Presentation
Pain and swelling of the knee, after patellar dislocation.
Patient Data
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.
Case Discussion
This case illustrates typical findings in previous lateral patellar dislocation:
- medial patellofemoral ligament injury with a patellar avulsion fracture and a tear of the femoral origin
- bone bruises at the typical locations at the inferomedial patella and the anterolateral margin of the lateral femoral condyle
- a tear of the medial retinaculum
- a myotendinous tear of the vastus medialis oblique (VMO) muscle
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.