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The Natural History of Femoroacetabular Impingement

Wylie, James D. MD, MHS*; Kim, Young-Jo MD, PhD

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Journal of Pediatric Orthopaedics: July 2019 - Volume 39 - Issue - p S28-S32
doi: 10.1097/BPO.0000000000001385
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Wiberg1 described the first evidence of abnormal anatomy of the hip leading to osteoarthritis (OA) in his famous monograph on acetabular dysplasia in 1939. In this, he described the lateral center edge angle (LCEA) and the propensity for patients with an LCEA of <20 degrees to develop OA over time.1 Murray further confirmed Wiberg’s observations on acetabular dysplasia in 1965. In addition, this was the first description of a proximal femoral morphology leading to hip OA.2 Murray,2 a British radiologist, described the tilt deformity of the proximal femur, as well as the deformity of the slipped capital epiphysis and Perthes deformity as causes of hip OA. Resnick3 disputed Murray’s description of the tilt deformity and argued that the anatomic changes were due to femoral head remodeling and osteophyte formation caused by OA. The concept of femoral-sided deformity leading to OA gained more traction with Harris’ description of the pistol grip deformity in the 1980s.4 Ganz et al5 built on these early observations and described femoroacetabular impingement (FAI) in the early 2000s. They showed that epiphyseal extension leading to femoral cam deformity was present in young patients without any evidence of OA.6 Ganz and colleagues’ greatest contribution to furthering this work was the description of the surgical dislocation approach and the periacetabular osteotomy to allow for treatment of hip impingement and hip dysplasia.7,8 The surgical dislocation approach allowed safe treatment of FAI without a risk to the blood supply of the femoral head.8 More recently, hip arthroscopic instrumentation and methods have been developed to effectively treat some forms of FAI with hip arthroscopy.9


There are 3 distinct anatomic variations that predispose to FAI syndrome, that is clinically symptomatic FAI. Cam-type FAI is an abnormality of the proximal femur that leads to an asphericity of the femoral head and a lack of normal offset at the femoral head neck junction.5 This is thought to be caused by stress on the proximal femoral physis that leads to epiphyseal cupping as an adaptive response during growth.10 It is most commonly located in the anterosuperior femoral head/neck junction but can extend all the way from the anterior to lateral and even the posterior head/neck junction. Pincer-type FAI is caused by acetabular over-coverage and is sometimes referred to as a deep and/or retroverted acetabular socket. When both femoral-sided cam-type and acetabular-sided pincer-type FAI are coexistent this is referred to as mixed-type FAI.

The mechanics are different between cam-type and pincer-type FAI. Cam-type FAI causes an aspherical head to engage in the spherical acetabulum in deep flexion and/or with flexion and internal rotation, depending on the specific area of deformity on the femoral side. Repetitive trauma due to this anatomic mismatch leads to joint injury at the chondrolabral junction and over time can lead to cartilage delamination and degeneration to arthritis. Pincer-type FAI leads to an impaction of the acetabular labrum on the femoral neck and when further motion is forced a slight subluxation causing a contra-coup injury in the posterior inferior acetabulum.5 The primary injury in the weight-bearing area of the joint is labral in nature as opposed to the chondrolabral injury seen in cam-type FAI. The degree of injury seen with any of these morphologic patterns depends upon the activity level of the patient and the motions that the patient demands of the hip. These 2 factors determine the volume and force of the repetitive trauma that the joint sees and therefore likely represent key variables in the amount of joint damage that develops. Genetic factors intrinsic to cartilage may also play a role in the susceptibility to cartilage injury and degeneration.11


Hip OA was commonly thought of as a slowly degenerative “wear and tear” on the articular cartilage of the joint. Before our understanding of FAI, known causes of early hip arthritis were avascular necrosis, acetabular dysplasia and pediatric hip disease, including slipped capital femoral epiphysis and Perthes disease.1,2 However, many patients were diagnosed with idiopathic hip OA, even those who developed it at a young age. Clohisy et al12 performed a study of 2 tertiary centers in the United States looking at patients undergoing total hip arthroplasty (THA) before the age of 50 years. They found that of the 337 patients with a diagnosis of OA, 163 (48%) had acetabular dysplasia, 32 (10%) has residual of Perthes disease, and 21 (6%) had a prior slipped capital femoral epiphysis. The remaining 121 patients had idiopathic hip OA. On review of these patients radiographs 76 (63%) had cam-type FAI, 7 (6%) had pincer-type FAI and 35 (29%) had mixed-type FAI.12 This left only 3 radiographically normal hips that presented for THA before the age of 50 years. This study supports the idea that the vast majority of hip OA, especially in younger patients, is due to some anatomic abnormality that alters the normal function and homeostasis of the joint.


Studies trying to understand the natural history of a disease can be longitudinal in nature, where a group of patients are followed over-time, or cross-sectional, where an association is found between a diagnosis and a predictor variable at one time point. Longitudinal studies provide the best information between a causative factor and development of OA.

Longitudinal studies looking at the development of disease are preferred to understand associations between anatomical differences and OA development. Two prominent studies have followed patients longitudinally and reported an association between cam-type FAI morphology and OA development.13,14 The Chingford 1000 Women study is a prospective cohort of 1003 women in the United Kingdom.13 They were followed prospectively for 20 years with anteroposterior (AP) pelvis radiographs and cam-type morphology was determined by the alpha angle and the triangular index. Both the alpha angle and triangular index were highly associated with the development of radiographic OA and total hip replacement at 20 years follow-up.13 In regard to the alpha angle, for every degree over 65 degrees there was a 5% increased risk of radiographic arthritis and a 4% increase in the risk of total hip replacement in the study period.13 The cohort hip and cohort knee (CHECK) cohort was a similar study in the Netherlands that obtained baseline radiographs and then followed patients for 5 years and determined progression of radiographic hip OA in middle age patients presenting with hip pain.14 They also used the alpha angle on AP radiographs as their measure of cam-type FAI and found that an alpha angle >60 degrees had an adjusted odds ratio (OR) of 3.67 for the development of end-stage arthritis. An alpha angle >83 degrees had an adjusted OR of 9.66 for the development of end-stage arthritis over 5 years.14 When patients had both an alpha angle of >83 degrees and hip internal rotation in 90 degrees of flexion of <20 degrees the adjusted OR was 25.2 and a positive predictive value of 53% for the development of end-stage arthritis over 5 years.14 A third longitudinal study looked at adolescent athletes and found that those with restricted range of motion and magnetic resonance imaging (MRI) findings of cam deformity on radial sequence imaging were at risk of further joint injury and degenerative changes on follow-up MRI and radiographs five years later.15

There have been multiple cross-sectional studies as well on the association between cam-type FAI morphology and radiographic degenerative changes in the hip. Gosvig and colleagues reported that in the Copenhagen OA substudy a triangular index of >1.0 was associated with radiographic OA. Similarly, Amstutz and Le Duff16 showed that a lower head/neck ratio, indicating less head/neck offset, was associated with degenerative changes on standing AP pelvis radiographs. Conversely, Anderson et al17 showed that in a cross-sectional cohort of active older patients at the Huntsman Senior Games that cam morphology was not associated with hip OA. A summary of studies on the association between cam-type FAI and OA is presented in Table 1.

Natural History Studies on Cam-type Femoroacetabular Impingement

Despite the mounting evidence that cam-type FAI is associated with the development of OA, there are no studies showing that surgical intervention reverses this process. One preliminary study of 10 patients by Beaulé et al20 recently showed normalization of bone and articular cartilage parameters in the hip postoperatively on computed tomography and cartilage specific MRI after cam-type FAI correction. More studies are needed to understand the effect of surgery to correct the cam deformity on disease progression.


The data on pincer-type FAI is not nearly as definitive as cam-type FAI. Some studies suggest an association with OA, while others suggest a protective effect for the joint. Part of the reason for this may be the inconsistent definition of pincer-type FAI. This can be a heterogenous group of patients with pathology from global over-coverage to isolated cranial retroversion of the acetabulum.

The prospective longitudinal studies looking at disease progression have reported no association between pincer FAI and OA. In the Chingford cohort, there was no association between a LCEA >39 degrees and development of arthritis or hip replacement over a 20-year period.13 Similarly, in the CHECK cohort there was no association between an LCEA >40 or an anterior center edge angle >40 degrees and incident OA or development of end-stage disease.21 In fact, in patients with both a LCEA and an anterior center edge angle of >40 degrees there was an adjusted OR of 0.34 for developing incident OA, meaning they were three times less likely than others to develop OA in the 5-year follow-up.21 One cross-sectional study suggests an association between pincer-FAI and OA. Data from the Copenhagen OA substudy show that an LCEA >45 degrees had a relative risk of OA of 2.4.18 Other studies have shown that acetabular retroversion, which is commonly referred to as a pincer-type FAI mechanism is associated with the development of hip OA.22,23 Kim et al22 reported on patients undergoing pelvic CT and found that acetabular retroversion was correlated with hip joint space narrowing. Giori et al23 found that patients undergoing hip arthroplasty had 4 times the incidence of acetabular retroversion compared to controls without OA. A summary of studies on the association between pincer-type FAI and OA is presented in Table 2.

Natural History Studies on Pincer-type Femoroacetabular Impingement


Natural history studies have consistently shown that cam FAI as measured by the alpha angle or the triangular index, on the AP pelvis radiograph is associated with the development of hip OA. The data on the relationship of radiographic pincer FAI and hip OA has not shown a consistent association, with one study showing a protective effect. Part of the reason for these findings may be the heterogenous definitions of what constitutes pincer FAI in these studies. More studies are needed to understand the relationship between both cam deformity on lateral radiographs and more contemporary definitions of pincer FAI and the development of hip OA. In addition, more studies on the effect of surgical correction of FAI on the natural history of the disease are necessary to understand whether we have an ability to preserve the native hip.


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femoroacetabular impingement; natural history; hip osteoarthritis; cam; pincer; acetabular retroversion

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