Cats with unstable hind legs and arched back may be due to rickets, which is generally seen in cats with folded ears. rickets usually develop when they are about 2 to 6 months old, and may not develop until a year later. Cats with early onset of the disease There will be inactivity, abnormal movements, unsteady walking, arched back, etc., which will make the cat very painful. Therefore, if the cat has rickets, it is best to give the cat chondroitin from an early age.