Surface geometry of the human ankle joint

Synovial Joints. Tribology, Regeneration, Regenerative Rehabilitation and Arthroplasty

Synovial joints are unique biological tribosystems that allow a person to perform a wide range of movements with minimal energy consumption. In recent years, they have been increasingly called “smart friction units” due to their ability to self-repair and adapt to changing operating conditions. However, in reality, the elements of the internal structure of the joints under the influence of many factors can degrade rather quickly, leading to serious disease such as osteoarthritis. According to the World Health Organization, osteoarthritis is already one of the 10 most disabling diseases in developed countries. In this regard, at present, fundamental research on synovial joints remains highly relevant. Despite the fact that the synovial joints have already been studied fully, many issues related to their operating, prevention, development of pathology, diagnosis and treatment require more detailed consideration. In this article, we discuss the urgent problems that need to be solved for the development of new pharmacological agents, biomaterials, scaffolds, implants and rehabilitation devices for the prevention, rehabilitation and improvement of the treatment effectiveness of synovial joints at various stages of osteoarthritis.

Transient Elastohydrodynamic Lubrication Models for the Human Ankle Joint

An equivalent bearing was proposed to represent the normal human ankle joint. The geometry was based on measurements of dissected ankle joints and tissue properties were obtained from the work of previous investigators. Theoretical models were developed to estimate the cyclic variation in lubricant film thickness and coefficient of friction during repetitive activities such as walking. Solutions were obtained for various combinations of input parameters. For the conditions representing the walking cycle, film thicknesses of about 0.7 μm were calculated. Although this value was smaller than most previous measurements of the rms roughness of cartilage, it was not much smaller and suggested that transient elastohydrodynamic lubrication played a role in synovial joint lubrication. The possibility of full fluid film lubrication was supported only when a very high input viscosity was employed, based on values estimated from the previous experimental studies of the boosted lubrication mechanism. Also, an attempt was made to link the current findings to a published experimental study of whole joint lubrication.

Squeeze-film lubrication of the human ankle joint with synovial fluid filtrated by articular cartilage with the superficial zone worn out

A squeeze-film lubrication model of the human ankle joint in standing that takes into account the fluid transport across the articular surface is presented. Articular cartilage is a biphasic mixture of the ideal interstitial fluid and an elastic permeable isotropic homogeneous intrinsically incompressible matrix. The simple homogeneous model for articular cartilage models the case of early osteoarthritis, when the intact superficial zone of the normal articular cartilage, much stiffer in tension than the bulk material, has been already disrupted or worn out. The calculations indicate for this case that in normal approach motion the lubricating fluid film is quickly depleted and turned into a synovial gel film that is supposed to serve as a boundary lubricant if sliding motion follows

Lubrication of the human ankle joint in walking with the synovial fluid filtrated by the cartilage with the surface zone worn out: steady pure sliding motion

A mixture model of synovial fluid filtration by cartilage in the human ankle joint during walking is presented for steady sliding motion of the articular surfaces. In the paper the cartilage surface zone is assumed worn out. The same model has been recently applied to the squeeze-film problem for the human hip joint loaded by the body weight during standing (Hlavácek, Journal of Biomechanics 26, 1145-1150, 1151-1160, 1993; Hlavácek and Novák, Journal of Biomechanics 28, 1193-1198, 1199-1205, 1995). The linear biphasic model for cartilage (elastic porous matrix + ideal fluid) due to Prof. V. C. Mow and his co-workers and the biphasic model for synovial fluid (viscous fluid + ideal fluid), as used in the above-mentioned squeeze-film problem, are applied. For the physiologic parameters of the ankle joint during walking, a continuous synovial fluid film about 1 microm thick is maintained under steady entraining motion according to the classical model without the fluid transport across the articular surface. This is not the case in the filtration model with the cartilage surface zones worn out. On the contrary, this filtration model indicates that synovial fluid is intensively filtrated by such cartilage, so that no continuous fluid film is maintained and a synovial gel layer, about 10(-8) m thick, develops over the majority of the contact. Thus, if the cartilage surface zones are worn out, boundary lubrication should prevail in the ankle joint under steady sliding motion for the mean values of loading and the sliding velocity encountered in walking cycle.

The influence of articular surface incongruity on lubrication and contact pressure distribution of loaded synovial joints

In the model intended for short-term loading (such as during the walking cycle) of a human synovial joint in the lower extremities, cartilage lubricated by Newtonian synovial fluid is considered to be incompressible elastic and subchondral bone is considered to be rigid. The model is non-diffusional, i.e. no interstitial fluid flow occurs across the articular surfaces. A simple plane strain case of the human ankle joint is considered. For high steady loading applied in the centre of the stationary tibial arc and for steady sliding of the talar arc, this model shows that individual physiological variations in the geometry of the articular surfaces have only a small effect on the contact stress distribution and the fluid film thickness. If this load is applied eccentrically in the tibial arc, the contact pressure distribution varies more with surface geometry, but the minimum fluid film thickness differs little from that for symmetric loading. The maximum contact pressure is placed eccentrically in this case, but its value is changed only little when compared to the central loading of the same value. In order to explain different distribution patterns of subchondral bone mineralization, it is anticipated that the total load peaks of periodic time-dependent loads are transmitted centrally in some incongruent joints and eccentrically in others.