The Anatomy of the Fetlock Joint

The fetlock joint is a high motion, high load hinge joint that plays a major role in shock absorption, propulsion, and energy storage during locomotion. In this Anatomy of the Fetlock Joint guide, we take a deep dive into the structures that make the joint work, and introduce the key terminology you may hear during veterinary workups and in more advanced literature.

In veterinary manuals, the fetlock joints are typically described as diarthrodial, synovial, hinge type joints. In plain language, that means they have limited twisting and side to side movement, but a very large range of motion in the sagittal plane, mainly flexion and extension.

Here is what that looks like in practice:

Both fetlock joints are freely movable joints that contain lubricating synovial fluid, and they function much like a door hinge.They are designed to move mostly back and forth, bending and straightening with each stride, while allowing only small amounts of axial rotation and lateral motion.

Diarthrodial = freely movable. These joints are not fixed like the skull bones and not slightly movable like some spine joints: they move a lot.

• Synovial = filled with lubricating fluid. These joints have a capsule filled with slippery fluid that reduces friction and allows smooth movement.

• Hinge-type joints = move like a door hinge. They mainly bend and straighten.

• Limited axial rotation = very little twisting. They can rotate only slightly around their long axis.

• Limited lateral movement = very little side-to-side motion. They are not designed to swing outward or inward much.

• Very large sagittal-plane motion (flexion/extension) = lots of bending and straightening. The sagittal plane divides the body into left and right halves. Movement in this plane means:

  • Flexion = bending (like bending your elbow or knee)

  • Extension = straightening

Want to learn more about joint anatomy and terminology? Download the Joint Anatomy Flashcards

Anatomical Terminology

In more advanced texts, the fetlock joints may be referred as:

• Forelimb: Metacarpophalangeal joint (MCP)

• Hindlimb: Metatarsophalangeal joint (MTP) 

Main Bones That Form the Fetlock Joint

Forelimb (Front Leg)

The fetlock joint in the front leg is made up of:

• The bottom end of the cannon bone (distal third metacarpal bone or MCIII)

• The top of the long pastern bone (proximal phalanx, P1, long pastern)

• Two small bones at the back of the joint called the sesamoid bones (medial and lateral, one on the inside, one on the outside)

  
  

Hindlimb (Back Leg)

The fetlock joint in the back leg is made up of:

• The bottom end of the hind cannon bone (distal third metacarpal bone or MCIII)

• The top of the long pastern bone (proximal phalanx, P1, long pastern)

• Two small sesamoid bones at the back of the joint (medial and lateral, one on the inside, one on the outside)

Study the bones of the horse with:

Interactive Horse Skeleton

Bones of the Horse Falshcards

Important Point About Front vs. Hind Fetlocks

The overall shape and structure of the fetlock joint are almost the same in the front and back legs.

However:

• The hind fetlocks are usually a little narrower from side to side

• The hind fetlocks also experience more twisting forces because the hind legs provide most of the horse’s propulsion (push-off power)

 

Joint Surfaces of the Fetlock (Articular Surfaces)

These are the smooth areas where the bones meet and move against each other.

Bottom of the Cannon Bone

(Distal MCIII in the front leg / Distal MTIII in the hind leg)

• There is a raised ridge running down the center (sagittal ridge).

• On either side of that ridge are two rounded areas (medial and lateral condyles).

• The surface is covered in smooth, cushioning cartilage (thick hyaline articular cartilage), especially on the front side of the joint (dorsally).

This smooth cartilage helps the joint move easily and absorb impact.

Top of the Long Pastern Bone

(Proximal Phalanx / P1)

• The top surface has two shallow cup-like areas (concave articular facets).

• A groove in the center fits around the ridge of the cannon bone (central groove accommodating the sagittal ridge).

This design helps keep the joint aligned and stable during movement.

The Two Sesamoid Bones

(Proximal Sesamoid Bones)

These small bones sit at the back of the fetlock joint.

They connect with:

• The back side of the cannon bone (palmar aspect in the forelimb / plantar aspect in the hindlimb of MCIII/MTIII).

• Each other through a strong connecting band (intersesamoidean ligament).

What They Do

The sesamoid bones help improve the mechanical efficiency of the tendons that bend the limb by increasing their leverage (increasing the moment arm of the flexor tendons).

In simple terms, they act like pulleys — helping the tendons work more effectively when the horse bears weight and pushes off the ground.

Joint Cartilage of the Fetlock

(Articular Cartilage)

The surfaces of the fetlock joint are covered with a smooth, cushioning tissue called hyaline cartilage.

This cartilage:

• Allows the bones to glide smoothly over each other

• Absorbs shock

• Protects the bone underneath

  
  

Where the Cartilage Is Thickest

The cartilage is especially thick in areas that take the most pressure:

• The front lower part of the cannon bone (dorsal distal MCIII in the forelimb / MTIII in the hindlimb)

• The top surface of the long pastern bone (proximal articular surface of P1)

These areas experience heavy loading every time the horse bears weight.

What the Cartilage Is Designed to Handle

This cartilage is adapted to tolerate:

• Strong downward pressure (high compressive loads)

• Repeated stress from constant motion (repetitive cyclical strain)

Every stride places force through the fetlock, so the cartilage must be durable and resilient.

Clinical Importance

When this cartilage begins to wear down or break apart (cartilage degeneration), it is a major cause of joint disease in the fetlock, especially fetlock osteoarthritis.

In simple terms: The smooth protective lining of the joint is built to handle heavy and repeated stress — but when it deteriorates, pain and arthritis commonly develop.

 

Joint Capsule and Synovial Structures

 

Joint Capsule

The fetlock joint is surrounded by a soft tissue envelope called the joint capsule.

It is:

• Large

• Thin

• Very stretchable (highly distensible)

This flexibility allows the fetlock to extend significantly when the horse bears weight.

Where It Attaches

The joint capsule connects:

• Above, to the cannon bone (proximally to MCIII in the front leg / MTIII in the hind leg)

• Below, to the long pastern bone (distally to P1)

  
  

Special Feature

At the back of the joint, the capsule forms small pockets between the sesamoid bones. These are called:

• Palmar pouches (in the front limb)

• Plantar pouches (in the hind limb)

These pouches allow extra room for joint movement and fluid expansion.

Synovial Membrane

Lining the inside of the joint capsule is a thin tissue called the synovial membrane.

It is:

• Rich in blood supply (highly vascular)

• Responsible for producing the joint’s lubricating fluid (synovial fluid)

This fluid reduces friction, nourishes cartilage, and helps the joint move smoothly.

Clinical Importance

The synovial membrane is a common site of inflammation:

• Synovitis = inflammation of the synovial membrane

• Capsulitis = inflammation of the joint capsule

Both conditions can cause joint swelling, pain, and reduced performance in horses.

 

Ligaments of the Fetlock

(Ligamentous Anatomy)

Ligaments are strong bands of tissue that connect bone to bone and stabilize the joint.

Collateral Ligaments

There are two main side ligaments:

• Medial collateral ligament (inside)

• Lateral collateral ligament (outside)

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They:

• Start at the bottom of the cannon bone (distal MCIII in the forelimb / MTIII in the hindlimb)

• Attach to the top of the long pastern bone (proximal P1)

• Prevent the joint from bending too far inward or outward (valgus and varus stress)

In simple terms: they keep the fetlock from wobbling side to side.

Ligaments Between and Below the Sesamoid Bones

(Proximal and Distal Sesamoidean Ligaments)

These ligaments are part of the fetlock’s support system.

Intersesamoidean Ligament

• Made of tough, cartilage-like tissue (fibrocartilaginous)

• Connects the two sesamoid bones

• Forms part of the back joint surface (palmar/plantar articular surface)

Distal Sesamoidean Ligaments

(Continuation of the suspensory apparatus)

These run downward from the sesamoid bones:

Straight sesamoidean ligament

• Runs from the sesamoids to the top of the middle pastern bone (P2)

Oblique sesamoidean ligaments (medial & lateral)

• Run from the sesamoids to the back surfaces of P1 and P2

Cruciate sesamoidean ligaments

• Cross over each other

• Provide deeper support when the fetlock extends

Why These Ligaments Matter

All of these ligaments work together to prevent the fetlock from dropping too far when the horse bears weight.They prevent excessive straightening of the joint (hyperextension).

Tendons Around the Fetlock

(Tendinous Anatomy)

Tendons connect muscle to bone and create movement.

Flexor Tendons (Back of the Limb – Palmar/Plantar Side)

Superficial Digital Flexor Tendon (SDFT)

• Runs over the back of the fetlock

• Attaches to the middle pastern bone (P2)

• Helps:

  • Support the fetlock

  • Bend the limb

Deep Digital Flexor Tendon (DDFT)

• Passes between the sesamoid bones

• Attaches to the coffin bone (distal phalanx / P3)

• Mainly bends the lower part of the limb

Suspensory Ligament

(Interosseous Muscle)

This is one of the most important support structures of the fetlock.

It:

• Starts at the top of the cannon bone (proximal MCIII/MTIII)

• Splits into two branches (medial and lateral)

• Each branch attaches to a sesamoid bone

• Then continues forward to join the common digital extensor tendon

Learn more with the Tendons and Ligaments Flashcards

Key Point

The suspensory ligament is the main passive support structure of the fetlock. It holds the joint up when the horse stands and moves.

In the hind limb, it is often broader and flatter because the hind legs generate propulsion.

Structures on the Front of the Fetlock

(Extensor Structures – Dorsal Aspect)

At the front of the joint are:

• The common digital extensor tendon

• A thin joint capsule

• Small front fluid pockets (dorsal synovial pouches)

This area is prone to:

• Small bone fragments (chip fractures)

• Thickening of the joint capsule

• Bone spur formation at the front of the joint (dorsal osteophytes)

Blood Supply

(Vasculature)

Arterial Supply

Blood reaches the fetlock through:

• Palmar/plantar digital arteries

• These branch from:

  • The median artery in the forelimb

  • The femoral → popliteal → tibial arteries in the hindlimb

Venous Drainage

• A network of veins (venous plexus) carries blood away

• This helps nourish the joint fluid and regulate temperature

Nerve Supply

(Innervation)

The fetlock is supplied by:

• Palmar/plantar digital nerves

• These come from:

  • The median and ulnar nerves (forelimb)

  • The tibial nerve (hindlimb)

These nerves are commonly numbed during lameness exams (diagnostic anesthesia) to determine whether pain is coming from the fetlock.

Forelimb vs Hindlimb Anatomical Emphasis

Feature	Fore Fetlock	Hind Fetlock
Bone shape	Slightly wider	Slightly narrower
Loading	Compressive	Tensile + torsional
Suspensory role	Shock absorption	Propulsion control
Injury predisposition	Articular cartilage	Suspensory branches

Key Takeaway

The horse’s fetlock joint is a highly specialized, weight-bearing structure(load-bearing structure).

It combines:

• Strong bone surfaces that fit together securely (robust osseous articulation)

• Flexible but powerful supporting ligaments (elastic ligamentous support)

• A complex system of tendons and the suspensory ligament that act like a sling to hold the joint up (complex tendinous suspension)

Because this joint carries tremendous force with every stride, even small changes in structure or repeated excessive stress (repetitive overload)

can lead to serious injury or disease.

For this reason, the fetlock is one of the most frequently injured joints in performance and athletic horses.

FAQ: The Anatomy of the Fetlock Joint

What is the fetlock joint in a horse?

The fetlock joint is a high motion, high load hinge joint that contributes to shock absorption, propulsion, and energy storage during locomotion.

Is the fetlock joint a synovial joint?

Yes. In veterinary terms, the fetlock is a diarthrodial, synovial, hinge type joint with very large flexion and extension, and only limited twisting and side to side motion.

What bones form the fetlock joint?

The fetlock is formed by the distal end of the third metacarpal or metatarsal bone (cannon bone), the proximal phalanx (P1, long pastern), and the two proximal sesamoid bones at the back of the joint.

What is the difference between the front and hind fetlock?

The overall structure is very similar, but hind fetlocks are usually a little narrower side to side and experience more twisting forces because the hindlimbs generate most of the propulsion.

What is the sagittal ridge, and why does it matter?

The sagittal ridge is the raised ridge on the distal cannon bone’s joint surface. The top of P1 has a matching central groove that accommodates this ridge, helping keep the fetlock aligned and stable in motion.

What do the proximal sesamoid bones do in the fetlock?

They sit at the back of the fetlock and act like pulleys by increasing the moment arm of the flexor tendons, improving mechanical efficiency when the horse bears weight and pushes off.

What is synovitis or capsulitis in the fetlock joint?

Synovitis is inflammation of the synovial membrane and capsulitis is inflammation of the joint capsule. The article notes both can cause joint swelling, pain, and reduced performance.

What structures prevent the fetlock from dropping too far (hyperextension)?

A major role is played by the sesamoidean ligament system (including intersesamoidean plus straight, oblique, and cruciate distal sesamoidean ligaments) working with tendons and the suspensory apparatus to resist excessive extension under load.