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Patellar Luxation 763

Video
Patellar Luxation Available
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○ Grade 4: patella is luxated all the time and
BASIC INFORMATION
patellae are in place, but they are useful for
cannot be manually reduced. Resultant • Radiographs may be normal if taken when the
Definition limb malpositioning is severe. planning surgical intervention and identify-
Medial or lateral displacement of the patella ing changes with concurrent cranial cruciate
from the femoral trochlear groove is usually a HISTORY, CHIEF COMPLAINT ligament rupture. However, interpretation of
developmental disease. Medial patellar luxation • Low-grade luxation: transient skipping gait apparent angular and torsional deformities
(MPL) is the most common form. (holding hindlimb flexed and non–weight must be done with caution because natural
bearing for a few steps at a time) when curvatures of the femur and tibia can give
Epidemiology walking or running; grade 2 luxation may the impression of abnormal conformation,
SPECIES, AGE, SEX be associated with a consistent limp. especially if radiographs include a degree of
Occurs in dogs and cats of both sexes; onset • High-grade luxation: sustained lameness due rotational malpositioning.
usually during growth but condition persists to secondary limb deformity
throughout life • Weakness in jumping or reluctance to Differential Diagnosis
jump • Aseptic necrosis of the femoral head (Legg-
GENETICS, BREED PREDISPOSITION • Acute lameness or increase in lameness in a Calvé-Perthes disease): young, small dogs
• Most common in Newfoundlands and toy limb with chronic patellar luxation suggests only
and miniature dogs, but occurs in most recent cranial cruciate ligament rupture. • Coxofemoral luxation
breeds • Cranial cruciate ligament rupture (often
• Heritability has been established for many PHYSICAL EXAM FINDINGS concurrent)
dog breeds. • Patella can be manually displaced into and • Osteochondrosis of the lateral femoral
• Some reports of increased incidence among out of the trochlear groove during flexion condyle
Maine coon, Persian, Devon rex, and Abys- and extension with grades 1 and 2 luxations; • Immune-mediated polyarthritis
sinian cats crepitus may be palpable with grade 2. • Lumbosacral compressive disease
• With higher grades, patella may sometimes be
RISK FACTORS palpated medial or lateral to femoral condyle; Initial Database
• Varus/valgus deformities of the distal femur trochlear groove may be palpable. • Palpation of the stifle joints (in flexion and
or proximal tibia • Stifle may flex excessively when bearing extension) for patellar instability and cranial
• Trauma to the bone or soft tissues of the weight. cruciate ligament rupture
hindlimb or neurogenic muscle contracture • Animals with MPL may demonstrate genu • Radiographs
• Previous stifle surgery that included lateral varum and metatarsal varus (bow-legged and ○ Include the hip and/or tarsus to better
arthrotomy with patellar luxation pigeon-toed, respectively) when patella is assess angular and torsional deformities
luxated (see first Video) of femur and tibia.
ASSOCIATED DISORDERS • Animals with LPL demonstrate genu valgum ○ Take views with patellae in normal posi-
• Cranial cruciate ligament instability (present (walking knock-kneed), with external rota- tion if possible.
in 15%-42% of stifles with chronic patellar tion of the distal limb when the patella is ○ Craniocaudal views should be taken with
luxation) luxated (see second Video) the x-ray beam perpendicular to the thigh
• Hip dysplasia (weak association in dogs and because slight variations from the ideal
cats) Etiology and Pathophysiology can cause the femurs to appear falsely
Skeletal changes are associated with asym- angled.
Clinical Presentation metrical pull of the quadriceps mechanism.
DISEASE FORMS/SUBTYPES Which comes first is unknown because many Advanced or Confirmatory Testing
• MPL: the most common form of patellar of the changes in high-grade patellar luxation • Measurement of varus/valgus angles of femur
luxation in cats and dogs of all sizes result from the abnormal directional pull of and tibia
• Lateral patellar luxation (LPL): uncommon the quadriceps. • CT scan with three-dimensional reconstruc-
but seen in dogs of all sizes as well as cats • Medial patellar luxation tion
• Traumatic luxation: uncommon; usually the ○ Distal femoral varus/genu varum, external
patella displaces medially after retinacular rotation TREATMENT
injury. ○ Tibial internal rotation/metatarsal varus
• Grading system for degree of luxation ○ Shallow femoral trochlear sulcus Treatment Overview
○ Grade 1: patella is in place most of the ○ Tight, thick medial retinaculum; loose, • Only animals showing overt clinical signs
time and spontaneously reduces if it does thin lateral retinaculum should have surgery. Patellar luxation
luxate. • Lateral patellar luxation produces surprisingly little osteoarthritis,
○ Grade 2: patella moves in and out of the ○ Distal femoral valgus/genu valgum, which is typically slow to progress.
trochlear groove, often with every stride. internal rotation • In all but very young puppies and kittens,
This is the grade most commonly associ- ○ External tibial rotation correction of bone deformities as well as
ated with consistent, painful lameness. ○ Tight, thick lateral retinaculum; loose, soft-tissue repairs are necessary if surgery
○ Grade 3: patella is luxated all of the time thin medial retinaculum is performed. Without bone correction, a
but can be moved back into the trochlear soft-tissue repair alone, even if it feels stable
groove by an examiner. This and grade DIAGNOSIS on the operating table, usually allows patellar
4 luxation produce a mechanical lame- reluxation by 3 weeks after surgery.
ness due to deformity from abnormal Diagnostic Overview • Concurrent cranial cruciate rupture (p.
joint positioning, but it is not evidently • Diagnosis of patellar luxation is based on 218) must be addressed during the patellar
painful. physical exam. luxation operation because cruciate ligament

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