Dislocating kneecap, luxating patella, loose knee, trick knee
Function of the Patella:
To protect the large tendon of the thigh muscle when the quadriceps are used
to extend/straighten the stifle joint (knee).
Description of Luxated Patella (LP):
The kneecap, or patella, dislocates outside the normal trochlear groove (a
groove in the bone of the knee). Dislocation can be on the inside surface
(medial) or the outside surface (lateral) surface of the knee. Medial patellar
luxations are the more common form in smaller breeds. The dislocation results in
movement problems, lameness, and if left untreated, may result in arthritis or
other structural changes later in life. It may be unilateral (only affecting one
leg) or bilateral.
Clinical Signs:
LP often results in intermittent lameness. Animals with LP are often unable
to jump or walk normally and often walk with a characteristic "hitch"
or skip in their gait. Dogs may also frequently extend the knee out from the
body in order to get the patella to "pop" back into the trochlear
grove. Affected animals may have thick or swollen stifles. Often animals exhibit
pain when displaying full range of motion. In severe cases (especially of
lateral LP) there may be malformation of the femur, the large bone in the thigh,
and also of muscle groups in the affected leg(s). It should be noted that as the
condition progresses the structures that hold the patella in place become looser
and problems will be more frequently observed.
Diagnosis:
Made from a physical exam involving palpation of the joint and manual
luxation of the patella. Palpation can result in a reliable diagnosis at about 4
to 6 months of age. X-rays may be needed in order to determine the degree of
rotation, degree of arthritis, and to evaluate any malformation of the femur and
tibia.
Treatment:
Surgical correction of the deformities to allow the kneecap into the groove
in the knee to prevent it from popping in and out are required. The exact nature
of the surgery can vary. Often an imbrication is performed whereby the joint
capsule is tightened to prevent the patella from moving from an appropriate
position. In severe cases a synthetic suture is may be needed to keep the
patella in place. Alternately, a trochleoplasty can be performed where the
trochlear grove is deepened so that the patella no longer slips. This procedure
can only be performed with very young dogs. In cases where the tendon that
attached the kneecap to the lower leg is mis-aligned, a tibial tuberosity
transposition may be performed to realign the joint, kneecap, and tendon and to
prevent dislocation of the patella. In severe cases of LP, with malformation of
the tibia or femur corrective bone cuts known as osteotomies may be required.
Transmission and/or Cause:
The congenital condition is probably genetic in nature, although an exact
mode of transmission has not yet been identified. It is possible that some form
(especially less severe grades) may be the result of injury. The common
recommendation in the literature is that breeders ought to avoid using animals
with LP in their breeding programs whenever possible.
