In the chronic phase of laminitis, the angle between the coffin bone and the hoof capsule changes: this change is due not only to the coffin bone tilting, but due also to rotation of the hoof capsule.
Coffin bone rotation
Initially, the coffin bone will tilt relative to the hoof wall. In that case, the lamellar connection is only damaged in the dorsal part of the hoof (the toe). The coffin bone turns (pivots) around the still-intact caudal (rear) part of the lamellar connection. This happens mainly due to the force resulting from the downward pressure of the horse’s weight and, to a lesser extent, the pulling force of the deep digital flexor tendon during break over of the hoof. This phenomenon is called coffin bone rotation or, to be more precise, histological-mechanical coffin bone rotation.
Hoof capsule rotation
In the next stage, the angle between the coffin bone and the hoof capsule will increase further. In addition to the continuing coffin bone rotation, there is now hoof capsule rotation. The hoof wall is hereby pushed away from the coffin bone.
There is a distinction between anatomical-mechanical and histological hoof capsule rotation. Anatomical-mechanical hoof capsule rotation is the result of incorrect biomechanical force distribution, which results in the hoof capsule being torn away from the internal foot. Toes or a hoof wall that are too long, or heels that are too high all contribute to this. Histological hoof capsule rotation is the result of the formation of a lamellar wedge. In many cases, anatomical-mechanical and histological hoof capsule rotation occur together.
To increase the chances of recovery, it is important to realize that coffin bone and hoof capsule rotation are not the same. Apart from weight loss, nothing can be done to decrease the downward pressure of the horse’s body weight. Even in an anatomically perfect hoof, where the strain on the lamellar connection is minimal and the heels have the correct height to align the coffin bone in its optimum position, the latter will still tilt as soon as the lamellar connection in the toe region breaks down. Hoof capsule rotation on the other hand, can be largely corrected with one or more trims. As soon as the coffin bone returns to its anatomically correct position, the hoof will be better balanced and the force distribution across all anatomical parts of the hoof will be optimal. This approach will also lead to a reduction in pain, a better hoof mechanism and thus a faster recovery.
Lateral coffin bone rotation
In some cases, X-rays show that the angle between the coffin bone and the ground also changes in a lateral direction. This medio-lateral coffin bone rotation deserves as much attention during the treatment (read: trim) as a ‘normal’ rotation.
This phenomenon is less common than the normal dorso-palmar coffin bone rotation, although it is seen more frequently since veterinarians ask more often for dorsal radiographs. These photos show that the coffin bone tilts medially inside the hoof capsule. In the rare cases that the coffin bone tilts to the other side, it is called a latero-medial coffin bone rotation.
The changed position of the coffin bone can also be observed in the joint space between the short pastern bone and the coffin bone, which then becomes wider on the lateral side than on the medial. On the medial side of the hoof the sole thickness is usually reduced.
A medio-lateral coffin bone rotation puts higher pressure on the medial side of the coffin bone. As a result, the risk of osteoporosis in this bone increases.
Without X-ray’s, the following signs of lateral coffin bone rotation can be determined as well: there is reduced wall growth on the medial side of the hoof, the coronary band feels dented and the top of the hoof wall can be felt through the skin. These signs are less observable or entirely absent on the lateral side.