ABSTRACT
Objective To analyze the relationship between compression deformation and mechanical bearing characteristics of articular cartilage. Methods Indentation test was used to measure the compression displacement of bovine knee articular cartilage with different indenter diameters and under different loads. The fluid flow and bearing characteristics of the articular cartilage were simulated by the finite element model. Results The maximum relative error between simulated maximum compression displacement and experimental result was 1.73%. The elastic modulus and permeability coefficient of cartilage increased with indenter diameter increasing under the same load, while decreased with indentation load increasing at the same indenter diameter. Fluid flow was mainly in the internal cartilage when the load was exerted on cartilage. With the load sustaining, fluid flow was gradually moved to outside of the cartilage. Pore pressure, axial stress and radial stress on the cartilage surface then presented a nonlinear change due to the fluid flow. Conclusions Fluid flow, pore pressure and stress distribution on the cartilage surface have a great impact on its bearing characteristics, which varies greatly with different indenters and under different loads.
ABSTRACT
Objective To study the effect from the content change in hydroxyapatite (HA) on mechanical properties of polyvinyl alcohol/hydroxyapatite(PVA/HA)composite hydrogel and its load bearing characteristics based on experiment and finite element simulation. Methods The compression and stress relaxation tests of PVA/HA composite hydrogel were performed on the UMT test machine. Through the simulation and experimental results, the load-bearing characteristics of PVA/HA composite hydrogel and the effect of HA on PVA/HA composite hydrogel performance were investigated. Results With HA content increasing, the compression modulus of PVA/HA composite hydrogel was first increased and then decreased, while its permeability coefficient was first decreased and then increased. When the HA content was 3%，the compression modulus of PVA/HA composite hy drogel reached maximum (1.25 MPa)and its permeability coefficient reached minimum(1.59×10-3 mm4•N-1•s-1). The fluid bearing proportion of PVA/HA composite hydrogel was first increased and then decreased with the loading time increasing, which presented a nonlinear change. The proportion of the fluid bearing was significantly increased after HA added into PVA / HA composite hydrogel. The stress relaxation rate was first increased and then decreased with HA content increasing. Under the same loaded displacement, PVA / HA composite hydrogel with 3% HA content could disperse more contact stress. Conclusions The internal bearing characteristics of PVA / HA composite hydrogel have a great impact on its mechanical properties. When the HA content is 3%, the PVA / HA composite hydrogel has the best mechanical properties, which are closer to those of the natural articular cartilage.
ABSTRACT
Objective To cross-link the porous biological ceramics and PVA hydrogel to form a double layer construction between the artificial cartilage and hard joint, and to analyze its morphologies and mechanical properties. Methods With hydroxyl apatite (HA) as the substrate, the porous hydroxyl apatite biological ceramics with different porosities were prepared by using NH4HCO3 crystal grains as the pore-formed material. The poly vinyl alcohol (PVA) and epoxypropane were used as the primary material and cross-linking agent, respectively. The PVA hydrogel with double layer construction was cross-linked and prepared on the porous biological ceramics surface. The fracture appearances of the test specimen section were characterized. The performances of anti tensile strength and anti-shear strength for PVA hydrogel were analyzed. Results The cross-linked PVA hydrogel could permeate in the biological ceramics substrate, and the union between ceramic substrate and PVA hydrogel performed well. With the porosity of the porous biological ceramics increasing, the tension load and shear load of the PVA hydrogel samples both increased, and with the average porosity of 70%, the samples’ biggest tension load and shear load were 153.61 N and 64.46 N, respectively. But the corresponding tensile strength and shear strength both decreased and with the average porsity of 70%, the samples’ biggest tensile strength and shear strength were 2.12 MPa and 1.13 MPa, respectively. The failure mode of both tension and shear tests for PVA hydrogel samples was due to the crack propagation, and the fracture morphologies showed that obvious cracks and internal defects appeared on the fracture surface, while the source of the crack and the direction of the crack propagation could be observed. Conclusions Considering the compression strength of porous biological ceramics, the permeation effect on the porous biological ceramic substrate with the average porosity of 50% is moderate to be used, which ensures the appropriate shear and tensile strength of PVA hydrogel samples and the compression strength of porous biological ceramic.