Canine Phalanges - Anatomy & Physiology

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Bones

Metacarpals and Metatarsals

• The arrangement of the metatarsals are similar to those of the metacarpals.
• 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 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

• Proximal phalanx - of the main digits (II - V) they have a concave articular surface proximally. 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 seperated by a groove.
• Middle phalanx - roughly two-thirds the length of the proximal, 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.
• Distal phalanx - made up of a cone-shaped ungual process with a distinct collar, ungual crest. The deep ungual groove distal to the crest provides attachment for the proximal border of the claw. It 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 two phalanges that resemble the proximal and distal ones.

Joints

Metacarpophalangeal Joint

• Movement - flexion and extension.
• Each joint, bar the first digit, has a pair of sesamoid bones associated with the palmar aspect of the joint. They articulate with a concave area of the proximal phalanx.
• The joint capsule runs between the four bones of the joint. Dorsally it extends under the extensor tendon, this part also intermingles with 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.
• Collateral ligaments - bind the the metacarpal bone to the proximal phalanx, with a deep branch attaching to the sesamoid bone.
• Palmar/intersesamoidean ligament - a mass of fibrocartilage that embedd the sesamoid bones. The palmar aspect of it forms a groove for the deep flexor tendon to run in.
• Collateral sesamoid ligaments - connect the outer aspect of the sesamoids to the proximal phalanx.
• Distal sesamoid ligaments - a collection of ligaments that aren't well developed in the dog. They connect the distal surface of the sesamoids to the plamar aspect of the phalanx. They include the cruciate and short ligaments.

Proximal Interphalangeal Joint

• Movement - flexion and extension.
• The joint capsule attaches near the articular surfaces of the proximal and middle phalanges. It 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.
• Collateral ligaments - connect the distal end of the proximal phalanx to the proximal end of the middle phalanx. They lie in a vertical direction rather than along the bone axis.

Distal Interphalangeal Joint

• Movement - extension, slight flexion.
• The joint capsule attaches to the articular periphery of the bones and has a small fibrocartilagenous bead in the palmar aspect.
• Collateral ligaments - connect the distal part of the middle phalanx to the sides of the ungual crest of the distal phalanx.
• 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

• Function - support of the metacarpophalangeal joints.
• Origin - palmar aspect of the proximal end of the metacarpal bones.
• Insertion - sesamoid bones of the metacarpophalangeal joint, 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