Canine Phalanges - Anatomy & Physiology

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Dogs are digitigrade animals; this means that the weightbearing surface of their limbs is their digit. The canine phalanges are thus very important. They are virtually identical in their structure in the hindlimb and forelimb. The main differences are in the forelimb we have metacarpals and the metacarpophalangeal joint, the hindlimb equivalents are the metatarsals and the metatarsophalangeal joint. Also in anatomical planes we use the term palmar for forelimb and plantar for hindlimb. These terms are all interchangable in the below article depending on whether you want to talk about the fore or hindlimb.

Bones

Metacarpals and Metatarsals

The arrangement of the metatarsals are similar to those of the metacarpals in that they are rod shaped bones, numbered from I to V. The 1st is the most medial and is very small, the 3rd and 4th are the longest. The proximal base of each articulates with it's corresponding carpal bone and the adjacent metacarpal. The distal end is it's head, which is transversely cylindrical and articulates with the proximal phalanx. Metacarpals II - V possess a sagittal ridge on their palmar aspects.

Phalanges

The proximal phalanx of the main digits (II - V) have a concave articular surface and the palmar border has a groove to accomodate the articular surface of the metacarpus when the joint is fixed. The distal head has two convex areas separated by a groove. The middle phalanx is roughly two-thirds the length of the proximal phalanx and its base has a sagittal ridge on the articular surface which articulates with the groove of the proximal phalanx. The head resembles that of the proximal phalanx. The distal phalanx is made up of a cone-shaped ungual process with a distinct collar called the 'ungual crest'. The deep ungual groove distal to the crest provides attachment for the proximal border of the claw and articulates with the middle phalanx via a small sagittal crest. A bony sesamoid bone is found on the dorsal aspect of the metacarpophalangeal joint. The 'dew claw' (metacarpal I) is normally present in the forelimb but often not in the hindlimb. It normally only consists of two phalanges that resemble the proximal and distal ones.

Joints

Metacarpophalangeal Joint

The metacarpophalangeal joint is able to undergo flexion and extension movements. Each joint (except the first digit) has a pair of sesamoid bones associated with the palmar aspect of the joint which articulate with a concave area of the proximal phalanx. The joint capsule runs between the four bones of the joint extending dorsally under the extensor tendon and part also intermingling with an expanding area of the common digital extensor tendon. Distally it binds to the articular cartilage of the proximal phalanx and on the palmar aspect its dorsal attachment is level with the proximal end of the sesamoid bones. The collateral ligaments bind the the metacarpal bone to the proximal phalanx, with a deep branch attaching to the sesamoid bone. The palmar/intersesamoidean ligament is a mass of fibrocartilage that embeds the sesamoid bones. The palmar aspect of this ligament forms a groove for the deep flexor tendon to run in. The collateral sesamoid ligaments connect the outer aspect of the sesamoids to the proximal phalanx whilst the distal sesamoid ligaments, although not well developed in the dog, connect the distal surface of the sesamoids to the palmar aspect of the phalanx. These ligaments include the cruciate and short ligaments. The superficial transverse metacarpal ligaments surround the flexor tendons and their sheaths at the point of the sesamoid ligaments and the distal annular ligaments cross the surface of the flexor tendons and sheaths at the level of the proximal and middle phalanx.

Proximal Interphalangeal Joint

The proximal interphalangeal joint provides flexion and extension movements. The joint capsule attaches near the articular surfaces of the proximal and middle phalanges and extends slightly in a pouch dorsally and more extensively on the palmar aspect. The dorsal capsule is reinforced by a fibrocartilaginous nodule; the extensor tendon attaches to the capsule here. The collateral ligaments connect the distal end of the proximal phalanx to the proximal end of the middle phalanx and lie in a vertical direction rather than along the bone axis.

Distal Interphalangeal Joint

The distal interphalangeal joint allows extension and slight flexion movements. The joint capsule attaches to the articular periphery of the bones and has a small fibrocartilagenous bead in the palmar aspect. The collateral ligaments connect the distal part of the middle phalanx to the sides of the ungual crest of the distal phalanx. The dorsal ligament connects the proximal dorsal border of the extensor process of the distal phalanx. These are paired in dogs and function is to keep the claw raised until contraction of the deep digital flexor acts to overcome their tension.

Musculature

Interosseous
The function of the interosseous is to support the metacarpophalangeal joints. It originates from the palmar aspect of the proximal end of the metacarpal bones and inserts on the sesamoid bones of the metacarpophalangeal joint, where it then continues as extensor branches that wrap round the dorsal aspect of the digit to join the extensor tendons.

Forelimb

Superficial digital flexor

Function - flexion of the digits
Origin - medial epicondyle of the humerus
Insertion - palmar aspect of the middle phalanges

Deep digital flexor

Function - flexion of the digits
Origin - medial epicondyle of the humerus, proximal radius and ulna
Insertion - palmar aspect of distal phalanges

Common digital extensor

Function - extensor of the digits
Origin - lateral epicondyle of the humerus
Insertion - extensor process, distal phalanx

Lateral digital extensor

Function - extensor of the digits
Origin - lateral epicondyle of the humerus
Insertion - proximal phalanx

Hindlimb

Long Digital Extensor

Origin - extensor fossa of the lateral femoral condyle.
Insertion - flexor aspect of the tarsus and branches which continue onto the extensor process of each of the distal phalanges.
It divides into four branches at the level of the tarsus and are bound by the fibrous transverse bands of the proximal and distal retinaculum and are enclosed by a common synovial sheath.

Lateral Digital Extensor

Origin - proximal fibula and lateral collateral ligament of the stifle.
Insertion - It joins to the tendon of the long digital extensor to insert on the distal phalanx of the 5th digit.

Long Extensor of the first digit

Origin - Proximal fibula
Insertion - the metatarsophalangeal joint of the 2nd and 1st digit.

Superficial Digital Flexor

Origin - Supracondylar tuberosity of the femur and is firmly united to the lateral head of the gastrocnemius.
Insertion - Middle phalanx of the 2nd to 5th digits.
The muscle belly converges into a strong tendon at mid-tibial level and runs medially to the gastrocnemius to the point of the hock forming a broad cap. This cap is held in place by medial and lateral retinaculae.
A synovial bursa lies between this cap and the calcaneal tuberosity.
It then runs over the plantar aspect of the hock and splits twice at the level of the dorsal tarsal bones, thus forming it's four branches for insertion.

Deep Digital Flexor

This muscle consists of three seperate heads; the caudal tibial muscle, the lateral digital flexor muscle and the medial digital flexor muscle.
Lateral digital flexor
- Origin - caudal surface of the tibia and fibula
- Insertion - its strong tendon fuses with that of the medial digital flexor tendon on the plantar aspect of the tarsus forming the deep flexor tendon. This then branches and inserts on the distal phalanx of each digit.
Medial digital flexor
- Origin - the head of the fibula and the popliteal line of the tibia.
- Insertion - It fuses with the medial digital flexor tendon forming the deep flexor tendon.
Caudal tibial
- Origin - caudomedially surface of the tibia
- Insertion - the tendon radiates into the medial ligaments of the tarsus and doesn't form part of the deep flexor tendon.

Vasculature

Innervation

Innervation of the Hindlimb

The Canine Paw

Composed of four weight-bearing toes, each with a claw and footpad. In addition there is a metacarpal/metatarsal footpad.

Footpad

The epidermis is thick, pigmented, keratinised and hairless. It is arranged into conical papillae. Sweat/merocrine glands open onto the surface.
The dermis of the dense connective tissue is papillated.
The subcutaneous tissue is mainly adipose tissue with reticular, collagenous and elastic fibres. These help form digital cushions deep to the foot pad.
The digital pads are oval shaped and support the distal interphalangeal joints. The metacarpal/metatarsal pads are heart shaped, with the apex pointing distally and support the metacarpophalangeal joints.
A small carpal pad is present medial and distal to the accessory carpal bone.

Claw

This is the horny covering of the distal phalanx. The walls of the claw, which are continuous dorsally, embrace the soft flaky horn of the sole when they touch the ground.
The proximal part of the claw, coronary border fits between the ungual groove and the ungual crest, where it is continuous with the skin, making the claw fold.
The periosteum of the distal phalanx is continuous with the dermis, which is arranged in papillae. These cover the dorsal aspect of the phalanx and contain the germinative layer. These are flat keratinised epidermal cells which are considered responsible for horn formation.
The lateral and medial walls are formed by dermal laminae, instead of papillae.
The plantar aspect of the claw is produced by papillae but this horn is very friable.
The claws grow at rapid rate and so if they aren't worn by active wear they must be trimmed. Care must be taken as the dermis is highly vascular and bleeding will occur if cut into. To ensure that only the epidermis is cut take great care.