Digital X-ray of left stifle joint in lateral view

See figure 7 below. 

Horse’s Stifle Anatomy Integral for Rest and Snoozing While Standing

Hook-and-loop system allows equids to lock and unlock hind legs

Third in a Series

In this series, Dr. Deb Bennett examines the equine hind limb. In this installment, Bennett explores the stifle.

Farrier Takeaways

  • The function of stifle-locking is to allow the horse to rest the muscles of its hindquarters and allow it to snooze in a standing position. The stifle may, however, unexpectedly lock during locomotion.
  • Stifle unlocking is at least as important as stifle locking in permitting normal equine locomotion.
  • Sticking stifles and full upward fixation of the PPC unit are more frequent in horses with “post-legged” conformation, but dis-coordination can cause it to happen in a horse of any build, even one with highly angulated hind limb conformation.

At first glance, the stifle joint appears to be a huge mistake — the worst-designed part of the musculoskeletal system.

Unlike the shoulder joint, hip socket, hock, elbow, intervertebral joints or the coffin joint, the articular surfaces of the femur and tibia, which face each other to form the stifle joint, do not closely correspond in shape. There is no neat ball-and-socket fit, no tight tongue and groove geometry and because of that the joint appears to have been “post-engineered” with a whole slew of auxiliary rims, pads, and straps that work to stabilize it and hold it together (Figures 1 and 2).

In terms of embryology and evolutionary history, the stifle joint’s design is indeed something of an “afterthought.” Lungfishes (Figure 3), have several modes of survival under conditions of drought, all of which have been…

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Dr deb bennett

Deb Bennett

Dr. Deb Bennett has studied classification, evolution, anatomy and biomechanics of the horse. She worked at the Smithsonian Institution, until founding the Equine Studies Institute. She is an author who has published four books on horse-related topics, in addition to articles in most major equine magazines in North America.

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