What are the three layers of epidermis that make up the horny covering of the equine hoof?
The hard, horny outer covering of the hoof is non-innervated epidermis from epidermal cells. It is made up of three layers:
Stratum externum - includes the unpigmented periople (the intermediate horn in terms of hardness)
Stratum medium - tubular horn; this grows from the epithelial germ layer that covers the corium (the epithelium doesn’t grow corium, which comes from dermal germ layer)
Stratum internum - unpigmented laminae
What lies flush to the non-innervated wall, toe, periople, etc. -- the horny covering -- of the hoof? What does it derive from and how is it organised to "interact" with the hard, epidermal layer? Ie., how is it "connected" to the dermal layer?
The corium lies flush against the innermost stratum internum of the horny hoof covering.
It is derived from dermis, as it is a dermal layer that is highly innervated and vascularized.
The meeting of the epidermal and dermal layers occurs at the primary & secondary epidermal laminae and the primary & secondary dermal laminae. These points of contact provide much surface area for tissue contact for blood vessels and nerves. Parallel tissue fibres characterise the histological appearance of the laminae.
How does the horny hoof grow? Ie., from what layer does it grow? And what is its rate of growth?
It grows from the epithelial layer closest to the coronary corium, ie., the stratum internum. It grows outward to form the stratum medium, at a rate of 1 cm/month, which is why foot-trimming is required.
What are the different regions of the corium?
Since it literally "fleshes out" the horny hoof from the inside, it is named for the same regions on the outside. Ie., There is the periople corium, coronary corium, the lamellar corium, which is deep to the hoof wall, the solar corium and frog corium, etc.
What are the structural specialisations of the equine limb for weight-bearing and locomotion? Start proximally.
1. Origins of the hamstring muscles
2. Dorsal & palmar joint-cavity recesses
3. Navicular bone & bursa
4. Fat-filled digital cushion
5. Heavily keratinized hoof (epidermal outer layer)
6. Lateral cartilages
How do the origins of the hamstring muscles -- the biceps femoris, semitendinosus & semimembrinosus - facilitate specialised weight-bearing and locomotion abilities in the horse limb?
The origins on the caudal vertebrae, along with origins on the ischial tuberosity of the pelvis, give the horse’s hamstrings extra length and force for propulsion.
How do the dorsal and palmar recesses of the equine distal-limb joints -- the distal metacarpophalangeal joint (fetlock joint), the proximal interphalangeal joint (PIP, or pastern joint) and the distal interphalangeal joint (DIP, or coffin) -- act as specialised structures for load-bearing and locomotion?
Synovial-fluid-lubricated recesses facilitate greater movement by providing more articular surface and absorption of force distally.
Since there's greater vascularity in synovial membrane, more synovial fluid tends to be produced if there's chronic inflammation, so this can become a clue eg., distended flexor sheaths or distended dorsal recesses.
How do the navicular bone (the distal sesamoid) and the navicular bursa (aka podotrochlear bursa) act as specialised structures of equine weight-bearing and locomotion?
Absorb ground-force to aid in both locomotion & weight-bearing. Like the hard hoof wall, they are both anti- concussive.
How does the heavily keratinized hoof act as a specialised structure of equine weight-bearing and locomotion?
Wall is anti-concussive & protective, non-innervated, non-sensitive, to better provide protection and ground-force absorption.
Where is the fat-filled digital cushion and how does it contribute to weight-bearing and locomotion of the equine limb?
The digital cushion is situated beneath the rear part of the sole. It separates the frog and the bulb from underlying tendons, joints and bones, providing cushioning protection.
In foals and yearlings, the digital cushion is composed of fibro-fatty, soft tissue. In the adult horse, it hardens into a fibro-cartilagineous tissue when sufficient, consistent concussion stimulates the back of the hoof. Normal transformation of the digital cushion into fibrocartilagineous tissue is now considered a key goal, both for prevention of, and for rehabilitation of recovering cases of navicular syndrome.
What are the lateral cartilages and what contribution do they make as specialised structures of weight-bearing and locomotion in the equine limb?
The lateral cartilages are located both above and below the coronary band, extending around the front, the sides and back of the hoof.
Below the coronary band they extend out over the digital cushion and attach to the back of the pedal bone. The horn producing corium of the inner hoof wall attaches to the lateral cartilages at the back of the hoof where the pedal bone does not reach.
They keep lateral movement of hoof aligned but still allow this flexibility to absorb ground force and facilitate locomotion.
These cartilages provide resistance as the pedal bone descends during weight bearing, regulating the amount of pressure applied to the coriums. They also help to suspend the pedal bone in the correct position as well as acting as a spring, storing and releasing energy during locomotion.
Describe the microvasculature of the equine hoof and how changes in perfusion of this vasculature can lead to laminitis.
Make sure to mention:
1. blood supply to the equine foot
2. unique anastomoses
3. impact on the laminae
Blood supply to the equine foot / hoof comes from the from lateral & medial palmar digital arteries, the terminal ends of which enter canals in the distal phalanx.
The microvasculature is characterised by arteriovenous anastomoses in the dermal laminae in the corium of the hoof, which carry blood to & from the laminae tissues of the distal phalanx, including the epidermal cells that form the hoof wall.
These “AV” anastomoses are usually closed, but certain pathologies can cause these “shunts” to open, changing the arterial pressure such that high-pressure blood from the arteries is diverted directly into the veins, bypassing the capillaries that supply the most-distal tissues of the hoof, ie., the epithelia of the hoof wall and the dermal laminae of the corium.
The opening of the AV anastomoses and the change in perfusion to the tissues can result in ischaemia to both the epidermal and dermal laminae, causing separation of the (epidermal) hoof wall. In the most extreme cases in which the epidermal and dermal tissues are separated and move independently, the horse suffers laminitis.
What is the nervous supply to the equine distal limb and how does this differ from other species?
Innervation to the distal limb comes from the medial digital nerve (from median nerve) and the lateral digital nerve (from median & ulnar nerves).
These branches of the medial and ulnar nerves supply the whole of the single digit (dorsal/cranial and palmar sides) - this is unusual among species, eg., in humans, carnivores and ruminants, usually the dorsal side of most species are supplied by the distal distribution of the RADIAL NERVE and the palmar side, like the horse, is innervated by media and lateral digital nerves.
Ie. The radial nerve not involved in innervating the distal phalanx of the horse because it only extends to the carpus.
What are the some of the pathologies of the distal limb?
Navicular disease - inflammation of navicular bursa or area around navicular sesamoid bone; this bone is key to absorbing ground forces
Fracture - rarer
“Side bone” - lateral cartilages of distal phalanx become ossified so lateral movement of hoof becomes limited/painful/impossible.
What is the first of the four standard diagnostic nerve blocks? Where do you do it and what does it anaesthatise?
Start most distally, just above the hoof:
Lower palmar digital block aka Navicular Bursa nerve block - this anaesthatises the navicular bursa and the distal phalanx.
Palpate the lateral cartilage and insert the needle distally, along the edge of the deep digital flexor tendon. Follow dorsal-to-caudal VAN. See illustration and point "c".
What is the second of the four standard diagnostic nerve blocks after blocking the navicular bursa? Where do you do it and what does it anaesthetise?
The mid-long-pastern palmar digital block - this basically anaesthetises all the structures within the hoof (except for the lamellar corium), such as the navicular bursa, the distal phalanx & the distal interphalangeal joint (coffin). It also blocks the proximal interphalangeal joint (PIP, or pastern joint).
The area is midway between the metacarpophalangeal joint and the coronary band. The needle is inserted dorsal to the tightened ligament of the ergot. See illustration and point "a"
The ergot is a small mass of horny material on the palmar surface of the fetlock joint (the metacarpophalangeal joint).
What is the third nerve block, if you want to block the proximal interphalangeal joint and the distal sesamoidean ligaments?
Abaxial sesamoid block: Palpate the abaxial surfaces of the proximal sesamoids and insert the needles there. That's where the palmar digital nerves are after the vein and arteries. See "d" in illustration.
What is the fourth, and most proximal nerve block? What does it anaesthetize and how do you do it?
Low-palmar four-point block - it blocks the metacarphalangeal joint and proximal sesamoids.
Palpate the buttons of the splint bones - the palmar metacarpal nerves emerge there and are blocked just below the buttons. See "b" in illustration.
Which nerve innervates the horse forelimb distal to the carpus?
Palmar aspect: median (medially) and ulnar nerve (laterally)
Dorso-medial metacarpal region: musculocutaneous n
Dorso-lateral metacarpal region: ulnar nerve
The crus (the hoof) is innervated medially by the medial palmar digital nerve that comes off the median nerve and laterally by the lateral palmar digital nerve that comes off the ulnar nerve.
What is the innervation to the distal hindlimb?
The tibial nerve divides into lateral and medial, which become the lateral and medial plantar digital nerves more distally.