Locked Stifle in Horses – Upward Fixation of the Patella

By Garrett Metcalf, DVM

It is common for most horse owners to experience a horse having a locked stifle, but when you see if for the first time it can be quite surprising and concerning. This article will cover some of the causes and treatments for upward fixation of the patella (UFP).

First off, to understand why the stifle gets locked we have to understand some of the basic parts of the stifle in a horse. You can equate the horses stifle to the human knee to some degree but it has some extra parts that humans don’t have. They have a knee cap or patella and the same bones that make up the stifle joint (Femur and Tibia) but with some significant anatomical distinction. The difference is that horses have three patellar ligaments compared to the human knee which only has one and the horse has large trochlear ridges on the front part of the femur that the patella glides across. These ligaments tie the patella to the top of the tibia and when the quadriceps muscles contract it extends the stifle joint and the rest of the limb. When horses are resting or sleeping in the standing position they have a built in energy saving mechanism to keep the limb locked in extension using very little energy to do it with by perching the patella on top of the trochlear ridges of the femur. When this mechanism goes wrong is when we see the horse with a locked stifle.

When a horse locks or catches in the stifle it may appear like a sudden jerking motion of the limb or a period of dragging the limb with the fetlock joint flexed while the rest of the limb is in rigid extension. Riders will often feel a horse catching in the stifle as if they suddenly give out in one of the rear limbs and describe it as if the horse stepped in a hole. Some horses will be stuck like this for minutes to hours with the especially if they have been locked up in a stall and are predisposed to UFP. To get a horse to unlock there stifle they can be backed or turned in tight circles to help them unlock it.

There are several causes of upward fixation of the patella in horses and they are listed below.

  • Conformation – Straight leg or post leg horses
  • Young/Growing – Rapid growth or under developed muscle tone/fitness when starting into training
  • Muscle weakness – Neurologic diseases that leads to muscle weakness such as EPM (equine protozoal myeloencephalitis)
  • Breed – Miniature horses and Shetland ponies are more prone to UFP
  • Stall Rest – Keeping horses on stall rest can exacerbate an already predisposed horse to UFP
  • Chronic Lameness – Pain in the hind limbs can lead to muscle atrophy and subsequently UFP.

Read more in the September 2021 issue of Oklahoma Farm & Ranch.