Bones and Joints of the Distal Limb

Describe the relative position of the carpus in a dog vs. a horse?

• What changes in directional terminology occur?

• The sequence of the bones remains the same, but the relative position may change depending on the species

• Past carpus, utilize dorsal and palmar for front limb, dorsal and plantar for hindlimb

What is the carpus composed of?

• Composed of carpal bones arranged in a linear series, attached to one another by fibrous joints

Type = short bones

What are the components of the carpus, describe the:

• Proximal Row

• Distal Row

The carpal bones in all species are arranged in two rows.

• Proximal row (PR):

• Radial carpal bone/ Intermediate carpal bone (fused, RCB)

• Ulnar carpal bone (UCB)

• Accessory carpal bone (ACB)

• Palpable on palmar aspect

• Distal row:

• 1st, 2nd, 3rd and 4th carpal bones

• Numbered medial to lateral

What are the centers of ossification in the carpus?

Single centre of ossification except ACB and fused RCB/ICB in dogs/cats (Has two centers))

What are the species variations in the carpus, consider the following:

• Pig

• Horse

• Pig

  • Has complete, full set of individual bones

• Horse

  • 3rd CB largest (Largest in cats/dogs is 4)

  • 1st CB completely absent

What are the three main joints composing the carpus?

• What bones articulate?

• What are the species variation in the 3 joints in the carpus?

1. Antebrachio-carpal joint:

• Between antebrachium + proximal row of carpal bones

• Radius articulates with - RCB + UCB

• Ulna articulates with - UCB + ACB

  • **Equine does not have full length ulna so joint is not involved, could be called radiocarpal joint in this species

2. Middle carpal joint:

• Proximal + distal rows of carpal bones

3. Carpo-metacarpal joint:

• Distal row + metacarpal bones

Describe specific movement occuring within the carpus as well as the features, note the accessory carpal bone’s role in this as well.

• Movement:

• Standing position, the bones are “stacked” = extension

• Accessory carpal bone: 

• ACB acts as lever for distal limb +

• Insertion of major carpal flexor muscles

• Counteraction by palmar ligament

In the carpus, when are the joint spaces palpable and what does this allow for?

What are the joints between the carpal joints called?

palpable in flexion

• Carpal joint fluid sampling

Intercarpal joint = fibrous joints

Describe the ranges of flexion in the carpus joint.

Large range of flexion:

• Most movement antebrachio-carpal joint

• Least movement carpo-metacarpal joint

• Some movement between carpal bones

What type of joint is the carpus and where can the joint capsule be found?

• Typical synovial joint

• Single joint capsule extending proximally between radius and ulna, and distally between metacarpal bones

• Poor communication between compartments, fluid at top will not communicate with bottom

What provides the carpus joint the necessary stability?

1. Collateral ligaments

• Lateral: ulna (lat. Styloid process) to 5th metacarpal

• Medial: radius (medial styloid process of radius) to 2nd metacarpal

2. Cartilage plate on palmar aspect of joint capsule

   1. Reinforces the stacking of the bone

3. Retinaculum

• Another fibrous layer of tissue to help support, acts like a sock

What are the bones found in the distal limb and their corresponding joints?

• Note any species differences.

Long bones:

• Metacarpal / metatarsal bones

• Digit:

• Proximal phalanx

• Middle phalanx

• Distal phalanx

Joints:

• Metacarpo / metatarso - phalangeal joint (MCP / MTP)

• Proximal Interphalangeal joint (PIP)

• Distal Interphalangeal joint (DIP)

  • All typical synovial joints

Sesamoid bones:

• Proximal sesamoids (2, lateral and medial)

  • all species

• Dorsal sesamoids - in carnivores

• Distal sesamoid - in large animals

What are the species variations in the number of digits? Consider:

• Humans

• Dogs & Cats

• Unguligrades

  • Artyrodactyla

  • Perissodactyla

Humans & primates:

• 5 digits & associated metacarpals / metatarsals

• Numbered medial to lateral

Dogs & cats:

• Shed 1st digit for speed (dew claw)

• Have toes 2-5

Unguligrades:

Shed more digits for speed

• Artyrodactyla - ruminants

• Even number weightbearing digits

• 3rd & 4th digits remain

• Perissodactyla - equines

• Uneven number weightbearing digits

• 3rd digit remains

Plantigrade refers to:

A/ Plantigrade:

• Eg. rabbits, bears, humans

Bear weight on: Digits + metatarsals + tarsal bones (entire length)

Digitigrade refers to:

B/ Digitigrade:

• Eg. Dogs, cats

• Bear weight on: Digits / phalanges, protected by foot pads

Unguligrade refers to:

C/ Unguligrade:

• Eg. Ruminant, horses

• Bear weight on distal phalanx only, protected by hoof

Describe the number of centers of ossification in the phalanx.

• What exceptions are there?

Development:

• 2 centres of ossification

• Except:

• Distal phalanx = 1

Describe the equine distal limb bones, beginning with the metacarpals.

Metacarpal bones:

• 3rd Metacarpal / Metatarsal = "cannon" bone

• Weight bearing

• Distal articular surface

• Sagittal ridge

• 2nd & 4th MC / MT = 'splint' bones *

• Vestigial / non-weight bearing

• 'Button' at distal end - palpable

• Fibrous joints between bones

• (syndesmosis)

• Mineralise as age = 'splints'

What are features of the proximal sesamoid bones and what is their function?

• All species

• Paired, two Palmaro-distal to every MC3 / MT3, at the metacarpophalangeal joint

• Embedded in suspensory ligament

Function to protect:

• Superficial Digital Flexor Tendons (SDFT)

• Deep Digital Flexor Tendon (DDFT)

What are features of the dorsal sesamoid bones in dogs?

• Dogs and cats, unlike other species have dorsal sesamoid bones

• Single

• Dorsal aspect every MCP / MTP joint

• Visible on radiographs

• Embedded in & protect - extensor tendons

Describe the equine proximal phalanx.

• Proximal phalanx (PP):

• "Long pastern bone" / P1

• Raised 'V' on palmar surface

  • Important for insertion of Oblique distal sesamoidean ligament

• 2 x sagittal grooves

What are the components of the equine Metacarpo-phalangeal (MCP) / metatarso-phalangeal (MTP) joint?

• Components:

1. 3rd metacarpal / metatarsal bone

2. Proximal phalanx

3. Proximal sesamoid bones

• Typical synovial joint

• Large palmar pouch - incorporates proximal sesamoids

• Large dorsal pouch - cushions extensor tendons

What provides the equine Metacarpo-phalangeal (MCP) / metatarso-phalangeal (MTP) joint with stability?

• How does movement look in this joint?

• Collateral ligaments

• Sagittal ridge / groove

• Movement: limited to flexion & extension

'High motion' joint

What ligament supports the MCP / MTP joints in the horse? What is the main function of this ligament?

• Suspensory ligament *

• Originates proximal palmar MC3 / MT3

• Fills area between splint bones

• Divides in two

• Attaches abaxial aspect proximal sesamoids

• Passes to dorsal aspect limb

• Fuses with Common Digital Extensor tendon

• Function:

• Acts like a sling

• Returns joint to resting position

• Relies on stability of proximal sesamoids!

What ligaments provide stability to the distal equine limb especially concerning the sesmoidean ligaments?

• Sesamoidean ligaments:

• Provides stability of proximal sesamoids

• A/ Inter-sesamoidean ligament:

• Fibrocartilage plate between proximal sesamoid bones

• Smooth passage of DDFT and SDFT (tendons)

• B/ Collateral sesamoidean ligaments:

• Proximal sesamoids to MC / MT 3 and PP

• Medial & lateral

• C/ Distal sesamoidean ligaments:

1. Cruciate - PP

2. Short - PP

3. Oblique ligament - PP

• V-shaped trough on PP

4. Straight ligament - MP

• Inserts with SDFT

How does the suspensory apparatus in the equine distal limb function?

• Function relies on interaction between

• Suspensory ligament

• Proximal sesamoids

• Sesamoidean ligaments

• Common digital extensor tendon

Describe key features of the middle phalanx in the horse.

• Middle phalanx (MP):

• "Short pastern bone" / P2

• Sagittal ridge - proximal

• Sagittal groove - distal

• Bony bumps

• Attachment of collateral ligaments

What are the components of the equine proximal interphalangeal joint?

• Components:

• Proximal & middle phalanges

• Typical synovial joint

• Extensive joint capsule

• Forms dorsal bursa to cushion extensor tendon

• Collateral ligaments

What are the components of the equine distal phalanx?

• What does each process suport?

• What about each surface?

• "Pedal bone" / P3

• Complex shape

• Extensor Process

  • Attachment Common Digital Extensor tendon

    (CDE)

• Palmar processes

  • Support lateral cartilages

• Solar Surfaces

  • Attachment Deep Digital Flexor Tendon (DDFT)

• Dorsal Surface

  • Striations for hoof attachment

    Vascular channels for blood vessels

What are the features of the equine distal interphalangeal joint?

• What are it’s components?

• What provides the stability?

• What portion provides movement?

Distal interphalangeal (DIP) joint

• "Coffin" joint - because buried within hoof

• Components:

• Middle & distal phalanges

• Distal sesamoid & distal phalanx

• Distal sesamoid & middle phalanx

• Typical synovial joint

• Extensive joint capsule

• Forms dorsal bursa to cushion extensor tendon

• Accessible for injection into joint

• Stability:

• Collateral ligaments

• Movement :

• flexion & extension

• Some rotation to accommodate uneven ground

What are the components of the equine distal sesamoid?

• Note and species differences and important bursas.

Distal sesamoid:

• "Navicular" Bone

• 2 smooth articular surfaces

• Cancellous centre with vascular channels (spongy)

• Absent in dog & cat

• Navicular / podotrochlear bursa:

• Fluid filled pocket between distal sesamoid and DDFT

• Cushions DDFT

• Separated from joint space by impar ligament

What is navicular disease syndrome?

Navicular disease syndrome:

• Forelimb lameness

• Erosion visible on radiographs

• Poor correlation between radiographic changes and clinical signs

What is the stance?

Posture of distal limb when the animal is weightbearing, how the animal stands