Table 1 MRI checklist for the assessment of patellar maltracking

Trochlear dysplasia

Trochlear depth (Fig. 1), lateral trochlear inclination (Fig. 2), trochlear facet asymmetry (Fig. 3) (evaluated on most cranial axial image showing cartilage, approximately 3 cm above the joint line)

Geometric abnormality of the trochlear groove that can result in abnormal tracking of the patella along the trochlea

Patella alta

Insall–Salvati index (Fig. 4)

Caton–Deschamps index (Fig. 4)

Relates to a long patellar tendon. In order for the patella to engage with the trochlea, a higher degree of flexion than normal is needed

Lateralization of the tibial tuberosity

Tibial tubercle–trochlear groove distance (TT-TG) (Fig. 5)

High TT-TG would exert lateral pressure on the patella during extension, and if not counteracted by vastus medialis contraction, it may predispose to patellar subluxation

Lateral patellar tilt

Patellar tilt angle (Fig. 6)

Patellofemoral angle

Sensitive marker for patellar instability present in significant proportion of patients

Hoffa’s fat pad impingement

Edema at the superolateral aspect of Hoffa’s fat pad on MRI

Significant association with several patellar maltracking indicators

MPFL and medial patellar retinacular injury

Best evaluated on the axial fluid sensitive MRI sequence

Present in the majority of patellar dislocation cases

Chondral and osteochondral damage

MRI can show discrete osteochondral defect or various degrees of patellofemoral cartilage loss

Patellar maltracking is significant risk factor patellofemoral osteoarthritis. Patellar dislocation can result in discrete osteochondral defects at the patella or lateral femoral condyle