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Femoroacetabular Impingement (FAI)

Co-Authors:  Milford H. Marchant Jr., M.D.; Thomas P. Vail, M.D.; Steven A. Olson, M.D.

July 2006

Femoroacetabular Impingement (FAI)
       - Descriptive Diagnosis characterized by a combination of clinical signs, symptoms, and pathology that may be responsible for the progression of degenerative changes in patients with osteoarthritis of the hip
       - Distinct Mechanical entity defined as the abutment between the proximal femur and the acetabular rim.

Relation to Hip Arthritis
       - Types of Mechanical Arthritis
               - Osteoarthritis - Primary Idiopathic
               - Secondary (Trauma, Development Dysplasias)

       - Etiology
               - Prior Biomechanical theories suggest that cartilage damage is initiated by Concentric or Eccentric overload.
               - Eccentric overload
                       - easily explained by non-congruent articulations caused by developmental dysplasias and post-traumatic anatomy
               - Concentric overload
                       - not as easy to explain

               - Harris (1986) 
                       - Large majority of cases previously thought to be primary or idiopathic osteoarthritis (OA) are actually secondary to unrecognized mild developmental abnormalities
                       - Review of 75 patients with idiopathic OA, 79% had subtle abnormal femoral or acetabular  morphology
                               - Femoral deformity
                                       - Abnormal head/neck contour
                                               - Slipped Epiphysis, Legg-Calve-Perthes
                                               - More Common in Males - later onset (70’s)
                               - Acetabular dysplasia
                                       - Coxa profunda / Retroversion
                                       - More Common in Females - earlier onset (60’s)
                       - Fails to completely explain the development of arthritis is patients with non-dysplastic appearing skeletal structures 
                       - Etiology of osteoarthritis of the hip.

               - Ganz, et al. (2003) 
                       - Summarized the concept of FAI
                       - Mechanism for development of osteoarthritis based on subtle aberrant bony morphology
                               - Acetabular Retroversion / Coxa Profunda
                               - Femoral Head non-sphericity
                       - Abnormal Contact in Normal / Near Normal Appearing Hips
                       - Abutment of the Proximal Femur on the Acetabular Rim during terminal motion of the hip leading to lesions of the labrum and/or the adjacent cartilage
                       - Chondral and Labral lesions progress and result in degenerative disease 
                       - Femoroacetabular impingement: a cause for osteoarthritis of the hip.

               - Leunig, et al. (2004) 
                       - Acetabular Rim degeneration & early hip arthritis
                       - 30 cadavers and 18 hemiarthroplasty patients for displaced femoral neck fractures (Ages > 60)
                               - No patients with radiographic evidence of OA
                               - 30/30 cadavers had labral and acetabular cartilage damage
                               - 17/18 patients with labral injury and 16/18 with acetabular cartilage injury
                               - All but 1 femoral head cartilage was normal
                       - 80% found to have evidence of FAI
                       - Acetabular Rim Degeneration is a constant finding in the aged hip and FAI may be a significant trigger 
                       - Magnetic resonance arthrography of labral disorders in hips with dysplasia and impingement.

       - Impingement
               - Normal / Near-Normal hips
                       - Impingement occurs at excessive or supraphysiologic ranges of motion = Flexion / Adduction / Internal Rotation
                               - Minor Trauma
                               - Specific occupations (Carpet Layers)
               - Atypical Bony Morphology
                       - Less motion is required for impingement

       - 2 Mechanisms for FAI
               - CAM Impingement
               - Pincer Impingement

       - CAM Impingement
               - Anatomy
                       - Abnormal Femoral Head/Neck junction with increased radius at the waist
               - Motion
                       - Impingement occurs primarily during flexion, adduction, IR
               - Mechanics
                       - Contact between the femoral neck and acetabular rim induces compression
                       - Shear stress generated at the junction between the labrum and the cartilage and at the subchondral tidemark
                       - Outward avulsion of the labrum and/or an inward compression of the articular cartilage at Anterosuperior Rim
               - Etiology of CAM Impingement
                       - Elliptical Femoral Head
                       - Slipped Capital Femoral Epiphysis - SCFE 
                       - Legg Calve Perthes 
                       - Adult Osteonecrosis 
                       - Malunited Femoral Neck Fractures

        - Pincer Impingement:
               - Anatomy
                       - Excessive Acetabular Coverage
               - Motion
                       - Dependent on acetabular morphology
               - Mechanics
                       - Linear Contact between the labrum and femoral head/neck junction
                               - Anterior = Acetabular Retroversion
                               - Circumferential = Coxa Profunda
                       - Force from the femoral neck is transferred through the labrum to the acetabular cartilage
                       - Results is chronic degeneration of anterior labrum and subsequent ossification
                               - Further deepens the cup
                       - Resultant leverage of head in acetabulum with excessive ROM can result in contre-coup lesion
                         in posteroinferior acetabulum and posteromedial femoral head
                               - Leading to Circumferential involvement
               - Etiology
                       - Acetabular Retroversion
                       - Coxa Profunda 
                       - Protrusio Acetabuli 
                       - Iatrogenic overcorrection for retroversion/dysplasia
                       - Coxa Vara 
                       - Os Acetabuli

       - CAM and Pincer mechanisms produce differing mechanisms for labral injury
               - CAM

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