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Congruence angle

This is a measurement of the 'fit' of the kneecap (patella) into the underlying groove or 'trochlea' of the thighbone (femur), i.e. the angle showing the relationship between the two. Compare with the sulcus angle.

The congruence angle is one of the indices calculated by surgeons and radiologists when assessing the patella both before and after surgical procedures which attempt to re-align it.

‘Skyline’ X-rays of the patella and trochlea are taken with the knee bent at special angles (30, 60 and 90 degrees) or CT scans are taken (where the patella can be imaged with the leg straighter). The different angles allow one to evaluate subluxation of the patella as it engages in the trochlea as the knee bends.