The Patella is the equivalent to the knee-cap in humans
luxation (dislocation) of the patella in foals is usually an inherited condition caused by a recessive gene, The patella is situated over two prominent ridges on the lower extremity of the femur, the outer of the two ridges is smaller than the inner. There is a muscle the biceps fe-moris situated in the area of the outer part of the patella. the area between the inner and outer ridge on the femur is known as the trochlear groove the outer ridge is naturally smaller than the inner but in some cases may be abnormally shallow. The late Professor Varnell attributed cases to a smallness or want of the outer ridge, this permits the bone in certain movements to be displaced outwardly by the pull of the biceps fe-moris. There have been some post-mortem examinations to support this conclusion. The condition may vary in severity from intermittent to persistent. In severe cases foals are unable to extend the stifle and adopt a crouching position, In less severe cases foals or adults may be reluctant to flex the stifle and demonstrate a stiff hind limb.
A breeding experiment was carried out over a period of 20 years to investigate the heredity of congential lateral patellar sub luxation. The experiment consisted of stallions and mares, some were free others were affected by lateral patellar sub luxation, in either one or both femoropatellar joints. 49 foals were born over the 20 year period from different mating combinations. Some of the foals were free from the defect, others showed the abnormality. Though this was a limited experiment, there was sufficient evidence to suggest a monogenic autosomal recessive hereditary transmission of the defect. (Hermans WA, Kersies AW, van der Mey GJ, Dik KJ. )
In adult horses it is more likely to be traumatic in origin, or secondary to joint disease.
Dislocation may also be seen in immature animals with poorly developed thigh muscles or/and after debilitating diseases. The affected limb can be noticed to be suddenly jerked backward and held for a brief moment in an extended position, then resumes it place again, this would represent a partial displacement. This can be confined to one or both hind legs, both legs may not be of equal severity. Signs can be less dramatic, there may simply be a lack of hind leg forward movement with a jerky patellar action.
The problem can be very intermittent lasting a day or two, several weeks or months, then it passes away as the animal gains strength. There is some evidence to suggest that being inactive for example, stabled or transported may be a cause in susceptible ponies, but this usually passes quickly once exercise is commenced.
It occurs in some breeds ( Shetland Ponies for example) more than others.
Other symptoms can be stifle swelling, abnormal gait, and short stride, instability in the stifle joints.