FAI is increasingly recognized as a pathomechanical process that can lead to hip pain and early osteoarthritis of the hip in young and active adults.
The hip joint is a ball-in-socket joint.
The principle function of the hip is to enable weight bearing for locomotion.
The normal anatomy of the hip joint consists of the following: the femoral head is slightly more than half a sphere; the femoral neck is cylindrical (narrowest in the midpoint and widest laterally) and attaches the head to the shaft; the acetabulum is cup shaped and the labrum is triangular in cross section with its base attached to the acetabular rim.
This clinical syndrome occurs as a result of abnormal contact between the proximal femur and acetabular rim during hip motion,
particularly in flexion and internal rotation.
In this setting,
impingement can occur when there is an underlying abnormal morphology (femoral head-neck junction or acetabulum or both factors) or at increased level of activities when the hip joint is heavily solicited,
despite normal bony geometry.
There are two major types of FAI:
PINCER impingement is the result of an acetabular abnormality with focal or general overcoverage of the acetabulum around the femoral head.
PINCER impingement is most frequently seen in middle-aged active women (in their 40s and 50s).
Possible causes for PINCER impingement are: idiopathic; developmental (retroverted acetabulum,
coxa profunda,
protrusio acetabuli,
chronic residual dysplasia of the acetabulum) post-traumatic (post-traumatic deformity of the acetabulum) and iatrogenic (overcorrection of retroversion in dysplastic hips).
CAM impingement is due to an aspherical portion of the femoral head-neck junction.
It can be caused by an osseous bump on the femoral head-neck junction,
that is typically located either laterally (so called pistol grip deformity,
seen on an AP pelvic radiograph),
or anterosuperiorly (seen on an axial cross table view of the proximal femur).
This form of hip impingement is most often seen in young,
active men (between 20 and 40 years of age).
Suggested possible causes for CAM impingement are: idiopathic,
developmental (non spherical femoral head,
coxa vara),
post-traumatic (malunited femoral neck fracture,
post-traumatic retroversion of the femoral head),
childhood orthopaedic conditions (Perthes disease,
slipped capital femoral epiphysis) and iatrogenic angular abnormality after femoral osteotomy.
Participation in high-impact sporting activities during skeletal maturation is also associated with an increased risk of developing CAM morphology.
Both configurations lead to recurrent abutment between the proximal femur and the acetabulum with subsequent damage to the labrum and adjoining articular cartilage,
leading to early degenerative hip arthritis.
Most patients have a combination of both forms - referred to as "mixed type impingement".
Active young and middle aged adults typically experience insidious onset of groin pain often exacerbated by activity or prolonged sitting and no prior history of trauma,
with reduced range of motion particularly flexion,
adduction and internal rotation and positive impingement tests.
Clinical tests are generally unspecific with a resulting broad spectrum of differential diagnosis.
Diagnosis of FAI is challenging and is based on a combination of hip pain,
reduced range of movement,
positive impingement tests,
and specific radiological findings.
Although there are very dedicated imaging tests that can be aimed to detect FAI,
common imaging modalities such as the plain radiograph should be the baseline investigation.
The role of radiography is to evaluate the hip for abnormalities associated with impingement and to exclude other joint problems.
The radiographic evaluation of FAI is limited by the lack of visibility of non-osseous structural abnormalities of the hip joint.
MRI or MR arthrography can then be used to confirm or exclude labral tears,
cartilage damage,
and other pathologic signs of internal hip derangement if impingement is suspected.