• May be medial (more commonly) or lateral (rarely).
• Degree of patellar instability classified into 4 grades of increasing severity:
  • Grade 1: luxating patella that will not stay out of the groove. 
  • Grade 4: patella permanently luxated.
• Cause: developmental (rare); trauma (more commonly).
• Signs: acute/intermittent, unilateral/bilateral hindlimb lameness, usually non-painful (unless patellar cartilage damage/erosion (has caused synovitis)); may be other concurrent pelvic limb abnormalities (eg hip dysplasia).
• Diagnosis: manipulation of limb; radiography.
• Treatment: conservative, analgesia, surgery.
• Prognosis: varies depending on degree of deformity and management.

Presenting signs

• Intermittent/acute pelvic limb lameness Lameness: overview - patient often carries limb for a number of strides.
• Patella relocates spontaneously, resolving the problem.
• Pelvic limb gait abnormality/difficulty standing (bilateral developmental), crouched stance.

Concurrent pelvic limb abnormalities

• Lateral bowing of distal femur.
• Medial bowing of proximal tibia.
• Medial rotation of tibial tuberosity.
• Hypoplasia of medial femoral condyle.
• Increased femoral torsion.
• Hip dysplasia.
• May present with concurrent cruciate ligament injury.

Acute presentation

• Acute unilateral/bilateral pelvic limb lameness.

Age predisposition

• Unilateral developmental:
  • Young animals.
• Bilateral developmental:
  • Young animals - usually less than 6 months.
• Traumatic:
  • Any age.

Breed/Species predisposition

• Devon Rex Devon Rex.
• Maine coon Maine Coon.