Definition of femoroacetabular impingement (FAI)

Femoroacetabular impingement (FAI) has gained much attention over the past decade. It has been associated with hip and groin pain, decreased hip function and can ultimately lead to osteoarthritis.1 A recent consensus meeting defined FAI as “a clinical entity in which a pathological mechanical process causes hip pain when morphological abnormalities of the acetabulum and/or femur, combined with vigorous hip motion (especially at the extremes), lead to repetitive collisions that damage the soft-tissue structures within the joint itself.”2 However, there is a lack of clarity regarding the definition of morphological abnormalities.

Distinguishing between normal and abnormal morphology

The presence of a cam or pincer deformity is a prerequisite for defining cam-type and pincer-type of FAI. Although an obvious cam deformity is not difficult to recognise, there may be subtle differences between normal and abnormal bony morphology. This is illustrated by a study showing poor correlation between the subjective assessment of the head–neck junction and objective measures.3 Poor reliability of radiologists and orthopaedic surgeons in quantifying morphological abnormalities has also been demonstrated.4 Objective methods to quantify a cam deformity are, therefore, preferable. Several objective measures have been proposed and currently, the α angle is most commonly used. For anteroposterior pelvic radiographs, a threshold of 60° could distinguish between normal and abnormal α angles in two large cohorts.5 For other planes, views and modalities, a threshold between 55° and 60° seems appropriate; however, more research is required to validate the presence of a cam deformity in other modalities.

A pincer deformity constitutes a variety of acetabular morphological and/or orientational abnormalities and its quantification is, therefore, challenging. Similar to a cam deformity, a variety of quantitative measures have been proposed that can capture the morphological or orientational components of a pincer deformity. However, the most appropriate quantitative measure(s) and accompanying threshold value(s) remain to be determined.

It takes two to tango

The presence of femoral and/or acetabular morphological and/or orientational abnormalities is only one of the conditions required to fulfil the criteria of FAI. The positioning of the proximal femur and acetabulum relative to each other, and the dynamic interplay between these bones during motion is even more important. For example, a subtle cam deformity in the anterior portion of the femoral head–neck junction might not cause impingement if there is sufficient anteversion of the acetabulum to allow for a physiological amount of impingement-free motion. A cam deformity may result in FAI in the presence of a retroverted acetabulum or if the acetabulum is relatively deep or has a bony overgrowth (figure 1). In addition, the femoral orientation—that is, the version, torsion and neck-shaft angles—also determines how the cam deformity approximates the acetabulum in an anatomical position. The degrees of freedom in which the hip can move without impingement determines the ‘safe range of motion’, which can only be assessed from a full three-dimensional analyses.6 Dynamic imaging techniques, such as ultrasound and software that utilises dynamic simulations from CT or MRI, have now been introduced to capture the zones at which impingement occurs,7 but the additional clinical value of these techniques above conventional static imaging remains to be proven.

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Figure 1

Mechanism of cam impingement. (A) A hip joint with a cam deformity (asterisk) is shown. (B) The cam deformity is forced into the acetabulum, especially during flexion (approximately 80°) and internal rotation (approximately 20°), which is called cam impingement. (C) In the case of less acetabular anteversion, a similar cam deformity might already cause impingement at a lower motion excursion.

FAI is not a radiographic definition

It is important to note that the radiographic presence of a cam or pincer deformity does not equate to a clinical diagnosis of FAI. Morphological abnormalities only become clinically relevant when a patient presents with signs and symptoms resulting from their hip impingement. This is illustrated by a high prevalence of cam deformity that will remain asymptomatic. Development of symptomatic FAI in the presence of morphological abnormalities is codependent on the type, intensity and frequency of activities that an individual undertakes and the vulnerability of the soft tissues within the hip joint, such as the labrum and cartilage. It is likely that an athlete only presents with hip pain once these variables are present to a degree which creates failure in these soft tissues. There is usually a gradual onset of deep internal hip pain, which is indicated by the patient cupping their hand above the greater trochanter, described as the ‘C-sign’.8 Several physical examination signs have been described as associated with FAI, including limited internal rotation and flexion range of motion, and pain elicited by the flexion adduction internal rotation (FADDIR) test and/or the flexion abduction and external rotation (FABER) test. However, a recent meta-analysis showed that the outcomes of these tests do not alter the probability of diagnosing FAI.9 At best, they can assist in ruling out FAI, which is primarily due to the high pretest probability of these tests, that is, high clinical suspicion of the presence of FAI in the participants of the currently published literature. The proportion of people with morphological abnormalities who progress to clinically experienced symptomatic FAI is, therefore, difficult to estimate. Future studies aiming to identify athletes at highest risk of developing symptomatic FAI should, therefore, not only focus on the presence of morphological abnormalities, but also on the interaction between the proximal femur and acetabulum, and the activities undertaken by the athlete.