Cartilage risk
I enjoyed Nathan Seppa’s article “Cartilage creation,” (SN: 8/11/12, p. 22) about attempts to generate new cartilage from somatic stem cells. He writes that cartilage evolved “in ancestors who lived shorter lives, carried less body weight and roamed an unpaved world.” Implications: The risk of osteoarthritis increases with age, body weight and impact on concrete, such as a long-term runner might experience. Is there solid scientific experience for all three of these putative risk factors?
William Check, San Francisco, Calif.

Yes. First, osteoarthritis stemming from cartilage wear and tear is most common in the elderly. Second, studies show that extra pounds add to stress on weight-bearing joints, particularly the knees. Third, it’s certain that modern humans spend a lot of time in a paved world. Joint compression is natural and necessary for nutrient transfer in cartilage tissue, which isn’t supplied by blood or driven by circulatory pressure. But too much compression — through abrupt injury or long-term pounding — creates stress, inflammation and other damage. MIT’s Alan Grodzinsky puts it this way: “With compression above a certain rate and amplitude, you cross a threshold into microdamage” of cartilage tissue and the cells that make it. — Nathan Seppa

On Turing’s life and death
Out of the whole of Tom Siegfried’s well-written article on Alan Turing “A mind from math,” (SN: 6/30/12, p. 26), I especially want to commend Siegfried both for his honesty and for the language he chose to speak of Turing’s death. It is so important, even in a science magazine in a math-history article, not to shy away from social truth and to name what is evil as evil.
David H. Kehret, via e-mail