Viscoelastic menisci

It is well known in polymer rheology that a torsional parallel-plate flow cell develops certain secondary flow and meniscus distortion beyond some stress level [ 14]. For viscoelastic melts, this can happen at an embarrassingly low stress. The critical condition for these instabilities has not been clearly identified in terms of the shear stress, normal stress, and surface tension. It is very plausible that the boundary discontinuity and stress intensification discussed in Sect. 4 is the primary source for the meniscus instability. On the other hand, it is well documented that the first indication of an unstable flow in parallel plates is not a visually observable meniscus distortion or edge fracture, but a measurable decay of stress at a given shear rate [40]. The decay of the average stress can occur in both steady shear and frequency-dependent dynamic shear. 

Further work is being directed to computer-aided analysis of (1) flow in other roll configurations, especially roll-wall combinations and reverse-roll coating (2) stability of the flow to meniscus nonuniformity — "ribbing," for example and (3) effects of viscoelasticity. 

Kobayashi et al. were able to use PVA cryogels as an artificial meniscus in animal models. The mechanical properties and viscoelastic characteristics as well as biocompatibility of the material are beneficial for this application. PVA was processed in a DMSO/water solvent, vacuum dried, and heated for aimealing. It was then left in water, cut and processed into meniscus form, and used as a prosthesis in rabbits. The samples remained intact for up to 2 years and no fracture or degradation of its mechanical properties occurred. Biocompatibility was also found to be satisfactory [98]. 

The anisotropic viscoelastic properties in shear of the meniscus have been determined by subjecting discs of meniscal tissue to sinusoidal torsional loading [35](Table B2.8). The specimens were cut in the three directions of orthotropic symmetry, i.e. circumferential, axial and radial. A definite correlation is seen with the orientation of the fibers and both the magnitude of the dynamic modulus IG I and the phase angle 8. 

This book is aimed at teaching senior engineering students or orthopaedic residents the fundamental principles of biomechanics of the musculoskeletal system. The book contains several chapters on the mechanics of joints, and the properties and functions of joint tissues. The chapter devoted to articular cartilage and the meniscus includes a review of collagen-proteoglycan interactions, and how these directly affect the mechanical behavior of the tissue. The biphasic and the triphasic theories for the viscoelastic properties are also discussed. 

The applied signal can be either the pressure change at the opposite end of the capillary (Fig. 2) [16, 17] as in the previous case, or the volume variation in the cell produced by a pulsating rod or a piezodriver at constant pressure near the opposite capillary end (Fig. 3) [4, 14, 18-20]. In all cases from the comparison of the applied and the measured signals the complex dilational viscoelasticity c(i(o) can be obtained as a function of frequency after elimination of all contributions caused by the bulk phase behaviour. It is possible also to measure both the pressure in the cell and the meniscus volume and to compare them [21-23]. 

Mechanical properties of the knee components are derived from selected referred articles (See Table 1). Bone s density and its mechanical properties varied along its length, but in this study tibia and femur simplified as a non-linear isotropic material. Meniscus is composed of fibrocartilage, an anisotropic nonlinear viscoelastic material and its viscoelastic behavior make constant after 1500s [2]. Because an instance load applied in the dynamic analysis, so the properties variation is neglected. Articular cartilage and tibia plateau cartilage are assumed as isotropic elastic material. 

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