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KNEECAP
LUXATION
Medial
patella luxation, or kneecap luxation, may be congenital (present at
birth) or acquired. The congenital form is most common in the toy and
miniature breeds such as the Miniature Poodle, Yorkshire Terrier, Toy Poodle,
Chihuahua, Pomeranian, and Pekingese, and may occur simultaneously with other
pelvic limb deformities. While the definitive sequence of events which leads to
or allows these deformities has not yet been established, the age at which the
syndrome occurs does play an important role in the severity of the degenerative
changes in the joint.
When
patellar luxations are present early in life, the major muscle groups of the
thigh pull toward the inside of the leg, putting abnormal pressure on the knee
joint cartilage. The result is a bowlegged stance and an abnormal pull
on the patella (fig.1). Therefore, the statement "patellar luxation is an
anatomic malformation not merely of the knee but of the entire pelvic
limb"… accurately describes the complexity of a seemingly simple
luxation. Thus, a number of anatomic pelvic limb deformities can lead to or
result from the structural manifestation of medial patellar luxation. These
include bowed legs, coxofemoral (hip) joint abnormalities, and outward rotation
of the limb.
When
the patella is in its normal position, its cartilage surface glides smoothly
and painlessly along the cartilage surface of the trochlear groove with little
or no discomfort. As the patella "pops out" of its groove these
cartilage surfaces improperly rub each other. The animal may cry and try to
straighten (extend) the leg to "pop it back in" or may hold the limb
up until muscle relaxation allows the kneecap to reposition itself. This
resembles an intermittent lameness. There is little or no discomfort until the
cartilage is effectively "rubbed off" or eroded to a point where bone
touches bone. From this point on, each time the patella "pops out"
into its abnormal, luxated position it will cause pain. This explains why many
individuals have no clinical lameness until they reach adulthood. Often
progressive cartilage wear creates an acutely painful condition.
Because
there is great individual variation in the pathologic deformities seen, a
graded classification of medial patellar luxation (Putnam 1968) has been
formulated as a basis for recommending which type of surgical repair is most
appropriate for each individual. In the following description each
classification is addressed.
GRADE
IThe
anatomic alignment of the stifle is normal with the patella luxating only
when pushed out of the socket.
GRADE
II
returned
by manual pressure.
GRADE
III
The
patella is permanently dislocated but can be reduced manually with the limb
extended.
GRADE
IV
The
patella is permanently dislocated and cannot be manually reduced.
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