Of course, having recently completed a week of orthopedics, and already a bit in awe of bone and its awesome-ness (see my my "bone is cool" post), I may have gotten a bit excited about the subject and probably went way beyond what 99% of my readers really want to know about the subject.
So here's a warning - this post is for the geeks that want to know the why behind the obvious. I'm going to save the rest of you some time. The obvious is this: if you overload bone it breaks.
Feel free to skim this and pick and chose what stuff you want to understand and what stuff you just want to plunk a few "gee whiz" facts out of. :) My hope is that there is something interesting for everyone here, not that you are able to pass a nit picky multi-guess test on the subject........
****Necessary disclaimer for any nosy vet school administrators that come across this post....all pictures and text are either generated by me, or were honestly stolen off of google, which BTW seems to be where all the professors get their pics for lectures anyways. I haven't deliberately lifted any images or pictures from the vet school that didn't show up on the first couple pages of a google search.
Shall we begin?
First off, bone responds to stress and loading forces very similarly to any other solid object in this world (except it can adapt and get stronger in addition to just saying *bad word* and breaking). In explaining how and why a bone breaks, first we need know how a bone responds to loads.
Wolf's Law: Basically says that if loading on the bone increases, the bone will remodel itself and in order to resist the forces acting on it. ie, bone will become thicker and "stronger" when you increase the demand on it, like during weight bearing exercise. The converse is true too - if loading of the bone decreases, the bone remodels itself and less bone is present and the bone is "weaker" because less force is acting on it.
Wolf's law is the whole premise behind "conditioning" bone. Conditioning for endurance rides is all about not creating too many stress fractures (that can weaken the overall bone and eventually cause a "pathologic fracture", we will talk more about stress fractures later in the both). Eventually the little bone eaters (osteoclasts) will come along and remove the damaged bone that has cracks in it, and the the little bone makers (osteoblasts) come along and deposit more bone in its place. Voila! Bone has adapted and become stronger.
Once you've reached a certain level of conditioning, it's all about keeping a certain activity level on the bone so that the body doesn't think that all that extra bone isn't needed and start letting the bone eaters eat more bone than the bone makers are depositing.
I went into quite a bit of detail of how bone conditions and repairs itself in my "Bone is Cool" post so please refer to that if you want more information (and ask questions if you have any!!!). So...we are going to move onto the actual bio mechanics of bone. Let your inner engineer rejoice!
More ways to visualize/understand how bone biomechanics:
Let's say you take a bone (a long bone for the purposes of our visualization). Now you add a "load" that bends that bone........At a certain amount of load, if you remove the load, the bone will return back to it's original shape right? It is at this load/energy that the bone is elastic.
An example of a type of load that a long bone might be "elastic" is walking around under normal body weight. As I take a step, 130 pounds of load is applied to my leg bone and the leg bone flexes just a little. When I pick up my foot, the bone returns to it's original shape.
If you increase the load enough, the bone still bends....but doesn't quite return to it's original shape when the load is removed - there is micro damage!!!!! The bone didn't break or "fail" but it suffered some damage as a result of the load. This is called the plastic region.
The point between the elastic and the plastic region is called the "yield point". The point beyond the plastic region (where the bone still stays whole, but has some micro damage) is where the bone breaks is the "failure point".
In summary: at lighter loads the bone will change shape, but be able to return to original shape when the load is removed. At heavier loads the bone will change shape and while it won't break, can't fully return to the original shape. At a certain point, the bone cannot handle the load and it will break or fail. The "loads/weight" can also be thought of as "energy" being applied to the bone. Lower energies = elastic, higher energies = plastic. At a high enough energy applied to the bone, the bone breaks.
The big picture - let's put together the pieces and add a couple more!:
When a force is applied to a bone, how it reacts is based on the load (we've already talked about load and how it relates to elastic and plastic and failure points) AND based on the geometry of the bone's cross section.
I'm going to start with the summary point because I think you will be able to follow the "why" better if you have the big picture first.
1. Force/energy/work is applied to the bone.
2. It bent or otherwise displaced and changed shape.
3. And then, because the force wasn't too great, the bone returned to it's original shape, either with or without stress fractures.
4. Because of Wolf's Law, bone was deposited in a way to make the bone stronger and resist the load.
5. The next time the same load was applied, the bone didn't bend or displace/change shape as much!
6. Over time, the bone remodels to resist the load (ie - bending or torsion forces) and more and more energy is required to change to cause displacement/change of shape when applying a load to a bone.
Of course, this assumes you don't overload and break the bone (or cause so many stress cracks that you end up with a pathologic fracture - we will discuss later.....)
Take a look at the picture above. Ignore the captions and just focus on the fact that these are all cross sections of femur's (femurs are the big "thigh" bones"). Do you notice the different shapes of the cross sections? the cross section is different because in each case different forces (or lack of forces) are acting on the bone and the bone is responding according to Wolf's law.
How a bone bends under a load, and how well it resists torsion is related to it's cross sectional area and how the bone is distributed around it's neutral axis.
*"bend" and "torsion" are types of forces that occur when a load is placed on a bone
*for the real geeks out there, I'm now going to talk about polar and and area moment of inertia without actually mentioning those terms.......
Here are 2 important concepts:
1. The further the material is from the neutral axis, the better the bone is at resisting bending and torsion.
2. The larger the bone's x-section is, the better the bone is at resisting bending and torsion.
Our bones aren't perfectly round. How did the bone get distributed around the neutral axis assymmetrically? By Wolf's Law! Because the areas of the bone that experienced stressed got stronger and thicker and bigger!!!!!!!
Looking at the femur cross sections above, can you see that more bone is deposited on some sides than others? That means that the bone is more adapted to resisting bending forces in that particular direction. The stress we place on our bones during movement and exercise does not build up bone equally in all directions. It builds up bone in the specific areas of the bone that experience stress during that particular activity.
This is why you can have a horse that has really solid conditioned bones that can pound out the trot down hill with minimal bone damage even though there is a tremendous force being applied to the bones.....but if that horse falls and the bone bends in a direction it's not used to, or is started in a different sport, it only takes a relatively small amount of force to break that bone - perhaps in the case of a fall, only the body weight of the animal.
The very last physics concept I will bore you with!!!!!!! VISCOELASTICITY
The last thing you need to understand about bones before we get into types of fractures and how they occur is viscoelasticity.
I still don't understand this concept more than superficially.
The basic premise is this - How the bone responds to force/work/energy depends on the amount of that force/work/energy......but ALSO how FAST you load the bone. The bone can absorb more energy if you load it fast, than if you load it slowly.
The amount of ENERGY that the bone has absorbed will affect what the fracture looks like.
For example, this is a "low energy" fracture. The bone absorbed less energy before shattering, perhaps loaded "slowly".
These are "high energy" fractures. These bones absorbed much more energy before shattering, and thus were perhaps "rapidly" loaded.
Types of fractures
Beyond the rate of loading, you can tell what forces were acting on the bone, by looking at the fracture type.
Here are the different forces that can act on a bone:
A fracture because of compression will result in an oblique fracture - this is because the bone will shear along the line of the greatest tension - which happens to be 45 degrees to the long axis (the shear plane).
What about a fracture because of tension? That will result in a transverse fracture
Bending and torsion forces generate fractures that are a combination of oblique and transverse fractures, because the bending and torsion forces generate a combination of compression and tension on the bone.
Bending is a combination of tension on side of the bone and compression on the other - the bone initially fails along the tension side (because the bone is stronger against compression forces) and starts to crack transversely. This is the "plane of maximum tension". The fracture continues along the shear planes (shear plane of bone is 45 degrees) on the compression side.
The result is a "butterfly" fragment of bone on the compression side of the bone.
You can see that the fracture, although it "spirals" around the bone, still follows the basic "tension" and "shear" planes of the bones, creating transverse and 45 degree oblique breaks!
If you can't picture a bone subject to a torsion force....imagine this scenario......
- A horse falls on a downslope and because of a rock or other trail debris, the foot remains stationary. The horse is off balance, and instead of falling straight over the leg, the body twists around with the leg still in place. Voila! Torsion applied and a spiral fracture result.
In real life, fractures are caused by a combination of forces - for example torsion with a bit of bending thrown in, so you might see a spiral fracture with a butterfly fragment. And remember, the speed of the loading and the energy absorbed also matters!
We are going to briefly talk about stress fractures since I think it is a more relevant form of "bone pathology" to most horse owners than a catastrophic fracture.
"stress risers" = the fancy term that includes small/stress fractures as well as other defects in a material - in this case BONE.
According to Wikipedia (the BEST, most trusty source as well all know....no I'm not being serious), a stress riser occurs in an area of an object (like a bone....) where stress is concentrated. Why would stress be concentrated in one spot?
An object (like a bone....) is strongest when the force/energy/load being applied to it is distributed over the entire thing......a micro crack/defect/contaminant (ie a "stress riser") in the object concentrates the force/energy load and reduces the area that the force is distributed over within the object.
Because the force is concentrated in a smaller area within the object, eventually the material that the object is made of fails and the result is a small fracture. Which we can call a "stress fracture". Get enough stress fractures (whether they occurred through few reps of a high load - such as working the bone in the plastic region - or lots of reps of a low load) and they will decrease bone strength exponentially.
If the defect (fracture or other defect in the bone) makes up 10% of the bone's diameter, the bone experiences a less than 5% reduction in strength.
If the defect makes up 20% of the bone's diameter, the bone will decrease in strength by 34%.
Note the lack of a linear relationship. Small fractures and other defects in the bone are NO BUENO. Too many stress fractures = bone strength loss = pathologic fracture
If this section on stress fractures is your biggest interest in this post, I would encourage you to go back and read my other bone post.
Here is an excerpt from that post (italics are my additions in this post):
"Stress creates tiny stress fractures (these are the stress risers we just discussed, a better word for these are micro cracks since they are the precursors to stress fractures) in the bone. This is a normal and lets the body know that it needs to send little bone eater cells to take out the stress fracture, and then send in the bone builder cells to rebuild the bone. You can't make bone stronger until it's replaced...and to replace bone, you have to remove the bone that is present, a little bit at a time, while rebuilding."
...tiny little cracks (occur) because the bone is under an increased load, which are being replaced by stronger bone. These tiny little stress cracks are perfectly normal and not a concern unless..........
You overload the bone to the point where the bone building process can't keep up with the amount of bone being compromised by the micro cracks. The bone eater cells are frantically eating up bone to get ride of the cracks and the bone builder cells are working furiously to put bone back into place - but they can only do it so fast.
Resorption (bone eaters) takes days to weeks, formation (bone builders) takes 3 months. During the gap between resorption and formation is a period of time that the bone is potentially weaker.
The build up of micro cracks can lead to a stress fracture. Stress fractures can lead to a complete fractures, like the perforation of a stamp. FYI - this is considered a "pathological fracture" as opposed to a acute or traumatic fracture, since the cause of the fracture wasn't necessarily due to your decision to gallop down a stretch of load - the fracture was due to an underlying cause of overtraining and a build up of micro damage and stress fractures. BOOM. Broken leg, dead horse. BAD. "
J.J. Kruzic, D.K. Kim, K.J. Koester, R.O. Ritchie, Indentation techniques for evaluating the fracture toughness of biomaterials and hard tissues, Journal of the Mechanical Behavior of Biomedical Materials, Volume 2, Issue 4, August 2009, Pages 384-395, ISSN 1751-6161, http://dx.doi.org/10.1016/j.jmbbm.2008.10.008. (http://www.sciencedirect.com/science/article/pii/S1751616108001021)
The above pictures is from an unrelated experiment but do a good job showing micro cracks and stress fractures.
Some other good images (the holes are normal, the crack is not)
(above = normal, for your reference)
Any questions? :)