Femoro-acetabular impingement (FAI)
FAI is the pathologic abutment of bone & cartilage of the femur to the acetabulum. The two surfaces should not come into contact in normal physiological range but do so in FAI. Abnormal contact between the proximal femur and acetabular rim during terminal motion leads to labral +/- adjacent acetabular cartilage lesions. Repeated microtrauma causes reactive bony hyperplasia and cartilage calcification causing further impingement and eventual OA. FAI may result from hip dysplasia, a varus proximal femur (coxa vara), acetabular protrusion, reduced head-neck offset (pistol grip deformity) or in the non-dysplastic hip after subjecting it to excessive ROM.
It is present in 20% of the male population, and 80-90% of hip OA is 2° to FAI. Certain sports exacerbate this abutment e.g. kicking in martial arts & soccer; egg beat kick; squash (low shots); hurdling & hypermobile athletes like dancers or gymnasts.
Femoro-acetabular impingement usually presents in active young adults with slow onset of groin pain often following minor trauma. During the initial stages pain is intermittent and may be exacerbated by ↑ demand such as athletic activities or prolonged walking. The pain often is present after sitting for a prolonged period.
Hip examination often reveals ↓ ROM, particularly IR/adduction in flexion. The impingement test is almost always positive. Flexion and adduction leads to approximation of the femoral neck and acetabular rim. Forceful additional IR induces labral shearing forces creating a sharp pain when there is a chondral lesion, a labral lesion, or both. Occasionally, posteroinferior impingement also can occur. The provocative test is done with the patient lying supine on the edge of the bed with the legs hanging free from the end of the bed, to create extension. ER in extension causing severe deep seated groin pain is indicative of postero-inferior impingement.
X-ray abnormalities include an anterolateral head/neck junction bony prominence best seen on lateral views (known as a Ganz lesion), reduced offset of the femoral neck/head junction, and acetabular rim changes like os acetabuli or double-line seen with rim ossification. The femoral neck may reveal herniation pits.
Types of Impingement
Abnormality is on the femoral side. An abnormal femoral head jams the acetabulum during forceful motion, especially flexion. The resulting shear forces produce outside-in abrasion of the acetabular cartilage and/or its avulsion from the labrum and the subchondral bone in a constant anterosuperior rim area. Chondral avulsion in turn leads to tear or detachment of the principally uninvolved labrum.
There is over coverage of the femoral head by the acetabulum and linear contact between the acetabular rim and the femoral head-neck junction. The first structure to fail here is the labrum. Continued abutment results in labral degeneration with intra-substance ganglion formation, or rim ossification leading to additional acetabular deepening and worsening of the over coverage.
Pincer impingement is more common in middle-aged ♀ with morphologic acetabular abnormalities while cam impingement is more common in young ♂ with morphologic femoral head abnormalities.
Surgical treatment comprises mainly removing any non-spherical portion of the head, improving the neck offset and subsequent clearance. Femoral neck osteoplasty particularly is an important part of alleviating cam impingement. For pincer impingement, this includes reducing the anterior over coverage by excising the bony prominence at the rim. The torn or degenerate area of the labrum also is excised and the remainder of the labrum, if substantial, is reattached to the rim using suture anchors.
Anteriorly the labrum is equilaterally triangular in radial section. Posteriorly it is more bulbous and lip like, dimensionally square but with a rounded distal surface. It has a rich nerve supply which is thought to have both a proprioceptive and nociceptive role. Most of the labrum is composed of thick, type I collagen fibre bundles principally arranged parallel to the acetabular rim. The labrum merges with the hyaline cartilage of the joint surface over a margin of 1-2mm.
X-ray may show dysplasia, arthritis and acetabular cysts, but is not reliable for detecting labral tears. MRI +/- gadolinium enhancement is more promising but not as good as arthroscopy. MRI features suggesting a labral tear include irregular shape, a non-triangular labrum, a thickened labrum with no recess, ↑ labral T1 signal intensity, and a labrum that has detached from the acetabulum.
Positive hip quadrant, occasional clunk. There is pain with:
Flexion, adduction, and IR of the hip joint (with anterior superior tears)
Passive hyperextension, abduction, and ER (with posterior tears)
Acute flexion of the hip with ER and full abduction, followed by extension, abduction, and IR (anterior tears)
Extension, abduction, and ER brought to a flexed, adducted, and IR position (posterior tears)
Unilateral groin pain. Frequently low back pain.
Usually history of twisting injury, sometimes running or direct trauma.
Tears occur most commonly in the anterior part of the labrum, especially the antero-superior quadrant, but the posterior part may also be affected. Labral tears are thought to represent up to 20% of athletes presenting with groin pain. Tears occur in 3 groups:
- The aging hip: there is degeneration and weakening of the labrum with age, with ↑ vulnerability to traumatic tears, and they often occur in osteoarthritic hips.
- The dysplastic hip: the labrum may be hypertrophied & exposed to ↑ stresses with tears as inevitable outcomes. 2 types of hip dysplasia are described above.
- The normal hip: Usually has an insidious course, with long periods between onset of symptoms & diagnosis. Tears are more likely in an anterior position.
The natural history of a hip labral tear is unclear and whether some tears become asymptomatic or predispose to further degenerative change is unknown. Labral tears tend to occur on the articular non-vascular edge, and may therefore not heal with conservative treatment.
Operative intervention includes arthroscopy and debridement, and has been shown to have fairly good outcomes. Pudendal, peroneal and sciatic nerve neuropraxia is a potential complication of arthroscopy. Sometimes ORIF is necessary.
Chronic labral tears occur in association with acetabular dysplasia which in turn causes chronic ligamentous traction on the labrum, resulting in it coming away from the acetabulum.