Upward fixation of the patella: the stifle is locked, or rather the stay apparatus which allows the horse to sleep standing up is locked. Chronic is short, straight legs. This is a problem because when a horse can't move his leg he will struggle and may hurt himself very badly trying to move the effected leg(s). Surgery can be performed, cutting the ligament which lets the horse lock his hind legs - the horse will take a while to figure out that he must lay down to sleep. This is often done to race horses because they tend to have short, straight legs.
Bog spavin: soft swelling on the inside or front of the hock. Caused by conformation problems such as cowhocks as well as activities in which the horse has to sit back on his hocks (cutting, sliding stops, jumping).
Thoroughpin: Soft swelling of the tendon sheath of the deep flexor tendon just above point of hock. A blemish indicating stress. It can be pushed from one side of the leg to the other.
Bone spavin: the bones of the hock fuse together. Not a problem if the talus and the calcaneous retain their ability to move. Horse might not be able to perform at extremely high levels of work. Sometimes the hock bones of racehorses are fused on purpose early on so it won't prevent them from racing later.
Capped hock: swelling of the bursa on point of hock caused by trauma such as kicking at a wall.
Wind puffs: swelling of the fetlock joint capsule or deep flexor tendon. Called articular windpuff when the joint capsule is effected (also more common in front legs) and tendonous windpuffs when the tendon in involved (more common in hind legs). In the first case the swelling will be on the side of the fetlock and it is brought on by arthritic changes bone chips, steep pasterns, and certain types of exercise. The second type the swelling is on the back of the fetlock and is brought on by certain types of exercise (extreme flexion of fetlocks as in cutting horses). If it only happens a few times the fetlock distension can disappear permanently but if it happens chronically the joint capsule can be permanently stretched. Windpuffs is simply an indication of prior trauma and does not cause lameness.
Osselets: a hard swelling down the whole pastern. The hard swelling is actually a form of remodeling - because of direct trauma irritating the periostium or chronic overstretching of the suspensory and/or deep flexor tendon the body decides than more bone needs to be laid down in order to strengthen the pastern. A blemish only if the new bone does not interfere with the joint. If the bone grows in the joint it is a lameness.
Ringbone: bone changes in the pastern joints or around the pastern bones. When in pastern joint it is called high ringbone, when in the coffin joint it is called low ringbone. When it does not interfere with a joint it is called false ringbone and it is only a blemish. Ringbone is caused by increased stress on the bones - bad conformation, short, steep pasterns, or even age. With high or low ringbone the horse cannot return to full work load.