The horse’s knee is a complex joint, susceptible to trauma and progressive arthritis – especially in performance horses.

Knee (or carpal) lameness is characterized by a shuffling gait, shortened foreleg strides, tripping in front due to not being able to lift the hoof fully off the ground, and swelling or heat on the knee.

The causes of knee lameness are varied. It is most often seen in horses that take excessive weight or concussion onto their front legs, such as jumpers, barrel racers, reiners, and cutters.

Vigilance and early intervention are crucial to preserving your horse’s function through knee lameness. The first indication of knee lameness is usually reduced performance, so knowing your horse is key for early intervention. [1]

You can help to prevent carpal lameness in your horse with good management practices. Ensure your horse is at appropriate body weight, support their joint health with proper nutrition, and watch for early signs of soreness.

Signs and Symptoms of Knee Lameness

Because of the complexity of the knee, horses are susceptible to lameness of all degrees.

Common signs and symptoms of carpal lameness are:

  • Reduction in performance
  • Obvious favouring of the limb (horse raises their head when stepping onto that limb)
  • Shuffling gait
  • Shortened foreleg stride length
  • Tripping over the front feet
  • Abducting the forelimb (swinging the foreleg out to try to get the hoof high enough off the ground to avoid tripping)
  • Swelling and/or heat on the knee [8]

Diagnosing Knee Lameness

Horses showing signs of knee pain should be examined thoroughly by an equine veterinarian. The vet will likely perform a lameness exam including:

  • Knee flexion tests to simulate stress on the joint
  • Observing the horse at a walk, trot, and canter in a straight line and circle
  • Nerve blocks to isolate the source of pain [14]

Your veterinarian may want to obtain further diagnostic imaging, such as x-rays, ultrasound images, CT, or MRI images to visualize suspected injury.

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Anatomy of the Horse’s Knee Joint

The horse’s knee is one of the most complex joints in the equine body, consisting of numerous small bones and ligaments.

The three main joints are the:

  • Radiocarpal joint
  • Intercarpal joint
  • Carpometacarpal joint [2]

These joints are held together by a network of ligaments, with additional ligaments that connect bone to tendons.

These ligaments not only preserve the knee’s structure but also assist in concussion absorption (the absorption of forces from movement).

Radiocarpal Joint

The radiocarpal joint connecys the large upper bone of the radius (upper leg) to the small carpal bones of the interior knee. The radius sits on top of three carpal bones:

  • Ulnar carpal bone
  • Intermediate carpal bone
  • Radial carpal bone [2]

In between the radius and the carpal bones, there are cartilage pads that ensure the joint glides smoothly during motion without pain or grinding.

Intercarpal Joint

Sitting below the carpal bones is the intercarpal joint. As its name denotes, this joint is an articulation within all of the carpal bones. The ulnar carpal, intermediate carpal, and radial carpal bones sit on top of the fourth, third, and second carpal bones. [2]

You may have noticed that no first or fifth carpal bones are listed. This is because these bones are only present in a fraction of horses. The first carpal bone is present in 30% of horses, while the fifth carpal bone is present in 1.4% of horses. [3]

The carpal bones are held together by intercarpal ligaments, most notably the palmar carpal ligament, which gives rise to the accessory ligament of the deep digital flexor tendon. [4]

Although technically not included in the intercarpal joint, the accessory carpal bone that extends from the back of the horse’s knee provides anchor points for the ligaments of the knee and the flexor carpi ulnaris. [2]

Cartilage pads are also present within this joint.

Carpometacarpal Joint

Finally, the carpometacarpal joint is an articulation between the fourth, third, and second carpal bones and the metacarpals of the lower leg.

The metacarpals of the lower leg include:

  • The second metacarpal (medial splint bone)
  • The third metacarpal (cannon bone)
  • The fourth metacarpal (lateral splint bone) [4]

The first and fifth metacarpals do not exist in horses. The second and third metacarpals (splint bones) are remnants of toes that existed when horses had five toes.

The carpometacarpal joint of the horse does not provide movement to the knee. All knee movement is from the radiocarpal and intercarpal joints, while the carpometacarpal joint supports any shear forces the knee may encounter. [4]

Image from Denoix, J.M. Essentials in Clinical Anatomy of the Equine Locomotor System.

9 – radial carpal bone; 10 – intermediate carpal bone; 11 – ulnar carpal bone; 13 – second carpal bone; 14 – third carpal bone; 15 – fourth carpal bone; 16 – second metacarpal bone; 17 – third metacarpal bone; 18 – fourth metacarpal bone. [25]

Ligaments of the Knee

The horse’s knee is encased in a strong capsule, the majority of which is made up of the lateral and medial collateral ligaments. The horse’s knee is largely a hinge joint, meaning its motion is similar to a hinge moving front to back.

The collateral ligaments ensure the horse’s knee remains stable side to side. [4] These ligaments are critical to the structure of the knee and are susceptible to injury.

Another major knee ligament is the palmar carpal ligament, which is a portion of the joint capsule. This ligament gives rise to the inferior check ligament. [6]

The inferior and superior check ligaments do not provide structure to the knee but serve an important role in the knee’s function.

Superior Check Ligament

This ligament is an attachment between the radius and the superficial flexor tendon. The superior check ligament maintains joint angles, assists in load distribution, and ensures the superficial flexor tendon is not unduly stretched. [5]

Inferior Check Ligament

This ligament is an attachment between the palmar carpal ligament and the deep digital flexor tendon. This ligament assists with shock absorption and prevents excessive lengthening of the deep digital flexor tendon. [7]

Muscles that Move the Knee

Because the knee is a hinge joint, the muscles function to flex and extend the knee.

The muscles that extend the knee are:

  • Extensor carpi radialis (also assists with the stay apparatus, fixes the knee so the horse can sleep standing up)
  • Common digital extensor
  • Lateral digital extensor
  • Extensor carpi obliquus [2]

The muscles that flex the knee are:

  • Flexor carpi radialis
  • Ulnaris lateralis
  • Superficial digital flexor
  • Deep digital flexor [2]

Causes of Knee Lameness

Knee lameness is usually a result of excessive concussion over time, over-twisting the knee, or trauma directly to the joint.

Predisposing Factors

Conformation can predispose horses to knee pain. Ideally, a horse’s foreleg should sit straight on top of the lower leg, and the lower leg should extend straight below the knee.

Common conformation faults that contribute to carpal lameness include:

Back at the knee (or calf knee)

When viewed from the side, the horse’s knee falls behind an imaginary vertical line drawn from the middle of the foreleg; the leg takes on a slight backwards v-shape.

This fault puts undue strain on the ligaments at the back of the knee and increases the force to the front of the carpal bones.

Knock knees

When viewed from the front, the knees of the horse fall to the inside of an imaginary vertical line drawn in the middle of the foreleg, while the hooves travel back out to fall on the imaginary line.

This fault puts undue strain on the medial collateral ligament (on the inside of the knee).

Bow-legged

When viewed from the front, the knees of the horse fall to the outside of an imaginary vertical line drawn in the middle of the foreleg, while the hooves travel back in to fall on the imaginary line.

This fault puts undue strain on the lateral collateral ligament (on the outside of the knee).

Bench knee

The cannon bone is set too far to the outside of the knee, leaving the medial aspect of the carpal bones (medial splint and medial aspect of the cannon bone) to carry excessive weight.

This confirmation fault makes the horse vulnerable to “popped” splints. [9]

Over at the knee

This is the opposite of back at the knee; the knee falls in front of the imaginary vertical line. Although an anomaly, this is not necessarily considered a conformation fault, as it does not cause undue stress to the knee. [9]