Features and Functioning of the Distal Forelimb

Home , Epicondyle, Forelimb, Palmar carpal ligament

1/19/2018

FEATURES AND FUNCTIONING OF THE DISTAL FORELIMB

Horseman’s Clinic January 27, 2018 Scott Hexum

1. Identify the bones of the limb. 2. Describe the major muscles/tendons involved in movement of the distal limb and digit. 3. Demonstrate the blood supply to the distal limb. 4. Demonstrate the innervation of the distal limb.

THE FORELIMBS ARE RESPONSIBLE FOR SUPPORTING ABOUT 60% OF THE HORSE’S WEIGHT. THE HINDLIMBS ARE RESPONSIBLE FOR SUPPORTING THE OTHER 40% OF THE HORSE’S WEIGHT, AS WELL AS CREATING THE FORCES NECESSARY TO PROPEL THE HORSE FORWARD.

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Most muscles of the limb are included in one of two functional groups:

Extensors Flexors

These groups acting together, but in opposition to each other, allow for movement of the limb, and therefore, create locomotion.

Pelvis Femur Patella Tibia Fibula (fused to tibia)

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Metacarpal 3 (Cannon bone) Metacarpal 2 (medial splint bone) Metacarpal 4 (lateral splint bone) Long pastern bone Proximal sesamoid bones Short pastern bone Coffin bone Navicular bone

Metatarsal 3 (Cannon bone) Metatarsal 2 (medial splint bone) Metatarsal 4 (lateral splint bone) Long pastern bone Proximal sesamoid bones Short pastern bone Coffin bone Navicular bone

Forelimb Hindlimb  Shoulder (Glenohumeral)  Hip (coxofemoral)  Elbow  Stifle  Carpus  Tarsus  Fetlock  Fetlock  Pastern  Pastern  Coffin  Coffin

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 The distal limb, meaning everything after the carpus in the forelimb, and after the tarsus in the hindlimb, are the exact same with the exception of some names.  Anything with “palmar” in it=forelimb.  Anything with “plantar” in it=hindlimb.  Dr. Madsen, who will be discussing ultrasounding later this morning, is going to focus on the forelimb, so I will only talk about the forelimb in regards to the distal limb. Just keep in mind that, as I said, the hindlimb is almost exactly the same.

 Carpus is actually a set of three joints.  It is maintained in complete extension in a normal standing posture.  Capable of a high degree of flexion.

Lateral Medial

Dorsopalmar (A) radiograph of the carpus. 1, radius 2, accessory carpal bone Proximal row 3, radial carpal bone 4, intermediate carpal bone 5, ulnar carpal bone Distal row 6, first carpal bone (when present) 7, second carpal bone 8, third carpal bone 9, fourth carpal bone 10, second metacarpal (medial splint) 11, third metacarpal (cannon bone) 12, fourth metacarpal (lateral splint)

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90-100° of flexion

~45° of flexion

Very few degrees of flexion, if any.

 Found laterally and moving to the front of the limb as it proceeds farther down the limb.  Primary muscle responsible for bringing the flexed joints of the digit back into an extended position.  Begins at the lateral epicondyle of the humerus.  Inserts on the extensor process of the coffin bone.

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 These two tendons, originating as muscles, are responsible for flexing the carpal, fetlock, pastern, and coffin joints.  As the muscle belly of each tendon contracts, it shortens the length, lessening the angle between the bones of each joint (at the flexor surface).  The SDF and DDF tendons are able to glide against each other with a minimal amount of friction.

Deep digital flexor tendon Superficial digital flexor tendon

 The multiple bones that make up the carpus are held together by a complex series of ligaments. Their palmar surface (the “back of them”) are all attached to the palmar carpal ligament.  Forms a base which helps hold the carpus together.  Also forms a smooth surface for the deep digital flexor tendon to move across within the carpal canal.

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Lateral

D Ulnar Accessory Deep digital flexor tendon o r Superficial digital flexor tendon s Intermediate P a a l l m Radial a r

Palmar carpal ligament Medial

1, Large metacarpal bone; 2, proximal phalanx; 3, middle phalanx; 4, distal phalanx; 4′, digital cushion; 5, proximal sesamoid bone; 6, distal sesamoid (navicular) bone; 7, dorsal pouch of fetlock joint; 7′, capsular fold; 7″, palmar pouch of fetlock joint; 8, 9, dorsal pouch of pastern and coffin joints; 10, navicular bursa; 11, interosseus; 12, straight sesamoidean ligament; 13, deep flexor tendon; 14, digital sheath; 15, connective tissue bridge; 16, distal navicular ligament; 17, common digital extensor tendon; 18, superficial flexor tendon.

 A great deal of pressure can be put upon a tendon as it crosses a bone, which would result in damage to the tendon.  A bursa is similar to a fluid-filled balloon and cushions a tendon as it crosses certain parts of certain bones.

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MRI of the digit: 2. Long pastern bone 3. Short pastern bone 4. Coffin bone 4’. Digital cushion 6. Navicular bone 10. Navicular bursa 13. Deep digital flexor tendon 17. Common digital extensor tendon

You can see where the DDF rounds the navicular bone would be a spot where a great amount of pressure was created. The bursa provides a cushion to prevent damage.

 An extension of the joint capsule of the fetlock joint.  Fluid-filled.  It may become inflamed causing a distension of the joint known as “wind puffs” or “galls.”  This pouch is easily accessible by needle from either the medial or lateral side.

 Landmarks:  Interosseous ligament  Fetlock joint  Cannon bone  Buttons of the splint bones

Lateral radiograph of fetlock joint and digit. 1, Metacarpal 3 (aka cannon bone) 2, proximal sesamoid bones 3, proximal phalanx (aka long pastern bone) 4, middle phalanx (aka short pastern bone) 5, distal phalanx (aka P3 or coffin bone) 5′, extensor process of distal phalanx 6, navicular bone 7, wall of hoof.

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The weight of a horse, especially while running, generates incredible downward forces. There have to be mechanisms in place to hold everything together, as well as provide a “spring” type of action to allow the limb to quickly change the direction of forces and move forward. Annular ligaments help with this. 2, interosseus; 5, palmar annular ligament (at the fetlock joint) 6, proximal digital annular ligament 7, distal digital annular ligament

The SDF and DDF “switch” places just after the fetlock joint. The SDF flattens to form a sleeve (which attaches to the long and short pastern bones) and the DDF passes through it to attach to the coffin bone. The “tunnel” created is the flexor manica.

4, interosseous 5, deep digital flexor 6, superficial digital flexor 7, extensor branch of interosseous

Arises from the palmar carpal ligament, and attaches to the proximal sesamoid bones.

Continues as the medial and lateral extensor branches of the interosseous, which then attach to the common digital extensor tendon on the dorsal (front) of the limb.

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4, interosseous 5, deep digital flexor 6, superficial digital flexor 7, extensor branch of interosseous

Also known as the interosseous ligament, it is a strong and flat, mostly tendinous band of tissue. It begins as an extension of the palmar carpal ligament.

4, interosseous 5, deep digital flexor 6, superficial digital flexor 7, extensor branch of interosseous

Also known as the interosseous ligament, it is a strong and flat, mostly tendinous band of tissue. It begins as an extension of the palmar carpal ligament.

“Energy, stored within the apparatus (and in the flexor tendons) by stretching on hoof impact, is released at the end of the stride, which allows the joint to flex and impart forward impetus.” (Dyce, 4th edition).

 14’-the accessory check ligament is very difficult to palpate.  Connects palmar carpal ligament to the deep digital flexor tendon (mid-cannon).  Main function seems to be reinforcing the digital flexor tendons and strengthen the entire flexion-extension mechanism.

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M L

The major arteries of the right forelimb, palmar view. 11, collateral ulnar a. 12, transverse cubital a. 13, common interosseous a. 14, median a. 15, radial a. 16, 16′, medial and lateral palmar aa. 17, 17′, medial and lateral palmar metacarpal aa. 18, 18′, medial and lateral digital aa.

M L

The major arteries of the right forelimb. B, Palmar view. 11, collateral ulnar a. 12, transverse cubital a. 13, common interosseous a. 14, median a. 15, radial a. 16, 16′, medial and lateral palmar aa. 17, 17′, medial and lateral palmar metacarpal aa. 18, 18′, medial and lateral digital aa. The medial and lateral palmar arteries lie next to the tendons of the digital flexors. The medial and lateral palmar metacarpal arteries lie against the cannon bone, deep to the interosseous ligament.

M L

Therefore, the palmar arteries are easily damaged, but the palmar metacarpal arteries are well-protected from harm.

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M L

Therefore, the palmar arteries are easily damaged, but the palmar metacarpal arteries are well-protected from damage.

This redundancy of vascular supply is known as collateral circulation and helps ensure an area receives blood even when one of the vessels is damaged.

 Medial palmar/plantar nerve  Lateral palmar/plantar nerve  Communicating branch  Medial digital artery  Lateral digital artery

Distribution of the nerves in the right forelimb; medial view. 11′, caudal cutaneous antebrachial n. 11″, palmar branch 11′″, dorsal branch 12, median n. 12′, muscular branches 12″, lateral palmar n. 12′″, medial palmar n. 13, medial palmar digital n. 13′, 13″, dorsal branches.

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Distribution of the medial palmar nerve. 1, Medial palmar n. 2, communicating branch (connects the medial and lateral palmar nerves). 3, medial digital palmar n. 3′, dorsal branch 4, medial palmar artery and vein 5, medial digital artery and vein.

The lateral side would be a mirror image of this medial side.

 http://vetmed.illinois.edu/courses/imaging_anatomy/ index.html  You’re welcome to use this site to look up skeletal structures and as a reference when looking at radiographs.