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Femoroacetabular impingement: question-driven review of hip joint pathophysiology from asymptomatic skeletal deformity to end-stage osteoarthritis - PubMed

  • ️Tue Jan 01 2019

Review

Femoroacetabular impingement: question-driven review of hip joint pathophysiology from asymptomatic skeletal deformity to end-stage osteoarthritis

L Pierannunzii. J Orthop Traumatol. 2019.

Abstract

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: V.

Keywords: Contact mechanics; FAI; Femoroacetabular impingement; Hip osteoarthritis; Inflammation; Labrum; Normal stress; Shear stress.

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Conflict of interest statement

The author declares there are no competing interests.

Figures

Fig. 1
Fig. 1

Cam FAI. The cam intrusion is variable, depending on thickness and shape of cam, location, extent, lubrication, force, etc. (COR center of rotation, r radius, x difference between head radius and cam extended radius). a Neutral joint position with cam dismorphism in the peripheral compartment; b cam intrusion

Fig. 2
Fig. 2

α-Angle goniometry. The proximal femoral epiphysis may exhibit a regular morphology (a) or a cam morphology (b). Since in a the femoral head center is in line with the neck axis, the 3-point method (a.1) is as suitable as the anatomic one (a.2), and both obtain α = 35° and AOR = AO/HD = 0.217. In b the center of rotation of the femoral head is slightly behind the neck axis, thus α = 60° according to the 3-point method (b.1, inappropriate), while the anatomic method provides the correct value of 65° (b.2). The AOR calculated in b.2 along the pure mid-neck axis (i.e., disregarding the head position) is 0.108, and belongs to the 2.5% left tail of the normal distribution of AORs (AOR anterior offset ratio, HD head diameter, AO anterior offset)

Fig. 3
Fig. 3

Pincer FAI. The rim-to-neck collision is geometrically determined and represents a firm endpoint for joint range of motion (COR center of rotation, r radius). a Neutral joint position; b end range of motion

Fig. 4
Fig. 4

Femur-based pincer FAI morphology. This arthro-MRI axial reconstruction performed with neutral rotation of the limb shows a retroverted neck of femur and some particular features of pincer FAI (*degenerated labrum; **indentation line; §ilio-pectineal bursa dilated by contrast medium; §§no signs of cartilage delamination)

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