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Institute of Advanced Orthopedics, MOSC Hospital and Medical College, Kolenchery, Ernakulam, Kerala

Radiological Measurements in Patellar instability

It is important to assess the ligamentous structures around the knee joint for planning a patellar stabilizing procedure in recurrent dislocation of the patella.

Usually radiographs and MRI scan are done to do the measurement. You will require an expert musculoskeletal radiologist to do accurate measurements to identify alignment issues in the extensor mechanism and trochlear groove of the femur, in addition to Medial patellofemoral ligament tears. Addressing these pathologies, if present, will increase the success rate of a arthroscopic MPFL reconstruction procedure.

Caton Deschamps index

The Caton-Deschamps index is used to measure patellar height and identify patella alta (high riding patella) and patella baja (low lying patella). Patella alta predisposes to patella dislocation.

Normal 0.6 to 1.3

>1.3 – Patella Alta

<0.6 – Patella Baja

CD index on Xray
CD index on MRI

Lateral Trochlear Inclination

Normal >11 degrees

Trochlear facet asymmetry

Normal >0.4

Normal Trochlear facet
Abnormally small medial facet of trochlea

Trochlear depth

Normal >3mm

Normal trochlear depth
A. Normal knee; B. Trochlear dysplasia

Trochlear Sulcus Angle (TSA)

Normal 138 +- 6 degrees

TT-TG Distance

Tibial Tuberosity- Trochlear Groove distance

Normal <15mm

Trochlear groove marked on MRI
Normal TT-TG distance measured

TT- PCL distance

Tibial tuberosity – Posterior cruciate Distance

Normal 11.9 +-4.76mm (>20 is abnormal)

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