• Print

Fractures of Long Bones

Written by James Rooney, D.V.M.
Category: Hoof Mechanics and Physics
Hits: 8376

Fractures of bone have always been and will always be with us. Fracture can be good problems since they can usually be repaired and sometimes will do the repairing without human intervention. While much has been written about the treatment of fractures, try very little attention seems to have been directed to the actual mechanism of fracture - how and why they occur. Here we shall look at how and why on the gross, pharmacy macro level. The microscopic details of fracture are complex and mathematically difficult and will not be attempted.

We must begin with some basic mechanics (no math required). Bone is a plastic/elastic material which can be compressed, diagnosis bent, and twisted somewhat like rubber and completely recover from such deformation. Since bone is plastic/elastic rather than almost purely elastic like rubber, the recovery from the deformation may be slower than the almost immediate recovery of stretched rubber. If the force or forces causing the compression, bending, twisting exceed the normal range, however, the bone will fail - break - as if it were a brittle material. This is a most useful property helping us to understand the mechanics of failure since many fractures of bones can be mimicked by applying appropriate forces to pieces of chalk. Glass tubing fractures in the same manner, but I cannot recommend it as a study material.

An imperfect analogy for the elastic/plastic - brittle transition of bone could be a piece of rubber or plastic tubing which is bendable and flexable until it is sufficiently chilled or frozen and shatters (brittle failure). Figure1, below, is a generic stress/strain curve illustrating what has been said.


The stress on the vertical axis is the force per unit area being exerted on/experienced by the bone. The strain on the horizontal axis is the deformation of the bone relative to the stress. In the elastic region the stress and strain are directly (linearly) related to each other. In the plastic region the strain increases more rapidly than the stress, and failure of course is fracture.

Next we deal with the types of forces experienced by any bone to one degree or another and shall just use a generic "bone" to start with. If you wish to follow along using pieces of chalk to represent the bone, you will be that much farther ahead. I use the terms simple and complex to describe fractures. These are not the descriptive terms simple, compound, comminuted usually used. As will be seen my terms simple and complex are based on the manner in which the fracture occurred - the pathogenesis.



The block of bone (or chalk) to the left, A, is subjected to a compression force along its long axis - an axial force - which causes a shearing stress in the block as indicated by the diagonal arrow. If that axial force is sufficiently large (you hit the block of chalk with a hammer), the block will break, fail, as shown in, B, to the right. This is shear failure, and when a material fails in a brittle manner because of compression, it fails in shear. This is an important type of failure of bones and will be discussed in more detail below. Please note that while the fracture lines shown in this essay may be drawn as smooth lines; in fact, because of imperfections in chalk and in the microstructure of bone, the fracture lines will always be somewhat irregular and jagged. The main fracture lines will be, however, as shown.

It may be noted that shear failure is theoretically at 45o to the compressive force. In real materials such as bone, however, the angle is more nearly 30o or 60o depending upon the direction you look.



Simply pulling on each end of the chalk will cause it to fail (fracture) across its length (transversely) as shown. Such tensile failure is rather difficult to demonstrate with chalk and is never seen as the only cause of failure of horse bone. We shall see, however, that bending failure always has a tensile component at right angles to the long axis of the bone. Bone is stronger in compression than in tension and, so, fails in tension before compression.


The remaining type of simple failure is torsion, Figure 4, below.


The column is twisted as shown on the left and the fracture plane is as shown both together and apart as on the right. The curved plane of the fracture is theoretically at 45o to the curving lines of the twisting being exerted on the column. As already noted the angle is more nearly 30o or 60o in real materials. This is easily demonstrated with a piece of chalk

This is an important and almost invariable component of fractures of long bones as will be seen. It probably never occurs alone in fractures of horse bones.


Long bones of horses are normally subjected to bending, and bending entails compression, tension, and shear. Figure 5, below. The bending may be present because of the angulation of a bone relative to the ground but is largely caused by muscular and tendinous forces exerted on a bone.


In Figure 5 the beam is bent downward with compression of the upper part and tension of the lower part. Since long bones are more nearly columns than beams, we have Figure 6, below:


In Figure 6 the red lines indicate the column before the slightly eccentric load is applied, causing the bending of the column and compression and tension as indicated.

Failure in bending alone is uncommon in horse bones but does occur, often as the result of an external force such as a kick to the fore or hind cannon bone (Mc3, Mt3). Such fractures are more often seen in foals kicked by older horses. An example is shown in Figure 7, below. The arrow indicates that a force was applied from the left.. Tensile failure occurred on the right side of the bone - the side in tension. - and is at right angles to the long axis of the bone. As the bone failed, the tensile crack progressed across the bone, the bone became weaker and weaker and finally completed the failure on the left side because of shear in compression. This is immediately shown by the near 45o angulation of the fracture on the left side.


It is evident that as the crack progresses (very rapidly) across the bone that compression is increasing on the intact bone remaining at each instant1. As the crack nears the left side, the stress is sufficient to cause shear failure of the bone before the transverse tensile crack reaches the left edge.

Using the chalk model one can cause pure tensile failure, a transverse break without a shear component, by pulling on the ends of the chalk while bending enough to initiate the break. Failing that, simply bending a piece of chalk will cause failure as in Figure 7 as it occurs in real bones.

Complex Failure

Most long bone fractures in horses are complex. Once again the chalk model will be useful. The commonest long bone fractures are of the humerus, tibia, femur, radius, and 3rd metacarpal/metatarsal bone in about that order of occurrence. That is obvious, to say the least - what other long bones are there?

It is evident that long bones always experience three forces:

  1. an axial compressive force along the long axis of the bone which is generated by the body weight on that leg
  2. a bending force which is generated by the axial compressive force and by the muscles and tendons around the particular bone
  3. a twisting or spinning force which is generated by the movement of the bone ends - the joints

The first two forces are easily visualized. The third may not be. That spinning/twisting occurs because the joints are not simple hinge joints but, rather, are cams which direct both the large scale extension/flexion and, at the same time, a spin of the bone around its own long axis, three dimensional movement, in other words.


Figure8, above, is a schematic view looking down on one half of a joint surface. The medial, inside, part of the surface is larger than the lateral, the outside1. We take the straight line, a, to represent the other, mating, half of the joint. When a rotate/slides on the other joint surface as indicated by the small arrows, a goes to b. The arrowhead on a obviously changes direction as it goes to b. That is, as the bone swings into flexion the uneven size of the joint surfaces (cam) insures that the bone spins around its long axis from medial to lateral.

We show that again in Figure 9, below, that for the proximal phalanx (long pastern bone) dorsiflexing, and spinning as it does so, from medial to lateral on the distal end of the cannon bone. This is the movement of the fetlock joint as the load comes onto the leg.


Figure 9a

rooney_fracture9bFigure 9b

rooney_fracture9cFigure 9c

rooney_fracture9dFigure 9d

As the proximal phalanx is dorsiflexing from 9a to 9d, the larger medial side of the distal end of Mc3 causes it to spin as shown from medial to lateral, eventually close-packing at 9d.

Having done with that, we can return to fractures per se. Long bone fractures are a combination of axial compression, bending, and twisting. One can with careful examination of the broken bone or radiographs taken from at least two positions determine which parts of the bone were in tension and compression when the fracture occurred. That is clearly shown in Figure 10, below.


This is a so-called screwdriver fracture of the proximal phalanx. The dorsal surface -left picture- has a spiral fracture line while the palmar surface (right picture) has a nearly vertical fracture line. This says that the dorsal surface was in tension and the palmar surface in compression when the fracture occurred. The fracture in this case began with tensile failure because of bending on the dorsal surface combined with twisting . When the fracture had progressed to the palmar surface, the two ends of the spiral crack are joined by a vertical, or nearly so, crack as the bone opens like a book. In non comminuted fractures such as this one the vertical crack has intact periosteum, so that repair is a matter of closing the book again, Figure11, below.


Twisting in the direction of the arrows opens the fracture like a book around the heavier black line "hinge." Obviously, realigning the fracture is accomplished by twisting the two bone ends in the opposite direction.

When the spiral cracks reach the compressed side of the bone, they are "blocked" by the compression. As the crack lines gape open, the compressed area fractures along a vertical, or nearly vertical line, failing in tension on the inner surface, Figure 12, below.


Figure 13, below, is a fractured tibia. The spiral component (the right picture) on the cranial surface was the part in tension when failure occurred. The vertical component (the left picture) on the caudal surface was in compression.


Clearly, the vertical failure in Figure13 is not as obviously vertical as in Figure10. This illustrates the important fact that in complex fractures there are variable contributions by compression, bending, and twisting. One or two components may be more significant than the third and, so, the appearance of the fracture lines can vary somewhat from example to example. In my experience, at least, the spiral component is always clear and obvious while the vertical component may vary. This suggests, of course, that the twisting force is always strongly present while bending and compression are always present but more variable.

Chip Fractures

While fractures of long bones certainly do occur, so-called chip fractures are by far more common in working horses, particularly horses that work at speed. The common sites are the dorsal aspects of the fore and hind fetlock joints, the carpus, and, less commonly, the tarsus. There are two types of fracture and one nonfracture. The first is a single event shear failure as the result of excessive compression force. The second, and most common, is a shear failure which is pathological, occurring after the articular cartilage has been damaged and eroded (arthrosis) and the underlying cancellous bone has become sclerotic (denser). The third, nonfracture, may appear as a fracture on radiographs but is, in fact, new bone formation (osteophytes) formed because of arthrosis. Obviously, the second and third types can overlap. Thus, the articular cartilage is damaged and new bone begins forming in response. The cancellous bone becomes denser, and a shear fracture may occur, so that both osteophytes and fracture are present. All of these conditions cause swelling, pain, and eventual thickening on the dorsal aspect of the fetlock, carpus, and tarsus and, in the case of the fetlock at least, are often called osselet.


Figure 14 is a postmortem specimen of a single event shear fracture of the third carpal bone. Such fractures also occur in the radial carpal bone.


Figure 15 is a radiograph showing a "lump" of bone on the dorsal aspect of the fetlock. This could be either a chip fracture from the dorsal lip of the proximal phalanx or osteophyte formation without obvious attachment, on the x-ray, to bone.


The subject of fracture has hardly been exhausted even on the gross level of mechanics that we have been considering in this essay. I have not dealt with fractures of vertebrae, sesamoid bones, splint bones nor considered in any detail what one can learn of the kind of step or misstep which led to the fracture.

For those who might be interested there is more to be found in my The Lame Horse and in the book Equine Pathology. For those even more seriously interested a search of the internet for fracture mechanics will reveal a wealth of information, much of it highly technical, not say mind-boggling, in nature.