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What is the difference between sickle-hocked, cow-hocked, and post-legged, and how do they affect soundness?

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Sickle-hocked
This abnormal hock angle places the foot too far forward and tends to stress the structures at the back of the hock and cannon bone. The long plantar ligament running down the back of the leg from the hock can be strained or injured during heavy work. Other issues include the development of curbs (inflammation in the ligament of the accessory metatarsal bone), arthritis, bog spavins (swelling in the soft tissues), and thoroughpin (swelling of the tendon sheath). Sickled-hocked horses also tend to interfere at the trot.

Cow-hocked
This flaw – a result of the medial section of bones of the tarsus growing more quickly than the lateral part – causes the hind legs to be narrow at the hocks and base-wide from the hocks to the hooves. This structure creates strain on the inside of the hocks and stifles and a wrenching effect to the hip joint. Bone spavins are not uncommon, and the torsional forces to the joints of the hock and pastern may cause lameness.

Post-legged
When there is insufficient angle between the tibia and cannon bone (hock) and tibia and femur (stifle), the increased stress on tendons and ligaments can cause bog spavin, bone spavin, and osteoarthritis in the hock. This type of hock is easily injured during hard work because of strain on the flexor tendon and upper part of the suspensory ligament. Locked stifles may also occur, as the straight stifle joint causes the patella (kneecap) to slip out of position, ‘locking’ the leg. As it travels, the post-legged horse tends to stab its hind feet into the ground at each stride, leading to cracked hooves, bruised soles, and other hoof problems related to excessive concussion.