Why do we order X-rays? X-rays are utilized to rule out patella fractures or loose bodies, assess alignment, patella position and trochlear dysplasia (groove morphology).
When do we order a CT scan? CT scans assess the TT-TG distance –> Patellar translation can be measured using the tibial-tuberosity to trochlear groove distance (TT-TG distance) by calculating the transverse length between the trochlear groove on the femoral and tibial tuberosity on axial images. The measurements are used to quantify patellar instability. A TT-TG >20mm is considered abnormal.
What about ordering an MRI? MRI can assess for injury to the cartilage, an MPFL tear and bony bruising.