February 10, 2019
femoroacetabular impingement cover

Femoroacetabular Impingement Syndrome

Femoroacetabular impingement (FAI) has become an increasingly recognized cause of hip pain, especially in young active adults (Ganz, 2003) Accordingly, the body of knowledge surrounding the understanding and management of FAI has grown substantially (Tucker, 2015). It is imperative for sports medicine clinicians to understand current principles and best practices regarding the pathophysiology, diagnosis, and treatment of FAI

Pathophysiology. FAI is defined as “a pathologic mechanical process by which morphologic abnormalities of the acetabulum and/or femur combined with vigorous hip motion lead to repetitive collisions that damage the soft-tissue structures within the joint itself” (Sankar, 2013). Notable soft-tissue structures include the acetabular labrum and the articular cartilage of the femoroacetabular joint.
FAI has been traditionally classified into two distinct types: pincer impingement (Figure 1), referring to the contact between the acetabular rim and femoral head or neck, and cam impingement (Figure 2), referring to abnormal contour of the femoral head or neck. However, it is common to find both types of FAI in combination within the same hip (Beck, 2005)  Both cam and pincer deformities are characterized by a unique set of pathological bony malformations and abnormal hip biomechanics (Lamontagne, 2009). The epidemiology of FAI has been difficult to quantify. This is due to diagnostic challenges, including the presence or absence of radiographic morphologies and difficulty quantifying the exact degree of impingement (Brukner & Khan).
Pincer impingement may occur due to a bony overgrowth located along the anterolateral rim of the acetabulum, exaggerated retroversion of the acetabulum (anterior overcoverage), or a deepened acetabular socket (coxa profunda) (Ganz, 2003). With repetitive hip flexion, the acetabular labrum is crushed between the abutment of the acetabular rim and the femoral head or neck (Byrd, 2014). The leverage produced by this persistent abutment may even cause a contrecoup injury to the the opposite region of the acetabular cartilage in some instances (Ganz, 2003). Few studies have specifically investigated the process by which these anatomical anomalies develop, thus the etiology of pincer impingement is not well understood (Brukner & Khan). Pincer impingement is commonly observed in active middle-aged women (Samora, 2011).
Cam impingement occurs due to the rotation of a non-spherical femoral head within the acetabulum. The term “cam” refers to the projection on a rotating part of machinery, similar to that of a camshaft in an automobile. Cam deformities may stem from multiple etiologies, the first of which may arise from a slipped capital femoral epiphysis. The slipped epiphysis causes an asymmetry of the anterior femoral neck, thus leading to a misalignment within the joint (Byrd, 2014). Cam impingement is more common in young, athletic males compared to any other demographic (Ng, 2010). It’s incidence ranges from about 15 to 25% in this group, in contrast to 5% in the general population (Gosvig, 2008; Gosvig, 2010; Hack, 2010; Jung, 2011; Reichenbach, 2010).
Another etiology of cam impingement is based on the theory that vigorous sporting activity in adolescence may cause premature closure of the capital physis, but more evidence is needed to validate this theory (Byrd, 2014). Murray and Duncan (1971) first identified a group of adolescents who were highly involved in athletics beginning around age 14. The authors found that the athletic participants had a greater incidence of cam-type deformities compared to their non-athlete peers (Murray, 1971).
For cam impingement, failure of the articular cartilage typically occurs before damage to the labrum. Articular cartilage damage occurs due to the cam deformity being forced into the acetabulum. This phenomenon produces shear forces on the anterolateral edge of the acetabular articular surface, eventually leading to damage of the articular cartilage (Byrd, 2014).  In pincer deformity, there is some debate on whether the labrum or cartilage breaks down first. One theory is that cyclical, submaximal microtrauma leads to deterioration of the acetabular labrum with eventual articular failure within the adjacent acetabulum over time (Byrd, 2014). Another proposed theory is that most damage occurs at the level of the acetabular cartilage at its junction with the labrum. The cartilage typically remains in continuity with the labrum in the early stages and then can progress into separation of the labrum and cartilage leading to unstable flaps (Sabetta, 2015).   
New evidence is emerging to support a hypothesis that subtle changes to the hip joint, as seen in FAI, are responsible for the development of osteoarthritis (OA). Ganz et al. argued that many cases of primary OA, originally labeled as “idiopathic”, might actually stem from altered architecture of the femoroacetabular joint. FAI may account for both subtle and gross abnormalities, which may alter hip mechanics, worsen deformities, and eventually lead to OA (Ganz, 2008).
femoroacetabular impingement illustration

Image 1. Illustration of types of FAI (courtesy of cnos.net)

Clinical Presentation. In order to make a diagnosis of FAI, a triad of symptoms, clinical signs, and imaging findings are all required. The primary symptoms of FAI are hip or groin pain associated with activity, motion, or positional changes (Griffen, 2016). Patients typically complain of insidious-onset pain, but it is common for a mild injury or specific event to elicit symptoms (Samora, 2011). Concomitant labral tears may cause mechanical symptoms such as clicking, catching, or locking. Patients may report exacerbation of pain with prolonged sitting, standing, and hip flexion-type movements (Jaberi, 2007).
On physical examination, range of motion is often limited in both internal rotation and passive flexion of the hip to more than 90 degrees while adducted (Wyss, 2007; McCarthy, 2001). The patient’s pain is usually reproducible with hip impingement tests. The flexion adduction internal rotation (FADIR) test, also known as the anterior impingement test, is sensitive but not specific for FAI (Ganz, 2008). Less commonly, posterior impingement can be assessed by passive hip extension and external rotation  (Samora, 2011).
Restricted range of motion due to FAI may lead to compensatory biomechanical changes in other parts of the body. For example, cam impingement has been proposed to increase motion within the pubic symphysis. This mechanism may overload surrounding extra-articular structures and muscle, potentially leading to the development of sports hernias and athletic pubalgia (Munegato, 2015). Recent studies have suggested that FAI and core muscle injuries frequently coincide in athletes with groin pain (Strosberg, 2016).
femoroacetabluar impingement cam deformity xray

Image 2. Radiographs demonstrating a cam type deformity (courtesy of ortho info)

Imaging. The initial studies of choice in patients with hip pain should be an anteroposterior (AP) radiograph of the pelvis and lateral femoral neck view of the symptomatic hip (Griffen, 2016). Common choices for lateral radiographs include a frog lateral view or variations of a Dunn lateral view. Radiographs can be used to assess impingement as well as evaluate joint space preservation and bony changes within the hip. Pincer and cam deformities each have their own distinct radiographic findings.
Pincer impingement can be identified by presence of the “crossover sign (acetabular retroversion), crossing of the medial wall of acetabulum over ilioischial line, or center of femoral head medial to posterior acetabular wall (coxa profunda) on AP x-ray”. Cam impingement can be characterized by an anterolateral bony prominence on the femoral neck (Ng, 2010). This prominence is often accompanied by a flattened head-neck junction, also known as a “pistol grip deformity.” The deformity is said to be reminiscent of a flintlock pistol from old pirate movies (Tannast, 2007), first described by Stulberg et al. (Stulberg, 1975).
Several methods have been proposed to quantify the amount of femoral head and neck asphericity. Of the various methods proposed, measurement of the alpha angle (Figure 3) is one of the most frequently cited parameters.  The alpha angle was originally intended to describe axial magnetic resonance imaging (MRI) scans, but can be used with lateral radiographs as well (Clohisy, 2008). A study by Wright et al. (2015) concluded that labral tears and the development of early osteoarthritis could be predicted by increased alpha angles (Wright, 2015).
The alpha angle is calculated by measuring the angle between the center of the femoral neck and a line drawn from the center of the femoral head to the point where the femoral head loses sphericity (Clohisy, 2008). Values of ≥ 55 degrees are generally considered indicative of a cam deformity (Notzli, 2002; Johnston, 2008). However, some authors consider an angle >60 degrees for reduced false positive diagnoses (Sutter, 2012).
MRI is not required to diagnose FAI, but may be utilized to better evaluate surrounding soft tissue pathology. MRI is superior to radiographs for evaluating soft tissue damage, but bony and articular pathology are easily missed (Jaberi, 2007). MRI, particularly an MRI arthrogram with gadolinium, is the most accurate imaging study for evaluating soft tissue labral tears (Pfirrman, 2008; Kassarjian, 2005). MRI arthrogram has been shown to have greater sensitivity at detecting intra-articular abnormalities but is still not completely reliable (Byrd, 2004). It has been proposed that computed tomography (CT) and MRI imaging with secondary radial reformats better characterize cam-type impingement because of the three-dimensional character of the deformity (Agricola, 2012; Dudda, 2009).
femoroacetabular impingement hip xray radiograph

Image 3. Radiographs demonstrating the alpha angle (a) and radiologic lateral center edge angle (b)

Nonoperative Treatment. When managing FAI, patient goals should be to decrease pain, restore range of motion, improve function, return to sport, and prevent osteoarthritis (Lebrun, 2018). Although numerous conservative therapeutic options have been extensively studied in athletes with generalized hip pain, a limited amount of research has focused specifically on the treatment efficacy of FAI-related hip pain. Activity modification, nonsteroidal anti-inflammatories (NSAIDs), core strengthening, and physical therapy are foundations of the conservative rehabilitation and treatment of FAI (Samora, 2011). A systematic literature review of 53 articles pertaining to the nonoperative treatment of FAI revealed that 65% of the articles recommended an initial trial of conservative treatment. Furthermore, 81% of the articles recommended activity modification and 48% promoted physical therapy (Wall, 2013).
Activity Modification. There is limited, but positive evidence supporting lifestyle modifications for treatments of FAI. A study of 37 young athletes with mild FAI found that 33 of the participants had a significant improvement in their mean Harris Hip Score (HHS) and Non Arthritic Hip Score (NAHS) after two years of hip range of motion limitations. These patients were taught to avoid activities that caused internal rotation of the hip including treadmill exercise, narrow trail running, and some regular activities of daily living. They were also taught new squatting techniques and given a stretching program. Six of the 33 patients had recurrent hip pain and discomfort (Emara, 2011). One case study found that avoiding activities which cause excessive hip adduction and internal rotation resulted in an immediate reduction in anterior-medial groin pain in the setting of FAI (Austin, 2008).
Physical Therapy. Physical therapy is a key component to the treatment of FAI and tends to reduce symptoms. A cohort study of military personnel in the UK with “prearthritic hip pain” found that participants who completed a “comprehensive multidisciplinary residential intervention” program experienced significant improvements in Y-balance scores, hip range of motion, and hip internal rotation, but no improvements in pain (Coppack, 2016). A randomized control trial (RCT) of 30 adults with FAI found that participants assigned to a personalized exercise program saw a significant improvement in a NAHS and Lower Extremity Functional Scale (LEFS) compared to a control group (Smeatham, 2017).
Not all studies show benefit. Another RCT randomized patients with FAI into three groups: one receiving a combination of manual therapy and supervised exercise, one receiving supervised exercise plus advice and home exercise, and the final receiving advice and home exercise alone, each over 6 weeks. The investigators found no significant difference in pain or physical function between groups (Wright, 2016).
Corticosteroid & Hyaluronate Injections. Intra-articular injections can provide both diagnostic and therapeutic value. Diagnostically, combinations of corticosteroid and anesthetic (CSI), or even anesthetic alone, can be injected within the hip capsule of individuals with suspected FAI-related pain. If the injection successfully reduces pain, this suggests an intracapsular pain source as opposed to extracapsular. The differential diagnosis is thus narrowed to disorders within the femoroacetabular joint and hip capsule, including FAI. Extra-articular diagnoses such as muscle injuries and extracapsular bone pathology can likely be excluded.
The role of intra-articular injections for therapeutic applications in FAI is still being investigated. A retrospective analysis of 18 patients and 19 hips that underwent fluoroscopic guided intra-articular hip injections of CSI for hip pain secondary to FAI found that 52% of hips eventually required surgery, on average 12.8 months following injection. Ninety percent of hips that required surgery had presence of cam or pincer impingement morphologies. For patients with no evidence of bony abnormalities, 90% improved with CSI and did not require any further treatment within an average of 2.4 years (Tangtiphaiboontana, 2017)
A systematic review of 8 studies and 281 hips found that: 1) patients with FAI and associated chondral damage obtained greater symptomatic relief from intra-articular hip injections compared to patients without chondral damage, 2) injections performed under ultrasound guidance were less painful than those performed under fluoroscopic guidance, 3) hyaluronic acid injections were most effective at providing pain relief compared to corticosteroid injections at 12 months, 4) corticosteroid injections resulted in pain relief in 15% of patients at 6 weeks, and 5) a poor response to intra-articular hip injections is a strong predictor for poor surgical outcomes (Khan, 2015).
Additionally, a RCT of 30 patients with FAI were randomized into two groups: one receiving intra-articular steroid injections of the hip and the other receiving hyaluronic acid injections. Patients without a decrease in pain intensity after two weeks received a “cross-over” injection of the opposite treatment. Results suggested that steroid injections were better at reducing pain at 2 weeks but hyaluronic acid were better at 4 weeks (Lee, 2016).
Overall, it appears as if intra-articular injections are useful in both the diagnosis and treatment of patients with FAI (Khan, 2015). Although there is some disagreement within the literature as to which indications are most appropriate for intra-articular injections, it does appear that injections have the potential to reduce pain without significant major adverse effects. Future studies are needed to examine the efficacy of intra-articular injections, especially compared to alternatives treatments for FAI.
arthroscopic approach to femoroacetabluar impingement surgery

Image 4. Illustration and clinical example of arthroscopic approach to FAI (courtesy of nsmoc.com)

Surgical Treatment. In patients requiring surgical intervention, outcomes are generally favorable. Philippon et al. (2009) studied 122 patients who underwent arthroscopic surgery of the hip for FAI with a mean follow-up of 2.3 years. Patient’s HHS improved by an average of 24 points and their median satisfaction was on average a 9 out of 10 following surgery. Pre-operative HHS, joint space narrowing ≥ 2mm, and labral repair instead of debridement predicted superior outcomes. Nine percent of participants underwent total hip arthroplasty (THA) within 8 to 26 months following the initial procedure (Philippon, 2009).
A similar study by Larson and Giveans (2007) examined 100 hips with FAI that underwent arthroscopic management. In this study, the average HHS improved by 21.9 points and a positive impingement test was appreciated in 100% of preoperative patients versus 14% of postoperative patients (Larson, 2008). Byrd and Jones (2009) investigated outcomes of 158 patients who underwent arthroscopic correction of cam or pincer-type FAI. This group reported an average improvement in HHS of 20 points. One patient required THA at 8 months post arthroscopy for extensive articular cartilage loss (Byrd, 2009).
Labral repair is likely superior to debridement. A cohort study of young patients with FAI performed labral debridement on a group of 44 hips and labral refixation on a group of 50 hips with mean follow-up in 3.5 years. Mean HHS, short form 12 (SF-12), and visual analog scale (VAS) scores were all significantly better for the refixation group compared to the debridement group (Larson, 2012).
These studies suggest that surgery is a reasonable treatment option for FAI when appropriately indicated. Patients who do not respond to conservative measures are likely good candidates for surgical intervention. The literature seems to agree that patients typically experience improved outcome scores and decreased pain following surgery. Also, the conversion rate to THA compared to open surgical dislocation and overall surgical complication rate seem to be within acceptable standards (Meredith, 2018).
Nonoperative Treatment vs. Surgery. There are no clear guidelines on which patients are candidates for nonoperative management and which patients should be referred for surgical intervention. A Cochrane review of surgery in 2014 could not support or discourage surgical intervention for patients with FAI due to lack of randomized controlled trials (Wall, 2014). However, clinical experience and case reports have shown success with both surgery and nonoperative treatment. One RCT randomized 348 patients with FAI into one group that received personalized hip therapy and another that underwent hip arthroscopy. After 12 months, results showed that hip related quality of life, as measured by the International Hip Outcome Tool (iHOT-33), had improved by 19.6 points in the hip arthroscopy group and 14.1 in the personalized hip therapy group. This data suggests that both therapy and surgery improve hip-related quality of life for patients with FAI. However, primary analysis of the data revealed that the mean difference in iHOT-33 scores was statistically significant in favor the arthroscopic group (Griffin, 2018).
Another RCT randomized 80 patients from at a large military hospital to either a rehabilitation group that underwent 12 sessions of physical therapy or a group that underwent arthroscopic surgery. At follow-up after two years, significant improvements were seen in both groups with regard to the Hip Outcome Score (HOS) and iHOT-33. However, there was no significant difference between groups (Mansell, 2018).
A multicenter RCT examined 222 patients between the ages of 18 and 60 years with symptomatic FAI that were referred to secondary or tertiary care. Participants were randomized into one of two groups: the first group of 110 patients received up to eight personalized physiotherapy sessions over five months which focused on core stability and movement control, the second group underwent hip arthroscopy. After eight months, the physiotherapy group was found to have an average Hip Outcome Score Activities of Daily Living (HOS ADL) of 78.4 and the hip arthroscopy group had an average HOS ADL of 69.2. After adjusting for variables such as age, sex, and baseline HOS ADL, the improvement of the arthroscopic surgery group’s HOS ADL compared to that of the physiotherapy group was statistically significant (Palmer, 2019).
Conclusion. Femoroacetabular impingement is a disorder characterized by hip pain in the setting of irregular architecture of the femoral head, femoral neck, or acetabulum. Repetitive motion of the hip may subject the joint to abnormal biomechanical forces, potentially leading to damage of the labrum or articular cartilage. Patients with FAI are typically young active athletes or active middle-aged adults. Individuals will complain of either insidious-onset hip pain or hip pain after a minor injury, often accompanied by diminished range of motion or a clicking, locking, or catching sensation.
Initial evaluation of patients with a potential presentation of FAI should begin with a meticulous history and physical examination, followed by radiographs of the hip and pelvis. MRI with arthrogram is recommended in patient with suspected soft tissue labrum damage. Conservative therapy, including NSAIDs, activity modification, physical therapy, and intra-articular injections, is considered first-line treatment for FAI. Arthroscopic surgical intervention can be considered when conservative treatment fails. Although nonoperative and surgical management have both been shown to be safe and effective in improving symptoms, there are no clear guidelines regarding who will benefit from either approach. More research is needed to help clinicians determine optimal management strategies.

Author Information

Jeff Fleming, DO , PGY1

Rowan University Family Medicine Residency Program


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