I am sorry to hear that your Staffy is having trouble with her knees. Five years of age is very young to have arthritis, is there any underlying mechanical problem?
Your little girl needs a visit to the vets. After a thorough examination and possible X-rays they will be able to ascertain the cause of any knee pain and provide the best recommendation on surgical or medical options to treat the problem.
There are many products on the market now to treat arthritis in dogs prior to or in conjunction with anti-inflammatory tablets. These include pentosan polysulphate injections, glucosamine and chondroitin supplements and even mesenchymal stem cells. Your vet will be able to advise you on their application or relevance to your girl’s knee disease.Best of luck.