Almost any joint in the skeleton of the horse could be potentially hyperextended through some abnormal movement or injury. Hyperextension basically refers to any movement that causes the joint to move past its normal range of motion, resulting in pain, swelling and restriction of movement either for short or long periods of time.
Hyperextension in horses is common in the legs, in particular the hindquarters. Often horses that are defined as "post-legged" or have hindquarters that are too straight in conformation or horses with very long pasterns are more likely to have hyperextensions during movement. Hyperextensions can happen at any gait but are more likely at faster gaits than at walks and jogging gaits. Post-legged horses have limited impulsion or forward thrust because of the straight up and down structure of the legs, and may use the joints to generate the forward motion, resulting in strain on the joints and the greater likelihood of over-extension.
Another common hyperextension injury is to the fetlock. This condition can happen if the horse has joint problems or is worked on uneven surfaces or even in slippery conditions. Even horses in great physical health with no history of joint problems can hyperextend a fetlock by turning suddenly or slipping. Often cutting horses, gymkhana horses or jumping or dressage horses will slip and hyperextend a fetlock or joint. Occasionally walking up or down very steep inclines or slipping when walking downhill can resulting in a hyperextension of a joint, particularly the fetlock.
Hyperextension is just like any other type of muscle injury, it does require time to fully heal and recover. There are many different products available on the market to help support joints that have been hyperextended. Fetlock boots are often used to support the fetlock and prevent hyperextension in competition and working horses. Leg wraps are also used in competition horses to provide additional support. Often fetlock hyperextension is very debilitating and may prevent the horse from being used in any type of competition or even as a pleasure riding horse. Many horse breeds with longer pasterns are more likely to have fetlock problems and will benefit from the support boots prior to any injury.
A lot of the problems with hyperextension in horses are found in animals with poor conformation. Issues such as calf-knees, spayed legs and feet that toe-in or out are most commonly seen in horses with hyperextensions. In addition racing horses or performance horses that are stressed or overworked are more likely to hyperextend a joint, especially towards the end of the competition or event when the muscles are already fatigued. Monitoring the horses and gradually working to strengthen the supporting muscles rather than over stressing an out of shape or even in-shape horse is essential.
Once a joint is hyperextend the treatment usually involves rest, pain medications and non-steroid anti-inflammatories as well as therapy to strengthen the joint. A vet will be able to advise on the possibility of using the horse in further competitions or events and the likelihood of the condition reoccurring.