Good question Son,
Personally I think it is all about the clinical physical exam performed by aq sports and exercise physician, orthopod or physiotherapist and the plain radiographs.
The ultrasound is a secondary concern,
BUT
I still look for it every time I do a hip ultrasound.
There are 2 types of FAI, Pincer and Cam
In Pincer there is no signs on the ultrasound except early onset OA changes,labral pathology such as paralabral cysts.
In Cam you casn also see early onset OA changes,labral pathology such as paralabral cysts, but you may also note that the femoral head / neck has an abnormal shape with the femoral head seemingly extending into the femoral neck territory and you lose the characteristic “slippery dip” femoral head to neck shape.
I often notice this just lateral to the midline of the femoral neck.
I might also notice some irregularity of the bone at the head / neck junction area.
Then I look at the acetabular rim (iloppectineal eminence) and just lateral to the iliopsoas tendon you may see some bony irregularity.
In your case the bony change at the acetabular rim is worth noting and your patient should have an AP pelvis / lateral hip radiograph and a physical examination as described.
Nice one.
The trick bit is how do they manage someone this age with FAI???
I don’t think anyone really knows what is best,
Steve