Pelvic tear drop
The pelvic tear drop (also known as U-figure) is a radiographic feature seen on pelvic x-rays and results from the the end-on projection of a bony ridge running along the floor of the acetabular fossa.
This was shown to be the case by demonstrating that sawing away this bony plate made the feature disappear. It has also been confirmed by Vare in 1952 by applying a thin "strip of lead foil applied to the acetabular fossa and curving round the inferior margin of this, up over the inner wall of the lesser pelvis as far as the ilio-pectineal eminence."
There are several other tear drops, please see the disambiguation article here.