More Osteons

Osteons appear to have a set size. Instead of growing larger in response to exercise stress, they maintain their pattern and space. How do we know? There are some limiting factors on osteon growth.

Notice in the photo the outer "cement" which certainly would seem a limiting factor on expansion. Note also the crucial osteon structure named "lamella"which are the concentric circles of bone collagen that layer one on top of the other from the middle outward to form the main structure and body of the osteon. We know in humans that depending on the bone being discussed that there are a certain specific set number of these lamella circles ranging from four to twenty.

Does a process perhaps occur, where in response to stress, an extra lamella layer or two might develop on the outer portion just under the cement? Simple logic indicates that such extra layering on the outer portion of the osteons would immediately bump into neighboring osteons and structures. Conclude that osteons then have a set predetermined size that varies very little regardless of exercise.

The next question: does exercise stress increase the number of osteons and therefore increase bone thickness or size by sheer numbers? Again, I believe the answer to be in the negative.

We know from Nunamaker's shin study that exercise will cause new bone to be laid under the periosteum in the shin area. We may thus surmise that there will be some increase in the number of osteons outward, and I'd further suspect in the race horse there will be substantial osteonal thickening inward as soft spongy trabecular bone near the medullar (marrow) cavity is replaced by hard cortical bone in response to stress.

But, this sort of growth would appear to have very strict logical limits--i.e. we get a small amount of bone growth only to a certain point. When that point is reached we are without any further growth regardless of amount of stress (aside: I'd be interested to feel the cannons of one of those 100 mile Arabian racers.)

What are the limits on osteon numbers? I'd suspect there is a close relationship between the genetic pattern by which bone generates itself and the form, function and construction of the overall skeleton. Cannon bones e.g. must have a certain size including thickness in order to properly adjoin with the knee carpal bones above them and the ankle-fetlock joint below. In this sense bone size is related to and controlled by function. And thus, the explanation why the cannons of my much worked 14 year old, though they feel a little stouter than my three year olds, feel to the touch comparatively the same size.

Since osteons appear to grow only marginally in size and number, is there anything about our exercise program which does affect the osteons, next post.

Training:
Mon. 8/10: Off
Tues. 8/11:
Art: flunks his second trot test showing as much lameness as a week ago. We're now in the 9th day. He keeps re injuring by romping and playing. This is where a stall at the farm would be helpful. Probably looking at another week here. Needless to say, a big interruption.

Rod: After Monday's .75 inch rain it's too wet to gallop. We decide on riderless play in the paddock--about 3 miles mostly in 3f heats in :14s with some faster spurting. About a 3/4 fast workout to permit galloping to continue tomorrow.