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Fluid of Articular Cartilage

As noted earlier in the air-tent analogy, the inflation medium of articular cartilage is synovial fluid, which essentially is an ultrafiltrate of plasma plus hyaluronan. The hyaluronan molecules are too large to enter cartilage through its surface pores (diameter, 6 nm), but most of the remaining ions and molecules of normal synovial fluid, such as water, sodium, potassium, and glucose, are sufficiently small to pass easily through these pores.

Movement of fluid into and out of cartilage occurs, to some extent, by diffusion, but diffusion does not seem adequate in and of itself to provide for cartilage health. The percentage of water in cartilage ranges from more than 60% to nearly 80%. The water is bound by a variety of weak forces, such as hydrogen bonding to proteoglycan and collagen or simple hydration shell formation, but is relatively mobile.

Net flow into and out of cartilage is induced by the normal weight-bearing function of synovial joints. Maroudas et al. It has calculated that for normal articular cartilage, the sum of swelling pressures is greatly exceeded (10-fold) by loading conditions, such as walking.

The implications would seem to be that under loading conditions, cartilage would be compressed rapidly and completely, much as a wet sponge is compressed by weight. The rate of fluid movement permitted by the small pore size and the cartilage microarchitecture is sufficiently slow, however, that cartilage is only partially compressed even after loading for hours.

Those investigators used an apparatus designed to fit into a centrifuge capable of forcing fluid out of cartilage and into a receptacle. The cartilage fits into a porous, basketlike container into which a plunger rests. The unit is placed into a centrifuge, and the faster the centrifuge revolves, the greater the pressure on the cartilage in the basket.

In this way, the effect of varying loads over varying periods of time on the rate of fluid expression from cartilage can be evaluated. These experiments showed that the amount of fluid that can be expressed (~30%) is extremely small in relation to the total water conten.

Subsequent experiments using an animal joint demonstrated the processes of fluid movement in cartilage more directly; the device constructed for that experiment was termed an arthrotripsometer. By developing the necessary design criteria, it was possible to vary loading conditions with respect to amplitude of load and to stationary versus cyclic conditions.

The joint was immersed in synovial fluid during testing, and deformation versus time was seen to be greater for stationary than for cyclic loads. The explanation for this observation is that in the cyclic condition, partial recovery occurs because of the effect of swelling pressure in pulling fluid back into cartilage during the phase of the cycle when the cartilage is unloaded.

The effectiveness of this unparalleled system of load bearing by articular cartilage depends on functional integrity and detailed interaction of each architectural, biomechanical, and biochemical element within the system. Details of viscoelastic properties of articular cartilage, including the Poisson ratio and compressive modulus, have been concisely summarized in recent reviews.

The fluid movement that occurs during the loading process as described appears to be important for lubrication of the joint surfaces as well as for load carriage. On the basis of calculations obtained from complex mathematical models, Mow et al.

They have proposed that fluid is expressed out of cartilage in front of the advancing contact surfaces of cartilage. This process provides a fluid film that minimizes cartilage–cartilage contact, thus also minimizing wear. Indeed, if this analysis is correct, then in a sense, we walk on water.

Issues of cartilage nutrition and metabolism are beyond the scope of this chapter. However, it its notable that the cellular population of articular cartilage is sparse and that the metabolic domain of a single chondrocyte is huge. Wong and Hunziker  and Hunziker et al.

They have calculated the volume of matrix that must be maintained by a single chondrocyte to be 180,000 µm. Given the turnover of matrix components at a level not dissimilar to that of other connective tissues, the miracle of articular cartilage continues to amaze all students of the subject.

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