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 Location:   Starting Out | The Horse  

The Functions and Locations of Tendons, Ligaments and Bones in the Lower Leg

Natural selection has fashioned the horse from a short-legged five-toed rodent to the tall single-toed horse that we know today. The lower leg consists of tendons, ligaments, and bone; which take the majority of the strain and are sometimes the ‘weakest link’.

Nature intended the horse roam free over the plains, browsing on grasses, with short bursts of speed if danger approached. When man domesticated the horse, we changed their way of living and their way of moving. Lameness is more common in the front legs than the hind legs, because of the weight-bearing function of the forelegs and riding styles. Jumping on hard ground or a horse that hasn't been trained to use his hindquarters properly all place more strain on the front legs.

The strength of a chain is measured by its weakest link, so is the power and endurance of a horse limited by the weakest point in his structure. Worked with care a horse is unlikely go lame; but this is dependent upon the conformation of an individual horse. Circumstances may cause excessive strain on a part of the horse that is not considered ‘weak’, while a conformation fault being subjected to strain, continues sound.

Ligaments help to limit the movement of joints according to their functions; e.g. the fetlock, pastern and coffin joints all have ligaments that allow the joint to move forward and backward only. They are poorly supplied with blood and are very slow to heal after injury and do not withstand prolonged stretching. They are made of bands of white and yellow fibrous tissue, the white being inelastic, and the yellow elastic. There are four different types of ligament: -

  • Supporting or suspending - the suspensory ligament.
  • Annular - a broad band of ligament, which directs the pull on a tendon.
  • Inter-osseus - ties bone together, e.g. the pedal and navicular.
  • Funicular (or cord like) - holds bones together.

Tendons are fibrous cords of connective tissue attaching muscle to bone, cartilage or other muscle. Tendons insert into bone or cartilage by means of small spikes known as ‘Sharpey’s fibres’.

Check Ligament Deep Digital Flexor Tendon Superficial Digital Flexor Tendon Long Pastern Bone  (First Phalanx) Branches of the Suspensory Ligament Long Pastern Bone  (First Phalanx) Short Pastern (Second Phalanx) Pedal Bone (Third Phalanx) Suspensory Ligament Annular Ligament (surrounding the Sesamoid Bones) Superficial Digital Flexor Tendon Splint Bones (Small Metacarpal) Cannon Bone (Large Metacarpal) Extensor Tendons Navicular Bone (Distal Sesamoid) Deep Digital Flexor Tendon


  Description

Cannon Bone

Large Metacarpal

The shaft of the cannon bone is slender but capable of carrying a great deal of weight. Its strength depends upon the thickness of the bone and is therefore almost completely solid bone with only a small spongy centre.

The term ‘bone’ is used when referring to the horses lower leg, this description includes the cannon bone, tendons, blood vessels, nerves, ligaments, connective tissue, and the overlying skin. The measurement is taken just below the knee, (e.g. 22.5cm or 9” of bone). The number of inches is a guide to ability of the horse to bear weight and stand up to work on hard surfaces, light-bones types are less likely to stand up to the rigours of hard work, especially when young.

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Splint Bones

Small Metacarpal

Remnants of an earlier time when horses had five toes. The inner bone is usually slightly larger than the outer bone.

The top of the splint bone supports the lower row of knee (carpal) bones. Up to 3 – 4 years of age, the cannon and splint bones are not fully joined and some movement is transferred to the splint bones from the activity of the knee. This movement, especially near the joint frequently causes lameness in young horses. However, this problem usually improves when the bones become firmly fused either from age or post-inflammatory union

The bottom of the splint bone ends in a small round button.

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Long Pastern Bone

First Phalanx

A long bone because it has a central marrow, it is angled between the cannon bone and the short pastern bone and acts as part of the shock absorbing mechanism of the legs.

The top of the bone is deeply grooved and divided into two articular surfaces, which fit snugly together with the lower end of the cannon bone forming the fetlock joint.

The inner section of the bone is usually larger and wider and as a result the foot travels forward in a straight line. However slight abnormalities do occur, which may give rise to faulty action of the lower leg.

The lower end of the long pastern is also divided into two articular surfaces with the inner surface larger, but the line of division is not as distinct as the upper end. On the front surface of the bone is a bulge where the common digital extensor tendon attaches.

The superficial flexor tendons are attached at back, on either side.

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Short Pastern Bone

Second Phalanx

A short solid bone with no central marrow, it is important because it is partly inside and partly outside the wall of the hoof and also because it is the first bone to sustain concussion as the foot hits the ground.

The short pastern is also an angled bone and is supported at the back by the deep flexor tendon, which runs along a groove covered by cartilage. As with the long pastern bone the common extensor tendon is also attached to the front surface of this bone and the superficial flexor tendon is attached to the back surface.

The fact that this bone is not upright, and that it is supported by the tendons, probably does a great deal to lesson the degree of concussion it is obliged to withstand.

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Annular Ligament

(Surrounding the Sesamoid bones)

Increases the area of the fetlock joint and is able to withstand the forces transmitted down from the cannon bone.

There is a great deal of stress and strain put upon this joint, however nature has provided it with a strong system of bonding to protect and support these two little bones.

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Sesamoid Bones

Proximal Sesamoids

Two small bones on either side at the back of the fetlock joint.

Each bone is shaped like a pyramid.

These bones act together to form a fulcrum or pulley over which the deep flexor tendon runs. The sesamoid bones are also bound into the joint by a very strong ligament called the Annular Ligament.

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Pedal Bone

or

Coffin Bone

Third Phalanx

The last bone of the leg, which fits the shape of the hoof, but occupies only a small section of the cavity within the hoof.

The wall of the bone slopes downwards and forwards and is rough and pitted by numerous holes. The under surface is divided by a curved line into a crescent-shaped sole and the deep flexor tendon is attached to the edges on the crescent.

Within the ‘well’ of the crescent is the navicular (distal sesamoid) bone.

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Navicular Bone

Distal Sesamoid

This small bone lies between the short pastern and the pedal bone.

The tendon surface on this bone is directed downwards and backwards, is covered by cartilage, and allows the deep flexor tendon to gain greater leverage, to act as a fulcrum or pulley.

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Deep Digital Flexor Tendon

 

 

 

 

Running over the back of the knee in the carpal canal and held in position by a carpal check ligament.

It then extends down the back of the cannon bone between the superficial digital flexor tendon and the suspensory ligament.

In the middle of the cannon bone the deep digital flexor tendon is joined by the carpal check ligament, known as the inferior check ligament.

The tendon then passes over the sesamoid bones, before passing between the two extensions of the superficial digital flexor tendon.

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Branches of the Deep Digital Flexor Tendon

At this point, the deep digital flexor tendon becomes broad and fanlike, passing over the navicular bone before inserting into the lower surface of the pedal bone. This takes some of the strain from the muscles situated above the knee in the forearm or above the hock in the gaskin.

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Superficial Digital Flexor Tendon

Running down the back of the humerus over the back of the knee (carpus) then down the back of the cannon bone, completely covering the deep digital flexor tendon.

At the lower end of the cannon bone the superficial digital flexor tendon widens and encircles the deep digital flexor tendon, forming a ring known as the annular ligament of the fetlock joint, extensions from which then attach to the short and long pastern bones.

Their other chief function, when the horse is in motion, is to flex the joints of the lower leg.

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Branches of the Superficial Digital Flexor Tendon

The superficial and deep flexor tendons extend down from their muscles in the forearm/gaskin through to the foot, providing weight-bearing support and preventing the over-extension of the fetlock joint, (a role in which they are helped by their check ligaments).

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Check Ligament

The function of this ligament is to prevent undue strain being applied to the flexor tendons and to assist in supporting the horse, thus allowing him to sleep whilst standing. This ligament is connected to the deep flexor tendon and attached to the cannon bone, at the back, and angles downward and backward.

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Suspensory Ligament

Lies between the two splint bones close to the back of the cannon bone, starts close to the knee and going down to the fetlock joint, where it divides into two branches.

Each branch attaches to the corresponding sesamoid bone while some fibres combine with the common digital extensor tendon.

The suspensory ligament provides support for the fetlock joint, preventing it from extending down too far towards the ground, which would increase the risk of strains.

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Branches of the Suspensory Ligament

Leave the Annual Ligament either side of the lone pastern bone and joins onto the extensor tendon at the front of the cannon bone, therefore joining ligament to tendon.

Particularly in the front leg, they take all the strain on landing from a jump and which is normally about 60% of the horse’s weight.

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Extensor Tendons

This tendon is attached to all the bones in the foot except the Navicular bone, it bears no weight and is therefore slim and generally trouble-free.

At the lower end of the long pastern bone it receives reinforcement on either side from the branches of the Suspensory ligament, which increase its width.

The function for the extensor tendon is to extend the bones of the foot and lift the toe.

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