A novel, hydroxyapatite-based screw-like device for anterior cruciate ligament (ACL) reconstructions.

BACKGROUND: Rupture of the anterior cruciate ligament (ACL) is one of the most common injuries of the knee. Common techniques for ACL reconstruction require a graft fixation using interference screws. Nowadays, these interference screws are normally made of titanium or polymer/ceramic composites. The main challenge of application of a fixation device made entirely of bioactive ceramic is in relation to the low strength of such materials. The purpose of this study was to evaluate a novel geometry for a fixation device made of pure hydroxyapatite for ACL reconstructions that can overcome some problems of the titanium and the polymer/ceramic screws. METHODS: Finite Element Analysis was used for optimization of the stress distribution in conventional interference screw geometry. For experimental evaluation of the new fixation device, ex vivo tests were performed. RESULTS: The innovative screw-like fixation device is characterized by multiple threads with a large thread pitch. The novel design enabled the insertion of the screw into the bone without the application of an external torque or a screwdriver. In turn, it also allowed for the use of low-strength and high-bioactivity materials, like hydroxyapatite. Ex vivo tests showed that the novel screw can sustain pull-out forces up to 476 N, which is comparable to that of the commercially available BioComposite™ interference screws (Arthrex Inc., Germany), as a reference. CONCLUSIONS: In summary, the novel screw design is a promising strategy to develop all-ceramic fixation devices for ACL reconstructions, which may eliminate some drawbacks of the current interference screws.

Hierarchical Porous Zeolite Structures for Pressure Swing Adsorption Applications.

Porous adsorbents with hierarchical structured macropores ranging from 1 to 100 µm are prepared using a combination of freeze casting and additional sacrificial templating of polyurethane foams, with a zeolite 13X powder serving as adsorbent. The pore system of the prepared monoliths features micropores assigned to the zeolite 13X particle framework, interparticular pores of ∼1-2 µm, lamellar pores derived from freeze casting of ∼10 µm, and an interconnected pore network obtained from the sacrificial templates ranging from around 100 to 200 µm with a total porosity of 71%. Gas permeation measurements show an increase in intrinsic permeability by a factor of 14 for monoliths prepared with an additional sacrificial templated foam compared to monoliths solely providing freeze casting pores. Cyclic CO2 adsorption and desorption tests where pressure swings between 8 and 140 kPa reveal constant working capacities over multiple cycles. Furthermore, the monoliths feature a high volumetric working capacity of ∼1.34 mmol/cm(3) which is competitive to packed beds made of commercially available zeolite 13X beads (∼1.28 mmol/cm(3)). Combined with the faster CO2 uptake showing an adsorption of 50% within 5-8 s (beads ∼10 s), the monoliths show great potential for pressure swing adsorption applications, where high volumetric working capacities, fast uptakes, and low pressure drops are needed for a high system performance.

Synthesis and mechanical evaluation of Sr-doped calcium-zirconium-silicate (baghdadite) and its impact on osteoblast cell proliferation and ALP activity.

For the first time the successful preparation of Sr doped baghdadite (Ca3-x Sr x ZrSi2O9 x = 0.1 and 0.75) is shown. Sr-doped as well as pure baghdadite are prepared via a versatile solid-state synthesis and conventional sintering at 1400 °C. XRD measurements and crystal structure refinements reveal that a substitution of Ca atoms with Sr and a high purity (>99%) is achieved. The physical, mechanical, and biological properties of these novel bioceramics are presented in relation to the dopant concentration. Incorporating Sr into the baghdadite crystal caused only minor changes to the grain size and the mechanical properties. The characteristic strength ranges from 145 to 168 MPa and a Weibull modulus of 4.9 to 9.2 is observed. Other mechanical properties like fracture toughness and hardness vary from 1.23 ± 0.07 MPam(0.5) to 1.31 ± 0.12 MPam(0.5) and 7.3 ± 0.6 GPa to 8.0 ± 0.7 GPa, respectively. The in vitro cellular response of human osteoblasts showed an increase in the cell proliferation and a significantly higher alkaline phosphatase (ALP) activity with an increase in the Sr content. From the improved biological properties and the suitable mechanical performance we conclude that this material is a highly promising candidate for bone replacement material and bioactive implant coatings.

Effect of silica on porosity, strength, and toughness of pressureless sintered calcium phosphate-zirconia bioceramics.

The preparation of dense, high-strength calcium phosphate-zirconia (CaP-ZrO2) composed bioceramics is realized via versatile pressureless sintering by adding silica nanoparticles. Two different weight ratios of HAp:ZrO2, 9:1 and 1:1, are used with varying silica contents from 5 to 20 wt%. After sintering at 1200 °C, the phase composition, microstructure, porosity, biaxial bending strength, and fracture toughness as well as SBF in vitro bioactivity are characterized. We show that the addition of silica altered the crystal phase composition, inhibiting the formation of non-favourable cubic ZrO2. Furthermore, SiO2 addition leads to an increase of the biaxial bending strength, and the fracture toughness of CaP-ZrO2-containing materials. With the addition of 20 wt% silica we find the highest characteristic strength (268 MPa) and toughness (2.3 ± 0.1 MPam(0.5)) at <1% porosity. Both mechanical properties are 2 times higher than those of pure hydroxyapatite. At the same time we observe for the very same composition similar bioactivity to that of pure hydroxyapatite.

Mechanical evaluation of calcium-zirconium-silicate (baghdadite) obtained by a direct solid-state synthesis route.

Ca3ZrSi2O9 (baghdadite) has become a major research focus within the biomaterial community due to its remarkable in-vitro and in-vivo bioactivity. Although baghdadite seems to exhibit interesting biological properties, as yet there has been no data published concerning its mechanical properties. This lack of knowledge hinders targeting this novel bioactive material towards potential applications. In this study we prepare dense Ca3ZrSi2O9 bulk ceramics for the first time, allowing the evaluation of its mechanical properties including hardness, bending strength, Young׳s modulus, and fracture toughness. The preparation of baghdadite has been accomplished by a direct solid-state synthesis in combination with conventional sintering at 1350-1450°C for 3h. Our results show that samples sintered at 1400°C exhibit the best mechanical properties, resulting in a bending strength, fracture toughness, and hardness of 98±16MPa, 1.3±0.1MPam(0.5), and 7.9±0.2GPa. With a comparable mechanical strength to hydroxyapatite, but with an increased fracture toughness by 30% and hardness by 13% baghdadite is highly suitable for potential applications in non-load bearing areas (e.g. coatings or filler materials).