Background
The weight of the horse is borne by the limbs. As such, each foot exerts a force on the ground. When the foot is stationary, the force on the ground (weight born by the limb) is opposed by an equal and opposite force, the ground reaction force (GRF). Force is exerted on the foot at every point of contact and for ease of study and discussion, these forces are summed and represented by a single force, the ground reaction force vector (GRFV). As a vector, it has magnitude and direction, but it also acts through a point, frequently termed the center of pressure (CoP). The CoP is typically approximately in the middle of the healthy foot. The center of rotation (CoR) is 10-15 mm palmar to the CoP (that means it is also palmar to the widest part of the foot, for the front foot at least)
Why is the CoP important?
The CoP is important because it determines the distribution of stresses within the hoof and other structures in the distal limb (the effect becomes reduced the further up the limb you look). For example, if you move the CoP to one side of the foot, it will increase the load in that wall and decrease the load in the opposite wall. Similarly, it will increase the load in the ipsilateral osseous and articular structures, but decrease the load on the ipsilateral ligaments. It will also increase the load on the ipsilateral lamellae (because they are part of the wall).
How can you tell where the CoP is?
The scientific answer to this would be to use a force plate or a pressure mat, but the foot itself can give you clues. The foot responds to stress in three main ways: 1) increased stress will tend to cause the coronary band to move proximally; 2) increased stress will cause the growth rate of the wall to decrease; and 3) increased stress will cause distortion in the hoof wall - this is seen as flaring in the dorsal foot and underruning in the palmar/plantar foot. Therefore, by looking at the position of the coronary band, spacing of the growth rings, and flares/underruning, you can make an intelligent prediction as to which direction the CoP has been displaced from where it should be located.
How can you use knowledge of the CoP therapeutically?
Therapy is either directed at a specific disease/tissue combination or is symptomatic. For specific disease/processes, you know the structure involved. You must determine if it is stressed under tension or compression. Then you must decide what manipulation will reduce that stress. Typically, this involves moving the center of pressure. For example, if a medial collateral ligament is stressed, moving the CoP towards the medial side will reduce the stress in that ligament. If you are confronted with a medial sinker, the stressed tissue is the medial wall, then to reduce that stress you should move the CoP away from that wall. Whenever the CoP is manipulated in this manner, there are always tradeoffs because other structures will become overloaded, though hopefully not sufficient to cause another problem. For symptomatic treatment, you should move the CoP away from the side of the hoof capsule that is telling you it is most stressed.
How can you move the CoP?
There are many ways to move the CoP. It depends on circumstances, particularly what substrate the horse is standing on. That said, any asymmetrical redistribution of pressure on the ground surface of the foot is likely to change the CoP: extensions, shoe branches of differing widths, wedges, partial pads, etc. Wedges will work on a flat surface - elevating one side of the foot always moves the CoP to the elevated side. Most of the other techniques rely on the fact that a change in surface area/leverage causes one side of the foot to sink into the substrate a bit more than the other - i.e. they function as a mild wedge.
Why doesn't moving the CoP always work as expected?
There are several reasons moving the CoP may not have the desired effect. 1) The disease is too severe for it to be beneficial. 2) The CoP has been moved in the wrong direction. 3) The right technique has been used but to an insufficient extent (but changing things gradually is usually better than drastically). 4) The CoP has been moved in the right direction, but too drastically - sufficient to damage other tissues.
What about the CoP in motion?
This is of course a whole lot more complicated. Extended discussion for another day. But factors such as break over, which means a long discussion about moments, become important. To keep it short, because the CoP is 10-15 mm dorsal to the center of rotation, the GRFV creates an extensor moment. For the foot to be stable on the ground this must be opposed by an equal magnitude flexor moment, and the flexor moment is related to tension in the DDF. It is when these two moments are not balanced that the foot moves. As the moments become unbalanced, the CoP also moves.