Together with the femur and the tibia the Patella, or kneecap, forms the knee joint. The Patella is held in place by the patella tendon. This tendon is attached to the tibia. In a healthy situation, the Patella will move up and down in a groove, called the trochea that is located at the lower hand and frontside of the femur. When the trochea in the femur is too shallow, the attachement of the knee tendon is not in the right place or the position of the tibia in relation to the femur deviates then the Patella could slip out of the trochea. This is called Patella Luxation.When examining the knee, the depth of the groove, the placement of the Patella, the alignment of the tibia with regard to the femur and the curve of the tibia all must be checked. Behind the Patella lays a number of cruciate ligaments. Because of the movement of the Patella, these may be damaged too. In the severe cases surgical correction may be needed. PL is graded from 1 to 4, in which 4 is the most severe case. Grade 1 can become a grade 4.


A method of classifying the degree of luxation and bony deformity is useful for diagnosis,

and can be applied to either medial or lateral luxations by reversing the medial-lateral
directional references. The position of the patella can most easily be palpated by
starting at the tibial tubercle and working approximately along the patella ligament
to the patella.
Grade 1
The patella easily luxates manually at full extension of the stifle joint, but
returns to the trochlea when released.No crepitation is apparent. The medial,
or very occasionally, lateral deviation of the tibial crest (with lateral luxation
of the patella) is only minimal, and there is very slight rotation
of the tibia. Flexion and extension of the stifle joint is in a straight line with
no abduction of the hock.
Grade 2
There is frequent patellar luxation which, in some cases, becomes more
or less permanent. The limb is sometimes carried, although weight bearing
routinely occurs with the stifle remaining slightly flexed.
As much as 30 degrees of medial tibial torsion and a slight medial deviation
of the tibial crest may exist.
When the patella is resting medially the hock is slightly abducted. If the condition
is bilateral, more weight is thrown onto the forelimbs.Many cases in
this grade live with the conditional reasonably well for many years,
but the constant luxation of the patella over the medial lip of the
trochlea causes erosion of the articulating surface of the patella and
also the proximal area of the medial lip. This results in creptitation
becoming apparent when the patella is luxated manually.
Grade 3
The patella is permanently luxated with torsion of the tibia and
deviation of the tibial crest of between 30 degrees and 50 degrees
from the cranial/caudal plane.Although the luxation is not intermittent,
many animals use the limb with the stifleheld in a semi-flexed position.
Flexion and extension of thejoint causes abductionand adduction of the
hock. The trochlea is very shallow or even flattened.
Grade 4
The tibia is medially twisted and the tibial crest may show further
deviation medially with the result that it lies 50 degrees to 90 degrees
from the cranial/caudal plane. The patella is permanently luxated.
The patella lies just above the medial condyle and a space can be palpated
between the patellar ligament and the distal end of the femur. The trochlea
is absent or even convex.The limb is carried, or the animal moves in a
crouched position, with the limb partly flexed.
Dogs with a grade 3 and 4 may have to deal with various degrees of paralysis.

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