Compressive Mechanics Of A Poly (Vinyl Alcohol)-Based Hydrogel System For The Replacement Of The Knee Meniscus

Abstract

Osteoarthritis and cartilage deterioration are favored by meniscetomy, which is the ablation of the meniscus from the knee joint. Meniscectomy can be partial or total. This procedure is performed when meniscus lesions and tears or the degeneration of the meniscus caused by its natural dehydration occur. There is a peak of meniscal lesions observed between 20 and 29 years old. Alternative methods such as sutures fail in that they present a short term solution which is ideal for a less active, older generation. A long term solution is needed for a younger population to reduce the number of surgical procedures over the lifetime of this active group. There is a crucial need for a functional implant designed in the image of the native meniscus. Blends of poly (vinyl alcohol) PVA and poly (vinyl pyrrolidone) (PVP) present a potential solution. PVA has shown similar characteristics to soft tissues. PVP further stabilizes the hydrogel network. This work is the mechanical characterization of PVA/PVP (99:1) hydrogels under physiological conditions. Equilibrium swelling in a medium replicating the ionic and the osmotic content of the synovial fluid was investigated during 35 days. The mass retention of hydrogels was characterized using data obtained from the swelling study and was examined as a function of the cross link density and the polymer content. The modulus of hydrogels was obtained in unconfined compression, first at a strain rate slow enough to ignore fluid flow in and out of the gels, and subsequently at a physiological strain rate of walking. Results indicate that PVA/PVP hydrogels volume swelling ratio and weight swelling ratio show no significant difference for most formulations by the 14th day of immersion. A few hydrogels would reach equilibrium by day 21. Additionally, percentage polymer mass retention increases with the cross link density. However, there is no consistent trend with the polymer content. All formulations with 10% wt of polymer show the highest mass retention while 15% wt show the lowest. Interestingly, the mechanical characterization of hydrogels at 100%/min strain rate shows that 15% wt is the only formulation whose compressive modulus falls within the targeted range whereas 10% wt proves to not be stiff enough. 20% wt and 25% wt are always too stiff. Results obtained from unconfined compression at the physiological strain rate, that is 1920%/min, are rather inconclusive. There is not enough consistency in the literature to narrow the results down to one successful candidate formulation. The modulus range obtained at physiological strain rate encompasses the range obtained at 100%/min strain rate. The highest modulus value obtained is 10 times higher at physiological strain rate than the modulus of a real human meniscus obtained at 100%/min strain rate. It is not reasonable at this time to make a choice of a formulation at physiological strain rate due to high variability of the modulus of a human meniscus as a result of its intrinsic anisotropy. All formulations tested would be considered successful candidates, which is irrational considering the difference in their stiffness.