3- Thoracic Limb (Large Animal).docx

- **Large Animal: General Info** - Large animals (such as horses) constantly experience a downward force, due to their body weight. - The "**stay apparatus**" is a fibrous mechanism that allows them to oppose gravity without expending energy. - Approxim...

- **Large Animal: General Info** - Large animals (such as horses) constantly experience a downward force, due to their body weight. - The "**stay apparatus**" is a fibrous mechanism that allows them to oppose gravity without expending energy. - Approximately 60% of the horses' body weight is placed on the thoracic limbs. - If the thoracic limbs were left unsupported, the jointed boney column of the thoracic limb would collapse under the weight of the animal. - The body weight is transferred to the thoracic limbs via the pectoral muscles and serratus ventralis muscle. - **Walking postures** - **Digitigrade** walkers bear weight on their digits - Examples include: dogs and cats. - **Unguligrade** walkers bear weight on the distal phalanx. - Examples include: horses and cattle. - **Plantigrade** walkers bear weight on their digits, metatarsals, and plantar tarsus. - Examples include: humans, bears, squirrels, and skunks. - If a dog or cat walks plantigrade, this can indicate pathogenesis. - **Bones: Thoracic Limb** - Bones of the large animal thoracic limb include the" scapula, humerus, radius, ulna, carpus, metacarpus, phalanges, and proximal and distal sesamoid bones. - In horses, - The distal sesamoid bone is referred to as the "Navicular" bone. - The radius and ulna are almost completely fused, and the ulna is no longer discernable at the distal radius. - The point of the shoulder can be referenced by the Greater Tubercle of the humerus. - Their approximate center of gravity is located at the 15^th^ thoracic vertebrae. - The metacarpal 3 bone has the common name of the "Cannon" bone. - This is the bone that they primarily weight bear on. - The metacarpal 2 and 5 bones have the common name of the "Splint" bones. - In **dogs**, - The radial and intermediate carpal bones are fused, forming the "radiointermediate carpal bone". - In **ox (cows),** - They weight bear on their metacarpal 3 and 4 bones. - The carpal bones 2 and 3 are fused. - The radius and ulna are partially fused. - The carpus has the common name of the "Knee". - Flexion of the carpus requires elongation of the extensor carpi radialis muscle. - **Joints: Horse Thoracic Limb** - The Shoulder joint is referred to as the "Glenohumeral" joint. - The Elbow (or cubital) joint is referred to as the "Humeroradioulnar" joint. - The Proximal Interphalangeal joint has the common name of "Pastern" joint. - The Distal Interphalangeal joint has the common name of "Coffin" joint. - The Metacarpophalangeal joint has the common name of "Fetlock" joint. - The Carpal (or antebrachiocarpal) joint is composed of the middle carpal joint, and the metacarpophalangeal joint. - **Stay Apparatus: General Info** - The **functions** of the stay apparatus include: - Reducing the energy required to stand, which allows rest while standing - Making movement efficient via specific groups of muscles and tendons/ligaments working in synchrony to conserve energy. - The stay apparatus allows horses to doze while standing up, while using minimal muscular energy. - The coordination of many individual structures act to "lock" the limb in extension, which prevents collapse, despite putting in minimal effort. - **Stay Apparatus: Shoulder Joint Stabilization** - The shoulder joint is stabilized via prevention of the shoulder flexing. - The tendon of the biceps brachii muscle prevents the shoulder from flexing - The tendon of origin of the biceps brachii muscle engages on the intermediate tubercle of the humerus (at a fixed distance), making the shoulder joint unable to flex. - Tension on the origin of the biceps brachii tendon "interdigitate" with the intermediate tubercle of the humerus, which locks the shoulder joint into place. - **Stay Apparatus: Elbow Joint Stabilization** - Elbow joint stabilization involved preventing elbow flexion while under weight. - Since the shoulder is "locked", forces of the humerus rests on a nearly vertical radius. - The body weight is balancing on the radius in a "crooked post". - Only small corrections are necessary to keep weight balanced on the radius. - There is minimal force acting on the elbow to flex or extend (if balanced). - The triceps muscle remains relatively flaccid during standing rest. - **Stay Apparatus: Carpal Joint Stabilization** - Carpus stabilization involves preventing the carpus from flexing or extending. - The tension on the biceps brachii muscle is transmitted to the extensor carpi radialis muscle via the "**lacertus fibrosus**". - The carpus cannot flex unless the extensor carpi radialis muscle is relaxed. - The strong **palmar carpal ligament** aids in preventing hyperextension of the carpal joint. - The palmar carpal ligament extends from the distal radius to the proximal metacarpal 3 bone, and can be found at the dorsal border of the carpal canal. - **Stay Apparatus: Check Ligaments** - Tension on the SDF (superficial digital flexor) and DDF (deep digital flexor) tendons are "checked" by the accessory ligaments, which are attached to these tendons. - The **proximal check ligament** is attached to the tendon of the SDF, above the carpus. - The **distal check ligament** is attached to the tendon of the DDF, below the carpus. - A downward force on the fetlock joint (hyperextension) puts tension on the SDF and DDF tendons. - This downward force is counteracted by the proximal and distal check ligaments. - **Stay Apparatus: Fetlock, Pastern, and Coffin Joint Stabilization** - The stabilization of the fetlock, pastern, and coffin joint is due to the suspensory apparatus, and involves the prevention of hyperextension of these joints. - The suspensory apparatus involves: - **Interosseus muscle (AKA: suspensory ligament)** - The medial and lateral extensor branches unite with the tendon of the common digital extensor muscle. - The downward force of the fetlock stretches the suspensory ligament, which inserts on the proximal sesamoid bones. - Thus, opposing hyperextension of the fetlock. - The suspensory ligament originates at the proximal metacarpus, and it inserts on the proximal sesamoids. - The extensor branches of the suspensory ligament, join the common digital extensor tendon. - Proximal sesamoids - The proximal sesamoid bones are covered by the **scutum**. - (Distal) Sesamoid ligaments - Straight sesamoidean ligament: have an "I" shape - Oblique sesamoidean ligament: have a "V" shape - Cruciate sesamoidean ligament: have a "X" shape - SDF and proximal check ligament - DDF and distal check ligament - The **distal sesamoidean ligaments** (straight, cruciate, oblique) help to counteract hyperextension of the fetlock joint. - All originate from the proximal sesamoid bones, and have distal attachments to the proximal phalanx. - The straight sesamoidean ligament attaches to the middle phalanx and the proximal phalanx.