Young and Hip.

A teenager presented with pain in his right hip.  It had been present for a couple of weeks after kicking a ball.  There was a reduction in hip flexion to about 90 degrees, although an otherwise reasonable ROM.  He was tender over the antero-lateral hip.  He had an X-ray to exclude Slipped Upper Femoral Epiphysis (SUFE).  What do you think?

No SUFE, but an Anterior Inferior Iliac Spine (AIIS) avulsion on the right.  

This lad is the right age group for SUFE, and it can present in a similar fashion; SUFE classically affects Boys (OR 2.4), 10-16 years, often after sport and often presenting some time after the pain started.  While they may be unable to bear weight, they usually can but with an antalgic and out-toeing gait - the fact that they have "obligatory" external rotation (even mild) is pathognomic.  SUFE is associated with higher BMI and hypothyroidism/pituitarism.  The Xray findings (Line of Klein not intersecting the lateral part of the femoral head etc)  are discussed further here.

AIIS avulsion is the second commonest of the 6 main pelvic avulsion injuries and classically happens after kicking.  You can get an overview of the treatment options here.

These are the main sites of avulsion - sometimes hard to tell from the apophyses (as seen at the ischial tuberosities and iliac crests in the above Xray) which are a normal growth appearance...

(From Radiopaedia.org)