And a fast answer!
Those swellings are hygromas (fluid filled sac around the joint). In Kruger they are generally associated with TB in lion (and often found on older lion correlating with their TB incidence) but in areas where there is no TB they are common in young lions. To my knowledge the exact causative agent has not yet been isolated but they often regress as the lion gets older. In these cases the swellings do not seem to be painful all.
A hygroma is a false bursa that occurs over bony prominences and pressure points, especially in large breeds of dogs. Repeated trauma from lying on hard surfaces produces an inflammatory response, which results in a dense-walled, fluid-filled cavity. A soft, fluctuant, painless swelling develops over pressure points, especially the olecranon. If long-standing, severe inflammation may develop, and ulceration, infection, and fistulas may be present. The bursa contains a clear, yellow to red fluid.
In layman's terms it is a swelling that often develops when a joint, usually the elbow, keeps getting smacked on a hard surface. Think of it this way. If you walk through your living room and "whomp" (really hit) your shin on your coffee table as many times a day as your dog goes down on its elbows, your shin is going to swell up. If you do it long enough your body will start to produce a more permeant protective solution then the skin swelling called a bursa (fluid filled sack). Once you stop hitting your shin on the coffee table and the shin bone begins to heal the swelling will begin to go down. The same is true with your dog's elbow, once they stop traumatizing their elbows, the joint will begin to heal, as it heals, the body will absorb the fluid the joint has produced (hygroma) as a response to the repeated trauma.
So we need to put some soft beds in Kruger...
: The photos from our trip! Overhere! Feel free to use any of these additional letters to correct the spelling of words found in the above post: a-e-t-n-d-i-o-s-m-l-u-y-h-c