Lecture 9 -- Equine Forelimb Anatomy

What is another name for proximal interphalangeal joint?

Pastern joint

Where do the collateral ligaments of the proximal Interphalangeal Joint run?

Collateral ligament x2 pairs
1. Runs from the Proximal Phalanx (P1) to the Middle Phalanx (P2)
2. Proximal Phalanx (P1) to navicular bone

What structures make up the Distal Interphalangeal Joint (DIP)?

Middle Phalanx (MP), Distal Phalanx (DP) and navicular bone

What is another name for distal interphalangeal joint?

Coffin joint

What are the ligaments of distal sesamoid?

• Collateral ligaments

  • Middle phalanx to Distal phalanx

• Impar ligaments

  • Navicular bone to Distal phalanx

Where is the Navicular / podotrochlear bursa located?

Between the Navicular and Deep Digital Flexor Tendon (DDFT)

Different from dogs and cats, what ligament is absent in horses in their distal interphalangeal joint?

No dorsal elastic ligament in horses

What bones are shown in the picture?

Lateral / Ungual Cartilage

Where are the ungual cartilages located? What are the function of ungual cartilages?

Located at the lateral + medial aspect of the palmar processes of distal phalanx

Function: Shock absorption

Are ungual cartilages able to be visualise in radiograph?

Normally not → Cartilage not visible (Slightly darker than bone) → BUT Mineralise as age (Able to identify in radiograph) = Sidebone

Where does the Common digital extensor (CDE) insert?

Crosses the dorsal aspect of the carpus and inserts on PP, MP & DP

Different from dogs, where does the Lateral digital extensor (LDE) insert?

Inserts on the Proximal Phalanx (P1)

P.S. In dogs, lateral digital extensor inserts into 4th and 5th digits

How does the function of the extensor carpi ulnaris differ between horses and dogs?

Horses:

• Acts as flexion of carpus joint

Dogs

• Acts as extension of carpus joint

How does the flexor carpi ulnaris tendon in horses differ from dogs?

Horses:

• Lie on top of the superficial digital flexor tendon (SDFT) on the caudal aspect of the forelimb

Dogs:

• Beside the extensor carpi ulnaris

How do the deep digital flexor tendon and the superficial digital flexor tendon interact?

In horses:

• SDFT + DDFT pass through carpal tunnel → SDFT splits to allow the DDFT pass through it → SDFT inserts into MP, while DDFT inserts into DP

In dogs:

• SDFT and DDFT runs parallel over the phalanges

What are the accessory check ligaments?

• Superior check ligament

  • Locate above the carpus

  • Originates proximal to carpal canal

  • Fuses with SDFT

• Inferior check ligament

  • Locate below the carpus

  • Originates from fibrocartilage plate on palmar aspect of carpus

  • Fuses with DDFT

If we perform an ultrasound examination of the proximal metacarpus in horses, which structures can we visualize?

If we perform an ultrasound examination of the distal end of metacarpus in horses, which structures can we visualize?

NO inferior check ligament

What is scutum? Where is the intersesamoidean ligament located in relation to the scutum?

• Cartilage shield

• Located on top of the intersesamodiean ligament, which sits between the proximal sesamoid bones

• Allow smooth passage for DDFT and SDFT

What structure holds the Deep Digital Flexor Tendon (DDFT) and Superficial Digital Flexor Tendon (SDFT) against the bones in the digital region?

Annular ligaments

List out all the annular ligaments in digital region

1. Proximal annular ligament

   • Attach around the metacarpo-phalangeal joint

2. Proximal phalanx annular ligament

   • Attach at the proximal phalanx

3. Distal annular ligament

   • Attach around the distal interphalangeal joint

What structure is shared by the SDFT and DDFT and extends distally from the annular ligament?

Tendon sheath

Order of fetlock structures from palmar/plantar to dorsal:

Intersesamoidean ligament → Scutum → DDFT → SDFT → Annular ligament → Tendon sheath

What is the stay apparatus?

Mechanism for passive weight bearing → Horses can stand for extended period of time without using energy

Which structure cooperates in maintaining extension and preventing flexion in the stay apparatus?

Shoulder:

• Biceps brachii tendon

Elbow:

• Collateral ligament (between humerus and radius and ulna) → Support the attachment of radius and humerus

Carpus:

• Lacertus fibrosus

  • Connect the biceps to the extensor carpi radialis

  • If one joint is extended, the other joint atomically become extended e.g. shoulder joint is extended, it pulls the carpal joint to be extended

Which structure works to prevent hyperextension of the forelimbs in the stay apparatus?

Carpus

• SDFT

• Check ligament

• Palmar fibrocartilage joint reinforcement

• Retinaculum

MCP joint:

• Suspensory ligament

• Proximal sesamoids

• Distal sesamoidean ligament

• Common digital extensor

MCP, PIP and DIP joint:

• DDFT, SDFT

• Check ligament

• Annular ligaments

Describe the arterial supply in horses.

Similar to dog

Subclavian artery → Axillary artery → Brachial artery → Median artery

Is the nerve supply of horses similar to dogs?

Same beaches as dog

Same motor function as dog

BUT different sensory areas

The brachial plexus in horses has the same motor functions but supplies different sensory areas. Describe which regions are innervated by which nerves.

Musculocutaneous nerve: Medial aspect of antebrachium

Axillary nerve: Cranial aspect of the shoulder region and lateral aspect of elbow joint

Medial and ulnar nerve: Palmar aspect of carpus, metacarpus and digits

Radial: Lateral aspect of elbow joint + a bit of antebrachial joint

Median and ulnar nerves combine with musculocutaneous nerve → Supply the pastern and foot + medial aspect of metacarpal

Which two major branches that the median and ulnar nerves that branches off? What are their functions?

Medial and ulnar nerves combines then branches off as …

1. Palmar metacarpal nerve (Yellow)

   • Paired (Medial and lateral)

   • Runs between MC3 and splint bones (MC2 + MC4)

   • Emerges at the distal end of splint bone

   • Supply the dorsal aspect of P1

2. Palmar nerves

   • Paired (Medial and lateral)

   • Runs between suspensory ligament and SDFT/DDFT → Cross abaxial aspect of the two proximal sesamoids → Become palmar digital nerves → Divides into dorsal and palmar branches of palmar digital nerves → Supplies hoof components

What structures does the palmar nerve usually accompany?

The palmar nerve usually runs with the palmar artery and palmar vein, forming the neurovascular bundle

List the three major types of nerve blocks of the hoof.

1. Palmar metacarpal nerve block = 4 point block

2. Abaxial sesamoid nerve block

3. Palmar digital nerve block

Describe the palmar digital nerve block in detail.

• Aim to block the palmar branch of the palmar digital nerves

  • = Caudal ½ foot

• Locations:

  • 1cm above coronary band

• 2 points of injections (Medial and lateral injection)

  • Medial palmar branch of palmar digital nerve

  • Lateral palmar branch of palmar digital nerve

• Check for function of distal phalanx, navicular bone and distal sesamoid

Describe the abaxial sesamoid nerve block in detail.

• Aim to block the palmar digital nerve before it branching off dorsal and palmar branches

  • = Block the entire foot (Includes the dorsal branches of the palmar digital arteries)

• 2 points of injections

  • Medial and lateral palmar digital nerves

• Location:

  • Right beside the abaxial aspect of sesamoids = Around the fetlock joint

Describe the palmar metacarpal nerve block = 4 point nerve block in detail.

• Aim to block and palmar metacarpal nerve and the palmar digital nerve

  • = Block the entire digit (P1, P2 and P3)

• 4 points of injections

  • Medial palmar metacarpal nerve

  • Lateral palmar metacarpal nerve

  • Medial palmar nerve

  • Lateral palmar nerve

• Location:

  • Distal end of the two splint bones (Named button)