Stifle joints are injected the same way hock joints are. It’s quite common. My gelding had his stifles injected Monday and my instructor just had her horse’s stifle injected too. Digital x-rays can show a lot. My guy has OCD of the R stifle but they injected both sides to help minimize any issues.
I’d start with a course of Adequan and a good lameness eval, and move up to joint injections if warranted and if Adequan fails to help.