We see that wear and weight pressure form a horse's foot. This depends not only on how the horse is kept and used, but also on its individual anatomy and thus on its pattern of movement (e.g. slim horses tend to put more weight on the medial hoof walls of their front feet, broad horses often put more weight on the lateral hoof walls - consequently these hooves are "stressed" differently and wear off differently).
Trimming hooves according to hooforthopaedic principles means taking these factors into account. The way the horse uses its feet shapes them and therefore the hoof itself provides the information we need to decide how it needs to be trimmed. When trimming, we make use of the natural process of growth and wear: We try to establish a balance, so the hoof walks itself into the healthiest shape possible for each horse during the given time between two trimmings. That means we let time and the horse work for us, the trimming gives an impulse for the development in the right direction. Ideally, hooves should be trimmed this way, before they turn worse again (every 4-6 weeks or sometimes even more frequently).
We believe this to be a less stressful way of influencing the hoof (and the whole anatomy on top of it) than making "corrections" every few weeks in a "worse again"-situation. Why is that?
I will try to explain this by an example: In adult horses, bones and joints have been formed into their final shape and position. If a horse is not standing correctly because, e.g. opportunities for early corrections at foal's age have been missed, the horse and the trimmer have to deal with this given situation rather than being able to "fix" it. Commonly, a method used to "correct" asymmetrical hooves with single-sided heavier wear would be to shorten the longer hoof wall. Consequently the hoof will tilt into a "straight" position, meaning the horizontal axis of the hoof in relation to the vertical (or leg). It can only do so to an extend the hoof joint allows to. But: This joint is usually intended to flex backward and straighten forwards only, not to flex sideways! There is some flexibility allowing for sideways movement, but this is needed for the horse to cope with uneven grounds. If we used this flexibility to change a horse's foot-leg-position in one single event, an uneven joint space in (at least) the lowest joint would be the consequence (there is radiographic evidence to this fact). This leads to an unbalanced impact (by vertical weight pressure) on the joint surfaces. Also there would be more stress e.g. on ligaments on one side and less stress on ligaments on the other side. The joint capsule and other tissues around are affected as well. Moreover, there is a higher danger of injury when the horse walks on uneven grounds. As a reaction to this imbalance in the joint and the tissue surrounding it the body tries to adapt by reinforcing less stressed tissues by calcification. Therefore, these "corrections" increase the danger of joint damage (arthritis) and joint capsule, ligament, or tendon calcifications if conducted repeatedly. This is by no means new knowledge and the reason why everybody says corrections need to be done very carefully and slowly over several trimmings. However, we often see horses walking worse right after a correction has taken place, which is an important sign of overstraining. But how can we know how much would have been tolerable for the horse in a certain situation?
We prefer to create a situation in which the horse gets a chance to walk itself - slowly over time - into a comfortable position if that is still possible. The anatomy of the whole body (sometimes physiotherapy is the key success factor), the joints and the surrounding structures allow for harmless changes. Since these changes take place virtually step by step, the body has a chance to adapt over time. Sometimes (or in adult domestic horses: often) there are "bony reasons" that do not allow for all the desired changes (e.g. the coffin bone has become asymmetrical). We have to accept that and find a way to help the horse to balance the hooves' growth and wear under the given circumstances (including keeping and use), so the hooves themselves stay healthy and comfortable for the horse to use.
How do we define a hoof as healthy?
We assume a hoof is healthy if its wear is more or less balanced over the complete hoof capsule (=> walls are not considerably flared), and the white line is intact and closed (not stretched, showing red marks or rotten parts). Ideally the distal part of the hoof wall should be intact all around. There are many more signs we can use to "read" a hoof and to determine its actual situation, load bearing et cetera. But the aforementioned are the most obvious to see and recognise. Some examples of additional signals the hoof gives: growth lines (horizontal lines: how many, evenly distributed, shape?), the frog (size, shape, hardness, rottenness?), the heels (squeezed, position?), coronary line (shape, hairy line?), cracks or gaps in the hoof walls, general quality of the hoof capsule et cetera. Last but not least: The horse should be able walk comfortably.
What do we do and why?
First of all we have a look on the horse as a whole and "read" the hoof to determine its actual state. It is very important to understand why the hoof looks like it does and to estimate which influence the keeping, the usage of the horse and its general health has in this situation. If, for example, there is a very heavy horse on inherited small feet (e.g. a Quarter Horse) with flared walls all around, living on a soft pasture permanently, we might not be able to change a lot by trimming its hooves every 6 weeks. Only if the horse looses weight and is also offered a somewhat abrasive ground during training or in front of the water place corrections are possible. Generally, when we trim hooves, we try to balance growth and wear.
The hoof shows us where it wears more or less. It also shows us how the horse loads it with weight. Some horses distribute more weight to their heels, some more to their lateral and some more to their medial hoof walls. Thus, the parts being worn more show shorter distal wall lengths (which are sometimes damaged because of "overloading"). These walls are usually straight, tightly attached, sometimes even undergrowing the hoof, the white line is not stretched. Parts wearing less are highlighted by longer distal wall lengths. These lead to a tilt of the hoof capsule as a whole, inside the boundaries permitted by the joints. Therefore they have to move outward, leading to an inclining wall angle (in relation to the vertical or more heavily loaded hoof side) and the longer the walls grow the more flare develops. The lever on the distal hoof wall leads to the flare and also stretches the white line, allowing contaminants to enter and cause mechanical and biological decay or even abscesses. Eventually, the walls grow so long that they break out. The frog shows us how the horse distributes its weight over the hooves by squeezing the frog to the "less stressed" side.
We cannot prevent wear directly but the hooves of most domestic horses show a general lack of wear. We are able to reduce exaggerated wear of some parts by provoking/causing it in other parts of the hoof. For example: I trimmed a horse that wore his lateral walls much more than his medial hoof walls. Consequently his hooves tilted to the lateral which affected his gait even more. This vicious cycle lead to long flared medial walls and he literally walked around his toe while the lateral wall and the heel were very short. By causing wear on the medial walls and on the toe, his hooves stopped tilting to the lateral. This enabled the horse to walk straight again while reducing the overwearing of the lateral walls.
How do we cause wear? By carefully reducing some material around the distal hoof walls, not by completely removing them. This could mean to use the rasp on the hoof wall, it may be removing some material from the white line (the rubbery texture prevents wear quite effectively) or sometimes even removing a bit from the peripheral sole. The result is newly shaped material that can be worn more easily in order to compensate for the imbalance in "natural wear" the horse would suffer from without interference.
By the way: using the rasp on the hoof wall has several functions. It is not only used to reduce excess material but it also creates the effect of a "laminated" leaf spring which makes it a very effective instrument against flaring walls by minimising leverage effects. It does not have anything to do with "thinning" or "straightening" walls. In addition to balancing growth and wear of the hoof capsule, we remove excess material which would foster decay or rotting processes.
In our understanding of the anatomy of the hoof, the most important load bearing structure is the lamellar architecture (as it provides a comparatively huge surface area for the coffin bone to hang from and to carry the weight of the horse), ideally supported somewhat by the frog. This must not always be the case though. Narrow hooves with strongly arched soles often do not have significant ground contact.
We do not believe that the sole is a very suitable structure for the main load bearing, as the "hanging" coffin bone has a completely different structure than e.g. vertically "carrying" hollow/long bones and because there is no subcutis to buffer any pressure between sole and coffin bone. This is why we always leave the distal hoof walls a bit longer than sole level and the reason why walls should not be rasped too much. Because of this one should not shorten flared walls to an extend where they do not touch the ground any more (done by NHC-trimmers to eliminate the lever). We reduce the leverage effects by using the rasp on the wall to create the "lamination" of the leaf spring but we want the lamellar structure to be loaded all around the hoof. If we shortened the "less loaded" and therefore long-flared walls to the extend where they would not be stressed any more, it would result in even more stressing of the walls and lamellar structure in "heavier loaded" parts and henceforth raise the risk of traumatic laminitis.
Reasons to work on a sole can be reducing excess material, emulating the natural wear or just catching dirt (many domestic horses do not even wear off their soft, naturally decaying sole layer). On top of this sole calluses may develop which can then bruise the sole due to the lack of subcutis underneath the coffin bone and should therefore be prevented.
As with all techniques: The quality of barefoot orthopaedics depends on the ability of the trimmer to evaluate a given situation, understand causes and symptoms and to choose proper and effective measures. In our non-profit organisation we offer a 2-years training program during which our trainees get equipped with profound information about hoof anatomy and horse anatomy in general and about any kind of food- or illness-caused impacts on hooves. We practice how to reasonably judge situations and practical trimming. As a quality control instrument and to further develop these techniques by learning from each other, each member has to commit himself/herself to participate in at least two training events per year and is asked to hand in one well-documented case study. Customer complaints have to be filed with our president. Currently the organisation has about 100 active members. There is another organisation practicing barefoot orthopaedics named "Deutsches Institut für Huforthopädie" (difho.de) which has a similar member count.