Ratcheting Behavior Of Articular Cartilage Under Unconfined Cyclic Compression

As a important component of diarthrodial joints,articular cartilage is a kind of hydrous connective tissues with nonlinearity and viscoelasticity covering the surface of subchondral bones.It plays a key role in transmiting loads,absorbing shock and sustaining daily loading histories.Under cyclic compression,although the deformation of articular cartilage in a single stress circulation maybe very small,but it can accmulate up to a very high level with the increasement of the number of cycles,which may result in fatigue damage or degenerative diseases.Since the mechanical fatigue was assumed as the original cause of osteoarthritis.So it is very important to investigate the ratcheting behavior of articular cartilage under cyclic compression for prevention and cure of cartilage diseases and the development of artificial cartilage.In this paper,ratcheting loading tests of articular cartilage under unconfined cyclic compression were conducted.Different layers of cartilage were observed by using non-contact digital image correlation technique and the ratcheting behavior of articular cartilage was investigated.The results of the soaked tests and unsoaked tests were compared.It is found that the ratcheting strain increase with the increasement of the number of cycles and the evolution of ratcheting strain can be divided into two stages.The first stage is very short and characterized by an initial sharp increase in ratcheting strain.The second stage has a long duration and shows a small rate of strain accumulation.The ratcheting strain rate decreases rapidly at first,and exhibits a relatively stable and small value later.The initial ratcheting strain of online soaked cartilage is smaller than that of unsoaked cartilage,but with the increase of the number of cycles,the ratcheting strain of online soaked cartilage approaches to that of unsoaked sample with increase of number of cycles.The ratcheting strain and its rate decrease along the depth of cartilage,the values of superficial layer are the largest,the values of deep layer are the smallest and the values of middle layer are between the superficial layer and deep layer.The trend of the Young’s modulus of cartilage is opposite to the ratcheting strain along cartilage depth.The tests under different loading conditions were conducted and the influencing factors of the ratcheting behavior of articular cartilage were investigated.It is found that the ratcheting strain evolution and its rate evolution are not coincident for different tests,which means that stress range,loading rate,peak time and creep prestrain have great influence on the ratcheting behavior of cartilage.The ratcheting strain and its rate of articular cartilage increase with the increasing stress range,increasing peak time and decreasing loading rate.The ratcheting strain with prestrain is much larger than that without prestrain,which can become smaller after 30 minutes’ recover in saline but still larger than that without prestrain.Ratcheting constitutive model of articular cartilage was built and the ratcheting strain ofarticular cartilage was predicted.The prediction has a good agreement with expermental data,which means that the ratcheting constitutive model is reasonable.