The Horses Foot and Distal Limb

Horse

Structures of the distal limb

• horse (unguligrade) walks on tip of distal phalanx

• lack of muscle tissue below carpus (long tendons)

• hoof contracts and expands with loads imposed

  • dynamic NOT STATIC strucuture

elongated 3rd metacarpal (with proximal sesamoid bones → Proximal phalanx → middle phalanx → distal phalanx (sits in the hoof capsule)

• note extensor process of distal phalanx

• navicular (distal sesamoid bone) - palmar/plantar aspect + sits inbtw DP and MP

• DDFT inserts on the distal phalanx

cranial to caudal

navicular bone → DDFT → digital cushion

Distal phalanx (pedal bone)

• Mirrors shape of hoof

• Bound to hoof wall via lamellae

• Collateral cartilages

  • projections of distal phalanx

  • Shock absorptive, blood flow role

  • Calcify with age in larger heavier breeds → fracture

Synovial structures

• Large joint spaces with dorsal and palmar recesses

• Very large dorsal + palmar recesses

  • Easy access for synovial fluid sampling or injection

  • Inject corticosteroids → arthritis treatment

• Located at distal interphalangeal joints

Digital cushion

• Role: shock absorption and circulation

  • Hydrostatic cushion- minimises pressure changes in navicular apparatus

    • Complex loading env: Compressed by middle phalanx and DDFT (via collateral ligaments)

  • Monitors load in navicular region via pain and pressure receptors

• Tissue: myxoid tissue + fibrocartilage

  • collagen

  • elastic tissue

  • fat

• Extends under distal sesamoid bone

• Provides support for navicular bone and palmar pedal bone

Navicular bursa

Plus navicular ligaments

• Visible when DDFT reflected

• Filled with synovial fluid

• Navicular bone has fibrocartilage where it meets the bursa

  • hyaline cartilage when meeting phalanx

• navicular suspensory ligament (collateral sesamoidean ligament)

  • suspends navicular bone

  • attaches proximal border to distal second/middle phalanx

• distal navicular ligament

  (distal sesamoidean ligament)

  • suspends navicular bone

  • attaches distal border to distal/third phalanx

Navicular syndrome/disease (palmar foot pain)

• Common cause of lameness

• Involves navicular bone + surrounding structures

  • Lesions formed in navicular bursa → friction with DDFT → pain

  • No reversal just management

shoeing horse advantages

• protection against penetration of sharp objects

• less likely to slip on wet/muddy surfaces

• feet unlikely to wear unevenly

shoeing horse disadvantages

• farriers regulated by farriers regulation council

• expensive

• potential to increase lameness

  • entirely dependent on horse activity

• unnatural → barefoot has greater contact area with ground

  • shoe bears most of weight peripheral through hoof wall

  • nails placed into white line → between sensitive lamellae + dermis and insensitive hoof wall

  • nail ends are bent outwards to side of hoof → secures nails in place

Barefoot hoof care

• Bearing weight through solar surface and hoof wall

• Correct diet and environment important for good hoof growth

• Hoof boots → provide protection (e.g. stony ground)

Effect of barefoot trimming V shoeing

Studies have shown:

• Shoeing changes hoof shape

• Shoeing decreases digital cushion thickness and increases stride length

• Shoeing decreases hoof temp

• Shoeing improves lameness scores

Epidermal and dermal structures

• Hoof is specialised skin

  • Epidermis (outer → inner)

    • Stratum externum

    • Stratum medium

    • Stratum internum (interdigitates with dermis)

  • Dermis (corium) [inner vascularised and innervated]

    • Peri-oplic corium (above the coronary band)

    • Coronary corium

    • Lamellar corium (majority)

    • Solar corium

Epidermal/Primary lamellae

• Projections from dermis and epidermis

• Primary lamellae- interdigitate

  • provides high surface area to bind dermis and epidermis

Dermal/Secondary Lamellae

Cross-section

hoof wall (stratum internum of epidermis)→ lamellae → dermis → distal phalanx

• Each primary lamellae have secondary lamellae

• Highly vascularised

• Surface friction prevents lamellae pulling apart

Hoof wall growth

• Growth to replace hoof lost to wear

• Regeneration occurs at coronary band

• Germinal cells in epidermis → daughter cells → mature + keratinise, add to proximal (deepest) hoof wall

• More proliferative cells in proximal V distal lamellae

• New hoof wall moves past stationary distal phalanx via remodeeling in epidermal lamellae

Horse foot blood supply (inside)

• Extensive blood supply: palmar digital arteries

• Drainage: lateral & medial digital veins

Horse blood supply (outside)

Hoof wall → epidermis → lamellae [axial veins and arteries] → sublamellar dermis

[sublamellar venous plexus + parietal branches of palmar/plantar digital arteries]

• sublamellar venus plexus drains dermis/corium

• supplies by parietal braches of terminal arch of palmar/plantar digital arteries

• Axial veins and arteries are within the primary dermal lamellae ABOVE the sublamellar dermus

Dermal microcirculation

• Capillary beds in 2ndary dermal lamellae

• Arteriovenous anastomoses (AVA) regulate blood supply

  • Directly connect arteries and veins

  • Normally closed → forces blood into capillary network

  • open→ bypasses capillary network

Laminitis- loss of lamellar integrity

• Stretching of white line

• Sole prolapse

• Mostly front limb condition

• Rocked back position → shift limbs from front to hind limb

• Recognition

  • Heat within hoof wall

  • Strong digital pulse

  • Change in behaviour

    • reluctance to turn

    • reluctance to walk on hard ground

• Chronic laminitis

  • Divergent lines on hoof wall

  • Wider in quarters (proximally) vs toe

Laminitis causes

• Underlying endocrine condition + high sugar intake (lush grazing → bouts of laminitis)

  • Equine metabolic syndrome

  • Pituitary Pars Intermedia disorder

    • degeneration of dopmine producing neurones in pars intermedia

    • Less inhibition of ACTH

    • leads to excessive cortisol production → hyperadrenocorticism

• Sepsis associated (systemic infection)

• Supporting limb laminitis

  • Overuse of healthy limb to compensate of an injured limb for weight bearing (overwork)

Laminitis histopathology

• Lesions form at tips of primary (epidermal lamellae)

• Loss of secondary lamellae OR secondary lamellae may become longer

  • Primary lamellae pulled apart

• Underlying mechanisms unclear- endothelial cell dysfunction?

• lesion formation at tip of the epidermal lamellae

• pulling apart of primary lamellae due to secondary elongation

Complete epidermal and dermal lamellar separation when lamenitis progresses

Lack of connection btw hoof and underlying dermis

Pedal bone rotation and solar penetration

1. DDFT pulls on pedal bone

2. Pedal bone pulled away from hoof wall

3. Pedal bone penetrates sole

4. Can be reversed by careful trimming and management (but takes long time)

Diagnostic nerve blocks

• Localises site with pain → lameness

• Sequential local anaeshesia of sensory nerves from distal → proximal (upwards)

• Nerves part of neurovascular bundle

  • palmar to vein and artery

  • dorsal → palmar/plantar = VAN

    • surface → underside

Hoof

- lateromedial section through middle

 and distal phalanx

• Ventral to the synovium of dorsal recess of interphalangeal joint = middle phalanx

• coronary dermis/band (coronet) → proximal ring around the hood where stratum medium and stratum internum mmet

4 distinct targets of nerve blocks

(1,2,3,4a, 4b)

• block 1 first (work distally → proximally)

• nerve block both medial and lateral → communicating branch

1. Low palmar/plantar digital block → navicular bursa and distal phalanx

2. Midpastern palmar/plantar digital block → DIP joint + all deep structures EXCEPT lamellar corium

3. Abaxial sesamoid block → PIP joint, distal sesamoidean ligaments, lamellar corium

4. Low palmar/plantar block → MCP/MTP joints and proximal sesamoids

   • 4a → distal ends of both splint bones

   • 4b → distal to communicating branch

DP → DIP → PIP → MCP/MTP

Low digital → midpastern → abaxial sesamoid → low palmar/plantar

Forelimb mid fetlock

(dorsal to palmar)

Common + lateral extensor tendon

lateral joins common above the fetlock

Just before fetlock

• sdft wraps around ddft forming flexor manica/manica flexoria → (bifurcated sdft)

• oblique ligaments adjacent and caudal to straight [distal sesamoidean] ligament

• straight ligament deep to the ddft

• cruciate sesamoid ligamants btw oblique ligaments deep to straight → resting on the palmar bone surface

Common digital extensor tendon (lateral already inserted) → cannon bone 

→ paired proximal sesamoid bones → intersesamoid ligament → 

 cruciate sesamoid ligaments (cross-shaped) 

→ 2 oblique distal sesamoidean ligaments 

→ straight distal sesamoidean ligament 

→ DDFT → SDFT (@ level of fetlock it has bifurcated and surrounds DDFT)

Mid metacarpal

(from dorsal to palmar)

• before fetlock

Lateral & common digital extensor tendon → cannon bone → splint bones 

→ suspensory ligament

→ Accessory/inferior check ligament → DDFT → SDFT

check ligament = accessory/branch of ddft

• plantar + proximal to suspensory ligament

• fuses with ddft before origin of the suspensory ligament
  

Above the MCP

deep → superficial

• check/accessory → ddft → sdft

Below MCP but proximal metacarpal (add suspensory)

• suspensory → check/accessory → ddft → sdft

What wraps around the SDFT and DDFT

Palmar annular ligament wraps over SDFT and DDFT superficially and mediolaterally

• Intersesamoid ligament continuous but deep to S/DDFT = straight distal sesamoidean ligament = straight ligament

Hindlimb - dorsal to plantar

Mid Fetlock

(lateral fuses with the long digital extensor tensor)

Long digital extensor tendon → cannon bone → paired proximal sesamoid bones → intersesamoid ligament

cruciate sesamoid ligaments (cross-shaped) → 2 oblique distal sesamoidean ligaments → straight distal sesamoid ligament → DDFT → SDFT

Mid-metatsal - dorsal to plantar

Lateral & common digital extensor tendon → cannon bone

 → splint bones → suspensory ligament

→ Accessory/inferior check ligament → DDFT → SDFT

Forelimb

Before fetlock:

lateral & common digital extensor tendons

After fetlock

fuses } common digital extensor tendon

(PTO for hindlimb)

HINDLIMB

Before fetlock:

lateral and LONG digital extensor tendons

After fetlock

fuses } LONG digital extensor tendon

Forelimb vs hindlimb - metacarpal/tarsal differentiation

PART 1

Solar surface of hoof

• central sulcus (of the frog)

• collateral sulcus

• heel bulb

• bars

• white line

• hoof wall

Other key hoof terms

• coronary band/corium

• digital cushion

• lamellar and solar dermis (corium)

More important external hoof structures