Anatomy

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Pelvic Girdle and Hip

Bones

The pelvic girdle is formed by two hip bones which are joined ventrally at the cartilagenous pelvic symphysis and articulate dorsally with the sacrum. The three components of each hip bone are the Ilium, Pubis and Ischium.

The bone that articulates with the hip bones to form the hip joint is the Femur.

Equine Bone Specifics

  • Hindlimb - Anatomy & Physiology - Ilium
    • In the horse the tuber coxae is visible and an important palpable landmark.
    • The iliac crest is thin and concave.
    • The ileal wing is orientated in an vertical manner.
  • Pubis
    • Presence of the pubic groove. This crosses the ventral surface of the iliopubic eminence towards the acetabulum. In the groove lies the accessory ligament of the head of the femur.
    • A dorsal pubic tubercle exists on the stallion.
  • Ischium
    • The ischial tuberosity is linear in shape.
    • The ischial arch is shallow and irregular.
  • Hindlimb - Anatomy & Physiology - Femur
    • The femoral head notch is wedge-shaped and opens towards the medially side of the head.
    • The greater trochanter extends beyond the the dorsal limit of the femoral head.
    • The horse has an extra process called the third trochanter and is situated on the lateral aspect of the proximal third of the shaft. The superficial gluteal inserts on here.
    • The caudal distal aspect of the femoral shaft is hollowed out forming the supracondylar fossa. This provides a larger area of origin for the superficial digital flexor muscle.
    • The medial trochlear ridge is larger than lateral trochlear ridge. The medial trochlear ridge also has a protuberance projecting proximally.

Joints and Synovial Structures

Sacroiliac Joint

  • In horses the short branch of the dorsal sacroiliac ligaments connects the sacral tuberosity to the spinous processes of the sacrum.
  • The sacrotuberous ligament is a broad sheet-like ligament, which extends between the transverse processes of the first caudal vertebrae and the dorsal border of ischium and ilium. In this ligament are the lesser and greater ischiatic foramen, these are present to allow for blood vessels, nerves and tendons.

Coxafemoral/Hip Joint

  • The horse has a limited range of movement compared to a dog in its hip. This is mainly restricted to flexion and extension and is a result of the conformation of its femoral head, intraarticular ligaments and a large muscle mass around the joint.
  • Unlike other species the horse also has an extra ligament present in the joint; the accessory ligament.

Musculature

The muscles affecting the pelvic girdle and hip can be divided into two distinct groups:

  • Girdle Musculature
    • Psoas minor - the muscle is marked by many tendinous intersections
    • Iliac muscle - a fleshy muscle that is cross-sectionally flat cranially and round caudally. It originates from two heads; a strong lateral head from the wing of the ilium and a small medial one from the ileal shaft. They enclose the greater psoas and will unite to form a common tendon for insertion.
    • Quadrate Lumbar - a thin tendinous muscle whose origin is at the proximal end of the last rib and the transverse process of the cranial lumbar vertebrae. Its insertion is the transverse processes of the caudal lumbar vertebrae and the sacral wing.


  • Rump Muscles
    • Superficial gluteal
      • Origin - gluteal fascia.
      • It unites with the tensor muscle fascia lata, passes over the greater trochanter and attaches onto the third trochanter and radiates into the femoral fascia.
      • A Synovial bursa exist between the tendinous insertion and the third trochanter.
    • Middle Gluteal
      • Origin - 1st lumbar vertebrae, aponeurosis of the longissimus muscle, the sacrum and the broad sacrotuberous ligament.
      • Insertion - The short portion inserts on the greater trochanter, the deep portion inserts on both the greater trochanter and intertrochanteric crest.
    • Tensor Muscle of the Fascia Lata
      • Origin - coxal tuberosity
      • Insertion - it joins the fascia lata and so indirectly attaches to the patella, lateral pattelar ligament and cranial border of the tibia. A caudodorsal detachment joins the superficial gluteal and so attaches to the greater trochanter.
    • Biceps
      • Origin - Vertebral head - spinous processes of the sacral vertebrea, the caudal border of the sacrotuberous ligament and the ischial tuberosity. Pelvic head - ventral aspect and caudal rim of ischium.
      • Insertion - the two bellies then unite and redivide into three parts that form an extensive aponeurosis.
        • The cranial branch inserts just distal to the third trochanter on the caudal aspect of the femur, to the patella and to the lateral patellar ligament.
        • The middle branch inserts onto the crural fascia, the lateral patellar ligament and the cranial aspect of the tibia.
        • The caudal branch radiates into the crural fascia and forms the tarsal tendon. This once combined with the detachment from the semitendinous inserts on the calcaneus.
    • Semitendinous
      • Origin - Pelvic head - ventral aspect of ischial tuberosity. Vertebral body - sacrum, 1st caudal vertebrae and the sacrotuberous ligament.
      • Insertion - The two bellies unite and run as a flat tendon to the medial side of the leg. It then partially fans out into the crural fascia and partially inserts onto the cranial aspect of the tibia. The remaining tendon joins the tarsal tendon.
    • Semimembranous
      • Origin - Vertebral head - sacrotuberous ligament and 1st caudal vertebrae. Pelvic head - ventral aspect of ischial tuberosity.
      • Insertion - the bellies unite and insert at three places. The medial femoral condyle via a short tendon, the medial collateral ligament of the femerotibial joint and to the medial condyle of the tibia via an aponeurosis.
    • Sartorius
      • Origin - iliac fascia and the tendon of the psoas minor.
      • Insertion - medial patellar ligament, crural fascia and so inserts on the tibial tuberosity.
    • Gracilis
      • Origin - the horse's gracilis also originates from the accessory ligament of the femoral head.
    • Adductor Muscles
      • The horse has a short adductor and a greater adductor.
      • Insertion - they both insert along the entire medial aspect of the femur from the lesser trochanter to the medial condyles and the medial collateral stifle ligament.
    • Internal Obturator
      • Origin - cranial and medial border of the obturator foramen, the pelvic symphysis and pelvic aspect of the body of the ilium.
      • Insertion - trochantic fossa having passed over the lesser sciatic notch.

Proximal Hindlimb including Stifle and Tarsus

Bones

The bones immediately distal to the femur are the Tibia, Fibula, Patella and some minor sesamoid bones. Some of these are involved in the stifle joint, weight-bearing and providing attachment for muscles.

Distal to these bones are the complex series of bones that make up the tarsus, these are the Tarsal bones and Metatarsals.

Equine Bone Specifics

  • Patella
    • The patella is extended medially by fibrocartilage.
  • Tibia
    • The caudal aspect of the tibial shaft has several grooves where the popliteal and digital flexor muscles attach.
    • The central ridge of the cochlea is orientated cranio-laterally.
    • The horse has a lateral malleolus formed due to the fusion of the distal end of the Fibula to the Tibia.
  • Fibula
    • The head articultes with the Tibia and as the shaft continues distally it merges with the Tibia.
    • The distal portion is entirely incorporated into the Tibia as the lateral malleolus. This has a seperate ossification centre.
  • Talus
    • The trochlear ridges are alligned in a mediolateral direction. Thus when the hock flexes the digit moves forward and out.
    • The horse articulates with the central tarsal bone via flat surface rather than a small trochlear as in other species.
  • Calcaneus
    • The calcaneal tuberosity is highly pronounced and is marked by a groove on its proximal aspect.
    • There is a pointed process at the base that articulates with the fourth tarsal bone.
  • Distal Row of Tarsal Bones
    • The horse only has four bones. The central, 3rd and 4th tarsal bones remain but the 1st and 2nd tarsal bones are fused together.

Joints and Synovial Structures

Stifle Joint

  • The horse has three patellar ligaments; the middle patellar ligament, the lateral patellar ligament and the medial patellar ligament.
  • The middle patellar ligament connects the the cranial aspect of the patella apex to the tibial tuberosity. It has two associated bursae; one between the ligament and the groove on the tibial tuberosity and one between the proximal part of the ligament and the patella apex. It is palpable just proximal to the tibial plateau.
  • The medial patellar ligament connects the parapatellar fibrocartilage to the medial aspect of the tibial tuberosity.
  • The lateral patellar ligament the cranio-lateral aspect of the patella to the lateral aspect of the tibial tuberosity.
  • This arrangement is crucial to the stay apparatus and provides a locking ability. Contraction of the quadriceps while in rest enables the patella to be hooked over the trochlea of the femur due to the binding loop formed by the patella, middle and medial patellar ligaments.
  • The femeropatellar joint capsule sometimes communicates with the lateral femerotibial cavity and usually with the medial. No communication exists between the femorotibial joints.

Tarsal Joint/Hock

  • The horse has no movement in its proximal intertarsal joint.

Musculature

Muscles of the Stifle

  • The straight muscle of the quadriceps is larger than in other species and covers the lateral aspect of the femur too. It originates with two tendons from the cranial side of the acetabular rim. A synovial bursa lies between the middle patellar ligament and the tibial tuberosity.
  • The popliteal is triangular in the horse and is covered by the gastrocnemius and the superficial digital flexor. Its tendinous origin is continuous with the femerotibial joint capsule thus acting as a tendon sheath.

Muscles of the Equine Crus

Vasculature of the Hindlimb

Arteries of the Hindlimb

  • The essential difference between the equine arterial system of the hindlimb and that of the dog is the supply to the distal hindlimb.
  • The dorsal pedal artery forms the great metatarsal artery after the perforating branch branches off. This is the largest artery of the foot and travels between the cannon bone (metacarpal III) and the lateral splint bone (metacarpal IV) . It becomes the distal perforating branch once it passes under the splint bone 'buttons' and so reach the plantar aspect of the foot.
  • The artery branches forming the medial and lateral digital arteries which run along their respective aspects of the foot ending in the terminal arch.
  • The saphenous artery gives off medial and lateral plantar and medial and lateral metatarsal arteries that join the great metatarsal artery.

The Stay Apparatus

The equine hindlimb skeleton supports a massive amount of weight from the main body of the horse. This weight is centered on the femoral head and then continues in a vertical line down to the ground by intersecting the hoof. This means that the stifle joint lies cranially of this line and the hock, fetlock and pastern lie caudal to the line. This would result in a collapse of the leg by flexion of the stifle and hock, and overextension of the fetlock and pastern, if there wasn't some supporting mechanism. The stay apparatus is a collection of tendons and ligaments that provide this support, whilst using minimal muscular effort. The stay apparatus has three main components; patellar locking, the reciprocal mechanism and the check apparatus.

Patellar Locking

This is not strictly part of the stay apparatus as there is no increase of collagenous tissue of muscle, but it is so important in reducing muscle fatigue by immobilising the stifle joint it is considered here. It is the specific anatomy of the equine stifle that enables the locking to occur, in particular:

  • The trochlea groove is wide for the gliding movement of the patella.
  • The medial trochlear ridge is the larger of the two ridges. It is wide and round proximally and surmounted by the tubercle of the trochlea.
  • The articular aspect of the patella has a vertical ridge which sits in the trochlear groove and is flanked by two concave areas which relate to the trochlea ridges.
  • The medial concavity is continued by the patellar cartilage. This is a strong curved plate of fibrocartilage that is adapted to the medial ridge.
  • The horse has three patellar ligaments. The lateral and middle patellar ligaments bind the patella to the tibia, whilst the medial patellar ligament attaches the patellar cartilage to the tibia.

During normal flexion and extension of the stifle the patella will glide in the trochlear groove.

When the horse rests its weight on the hindlimb the stifle can extend beyond a certain point. This causes the patella to move to the proximal extremity of the trochlear groove, a medial twist then results in the medial ridge protruding between the middle and medial patellar ligaments with the patellar cartilage hooking over the trochlea tubercle. Thus the patella is locked and further flexion is not possible.

This results in an immobile stifle joint and so no muscle activity is needed to maintain the leg in this position. Thus allowing the horse to rest a lot of it's weight on this leg.

To release the patella, the horse shifts its weight to the other limb and the quadriceps contract drawing the patella proximally. It then twists laterally and is returned to the trochlear groove.

Reciprocal Mechanism

This modification is made of largely tendinous muscles on either side of the tibia.

On the cranial aspect is the peroneus tertius. It's origin is between the trochlea and lateral condyle. It bifurcates distally, both parts inserting in the tarso-metotarsal region. This fibrous muscle ensures that flexion of the stifle is accompanied by flexion of the hock.

On the caudal aspect are the superficial digital flexor and the associated fibrous band of the gastrocnemius. The band of the gastrocnemius orginates on the lateral supracondyloid tuberosity of the femur and inserts on the calcaneus. The superficial digital flexor originates as a strong tendon from the caudal aspect of the femur between the two heads of the gastrocnemius. It runs distally covering the gastrocnemius tendon and partially attaches to the point of the hock. It continues distally and inserts as in the forelimb. These two muscles ensure that when the stifle is in extension the hock is also in extension

These structures thus receive their name from the reciprocating action of when the femur moves cranially the distal limb swings caudally and visa versa. This apparatus also ensures that when the patella is locked the hock is also immobilised. These results are obtained with no muscle fatigue.

Check Apparatus

The check apparatus of the accessory ligament of the deep digital flexor tendon is very similar to that of the forelimb. The superficial digital flexor muscle is entirely tendinous and so there is little/no muscle to be fatigued, thus a 'check' ligament would be of no advantage and so there is none in the hindlimb.

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