Equine - Anatomy & Physiology
Proximal Phalanx
The proximal phalanx is shaped like an hourglass and is wider proximally than distally. Proximally, it has two shallow articular surfaces separated by a small sagittal groove; the medial cavity is larger than the lateral cavity. The saggital groove accepts the saggital ridge of the distal third metacarpal (cannon) bone. Distally there are two convex areas separated by a sagittal groove to accept the proximal articulation of the middle phalanx. The proximal phalanx is approximately twice the length of the middle phalanx.
Middle Phalanx
The middle phalanx is half the length of the proximal phalanx, its proximal articular surface is ridged so it can articulate with proximal phalanx and the distal end resembles that of the proximal phalanx.
Distal Phalanx
The distal phalanx is rounded to a point distally; articulating with another bone only at the proximal end. It does not have a cortex or medullary cavity, but has three surfaces: articular surface, parietal surface and solar surface.
- The proximal articular surface has two concavities separated by a ridge. The palmar border meets with the distal sesamoid (navicular) bone. The dorsal aspect has a marked extensor process, where the common digital extensor attaches.
- The solar surface has two distinct areas, the more palmar area is the roughened flexor surface and the remainder is smooth, concave and crescent-shaped. There is a solar groove on each side of the flexor surface, which leads to the solar canal. The palmar aspect extends either side to form the medial and lateral palmar processes, which each have a foramen. The juction between the solar surface and the parietal surface forms the solar border. There is a notch in the dorsal solar border, known as the crena marginis solearis.
- The parietal surface of the distal phalanx is that which conforms to the hoof wall. It is convex, rough, porous and has processes on each side heading in a palmar direction. There are many foramina and grooves on this surface for vasculature and nerves to pass. The ungual (collateral) cartilages attach to these processes, they are squarish plates which are palpable above the level of the hoof. They become more fibrous in life and may ossify.
The distal sesamoid in horses is known as the navicular bone. It is elongated transversely and articulates with both the distal and middle phalanx, lying palmar to the distal interphalangeal joint. Dorsally, the articular surface is covered by hyaline cartilage. It articulates with the palmar aspect of the middle phalanx. The palmar flexor surface is characterised by a prominent sagittal ridge and is covered by fibrocartilage; providing a smooth surface for the deep digital flexor tendon to glide during weightbearing. The distal border contains a small articular facet of hyaline cartilage for articulation with the distal phalanx. The distal border contains numerous, synovium-lined, nutrient foraminae.
Ligamentous Support
Three ligaments support the navicular bone:
- Paired collateral sesamoidean ligaments: Originate from depressions on either side of the distal aspect of the proximal phalanx. They extend in a palmar direction to insert on the extremities and proximal border of the navicular bone; thereby acting to suspend the navicular bone.
- A branch from each collateral sesamoidean ligament originates at the palmar process (angle) of the distal phalanx (pedal bone)and inserts on the axial surface of the hoof cartilage.
- The distal sesamoidean (impar) ligament originates from the distal margin of the navicular bone and deep digital flexor tendon. It extends from the navicular bone proximally for 1.0-1.5cm and distally to the insertion of the deep digital flexor tendon on the distal phalanx (pedal bone).
The navicular bone is separated from the deep digital flexor tendon by the navicular bursa. There is no communication between the navicular bursa and the distal interphalangeal (coffin) joint.
Innervation
Sensory innervation to the navicular bone is supplied by the digital nerves. Nerves run distally through the collateral sesamoidean ligaments and within the distal sesamoidean (impar) ligament. Sensory innervation to the navicular bursa is also via digital nerves.
Vasculature
Arterial supply to the navicular bone is via anastomoses between medial and lateral palmar digital arteries. Proximally, a transverse plexus joins these arteries and gives rise to several small arteries that enter the formania of the navicular bone along its proximal border. Distally, branches connecting the medial and lateral palmar digital arteries form a distal navicular plexus; which give off additional small arteries that enter foramina along the distal border. These digital arteries supply most of the blood to the navicular bone. Anastamoses between the proximal and distal blood supplies occurs in the adult horse.
Venous drainage occurs via medial and lateral palmar digital veins.
References
- Konig, H.E., Liebich, H.G. Veterinary Anatomy of Domestic Mammals: Textbook and Colour Atlas (2009), Schattauer
- Pasquini, C, Spurgeon, T.L., An Anatomy of Domestic Animals: A Systemic and Regional Approach, 10th Edition (2003), Bowker
- Parks, A., Form and Function of the Equine Digit, Veterinary Clinics of North America: Equine Practice 19 (2003) 285-307