Markus Balkenhol

Fracture strength of temporary fixed partial dentures: CAD/CAM versus directly fabricated restorations

OBJECTIVES This study aimed at investigating the influence of fabrication method, storage condition and material on the fracture strength of temporary 3-unit fixed partial dentures (FPDs). METHODS A CrCo-alloy master model with a 3-unit FPD (abutment teeth 25 and 27) was manufactured. The master model was scanned and the data set transferred to a CAD/CAM unit (Cercon Brain Expert, Degudent, Hanau, Germany). Temporary 3-unit bridges were produced either by milling from pre-fabricated blanks (Trim, Luxatemp AM Plus, Cercon Base PMMA) or by direct fabrication (Trim, Luxatemp AM Plus). 10 FPDs per experimental group were subjected either to water storage at 37 °C for 24h and 3 months, respectively, or thermocycled (TC, 5000×, 5-55 °C, 1 week). Maximum force at fracture (Fmax) was determined in a 3-point bending test at 200 mm/min. Data was analyzed using parametric statistics (α = 5%). RESULTS Fmax values ranged from 138.5 to 1115.5N. FPDs, which were CAD/CAM fabricated, showed a significant higher Fmax compared to the directly fabricated bridges (p < 0.05). TC significantly affected Fmax for Luxatemp (p < 0.05) but not for the PMMA based materials (p > 0.05). CAD/CAM milled FPDs made of Luxatemp showed significantly higher Fmax values compared to Trim and Cercon Base PMMA (p < 0.05). SIGNIFICANCE CAD/CAM fabricated FPDs exhibit a higher mechanical strength compared to directly fabricated FPDs, when manufactured of the same material. Composite based materials seem to offer clear advantages versus PMMA based materials and should, therefore, be considered for CAD/CAM fabricated temporary restorations.

Fracture toughness of cross-linked and non-cross-linked temporary crown and fixed partial denture materials

OBJECTIVES Temporary crowns and fixed partial dentures are exposed to considerable functional loading, which places severe demands on the biomaterials used for their fabrication (= temporary crown & bridge materials, t-c&b). As the longevity of biopolymers is influenced by the ability to withstand a crack propagation, the aim of this study was to investigate the fracture toughness of cross-linked and non-cross-linked t-c&bs. METHODS Four different t-c&bs (Luxatemp AM Plus, Protemp 3 Garant, Structur Premium, Trim) were used to fabricate bar shaped specimens (2mmx5mmx25mm, ISO 13586). A notch (depth 2.47mm) was placed in the center of the specimen using a diamond cutting disc and a sharp pre-crack was added using a razor blade. 60 specimens per material were subjected to different storage conditions (dry and water 37 degrees C: 30min, 60min, 4h, 24h, 168h; thermocycling 5-55 degrees C: 168h) prior to fracture (3-point bending setup). The fracture sites were inspected using SEM analysis. Data was subjected to parametric statistics (p=0.05). RESULTS The K(IC) values varied between 0.4 and 1.3MPam(0.5) depending on the material and storage time. Highest K(IC) were observed for Protemp 3 Garant. Fracture toughness was significantly affected by thermocycling for all dimethacrylates (p<0.05) except for Structur Premium. All dimethacrylates showed a linear-elastic fracture mechanism, whereas the monomethacrylate showed an elasto-plastic fracture mechanism. SIGNIFICANCE Dimethacrylates exhibit a low resistance against crack propagation immediately after curing. In contrast, monomethacrylates may compensate for crack propagation due to plastic deformation. However, K(IC) is compromised with increasing storage time.

Effect of different retraction and impression techniques on the marginal fit of crowns

OBJECTIVES Periodontal conditions in restored teeth are strongly influenced by the marginal fit of the restoration which is closely related to the quality of the impression. However, the influence of the retraction and impression technique on the outcome of the impression has only sparsely been investigated. Thus, it was the objective of this study to compare the marginal fit in fixed restorations using two modes of gingival retraction and two different impression techniques in an animal model. METHODS To simulate clinical conditions, 6 teeth in each of 10 lower jaws of freshly slaughtered cows were prepared with subgingival finish lines. Two different retraction techniques were used to expose the finish line: retraction cords containing epinephrine (Surgident) and electro-surgery were applied contra-laterally at 3 teeth per quadrant. Two impressions per jaw were taken in a two-step putty-wash technique (TPW) and a one-step putty-wash technique (OPW), respectively. On the casts, measurement copings were fabricated and seated on the extracted original tooth. In each coping the marginal discrepancy was assessed at 8 reference marks. Since the data was normally distributed, results were subjected to parametric statistics (T-test; p=0.05). RESULTS Overall marginal discrepancies ranged between 0 and 200 microm. There was a small but not significant difference between electro-surgery and the retraction cords whereas TPW produced significantly better results than OPW (p<0.05). CONCLUSIONS Within the limits of the study it can be concluded that the use of gingival retraction cords as well as electro-surgery lead to acceptable results. The difference between TPW and OPW concerning the marginal discrepancies can be regarded as clinically insignificant.

Correlation between polymerization shrinkage and marginal fit of temporary crowns

OBJECTIVES During polymerization, temporary c&b materials (t-c&b) undergo shrinkage which may lead to marginal inaccuracies of the temporary restoration. The degree of correlation between these two factors is still unknown. Hence, the objective of this study was to determine the polymerization shrinkage of t-c&bs and to evaluate, to which degree the shrinkage is correlated to the width of the marginal gap. METHODS Six different t-c&bs (2 monomethacrylates and 4 dimethacrylates) were used to fabricate temporary crowns (n = 12) on two prepared teeth (stainless steel) of different diameters (5 and 7 mm). The crown diameters and marginal discrepancy were measured at various storage times after mixing (10, 30, 60 min) using a travelling microscope. In addition, the shrinkage (bonded-disk method) and content of inorganic filler (ashing method) were recorded. The statistical analysis was carried out using parametric tests, the F-test and Pearsons correlation (p = 0.05). RESULTS The marginal discrepancies increased as a function of increasing storage time (10 min: 0.162-0.218 mm; 60 min: 0.271-0.521 mm). Lower values were recorded for monomethacrylates than for the dimethacrylates (greatest changes recorded during the first 30 min). The shrinkage values recorded 10 min after mixing varied between 3.25 and 4.10%. There was no significant relationship between the width of the marginal gap and shrinkage (p > 0.05). SIGNIFICANCE Shrinkage values are not suitable to predict the marginal accuracy of a temporary restoration. At least 30 min should elapse between fabricating and trimming temporary crowns to avoid further inaccuracies.

Bonding of acrylic denture teeth to MMA/PMMA and light-curing denture base materials: The role of conditioning liquids

OBJECTIVES The connection between resin denture teeth and the denture base is essential for the integrity of partial and full dentures. The aim of the present study was to analyse the bond strength of acrylic denture teeth to two light curing denture base materials compared to the gold-standard (MMA/PMMA) using different conditioning liquids. METHODS The ridge laps of 220 identical denture teeth were ground and pre-treated using different conditioning liquids (MMA, an experimental conditioning liquid as well as the two commercially available liquids Palabond and Versyo.bond). The denture base materials (PalaXpress, Versyo.com, Eclipse) were applied using a split mould to obtain tensile bond strength specimens of identical shape. Ten specimens per test group were either stored in water for 24h or thermocycled (5000×, 5-55°C) prior to tensile bond strength testing (cross-head speed 10mm/min). Data was subjected to parametric statistics (α=0.05). RESULTS The three-way ANOVA revealed a significant influence of the material, pre-treatment as well as the storage. PalaXpress showed the highest bond strength (24.3MPa) of all materials tested after TC, whereas the use of MMA led to the most constant results. Lower values were recorded for Versyo.com (17.5MPa) and Eclipse (10.4MPa) bonded with Versyo.bond. CONCLUSIONS The results indicate that MMA/PMMA based denture base resins provide reliable and durable bond strength to acrylic denture teeth. Using light-curing denture base materials requires the application of appropriate conditioning liquids to obtain acceptable bond strength. The use of MMA affects bond strength to light-curing denture base materials. CLINICAL SIGNIFICANCE The pre-treatment of denture teeth is critical regarding their bond-strength to denture base materials and in turn for the integrity of removable full and partial dentures. Light-curing denture base resins are more sensitive to the correct tooth pre-treatment compared to conventional MMA/PMMA materials, requiring specific conditioning liquids.

Influence of prolonged setting time on permanent deformation of elastomeric impression materials.

STATEMENT OF PROBLEM Upon removal, tear forces occur in various areas of an impression. As a result, thin sulcus details may be permanently deformed, affecting the impressions accuracy. PURPOSE The purpose of this study was to investigate the effect of the chemistry and prolonged setting time on the permanent deformation of light-body impression materials after stretching. MATERIAL AND METHODS A dumbbell-shaped mold (2 mm x 4 mm x 75 mm) was used to prepare specimens (n=20) of 6 different impression materials (Affinis Light Body, Aquasil Ultra XLV, Express 2 Light Body Flow Quick, Flexitime Correct Flow, P2 Polyether Light, and Impregum Garant L DuoSoft). After water storage (35 degrees C), either for the manufacturers suggested setting time or for 5 minutes, specimens were stretched by 80% using a universal testing machine at a crosshead speed of 200 mm/min. The permanent deformation (Deltal (%)) was determined after 2 hours of storage in ambient conditions. A 2-way ANOVA followed by a Games-Howell test was used to analyze the influence of material and storage time (alpha=.05). RESULTS Polyether materials showed a significantly higher permanent deformation (Deltal>4% to 5%) than vinyl polysiloxane materials (P<.05). Extending the manufacturers suggested setting time resulted in clinically relevant improvements in elastic recovery for products with a polyether backbone only. CONCLUSIONS Increasing the setting time might be necessary for impression materials with a polyether backbone to improve elastic recovery.

Surfactant Release from Hydrophilized Vinylpolysiloxanes

Vinylpolysiloxane impression materials (VPS) exhibit an apolar (hydrophobic) backbone chemistry. Hence, surfactants are added to improve their hydrophilicity for impression-taking in moist environments. However, the mechanisms at the liquid-VPS-interface regarding the surfactant are unknown. We hypothesized that surfactant is leached from the VPS. Four experimental VPS formulations were fabricated containing 0 (control), 1.5, 3, and 5 wt% non-ionic surfactant. Samples were prepared (n = 6) and contact angles determined 30 min after mixing. After 60 sec, droplets were transferred onto the control. Mass spectrometry was used to analyze the droplets. Contact angles were inversely correlated with the surfactant concentration (p < 0.05). Droplets transferred from hydrophilized specimens onto the control showed similar contact angles. Surfactant could be clearly identified inside the droplets from the hydrophilized samples, however, not inside the control. Surfactants reduced the surface tension of the liquid in contact and did not change the surface properties of the VPS itself.

Effect of surface condition and storage time on the repairability of temporary crown and fixed partial denture materials

OBJECTIVES The aim of the present study was to investigate which parameters (chemical nature, time after mixing, surface characteristics) might affect the repair strength of temporary crown and bridge materials (t-c&b). METHODS Four different t-c&bs (Cool Temp Natural, Protemp 3 Garant, Structur Premium, Trim) were investigated using a shear-bond strength (SBS) setup. A cylinder (2 mm x 2.37 mm) of identical t-c&b (n=10) was bonded onto a specimen surface of either freshly set t-c&b (10 min after mixing) or onto specimens that were stored for 24h (37 degrees C, distilled water) and 7 days (thermocycling x 5,000, 5-55 degrees C=TC), respectively. The specimen surface was roughened with SiC paper (grit size 320) or left as it was (specimens stored for 10 min) prior to repair to retain the oxygen-inhibition layer. In addition, mono-block specimens were fabricated as control. The thickness of the oxygen-inhibition layer and the surface morphology was determined. Statistical analysis was carried out with an ANOVA followed by parametric tests (p=0.05). RESULTS SBS values ranged from 10 to 40 MPa. Trim showed lowest SBS values for most storage conditions. Material, surface characteristics and time after mixing significantly affected the SBS (ANOVA p<0.001). TC significantly reduced the SBS (p<0.05) for all t-c&bs except for Trim (p>0.05). CONCLUSIONS In case of monomethacrylates, storage and surface condition do not affect the repair strength. In contrast, the repair quality of dimethacrylates greatly depends on the material. In any case, roughening the surface is recommended, even if an oxygen-inhibition layer is present.

Repairability of Cross-linked Biopolymers

Repair of biopolymers is a critical issue, especially with aged restorations. Obtaining a chemical bond to the repair surface might solve this problem. We hypothesized that certain repair liquids are suitable to establish a strong bond to an artificially aged dimethacrylate-based biopolymer for temporary restorations. Specimens made of a self-curing temporary crown-and-bridge material were prepared and thermocycled for 7 days (5000x, 5–55°C). Cylinders made of light-curing composites (n = 10) were bonded onto the specimen surface, either after grinding or after the application of 4 different experimental repair liquids (Bis-GMA:TEGDMA mixture = bonding, methylmethacrylate = MMA, bonding & acetone, bonding & MMA). A shear bond strength test was performed 24 hrs after repair. The highest bond strength was obtained with the bonding & acetone liquid (20.1 ± 2.2 MPa). The use of MMA significantly affected the bond strength (6.8 ± 1.9 MPa). MMA is inadequate as a repair liquid on aged composite-based biopolymers.