Part 5 of Equine Foot Balance

When comparing the bioengineering of the pedial bone, we must consider two factors. These two forces are the loading on the dome-shaped pedial bone and that transferred to the hoof capsule that acts as its supporting beam to cancel out the tensile and shearing stresses (resistance of an object to force tending to tear it apart) that prevent the pedial bone from being deformed or pushed apart when meeting the reactionary ground forces.

Still working with the pedal bone, we can see it is not a complete dome shape, as in having all its sides contacting the ground. However, it does have a supporting structure that it relies on to support the structure and transfer the loading and gravitation forces to its outer edges without compromising or deforming the shape of the bone and allowing the structures above the mobility to adjust to the terrain it has to work on, and that is the navicular bone or distal sesamoid bone.

The navicular bone plays a very important role in the loading properties of the distal interphalangeal joint. It allows the distal phalanx (pedal bone) to conform to the changing surface under the foot by freeing up movement between the wings of the distal phalanx while maintaining the strength within the distal interphalangeal joint and room for the descending arc of the middle phalanx.

The navicular bone achieves this by connecting to the distal phalanx with its Collateral, Impar and Suspensory ligaments of the navicular bone so it can remain stationary with the distal phalanx and only have a slight articulation. Remember that most of the articulation of the distal interphalangeal joint and the navicular bone occurs between the proximal inner articulation surface of the navicular bone and the distal palmar aspect of the middle phalanx. This arrangement allows the distal phalanx to maintain the dome's curvature and transfer the additional compressive stress to its ground surface and the hoof capsule as its supporting beam.

The interesting thing about the navicular bone is when you use a Dremel and open it through its length, and you can see it forming a dome shape with its outer edges curving down towards the ground. The internal dome shape of the navicular bone allows its loading from the deep digital flexor tendon to be transferred from its centre to its outer edges, where it is held and connected to the distal phalanx by the ligament system, and the transferred force of the movement of the distal interphalangeal joint is relocated to the distal phalanx and its wings. It is the same for the navicular bone's articulation surface, as it allows the articulating middle phalanx's forces to be transferred from its centre to its outer edges, where the ligament system holds and connects to the distal phalanx.

The bioengineering aspect of the navicular bone, in conjunction with the distal phalanx's dome-shaped structures, means the palmar loading properties of the distal joints of the limb can cope with the equine body's locomotion and counteract the opposing ground reaction forces appropriately. 

When we take the time to look at how nature has designed the equine and how the structures are interconnected through the ligament structures, including the neuro-muscular system, we can start to understand how the equine balance works. It also allows us, the farrier and bodywork, a checking system for the correct orthopaedic balance throughout the entire animal and why we are left with the shape of the hoof capsule we are meant to work with. What other structures can we use to trim or she the equine foot? We will review them in our next blog and continue this learning experience. 

Nature's design, not ours.