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It is sometimes associated with fracture of the edge of the trochlear surface, which renders retention in position difficult.

It is often found between the patella and the trochlear surface of the femur in tuberculous disease of the knee.

When the patella is situated upon the inner trochlear rim, the tibia must be extended because of the traction exerted by the straight ligaments.

The patella is raised from the front of the femur by the collection of fluid in the joint—“floating patella”—and, if firmly pressed upon, it may be made to rap against the trochlear surface.

The bones also are altered in their shape and mutual relations, the talus being rotated forwards so that a large portion of its trochlear surface protrudes from the tibio-fibular socket.