Canine Forelimb - Anatomy & Physiology

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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 seperated 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. This roughened area is very variable in size in dogs and can be non-exsistent.

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 betwwen 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 bewteen 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 proximally attach to the epicondyle and distally attaches 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 accesory carpal bone makes up the carpal canal. It houses just the deep digital flexor tendon in the dog.
The intermetacarpal joint are tight joints between the proximal ends of the metacarpals. The joint capsules are continuous with that of the carpal joint. They are hold together by 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: 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.: Accessory n.

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 limb is in motion
- draws head and neck ventrally when limb is fixed

Omotransversarius: Accessory n.

Origin: transverse processes of the atlas
Insertion: acromion and distal spine of the scapula
Action: advancing the limb
Innervation

Latissimus dorsi: local branch of brachial plexus

The broadest muscle of the back
Origin: thoracolumbar fascia
Insertion: teres tuberosity of the humerus
Actions: antagonist to the brachiocephalic m.
- cranial fibers strap scapula to the chest
- retracts free limb and flexes shoulder joint
- draws trunk forward over the fixed limb

Pectoral mm.: 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 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
  - draw trunk forward when limb is fixed

Serratis ventralis: branch of brachial plexus

Origin: C4 to 10th rib
Insertion: medial scapula and scapular cartilage
Action: supporting the weight of the trunk
- reinforced by strong fascia
- cervical portion can retract the limb
- caudal portion can advance the limb

Rhomboids: 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, Suprascapular n. of the brachial plexus
- Origin: the fossae of the scapula
- Insertion: both tubercles of the humerus
- Action: brace the shoulder
- Clinical significance: bursa between the tendon of the infraspinatus and lateral tubercle of the humerus can be the site of inflammation
Medial:
- Supscapularis: 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: Musculocutaneous n. of the brachial plexus
  - Origin: medial supraglenoid tubercle
  - Insertion: proximal shaft of the humerus
  - Action: fixator
Caudal (Flexors): Axillary n. of the brachial plexus
- Deltoids
  - 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
  - Origin: dorsal part of the caudal scapula
  - Insertion: teres tuberosity midway down humerus

Muscles of the Elbow

Extensors: Radial n. from the brachial plexus

Triceps brachii: Has four heads in the dog
- Long head: caudal margin of the scapula
- Lateral, medial, and accessory heads: shaft of the humerus
- Insertion: olecranon, protected by tricipital bursa against the bone and subcutaneous bursa against the skin
Tensor fasciae antebrachii
- Origin: tendon and lateral surface of the latissimus dorsi

Flexors: Musculocutaneous n. from the brachial plexus

Biceps brachii
- Origin: supraglenoid tubercle of the scapula
- Insertion: medial tuberosity of proximal radius and adjacent ulna
- Runs through the intertubercular groove of the humerus
Brachialis
- Origin: proximocaudal humerus
- Insertion: spirals to insert just proximal to biceps

Muscles of Supination and Pronation

Supinators: Radial n. from the brachial plexus

Brachioradialis
- Origin: lateral epicondyle of the humerus
- Insertion: distal part of medial radius
- Often much reduced and sometimes absent
Supinator
- Deep to extensor muscles, passing from lateral humeral epicondyles to upper medial radius

Pronators: Median n. from the brachial plexus

Pronator teres
- Origin: medial epicondyle of the humerus
- Insertion: dorsal surface and medial border of radius
Pronator quadratus
- Origin: palmar surface of radius and interosseous ligament
- Insertion: interosseous border of the ulna

Muscles of the Carpal and Digital Joints