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Articular Cartilage

Key Points

  • Few mechanical devices even remotely approach the durability and efficiency of cartilage.
  • The typical response to cartilage injury in which the subchondral plate is fractured is the formation of fibrocartilage, a scarlike tissue unsuited to the support of compressive loads and shear forces.
  • The pattern of collagen fibrils within articular cartilage is well suited to the functional requirements of the tissue.
  • Loss of the densely packed collagen mat at the surface of cartilage in weight-bearing regions is the prelude to fibrillation, accelerated wear, and ensuing degenerative arthritis.
  • Collagen is the key protein in musculoskeletal stability. It provides the mechanical properties to connective tissue, and it constitutes 65% to 80% of the mass by dry weight of such connective tissues as tendons, ligaments, skin, joint capsules, and cartilage.
  • Attempts to achieve cartilage repair, as in surgical arthroplasty, do not successfully regenerate cartilage and seldom produce completely satisfactory clinical results. The collagen fiber architecture of the arthroplasty repair tissue is disordered throughout the deep layers and lacks the membranelike characteristics so important to the surface layer of articular cartilage. These are major factors contributing to failure of cartilage regeneration.

Cartilage is a unique tissue that unless injured, provides virtually frictionless mechanical motion throughout the latter decades of life. Few mechanical devices even remotely approach the durability and efficiency of cartilage.

The purpose of this chapter is to provide a brief account of the structure, composition, and mechanical properties of articular cartilage. Such an account is basic to understanding both the function of cartilage and the therapeutic goals of cartilage restoration by any of the present (and future) attempts at surgically induced regeneration.

The typical response to cartilage injury in which the subchondral plate is fractured is the formation of fibrocartilage, a scarlike tissue unsuited to the support of compressive loads and shear forces. If the subchondral plate is not fractured, attempts at healing rely on articular cartilage cells, the response of which to injury is consistently and completely ineffectual.

Given the elegant precision of the morphological and compositional interdependence of articular cartilage, it is not surprising that attempts at effective regeneration of articular cartilage have frustrated clinicians and basic scientists alike. Mature mammalian cartilage cells, which constitute only 2% of the total volume of the tissue, have lost the ability to dedifferentiate.

This property of cartilage cells is shared with the universe of other mammalian cells resulting from evolutionary progress. This circumstance likely is a controlling factor in the limitation of the biological response of articular cartilage to injury.

As mentioned, articular cartilage is a unique tissue in many respects, but especially with regard to its structural, metabolic, and functional interactions. Articular cartilage possesses unparalleled biomechanical functional efficiency, and this efficiency is derived from design features that are marveled at by physicians and engineers attempting to design artificial substitutes for diseased joints.

For example, the articular cartilage lubrication efficiency is an order of magnitude superior to the best bearing surfaces known to modern engineering. Such efficiencies are achieved in spite of stringent limitations imposed on the tissue, such as the lack of blood supply and a tissue thickness that measures a few millimeters at most.

Couple these points with a limited repair capability and the consequent requirement that the tissue survive a lifetime of use and then the question becomes, “How can synovial joints survive as long as they do?”

The thrust of this chapter will be to describe the morphological, biochemical, and physiological interactions of the cartilage matrix to provide insight regarding the basis for successful long-term survival of cartilage and the requirements for its successful repair or regeneration.

A useful phenomenological concept that is helpful in understanding the function is air tent, which is a structure used as a cover for recreational areas, such as swimming pools and tennis courts, or as a temporary cover for exhibitions. The functional requirements for the air tent are an inflation pump or fan, an intake tube for the inflation medium, (c) the inflation medium (air), and the fabric required to contain the pressurization and to provide the cover.

The pump must be working constantly to maintain expansion of the system because of inevitable leaks through the fabric. In the case of cartilage, the surface membrane (i.e., the fabric) consists of the fine collagen fibril network concentrated at the articular surface.

The inflation pump of cartilage is the proteoglycan molecular structure, and the inflation medium is an ultrafiltrate of synovial fluid. Cartilage, of course, has no single intake vent for the inflation medium to enter. Rather, fluid inflating the tissue enters through a myriad of microscopic pores at the surface; these are the same pores from which the fluid exits when compressed.

These elements are interrelated, and a deficiency in any of them will result in failure of the system. In the case of the air tent, a tear in the fabric for which the pump is not able to compensate will result in collapse of the tent. Or, if the pump fails, the tent will gradually collapse as pressurized air leaks through the pores of the fabric.

More specifically, the fabriclike structure at the cartilage surface, consisting of fine collagen fibrils packed tightly in a matted pattern parallel to the surface, is much different from that seen in the deeper layers, where fibers become thicker, their orientation becomes more vertical, and the spaces between the fibers increase.

The surface “fabric” of cartilage has tiny pores that permit fluid and small molecules access to and egress from the tissue but that block the movement of large molecules. The inflation medium in articular cartilage is, of course, fluid rather than air. The cartilage fluid is in equilibrium with the synovial fluid, which is essentially an ultrafiltrate of plasma.

The fluid in articular cartilage is significantly pressurized. Calculations by Ogston led him to conclude that articular cartilage is inflated to the equivalent of “motor tire pressure.” The pump for this pressurized system is not intuitively obvious, but its presence has been established without doubt by modem techniques of rheology and biophysics.

The pump for the articular cartilage system is chiefly aggrecan, a proteoglycan molecule that becomes linked to hyaluronan to form a huge molecule designated as a proteoglycan aggregate. This huge macromolecule is locked within the articular cartilage fibrillar matrix by its large size and volume.

In its state of equilibrium, the expansion pressure in the articular cartilage system is in balance with the resisting tension of the collagen fibers; however, the balance can be upset by an externally applied load. If the external pressure exceeds the internal pressure, fluid will flow outward until a new equilibrium is reached.

As the proteoglycan molecules become compressed, their charges become more concentrated, and this causes the fluid pressure within the cartilage to be increased until a new equilibrium is reached. The theoretical analysis of the fluid flow patterns and viscoelastic properties under various loading conditions has been studied extensively. This fluid movement is of great interest, because it explains the mechanism of several fundamental properties of the articular cartilage system, including lubrication, load bearing, and nutrition.

As indicated above, the proper evaluation of attempts at repair requires fundamental knowledge of articular cartilage form, composition, and biomechanical characteristics. The collagen matrix of normal articular cartilage, its proteoglycan and proteoglycan aggregate, and the movement of fluid within cartilage are described in greater detail in the following sections with respect to its morphologic, biochemical, and functional features.

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