Canine Forelimb - Anatomy & Physiology

Structures of the Proximal Forelimb and Shoulder

Scapula

The Scapula forms the basis of the shoulder region, providing points of attachment of extrinsic and intrinsic muscles. It is held in place by a synsarcosis of muscles and does not form a conventional articulation with the trunk. The spine culminates in the acromion. The scapular cartilage is comprised of only a very narrow rim.

Clavicle

In the cat, a remnant of bone may remain embedded in the fibrous intersection in the brachiocephalicus, which may prove misleading in radiographic images.

Humerus

The Humerus is the long bone of the forearm, articulating with the scapula to form the shoulder and the radius and ulna to form the elbow. In situ, it lies obliquely along the ventral thorax and is more horizontal in larger species. The greater tubercle is not separated into two parts like in other species. In dogs and cats, it articulates with the ulna medially via a trochlea and the radius laterally via a capitulum.

Radius

A radial tuberosity provides a site of attachment for brachialis and biceps brachii mm. This roughened area is very variable in size in dogs and can be non-existent.

Ulna

The ulna shaft tapers distally, lying oblique to the radius, i.e. the proximal end lies medial and the distal end lateral to the radius. There is a distinct gap between it and the radius, which is filled by the pronator quadratus muscle. The distal styloid process is blunt and articulates with the ulnar carpal bone, accessory carpal bone and ulnar notch of the radius.

Joints of the Proximal Forelimb

Shoulder Joint

The joint capsule barely extends past the areas of articulation, except where it continues distally into the intertubercular groove of the humerus. This provides cushioning and synovial support for the bicipital tendon. The bicipital tendon and the joint capsule pouch are held in place by the transverse humeral retinaculum, which lies between the greater and lesser tubercles of the humerus.

Elbow Joint

The radius articulates with the capitulum and lateral trochlear surface of the humeral condyle. Whilst the medial part articulates with the trochlear notch of the ulna. To allow the radius to rotate slightly the radial head has a marginal band of cartilage making the articular circumference. It faces the radial notch of the ulna between the medial and lateral coronoid processes. The joint capsule runs from just proximal to the articular surface of the condyle of the humerus to the periphery of the olecranon fossa; it pouches between the ulna and radius, and under the tendinous attachments of some muscles.

Collateral ligaments are paired and lie medially and laterally. They both attach proximally to the epicondyle and distally to the tuberosities of the radius and ulna.

Annular ligament of the radius attaches to the sides of the coronoid process of the ulna. This runs deep to the collateral ligaments and forms a ring for the radial head to turn in during pronation and supination.

The radius and ulna are joined mid-shaft by the interosseous ligament, the remainder is filled by the interosseous membrane.

Structures of the Distal Forelimb

Carpal Bones

Carpal bones comprise two rows:

Proximally - the radial and intermediate bones are fused to form the radial carpal bone. The accessory carpal bone articulates with both the ulnar carpal bone and the distal ulna.

Distally - bones I-IV are present.

Metacarpal Bones

These are covered in detail in the canine phalanges section.

Joints of the Distal Forelimb

Carpal Joint

The synovial membranes form three compartments corresponding to each joint. The proximal is the largest whilst the middle carpal and carpometacarpal sacs communicate and extend into the intermetacarpal articulations. The fibrous joint capsule is common to all three joints and attaches to the individual bones and various intercarpal ligaments. The extensor retinaculum is fibrous collagenous tissue on the dorsal aspect that allows passage of the extensor tendons. On the palmar aspect lies the palmar carpal fibrocartilage, that provides attachment for some metacarpal bones. Paired collateral ligaments bridge the sides of the three main articulations.

The flexor retinaculum is the carpal fascia on the palmar aspect, and lies between the accessory carpal bone and the medial aspect of the carpus. This with the joint capsule and medial surface of the accessory carpal bone, makes up the carpal canal. It houses just the deep digital flexor tendon in the dog. The intermetacarpal joints, are tight joints between the proximal ends of the metacarpals. The joint capsules are continuous with that of the carpal joint. They are held together by the interosseous metacarpal ligaments.

Muscles of the Forelimb

Extrinsic Musculature

These muscle are responsible for joining the forelimb to the trunk, forming a synsarcosis rather than a conventional joint. Collectively, they act to transfer the weight of the body to the forelimbs as well as stabilize the scapula.

Trapezius:

Innervated by: Accessory n.

Origin: mid-dorsal raphe and supraspinous ligament

Insertion: spine of the scapula

Body: two parts, cervical and thoracic separated by aponeurosis

Action: raises scapula against the trunk and swings cranially to advance the limb.

Brachiocephalic m.:

Innervated by: Accessory n.

There are two parts separated by the clavicle.

Origin: clavicle

Insertion: median raphe of the neck and the occipital bone. The ventral part attaches to the mastoid process.

Actions: advances the limb and extends the shoulder joint when the limb is in motion; draws the head and neck ventrally when the limb is fixed.

Omotransversarius:

Innervated by: Accessory n.

Origin: transverse processes of the atlas

Insertion: acromion and distal spine of the scapula

Action: advancing the limb

Latissimus dorsi:

Innervated by: local branch of brachial plexus

It is the broadest muscle of the back.

Origin: thoracolumbar fascia

Insertion: teres tuberosity of the humerus

Actions: antagonist to the brachiocephalic m. It retracts the free limb and flexes the shoulder joint. It also draws the trunk forward over the fixed limb.

Pectoral mm.:

Innervated by: brachial plexus

Two superficial parts; cranial and caudal

Origin: cranial sternum

Insertion: Cranial (descending): crest of the humerus distal to the deltoid tuberosity. Caudal (transverse): covers the elbow joint to insert on the medial fascia of the forearm.

Action: adduct the forelimb, assist in protraction and retraction.

One deep part (pectoralis profundus), with cranial and caudal parts.

Origin: ventral sternum and adjacent cartilage

Insertions: Cranial (subclavius): supraspinatus m., Caudal (pectoralis ascendens): lesser tubercle of the humerus.

Actions: slinging trunk between forelimbs, may also retract free limbs. It draws the trunk forward when the limb is fixed.

Serratis ventralis:

Innervated by: branch of brachial plexus

Origin: C4 to 10th rib

Insertion: medial scapula and scapular cartilage

Action: supporting the weight of the trunk. It is reinforced by strong fascia. The cervical portion can retract the limb and the caudal portion can advance the limb.

Rhomboids:

Innervated by: brachial plexus

Origin: nuchal ligament, 4th - 6th thoracic spine

Insertion: dorsal border and adjacent scapula

Action: retracting the limb, may also raise limb.

Intrinsic Musculature

Muscles of the Shoulder

These muscles are grouped:

Lateral

Supraspinatus and Infraspinatus:

Innervated by: Suprascapular n. of the brachial plexus

Origin: the fossae of the scapula

Insertion: both tubercles of the humerus

Action: brace the shoulder

Clinical significance: the bursa between the tendon of the infraspinatus and lateral tubercle of the humerus, can be the site of inflammation.

Medial

Supscapularis:

Innervated by: Subscapular n. from the brachial plexus

Origin: Deep surface of the scapula

Insertion: medial tubercle of the humerus

Action: braces medial shoulder joint, potential adductor

Coracobrachialis:

Innervated by: Musculocutaneous n. of the brachial plexus

Origin: medial supraglenoid tubercle

Insertion: proximal shaft of the humerus

Action: fixator

Caudal (Flexors)

Deltoids:

Innervated by: Axillary n. of the brachial plexus

Origin: acromion, the length of the scapular spine

Insertion: deltoid tuberosity on the humerus, fascia of the lateral arm

Action: Flexor of Shoulder, Abductor and outward rotator of the arm

Teres Major:

Innervated by: Axillary n. of the brachial plexus

Origin: dorsal part of the caudal scapula

Insertion: teres tuberosity midway down humerus

Muscles of the Elbow

1. Extensors

Triceps brachii:

Innervated by: Radial n. from the brachial plexus

Has four heads in the dog

Origin:

Long head: From the caudal margin of the scapula

Lateral, medial, and accessory heads: From the shaft of the humerus

Insertion: olecranon, protected by tricipital bursa against the bone, and subcutaneous bursa against the skin

Tensor fasciae antebrachii:

Innervated by: Radial n. from the brachial plexus

Origin: tendon and lateral surface of the latissimus dorsi

2. Flexors

Biceps brachii:

Innervated by: Musculocutaneous n. from the brachial plexus

Origin: supraglenoid tubercle of the scapula

Insertion: medial tuberosity of proximal radius and adjacent ulna

Runs through the intertubercular groove of the humerus

Brachialis:

Innervated by: Musculocutaneous n. from the brachial plexus

Origin: proximocaudal humerus

Insertion: spirals to insert just proximal to biceps

Muscles of Supination and Pronation

1. Supinators

Brachioradialis:

Innervated by: Radial n. from the brachial plexus

Origin: lateral epicondyle of the humerus

Insertion: distal part of the medial radius

Often much reduced, and sometimes absent

Supinator:

Innervated by: Radial n. from the brachial plexus

Deep to the extensor muscles, passing from the lateral humeral epicondyles to the upper medial radius

2. Pronators

Pronator teres:

Innervated by: Median n. from the brachial plexus

Origin: medial epicondyle of the humerus

Insertion: dorsal surface and medial border of radius

Pronator quadratus:

Innervated by: Median n. from the brachial plexus

Origin: palmar surface of radius and interosseous ligament

Insertion: interosseous border of the ulna

Muscles of the Carpal and Digital Joints

1.Extensors

They are located at the craniolateral position on the forearm. They almost all originate from the lateral epicondyle of the humerus and are all innervated by the radial n. from the brachial plexus.

Extensor carpi radialis

The most medial, inserts on middle metacarpal bone.

Ulnaris lateralis

The most lateral, inserts on accessory carpal bone and 5th metacarpal.

Extensor carpi obliquus (aka abductor pollicis longus)

Origin: cranial radius

Insertion: 1st metacarpal

Last two may also serve in medial deviation of the paw.

Common Digital Extensor

Insertion: extensor process of the distal phalanx of each digit

Sends a medial branch to the dew caw.

Lateral Digital Extensor

Inserts on the dorsal proximal phalanges of the 3rd to 5th digit.

2. Flexors

They are at the caudal position on the forearm, originate from the caudal medial epicondyle of the humerus and all are innervated by the median or ulnar n. of the brachial plexus.

Flexor carpi radialis

The most medial, inserts on the upper 2nd/3rd metacarpal bone.

Flexor carpi ulnaris

The most lateral, inserts on the accessory carpal bone.

Superficial Digital Flexor

It divides into four branches, which insert on the middle phalanges of all digits.

Deep Digital Flexor

Passes through the carpal canal before branching, and continues to the palmar distal phalanges.

Interosseus muscles

Support the metacarpophalnageal joints. They arise from the palmar proximal metacarpal bones, and insert on the sesamoid bones within the joints. They are continued by ligaments to the phalanges.

Vasculature of the Forelimb

Arteries of the Forelimb

Veins of the Forelimb

Lymphatics of the Forelimb