Muscles and Tendons of the Distal Limb

What are the general features of muscles found in the antebrachium?

• Affect joints of carpus and digits

• Originate epicondyles of humerus

• Muscle belly located in antebrachium

• Tendons of insertion - are distal to carpus

Describe the following features of the extensor carpi radialis muscle:

• Origin

• Insertion

• Function

• Nerve Supply

• Useful landmark

• Extensor carpi radialis muscle

•  O = lateral epicondyle of humerus

•  I = Metacarpal bones

• Location:

• Cranial aspect of antebrachium

• Function:

• Crosses dorsal aspect carpus

• Carpal EXTENSOR

• Nerve supply

• Radial nerve

• Oblique muscle

  • Crosses at level of carpus

  • Useful landmark

Describe the following features of the common digital extensor muscle.

• Origin

• Insertion

• Function

• Nerve Supply

Common digital extensor muscle

• O = lateral epicondyle of humerus

• I = all digits

• distal phalanx - extensor process

• Location:

• Cranio-lateral aspect of antebrachium

• Function:

• Crosses dorsal aspect carpus

  • Carpal EXTENSOR

• Crosses dorsal aspect metacarpo-phalangeal joints & interphalangeal joints

  • Digital EXTENSOR

• Nerve Supply:

• Radial nerve

How does the branching of the common digital extensor muscle differ between species?

• Dogs

  • 4 branches (+1 to the dew claw if present)’

• Horse

  • 1 branch, attaches dorsal aspect of phalanges and protected by joint capsules'

  • Receives branches of the suspensory ligament

Describe the following features of the extensor carpi ulnaris / ulnaris lateralis muscle.

• Origin

• Insertion

• Function

• Nerve Supply

extensor carpi ulnaris / ulnaris lateralis muscle

• O = lateral epicondyle of humerus

• I = Metacarpal bone + ACB

• Location:

• Lateral aspect of antebrachium

• Function:

• Crosses lateral aspect carpus

• Carpal EXTENSOR & FLEXOR

  • Depends on position of limb

• Nerve supply:

• Radial nerve

Describe the following features of the flexor carpi ulnaris muscle. 

• Origin

• Insertion

• Function

• Nerve Supply

flexor carpi ulnaris muscle

• O = medial epicondyle

• O = Olecranon process of ulna

• I = ACB

• Location:

• Caudal aspect of antebrachium

• Function:

• Crosses caudal aspect carpus

• Carpal FLEXOR

• Nerve supply

• Median & Ulnar nerves

Describe the following features of the superficial digital flexor muscle. 

• Origin

• Insertion

• Function

• Nerve Supply

superficial digital flexor muscle

• O = medial epicondyle

• I = all digits (middle phalanx)

• Location:

• Caudal aspect of limb

• Function:

• Crosses palmar aspect carpus

  • Carpal FLEXOR

• Crosses palmar aspect of Metacarpo-phalangeal joints & Proximal interphalangeal joints

  • Digital FLEXOR

• Nerve Supply:

• Median & Ulnar nerves

Describe species differences in the branching of the superficial digital flexor muscle.

• Dog:

• 4 branches (+1 to dew claw)

• Horse:

• 1 branch

• Receives Accessory Check ligament (ACL) from radius

  • Proximal to carpus

  • Limits length of tendon / protects muscle belly from over-stretching

• Both:

• Splits to allow passage of DDFT -

  • Inserts middle phalanx

Describe the following features of the deep digital flexor muscle. 

• Origin

• Insertion

• Function

• Nerve Supply

deep digital flexor muscle

• O = medial epicondyle

• O=radius & ulna

• I = all digits(palmar process distal phalanx)

• Location:

• Caudal aspect of limb

• Function:

• Crosses palmar aspect carpus

  • Carpal FLEXOR

• Crosses palmar aspect Metacarpo-phalangeal joints & both interphalangeal joints

  • Digital FLEXOR

• Nerve Supply:

• Median & Ulnar nerves

Describe species differences in the branching of the deep digital flexor muscle. 

• Dog

• 4 branches (+ 1 to dew claw) *

• Horse:

• 1 branch

• Receives Accessory check ligament

  • Extension of carpal joint capsule

  • Distal to carpus

  • Limits length of tendon / protect muscle belly

• Both:

• Pass through split in SDFT to insert

• Insert distal phalanx

• Horse - runs over distal sesamoid

Summarize the muscles of the distal forelimb, dividing them into the:

• Extensors

  • Origin

  • Location

  • Nerve Supply

• Flexors

  • Origin

  • Location

  • Nerve Supply

What regions of the equine distal limb are the extensor and flexor tendons found?

• Where do they run?

• Carpus:

• Extensor tendons - dorsal aspect

• Flexor tendons - palmar aspect

• Run through carpal canal

What are the boundaries of the carpal canal?

• Boundaries of carpal canal:

• Dorsal - Palmar aspect of carpal joint capsule

• Lateral - ACB

• Palmar - palmar / flexor retinaculum (Compression band or sleeve of fibrous tissue encasing)

What are the contents of the equine carpal canal?

• How about the carnivore carpal canal?

• Contents of carpal canal in equines:

• Deep digital flexor tendon (DDFT)

• Superficial digital flexor tendon (SDFT)

• Blood vessels & nerves

In Carnivores:

• SDFT runs OUTSIDE the carpal canal

• DDFT inside

What are the purpose of tendon sheaths in the equine distal limb.

• What effect can inflammation have in this location?

• Tendon Sheath:

• Fluid filled

• Protect tendons where pass through confined spaces

• Inflammation:

• increased fluid volume / pressure

• bulges proximally & distally

What other supportive structures are found in the distal equine limb?

• Consider dorsal aspect

• And palmar aspect

• Dorsal aspect limb:

• CDE held in place by retinaculum-

• Palmar aspect limb:

• SDFT & DDFT held in place by

  • Carpal canal

• Annular ligaments

• Fetlock & pastern regions

• Length limited by check ligaments

• Accessory Check Ligaments

• Provide palmar support for carpus & all distal limb joints

• Protected by tendon sheaths

Summarize the type of tendons on the:

• Dorsal Aspect

• Palmar Aspect

• Dorsal aspect = extensor tendons

• Palmar aspect =

• Skin

a. SDFT

b. DDFT

c. Check ligament (Fuses with DDFT)

d. Suspensory ligament (Splits into 2 branches)

What are “chestnuts” on the equine limb?

• Chestnuts are vestigial structure found on the medial aspect of the antebrachium, thought to be remnant of 1st metacarpal bone

• Vestigial horn pad

What are “ergots” on the equine limb?

• Lump in skin found on palmar aspect of MCP joint hidden by feathers

• Thought to be remnant of metacarpal pad

What is the purpose of the equine stay apparatus?

A mechanism for passive weight bearing, allowing horses to sleep while standing, stand without effort.

• Most of weight is borne on the forelimb, hindlimb is more fore propulsion

HOW does the stay apparatus work?

• Requirements:

• Maintenance of all joints in weight- bearing extension

• Proximal limb joints:

• Prevention of flexion in shoulder and elbow

• Carpus:

• Prevention of flexion

• Prevention of hyperextension

• Distal limbs joints:

• Prevention of hyperextension

What key muscles are important parts of the equine stay apparatus and what actions or purpose does each have?

Thoracic (Forelimb) Stay Apparatus

Purpose: prevents shoulder/elbow/fetlock collapse

• Serratus ventralis (supports trunk between forelimbs - acting like a sling)

• Biceps brachii + lacertus fibrosus (locks shoulder, preventing flexion, and carpus in extension)

• Collateral Ligaments maintain alignment in the elbow

• Suspensory apparatus of the fetlock: → Prevent fetlock overextension (dropping too low)

  • Suspensory ligament

  • Proximal sesamoid bones

  • Inter, Collateral & Distal sesamoidean ligaments

• Check ligaments (accessory ligaments of DDFT & SDFT)

  • → prevent excessive stretching of the flexor tendons

• Flexor tendons (SDFT, DDFT)

  • SDFT- acts as a major support against fetlock overextension.

  • DDFT - Adds additional support to the distal limb and helps resist flexion/buckling of the coffin joint.

    • Annular ligaments

      • Hold the flexor tendons close to the bones, preventing them from bowstringing.

      • This stabilizes the tendons so they can efficiently counteract the forces that would otherwise hyperextend the fetlock.

How do the carpus and distal limb joints prevent hyperextension in the equine stay apparatus?

• Carpus:

• Palmar fibrocartilage joint reinforcement at level of the carpus

• SDFT and check ligament

• Retinaculum

• MCP, PIP & DIP joints:

  • DDFT & SDFT

    • Check ligaments

    • Annular ligaments

• MCP / Fetlock joint :

• Suspensory ligament

• Common digital extensor

• Proximal sesamoids

• Distal sesamoidean ligaments

  • (short, cruciate, oblique & straight)