What We Don’t Know about FAI

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What We Don’t Know about FAI

Despite advances in our understanding, certain aspects of femoroacetabular impingement remain unclear. The authors discuss four issues that are controversial or in need of further evidence.

By Javad Parvizi - October 7, 20130FAIFeature Articles


Authors

Javad Parvizi, MD, FRCS, and Claudio Diaz-Ledezma, MD

Introduction

A decade after it was first formally reported in the orthopaedic literature, [1] femoroacetabular impingement (FAI) is now recognized as a prevalent cause of labral tears and chondral damage in the hip joint. It is also accepted that FAI may lead to premature osteoarthritis. [2]

Despite remarkable advances in our understanding, we remain unclear on certain aspects of FAI. In this article, we concentrate on four main areas related to FAI that are controversial or in need of further evidence.

Can We Rely on the “Classic” Radiographic Standards for Diagnosis of FAI?

Based on current understanding, three types of FAI are recognized:
Cam
Pincer
Combination of both (mixed type) [3]

Cam FAI

An altered offset at the level of the femoral head-neck junction (bump) is pathognomonic of a cam-type FAI. It generates a dynamic impact between the femur and the labro-acetabular complex, leading to labral tears and chondral delamination.

The alpha angle described by Nötzli is the most common measurement to assess cam-type deformities. [4] It is important to know, however, that the limit of 50° established by Nötzli et al in their MRI study involved the evaluation of only 74 patients (39 symptomatic versus 35 asymptomatic).

Although the concept of the alpha angle is easy to understand and integrate, it seems that a universally accepted limit to define normality is not available. We agree with Sutter and colleagues, who, after studying the MRI of 106 symptomatic and asymptomatic individuals, concluded that “because of the large overlap between alpha angles in patients with cam-type deformities and in volunteers, it is difficult to describe an optimal alpha-angle threshold value that is both highly specific and sensitive.”

Multiple authors have used the alpha angle to define abnormality in conventional radiographs. Dudda et al demonstrated that an altered anterosuperior alpha angle may be present in the MRI even if conventional radiographs are normal; [5] therefore, the utility of radiographs for diagnosis of FAI may be limited.

Despite the latter notion, large population studies have used AP pelvis view to define subtypes of cam morphology. In a 19-year follow-up study, Nicholls et al established that the alpha angle could predict whether patients would need total hip arthroplasty. However, a cut-off point to discriminate normality was not presented. [6]

Additionally, a Danish study described three categories to classify the alpha angle: pathological, borderline, and normal. [7] Using those criteria, a publication from our institution demonstrated that 13.9% of the asymptomatic males and 5.6% of asymptomatic females had a pathologic alpha angle. [8]

Data suggest that the Dunn’s view would be the most sensitive radiographic projection for studying cam deformity (considering CT-scan axial images as the gold standard), while the frog lateral view is the most specific. [13]

We concur with the ANCHOR group that for diagnostic and treatment purposes, a three-view radiographic hip series (AP pelvis, 45° Dunn, and frog lateral) efficiently portrays femoral head-neck junction abnormalities. These three views should be considered the optimal radiographic evaluation when cam-type FAI is suspected. In some cases patients may need axial images (CT scan or MRI) to completely assess their pathology.

High-quality studies are required to create an unequivocal definition of cam-type FAI, considering that:
The radiographic evaluation of the hip is highly variable [14]
A more-than-probable superposition of diagnosis between DDH and FAI [15] or between FAI and SCFE [16]
The equivocal utility of the alpha angle

Although other radiographic measurements besides the alpha angle have been proposed to study cam deformities, [9-12] their clinical usefulness is yet to be proved.