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Cats with arthritic joints often have difficulty jumping or even moving around so make sure that the litter tray is close to where your cat likes to spend their time and that it has low sides. It's also helpful to make sure there are sleeping places, food and water bowls on the ground level. If your cat likes to be up high (to get away from a playful dog for example) then consider providing a ramp up to their favourite spot.
If your cat is not as active these days then adjust the diet accordingly to ensure excess weight is not being carried on those little joints. Your vet can help advise you about the ideal weight for your cat, and what food might be the best for them.
Joint supplements containing neutraceuticals like glucosamine and chondroitin can be very helpful and for some cats alternative therapies such as acupuncture and even massage can ease the discomfort
If you find that your kitty's mobility worsens with age then talk to your vet as a course of medication may be worthwhile. These medications aim to reduce both swelling and pain and can significantly improve the quality of life of a cat suffering from arthritis.
-Dr. Alex Hynes