Equine Forelimb

Contents

Structures of the Proximal Forelimb and Shoulder

Scapula

  • The horse lacks an acromion.
  • It has a tuber scapular half way along the spine.
  • The scapular cartilage is reasonably extensive compared to other species.
  • supraglenoid tubecle large and separate from glenoid cavity

Humerus

  • Greater tubercle have a cranial & caudal part
  • Intermediate tubercle
  • lesser tubercle similar size
  • Double intertubercular groove.

Radius and Ulna

  • The two bones are fused, so all movement is in a sagittal plane with very little /if any pronation and supination.
  • Radius is weight bearing
  • The ulna tapers out distally at the middle of the radius shaft.
  • For the proximal quarter of the radius the bones are joined by ligaments, then there is an interosseous space for the passage of blood vessels, distal to this the bones are fused.
  • The lateral styloid process of the radius is in fact developmentally the distal end of the ulna. It appears as a seperate ossification centre in a foal until it fuses at about a year old.

Joints of the Proximal Forelimb

Shoulder Joint

  • The joint capsule attaches a very short distance from the periphery of the articular surfaces.
  • The intertubercular (bicipital) bursa lies between the humeral tubercles cushioning the bicipital tendon.
  • The bursa and tendon are held in place by the transverse humeral retinaculum running between the greater and lesser tubercles of the humerus.

Elbow Joint

  • The joint capsule attaches to the articular surface of the condyle, the periphery of the olecranon fossa and the articular cartilage of the trochlear notch of the ulna. It fuses with the collateral ligaments.
  • Paired collateral ligaments attach the epicondyles to the tuberosities of the radius and ulna.

Structures of the Distal Forelimb

Carpal bones

Carpal bones comprise two rows:

  • Proximally, (mediolaterally), radial, intermediate, ulnar and accessory bones
  • Distally, 1st carpal bone is small or absent, II-IV are all present

Metacarpal and digital bones:

  • 3rd MC is largest and weightbearing = canon bone
  • 2nd & 4th = splint bone
  • 1st= absent or chestnut
  • 5th =absent
  • Proximal phalanx= long pastern bone
  • Midle phalanx= short pastern bone
  • Distal Phalanx = Coffin Bone
Sesamoids:
  • 2 x Distal sesamoids on the palmar part of MC3
  • 1 x DIP joint= Navicular bone – guides DDFT over DIP joint

Joints of the Distal Forelimb

Carpal Joint

The carpal joint is a compound joint composed of:

  • The antebrachiocarpal joint between the radius/ulna and the proximal carpal bones – most movement
  • The middle carpal joint between the two rows of carpal bones
  • The carpometacarpal joint between the distal carpal bones and the proximal metacarpals- least movement

The joint is a synovial joint, comprised of a common outer fibrous capsule and three inner synovial pouches, one for each joint.

  • Collateral ligaments extend from the radius to the metacarpal bones on the medial and lateral aspect of the carpus.
  • The carpal canal houses both the superficial and deep digital flexor tendon within a common synovial sheath.

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 to 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 where it exists
  • Origin: mastoid process of temporal bone, nuchal crest, wing of atlas, transverse processes of the 2nd-4th cervical vertebrae
  • Insertion: deltoid tuberosity and crest of humerus
  • Actions:
    • advances the limb and extends the shoulder joint when limb is in motion
    • draws head and neck ventrally when limb is fixed

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, although in some species, dorsal spinal nerves can innervate

  • Origin: median connective tissue from poll to withers, lying deep to the trapezius
  • 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: caudal border and spine of the scapula
        • One head in the horse and two in species with an acromion (site of second origin)
      • Insertion: deltoid tuberosity on the humerus
    • Teres Major
      • Origin: dorsal part of the caudal scapula
      • Insertion: teres tuberosity midway down humerus
    • Teres Minor
  • There are no defined extensors of the shoulder. Those involved (brachiocephalic m., biceps brachii, supraspinatus, and ascending pectorals) have other, more primary roles.

Muscles of the Elbow

Extensors: Radial n. from the brachial plexus

  • Triceps brachii: Three heads
    • Long head: caudal margin of the scapula
    • Lateral and medial heads: shaft of the humerus
    • Insertion: olecranon, proteced by tricipital bursa against the bone and subcutaneous bursa against the skin
  • Tensor fasciae antebrachii
    • Overlies triceps extending from scapula to olecranon
  • Anconeus
    • Origin: distal humerus
    • Insertion: lateral olecranon

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 next to biceps

Muscles of the Carpal and Digital Joints

Extensors: Radial n. from the brachial plexus

  • Craniolateral position on the forearm
  • Almost all originate from the lateral epicondyle of the humerus
  • Extensor carpi radialis: most medial, metacarpal tuberosity
  • Ulnaris lateralis: most lateral, inserts on accessory carpal bone
  • Extensor carpi obliquus: aka abductor pollicis longus
    • Origin: cranial radius
    • Insertion: head of the metacarpal II
  • Common Digital Extensor
    • Insertion: extensor process of the distal phalanx
  • Lateral Digital Extensor
    • Insertion: dorsal proximal phalanx

Flexors: Median or Ulnar n. of the brachial plexus

  • Caudal position on the forearm
  • Originate from the caaudal medial epicondyle of the humerus
  • Flexor carpi radialis: most medial, inserts on proximal part metacarpal II
  • Flexor carpi ulnaris: most lateral, inserts on the accessory carpal bone
  • Superficial Digital Flexor
    • Insertion: promial extremity of middle phalanx, distal extremity of proximal phalanx
  • Deep Digital Flexor
    • Passes through carpal canal before branching and continues to palmar distal phalanx

Interosseus muscles/Suspensory ligament

Stay Apparatus of the Forelimb

When a horse is standing muscles on opposite sides of joints delicately contract and relax so that it stays in balance. This obviously buts considerable strain and stress on the muscles. To combat this the horse has increased the fibrous content of many muscles. These collagenous muscles have been named collectively as the stay apparatus. This is further divided into the suspensory apparatus and the check apparatus.

Suspensory Apparatus

Interosseous medius/Suspensory ligament

  • Proximally this attaches to the distal row of carpal bones and metacarpus. It runs distally on the palmar surface of the metacarpal bone and then bifurcates proximal to the fetlock, with a small branch connecting to each sesamoid.
  • It then continues distally on the dorsal surface of the proximal phalanx and joins the common digital extensor, which continues to insert on the middle and distal phalanges.

Associated ligaments

  • Collateral sesamoid ligaments – connect the abaxial surface of the sesamoid to the metacarpus and proximal phalanx.
  • Palmar ligaments – connect the sesamoid bones together.
  • Distal sesamoidean ligaments – connect the sesamoids to the proximal and middle phalanges.

This ligaments can be seen as a direct continuation of the interosseous muscle with the sesamoids emdedded in it.

Thus by virtue of the ligamentous arrangement, the suspensory apparatus without any muscular action can resist extension of the fetlock and pastern through the distal sesamoidean ligaments and resist flexion of the pastern and coffin joints through the common digital extensor tendon.

Stay Apparatus

Serratus ventralis support the cranial part of the body and connect the costal side of the scapula to points of attachment on the caudal caervical vertebrae and cranial ribs. It contains a tendinous layer that suspends the body once the muscle relaxs. This though will cause flexion of the shoulder which needs to be balanced by the extensors of the shoulder.
The Biceps brachii fulfils this role. It’s collagenous tendon extends the length of the muscle and divides near the elbow. The short tendon inserts on the radial tuberosity, the long tendon (lacertus fibrosus) continues distally and blends with the tendon of the extensor carpi radialis and forearm fascia; combined these insert on the proximal end of the metacarpus.
Thus the biceps has provision to be able to relax without the collapse of the shoulder, elbow and carpal joints.

Check Apparatus

The superficial digital flexor muscle continues past the carpus as a thick tendon. The accessory or superior check ligament is it’s tendinous radial head that joins the muscle at this point. The tendons continue distally and bifurcate by the fetlock into slips that attach to the medial and lateral aspect of the pastern joint. There is therefore a ligamentous connection from the distal radius to the pastern that can help prevent extension of the carpal and fetlock joint without muscle contractions.

The deep digital flexor also forms a thick tendon just proximal to the carpus. The accessory ligament or inferior check ligament joins the tendon at the middle of the metacarpus. They then continue through the bifurcation of the superficial flexor tendon, through the intersesamoidean groove at the fetlock joint, over the pastern and inserts on the distal phalanx. This ligamentous tissue between the metacarpal region and distal phalanx helps prevent extension of the fetlock, pastern and coffin joints.

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About Annettevet

I'm a 27 year old Norwegian Girl at my final year doing veterinary. I've decided to keep a blog of my experiences as a vet student and my life on the way. “The greatness of a nation and its moral progress can be judged by the way its animals are treated”
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7 Responses to Equine Forelimb

  1. Pingback: Joints and muscles | Vet ressources

  2. beth says:

    second paragraph, on the Interosseous m. should it be .. joins the common digital flexor

    • RUSVM Student says:

      I have that the common digital extensor t.and lateral digital extensor t. fuse before the suspensory ligament joins them via its extensor branches.

  3. kathy gentry says:

    Im looking at a horse that, has a problem with its upper scapula, when he is walking you hear a snapping noise in the shoulder. You can feel it snap aswell. Is this a correlate problem?

  4. I’m a first year vet student and this is so so helpful, thank you so much!

  5. Dustie says:

    Could you recommend a good muscular-skeletal book, offering origins, insertions, nerve supply and actions – for equines and canines. Thanks.
    Love the blog by the way. Very useful.

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