'Sticky Stifle' is a result of a slight problem with the locking mechanism which fixes the stifle joint and allows the horse to stand sleeping - as a result, the rest of the horse's hind leg is affected.
Sometimes this locking mechanism in the stifle gets "stuck" and the horse or pony drags his hind leg and toe.
This common condition is also called upward fixation of the patella.
A horse affected by a locked stifle joint can become quite distressed and tends to hold the leg out straight. "Popping" sounds can sometimes be heard coming from the stifle joint.
An 'upward fixating patella' can usually be diagnosed with ultrasound or X rays. If necessary , a bone scan or MRI scan can confirm this diagnosis.
4 bones make up the stifle joint in the horse - the patella, femur, tibia and fibula.
The patella (knee cap) has 3 ligaments extending from its base to the to the tibia (or shin bone).
On the end of the femur (thigh bone) there is a notch and it is the interaction between the ligaments and this notch that causes the upwards fixation or stuck stifle.
There is a groove between the inside and centre ligaments of the patella, and this groove hooks over the notch of bone on the femur
The problem in some young horses is that the notch has too much of a lip on the front and the patella ligaments get stuck.
With age and growth the notch is re-modelled by the horse's body and it is reshaped into the most effective form.
Unfortunately, we come along and upset nature's balance by getting the horse fitter than the skeleton is ready for and toning up the quadriceps muscle, which means that the patella is held fractionally higher.
Then if the horse is rested the quadriceps get flabbier and consequently the patella gets stuck in the notch.
A locked stifle is most often seen in younger horses, especially if they have been worked for a while and then turned away.
It is also seen in horses who are growing abnormally fast and disproportionately as the bones and ligaments become mismatched.