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 border 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
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