Medial patellofemoral ligament avulsion injury

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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:

• medial patellofemoral ligament injury with bony avulsion of the patellar insertion and tear of the femoral origin
• tear of the medial retinaculum and vastus medialis oblique (VMO)
• bone contusion of the anterolateral femoral condyle
• decreased trochlear depth indicating trochlear dysplasia
• patella alta

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

• Insall-Salvati-ratio (blue) was > 1.5
• modified Insall-Salvati-ratio (orange) > 2

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.