Femoroacetabular impingement: question-driven review of hip joint pathophysiology from asymptomatic skeletal deformity to end-stage osteoarthritis
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Femoroacetabular impingement (FAI), together with its two main pathomechanisms, cam and pincer, has become a trending topic since the end of the 1990s. Despite massive academic research, this hip disorder still conceals obscure aspects and unanswered questions that only a question-driven approach may settle. The pathway that leads a FAI asymptomatic morphology through a FAI syndrome to a FAI-related osteoarthritis is little known. Contact mechanics provides a shareable and persuasive perspective: cam FAI is based on shear contact stress at joint level with consequent cartilage wear; pincer FAI, contrariwise, determines normal contact stress between acetabular rim and femoral neck and squeezes the labrum in between, with no cartilage wear for many years from the onset. Pincer prognosis is then far better than cam. As a matter of fact, cartilage wear releases fragments of extracellular matrix which in turn trigger joint inflammation, with consequently worsening lubrication and further enhanced wear. Inflammation pathobiology feeds pathotribology through a vicious loop, finally leading to hip osteoarthritis. The association of cam and pincer, possibly overdiagnosed, is a synergic combination that may damage the joint rapidly and severely. The expectations after FAI surgical correction depend strictly on chondral layer imaging, on time elapsed from the onset of symptoms and on clinic-functional preoperative level. However, preemptive surgical correction is not recommended yet in asymptomatic FAI morphology.
Level of evidence
KeywordsFAI Femoroacetabular impingement Contact mechanics Shear stress Normal stress Inflammation Labrum Hip osteoarthritis
slipped capital femoral epiphysis
anterior offset ratio
lateral center–edge angle
range of motion
center of rotation
Cam and pincer, similarities and differences
Femoral head–neck junction
Acetabular cartilage wear
Prevailing gender and age
Development of osteoarthritis
Sliding friction (shear contact stress)
Rim-to-neck impact (normal contact stress)
Primary cam: physeal scar hypertrophy or subclinical SCFE
Secondary cama: malunion of femoral neck fracture; SCFE; Legg–Calvè–Perthes disease; femoral head and neck neoplasms; malrotation of the femoral epiphysis; coxa vara
Primary pincer: Protrusio acetabuli (global overcoverage); acetabular retroversion (anterolateral overcoverage)
Secondary pincer: acetabular retroversion after periacetabular osteotomy
FAI morphology, FAI syndrome and FAI-related hip osteoarthritis
“FAI morphology” is a deformity (or a combination of deformities) that may be detected through an X-ray examination or dedicated CT/MRI, but only seldom becomes symptomatic and deserves to be termed “FAI syndrome” . The factors that keep this condition silent are incompletely acknowledged, but the lumbo-pelvi-femoral complex is suspected to play an important role. This multiple joint complex (to which hips belong) might compensate hip sagittal ROM restrictions with coordinated lumbar spine enhanced curvature (lordosis/kyphosis) and pelvic tilt (anterior/posterior): as far as the restriction is functionally compensated, the subject may be asymptomatic despite positive radiological findings . Athletes might have higher functional demands and develop symptoms more frequently and sooner than non-athletes. Nowadays, no evidence supports surgical correction (arthroscopic or open) of asymptomatic FAI morphology, since neither eventual symptoms onset without surgery nor osteoarthritis prevention with surgery have yet been confirmed. However, the panel of experts participating in the Warwick agreement admit that professional athletes might belong to a very high risk category and case-by-case management should be considered .
Cam morphology: α-angle goniometry and cut-off values
Given that most cam deformities are located on the anterior or anterolateral aspect of the epiphysis just below the equator of the femoral head, many common hip movements (notably flexion, adduction and internal rotation) drive the lesion or part of it into the central compartment of the joint. Moreover, the typical tangential or quasi-tangential cam profile allows thin deformities to slip smoothly below the limbus. Hence cam intrusion is not a rare event, and is suspected to be responsible for the progressive chondral damage on the antero-superior quadrant of the acetabular surface.
The preferential location of the cam explains why lateral or dedicated oblique views (notably the 45° Dunn view ) of the proximal femur are more reliable than standard antero-posterior views in detecting cam deformities. A minimal back-shift (< 1 mm) of the head center with respect to the mid-neck axis is extremely common even in normal hips; significant posterior or postero-inferior shift/tilt of the femoral head is often documented in cam FAI, where the anterior head–neck offset, physiologically smaller than the posterior one, is minimal or zero (flat anterior contour) . As anticipated in the previous paragraph, the anterior offset ratio (AOR, or distance between 2 straight lines parallel to the mid-neck axis, tangent to the anterior femoral head and the anterior contour of the thinnest point of the neck, respectively, and normalized for head diameter) should be associated with α angle when the angular measurement method is not anatomic and the lack of convexity is likely underestimated (if the head center is positioned behind the mid-neck axis, the α-angle measured with the 3-point method results in a smaller value). In normal hips mean AOR is 0.19 ± 0.04 and mean overall head-to-neck ratio is 1.75 ± 0.11 . To avoid unnecessary radiation exposure for α measurement and to obtain superior anatomical detail at the same time, MRI and arthro-MRI are progressively replacing multiple plain radiological views. Dedicated radial hip reconstructions allow precise assessment of cam deformities, wherever located, and objective detection of consequent joint injuries.
Pincer morphology: coxa profunda does not mean acetabular overcoverage
Coxae profundae were traditionally considered examples of typical pincer morphology with global overcoverage of the femoral head. They are defined as deep sockets where the fossa acetabuli is medial to the Kocher’s line and differ from protrusio acetabuli because the femoral head is still lateral to this landmark. If the head touches or even protrudes across Kocher’s line, then a true protrusio is diagnosed. Recently, no correlation between coxa profunda and overcoverage (i.e., LCE or lateral center–edge angle > 40°) could be demonstrated . Thus, many authors discourage considering coxa profunda as a radiographic sign of pincer and recommend quantification of the real head coverage by using 2D reconstructions passing through the center of the virtual sphere best fitting the acetabular cavity, and calculating the angle to the center determined by diametrically opposite points of the rim; alternatively, conventional radiological parameters (such as extrusion index, LCE, acetabular arc, acetabular index, Sharp angle, etc. ) retain their validity.
Cam FAI: pathotribology and inflammation pathobiology
Overcoming the simplified rigid-body normal-stress contact mechanics and embracing a more complex but realistic model including shear stresses, viscoelastic bodies and a fluid-lubricated environment, the resulting biotribological perspective may provide additional information , useful for in-depth understanding of cam pathomechanics. As a matter of fact, cam intrusion disrupts the fluid film that separates the femoral head and acetabulum, increasing friction. Theoretically, the lubrication regimen of a synovial “ball-and-socket” joint may be boundary (i.e., a monolayer of macromolecules such as lubricin, phospholipids, hyaluronic acid, etc. is linked to each surface, reducing friction even with no fluid interposed) or fluid film-mediated (hydrostatic, or depending on an external device such as a “pump” that pressurizes synovial fluid between joint surfaces; hydrodynamic, where the thick fluid film is gathered by the relative sliding of the articular surfaces, as a function of speed, roughness, curvature, normal load and viscosity; elastohydrodynamic, whose pressurized fluid film is partially determined by the elastic properties of the joint surfaces) or mixed (when the two regimens—boundary and fluid film-mediated—alternate). In the healthy human hip, the acetabular labrum guarantees the pressurization of a supportive film of synovial fluid in the central compartment. Cam intrusion lets synovial fluid escape from the central compartment through the labrum, worsening the lubrication and then favoring wear . For this reason most hip arthroscopists are concerned about limbectomy and aim at preserving the labrum as much as possible, even considering reconstruction with tendon grafts whenever the preservation is not feasible. In a synovial spherical joint model the labrum is everted by cam intrusion and the seal temporarily compromised. Over time, the transition between labral fibrocartilage and hyaline articular cartilage lining the facies semilunata is peeled from the subchondral bone (“delamination”) and subsequently torn and worn off. This is how osteoarthritis begins in a mechanistic “wear and tear” scenario. The pathobiological theory of inflammation contributes to support the pathotribological perspective in cam FAI-related osteoarthritis, since fragments of cartilage extracellular matrix released by acetabular wear might trigger and maintain a chronic-recurrent joint inflammation [15, 16].
Inflamed hips are not only poorly lubricated by inflammatory synovial fluid (whose viscosity is decreased because of augmented proteolytic activity), but also slowed down (due to joint pain) and heavily loaded (as joint pain leads to a sedentary lifestyle and often to overweight). All these factors result in increased friction and wear, feeding a vicious loop towards osteoarthritis. Moreover, adipokines released by fat tissue might specifically contribute to joint inflammation .
Pincer FAI and normal-stress contact mechanics
In other words cam and pincer FAI are respectively due to sliding friction and collision, while the location of the deformity, femoral or acetabular, is not pathognomonic. Then occasionally a mismatch may occur (i.e., femur-based pincer FAI).
Lastly, cam and pincer may coexist—in this case FAI is defined as “mixed-type”  or “combined.” It is likely that such occurrence is rarer than hip arthroscopists report, but cam and pincer are synergic, and a joint affected by a mixed-type FAI should rightly receive prompt and targeted attention as soon as symptoms appear. No wonder the threshold for such a diagnosis is cautiously very low.
Cam and pincer FAI are quite different issues: both are FAI pathomechanisms, but the former is also a severe pathotribological disorder, dictated by a morphological incongruence of joint surfaces with generation of frictional stress able to seriously injure hip cartilages , while the latter preserves the concentric spherical shape of the articular surfaces and behaves as a substantial range of motion restraint with generation of normal stress at sites of rim-to-neck collision . The different contact mechanics make the prediction of ROM more accurate for pincer FAI (where boundaries depend on normal impacts) than for cam FAI (where boundaries depend on maximum cam penetration, which in turn hinges on multiple factors like thickness and shape of cam, location, extent, lubrication, force, etc.). Both mechanisms can be corrected surgically with extensive approaches (surgical dislocation  and anterior approach ) or minimally invasive techniques (hip arthroscopy ), but chances of a persistent satisfactory outcome (i.e., no relapse of symptoms and osteoarthritis prevention) seems to be limited to hips with no or very early arthritic changes, short time elapsed from symptoms onset, and good clinic-functional preoperative performance , since chondral status is the weakest link of the chain. Surgical technique, extensive or minimally invasive, has a questionable role, if any. On the other hand, if the condition is asymptomatic, no preemptive surgical correction, neither open nor arthroscopic, is supported by sufficient evidence. In future randomized controlled trials and other high-level-of-evidence studies—some of which are ongoing [26, 27]—might answer these residual big questions (such as what is the natural history of asymptomatic FAI morphology, whether and how we may interfere with it, etc.), that cannot be addressed by simply rearranging the evidence collected so far.
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- 2.Davidson EJ (2015) Question-driven methods or method-driven questions? How we limit what we learn by limiting what we ask. J MultiDiscipl Eval 11(24):1–10. ISSN 1556‐8180 http://www.jmde.com
- 3.Griffin DR, Dickenson EJ, O’Donnell J, Agricola R, Awan T, Beck M, Clohisy JC, Dijkstra HP, Falvey E, Gimpel M, Hinman RS, Hölmich P, Kassarjian A, Martin HD, Martin R, Mather RC, Philippon MJ, Reiman MP, Takla A, Thorborg K, Walker S, Weir A, Bennell KL (2016) The Warwick Agreement on femoroacetabular impingement syndrome (FAI syndrome): an international consensus statement. Br J Sports Med 50(19):1169–1176. https://doi.org/10.1136/bjsports-2016-096743 CrossRefPubMedGoogle Scholar
- 5.Stulberg SD, Cordell LD, Harris WH, Ramsey PL, MacEwen GD (1975) Unrecognized childhood hip disease: a major cause of idiopathic osteoarthritis of the hip. In: The hip. Proceedings of the third open scientific meeting of the hip society. C.V. Mosby, St. Louis, pp 212–228Google Scholar
- 7.Agricola R, Waarsing JH, Thomas GE, Carr AJ, Reijman M, Bierma-Zeinstra SM, Glyn-Jones S, Weinans H, Arden NK (2014) Cam impingement: defining the presence of a cam deformity by the alpha angle: data from the CHECK cohort and Chingford cohort. Osteoarthr Cartil 22(2):218–225. https://doi.org/10.1016/j.joca.2013.11.007 CrossRefPubMedGoogle Scholar
- 14.Bsat S, Frei H, Beaulé PE (2016) The acetabular labrum: a review of its function. Bone Joint J. 98-B(6):730–5. https://doi.org/10.1302/0301-620x.98b6.37099. Erratum in: Bone Joint J. 2017;99-B(5):702–704
- 18.Satpathy J, Kannan A, Owen JR, Wayne JS, Hull JR, Jiranek WA (2015) Hip contact stress and femoral neck retroversion: a biomechanical study to evaluate implication of femoroacetabular impingement. J Hip Preserv Surg. 2(3):287–294. https://doi.org/10.1093/jhps/hnv040 CrossRefPubMedPubMedCentralGoogle Scholar
- 21.Agricola R, Heijboer MP, Roze RH, Reijman M, Bierma-Zeinstra SM, Verhaar JA, Weinans H, Waarsing JH (2013) Pincer deformity does not lead to osteoarthritis of the hip whereas acetabular dysplasia does: acetabular coverage and development of osteoarthritis in a nationwide prospective cohort study (CHECK). Osteoarthr Cartil 21(10):1514–1521. https://doi.org/10.1016/j.joca.2013.07.004 CrossRefPubMedGoogle Scholar
- 25.Pierannunzii L, Di Benedetto P, Carulli C, Fiorentino G, Munegato D, Panascì M, Potestio D, Randelli F, Della Rocca F, Rosolen V, Giangreco M, Santori N (2018) Midterm outcome after arthroscopic treatment of femoroacetabular impingement: development of a predictive score. Hip Int. 29(3):303–309. https://doi.org/10.1177/1120700018786025 CrossRefPubMedGoogle Scholar
- 26.Simunovic N, Heels-Ansdell D, Thabane L, Ayeni OR, Investigators FIRST (2018) Femoroacetabular Impingement Randomised controlled Trial (FIRST)—a multi-centre randomized controlled trial comparing arthroscopic lavage and arthroscopic osteochondroplasty on patient important outcomes and quality of life in the treatment of young adult (18–50 years) femoroacetabular impingement: a statistical analysis plan. Trials. 19(1):588. https://doi.org/10.1186/s13063-018-2965-0 CrossRefPubMedPubMedCentralGoogle Scholar
- 27.Griffin DR, Dickenson EJ, Wall PDH, Achana F, Donovan JL, Griffin J, Hobson R, Hutchinson CE, Jepson M, Parsons NR, Petrou S, Realpe A, Smith J, Foster NE, FASHIoN Study Group (2018) Hip arthroscopy versus best conservative care for the treatment of femoroacetabular impingement syndrome (UK FASHIoN): a multicentre randomised controlled trial. Lancet 391(10136):2225–2235. https://doi.org/10.1016/s0140-6736(18)31202-9 CrossRefPubMedPubMedCentralGoogle Scholar
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