In response to two enquiries: “(1) unable to stand up on knee” and “(2) healing dry skin on knees that is white/grey.”
(1) If we focus only on the knee and possible causes directly related to the knee or originating directly from the knee, there would most likely be (1-a) severe arthritis in the knee; (1-b) degeneration of the cartilage in the knee; (1-c) shattered knee joint; (1-d) disjoint of the knee; (1-e) dislocation of the joint; (1-f) luxation of the knee.
(2) What is the cause of the dry white and grey skin on the knees? The condition may be from (2-a) dead tinea corporis; (2-b) Does the skin at that location itch? If so, it may be due to localized psoriasis due to stress; and (2-c) Are there any other patches of dry and grey skin covering other parts of the body like the elbows? The elbows are usually covered with more dead skin, shows a darker color than the surrounding skin of the arms, and the elbow skin feels rougher to the touch.