04: Forelimb 1

Modifications in cows and horses that makes weight bearing less energetically expensive

• CT has replaced muscle in the distal limb

• There are less bones and less bone mass at the distal limb

Where is most of the weight carried in a horse

Forelimbs

What happens if a rider is not aligned with the horse’s center of gravity

They eat dirt :)

When a horse lands from a jump, how is the force from the torso transferred to the forelimb

Wt goes down → travels up the serratus ventralis and other ventral muscles to the medial scapula → down the leg

New/significant scapular structures in the horse

• Spinal tuber

• Scapular cartilage

New/significant scapular structures in the ox

• Acromion

• Scapular cartilage

Scapular cartilage function

Increases surface area for attachment of serratus ventralis and rhomboideus muscles

New/significant structures of the humerus in large animals

• Cranial part of the greater tubercle

• Caudal part of the greater tubercle

Extra structure of the horse humerus and function

Intermediate tuberosity; stabilizes the biceps tendon

Structure that reduces friction of the proximal biceps tendon in the bicipital/intertubercular groove

Intertubercular/bicipital bursa

Additional significant shoulder muscle in the horse

Subclavius muscle

Where is the subclavius

Just cranial to the supraspinatus

Subclavius function at rest and in movement

• Rest: brings trunk forward

• Movement: brings leg forward; adducts limb

How is the deltoideus different between the horse and ox

In the horse, the two heads of deltoideus are fused. In the ox, they are split by the acromion process of the scapular spine

Non-muscular structure in the horse that works with the serratus ventralis for shock absorption

Dorsoscapular ligament

Muscle that is key for keeping the scapula in place

Infraspinatus

Structure that reduces friction at the distal aspect of the infraspinatus muscle

Infraspinous bursa

Muscle that is key for standing up from recumbency

Triceps muscle

Muscle that lies caudomedial to the triceps

Tensor fascia antebrachii

T/F: the triceps muscle is kept engaged while the horse/ox is standing still

False, that’s too much energy

Triceps innervation

Radial nerve

Clinical signs of proximal radial nerve damage

• Loss of extensor function

• No weight bearing

• Dropped elbows

• Knuckling

• Abn movement

Biceps brachii attachments

• Supraglenoid tubercle (scapula)

• Radial tuberosity (radioulna)

• Metacarpal tuberosity (cannon)

What part of the biceps brachii attaches to the metacarpal tuberosity

Lacertus fibrosus

What muscle does the lacertus fibrosus join as it runs distally

Extensor carpi radialis

Where does the lacertus fibrosus come from

The CT core in the biceps

Lacertus fibrosus function in the stay apparatus

• Limits flexion of the shoulder

• Maintains extension of elbow and carpus

Why does the elbow joint favor extension in large animals

The collateral ligaments are located caudal to the axis of rotation, pulling the joint towards extension

Key muscles of the antebrachium

SDF and DDF

Extra muscle belly in the cow antebrachium that horses have reduced to CT

Pronator teres

What part of the digital flexors is most important

Tendons! We can’t really see the muscle bellies

SDF distal attachment

Distal P1 and proximal P2

SDF distal attachment in the ox

P2

DDF distal attachment

P3

DDF distal attachment in the ox

P3

Which forelimb joint commonly gets hyperextended

Fetlock/metacarpophalangeal joint

Which tendons stretch when the fetlock joint overextends

SDF and DDF

Components that keep the SDF and DDF from tearing under stress

• Elasticity

• Check ligaments

Check ligament function

Transfers some of the force from the muscle belly to the bone structure

Where does the SDF check ligament attach

Distal radius

Where does the DDF check ligament attach

Palmar carpal ligament

Which species has a full complement of carpal bones

Pigs

Carpal bones in the pig

• Top row: radial, intermediate, ulnar, accessory

• Bottom row: I, II, III, IV

Carpal bones in the horse

• Top row: radial, intermediate, ulnar, accessory

• Bottom row: maybe I, II, III, IV

Carpal bones in the cow

• Top row: radial, intermediate, ulnar, accessory

• Bottom row: II + III (fused), IV

How can you tell which side of the carpus is which

Accessory carpal bone is always lateral

List the carpal joints

• Radiocarpal/antebrachiocarpal

• Intercarpal

• Carpometacarpal

Which carpal joint has the highest range of motion

Radiocarpal

Which carpal joint has the lowest range of motion

Carpometacarpal

Which carpal joints communicate with each other

Intercarpal and carpometacarpal joints

Structure that prevents hyperextension of the carpus

Palmar carpal ligament

Reduced carpal pad analog

Chestnut

Reduced metacarpal pad analog

Ergot

What bones make up the cannon bone in the cow

Met III and IV

Where are the other metacarpal bones in the cow

I and II are gone, V is rudimentary but visible

What bones make up the cannon bone in the horse

Met III

Where are the other metacarpal bones in the horse

I and V are gone, II and IV make up the splint bones

Landmarks at the end of the equine splint bones

Buttons (bulges at the distal splint bones)

Anatomic and layman’s terms for the digital bones

• Proximal phalanx/P1 or long pastern bone

• Middle phalanx/P2 or short pastern bone

• Distal phalanx/P3 or coffin/pedal bone

Anatomic and layman’s terms for the digital joints

• Metacarpophalangeal or fetlock joint

• Proximal interphalangeal or pastern joint

• Distal interphalangeal or coffin joint

Where are the hoof/ungual cartilages

Medial and lateral to the coffin bone, above the hoof capsule

Age related problems that can arise with the hoof cartilages

Ossification and overgrowth, AKA “sidebone”

Which digital joint has the highest range of motion (and rate of injury)

Fetlock/metacarpophalangeal joint

Why can’t we visualize the coffin joint

It is withing the hoof capsule

Where do we find sesamoid bones in the horse and cow

• 2 proximal sesamoid bones at the distal palmar aspect of the cannon bone

• 1 distal sesamoid bone between palmar P2 and the coffin bone

Sesamoid bone function

Helps redistribute force for shock absorption and joint stability

Equine term for the distal sesamoid bone

Navicular bone

Dorsal pouch of fetlock joint function

Reduces friction between the common digital extensor and the fetlock joint (P1 and P2)

Dorsal pouch of pastern joint function

Reduces friction between the common digital extensor and the pastern joint (P2 and p3)

Navicular bursa function

Reduces friction between the DDF and navicular bone

What other space is the navicular bursa connected to

None, it is isolated

Ligaments attached to the navicular bone

• Collateral ligaments (medial and lateral)

• Impar/distal navicular ligament

Collateral navicular ligament attachment

P1 → navicular bone

Distal navicular ligament attachment

Navicular bone → P3

Extra shock absorbing structure that supports the navicular and coffin bones

Digital cushion

Extra pedal structures in the cow

• Dewclaws: rudimentary digits II and V attached by ligaments

• Interdigital ligaments

Functional group associated with the suprascapular nerve

Lateral stabilizers of the shoulder

Functional group associated with the subscapular nerve

Medial stabilizers of the shoulder

Functional group associated with the musculocutaneous nerve

Flexors of the elbow

Functional group associated with the axillary nerve

Flexors of the shoulder

Functional group associated with the radial nerve

Extensors of the elbow, carpus, and digits

Functional group associated with the median nerve

Flexors of the carpus and digits

Functional group associated with the ulnar nerve

Flexors of the carpus and digits