What We Don’t Know about FAI

    In this article, the authors focus on 4 main areas related to femoroacetabular impingement that are controversial or in need of further evidence.


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


    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” Radiologic Standards to Diagnose FAI?

    Based on current understanding, three types of FAI are recognized: [3]

    • Cam
    • Pincer
    • Combination of both (mixed type)

    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] and 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 was able to predict the requirement of THA. 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 had a pathologic alpha angle, with the same phenomenon observed in 5.6% of asymptomatic females. [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] with a more-than-probable superposition of diagnosis between DDH and FAI [15] or between FAI and SCFE. [16]
    • The alpha angle has equivocal utility. Although radiographic measurements other than the alpha angle have been proposed to study cam deformities, [9-12] their clinical usefulness is yet to be proved.

    Pincer FAI

    In relation to the definition of pincer FAI, there are three concepts that we would like to discuss.

    First is the ample diffusion of “coxa profunda” as one of the radiographic markers of pincer FAI. Two independent studies evaluating this topic have concluded that “coxa profunda” is not a reliable sign to describe a deep acetabulum, so it should not be considered a marker of overcoverage. [17,18] Therefore, the relevance of this radiographic sign appears insignificant.

    Second is the center edge angle (CEA). A CEA greater than 39° was proposed as an indicator of deep acetabulum, and a value greater than 44° as corresponding to acetabular protrusion. [19] Subsequently, some authors have included the criteria of CEA greater than 39° as:

    • A characteristic of pincer-type FAI [20]
    • A synonym for overcoverage [21,22]
    • A diagnosis of FAI [23-25]
    • A limit for normality [26]

    The original descriptions of FAI did not include CEA as a radiographic parameter. [1,27-29] We believe that CEA should be used to appraise acetabular dysplasia (its original purpose) or to avoid iatrogenic hip instability while performing an acetabuloplasty.[30] It should not be used to define or categorize acetabular overcoverage or pincer FAI until its upper limit of normality is appropriately defined.

    Third, the radiographic signs of acetabular retroversion or focal overcoverage, such as cross-over signs (COS), posterior wall sign (PWS), and ischial spine sign (ISS), have been used indistinctly to categorize this entity.

    In an article from our group presented at the 2013 meeting of the AAOS, we demonstrate that acetabular retroversion diagnosed on radiographs cannot be described as a single concept because the various radiologic findings associated with it have neither the same prevalence nor relevance within a single population.

    We found that acetabular retroversion index greater than 20% (ARI, a quotient between the length of overlap of the anterior rim in comparison to the entire length of the lateral acetabular opening) was able to predict the presence or absence of intraoperatively confirmed chondral lesions in FAI independent of the age, grade of osteoarthritis, or pathologic alpha angle.

    Although in our study PWS was irrelevant, its clinical and prognostic value has been highlighted by Bardakos and Villar [26] and by Leunig et al. [31] Additionally, Hansen et al described the importance of a negative posterior wall distance. [32] On the basis of this evidence, we affirm that PWS may have a role in diagnosis and preoperative planning in FAI.

    Importantly, Hartofilakidis et al observed that 82.3% of the patients classified as FAI using conventional radiographic criteria will not progress to end-stage osteoarthritis, without differences between the three subtypes of FAI [27] This study supports the theory that an “abnormal” hip according to FAI criteria is not necessarily indicative of future osteoarthritis. Therefore, it may be useful to reconsider the radiographic diagnostic criteria for FAI in order to enhance its prognostic value on the prediction of osteoarthritis.

    In summary, the clinical relevance of the “traditional” radiographic markers of FAI has been difficult to evaluate when rigorous scientific methods are applied. We believe that strong methodologic designs are required to define a clear limit between a normal and a pathologic hip morphology (FAI). As surgeons, we should not rely on measurements that appear conceptually powerful and logical, but scientifically suboptimal.

    What Are Non-operative Treatments for FAI?

    There is scarce evidence on this topic. According to a recently published systematic review, the level of evidence is low and very low, [33] and there are also discrepancies regarding the strategies used.

    The methods of non-operative treatment more commonly mentioned in the FAI literature are (in order): [33]

    • Activity modification
    • Non-steroidal anti-inflammatory drugs
    • Physical therapy
    • Avoidance of excessive movement of the hip and/or rest
    • Intra-articular corticosteroid injection
    • Osteopathy and chiropractic care

    The authors of this systematic review concluded that “literature is saturated with messages that promote the use of non-operative care for FAI despite the clinical evidence that supports these messages being weak.”

    Hunt et al presented the results of a prospective observational study on 56 patients with pre-arthritic, intra-articular hip disorders. [34] They found that 44% of the patients who had non-operative treatment reported satisfaction. The other 56% decided to undergo surgical treatment. The most interesting finding was that at 1-year follow-up, both groups demonstrated equivalent improvement in all outcome measures, even though the group that underwent surgery was more active at baseline.

    The non-operative treatment utilized in this study included restriction of some ranges of motion, patient education, and a home exercise program. Selectively, a fluoroscopically guided intra-articular hip injection (with 4 mL of 1% lidocaine) was recommended for patients who had persistent pain at 3 months.

    In summary, non-operative treatment for FAI is a neglected topic in orthopaedic literature. Well-designed studies will improve the evidence and decrease the variability of our current strategies.

    Is There High-quality Evidence to Support Surgical Treatment as the “Gold Standard”for FAI?

    There are no randomized controlled trials comparing surgical versus non-surgical treatment for FAI. Palmer et al [35] demonstrated that 77% of the hip surgeons who routinely perform FAI surgery were willing to recruit patients for a randomized controlled study. Furthermore, 90% of patients would agree to participate in such a study. The authors concluded that an “RCT comparing operative and non-operative management of FAI is feasible and should be considered a research priority.” We concur.

    Some systematic reviews have analyzed the results of surgical treatment for FAI and compared three modalities:

    • Hip arthroscopy (currently the most popular modality)
    • Mini-open anterior approach
    • Surgical hip dislocation [36-39]

    The cited studies showed that surgical treatment can significantly improve hip function and reduce symptoms in most patients. In addition, these studies demonstrated that patients with preexistent radiographic osteoarthritis were more likely to have poor outcome after treatment for FAI.

    One major issue that continues to be debated relates to definition of “success” In one article, Impellizzeri et al stated that “the definition of success after surgery is not universal and depends on how it is measured.” The authors wisely used the concepts of minimal clinical important change and patients’ acceptable symptoms state [40] to analyze their results and make such a relevant conclusion. [41]

    The same group showed that patients’ expectations are “overly optimistic” in more than a half of the cases undergoing FAI surgery. [42]

    Hip arthroscopy is currently the most popular technique to treat FAI. However, there are no randomized trials comparing this modality with other forms of surgical treatment, including mini-open approach or surgical hip dislocation. The low rate of complications of hip arthroscopy, 0.5% according to a recent systematic review, [43] is frequently cited as a major advantage of this technique compared with open surgical approaches.

    Although hip arthroscopy has been demonstrated to be an effective surgical modality, [44] its higher cost should be considered by the surgeon if equally efficient but less expensive alternatives are available. Using a formal method of decision making, we showed that the overall performance of the mini-open approach may exceed the performance of hip arthroscopy in pure cam-type FAI, mainly due to its lower cost. [45]

    In summary, there are no Level I studies to demonstrate the “superiority” of one surgical approach for FAI over another.

    Which Score Should Be Used to Evaluate the Results of Surgery for FAI?

    Evaluating the outcome of a surgical procedure is critical for measuring the “success” of that particular procedure.

    There continues to be a debate as to which outcome instrument constitutes the best way to measure the outcome of surgical management of FAI. The modified Harris hip score presented by Byrd and Jones [46] is the most commonly used functional scale to measure outcome of surgical care for FAI. [39,47] To our knowledge, the modified Harris Hip score has not been validated.

    Recently, Aprato et al demonstrated that the modified Harris hip score has limitations in measuring outcome – in particular, patient satisfaction – after surgical treatment for FAI. [48] At 1-year follow-up, 56% of patients with low scores were satisfied with the results of hip arthroscopy, while 9% of patients with higher scores were dissatisfied.

    Other scores have been used to evaluate baseline and outcome after FAI surgery, including:

    • SUSHI [49]
    • Hip Outcome Score (HOS) [50]
    • Non-arthritic Hip Score (NAHS) [51]
    • 12-item WOMAC [52]
    • Rosser Index Matrix [53]
    • Modifications of the WOMAC [54]
    • SF-36 [55]

    More recently, a new score was presented: the International Hip Outcome Tool (iHOT). Large (33 questions) [56] and a short (12 questions) [57] versions were presented. The methodology and purposes of these scores make them a promising option to better understand the results of FAI surgery.

    A study from Lodhia et al described the clinimetric qualities of different hip scores for FAI. [58] They found that only three scores reported clinimetric evaluations: HOS, NAHS, and WOMAC Osteoarthritis Index.

    From our perspective, the modified Harris hip score still has a role in evaluating the outcome of FAI surgery, especially in those clinical series in which it was originally used. We speculate that in the coming years, new instruments with good clinimetric qualities will replace the older, crude outcome instruments.


    Despite the enormous advances achieved in the understanding of FAI, numerous “pending tasks” or “in need of evidence” issues remain. The absence of high-quality evidence to support our current management strategies is troubling.

    Moving forward, the joint preservation societies of the hip, such as International Society of Hip Arthroscopy, should identify areas in need of further research and provide avenues to their members to execute these studies.

    Author Information

    Javad Parvizi, MD FRCS and Claudio Diaz-Ledezma, MD, are with The Rothman Institute at Thomas Jefferson University, Philadelphia, Pennsylvania.


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