 3-unit FPDs were fabricated by traditional casting, computer-aided design and computer-aided manufacturing (CAD-CAM) milling, and selective laser melting (SLM), with n=10 in each group. The frameworks were weighed, and distal and mesial connector areas measured. The frameworks were cemented and loaded centrally (0.5 mm/s) until deformation above 1 mm occurred. Stiffness was measured as the slope of the axis between 500 and 2000 N. Microhardness was measured on sectioned specimens by Vickers indentation. The microstructure was also analyzed by scanning electron microscopy. One-way ANOVA with Tukey post hoc analysis was used to compare the groups (α=.05). RESULTS: The framework design differed among the groups, making a comparison of strength impossible. The milled frameworks appeared bulky, while the cast and SLM frameworks were more slender. Statistically significant differences were found in microhardness, stiffness, wall thickness, weight, and connector size (P<.05), and a significant correlation was found between hardness and stiffness (-0.4, P<.005). CONCLUSIONS: Fabrication method affects the design, stiffness, microhardness, and microstructure of cobalt-chromium FPD frameworks. The SLM frameworks were stiffer and harder than the cast and milled specimens.

Novel Nanoparticulate and Ionic Titanium Antigens for Hypersensitivity Testing.

Titanium is used in a wide variety of materials ranging from medical devices to materials used in everyday life. Adverse biological reactions that could occur in patients, consumers, and workers should be monitored and prevented. There is a lack of available agents to test and predict titanium-related hypersensitivity. The aim of this study was to develop two bioavailable titanium substances in ionic and nanoparticulate form to serve as antigens for hypersensitivity testing in vitro. Peripheral blood mononuclear cells from 20 test subjects were stimulated with the antigens and secretion of monocytic and lymphatic cytokines and chemokines were measured by a multiplex bead assay. Lymphocyte stimulation indices were also determined in a subset of test subjects by measuring CD69 and HLA-DR expression by flow cytometry. Cytokine profiling revealed that both antigens increased production of typical monocyte and macrophage secreted cytokines after 24 h, with significant increases in IL-1ß, IL-7, IL-10, IL-12, IL-2R, IL-6, GM-CSF, TNF-α, IL-1RA, MIP-1α, MIP-1ß, IFN-α, and IL-15. Lymphatic cytokines and chemokines were not significantly induced by activation. After seven days of stimulation, ionic-Ti (2.5 µg/mL) caused proliferation (stimulation index > 2) of CD4+ cells and CD8+ cells in all persons tested (N = 6), while titanium dioxide nanoparticles (50 µg/mL) only caused significant proliferation of CD4+ cells. Our preliminary results show that the experimental titanium antigens, especially the ionic form, induce a general inflammatory response in vitro. A relevant cohort of test subjects is required to further elucidate their potential for predictive hypersensitivity testing.

Monolithic zirconia dental crowns. Internal fit, margin quality, fracture mode and load at fracture.

OBJECTIVE: Dental all-ceramic restorations of zirconia, with and without an aesthetic veneering layer, have become a viable alternative to conventional metal-ceramic restorations. The aim of this study was to evaluate whether factors of the production methods or the material compositions affect load at fracture, fracture modes, internal fit or crown margins of monolithic zirconia crowns. METHODS: Sixty crowns made from six different commercially available dental zirconias were produced to a model tooth with a shallow circumferential chamfer preparation. Internal fit was assessed by the replica method. The crown margin quality was assessed by light microscopy on an ordinal scale. The cemented crowns were loaded centrally in the occlusal fossa with a horizontal steel cylinder with a diameter of 13mm at 0.5mm/min until fracture. Fractographic analysis was performed on the fractured crowns. RESULTS: There were statistically significant differences among the groups regarding crown margins, internal fit and load at fracture (p<0.05, Kruskall Wallis). Fracture analyses revealed that all fractures started cervically and propagated to the occlusal surface similar to clinically observed fractures. There was statistically significant correlation between margin quality and load at fracture (Spearman's rank correlation, p<0,05). SIGNIFICANCE: Production method and material composition of monolithic zirconia crowns affect internal fit, crown margin quality and the load at fracture. The hard-machined Y-TZP zirconia crowns had the best margin quality and the highest load at fracture. Reduction of margin flaws will improve fracture strength of monolithic zirconia crowns and thereby increase clinical success.

Load at fracture of monolithic and bilayered zirconia crowns with and without a cervical zirconia collar.

STATEMENT OF PROBLEM: The effect of anatomic contour design in all or parts of zirconia crowns is uncertain regarding clinical reliability and survival rates. PURPOSE: The purpose of this in vitro study was to compare the load at fracture of monolithic, anatomic contour zirconia crowns with bilayered crowns with and without a cervical zirconia collar. MATERIAL AND METHODS: Thirty zirconia crowns were fabricated for a shallow chamfer molar preparation, 10 with a normal core-veneer design, 10 with a core-veneer design with an additional cervical collar of zirconia, and 10 with a monolithic, anatomic contour design. Veneering ceramic was applied to the first 20 specimens to create an anatomic form. All crowns were cemented to epoxy abutments and loaded until complete fracture with a clinically relevant test method. The fracture modes and load at fracture were recorded. RESULTS: Statistically significant differences were found in the load at fracture and fracture modes among the test groups (P<.001). All fractures except one initiated in the crown margin, mostly in the proximal region. The mean load at fracture was 4091 N for the normal core-veneer design, 4712 N for the collar design, and 6517 N for the monolithic, anatomic contour design. CONCLUSIONS: Monolithic, anatomical contour design gave higher loads at fracture than traditional core-veneer design. Crowns with a cervical zirconia collar had higher load at fracture than the core-veneer design, but lower than the monolithic crowns.

The Effects of Margin Curvature on Load at Fracture of Ceramic Crowns.

PURPOSE: Despite the high fracture strength of modern dental core ceramics, fractures are a common cause of clinical failures. The aim of this study was to use a clinically relevant test method to evaluate the effect of the curvature of the cervical crown margins on load at fracture. MATERIALS AND METHODS: Thirty zirconia crowns and 30 glass-ceramic crowns were produced for three premolar preparations with variation in the approximal crown margin curvature: low curvature (r=12 mm), moderate curvature (r=2.5 mm), and high curvature (r=1.5). The crowns were loaded until fracture with a method that mimics clinical fracture modes. RESULTS: Statistically significant differences were found among both crown margin curvature and material groups (P<.001). Most fractures originated from pre-existing flaws. CONCLUSION: A moderate to low curvature of the crown margins increases crown strength compared with a high curvature.

Can bone apposition predict the retention force of a femoral stem? An experimental weight-bearing hip-implant model in goats.

BACKGROUND: The increasing incidence of prosthesis revision surgery in the Western world has led to an increased focus on the capacity for stem removal. We previously reported on a femoral stem implanted in goats with an approximate 15% reduction in retention force by drilling longitudinally orientated grooves on the side of the stem. In this current study, we aimed to histologically evaluate the bony apposition towards this stem and correlate this apposition with the pullout force. METHODS: We analyzed the femora of 22 goats after stem removal. All stems remained in place for 6 months, and the goats were allowed regular loading of the hip during this time. For histological evaluation, all femora were immersed in EDTA and decalcified until sufficiently soft for standard technique preparation. We evaluated bone apposition, the presence of foreign particle debris and other factors. The apposition was evaluated with a scoring system based on semi-quantitative bone apposition in four quadrants. Kappa statistics were calculated for the score. We correlated the retention force with the amount of bone apposition. RESULTS: The stem drilling was the only significant factor influencing the retention force (p = 0.020). The bone apposition Kappa score comparing poor and good apposition scores was fair (κ = 0.4, 95% CI 0.00-0.88). Signs of foreign body reaction were noted in 5 of 22 goats. CONCLUSIONS: Based on the current findings in an experimental goat model, it appears that the effect of drilling tissue/bone out of the longitudinal grooves has a more significant impact on the retention force required to remove the stem than the amount of bone apposition outside the stem grooves. This observation may be further explored in the research of stem designs that are potentially easier to remove.

Clinically relevant fracture testing of all-ceramic crowns.

OBJECTIVES: Fracture strength measured in vitro indicates that most all-ceramic crowns should be able to withstand mastication forces. Nevertheless, fractures are one of the major clinical problems with all-ceramic restorations. Furthermore, the fracture mode of all-ceramic crowns observed in clinical use differs from that found in conventional fracture strength tests. The aim of the present study was to develop and investigate a method that simulates clinical fracture behavior in vitro. METHODS: 30 crowns with alumina cores were made to fit a cylindrical model with a molar-like preparation design. These crowns were randomly allocated to 3 tests groups (n=10). The crowns in group 1 were cemented to abutment models of epoxy and subsequently fractured by occlusal loading without contact damage. The crowns in group 2 were fractured by cementation with expanding cement. The crowns in group 3 were cemented on an abutment model of epoxy split almost in two and fractured by increasing the diameter of the model in the bucco-lingual direction. The fractured crowns were analyzed by fractographic methods and compared to a reference group of 10 crowns fractured in clinical use. RESULTS: The fracture modes of all the in vitro crowns were similar to clinical fracture modes. The fracture modes in group 1 were most closely matched to the clinical fractures. These crowns also fractured at clinically relevant loads. CONCLUSION: Laboratory tests that induce a distortion of the abutment model during occlusal loading without occlusal contact damage can simulate clinical fractures of all-ceramic crowns.

Fractographic analyses of all-ceramic crowns: a study of 27 clinically fractured crowns.

OBJECTIVES: The use of all-ceramic restorations has been steadily increasing in the decades since the introduction of high-strength core ceramics as an alternative to metal cores. Even with high-strength ceramic cores, the crowns are susceptible to fractures during clinical function, probably the major concern associated with all-ceramic restorations. The aim of this study was to analyze fracture features of clinically fractured crowns in order to determine crack initiation sites and crack propagation paths. METHODS: Fractographic methods were used to analyze 27 all-ceramic full coverage crowns with alumina cores and feldspatic veneering ceramic. The fractured crowns were supplied by dentists in private and public practices with information of type of cement and time in function if available. RESULTS: The fractured crowns comprised 13 incisors, 3 premolars and 11 molars. The results revealed that all fractures initiated in the cervical margin of the crowns and usually from the approximal area close to the most coronally placed curvature of the margin. There was no statistically significant association between the cement used and time in function. SIGNIFICANCE: Fractography of clinically failed all-ceramic crowns can provide information on the fracture modes and design considerations. The findings are in contrast to fracture modes from in vitro tests. The results suggest that more clinically relevant in vitro test methods to study the durability of ceramic crowns should be developed.

Coordinating retrieval and register studies improves postmarket surveillance.

BACKGROUND: The relative risk of revision of the Titan(®) femoral stem due to aseptic loosening increased after 2000; however, the reasons for this have not been established. A retrieval analysis was initiated with the aim of delineating the failure mechanism. QUESTIONS/PURPOSES: We asked whether aseptic loosening in stems after 2000 was associated with (1) appearance of osteolytic lesions, (2) wear particle exposure, (3) stem damage, or (4) changes to the implant or surgical instrumentation. METHODS: Femoral stems, cement, tissue, and radiographs were collected from 28 patients. We assessed the development of osteolytic lesions in 17 patients. Exposure to wear particles was quantified in 18 patients. Stem damage was assessed in 15 patients. We observed differences in the implants by examination of 24 retrieved stems. Information concerning changes to instrumentation was requested from the manufacturer. RESULTS: We found osteolysis in all patients receiving implants after 2000, which was associated with a median dose of cement and stem particles of 14,726/mm(2). Abrasion covered 59% of the surface of stems implanted from 1999. We identified geometric changes to the stem, the percent weight of aluminum in the stem's oxide layer decreased from 25% to 14% after 1997 and the rasp used to prepare the femoral cavity changed to a broach in 1999. CONCLUSIONS: Stems implanted from 2000 failed through osteolysis induced by particles released from the cement and implant. Changes to implant geometry, surface oxide layer, and surgical tools occurred in the same time frame as the reduction in survivorship.

An experimental study of low-level laser therapy in rat Achilles tendon injury.

The aim of this controlled animal study was to investigate the effect of low-level laser therapy (LLLT) administered 30 min after injury to the Achilles tendon. The study animals comprised 16 Sprague Dawley male rats divided in two groups. The right Achilles tendons were injured by blunt trauma using a mini guillotine, and were treated with LLLT or placebo LLLT 30 min later. The injury and LLLT procedures were then repeated 15 hours later on the same tendon. One group received active LLLT (λ = 904 nm, 60 mW mean output power, 0.158 W/cm(2) for 50 s, energy 3 J) and the other group received placebo LLLT 23 hours after LLLT. Ultrasonographic images were taken to measure the thickness of the right and left Achilles tendons. Animals were then killed, and all Achilles tendons were tested for ultimate tensile strength (UTS). All analyses were performed by blinded observers. There was a significant increase in tendon thickness in the active LLLT group when compared with the placebo group (p < 0.05) and there were no significant differences between the placebo and uninjured left tendons. There were no significant differences in UTS between laser-treated, placebo-treated and uninjured tendons. Laser irradiation of the Achilles tendon at 0.158 W/cm(2) for 50 s (3 J) administered within the first 30 min after blunt trauma, and repeated after 15 h, appears to lead to edema of the tendon measured 23 hours after LLLT. The guillotine blunt trauma model seems suitable for inflicting tendon injury and measuring the effects of treatment on edema by ultrasonography and UTS. More studies are needed to further refine this model.

Extensive composite molar restorations: 3 years clinical evaluation.

Abstract Objective. To evaluate the clinical performance of extensive direct composite restorations in molars after 1 and 3 years and to find out the importance of extent of the restorations and other factors related to their performance. Materials and methods. Seventy-four patients with a molar tooth in need of a restoration covering at least three surfaces and one cusp were selected. Patient-related factors were registered and the tooth was prepared and restored by using a nano-filled composite. A topographic system for classification of extensive posterior restorations was developed. At baseline, the operator recorded a clinical evaluation, using modified USPHS-criteria. After 1 and 3 years, an independent observer evaluated the restorations. Post-operative problems arising during the observation period were registered. Results. A change in clinical score from baseline to the 1 and 3 year recall was recorded for all clinical criteria. A total of nine restorations were graded as unacceptable after 3 years (3-year survival rate of 87.7% and a mean annual failure rate of 4.2%). Except for gender (p = 0.022), none of the patient-related factors investigated (age, caries risk, extension of the restoration and presence of cervical enamel) had a significant influence on the survival of the restorations. Conclusions. Extensive direct posterior composite restorations showed an acceptable clinical performance after 3 years. Men had a significantly greater restoration failure rate than women.

Class II composite restorations: importance of cervical enamel in vitro.

OBJECTIVE: This study evaluated the importance of enamel at the cervical margin for support and retention of a class II composite restoration in relation to fracture strength, fracture mode, and leakage. METHODS: Sixty-five newly extracted teeth were randomly divided into five groups. Within each group, standardized class II preparations were made at the mesial surface of the tooth with four different preparation designs. Group D (n=15) had the cervical margin placed below the cemento-enamel junction (the dentin group), and in the other three groups (the enamel groups: E1, E2, and E3), the cervical margin was within the enamel (n=15 each). Group E3 had restorations with cuspal coverage, while groups E1 and E2 differed in vertical dimension. Intact teeth without preparation or restoration were tested as controls (n=5). The area of the horizontal part of enamel at the cervical margin of the preparation (available cervical enamel) was calculated. The teeth were restored with a nanofilled composite material and an etch-and-rinse adhesive system. The teeth were subjected to artificial aging consisting of thermocycling and mechanical cyclical loading. The restorations were subsequently loaded until fracture. The teeth were examined microscopically to assess fracture mode and leakage at the interface between the restoration and the tooth substance. RESULTS: The fracture strength in group D (without cervical enamel) and E3 (with cuspal coverage and cervical enamel) was lower (p<0.01) than in the other two groups (with cervical enamel). There was a correlation between the area of available cervical enamel and fracture strength (p<0.01). The median fracture strength for the control teeth was not significantly different from groups E1 and E2. Group D exhibited a predominance of adhesive fractures, while the other groups revealed more cohesive fractures. CONCLUSION: The results from this study indicate that available cervical enamel has an impact on the performance of class II composite restorations.

Alumina additions may improve the damage tolerance of soft machined zirconia-based ceramics.

The aim of this study was to evaluate the damage tolerance of different zirconia-based materials. Bars of one hard machined and one soft machined dental zirconia and an experimental 95% zirconia 5% alumina ceramic were subjected to 100,000 stress cycles (n = 10), indented to provoke cracks on the tensile stress side (n = 10), and left untreated as controls (n = 10). The experimental material demonstrated a higher relative damage tolerance, with a 40% reduction compared to 68% for the hard machined zirconia and 84% for the soft machined zirconia.

Formation of potential titanium antigens based on protein binding to titanium dioxide nanoparticles.

Degradation products of titanium implants include free ions, organo-metallic complexes, and particles, ranging from nano to macro sizes. The biological effects, especially of nanoparticles, is yet unknown. The main objective of this study was to develop Ti-protein antigens in physiological solutions that can be used in testing of cellular responses. For this purpose, 0.1% TiO2 nanoparticles less than 100 nm were mixed with human serum albumin (HSA), 0.1% and 1%, in cell culture medium (DMEM, pH 7.2). The Ti concentrations in the resulting solutions were analyzed by inductively coupled plasma mass spectrometry. The stability of the nanoparticles in suspension was analyzed by UV-vis spectrophotometer and Dynamic Light Scattering. The concentration of Ti in suspension was dependent on the presence and concentration of HSA. Albumin prevented high aggregation rate of TiO2 nanoparticles in cell culture medium. It is shown that nano TiO2-protein stable aggregates can be produced under physiological conditions at high concentrations, and are candidates for use in cellular tests.

Should the galvanic combination of titanium and stainless steel surgical implants be avoided?

It is recommended that one should not combine different metals in orthopaedic devices. The least noble metal in such a galvanic coupling is more likely to corrode. However, some studies have failed to show increased corrosion when titanium and stainless steel are combined. The aim of this study was to determine the fretting corrosion of the contact areas between screws and plates made of these dissimilar metals used for internal fixation of bone fractures. The plates were fixed to a bone-simulating material and subjected to tensile and compressive forces in both human serum and Hank's solution. The outcome variables included in the analyses were weight loss, and release of Ti, Cr, Ni and Mo to the different media. Results from the multiple combinations were subjected to multivariate statistics. Principal component analysis visualised our findings and allowed classification of similar samples and separation of discrepant groups of samples. We found a significant effect of the test medium, but no dramatic effect due to mixing of metals. The titanium screws and plates corroded more in serum than in saline, while the opposite was true for stainless steel. Combination of dissimilar screws and plates did not cause higher weight loss or metal release than the single-material constructions, indicating comparable clinical safety.

The firing procedure influences properties of a zirconia core ceramic.

OBJECTIVES: High-strength ceramics for dental restoration are used as an understructure (core) that subsequently is covered by veneering ceramic. The veneering process involves a firing procedure at high temperatures at least once, usually two to five times. The aim of this study was to investigate whether these firing procedures affect the mechanical properties of a zirconia ceramic. METHODS: Thirty-three specimens of an industrially sintered yttria-stabilized zirconia ceramic (DC Zircon, DCS Dental AG, Allschwil, Switzerland) were cut into bars (1.2mmx4mmx20mm). One set of specimens (n=13) remained untreated (controls). Another set of specimens (n=10) was heat-treated once, corresponding to the first step of the veneering process. The third set of specimens (n=10) was heat-treated five times to mimic the full veneering process. Flexural strength, microhardness, dimensions and surface roughness were measured. The fracture patterns were assessed by light microscopy. RESULTS: The untreated specimens showed a statistically significant higher flexural strength (20%) and microhardness (9%) than both of the test groups (p< or =0.001). No significant differences were found for fracture patterns, dimensions or surface roughness. SIGNIFICANCE: The heat treatment associated with the veneering procedure on a zirconia core material reduced the flexural strength of the core after the first firing. Subsequent firings were not detrimental to the properties measured.

[Health complaints related to dental filling materials]. / Pasienter med helseplager relatert til tannfyllinger.

BACKGROUND: A wide range of materials is used in dental treatment. To what extent these materials lead to adverse reactions is under dispute. The aim of this study was to investigate whether patients with suspected adverse reactions to dental materials experienced an improvement in health after these materials were replaced. MATERIAL AND METHODS: Information on health complaints related to dental materials was obtained from the Dental Biomaterials Adverse Reaction Unit in Bergen, Norway for 142 patients. A follow-up questionnaire regarding subjective health was sent to these patients 1(1/2) to 2(1/2) years later. A similar questionnaire was sent to a reference group of 800 persons drawn from the general population. RESULTS: The patient group had more health complaints than the reference group (p < 0.001) at baseline. Of the 84 patients who completed the questionnaire (59%) 35 had changed dental materials. Amalgam fillings had been replaced in most of these patients. 23 patients (66%) reported improved health after replacement. Intraoral complaints decreased significantly (p = 0.022), and were most pronounced in patients with lesions in contact with dental materials. The intensity of various health complaints decreased slightly in most patients with replaced dental materials, but the patient group still had significantly higher health complaint indices than the reference group. INTERPRETATION: The intensity of subjective health complaints was reduced after replacement of dental materials, but it was still higher than for a comparable group in the general population. The results indicate that there may be a specific health effect of replacing dental materials, but normal symptom fluctuations over time and placebo effects such as positive effects from expectations and general care from the health personnel may have had an influence.

Time dependent mechanical properties of bone cement. An in vitro study over one year.

Changes in mechanical properties of bone cements over time are of clinical importance, but not well documented. Specifications for testing do not address the time factor. This study recorded changes in compressive properties and microstructure of one bone cement stored under simulated physiological conditions (water at 37 degrees C) from 20 min up to 1 year and in dry air at 37 degrees C for comparison. Compressive strength increased within the first week (p < 0.001), decreased at 1 month (p < 0.001), and remained at that level at 1 year. Elastic modulus showed a similar development. Maximum strain values, indicating plastic deformability, increased continuously over 1 year. Microscopy revealed microcracks between the pre-polymer beads and the matrix in specimens tested after 20 min, whereas there were less cracks in 1 year specimens. Increase in strength during the first week is due to polymerization and formation of interpenetrating molecular networks. The subsequent decrease could be due to the plasticizing effect of water uptake, as supported by higher values for dry specimens. It can be speculated that microcracks which could be initiated while reducing an arthroplasty at 15 min, acting as initiators for fatigue fractures in the cement mantle, contribute to cement failure. It is recommended that testing of bone cements should be performed after extended ageing at simulated physiological conditions, for the present cement at least 5 weeks. Results obtained at less than one week could be influenced by ongoing polymerization, as well as microcracks and lower coherence between the prepolymer beads and the matrix.