the chihuahua breed is known to have the knee condition known as patellar luxation and is one of the most common health problems associated with chihuahuas.   the knee is a complex structure consisting of muscles, ligaments, tendons, cartilage, and bones.  these components must align properly and interact harmoniously in order to function properly.  three bones are included in the knee: the femur, the tibia, and the patella (kneecap).

the lower front portion of the femur (thigh bone) in a normal chihuahua has two bony ridges that form a fairly deep groove in which the patella is supposed to slide up and down.  these structures limit the patella’s movement to one restricted place, and in doing so, control the activity of the quadriceps muscle.  the entire system is constantly lubricated by joint fluid.  it works so that there is total freedom of motion between the structures.

diagram of the anatomy of a normal knee cap

patellar luxation (or trick knee, subluxation of patella, or floating patella), occurs when the patella, or kneecap, dislocates or moves out of its normal location.

when the patella is luxated in a chihuahua, the ridges forming the patellar groove are not prominent, and a too-shallow groove is created.  in a chihuahua with shallow grooves, the patella will luxate (jump out of the groove) sideways, especially toward the inside.  this can causes the leg to ‘lock up’ with the foot held off the ground.

chihuahua with a luxating patella on both hind legs
(like teaka the famous chihuahua has as seen in the video above)

when the patella luxates from the groove of the femur, it usually cannot return to its normal position until the quadriceps muscle relaxes and increases in length.  this is why a chihuahua may be forced to hold their leg up for a few minutes after this occurs because the muscles are contracted, the patella is luxated from its correct position and the joint is held in a flexed or bent position.  pain is caused by the knee cap sliding across the bony ridges of the femur.

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