A basic knowledge of anatomy is needed to understand how the hoof functions and why certain problems can develop. Understanding anatomical terms will help you when discussing issues with your trimmer, farrier or vet. This is a brief overview of the anatomy of the hoof.
THE BONES OF THE FOOT.
Long Pastern Bone – Also called the first phalanx, proximal phalanx or P1. Articulates with the short pastern to form the pastern joint.
Short Pastern Bone – Also called the second phalanx, middle phalanx, or P2. It sits with its lower end inside the hoof capsule. It articulates with the pedal bone to form the coffin joint and the long pastern to form the pastern joint.
The long and short pastern bones should have the same angle as the toe wall, this is known as the hoof-pastern axis.
Pedal Bone – Also called the coffin bone, distal phalanx, third phalanx or P3. It sits within the front half of the hoof capsule and articulates with the short pastern bone and navicular bone. The common digital extensor tendon runs down the front of the leg and attaches to the extensor process at the front of the pedal bone. The deep digital flexor tendon runs down the back of the leg, passes over the navicular bone and attaches to the underside of the pedal bone. The 2nd illustration shows the shape of the pedal bone in cross section. This is the shape seen on x-ray (which shows the densest part of the bone). Surrounding the coffin bone is a highly vascular layer called the corium. The horn of the hoof capsule grows from the corium.
Navicular Bone – Also called the distal sesamoid bone. It sits behind and articulates with the pedal bone. It acts as a fulcrum point for the deep digital flexor tendon, ensuring that it maintains a correct angle of insertion onto the pedal bone. Below: Navicular bone is highlighted in green, deep digital flexor tendon is highlighted in purple.
Hoof Wall – This is made up of 2 layers. The outer layer is pigmented and harder, forming a protective outer shell. The inner layer is called the water line. This is white (it has no pigment) and is more durable. This is the part of the wall that bears weight in the naturally trimmed hoof. The hoof wall grows down from the coronary band at a rate of approximately ½ cm to 1cm a month.
Periople – A thin skin of soft horn which forms a layer over the join between the coronary band and the hoof wall. This is the same as the cuticle on your own nails. Sometimes it extends further down the hoof and becomes more obvious (below), and in wet weather it will swell up and turn whiter.
Sole – Grows down from the underside of the pedal bone. Barefoot trimmers talk of live sole, which is hard and waxy in appearance. A shod or overgrown bare hoof will have a covering of unexfoliated sole, which is flaky or chalky in appearance. This forms when the sole is not making contact with and being abraded by the ground, because of the shoe or excessively long hoof walls.
Frog – Functions as a shock absorber and traction aid and is a hinge allowing the hoof wall to flex and expand with the hoof mechanism. The central cleft (or central sulcas) should be open. If the hoof is contracted it will be a narrow, deep crack which is a perfect home for fungus and thrush. The frog contains sensory nerves called messner receptors. We have them in our finger tips. These highly sensitive nerves allow the horse to feel what he is standing on another reason to ensure the frog makes contact with the ground.
Bars – These are a continuation of the hoof wall. They strengthen the hoof preventing it from flexing too much. The bars should be straight and end about halfway along the frog.
Seat Of Corn – The angle between the hoof wall and bars. If the shoe is left on too long its heel presses into this area causing a corn (bruise), hence the name.
Collateral Grooves – These are the grooves running along either side of the frog. Their depth is a guide to the position of the pedal bone within the hoof. If the collateral groove is very shallow it indicates that the pedal bone is sitting low in the hoof.
White Line – The white line is the join between the pedal bone and hoof wall. Growing out from the wall of the pedal bone are the dermal laminae. These look like the flukes on the underside of a mushroom and contain the blood supply (below left). Sticking inward from the hoof wall are the epidermal laminae (below right), these interlock with the dermal laminae to form a strong bond.
Lateral Cartilage – Extends from the palmar process of the pedal bone into the rear of the hoof and above the hair line where it can be felt (shown as the green area in the photo below). It is involved in hoof mechanism, flexing as the hoof moves and absorbing concussion.
Digital Cushion – A fibro-fatty pad that sits in the back half of the hoof, above the frog and between the lateral cartilages (outlined in red in the photo below). Functions in the hoof mechanism and as a shock absorber.
Hoof Mechanism – The hoof mechanism is a complex process that pumps blood around the hoof and back up the leg. The movement of blood in the hoof also absorbs concussion. Basically during the support phase of the stride (when the horses’ weight is on the hoof) the hoof expands and the blood vessels within the hoof fill with blood. During the flight phase (when the hoof is off the ground) the hoof contracts and the blood is squeezed out and back up the leg. The pumping action of the hoof aides the heart in circulating blood around the body.
Breakover – This is the moment that the heels leave the ground and the foot rotates over the toe. The point on the toe that the horse “breaks over” can be seen as an area of greater wear on the shoe, or with the bare hoof, an area that the horse keeps neatly bevelled by himself, and is often called the breakover point. A healthy bare hoof has a short toe which requires less effort to breakover. The longer the toe, the more effort is needed to breakover, and the greater the strain on the tendons and ligaments.
Watching the horse from the side the foot should hit the ground heel first. This is because the shock absorbing structures are in the back of the foot. If the horse places the foot down toe first he is protecting the structures in the back of the foot due to soreness.
Watching from the front on a flat, level surface, ideally the foot should be put down flat. However, it is common for the foot (particularly the hind) to meet the ground outside first.