If the groove is uneven or too shallow, the kneecap could slide off, resulting in a partial or complete dislocation. A sharp blow to the kneecap, as in a fall, can also pop the kneecap out of place.

The Orioles placed Rogers on the 15-day injured list Thursday with a right knee subluxation, Jake Rill of MLB.com reports. Rogers suffered the injury back in January and, at last check, remained ...