Patellar instability with medial patellofemoral ligament tear

Case contributed by Dr Derek Smith

Presentation

Twisting injury with medial joint line pain. Normal knee radiograph. MRI to assess for meniscal tear.

Patient Data

Age: 30 years
Gender: Female

Small joint effusion.

Pattern of bone bruising (high signal on PD fat sat sequence) in the medial patellar facet and lateral femoral condyle suggests recent lateral dislocation.

A number of features supporting patellar instability: shallow trochlea, borderline tibial tuberosity - trochlear groove (TT-TG) distance, and lateral patellar tilt.

Signal change in the medial patellar retinaculum, with discontinuous/torn medial patellofemoral ligament. Edema extends along the medial supporting structures, including the MCL.

Normal menisci, ACL, PCL and lateral collateral complex. Normal patellar and included quadriceps tendon.

Case Discussion

There are a number of "typical" features for patellar instability/dislocation in this case including a flattened trochlea, malalignment, and bone edema consistent with recent dislocation. The medial patellofemoral ligament and retinaculum are review areas in MRI knee, and when torn can continue to leave the patella unstable. Instability may be initially managed with physiotherapy.

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