AAEP 56th Annual Convention: Baltimore, Maryland 2010

Standing without effort (mostly)

Dr.KathrynCarmalt,DVM,MSc,BA,AS

UNIVERSITY OF SASKATCHEWAN Saskatoon, Saskatchewan, Canada. www.usask.ca

Stifle

Hock

Suspensory Ligament

Cannon

Knee

Ergot

ElbowGaskin

Flexor tendons

Forearm

Point of Shoulder

Parts of the Horse Leg

Hoof

Shoulder

Point of hip

Pastern

Fetlock

Hip

Thigh

Chestnut

Special adaptations needed

As size doubles -> wt3, yet cross-sectional area2… which means the animal would have to have huge muscle mass to support its wt.

Evolutionary adaptations •  Increase limb length •  Restriction of range

of motion (osteology)

•  Muscle replaced by tendons.

•  Muscles located at proximal end of limb

Passive Stay Apparatus

Increase efficiency of locomotion as well as efficiency at rest.

Passive stay apparatus In the forelimb

•  Syncarcosis •  Biceps brachii •  Lacertus fibrosis •  Suspensory apparatus

In the hindlimb

•  Stifle-locking mechanism •  Reciprocal apparatus/

mechanism of stifle/hock •  Suspensory apparatus

The forelimbs support 60%

of body weight

The Stay Apparatus

Adapted from Dyce, Sack and Wensing (2001)

The Stay Apparatus Fixing the Shoulder (3 parts)

Muscle mass of the shoulder –  Extrinsic (1° serratus ventralis)

Adapted from Dyce, Sack and Wensing (2001)

The serratus group, trapezius group, rhomboid group, and pectorals suspend the trunk between the forelimbs and stabilizes the shoulder.

Extrinsic Muscle Mass of the Shoulder

From Dyce, Sack and Wensing (2001)

Rupture of the synsarcosis (mainly the serratus) is mercifully rare

When this fails!

Adapted from Dyce, Sack and Wensing (2001)

The Stay Apparatus Fixing the Shoulder

•  Muscle mass of the shoulder •  Tendon of the biceps brachii

Specimen, Tuskegee University

Craniolateral view of the shoulder joint

Lesser tubercle

Intermediate tubercle

Scapula

HumerusGreater tubercle

Superglenoid tubercle

Tendon of the biceps brachii

This is what it looks like in

the fresh state

Origin

Fibro-cartilaginous pad

Muscle belly Image courtesy of Peter Flood

Plastinated specimen, Tuskegee University

Tendon of origin of the biceps has a strong,

curved, fibro-cartilaginous pad with a depression on caudal surface that matches

the intermediate tubercle

When the tubercle and the depression are tightly engaged, the shoulder is fixed

Adapted from Dyce, Sack and Wensing (2001)

Locking of the shoulder joint is essential to fixation of the forelimb.

This line represents the biceps brachii

Simplified from Dyce, Sack and Wensing (2001)

Assume that the shoulder is locked and that it can no longer flex

In this case, the only action of the triceps is to extend the elbow holding it in the fully extended position

This action is opposed by the biceps

The Stay Apparatus Fixing the Elbow

•  Long head of Triceps –  Innervation? Why is this important?

Differential Diagnoses?

5.5 mm Cortical Screw (medium-size threads on screws)

6.5 Cancellous Screw (bigger threads on screws)

4.0 Locking Screws(smallest threads can hardly see here)

Tension band wire

Courtesy of Dr. James Carmalt

Adapted from Dyce, Sack and Wensing (2001)

The Stay Apparatus Fixing the Shoulder

•  Muscle mass of the shoulder – Extrinsic

•  Tendon of the biceps brachii •  Lacertus fibrous

Lacertus fibrosus: a substantial structure in the horse (less so in ruminants, absent in small animals)

Biceps

Extensor carpi radialis

Lacertus fibrosus

Simplified from Dyce, Sack and Wensing (2001)

Continuous fibrous band from the scapula to the metacarpus: •  Tendon running through biceps is

continued distally by the lacertus fibrosus to the tendon of insertion of the extensor carpi radialis

•  Under tension in the standing horse - created by opposing forces of biceps and triceps

Thus, when the shoulder is fixed the carpus is held in full extension

•  Superficial digital flexor tendon

•  Deep digital flexor tendon

•  Suspensory ligament – Aka interosseous ligament

•  Sesamoidean ligaments

The Suspensory Apparatus of the Digit

Simplified from Dyce, Sack and Wensing (2001)

Imagine a horse landing from a jump . . . .

The suspensory apparatus.

Superficial digital flexor Medial view

Single origin from medial epicondyle

Adapted from Ellenberger Baum (1912)

Strong accessory (“check”) ligament above the carpus on the medial side

Inserts on proximal end of middle phalanx

Deep digital flexor Medial view

Three heads

Strong accessory (“check”) ligament below the carpus

Inserted on distal phalanx just as in the hindlimb

Adapted from Ellenberger Baum (1912)

And if the system fails:

Bowed tendons and related conditions

Interosseus and sesamoidean ligaments

Interosseus arising from palmar surface of the third metacarpal bone (between MC 2 & 4)

Sesamoidean ligaments

Extensor branch

Adapted from Ellenberger Baum (1912)

What about the hind limb?

Adapted from Robert Kainer in Adams’ Lamness in Horses (1987)

Sacroiliac joint

Hip (flex)

Stifle (flex)

Fetlock (over extend)

Hock (flex)

Effect of body weight on joints of the hind limb

You can’t use muscle to defy gravity for

very long

The Reciprocal Apparatus & Patella Lock

Adapted from Robert Kainer in Adams’ Lamness in Horses (1987)

Superficial flexor

musculotendon unit

Peroneus tertius

The Reciprocal Apparatus

Gastrocnemius mm

Adapted from Robert Kainer in Adams’ Lamness in Horses (1987)

Hip

Stifle

Fetlock

Hock

The Reciprocal Apparatus

The hock and stifle MUST flex together

Which of these limbs has a reciprocal apparatus?

With rest, recovery is spontaneous and complete

Ruptured peroneus tertius

The horse can still bear weight in the limb

Falls in which the distal hindlimb is forcibly retracted

From Adams’ Lamness in Horses (1987)

Wrinkles

Failure of the Reciprocal Apparatus

Gastrocnemius rupture Peroneus tertius rupture

The Stay Apparatus Fixing the hip & pelvis

•  Muscle mass of the hip •  Locking of the stifle

Adapted from Robert Kainer in Adams’ Lamness in Horses (1987)

The Stay Apparatus Fixing the Stifle

Adapted from Robert Kainer in Adams’ Lamness in Horses (1987)

•  The “patella lock” –  The patella is brought proximal and

lateral by quadriceps and tensor fascaie latae

–  Medial patellar ligament “catches” over medial trochlear ridge of distal femur

•  The “patella unlock” –  Quadriceps bring patella proximal and

medial into normal position

Patella

Middle patellar ligament

Left equine stifle,

cranial view

Medial patellar ligament Lateral

patellar ligament

Medial ridge of trochlea

Quadriceps insert onto patella VIA the patellar ligaments

Patella

Medial miniscus

Left equine stifle, medial

view

Medial collateral ligament

Medial patellar ligament

Medial ridge of trochlea

Articular surface of the patella

Medial miniscus

Left equine stifle, cranio-medial view

Medial patellar ligament

Medial ridge of trochlea

Lateral ridge of trochlea

The condition can recur so probably restablishment of the ligament

Inability to release the locked patella

Cutting the medial patellar ligament gives immediate relief

Young horses in poor condition

From Adams’ Lamness in Horses (1987)

Adapted from Robert Kainer in Adams’ Lamness in Horses (1987)

Just a little more detail needed

Fixation of the hock

Trochlea of talus

Calcaneus

Right equine hock, lateral

view

3rd metatarsus

Calcanean tuber

Collateral ligament

Plantar ligament

4th metatarsus

Trochlea of talus

Calcaneus

Right equine hock, medial

view 3rd metatarsus

Calcanean tuber

Collateral ligament

Plantar ligament

2nd metatarsus

Groove for deep digital flexor tendon

Medial tendon of insertion of

the cranial tibial muscle

Equine hock, medial view

Calcanean tuber covered by superficial

flexor

Plantar ligament

Deep digital flexor tendon

The Suspensory Apparatus of the Hind Limb

•  Same basic components as front limb

•  No superior check in hind limb

Adapted from Robert Kainer in Adams’ Lamness in Horses (1987)

Straight sesamoid ligaments

Sesamoid ligaments

Superficial flexor insertion

Sesamoid bones embedded in fibrocartilage

Interosseus muscle!

Oblique sesamoid ligaments

Deep flexor insertion

Passive stay apparatus In the forelimb

•  Syncarcosis •  Biceps brachii •  Lacertus fibrosis •  Suspensory apparatus

In the hindlimb

•  Stifle-locking mechanism •  Reciprocal apparatus/

mechanism of stifle/hock •  Suspensory apparatus