Patellar Instability

Introduction

Acute traumatic

  • Direct blow to the knee.
  • Even prevalence by gender.

Chronic

  • Anatomical malalignment
  • Ligamentous laxity

Habitual

Epidemiology

  • 5.8 per 100,000 persons
  • Occurs 10-20 years

Pathophysiology

  • MPFL provides about 50% to 60% of restraint to lateral translation
  • MPFL is the primary restraint from 0 to 30 degrees flexion
  • Trochlea engages the patella beyond about 30 degrees flexion
  • Trochlear dysplasia is the commonest predisposing factor
  • Patella alta delays trochlear engagement
  • Raised TT TG distance increases the lateral force vector
  • Femoral anteversion, external tibial torsion and genu valgum add lateral force
  • Generalised ligamentous laxity

Classification

Dejour

Patellar Instability illustration

Clinical Evaluation

History

  • Twisting on a flexed knee with valgus
  • Number of dislocations, age at first episode and family history

Examination

  • Haemarthrosis after acute dislocation
  • Tenderness along the medial patellar border or adductor tubercle
  • Apprehension with lateral patellar translation at 20 to 30 degrees flexion
  • J sign on active extension
  • Lateral patellar translation graded in quadrants
  • Rotational profile, Q angle and Beighton score

Investigations

X-Ray

  • True lateral (30 degrees knee flexion)
  • Frontal
  • Sunrise / Merchant view (45 degrees knee flexion)

Image interpretation

Management

  • First time dislocation without osteochondral fracture managed non-operatively
  • Brief immobilisation then early motion
  • Quadriceps, hip abductor and core strengthening
  • Patellar stabilising brace or taping
  • Early surgery for a displaced osteochondral fragment or loose body

Surgical Management

MPFL reconstruction

TTT

Trochlearplasty

Videos

David Dejour – EFORT 2019

https://efortnet.conference2web.com/#!resources/tbd-d9c10653-2a0f-4b43-a962-4268106912e6

Philippe Tscholl – EFORT 2018

The Relevance Of Femoral Tunnel Positioning In Medial Patellofemoral Ligament Reconstruction On Postoperative Outcome

https://efortnet.conference2web.com/#!resources/the-relevance-of-femoral-tunnel-positioning-in-medial-patellofemoral-ligament-reconstruction-on-postoperative-outcome

Philip Schoettle – MPFL Reconstruction

Philip Schoettle – Trochlearplasty

References

Diduch, David R. MD; Kandil, Abdurrahman MD; Burrus, M. Tyrrell MD Lateral Patellar Instability in the Skeletally Mature Patient: Evaluation and Surgical Management, Journal of the American Academy of Orthopaedic Surgeons: June 15, 2018 – Volume 26 – Issue 12 – p 429-439 doi: 10.5435/JAAOS-D-16-00052

Thompson, Peter and Metcalfe, Andrew. Current concepts in the surgical management of patellar instability. The Knee November 11, 2019. Volume 26 (2019) p1171 – 1181. https://doi.org/10.1016/j.knee.2019.11.007

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.