Bones and Joints of the Distal Limb (Week 2, Mod 7)

What are the two MAIN components of the carpus?

1) Proximal row 

2) Distal row

What are the 3 main bones of the proximal row? List and describe them from medial to lateral:

• Radial carpal bone / intermediate carpal bone

  • Is one big, fused bone… largest bone of the carpus 

• Ulnar carpal bone

• Accessory carpal bone

  • Tucked CAUDALLY behind ulnar carpal bone (see image)

What are the 4 main carpal bones in the DISTAL row of the carpus? List them medially to laterally.

• 1st carpal bone

• 2nd carpal bone 

• 3rd carpal bone 

• 4th carpal bone

Which carpal bones only have 1 center of ossification?  Which have 2?

All carpal bones have 1 center of ossification, EXCEPT for the radial / intermediate carpal bone and the accessory carpal bone 

Species differences: How is the carpus of the pig different from the dog?

The radial and intermediate carpal bones are SEPARATE… has a complete set of proximal and distal rows 

Species differences: How is the carpus of the horse different from the dog? How is this significant?

Radial and intermediate bones are separate as well, BUT:

• 1st carpal bone is MISSING

• 2nd and 3rd carpal bones are FUSED 

  • Interacts the MOST with the 3rd metacarpal, which is weight bearing 

  • Is fused for support 

What are the 3 joints that can be found within the carpus?

1) Antebrachio-carpal joint

2) Middle carpal joint 

3) Carpo-metacarpal joint 

** also has intercarpal joints keeping the individual bones next to each other; are fibrous, and have no flexibility

Describe the antebrachio-carpal joint… where is it found in the carpus? How flexible is it?

Is found between the antebrachium and the proximal row of the carpus

• Facilitates the MOST movement of the carpus

Describe the middle carpal joint… where is it found in the carpus? How flexible is it?

Found between the proximal and distal rows of the carpus

• Allows for some flexibility, but not as much as the antebrachio-carpal joint

Describe the carpo-metacarpal joint… where is it found in the carpus? How flexible is it?

Is found between the distal row of the carpus and the metacarpal bones

• Has the LEAST amount of flexion here of the other carpal joints

What is the importance of the accessory carpal bone??  What ligament in particular attaches here, and what is its function?

Tendons of the foreleg muscles attach to the accessory carpal bone, allowing the bone to act as a sort of lever for FLEXION of the carpus

• However, when animals are standing, the legs are in extension

• To prevent hyperextension, the PALMAR LIGAMENT attaches to the distal portion of the accessory carpal bone, inserting at the METACARPALS to stabilize it 

What are 3 features that contribute to stabilizing the carpal joint?

1) Collateral ligaments

• Laterally: Styloid process of ulna to 5th metacarpal 

• Medially: Radius to 2nd metacarpal 

2) CARTILAGE PLATE on the PALMAR aspect of the joint capsule 

• Keeps bones stacked and supports them 

3) Retinaculum 

• A band of thickened connective tissue that surrounds the carpus, like a compression sock 

How many digits to dogs and cats have? How many to ungulates have, and which ones did they keep? How many do horses have, and which one did they keep?

Dogs and cats:

• Four (technically 5) digits… shed the 1st one (dew claw) for speed

Ungulates:

• Have only 2 weight-bearing digits… 3rd and 4th digits remain 

• Termed artyrodactyla 

Horses:

• Only 1 weight bearing digit, kept the 3rd digit 

• Termed perissodactyla 

What are the 3 different classifications of “stance”, and what are they based on?  Which species is unique?

1) Plantigrade

• Bears, humans

• Bearing weight on the digits, metatarsals, AND tarsal bones

2) Digitigrade

• Dogs, cats

• Bear weight on the digits / phalanges; protected by foot pads

3) Unguligrade

• Ruminants, horses

• Bear weight on the distal phalanx ONLY; protected by hoof

*** RABBITS can be both 1 and 2… sit plantigrade, run digitigrade 

How many centers of ossification to each metacarpal and phalanges have?  What is the exception?

Each bone has 2 ossification centers, EXCEPT

• Distal phalanx… only has 1

Describe the metacarpal bone of the equine… what is it also termed as?  What is its function?   What are 2 key features of the bone?

Is the 3rd metacarpal… also termed the canon bone

• Is WEIGHT BEARING 

2 key features:

• Has a sagittal groove on the distal articular surface; interacts with the proximal phalanx 

• Has 2 collateral SPLINT BONES at the proximal, palmar end (see image)

  • Are vestigial and non-weight bearing; are palpable 

Describe the proximal sesamoid bones… what species are they found in? What are their functions?

Paired proximal sesamoid bones are found in ALL species

• Are palmar-distal to every metacarpal/ metatarsal 

• Will ALWAYS be paired

• Are embedded in suspensory ligament 

Function to PROTECT THE SUPERFICIAL DIGITAL FLEXOR TENDONS (SDFT) AND THE DEEP DIGITAL FLEXOR TENDON (DDFT)

Describe the distal sesamoid bones… what species are they found in? What are their functions?

Are ONLY found in the canine, and are not paired; single

• Are found on the dorsal aspect of every metacarpal / metatarsal 

Protects the EXTENSOR TENDONS

Describe the proximal phalanx of the horse… how is it unique compared to other species? How is it kept stable?

Is also called the “long pastern bone” / P1

• Has a “v” shape on the palmar surface, which is where the oblique distal sesamoidian ligament attaches…

  • This provides support and stability to the fetlock by connecting the proximal sesamoid bones to the pastern bone

• Has 2 sagittal grooves… one proximal, and one distal

Describe the equine metacarpo-phalangeal joint… what are its 3 main components? What kind of joint is it? What are 2 unique features of the joint that contribute to the protection of surrounding structures?

Also called the “fetlock”; composed of

• 3rd metacarpal

• Proximal phalanx

• Proximal sesamoids

Is a typical synovial joint, but has two out-pouchings:

• Large palmar pouch - incorporates the proximal sesamoids

• Large dorsal pouch - cushions exterior tendons

What is the suspensory ligament in the equine? What does it attach to, where is it located, and what is its function?

Origin - proximal MC3… runs palmar, fills entire space between the two splint bones

• BRANCHES in 2, attaching to the ABAXIAL aspect of the proximal sesamoids and wrapping around the metacarpo-phalangeal joint 

• Passes around the DORSAL aspect of the limb and fuses with the common digital extensor tendon 

Function 

• Acts like a sling; prevents prolonged hyperextension of the limb and returns the joint to a resting position 

• Relies on the STABILITY OF THE SESAMOIDS 

There are 3 sesamoidean ligaments…. what are they called, where are they found, and what are their functions?

1) Inter-sesamoidean ligament 

• Is a fibrocartilage plate BETWEEN the proximal sesamoid bones 

• Allows for the smooth passage of the DDFT and the SDFT 

2) Collateral sesamoidean ligaments 

• Attach the sesamoids to the MC3 and the proximal phalanx (see image B)

3) Distal sesamoidean ligaments - there are 4

• Cruciate and short -

  • labeled C1 and C2, stabilize the sesamoids to the proximal phalanx

• Oblique - 

  • Attach the sesamoids to the “v” shaped trough on the palmar aspect of the proximal phalanx, offers great stability 

• Straight - 

  • Overlays entire palmar aspect of the proximal phalanx

  • Inserts with the SDFT

What is the suspensory apparatus and what structures is it made up of?

It is the set of ligaments and bones that stabilize the entire fetlock… very prone to injury, as every piece relies on the function of the other 

Is composed of:

• Suspensory ligament

• Proximal sesamoids 

• Sesamoidean ligaments 

• Common digital extensor tendon 

Describe the middle phalanx of the equine… what is it called? What is one unique feature that allows for the attachment of ligaments?  Describe its associated proximal interphalangeal joint, and what is unique about it that provides protection…

Middle phalanx = short pastern bone

• Has 2 sagittal grooves as well, 1 proximal and 1 distal 

• Has bony bumps along its surface that allow for better attachment of the collateral ligaments 

The proximal interphalangeal joint (PIP) is a typical synovial joint:

• Has an EXTENSIVE joint capsule; forms a DORSAL bursa to cushion the common digital extensor tendon 

Describe the distal phalanx of the equine… has a “complex shape”, allowing for the attachment of multiple ligaments.  What are these processes / aspects? (Hint: there are 4)

Extensor process -

• Found on the proximal / dorsal aspect of the pedal bone (DP)

• Allows for the attachment of the common digital extensor tendon (see image)

Palmar processes - 

• Are collateral processes at either end of the proximal DP… in the image, are proximal / palmar, kind of in the same line as the extensor process

• Support the lateral cartilages

Solar surface - 

• Is the BOTTOM aspect of the DP

• Deep digital flexor tendon attaches here 

Dorsal surface - 

• Has striations on its surface for hoof attachment 

• Can see vascular channels for blood vessels 

Describe the distal interphalangeal joint … What else is it called? What is it composed of? What is its function in terms of flexibility?

Called the “coffin”… is buried within the hoof 

Composed of:

• Middle / distal phalanges

• Distal sesamoid (AKA NAVICULAR BONE)

• Has an extensive joint capsule 

  • ANOTHER dorsal bursa for cushioning of the extensor tendon 

Is a typical synovial joint, stabilized by collateral ligaments 

Function:

• Flexion and extension 

• SOME ROTATION, unlike the rest of the interphalangeal joints 

  • Accounts for uneven ground

Describe the navicular bone… what other, protective structure is it associated with? What seems to separate it from the interphalangeal joint space?

The navicular bone has:

• 2 smooth articular surfaces 

• Is made of cancellous bone with vascular channels

Is associated with the podotrochlear bursa -

• A fluid filled pocket between the navicular bone and the DDFT

  • CUSHIONS the DDFT

The navicular bone is separated from the joint space by the IMPAR LIGAMENT, which can be seen in the image