It's a condition commonly found in large breeds like GSDs. Unfortunately, Trixie has MND. It's been apparent for about 6 months (starting June 2017), starting with her scraping her 'passenger-side' back paw. The vets were unsure whether it was arthritis or MND, but as the condition has progressed, they are certain it is MND.
MND is an incurable progressive paralysis disease. It usually starts at the base of the spine and works its way forwards. It can lead to total paralysis, whereby both the hind and front legs cease working. Usually, euthanasia is chosen before complete paralysis ensues.
Since Trixie has this condition, I want to share with you the measures I have taken to keep her active.

The first sign of MND was Trixie scraping her back paw, to the extent that it wore one of the claws down to the quick, which causes it to bleed. At the time, we thought it was probably arthritis. Anyway, to protect the claw, I bought Ruffwear socks and boots for her to wear on her walks. I have tried several different brands of boots, but I have found that these are by far the best and most comfortable for her. They're expensive, but you pay for quality and fit. See www.ruffwear.co.uk/dog-gear/dog-boots/grip-trex-ss16. The socks are important because they help to keep boots on and also improve comfort. See www.ruffwear.co.uk/dog-gear/dog-boots/bootliners
Initially I just had her wearing one boot, on the poorly paw. However, after experimenting, I found she walked more balanced with socks & boots on both back paws, although her 'driver-side' leg/paw is currently unaffected by MND.

The boots protect the claws from the scraping, but of course it doesn't stop the fact that the paw will scrape. This causes a lot of wear and tear to the boot, which I discovered as the condition worsened. It got to the point where one new boot would only last 4 days, because the ground scraping wore the top of the boot out and then the underside. Rather than but new boots every 4 days, which would work out very expensive, I looked around the internet for something that would enable me to repair her boots cost-effectively.