To fully evaluate the kneecap and its relationship to the femur below it, one needs to look at the bones from above.

It is easy to relate to this view, as it is the view you get of your own knees when sitting down.

knee from above

When the knee is bent, the undersurface of the patella (kneecap) lies snugly like this in the trochlear groove. The patella should be central in the groove, and not favouring one or other side, and it should not be tilted. The sides of the patella and the walls of the groove should be almost parallel. The apparent gap that one sees on X-rays from this angle (Merchant views) is not really a gap but the space is filled with the white joint cartilage that covers both of the bony surfaces where they are in contact.

Skyline view of patellaNote that you cannot see the rounded condyles of the femur in this view, nor can you see the tibia, as this angle looks down on the 'lap' and the condyles they are on the under-surface of the femur, while the tibia is at right angles to this view.

This view is also one that the surgeon can explore during arthroscopy, where he can get a superb opportunity to see how the patella glides in the trochlear groove. Many surgeons, unfortunately, miss this opportunity because they do not put the camera into this 'suprapatellar' region, and thus they fail to fully appreciate the anatomy here. This is also the best view to appreciate something called a 'plica', and problems with a plica may also be missed.