Rudolf Marx

In vivo fracture resistance of implant-supported all-ceramic restorations

STATEMENT OF PROBLEM Because of their specific mechanical properties, all-ceramic restorations demonstrate a lower fracture resistance than ceramic restorations supported by metal substructures. However, advances have been made in the fabrication of high-strength all-ceramic abutments for anterior implants. No previous study has compared the fracture loads between 2 different all-ceramic abutments restored by glass-ceramic crowns. PURPOSE The purpose of this in vitro investigation was to quantify the fracture load of implanted-supported Al(2)O(3) and ZrO(2) abutments restored with glass-ceramic crowns. MATERIALS AND METHODS Two ceramic abutments were tested: an Al(2)O(3) abutment (CerAdapt) and a ZrO(2) abutment (Wohlwend Innovative). The abutments (n = 10) were placed on Brånemark dental implants and prepared for restoration with glass-ceramic crowns (IPS Empress). After fabrication, in accordance with the manufacturers guidelines, the crowns were bonded to the all-ceramic abutments with a dual-polymerizing resin luting agent. The fracture loads (N) were determined by force application at an angle of 30 degrees by use of a computer-controlled universal testing device. The data were analyzed with the unpaired t test (alpha=.05). RESULTS Statistical analysis showed significant differences between both groups (P=.001) of all-ceramic abutments, with mean fracture load values of 280.1 N (+/- 103.1) for the Al(2)O(3) abutments and 737.6 N (+/- 245.0) for the ZrO(2) abutments. CONCLUSION Within the limitations of this study, both all-ceramic abutments exceeded the established values for maximum incisal forces reported in the literature (90 to 370 N). The ZrO(2) abutments were more than twice as resistant to fracture as the Al(2)O(3)-abutments.

Lifetime Prediction of All-ceramic Bridges by Computational Methods

There has been limited use of ceramic materials for all-ceramic posterior bridges. Major reasons are the low strength, the strength scatter, and the time-dependent strength decrease of ceramics due to slow crack growth. The objective of this study was to predict the long-term failure probability and loading capability of all-ceramic bridges (Empress 1, Empress 2, In-Ceram Alumina, and ZrO2) by computational techniques. The lifetimes of different bridge model designs were predicted by means of the NASA post-processor CARES. Bridges made of zirconia showed a very high mechanical long-term reliability. Empress I and InCeram Alumina seem to be insufficient as posterior bridge materials based on this prediction. The lifetime of the all-ceramic bridges can be significantly increased by improving the design in the connector area. We conclude that computational techniques can help to judge a ceramic material and a specific ceramic bridge design with respect to mechanical reliability before clinical use.

Surface pretreatments for medical application of adhesion

Medical implants and prostheses (artificial hips, tendono- and ligament plasties) usually are multi-component systems that may be machined from one of three material classes: metals, plastics and ceramics. Typically, the body-sided bonding element is bone.The purpose of this contribution is to describe developments carried out to optimize the techniques , connecting prosthesis to bone, to be joined by an adhesive bone cement at their interface. Although bonding of organic polymers to inorganic or organic surfaces and to bone has a long history, there remains a serious obstacle in realizing long-term high-bonding strengths in the in vivo body environment of ever present high humidity.Therefore, different pretreatments, individually adapted to the actual combination of materials, are needed to assure long term adhesive strength and stability against hydrolysis. This pretreatment for metal alloys may be silica layering; for PE-plastics, a specific plasma activation; and for bone, amphiphilic layering systems such that the hydrophilic properties of bone become better adapted to the hydrophobic properties of the bone cement. Amphiphilic layering systems are related to those developed in dentistry for dentine bonding.Specific pretreatment can significantly increase bond strengths, particularly after long term immersion in water under conditions similar to those in the human body. The bond strength between bone and plastic for example can be increased by a factor approaching 50 (pealing work increasing from 30 N/m to 1500 N/m).This review article summarizes the multi-disciplined subject of adhesion and adhesives, considering the technology involved in the formation and mechanical performance of adhesives joints inside the human body.

Nondestructive estimation of the strength of dental ceramic materials

OBJECTIVES The aim of this study was to prove a nondestructive method to evaluate the strength of dental ceramic materials with respect to its potentiality and limitations. METHODS The Youngs moduli of 13 dental ceramic materials were determined by the resonance frequency method. Additionally, the flexural strengths of eight of these materials were evaluated by the four-point bending test. Strength values for the other five ceramic materials were taken from the literature. The Youngs moduli were correlated with the strength values by Hooks law, respectively. RESULTS Youngs modulus for ceramics can be determined using a resonance frequency method. Fracture strain values of the ceramic materials tested (with the exception of Empress 2) have fracture strain values between 0.08 and 0.15%. A mean strain can be calculated (0.11%) and used with the value of Youngs modulus to estimate fracture strength of ceramics. For the materials evaluated, predicted strength was within 39% of the measured values. SIGNIFICANCE A non-destructive method to estimate strength of dental ceramic materials is possible, even though the accuracy of the predicted values is not very high. Nevertheless, the method permits new materials proposed for dental ceramic restorations to be screened for probable clinical success. Expensive and time-consuming testing would only need to be done on those materials which pass this initial criterion.

Human cell culture studies with dental metallic materials

The compatibility of cast non-alloyed titanium and eight dental castable alloys (precious, low noble, non-precious) was determined by means of human primary oral fibroblasts derived from the attached gingiva and the periodontium. Furthermore, cast non-alloyed Ti was tested in a human permanent cell culture (MOLT 4). All investigated metallic materials showed cytotoxic properties in primary cultures. The differences in cytotoxicity between the various materials were statistically significant. The best results were found for one noble alloy and for non-alloyed titanium, whereas all non-precious alloys almost completely inhibited cell growth. The cytotoxic potential of the investigated materials might contribute to the irritating effect of dental cast restorations adjacent to the gingiva and the periodontium.

R-Curve Behavior of Dental Ceramic Materials

Some technical ceramics exhibit the R-curve effect, i.e., an increasing fracture resistance with crack extension which is a desirable material property because more energy is necessary to propagate a microscopic crack. The objective of this study was to prove whether dental ceramic materials exhibit R-curve behavior. Nine dental ceramics were examined by the indentation-strength method. It was found that all of the tested ceramic materials exhibt a rising R-curve with crack extension. The R-curve behavior was more pronounced for the high-strength materials In-Ceram Alumina, monolithic alumina, and especially Empress 2. We conclude from our results that the mechanical behavior of a dental ceramic material can be judged more comprehensively, if the R-curve of the respective material is known.

Threshold intensity factors as lower boundaries for crack propagation in ceramics

BackgroundSlow crack growth can be described in a v (crack velocity) versus KI (stress intensity factor) diagram. Slow crack growth in ceramics is attributed to corrosion assisted stress at the crack tip or at any pre-existing defect in the ceramic. The combined effect of high stresses at the crack tip and the presence of water or body fluid molecules (reducing surface energy at the crack tip) induces crack propagation, which eventually may result in fatigue. The presence of a threshold in the stress intensity factor, below which no crack propagation occurs, has been the subject of important research in the last years. The higher this threshold, the higher the reliability of the ceramic, and consequently the longer its lifetime.MethodsWe utilize the Irwin K-field displacement relation to deduce crack tip stress intensity factors from the near crack tip profile. Cracks are initiated by indentation impressions. The threshold stress intensity factor is determined as the time limit of the tip stress intensity when the residual stresses have (nearly) disappeared.ResultsWe determined the threshold stress intensity factors for most of the all ceramic materials presently important for dental restorations in Europe. Of special significance is the finding that alumina ceramic has a threshold limit nearly identical with that of zirconia.ConclusionThe intention of the present paper is to stress the point that the threshold stress intensity factor represents a more intrinsic property for a given ceramic material than the widely used toughness (bend strength or fracture toughness), which refers only to fast crack growth. Considering two ceramics with identical threshold limits, although with different critical stress intensity limits, means that both ceramics have identical starting points for slow crack growth. Fast catastrophic crack growth leading to spontaneous fatigue, however, is different. This growth starts later in those ceramic materials that have larger critical stress intensity factors.

Improvement of Strength Parameters of a Leucite-reinforced Glass Ceramic by Dual-ion Exchange

An innovative dual ion-exchange process can improve the limited strength and the scatter-in-strength of technical glasses. The objective of this study was to prove whether such a two-stage ion-exchange process can also improve the limited strength and the problematic scatter-in-strength of dental ceramic materials. The first exchange was done in KN03 on a leucite-reinforced glass-ceramic material, the second exchange in 70 mol% KN03, 30 mol% Na N03 at different treatment times and temperatures. The dual-exchange process approximately doubled the Weibull strength. Moreover, the Weibull modulus showed a four-fold increase, i.e., the coefficients of variation were reduced from 18.3 to 4.7%. We conclude that the dual-exchange process may help significantly to increase the clinical reliability of glass-ceramic dental restorations, because the strength and the scatter-in-strength will be substantially improved by this treatment.

A new adhesive technique for internal fixation in midfacial surgery

BackgroundThe current surgical therapy of midfacial fractures involves internal fixation in which bone fragments are fixed in their anatomical positions with osteosynthesis plates and corresponding screws until bone healing is complete. This often causes new fractures to fragile bones while drilling pilot holes or trying to insert screws. The adhesive fixation of osteosynthesis plates using PMMA bone cement could offer a viable alternative for fixing the plates without screws. In order to achieve the adhesive bonding of bone cement to cortical bone in the viscerocranium, an amphiphilic bone bonding agent was created, analogous to the dentin bonding agents currently on the market.MethodsThe adhesive bonding strengths were measured using tension tests. For this, metal plates with 2.0 mm diameter screw holes were cemented with PMMA bone cement to cortical bovine bone samples from the femur diaphysis. The bone was conditioned with an amphiphilic bone bonding agent prior to cementing. The samples were stored for 1 to 42 days at 37 degrees C, either moist or completely submerged in an isotonic NaCl-solution, and then subjected to the tension tests.ResultsWithout the bone bonding agent, the bonding strength was close to zero (0.2 MPa). Primary stability with bone bonding agent is considered to be at ca. 8 MPa. Moist storage over 42 days resulted in decreased adhesion forces of ca. 6 MPa. Wet storage resulted in relatively constant bonding strengths of ca. 8 MPa.ConclusionA new amphiphilic bone bonding agent was developed, which builds an optimizied interlayer between the hydrophilic bone surface and the hydrophobic PMMA bone cement and thus leads to adhesive bonding between them. Our in vitro investigations demonstrated the adhesive bonding of PMMA bone cement to cortical bone, which was also stable against hydrolysis. The newly developed adhesive fixing technique could be applied clinically when the fixation of osteosynthesis plates with screws is impossible. With the detected adhesion forces of ca. 6 to 8 MPa, it is assumed that the adhesive fixation system is able to secure bone fragments from the non-load bearing midfacial regions in their orthotopic positions until fracture consolidation is complete.

Improvement of the long‐term adhesive strength between metal stem and polymethylmethacrylate bone cement by a silica/silane interlayer system

A loss of adhesive strength between metal stem and bone cement is clinically found to be a serious problem in total hip arthroplasty and causes many operative revisions. The objective of this study was to improve the long-term adhesive strength at this interface. A new silica/silane interlayer coating system is introduced. The layers are designed to bond the metal stem surface to the polymethylmethacrylate bone cement marginally leakage free. In vitro tensile tests were performed on specimens of TiAl6V4 and CoCrMo that were cemented by pairs with different bone cements with and without the new coating system. The specimens were stored in isotonic saline solution up to 150 days. The adhesive strength decreased about 75% within 30 days of storage on specimens of both metal alloys that were conventionally cemented without the new interlayer system. With the new coating, the high initial adhesive strength (40-50 MPa) could be stabilized for TiAl6V4 over 150 days. For the same 150-day storage period, the adhesive strength of the coated CoCrMo alloy still decreased but the decrease was only half that experienced by the uncoated CoCrMo. The loss of adhesive strength on CoCrMo specimens could be reduced if the metal surface was activated by a plasma treatment. The new coating interface system could help to considerably reduce revision operations caused by debonding effects at the interface metal/polymethylmethacrylate bone cement.