Joseph Norris, MD
Orthopedic Surgeon Sports Medicine
Home Patient Info Patellar Instability

Patellar Instability


  • Patellar instability means literally an unstable patella
  • This can either be an acute condition or chronic
  • Acute conditions are usually traumatic, meaning there was an injury
  • Chronic conditions are repeated events of patellar instability with no preceding event or injury

Dislocation vs. Subluxation

  • It is important to understand the difference between these two definitions

Dislocation– A dislocation is when the patella comes completely out of the trochlear groove and usually has to be put back in place manually

Subluxation– A subluxation is when the patella comes out of the trochlear groove but then spontaneously goes back into place without any assistance

  • Important to note that when a patella dislocates or subluxates , 99.9% of the time goes laterally or to the outside of the knee


  • Most commonly occurs in patients who are in their late teens- early 30’s
  • Acute or traumatic type occurs equally by gender (usually results from direct blow injury, ie. Football helmet to knee)
  • Chronic or atraumatic type usually occurs more in women

Risk Factors

  • Ligamentous laxity- (genetic, Marfan Syndrome, Ehlos-Danlos Syndrome)
  • Poor quad strength, specifically the vastus medialis oblique (VMO)
  • Patella alta (high riding patella)-causes patella not to track in groove properly, making it very unstable
  • Trochlear dysplasia- (groove is flat instead of being grooved)
  • Increased lateral patellar tilt (tight lateral retnaculum)- tissue pulls patella more laterally or to the outside predisposing it to coming out of place
  • Genu valgum (knock knees)
  • Pes planus (flat feet)
  • Increased femoral anteversion
  • Increased quadriceps or Q angle- measurement used to assess aligment of hip to patella to tibial tuberacle
    • Men- average is 10 degrees
    • Women- average is 15 degrees


  • If it is traumatic usually patient presents after sustaining some high impact injury (helmet to knee or knee to knee injury) and having patella reduced (put back in place)
  • If no injury, patient usually states doing something as simple as getting out of bed and patella came out and went back in (subluxates)
  • Knee may or may not have an effusion (swelling)
  • Motion may be limited due to swelling or pain

History/Physical Exam

  • History might include an injury resulting in a subluxation or dislocation or no injury but a history of patella feeling like it "goes out of place and comes back in" over several years
  • Physical exam will including:
    • Assessing whether or not there is an effusion or swelling
    • Palpate patella itself seeing if this provokes pain
    • Check for tenderness over the MPFL insertion site, which is a major medial (inside) patella stabilizer
    • Assess knee alignment (genu valgum)
    • Check for a flatfoot deformity
    • Measure the Q angle
    • Check for excessive femoral anteversion
    • Assess quad strength
    • Check for ligamentous laxity



  • Rule out fracture or loose bodies
  • Cartilage sometimes gets knocked off the medial (inside) patellar facet as it reduces (goes back in) as it hits the lateral (outside) femoral condyle


  • MRI is always needed to rule out loose bodies not seen on x-ray
  • Rule out cartilage injury
  • Check integrity of the MPFL (very important stabilizer of the patella)
  • Also to assess distance from the tibial tuberacle to the trochlear groove (TT-TG)
    • > 20 mm (2 cm) is indicative of malalignment



  • NSAIDs
  • Therapy (focusing on hip, core, and specifically VMO strengthening)
  • Brace wear (not necessary)
  • Foot orthotics (if flatfoot deformity)


  • Depends on the findings but in our practice always includes:
    • MPFL reconstruction with allograft with evaluation of articular cartilage
      • If cartilage defect can be repaired it is attempted
  • If cartilage cannot be repaired then cartilage may be harvested for a possible autologous chondrocyte implantion (ACI) at a later possible surgery. Sometimes a tibial tuberacle osteotomy is performed at the same time if there are any alignment issues with the patella and tibial tuberacle (Fulkerson procedure)
    • TT-TG > 20 mm (2cm)
  • Lateral renatcular release is done if tight, but not routinely done unless needed Reconstructed MPFL


  • Depends on everything done but generally:
    • 2-3 weeks in knee immobilizer and crutches or until quad strength is back (longer if cartilage fragment is repaired)
  • PT begins around 2 weeks mark working on quad strength and range of motion
  • Continue PT for 6-8 weeks until strong
  • Full recovery is anywhere from 3-6 months
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