The head has retained its normal shape but the cartilage is irregularly and unevenly thinned almost,
in some places, exposing the subchondral bone giving a brownish colour. At the periphery there is
osteophytic lipping where there are irregular bony outgrowths covered with cartilage. The cut
surface shows a thickwalled multilocular cyst; the cysts are lined by fibrous tissue and are said to
communicate with the joint surface. They may be present before other manifestations of
osteoarthritis; their genesis is not known.