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©2010 British Editorial Society of Bone and Joint Surgery

Focus OnFemoroacetabular impingement

IntroductionTears of the acetabular labrum are a recognised cause of hippain.1-4 Possible causes of labral tears include trauma, hypolaxityof the anterior capsule, dysplasia, bony impingement and reducedbiomechanical properties in the area most susceptible to tears.5,6

Impingement of the proximal femur against or within theacetabular rim is a common underlying cause of a labral tear inthe non-dysplastic hip.7,8

Leunig et al9 defined morphological abnormalities in theshape and orientation of the femur and acetabulum that charac-terised femoroacetabular impingement (FAI) and proposed acausative link between FAI and the development of osteoarthri-tis.9-12 Wagner et al13 showed that the cartilage from the non-spherical portion of the femoral head in FAI showed histologicalchanges similar to those of an arthritic hip, and different to aspherical head control group.

Chondral damage is associated with FAI in a predictable man-ner,14,15 and damage to the acetabular rim has been postulatedas part of a sequence of events causing hip osteoarthritis.16

The key morphological characteristics of FAI relate to abnor-malities in the shape and orientation of the femoral head andneck (cam impingement) or to abnormalities in the shape andorientation of the acetabulum (pincer impingement). While grossexamples of cam impingement (pistol-grip deformity, postSlipped Upper Femoral Epiphysis) and pincer impingement(severe acetabular retroversion) have been recognised for manyyears, the recognition of more subtle variants of these condi-tions and their links to the early development of arthritis hasallowed greater understanding of early adult arthritis and facili-tates the development of new treatment options.

Cam ImpingementThe basic hypothesis is that cam impingement arises when theradius of curvature of the femoral head or neck profile exceedsthe radius of curvature of the acetabulum, and can arise from anumber of basic abnormalities. A key element of cam impinge-ment is a reduced or absent offset between the femoral headand neck (Fig. 1). The degree of cam deformity has been defined

Offset ratio = x/y

y x

Alpha angle

Alpha angle

Ideal hip Retroverted Beta angle

Beta angle

Alpha angle

Beta angle

Alpha angle

Beta angle

Aspherical hipBroad hip

Fig. 1

Varieties of cam femoroacetabular impingement

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by measurements of offset-ratio, alpha and beta angles onradiographs17,18 or MRI.19

As the aspherical, expanded portion of the femoral head-neckcomplex encroaches on the acetabular rim, the labrum displacesoutwards resulting in increased tensile stress at the labrochon-dral junction resulting in labral type-1 tear separation from theadjacent articular cartilage at the watershed.20 As the asphericalportion rotates further within the acetabular rim, compressiveforces and shear stress at the osteochondral junction increase,resulting in delamination of the chondral surface from the under-lying bone and producing the so called 'wave sign' visible atarthroscopy. Fissure fractures created by traction forces at thelabrochondral junction expose the edge of the chondral surfaceto the advancing 'front' of the femoral cam lesion, and result inthe creation of chondral flap tears or 'carpet lesions' (Fig. 2). Thishypothesis is supported by McCarthy et al,21 who confirmed that

73% of hips with labral tears had adjacent acetabular chondraldamage. Johnston et al15 confirmed that cam impingers with highalpha angles were associated with labral injury, chondral damageand decreased range of movement.

Pincer ImpingementThe femoral neck of a hip with adequate head-neck offset canimpinge on the rim of the acetabulum at the extremes of itsrange of movement. However, if the shape or orientation of theacetabular rim results in impingement before the hip hasachieved its required functional range of movement, impactdamage to the labrum will occur.

Radiographic features suggesting a susceptibility to pincerimpingement include a deep socket (coxa profunda or protrusio),high centre-edge angle, low sourcil angle and signs of global or seg-mental retroversion or anteversion of the acetabular rim (Fig. 3).

Dynamic pincer impingement can occur in 'normal' hips if therequired range of movement is large (e.g. dancers and gym-nasts) or translated (e.g. hockey players22).

Labral impact damage frequently manifests signs of a thick-ened, patulous labrum with 'blunting' of the labral edge andcystic change. Alternatively, the labrum may be almost non-exist-ent as a result of intra-substance calcification,14 which com-pounds the problem of a reduced range of movement. Pincerimpingement is associated with a lower incidence of anterioracetabular chondral damage than cam impingement, although itstill occurs as a 'contrecoup' degeneration on the posterioraspect of the femoral head and acetabulum (Fig. 4).23

While isolated cam and pincer impingement can occur, themajority of hips present with a mixed pattern.

Presenting SymptomsFAI commonly presents in younger adults as a deep intermittentgroin discomfort during or after activity.24

Fig. 2

Chondral flap tear or 'carpet lesion'

Fig. 3

Abnormalities of acetabular version

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The cam variety of impingement presents with intermittentdiscomfort in the groin during or after repetitive or persistent hipflexion. Sprinting or kicking sports, ascending hills or stairs, pro-longed sitting in low chairs, driving or getting in and out of lowvehicles are common exacerbating activities. Pain may bereferred to the anterior thigh, to the region of the symphysispubis and to the ipsilateral testicle in men.

Anterior pincer impingement presents with similar symptoms ascam impingement. However, symptoms of posterior impingementmay be experienced in the buttock or sacro-iliac region. Thesesymptoms are more common in women, occur when the hip isrepeatedly hyperextended as in fast walking or walking downhilland are often difficult to differentiate from pain referred from thelow back and sacro-iliac joint. Women with posterior impingementfrequently complain of posterior hip pain during intercourse.

Mechanical symptoms of catching, clicking and a feeling ofgiving way are commonly associated with labral tears secondaryto hip impingement.

ExaminationThe hip is examined in a methodical manner, which should

include an assessment of rotational abnormalities in thefemur. Hip flexion, adduction and internal rotation most com-monly replicate the pain experienced during anterior hipimpingement.25,26

In flexion, the hip is progressively rotated from external tointernal rotation while moving from abduction to adduction.Using this technique, it is possible to identify the most likely posi-tion of impingement and the most likely area of acetabular riminjury. A similar examination of the hip in hyperextension mayhelp with the diagnosis of posterior impingement.

InvestigationsRadiology. A systematic approach to the plain radiographic evalu-ation of these patients is essential. An anteroposterior (AP) pelvisfilm should be supported by a cross-table lateral or Dunn view.18

Key acetabular features on the AP film include the presence ofa deep or shallow socket (dysplasia, coxa profunda, protrusio) andan assessment of acetabular version (anteverted, retroverted,focal or global). Pattern recognition of subtle abnormalities takesa long time to develop and is fallible; modern Picture Archiving andCommunications System (PACS) facilities allow accurate meas-urement of centre-edge and sourcil angles as a minimum. Keyfemoral measurements include head sphericity, neck-shaft angle,alpha angle and offset ratio. The Academic Network for Hip Out-comes Research (ANCHOR) study group27 has published an excel-lent systematic guide to evaluating plain images.MRI. To identify the most subtle cam imperfections in proximalfemoral anatomy, an MRI arthrogram with intra-articular contrast isrequired. This investigation should include radial sequence scans(Fig. 5) to allow a tangential view perpendicular to the acetabularrim and a detailed analysis of the proximal femoral morph-ology.28,29 Rakhra et al30 showed that the maximum alpha anglesobtained on radial scanning were abnormal in 54% of patients withapparently normal alpha angles on conventional oblique axialscanning. We also recommend acquiring cuts through the distalfemur to assess abnormalities of femoral torsion.

Fig. 4

Contrecoup posterior acetabular damage secondary to anterior pincer impingement

Fig. 5

A radial MRI scan

Fig. 6

Three-dimensional (3D) CT reconstruction showing a small cam lesion at the femo-ral anterior head-neck junction

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CT. The use of 3D reconstruction of CT scans has proved useful inthe recognition of subtle femoral deformities (Fig. 6) and in pre-operative planning during the management of complex deformi-ties.17 It is particularly helpful in achieving appropriate orienta-tion of the hip during arthroscopic femoral osteochondroplasty.

Management of FAIFemoroacetabular impingement is a mechanical problem thatrequires a mechanical solution. A common presenting feature isfailure to respond to analgesia and physiotherapy; stretchingexercises and yoga may aggravate the condition. Reduction inthe symptoms associated with pincer impingement might beamenable to sports therapy that focuses on modifying thedynamic hip flexion element of pincer impingement by maintain-ing core stability and a more upright stance during activity (e.g.skiing moguls or deep powder).

Surgical correction of the deformity remains the mainstay ofmanagement, and has been shown to be effective in the shortterm. Beaulé, Le Duff and Zaragoza31 showed that quality of lifescores such as the Short Form (SF)-12 score or Western OntarioMcMasters Universities (WOMAC) index increase after FAI surgery.

FAI comprises a broad spectrum of disorders that require a widerange of surgical options. With increased understanding of the con-

dition, and cross-fertilisation of surgical principles, it is clear thatboth hip arthroscopy and open surgery have a valuable role to play.

Surgical dislocation of the hip using the Ganz trochanteric flipapproach is based on a thorough understanding of the vascularanatomy of the femoral head32-34 and allows unparalleled accessto the femoral head and acetabulum. A full description of thetechnique has been summarised by Norton, Fern and Williams.35

The technique facilitates the key elements of basic FAI surgery,namely femoral osteochondroplasty, labral repair or resection orreplacement, reshaping of the acetabular rim and acetabularchondral debridement. With adequate experience, the versatilityof the technique allows extremely complex hip reconstruction(Fig. 7) to be performed safely and predictably.36,37

Clohisy and McClure38 reported good to excellent results in24/25 patients at 1.5 years after an anterior exposure of the hipthrough a limited Smith-Petersen approach (Fig. 8) combinedwith hip arthroscopy, despite the restricted exposure comparedwith formal hip dislocation.

Although favourable long-term results can be achieved withsimple arthroscopic labral resection,39 the early acceptance ofthe principles of FAI has resulted in considerable advances inboth the techniques and equipment associated with arthro-scopic hip surgery.

Fig. 7

The Ganz approach (top left) facilitates safe access to the hip and allows correction of the severe deformity which may be seen in slipped upper femoral epiphysis (toprow) and Perthes' disease (bottom row)

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Labral refixation provides better results than simple excision,40

and correction of the underlying structural deformity produces abetter result than dealing with the labrum in isolation.41,42 Theseresults mirror those achieved by open surgery.33 Philippon et al24

reviewed the arthroscopic treatment of FAI and concluded thatther technique had particular relevance to high-demand patients,particularly athletes seeking a return to high-level sport.

It is apparent that the key elements of surgery for FAI whichhave been developed through open surgery are achievable byskilled hip arthroscopists. Avoiding the need for trochantericosteotomies and prolonged periods of limited post-operativeweight-bearing has obvious advantages.

Zebala, Schoenecker and Clohisy43 summarised that FAI wasa diverse disease with evolving treatment options; the under-standing of the genetic elements of FAI44 might allow selectivescreening and earlier diagnosis of the condition. Long-term out-come studies showing a beneficial effect of peri-acetabular oste-otomy after 20 years support the use of biological solutions.45

Although FAI surgery is effective in the short term, evidence isstill required that early intervention prevents or delays the devel-opment of arthritis in the long term.

Surgically induced acetabular dysplasia as a result of inappropri-ate acetabular rim trimming has been linked with a poor outcomeafter FAI surgery,46 and has prompted a change in management,emphasising femoral correction and minimal removal of theacetabular rim and/or labral calcification in our centre.

Publication of definitive reference ranges for femoral camlesions43 and acetabular rim morphology in the near futureshould allow surgeons to become more accurate with patientselection in the future.

In return for professional services, 2 authors (MN and EDF) obtained or will obtain outsidefunding from a commercial group (Corin Group PLC). This is not directly related to thepreparation of material for this article. In addition, Corin Group PLC support a researchfund directed by the same 2 authors.

ED Fern MR NortonCornwall Hip FoundationTruro, Cornwall

Correspondence to:Darren FernPoint Quay House, Point, Devoran, Truro, Cornwall, TR3 6NLe-mail: edfern@btinternet.com

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Fig. 8

Limited access via a limited Smith-Petersen approach

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