Anatomy of the Equine Hind Limb

Original Editor - Rosanne Lejeune Top Contributors - Tarina van der Stockt, Jess Bell and Lucinda hampton

Introduction

The equine hind limb is also referred to as the pelvic hind limb. When working with horses, it is important to be able to accurately assess, diagnose and manage an equine patient. To do this, a good understanding of equine anatomy is essential.

Anatomy

Pelvic hind limb bears 40-45% of the weight and provides the majority of propulsion for locomotion.

Bones

• Os coxae
  • The Tuber coxae and tuber sacrale both palpable
  • Tuber ischii is located underneath the hamstrings
• Femur
  • Greater trochanter has a cranial and caudal part for gluteal attachments (deep & middle)
  • Third trochanter is prominent laterally for superior gluteal attachment
  • Medial trochlear ridge is enlarged
• Patella
  • Parapatellar fibrocartillage forms hook medially

• Tibia
  • Large and only weight bearing component of crus (stifle/ knee)
  • Large tibial tuberosity – patellar ligament
  • Medial tibia is subcutaneous
  • Cochlea is inclined craniolaterally. This causes the lower limb to move laterally on flexion

• Fibula is greatly reduced
  • Distally incorporated into tibia
  • Proximally tightly articulated with tibia
  • It has a short shaft
• Tarsals
  • Proximal row (calcaneus & talus)
    • Calcaneus is enlarged for muscle attachments
    • Distal sustentaculum tali forms the plantar groove
    • Talus has oblique trochlea
  • Middle row (central talus)
  • Distal row
    • 1st & 2nd bones fused (medially)
    • 3rd wedged shaped
    • 4th large and deep

Joints

Sacro iliac

• Type:
  • Synchondrosis (synovial joint – sacropelvic surface of illium)
• ROM:
  • Minimal. Designed for stability
  • Accessory movements - rotation
• Supporting structures:
  • Dorsal and ventral Sacroiliac ligaments
  • Sacrotuberous

Hip

• Type:
  • Ball and socket
  • Femoral head and acetabulum of the illium, ischium and pubis
  • A band of fibrocartilage on the rim of the acetabulum deepens acetabulum
• ROM:
  • Flexion and extention
  • Minimal – adduction and abduction
• Supporting structures:
  • Acetabular lip (fibrocartilage) continues as transverse ligament
  • Ligament of the femoral head
  • Accessory ligament (horse)
• Synovial structures and tendon sheaths:
  • Large joint capsule
  • Rectus femoris has a bursa at illium
  • Internal obturator
  • Bursa between gluteals and greater trochanter

Stifle

• Type:
  • Hinge joint with two cartilages/menisci
  • Femur & tibia – femorotibial (condylar)
  • Femur & patella – femoropatella (gliding joint)
• ROM:
  • Femur and tibia – Flexion and extension
  • End flexion there is internal rotation
  • End extension there is external rotation
• Supporting structures:
  • Meniscal ligaments
  • Menisicii attached cranially by Transverse ligament
  • Cranial tibial ligament (medial and lateral meniscus) attaches menisci to tibial spine
  • Lateral meniscus has extra ligament – meniscofemoral
  • Seven other ligaments
    • Lateral and medial collateral
    • Cranial and caudal cruciate ligament
    • Lateral, middle and medial patellar ligaments

• Synovial structures and tendon sheaths:
  • Three synovial sacs
    • Femoropatellar
    • Medial and lateral femorotibial
    • Link between femoropatellar and femorotibial
  • Joint capsule – extends to femoral extensor fossa
  • Bursa between tibial crest and semitendinosus attachment

Hock/Tarsal

Click on this link to see the sideview of the hock joint

• Type:
  • Tarsocrucal joint (TCJ) has the greatest movement
  • Proximal and distal intertarsal joint
  • Tarsometatarsal joint
  • Intertarsal
• ROM:
  • TCJ – flexion and extension and Lateral & rotatory accessory movements
  • Others – small amount of translatory and rotatory
• Supporting structures:
  • Medial and lateral collaterals – each have long and short pair
  • Annular ligament
  • Dorsal and plantar ligaments
  • Intertarsal ligaments
  • Proximal & distal transverse ligaments

• Synovial Structures and tendon sheaths
  • Each of 4 joints has own synovial sac
  • Bursa with most tendons crossing joints
  • Bursa separates SDF from gastrocnemius tendon at tuber calcanei
  • Most tendons have sheaths in this region

Muscles

Muscle	Origin	Insertion	Action	Nerve
Pectineus	Pubis	Femur	Adducts the limb and flexes the hip	Obturator
Psoas Major	Ventral surface of lumbar vertebrae	Lesser trochanter of femur	Flexes the hip and externally rotates the thigh	Ventral branches of thoracic and lumbar nerves and femoral nerves
Iliacus	Ventral surface of wing of the ilium
Gracilis	Pubic symphysis	Medial tibia	Adducts the limb	Obturator
Adductor	Pubic symphysis	Caudal surface of femur	Adducts the limb	Obturator
Internal Obturator	Pelvic floor	Trochanteric fossa of femur	Lateral rotation of the femur	Ischiatic
Gemelli	Ischium	Trochanteric fossa of femur	Lateral rotation of the femur	Ischiatic
External Obturator	Ventral surface of pubis and ischium	Trochanteric fossa of femur	Adducts the thigh	Ischiatic
Quadratus femoris	Ischium	Caudal surface of femur	Extends the hip and adducts the thigh	Obturator
Deep gluteal	Body of ilium	On or near greater trochanter	Extension and abduction	Gluteal
Superficial Gluteal	Dorsal to hip joint	Third Trochanter	Abducts the hip	Gluteal
Tensor fascia latae	Tuber coxae	Lateral femoral fascia	Hip flexion and stifle extension via tension of lateral femoral fascia	Gluteal
Biceps femoris	Ischiatic tuberosity (Tuber Ischii)	Greater Trochanter	Extends and abducts the hip	Gluteal
Semitendinosus	Ischiatic tuberosity	Tibia and calcaneal tub.	Extends the hip Flexes the stifle Extends the tarsus	Ischiatic
Semimembranosus	Ischiatic tuberosity	Femur and tibia	Extends the hip Flexes or extends the stifle	Ischiatic
Middle Gluteal	Wing of ilium	Greater trochanter of femur	Extension and abduction	Gluteal
Rectus femoris	Ilium (cranioventral iliac spine)	Patella and patella tuberosity	Flexion of the hip Extension of the stifle	Femoral
Vastus lateralis	Proximal femur		Extends the stifle
Vastus medialis
Vastus intermedius

Table of muscles of the Hip and Stifle

Muscle	Origin	Insertion	Action	Nerve
Gastrocnemius	Distocaudal surface of the femur	Calcaneal tuberosity	Extends the tarsus Flexes the stifle	Tibial
Long digital extensor (combined in this image with long digital extensor) Lateral digital extensor combined with long digital extensor (see below)	Extensor fossa of femur	Extensor processes of digital phalanges	Flexes the hip Extends the stifle	Peroneal
Soleus	Head of fibula	Tendon and lateral head of gastrocnemius	Extends the tarsus	Tibial
Lateral digital extensor (see long digital extensor image above)	Fibula	Lateral aspect of digit	Extends the digits Flexes the tarsus	Peroneal
Deep digital flexor	Tibia and fibula	Distal Phalanx	Extends the tarsus Flexes the digits	Tibial
Superficial digital flexor	Caudal distal femur, deep to gastrocnemius	Calcaneal tuberosity Middle phalanx	Flexes the stifle Extends the tarsus Flexes the digits	Tibial

Table of muscles of the Stifle, hock and pes

Nerves to the Pelvic Limb

• Femoral nerve (L4, L5 sometimes L3, L6)
  • Muscle Innervation – Stifle extensors (quadriceps), Iliopsoas
  • Cutaneous Innervation – Medial aspect of limb
• Obturator nerve (L5, L6)
  • Muscle Innervation– Adductors (obturator, adductors, gracilis)
  • Cutaneous Innervation– nil
• Gluteal nerve (L6, L7, S1)
  • Muscle Innervation– gluteals, TFL, bicaps femoris, semitendinosus
  • Cutaneous Innervation– nil
• Sciatic nerve (L6, L7, S1, S2)
  • Muscle Innervation– biceps femoris, semitendinosus, semimembranosis
  • Cutaneous Innervation– see tibial and fibula branches
• Tibial nerve (S1, S2)
  • Muscle Innervation– extensors of hock, flexors of digits
  • Cutaneous Innervation– caudal aspects of the limb below stifle
• Fibular nerve
  • Muscle Innervation– flexors of the hock, extensors of the digits
  • Cutaneous Innervation– cranial and lateral aspects of limb

    

    Nerves of the Equine Limb

Pelvic Limb Stay Apparatus

Function

The pelvic limb stay apparatus is used by the horse to support the weight of the caudal end of its body while using a minimal amount of muscular activity. When employed by one pelvic limb, the stay apparatus allows the other pelvic limb to be placed in a "resting" position with just the tip of the hoof touching the ground. Although the stay apparatus reduces the amount of energy required to remain standing, the amount of muscular effort is not reduced. This explains horses can be observed switching their weight from one hind limb to another.

How does the stay apparatus function?

The pelvic limb stay apparatus has three essential elements.

1. The stifle joint locking mechanism, which allows the weight of the caudal body to rest, essentially, on the locked joint
2. The reciprocal mechanism, which ensures that the stifle and hock joints will move in unison, and the leg will move in a smooth, coordinated manner
3. All other ligaments / tendons in the distal limb