Hip pain is a common cause of disability with chronic hip pain occurring in 30% to 40% of adult athletes resulting in a decreased quality of life (1). Hip injuries make up 10% of sports medicine outpatient visits. These injuries can be challenging to diagnose and treat because of the multiple possible sources of pain and dysfunction. Patients with intra-articular hip pathology see an average of three clinicians before establishing an accurate diagnosis (2). Among a wide differential diagnosis of potential causes, one of the most rapidly evolving etiologies of hip pain is femoroacetabular impingement (FAI) syndrome.
The first description of hip impingement in the literature appears in 1936 (3). In 2001, Ganz et al. (4) reported what is now called FAI syndrome as irregularities in femoral and acetabular anatomy, which lead to abnormal contact and mechanical forces across the joint, causing labral and chondral pathologies. Sankar et al. (5) later defined five elements essential for the diagnosis of FAI: abnormal morphology of the femur and acetabulum, abnormal contact between the two structures, vigorous motions that result in abnormal contact and collision, repetitive motion leading to continuous insult, and the presence of the ensuing soft tissue damage. Other studies of FAI syndrome describe not only soft tissue damage but also a correlation between FAI syndrome and the development of osteoarthritis (OA) (6).
A lack of clear diagnostic and management criteria of patients with FAI syndrome led to the 2016 Warwick Agreement consensus statement (7). The fundamental principles and terminology of the consensus statement are outlined in Table. The term FAI syndrome, and not FAI or FAI morphology, is the recommended term for patients with symptoms, clinical signs, and the imaging findings requisite to the various morphologies (7). Other terminologies should be avoided to limit the confusion and ambiguity of this clinical disorder.
2016 Warwick consensus statement questions.
|What is FAI syndrome?
||FAI syndrome is a motion-related clinical diagnosis of the hip that represents symptomatic contact between the proximal femur and the acetabulum.
|How should FAI syndrome be diagnosed?
||Symptoms, clinical signs, and imaging findings must be present to diagnose FAI syndrome.
||FAI syndrome commonly has motion-related or position-related pain in the hip or groin. Patients also may describe clicking, catching, locking, stiffness, restricted range of motion, or giving way.
||The diagnosis of FAI syndrome does not depend on a single clinical sign. The FADIR test is sensitive but not specific for FAI syndrome. Internal rotation of the hip is commonly restricted.
||Obtain an AP radiograph of the pelvis and a lateral femoral neck view of the symptomatic hip. Advanced imaging can be considered to further assess the morphological changes of the hip and the surrounding soft tissue.
|What is the appropriate treatment of FAI syndrome?
||FAI syndrome can be treated by various techniques discussed throughout the article to include conservative care, rehabilitation, or surgery.
|What is the prognosis of FAI syndrome?
||Most patients can return to full activity, including sports, with treatment. OA appears to be a long term outcome of cam morphology. However, it is currently unknown whether treatment for FAI syndrome prevents hip OA.
|How should someone with an asymptomatic cam or pincer morphology be managed?
||Rehabilitation techniques with core and pelvic stability can be recommended. However, it is unknown whether these individuals will progress to FAI syndrome.
There are three morphologies of the femoroacetabular joint associated with FAI syndrome: cam, pincer, and mixed (Fig. 1). A cam-type morphology has an aspherical femoral head due to adventitious bone formation along the head-neck junction of the femur. This morphology results in impingement of the superior acetabulum with hip flexion and internal rotation. A pincer-type morphology, commonly associated with acetabular retroversion, results from the overcoverage of the femoral head. A mixed-type morphology has both cam and pincer morphologies, which is more common than either in isolation.
Before the Warwick Agreement, it was difficult to determine the prevalence of this condition. In one study before the Agreement, FAI syndrome was described in about 40% of hip joint pathology, surpassing labral injuries (33%) and hip OA (24%) (8). No more recent prevalence data are available. The cam morphology accounts for 37% of the morphologies seen in FAI syndrome, and its prevalence is three times more likely in athletes than the general population, and more commonly described in men (9). The pincer morphology accounts for 67% of the morphologies and is found less frequently in athletes and more common in women (10). The higher incidence of the pincer morphology is likely because of the ambiguity of whether the pincer morphology includes populations with acetabular retroversion and focal overcoverage. The most common morphology is a mixed-type morphology. However, the true prevalence of this morphology is challenging to quantify in the literature.
Etiology and Natural History
There is not a consensus for the etiology of FAI syndrome. Current evidence suggests that this condition is multifactorial and based on the underlying morphological changes. In skeletally immature individuals, an adaptive response at the proximal femoral physis to repetitive and vigorous loading of the hip at extreme ranges of motion appears to be associated with the development of the cam morphology. Following physeal closure, there is a limited progression of cam morphologies (11). The prevalence of cam morphology is two to eight times more likely in athletes that participate in high-impact sports, such as football, hockey, basketball, and soccer than nonathlete controls (11,12). These sports involve aggressive, repetitive hip loading. A genetic association also exists. Sibling studies demonstrate a three-fold higher risk for the development of a cam morphology over controls, suggesting genetic predisposition in specific populations (13). Much less is known about the natural development of pincer morphologies and its associated long-term outcomes. Currently, research connecting the pincer morphology with athletic or developmental factors is lacking (14).
Cam morphologies are linked to soft-tissue damage along the superior anterior region of the acetabulum as the aspherical femoral head causes a shearing force when the hip is in flexion and internal rotation (15). An increased prevalence of labral tears and chondral defects demonstrates the effects of the shearing force in adolescents with cam morphology (16). Pincer morphologies are less likely to have localized cartilage injuries at the superior anterior acetabulum and typically demonstrate a circumferential labral injury (15).
The association between FAI syndrome and the development of premature hip OA is a controversial topic. Although cam morphology has a strong association with hip OA, only 25% of patients with cam morphology developed radiographic evidence of OA over a 20-year study (17). However, the severity of the cam morphology is associated with the severity of the future OA (18). There is an unclear relationship between hip OA and pincer morphology. Two extensive cohort studies did not demonstrate a correlation between pincer morphology and the development of OA (18,19).
As with most hip pathology, the physical examination to determine the diagnosis of FAI syndrome can be challenging. Patients frequently present with an insidious onset of mild and episodic chronic pain and dysfunction without trauma. Symptoms can arise with prolonged periods of sitting and upon rising to a standing position. Characteristically, patients present for evaluation after their symptoms progress, sometimes abruptly, to the point where their activities are adversely affected. The primary symptom of FAI syndrome is a specific motion or position that elicits hip or groin pain. Patients also report clicking, catching, locking, stiffness, restricted range of motion, or giving way of the hip (7). A “C sign” is often demonstrated by the patient when they cup their hand above the greater trochanter to describe the location of pain from anterior to lateral, thus making a “C” shape.
Physical examination should include an assessment of the patient's gait, single-leg balance, and direct palpation of the painful area. A systematic review shows that patients with FAI syndrome demonstrate deficits in hip strength and proprioception on single-leg balance (20). Range of motion testing is beneficial in discerning hip pathologies. In FAI syndrome, patients can have an obligate external rotation with active hip flexion as well as limited passive internal rotation of less than 30 degrees (21). Maximal active hip flexion with a flexed knee can reproduce pain in FAI syndrome. Reproducible pain in the groin area with flexion adduction internal rotation (FADIR) is neither sensitive (0.08% to 0.96%) nor specific (0.11%) for FAI syndrome but is commonly used to differentiate intra-articular from extraarticular pain (22). Likewise, there are many examinations designed to differentiate between intra-articular versus extraarticular pathology, but none are specific to FAI syndrome.
Initial radiographs for patients with symptoms and clinical signs consistent with FAI syndrome include anteroposterior (AP) pelvic and oblique radiographs of the hip, such as the modified Dunn, frog-leg, or cross-table lateral. The AP radiograph of the pelvis is centered on the pubic symphysis, free of rotation, or pelvic tilt to allow accurate measurements and bilateral comparison. The modified Dunn view best visualizes the anterior-superior aspect of femoral head-neck junction, where the cam morphology is most commonly found, by placing the patient in a supine position with the pelvis in a neutral position and the affected hip flexed to 90 degrees and abducted 20 degrees. Compared with a traditional frog-leg view, the modified Dunn view is more sensitive (84.2% vs 61.9%) and specific (90.9% vs 86.7%) in assessment of cam morphology (23).
Cam morphologies demonstrate a pistol grip appearance on an AP pelvis radiograph, where the proximal femur has a decreased offset along with an aspherical femoral head. An alpha angle is a radiological measurement evaluated on oblique radiographs, which can demonstrate the extent to which the femoral head deviates from a spherical shape in cam morphologies (Fig. 2). A standardized value for an alpha angle is controversial. A commonly used reference value for a normal alpha angle is less than 42 degrees. Multiple studies reference an alpha angle greater than 55 to 60 degrees, consistent with a cam morphology (24).
A pincer morphology is typically identified on an AP pelvis radiograph by the overcoverage of the acetabulum around the femoral head. This morphological change is commonly seen at the anterosuperior acetabulum, which can be identified by a crossover sign or the measurement of lateral center edge angle (LCEA) on an AP pelvis film (Fig. 2). An LCEA angle greater than 40 degrees is consistent with pincer morphology.
Advanced imaging modalities, such as magnetic resonance imaging (MRI) or computed tomography (CT), can characterize the soft tissue and morphological changes of FAI syndrome, respectively. With regard to hip labrum and cartilage, magnetic resonance arthrography has been shown to have higher sensitivity and specificity compared with conventional MRI with a sensitivity of 85% to 89% versus 77% to 89% and a specificity of 50% to 100% versus 50% when assessing the labrum and a sensitivity of 71% to 92% versus 58% to 83% and a specificity of 75% to 82% versus 64% to 79% when assessing the acetabular cartilage (25). CT scan, especially with 3D rendering, can reliably characterize and quantify the morphological changes of the hip. Despite the advancements in imaging and the improved characterization of the various morphological changes and associated soft tissue injuries of FAI syndrome, imaging is only one part of the diagnosis. The Warwick agreement emphasized that specific imaging measurements do not always correlate to the patient’s symptomatology (7).
There are multiple management strategies for FAI syndrome, including conservative care and surgical intervention. There is insufficient evidence in the literature comparing the various treatment options. Nonoperative management of FAI syndrome is evolving and currently includes patient education, activity modifications, oral anti-inflammatories, physical therapy, and intra-articular musculoskeletal injection therapies.
A wide range of physical therapy and rehabilitation protocols are described in the literature. Current physical therapy techniques demonstrate a significant reduction of pain and functional improvement for up to 24 months. These techniques include core stability, proprioception, and dynamic stability of the hip through correcting neuromuscular imbalance and strength deficits in the hip flexors, external rotators, abductors, adductors, and core musculature serving to decrease force or impact of the bony changes (26,27). A 2019 systematic review and meta-analysis of five randomized controlled trials (RCTs) compared different physical therapy protocols over 6 wk to 12 wk. This analysis demonstrated a statistically significant improvement of functional outcomes of patients with FAI syndrome through core strengthening, active physical therapy, and supervised physical therapy over the controls of no core strengthening, passive modalities, and unsupervised therapy, respectively (28). A prospective cohort study of 93 adolescent hips with FAI syndrome demonstrated 82% had improvement at 2 years with conservative therapy. Seventy percent of these cases were managed with physical therapy and activity modification alone, and 12% also received an intra-articular hip steroid injection. However, 59% of these patients did not return to their original sport. The exact reason for this is unclear because only 17% cited pain as the reason for not returning (29). Overall, this evidence supports an initial trial of physical therapy before considering surgery.
Therapeutic musculoskeletal injections are utilized for FAI syndrome, but evidence for them is limited. A prospective cohort study of 54 patients with FAI syndrome and a labral tear demonstrated a short-term benefit of an image-guided intra-articular hip corticosteroid injection, but the pain relief only averaged 9.8 d (30). Two other studies did not demonstrate a statistically significant positive correlation among postsurgical outcomes and presurgical response to anesthetic intra-articular hip injections with or without corticosteroid (positive likelihood ratio, 1.14 to 1.22). However, they did show a failure to respond to these injections is associated with a lack of postoperative improvement (negative likelihood ratio, 0.57 to 0.70) (31,32). These studies highlight the limited benefit of hip intra-articular corticosteroid injection for the treatment of FAI syndrome. However, limited evidence suggests an intra-articular hip diagnostic injection could guide presurgical patient counseling about postsurgical outcomes.
Multiple other musculoskeletal injection therapies have been described in the literature for hip OA, but only a few studies address FAI syndrome. In a small open, prospective trial of 23 hips with FAI syndrome, 2 mL of high-molecular-weight hyaluronic acid was used off-label and injected under ultrasound guidance into the hip at baseline, after 40 d, and after 6 months demonstrating a significant reduction of pain and improved function at 12 months (33). A systematic review and meta-analysis evaluating intra-articular injection therapies for FAI syndrome demonstrated that intra-articular corticosteroids and hyaluronic acid injections had a small effect size for pain and small to moderate effect size for function (26).
A small pilot study of eight patients that failed conservative management for hip labral tears to include intra-articular steroid injections underwent treatment with leukocyte-rich platelet-rich plasma through ultrasound guidance into the labral tear. These patients demonstrated a statistically significant reduction in pain and improved function (34). There is insufficient literature addressing the treatment of FAI syndrome with orthobiologics. Low-quality evidence demonstrates improved pain and function with intra-articular hip hyaluronic acid. Otherwise, there is insufficient evidence to support the use of injection therapies for the treatment of FAI syndrome at this time.
The goals of arthroscopic surgery for FAI syndrome are to correct the morphological changes and address the underlying soft tissue injuries to achieve impingement free range of motion and relieve pain. Surgery is rapidly increasing for FAI syndrome, which is demonstrated by a six-fold increase in surgery from 2006 to 2010 (35). Recent studies evaluated reliable indicators of surgical outcomes to advise physicians and patients in a shared decision-making process. Regarding athletic performance, a 2-year retrospective cohort study of 626 patients with FAI syndrome demonstrates negative predictors of return to high-level athletic function after surgery: the presurgical presence of chondral damage, higher alpha angles, mental health concerns, a higher body mass index, more than 2 years of symptoms, and the presence of a limp (36). Other studies demonstrate additional presurgical predictors of inferior outcomes to include moderate to severe hip dysplasia, increased age, pending workers’ compensation or litigation, chronic opioid use, and female sex (37–39).
Even though multiple indicators are being evaluated for surgical outcomes, the most reliable indicator is the presence of presurgical OA. A 10-year prospective cohort found a joint space of 2 mm or less has at least a four-fold increased risk of total hip arthroplasty within 10 years of arthroscopic surgery for FAI syndrome (37). These outcomes, based on the presurgical presence of OA, are consistent with a systematic review of 1556 patients, which demonstrates a higher risk of reoperation and lower patient-reported outcomes (PRO) in patients with at least Tonnis grade 2 hip OA, 2 mm or less of joint space, and a LCEA of fewer than 20 degrees (dysplasia) (40). A large retrospective study of 8267 hip arthroscopies demonstrates OA and increased age at the time of surgery increases the hazard ratio for reoperation, while labral repair and a higher-volume surgeon lower the hazard ratio (39). The surgical outcomes for FAI syndrome patients with preexisting OA are further defined in a meta-analysis of seven prospective studies showing cohorts with FAI syndrome and OA (total 310 subjects) demonstrate an odds ratio of 8.50 for failure (i.e., reoperation, patient dissatisfaction, or lack of improved PRO). Failure rates were 45.2% in the combined FAI syndrome and OA group compared with 13.2% in the FAI syndrome group alone (41).
Recent studies compare the efficacy of surgery to nonoperative management with physical therapy. The UK FASHIoN and FAIT are two large RCT that evaluated surgery versus physical therapy outcomes in patients with FAI syndrome. The UK FASHIoN RCT compared the PRO at 12 months in 171 patients managed with arthroscopy for FAI syndrome versus 177 managed with physical therapy. The surgically treated patients showed statistically significant improvement in symptoms and functional limitations when compared with physical therapy patients. If the patient had a cam morphology, the difference in surgical and physical therapy outcomes is more pronounced (42). The FAIT RCT included 222 patients between the ages of 19 and 60 years with FAI syndrome who were randomized to arthroscopic surgery or physical therapy and activity modification. Eight months postintervention, the arthroscopic surgery cohort demonstrated clinically and statistically significant improvement in the primary outcome of hip outcome score activities of daily living subscale when compared with the therapy group (43). Because of the advances in the physical therapy techniques in the literature, the physical therapy control group in these studies has been called into question (28,44). Although small, these studies demonstrate the effectiveness of surgery over physical therapy for FAI syndrome. Further studies are underway to evaluate the efficacy of surgery versus a sham control (45–47). Many ongoing studies include plans for a 10-year follow-up to evaluate the incidence of hip OA with and without surgery.
Appropriate patient education and shared decision making require knowledge of the risk associated with each treatment option and success rates of returning to sport after the chosen therapy. A 2016 compilation of nine previously published systematic reviews evaluated complications following arthroscopy for FAI syndrome. Complication rates ranged from 0.5% to 7.5%, including primarily minor risks of hip arthroscopy as well as rare cases of hip instability, septic arthritis, femoral neck fracture, and avascular necrosis (48). A systematic review and meta-analysis of 31 articles assessed 554 patients and found an 87.7% rate of return to sport (38). Another systematic review and meta-analysis of 35 studies demonstrated a 91% return to sport (95% CI, 88%–94%) in 4 to 10 months in postarthroscopy FAI syndrome patients. Thirteen of those studies report a 74% (95% CI, 67%–81%) return to sport at the patient’s preinjury level (49). A more recent meta-analysis of 809 patients across 15 case series demonstrated 88.3% (95% CI, 83.4%–92.4%) of patients returned to play and 85.5% (95% CI, 77.6%–91.6%) returned to play at preinjury level after arthroscopy. This meta-analysis included recreational through professional level athletes across a variety of sports (50). This data should be used to counsel patients whose goals include a return to high-level sports.
Despite the growing prevalence of arthroscopy for FAI syndrome, it is recommended to include a trial of nonoperative therapy before surgery due to a modest chance of improvement with low risk of harm. Despite progressive surgical techniques, surgical outcomes are not guaranteed and not without risks. Interpretation of the above studies is further complicated by the diversity of indications to pursue surgical intervention. A 2017 systematic review found only 56% of studies utilized the criteria proposed in the Warwick Agreement of symptoms, clinical signs, and imaging to make a diagnosis of FAI syndrome. Only 44% of studies reported a requirement of a failed trial of nonoperative management before surgery (51). A recent study of 74 patients showed hip extension and abduction strength in patients with FAI syndrome to be correlated with PRO after arthroscopy (52). This underscores the question of whether preoperative rehabilitation may in fact improve surgical outcomes. In patients with FAI syndrome, all treatment options should be considered, and a shared decision-making process to assist the patient in understanding the potential risks and benefits of surgery is essential (43). Surgery is not recommended for asymptomatic patients with cam or pincer morphology, as there is no evidence that it changes the course of the disease (53).
FAI syndrome is a constellation of symptoms, examination findings, and radiographic evidence that impacts athletic performance and activities of daily living. The Warwick Agreement confirms that all three are required to establish the diagnosis. Physical therapy and arthroscopic surgery demonstrate positive patient outcomes. The safety and efficacy of physical therapy warrant a trial before pursuing surgery. Additional research is needed to evaluate specific physical therapy protocols, but it should include active hip and lumbosacral core strengthening. Limited evidence supports consideration for an image-guided, intra-articular injection as part of the diagnostic or presurgical evaluation. Selection criteria and surgical outcomes need to be evaluated with improved study design. Further evaluation of patients with asymptomatic morphology and FAI syndrome is needed to identify whether FAI syndrome or morphology alone leads to hip OA and whether treatments change the overall risk of the patient with FAI syndrome for future hip OA.
The authors declare no conflict of interest and do not have any financial disclosures.
The views expressed herein are those of the authors and do not reflect the official policy or position of the Uniformed Services University, Defense Health Agency, Department of the Air Force, Department of the Army, Department of Defense, or the U.S. Government.
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