Study Of The Material Properties Of Articular Cartilage Under Compressive Loads And The Instantaneous Damages Of Articular Cartilage Due To Different Abnormal Stress Loading Protocols

Nonphysiological mechanical loading of articular cartilage is believed to be an important contributing factor in the development and progression of osteoarthritis. It's crucial to understand the mechanisms of this "abnormal loading - degaradation" course before, strategies that are prophylactic and remedial to the degradative disease could be established. The long-term progression of cartilage degradative disease is probably strongly influenced by acutely induced alterations to tissue integrity. So it is essential to make clear the instantaneous damages of articular cartilage under abnormal mechanical loadings.The destructive impacts of mechanical loadings are imposed on the two parts composing the tissue, i.e. the chondrocytes and the matrix. The matrix play a key role in bearing the compressive loadings. Whereas the chondrocytes, shielded from impacts by the collagen network, share little pressure. So it is important to make clear the mechanical circumstances of the matrix when studying the instantaneous damages of articular cartilage due to mechanical loadings.This study was carried out based on such idea that in order to determine the initiating mechanisms of the articular cartilage degradation, it is essential to make clear the material properties of articular cartilage under compressive loads and the instantaneous damages of cartilage due to abnormal stress loads with different loading rates. Two major parts dealing with such idea and anindependent part concerning the 3D finite element model construction of a human knee composed this thesis.PART I. The material properties of bovine articular cartilage1. Articular cartilage from the shoulder of a bovine was tested in a confined compression chamber. Two material parameters, aggregate modulus(HA) and permeability(ko) was acquired by successfully fitting the experimental data with the biphasic theory. The fitting results showed the HA as 0.53?.07MPa andkoas5.8xl0-16?.5xlO-16m4N-1S-12. The interstitial fluid support of the total stress, represented by p(0,t)/a(t), was calculated and analysed using the finite deformation theory. Results showed that fluid pressure was the major supporting factor of applied loads at beginning and dropped continuously during the stress relaxation stage, whereas the loads supported by the solid matrix increased.3. The deformation behavior of bovine articular cartilage under stress loads with various loading rates was studied by unconfined compression tests. Results showed that with the high loading rate(stress rate, 50MPa/s; peak stress, 3.2MPa), the cartilage exhibited a linear elastic property with a compressive modulus(E=17.43MPa) that was far greater than HA. Loads with a lower loading rate (stress rate, 0.2MPa/s; peak stress, 0.75MPa) produced a nonlinear stress-strain relationship. A parameter, instantaneous compressive modulus E(s) was introduced to represent the dynamic stiffness and calculated. Calculation showed that the dynamic stiffness became greater when the tissue strain increased. The test also showed that the stiffness was greater in the cartilage under a higher loading rate than that of a lower loading rate.PART II. "abnormal stress loads - instantaneous damages of cartilage"Two different stress loading protocols were applied on the bovine articular cartilage to find out the difference of matrix and cell injury due to different loads protocols. Single sub-impact loads characterized by a constant stress rate of 0.26MPa ?s-1 to a peak stress between 2MPa and 1 IMPa were applied to the adult bovine articular cartilage explants, defined as group SI. The explantssubjected to cyclic indentation impacts characterized by a constant stress rate of SIMPa ?s-1, 0.3Hz, to the same peak stress as group SI belong to group CI. No mechanical loads were applied to the control group. All the explants were maintained in culture for the next few days after loads. The spatial patterns and severity of chondrocytes injury were determined using fluorescein diacetate and propidium iod...