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
Lateral Trochlear Inclination
Normal >11 degrees
Trochlear facet asymmetry
Trochlear Sulcus Angle (TSA)
Normal 138 +- 6 degrees
Tibial Tuberosity- Trochlear Groove distance
TT- PCL distance
Tibial tuberosity – Posterior cruciate Distance
Normal 11.9 +-4.76mm (>20 is abnormal)