Evidence is presented that basophilic radial hairlines in the calcified layer of articular cartilage are in vivo microcracks. They were present in three of 25 normal patellas from subjects below the age of 50 years, in 20 of 25 recently fractured hips (average patient age, 77.3 years), and in 16 of 25 osteoarthritic tibial plateaus (71.7 years). It is hypothesized that extension of these microfractures beyond the calcified layer mediates remodeling of the osteochondral junction in aging and degenerative joint disease.