Hello! Our dog had TPLO surgery when she was 8 years old. Her ACL was completely severed and the surgery involved fusing the knee joint for strength and stability. She made a remarkable recovery, but within a few months she had the same trouble with a sit position that your dog is experiencing. It was just simple arthritis that was progressing really quickly. We put her on DeraMaxx and while she still had a bit of trouble with a sit, she was back to her old rambunctious self until she died at almost 14.
Any orthopedic injury or surgery is going to result in a certain amount of arthritis. That's a given. You have to treat the inflammation to slow down its progression and help with the pain. In addition to anti-inflammatories like DeraMaxx, MSM, chondtroitin and glucosamine are excellent supplements to keep on board.
Swimming is probably the best exercise for a dog with an ortho injury. They can build up the muscles without as much pain because the water takes off so much gravity resistance. About 15 to 20 minutes a day is plenty.
Welcome to the dog forum, we have wonderful people here to assist w/their many experiences and share with you, so check in from time to time and see you new comments...
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I personally do not have answer for your question, but wish you the very best..
My tip for scar tissue and joint and arthritis is: Surejell that you use in making jellies/jams is very good for humans and dogs...small dose daily, takes a month to see a difference, a human a tablespoon in glass of orange juice daily, a dog depends on size and how to you choose to administer in food to diguise it...