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Diseases of the Hoof, Distal Phalanx & Associated Structures -- Part I

By Ray Miller
I want to touch a bit on Diseases of the Hoof, Distal Phalanx and Associated Structures. The hoof/foot is very complex. "By definition, the foot of the horse includes the hoof and all structures contained therein, including the sole and frog. This hoof is only the cornified epidermis of the foot (wall, sole, frog), is no-vascular in structure, and has no nerve supply. Nutrition for the hoof is obtained from the combined coria".
The hoof is composed of the wall and it is approximately 25% water and is a modified cornified epithelium. It has three layers; the first outer layer is the periople, the second or middle layer composes the bulk of the hoof wall and is the densest portion of the wall. This is the layer that contains the pigment in pigmented feet. There is no difference in the make up of a pigmented hoof and a non-pigmented (white) hoof other then the pigments. The third layer or inner layer is the laminar layer that forms the epidermal laminae of the hoof.
The ground surface of the hoof is divided into sections called the toe, quarters and heel. Growth of the hoof wall is quite slow, about 6 mm or 1/4 inch per month. It takes about a full year for the hoof to grow down from the coronary band to the ground surface. This can be speeded up by feeding additional protein. You see this kind of growth when you feed supplements that have a protein base (bean meal, alfalfa meal, sea weed) or when you turn your horse out on lush pasture. Any change will be noted from the top of the hoof first. Not a the bottom.
The Bars, at the heels, where the wall turns anteriorly to from the bars that converge towards one another to meet at the apex of the frog. They run parallel to the collateral sulci of the frog. The sole then conforms to the inner curvature of the wall and to the angles formed by the bars. The sole comprises most of the ground surface of the hoof. It is approximately 33% water. The structure is similar to that of the wall and the tubules run vertically as formed by the papillae of the sole corium. These tubules curl near the ground surface, which accounts for the self-limiting growth of the sole and causes shedding of the sole. The frog is a wedge-shape Inverted V mass that occupies the angles bounded by the bars and sole. It is soft because it is about 50% water. The frog is divided into the apex which is at the front (anterior angle of the frog), base which is the hind (posterior aspect) and the frog stay which is the central ride of the internal surface.
The White Line is the junction of the wall and the sole. It is visible as a white line following the circumference of the wall around the hoof. The white line is the junction between the laminae of the wall and the tubules of the sole.
The Corium or Dermal layer, is modified vascular tissue that furnishes nutrition to the hoof. It is divided into five parts; Perioplic corium, coronary corium, laminar corium, sole corium and the frog corium.
The Digital Cushion is a fibroelastic, fatty, pale yellow, relatively avascular, and yielding pyramidal structure containing areas of cartilage in the posterior half of the foot. The primary purpose of this structure is to concussion to the foot. There are also other mechanism that also aid in the concussion such as the hydrology of the blood supply. The Coronary Cushion is the elastic portion of the coronary corium; it aids slightly in reducing concussion.
Lateral Cartilages are part fibrous tissue and part hyaline cartilage. When ossification of these cartilages takes place it is called "Sidebone".
The Coronary is the combined perioplic corium, coronary corium and coronary cushion; it is the primary growth and nutritional source of the bulk of the hoof wall. Injuries to this structure are serious and usually leave a permanent defect in the growth of the hoof wall/horn.
The Bulbs of the Heel are located in the posterior aspect of the foot where the perioplic corium covers the angles of the posterior aspect of the hoof wall. They are supported by the digital cushion.
The Distal Phalanx/third phalanx/p-3 bone/coffin bone, set on top of and with in these structures of the hoof.
There are two types of Diseases for the hoof: Diseases with Physical Causes and Diseases with Infectious & Immune.
I will cover in this first part, Diseases with Physical Causes.
Hoof Wall Cracks, especially those in the quarter and heel region, are a common cause of foot lameness. Hoof cracks have a multitude of causes. The horny hoof wall often fails internally before the crack propagates externally. Central toe cracks almost always are the result of rotation of the distal phalanx, such as in laminitis or deep flexor tendon contracture syndrome. Unattended feet that get to long. Excessively wet or dry condition or combinations of these may lead to hoof cracks. Conformation may also lead to hoof cracks and not being trim or shod to the conformation of the horse (Form To Function). Incorrect trimming and shoeing create focal foot imbalance, causing abnormal impact that may lead to hoof wall failure.
Keratoma Equine Keratoma is an uncommon hoof disease best described as a tumor of the keratin-producing cells of the hoof wall. The mass is interposed between the hoof wall and underlying third phalanx.
Sole Bruising is a common but often overlooked and underrated cause of lameness. Lameness can be acute or chronic in onset and the severity can vary from barely perceptible to non-weight bearing.
Coffin Bone/Distal Phalanx/Third Phalanx/P-3 Fracture are encountered occasionally. There are two main categories of fractures: articular fractures and nor articular fractures. Trauma is the predominant cause of distal phalanx fractures. High speed impack, kicking a hard object, stepping with high velocity on a nonyielding object.
Quittor is a lay term for necrosis of the collateral cartilage of the third phalanx. It is characterized by an intermittent purulent discharge and sinus tract formation at or proximal to the coronary band in the proximity of the collateral cartilage.
Sidebone is a lay term describing osification of the collateral cartilages of the third phalanx. This is a normal ageing process. The concern is that premature or abnormal osification of this structure may lead to lameness. Most likely this condition is the result of uneven foot impact caused by many factors, including continual work on hard, uneven surfaces, poor conformation and improper shoeing or trimming.
Pedal Osteitis is the demineralization of the solar margin of the distal phalanx/coffin bone/third phalanx/p-3. It is not fully understood and appears to be a secondary phenomenon that must be differentiated from the primary problems.
Sheared Heels is a relatively common cause of foot lameness. The name describes a structural breakdown and ultimate change in the shape of the foot that occur between the medial and lateral heels. The condition occurs as the result of single heel overload, either acutely or chronically. It may be found as a single entity or in combination with other foot problems.
Underrun Heels is the low heel/long toe configuration is perhaps the most important and common foot abnormality. The configuration is so common that is often thought to be with acceptable limits with many people. It is not a lameness in itself, but rather a major cause of foot problems and contributor to limb problems. The presence of the abnormality is noteworthy, regardless of the other problems. It should be corrected if possible.
If you have any of these problems they need to be addressed by your professional farrier and equine lameness veterinarian.
Next month I will cover Inflammatory, Infectious and Immune Disease of the foot.
Information for this article was taken from: O.R. Adams DVM, MS, "Lameness in Horses, Equine Medicine and Surgery, Patrick Colaham DVM Dipi ACVS, Alfred M. Merritt DVM, MS, James N. Moore, DVM, PhD, Dipi ACVS, I.G. Mayhew, BVSc, PhD, FRCVS, Dipi, ACVIM, ECVN. Manual of Equine Practice, Reuben J. Rose DVSc, DipVetAn, MaCVSc, PhD, FRCVS, David R. Hodgson, BVSc, PhD, FACSM, Diplomate ACVIM.