Aims: We studied by means of a magnetic resonance imaging (MRI) protocol, the junction area between supratrochlear (ST) surface and the femoral trochlear groove (FT). The variations of this junction area are they correlated with the patientñs functional signs and with the patellar cartilage injuries?Method: We practised on 87 patients (64 patellar instability, 23 patellar pains) and 25 witnesses, an MRI: DESS and MPR sequences. The trochlear bump was studied in the sagittal plan according to the aspect of the junction area and in measuring itñs height. Results: The junction area was dismembered in 4 types according to its slope with the ST surface: ÒßatÒ, ÒroundÒ, ÒobliqueÒ and ÒsquareÒ. No atÒ typeÒßwas found in cases of FT dysplasia. The ÒobliqueÒ and ÒsquareÒ types were more frequent in cases of important projection of the FT (p< 0.0001). These two types were more frequently associated with the patellar cartilage injuries (p< 0.08). The trochlear projection was maximum (p< 0.0001) in FT dysplasia with spur, with a maximum effect in this case on patellar instability (p< 0.01) and also on patellar pain (p< 0.05). Conclusion: The junction area between the ST surface and the FT groove was dismembered in 4 types. A þrst ßat type without trochlear bump, and 3 types deþning a trochlear Òstep of stairÒ, round, square and oblique in order of growing gravity. The latter two were more common when patellar cartilage injuries existed.