After a transient lateral dislocation the bone bruises (medial patella, lateral femoral condyle) and MPFL tear make the diagnosis; the report then measures the anatomical risk factors — trochlear dysplasia, patella alta, TT–TG distance — that decide stabilisation surgery.
Orient first
- Most dislocations reduce spontaneously; the patient may not know it happened.
- Kissing contusions: inferomedial patella and anterolateral lateral femoral condyle.
- Risk factors: trochlear dysplasia (Dejour), patella alta (Caton–Deschamps / Insall–Salvati), TT–TG > 20 mm (verify thresholds).
Acquire the study
- MRI knee: proton-density fat-saturated in three planes, T1 sagittal; axial images through the trochlea and tibial tubercle for TT–TG.
The manoeuvre
- Axial PD fat-saturated: contusions of the medial patellar facet and lateral femoral condyle.
- Medial patellofemoral ligament: patellar, midsubstance or femoral attachment tear on axial images.
- Osteochondral fragment: donor site on the patella or condyle and a loose body in the joint.
- Trochlea on the most proximal axial slice: flat or convex (dysplasia); sulcus angle in degrees.
- Sagittal: patellar height ratio; axial: TT–TG distance in mm.
What confirms it
- Kissing contusions with an MPFL injury after a twisting event.
What licenses you to exclude it
- No contusions and an intact MPFL make a recent dislocation unlikely.
The classic misread
- Missing a displaced osteochondral fragment — it needs early fixation.