•Femoroacetabular Impingement (FAI)

•Acetabular rim syndrome

•Cervicoacetabular impingement

•Young patients with chronic pain

•Reduced ROM in flexion and internal rotation

•Repetitive microtrauma

•Increased incidence of premature degenerative arthritis

•Etiology

•Abnormal acetabulum

•Abnormal femur

•Increased stress

•Two types

•Pincer (acetabular)

•Cam (femoral)

•Mixed – 86 %

•Clinical symptoms

•Groin pain

•Pain over trochanters

•Pain with flexion and internal rotation

•Usually unilateral

•Starts after mild trauma

•Younger patients than typical

•Maneuver

•Flex hip 90 degrees, adduct, and internally rotate

•Should compress labrum and cause pain

•Some predisposing factors to FAI

•Legg-Calve-Perthes disease

•Congenital hip dysplasia

•Slipped capital femoral ephiphysis

•Avascular necrosis

•Malunited fractures

•Acetabular protrusion

•Elliptical femoral head

•Retroverted acetabulum

•Prominent femoral head-neck junction

•Proposed etiologies

•Abnormal anatomy

•Prominent femoral head neck junction

•Acetabular overcoverage

•Unusual stesses

•Carpet layers – repeated flexion, adduction, internal rotation

•Pincer type of FAI

•Middle to older aged women (40)

•Seen in ballet dancers

•Close approximation of acetabular rim and femoral neck – acetabular abnormality

•Acetabular overcoverage

•Focal articular damage

•Acetabular damage can propagate

•Primary radiographic signs

•Coxa profunda

•Protrusio acetabuli

•Acetabular retroversion

•Decreased extrusion index

•Neutral acetabular index

•Posterior wall sign

•Posterior inferior cartilage abrasion due to contracoup injury

Normal

Pincer

•Coxa profunda – floor of fossa acetabuli overlaps ilioischial line medially

•Pincer type FAI

•Creates deep acetabulum

•General overcoverage

•Normal

•Protrusio acetabuli – occurs when the femoral head overlaps the ilioischial line medially

•Pincer type FAI

•Creates deep acetabulum

•General overcoverage

•Normal

•Lateral center edge angle – pincer type FAI

•Normal is between 25 and 39 degrees

•Increases with deeper acetabulum and more overcoverage

Protrusio acetabuli

•Decreased extrusion index – pincer type FAI

•(E / [A + E])

•25 % in normal subjects

•Decreases as femoral head becomes “more covered.”

•Acetabular index – pincer type FAI

•Should be positive

•Becomes negative as acetabulum “deepens”

Positive AI Negative AI in protrusio acetabuli

•Acetabular retroversion – pincer type FAI

•Cross over sign

•Focal acetabular overcoverage

•Cranial anterior wall line projects laterally

•Anterior/anterolateral labrum is obstacle to flexion and internal rotation

•Distinguish from deficient posterior wall

•Posterior wall sign – pincer type FAI

•PW line should descend through center of femoral head

•Medial – deficient

•Lateral – prominent

•Linear indentation sign – pincer type FAI

•Occurs due to mechanical injury and reactive change

•Associated with pincer type

•Os acetabuli

•Cam type of FAI

•Young males (32 years)

•Primary femoral abnormality

•Aspherical femoral head

•Femoral head jams into acetabular rim

•Shear forces on labrum and cartilage

•Diffuse articular damage

•Primary radiographic signs

•Pistol grip deformity

•CCD angle less than 125 degrees

•Horizontal growth plate sign

•Alpha angle greater than 50 degrees

•Femoral head-neck offset less than 8 mm

•Femoral retrotorsion

•Pistol grip deformity - Cam type FAI

•Loss of normal concavity

•Etiology

•Growth abnormality of the capital femoral epiphysis

•SCFE

•LCPD

•Fracture healing

•Horizontal growth plate sign - Cam type FAI

•Alpha angle – Cam type FAI

•Used as an objective representation of the prominence of the anterior femoral head-neck junction.

•Abnormal is greater than 50 degrees

Normal Abnormal

•Femoral head-neck offset (OS) – Cam type FAI

•Abnormal if less than 10 mm

•Femoral retrotorsion – Cam type FAI

•Congenital or post traumatic

•Calc by CT•Normal torsion

•Retrotorsion

•Coxa vara - Cam type FAI

•Abnormally located femoral neck

•Decreased caput collum diaphysis (CCD) angle

•Normal is 125 to 135

•Conventional radiographic findings

•Pincer

•“Deep” acetabulum

•Focal acetabular retroversion or posterior wall sign

•Cam

•Pistol grip deformity

•Coxa vara deformity

•Misshapen femoral head

•Prior trauma or deformity

•Secondary degenerative changes

•MR imaging

•Labral damage with corresponding damage to femoral head/neck junction

•Chondromalacia

•Superolateral in cam type

•Posteroinferior in pincer type

•CT imaging

•Similar to radiographic findings

•Secondary radiographic signs

•Labral ossification

•Bony impaction changes

•Synovial herniation pits

•Premature degenerative changes

•Secondary MR findings in cam FAI

•Superolateral changes

•Classic MR findings in pincer FAI

•Posteroinferior cartilage abnormality due to contracoup injury

•Treatment

•Intertrochanteric flexion-valgus osteotomy

•Arthroscopic debridement

•Remove any nonspherical portion of femoral head

•Reduce size of acetabular rim in pincer type

•Total arthroplasy in end stage disease

1. Tannast M, Siebenrock K, Anderson S. Femoroacetabular impingement: radiographic diagnosis--what the radiologist should know. AJR Am J Roentgenol. 2007 Jun;188(6):1540-52.

2. Pfirrman CW, Mengiardi B, Dora C, Kalberer F, Zanetti M, Hodler J. Cam and Pincer Femoroacetabular Impingement: Characteristic MR Athrographic Findings in 50 Patients. Radiology 2006 Sep; 240(3):778-85. Epub 2006 Jul 20.

3. Beall DP, Sweet CF, Martin HD, Lastine CL, Grayson DE, Ly JQ, Fish JR. Imaging findings of femoroacetabular impingement syndrome. Skeletal Radiol (2005) 34: 691 – 701