Patellar Instability
Timothy S. Brown
Rafael J. Sierra
Key Concepts
Patellar instability is a common cause of anterior knee pain and stiffness following total knee arthroplasty (TKA).
The most common underlying etiology is malrotation of the femoral, tibial, or both components.
Revision surgery to address patellar instability should address all malpositioned components and often requires both femoral and tibial revision.
Sterile Instruments and Implants
Revision TKA instrumentation and implants.
Preoperative Planning
Diagnosis
History
Prior patellar instability/dislocation
Prior valgus knee deformity with patellar subluxation
Difficulty with stairs, getting up from seated position
Predominantly anterior knee pain, different from preoperative knee pain
Signs/Symptoms
Anterior knee pain, especially with stair ascent/descent.
Lateral dislocation of the patella in a flexed position.
Difficulty actively extending the knee, but reduction of the patella allows active extension.
Extensor lag, may be present if patella is persistently subluxed or if associated with partial quad rupture.
Valgus deformity at the knee.
Excessive external rotation of the lower leg, may indicate internal rotation of the tibial component.
Tests
Examination
Apprehension with lateral motion of the patella
Pain with palpation of the patella throughout range of motion
Patient reduces a patellar dislocation manually as they bring leg into full extension
Excessively externally rotated foot
Valgus overall alignment
Radiographs
Anteroposterior (AP)/lateral of knee
Long-leg hip-to-ankle radiograph
Merchant view of bilateral patellae
Radiographs are reviewed for
component fixation (including patellar fixation)
polyethylene wear
osteolysis
component rotation
overall limb alignment
patellar tracking/tilt
patellar subluxation/dislocationRelated posts:
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