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Trochlear dysplasia classification according to D. Dejour. In type A, the crossing sign is present. Type B features include the crossing sign and trochlear spur. In type C, there is a crossing sign and the double contour sign. Type D combines the crossing sign, supratrochlear spur, and double contour sign (with permission of Springer: Dejour D, Saggin P. The sulcus deepening trochleoplasty—the Lyon’s procedure. Int Orthop 2010;34:313; Fig. 2)
