Science Literature

A notable feature of life in the Western World is the large number of people suffering from osteoarthritic pains and bad backs. It is not unusual to find people complaining about "bad design" and suggesting that walking upright is unnatural. People with attitudes like this would do well to read Jacob Klein's Perspective piece in Science. Here's a taster:

"The ultralow friction coefficients between the articulating cartilage surfaces in human hips or knees cannot be duplicated even by the most sophisticated technological means."

Osteoarthritis is a chronic disease of the joint cartilage and bone (source here)

The essential starting point for any analysis of the degeneration of hip or knee joints is an appreciation of the very efficient lubrication system in place. These joints have to withstand pressures of about 5 MPa (equivalent to about 50 times atmospheric pressure) and friction coefficients associated with the joints are about 0.001. These are very low values and are vital for avoiding wear and tear in use. Significantly, friction coefficients for man-made lubricants are far higher than the 0.001 typical of human joints.

The secrets of cartilage lubrication need to be probed at the nano-level of detail. Brush-like molecules and water combine to produce lubricating surface layers.

"Recent efforts to elucidate these molecular origins have focused on nanotribological studies of surface-attached molecules in aqueous media, seeking to emulate those at the cartilage surface. Raviv et al. showed that synthetic polyelectrolyte brushes attached to opposing surfaces can provide remarkable lubrication when mutually compressed to moderate pressures and made to slide past each other, with friction coefficients similar to those in healthy joints. Briscoe et al. found that boundary lubrication under water was far superior to that in air or oil, and mediated by the hydrated surfactant head-groups. These and other studies emphasize the importance of hydration layers surrounding charges in aqueous media as a basic lubrication element."

This lubrication mechanism demonstrates excellent results at low pressures, but it fails to mimic the natural system at pressures of 5 MPa. Clearly, more biomimetic work is needed to reach a better understanding of "the very efficient lubrication in living joints". This opens the door for a variety of repair and replacement measures to treat patients:

"Future materials challenges will be to design scaffolds that provide optimal environments for the progenitor cells that they bring to the damaged tissue, or to stimulate indigenous cells; and to develop bioadhesives that promote tissue integration and prevent scaffold detachment during joint articulation. In replacement strategies, surface treatments that suppress the wear leading to implant failure may allow a closer approach to "lifetime" prostheses for the most widely used polymer-metal implant combinations."

Approaching the problem with the premise of design functionality leads to the identification of numerous avenues worth exploring further. What factors have led to degeneration and how can they be addressed? What can be done to promote self-healing? (Regeneration and repair) What can be synthesised as a substitute for permanently damaged joints? (Replacement) All these leads follow from a design perspective. But what shall we say to those who persist with "bad design" arguments and evolutionary arm-waving? Do these particular emphases result in novel insights? As far as I can see, the 'evolutionary tinkering' perspective plays no part in the contemporary search for solutions.

Repair or Replacement - A Joint Perspective
Jacob Klein
Science, 323, 2 January 2009: 47-48.

Summary: Insights into the molecular origins of cartilage lubrication could lead to more efficient tissue repair and to longer-lasting prostheses for hip and knee joints.

See also:

Tyler, D. The remarkable phenomenon of biological lubrication, ARN literature blog (11 February 2008)