Medial patellofemoral ligament avulsion injury

Case contributed by Dr Joachim Feger

Presentation

Pain and swelling of the knee, after a patellar dislocation.

Patient Data

Age: 10 years
Gender: Female

Findings:

  • avulsion fracture fragment at the medial edge of the patella
  • joint effusion
  • patella alta

Findings:

  • large, predominantly suprapatellar joint effusion

Intercondylar Region:

  • unremarkable anterior and posterior cruciate ligaments

Medial compartment:

  • unremarkable cartilage and medial meniscus
  • soft tissue edema above the normal medial collateral ligament

Lateral compartment:

  • bone contusion at the anterolateral aspect of the lateral femoral condyle
  • unremarkable cartilage and lateral meniscus 
  • normal lateral collateral ligament and posterolateral corner

Patellofemoral compartment:

  • small osseous avulsion of the medial patella rim
  • thickened, edematous medial patellofemoral ligament 
  • fluid extending into the vastus medialis myotendinous junction
  • shallow trochlear groove, lateral trochlear inclination (LTI) was borderline 11-12°

Impression:

Typical findings of previous lateral patellar dislocation:

Annotated image

Key findings:

  • bone bruises of inferomedial patella (blue arrowhead) and the anterolateral femoral condyle (blue arrows)
  • medial patellar avulsion fracture (purple arrow)
  • medial patellofemoral ligament edematous and thickened (red arrow) also at the femoral attachment (red arrowheads)
  • a tear of the medial retinaculum at the inferomedial patellar attachment (orange arrow)
  • myotendinous tear of the vastus medialis oblique (VMO) muscle (green arrow)

Assessment for patella alta

Assessment for trochlear dysplasia

  • borderline lateral trochlear inclination (LTI) 
  • no trochlear facet asymmetry (FA)
  • decreased trochlear depth (2 mm) 

Lateral trochlear inclination (LTI) < 11°, medial vs lateral trochlear facet ratio of <40% 1-4, and/or a trochlear depth < 3mm 6, indicate trochlear dysplasia.

TT-TG distance (tibial tuberosity to trochlear groove distance)

  • was normal (<15mm) in this case

A TT-TG distance >20mm is considered a risk factor for patellofemoral instability.

Case Discussion

This case illustrates typical findings in previous lateral patellar dislocation:

It also reviews imaging assessment for patella alta, trochlear dysplasia and increased TT-TG distance which are predisposing factors for patellofemoral instability.

Surgical repair of bone avulsion injuries after prior lateral patellar dislocation is recommended 7,  especially in patients with risk factors and was done in this patient.

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Case information

rID: 75668
Published: 12th Apr 2020
Last edited: 18th Oct 2020
Inclusion in quiz mode: Included

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