The anatomy of the hind limb stay apparatus allows a horse to stand and rest with no muscular effort. Part of this stay apparatus relates to the patella’s (knee cap) ability to hang on a ledge of bone on the lower end of the upper limb bone (the femur).
Unfortunately, this apparatus can malfunction and the patella can remain caught in this location. When this happens, the entire limb cannot “fold” in a normal way. The limb is stuck out rigid behind the horse, they are reluctant to walk or cannot walk. When this occurs, often a horse’s fetlock will fold and knuckle over.
The patella is the front of the knee in the horse. Pressure from the flat of the hand inward will usually release it from the medial trochlear ridge, where it is hung up. You need to use firm strong pressure directed from outside to inside.
For the horse's left stifle, stand at the left side of the flank. Make contact with the horse and place the heel of your hand against the patella. Allow them to adjust to the feel of your hand and then push hard inward on the patella. At the same time as you press, have your handler walk your horse backwards a few steps. You should hear a "pop" and the limb will release.
Tips for safety & Success
You should not have to repeatedly perform this skill on your horse because it is not a good long-term solution. Talk to your vet about other treatment options.