Several months ago I was asked to look at a two year old gelding who was wearing a big ugly bump on his skull. He had banged his forehead full force, just above the eyes. The injury was the result of rearing. With the exception of the Spanish riding school and circuses, rearing is not a good thing. It can get a horse in trouble, especially when there is something hard overhead. This youngster had gone up while being wormed. If it hadn’t been for the overhead beam, he might have gone over. The owner said that his forehead from poll to eyes was very swollen and sore for several days. I saw him a few weeks after the collision. The swelling and pain were gone, but now the gelding was wearing that hard boney knot on his forehead. It stuck up nearly an inch. His owner was concerned that the bump might never go away.
Everyone trained in the medical professions learns about Wolff’s law. This law says that bone is deposited and reabsorbed according to the stresses placed upon it. We tend to think of bones as being hard, dry, and not as alive as other tissues. The fact is that bones are very active, have an excellent blood supply, and are constantly undergoing changes. When a bone is stressed by one big insult or many small repeated ones, the bone changes its structure.
This gelding had cracked his head as hard as if had been hit with a heavy hammer. The skin and connective tissue over the skull responded with the swelling that we call edema. Within a few hours the injured bone also responded. Cells in the bones called osteoblasts started to lay down new bone and when I saw him, he was wearing that big mass of new bone. So the initial “stress”, the bang on the head, caused the new bone to be laid down in response.
As things calmed down the bump began to remodel again, this time by the action of another type of specialized bone cell, called an osteoclast. These cells are active in the process of getting the bone back to its normal size. There is another law, which I don’t think has a name, that says, “use it or lose it.” If you’ve ever had a leg or an arm in a cast, you know that when a muscle isn’t being used it atrophies (shrinks). Soon after the cast is off and the muscles are back in use, they regain their size and strength. The same process happens with the underlying bone. It’s the bone obeying Wolff’s law.
An example of this in people is found in the sport of tennis. Compare the two arms of an active pro. You would expect the “active” arm to have bigger and stronger muscles, but the bone itself on that side, because of the constant use, will be denser and noticeably larger.
We see Wolff’s law in action with “splints” in youngsters in early training. The ligaments that connect the cannon bone to the splint bone are inflamed and are partially pulled off the bone. The bone reacts by becoming larger and thicker, which is what we see as a splint high under the inside of the knee. Ease off the training and the splint (boney reaction) gets smaller. I’m not sure that Wolff’s law works 100% because with severe skull trauma or big splints the increased bone never seems to go totally away. The gelding that cracked his head will probably have slight bump years from now.
So, if your horse gets a bone injury and subsequent bump, you can expect eventual healing and the remodeling that comes with time.
– David A.Jefferson, D.V.M.