Long toe, low heel syndrome

The following is a pictorial of the effects of trimming a horse to maintain a long toe and low heel. There have been many theories as to how this could be advantageous to the performance of horse. Commonly the horse is said to be delayed in "breakover" therefore allowing for a longer "reach". Over time, this hoof conformation will lead to bigger medical conditions. Another way that this conformation can manifest, is in a horse that already had an underlying medical condition causing the hoof not to grow appropriately. Anytime a hoof does not grow proportionately, an exam and radiographs may be indicated to assist the farrier in addressing the complicated hoof.
The above photo is an example of a horse with a crushed heel. Notice that the shoe (removed) has made an imprint into the back of the heel. The heel is now growing forward and is unable to grow in a more natural position. Also notice that at the quarter of the hoof there is a crack forming due to the mechanical stresses of the foot. The radiograph to the right is of the same foot. It demonstrates how a long toe and low heel leads to a distortion of the bony column. This in turn places added stress to the structures of the back of the foot and leg. This includes but is not limited to the Deep and Superficial digital Flexor Tendons, Suspensory Apparatus, and navicular bone. 
Due to the severe trimming of the foot, a negative plantar angle has formed. This has caused a backwards rotation of the coffin bone. Above there is a sterile abscess that has opened on the edge of the foot  (red circle). This has formed from too much pressure applied by the coffin bone to the caudal hoof structures. Note also that the hoof capsule has taken a more oval shape as opposed to the more round shape of a healthy foot.

The following is a series of hooves trimmed with excessive toe on left of the screen and a balanced hoof on the right of the screen:

The above radiograph is a demonstration of a severe negative plantar angle. The back of the coffin bone is lower than the toe. It was so severe that the dorsal hoof wall was separating. This horse also had a severe quarter crack. Also notice the severe bend in the bony column. It should form a straight line. (see third image)

This is a moderately severe example of a negative plantar angle. Notice the back of the coffin bone is even with the toe. While this is not as severe as the first example, this angle still places significant stress on flexor and suspensory structures, leading to a high risk of potential failure. Furthermore, notice the slight bend in the bony column, another clue that the hoof angle is incorrect.

This is an example of a good hoof confirmation. Notice the back of the coffin bone is higher than the toe. This is a more natural position that does not place undo stress on the supporting limb structures. Furthermore, notice the entire bony column forms a straight line from the fetlock joint down.

The above image is of a severely poorly trimmed hoof. It was intentionally trimmed to have a long toe and a low heel. This foot corresponds to the radiograph above it. 

The above foot is commonly seen when toes are allowed to grow long. Unfortunately, some horses have a tendency to grow significantly more toe than heel. This can make it difficult for the farrier to keep a healthy trim, and may require radiographs.

The above image is a foot trimmed well without excess toe. Notice that the heel grows in a more natural and downward angle. Also notice the abscence of cracks and stress lines.

The Quarter Crack

The above depicts a "blown quarter" or quarter crack. This occurs when too many mechanical stresses cause a shearing force. This force then causes the hoof crack. In more severe cases, these can split very deeply down to the lamina and cause bleeding and pain to the foot. These cracks can be so severe that horses must be rested and time taken out of performing in order for it to grow out and heal.However, these will not heal until the underlying condition is addressed.