Yesterday morning while doing chores, I thought I caught a glimpse of Larry moving oddly in his back end several times. But when I'd stop and watch him or ask him to move intentionally, everything appeared normal.
Later I had a chance to ride so I went to his stall to tack him up. Again, I thought I caught glimpses of something, but it wasn't until I picked out his hind feet that the problem walked up and shook my hand - his left patella locked! Fortunately, Rick came home for lunch so I had him come down to evaluate Larry. Nothing showed up at first - even when Larry ripped the lead rope out of my (ungloved; ahem) hands and took off running and bucking around the arena - until Rick picked up Larry's right hind foot. We both saw him lock his left hind then, and it stayed locked when Rick tried to pick it up.
The experts in ISELP (The International Society of Equine Locomotor Pathology, of which Rick is a member) believe a locking patella is caused by either sore muscles and/or lack of condition, which can be exacerbated by conformation (straight stifles). The recommendations are slow and careful conditioning/strengthening of the hindquarters (which I feel I have been doing) and NSAIDS. We gave Larry a loading dose of Bute and I lunged him - during which he took off and bucked repeatedly like a first-string rodeo bronc to the point of lathering his neck and chest (but only when circling right).
All this is definitely a huge new wrinkle in things. I'm in wait-and-see mode about our show entry on April 29. Meanwhile, I'm headed out to saddle up Larry for a long walk in long and low frame....