Subspine impingement is just a variation of FAI, specifically the pincer type. The anterior inferior iliac spine (AIIS) is a bony prominence just above the front of the acetabulum. It is a site of attachment of the rectus femoris, which is one of the muscles that makes up the quadriceps group in the front of the thigh. The area underneath the AIIS is the subspine region. There is much variation in the morphology of the AIIS, sometimes with a bony prominence that extends all the way down to the rim of the acetabulum. With sports in childhood, especially soccer and tennis, there can be overgrowth of the AIIS, accentuating bony impingement from this area. Also in adolescents, a piece of this bone can get pulled off with sprinting activities, and as it heals back, leave a bony prominence.
Subspine impingement can occur as an isolated subtype of pincer FAI or more commonly occurs in conjunction with a prominence of the acetabular rim. The presence of a prominence of the subspine region may not be evident on routine x-rays, but special views (false profile view) can detect it. In cases of FAI where surgery is planned, a 3D CT scan can more clearly delineate whether there is any contribution of a subspine problem.
The treatment of subspine impingement is the same as any other type of FAI. When surgery is needed, the only difference is taking into account the contribution of the subspine area and correcting this at the time of surgery. This is accomplished by simply shaving down the undersurface of the AIIS causing the impingement.