The most common site of primary osteoarthrosis is -
In general, weight bearing lower limb joints are affected more commonly than upper limb joints.
However, No textbook has mentioned which is the commonest joint : knee or hip.
According to me it is knee because of following facts :-


I have seen hardly one or two cases of primary OA of hip; while, I have seen hundreds of cases of Primary OA of knee in my OPD. At least 4 to 5 patients of primary OA of knee comes daily to my OPD.
"OA commonly affects the knee joint"   —Ebnezar
"OA of hip is second in frequency to the knee joint" —Ebnezar