IMAGING CONSIDERATIONS

– Written by Teresa B. Rodrigues and Vasco V. Mascarenhas, Portugal

INTRODUCTION

Hip and groin pain is a common concern among athletes of all genders and skill levels, affecting both elite professionals and recreational “weekend warriors.” It ranks among the top six most common athletic injuries, impacting up to a quarter of athletes annually. Intra-articular injuries, like femoroacetabular impingement syndrome (FAIS), account for approximately 6% of cases and can significantly hinder performance if not promptly diagnosed and managed^1,2.

FAIS is a motion-related clinical syndrome characterized by abnormal contact between the acetabular rim and the proximal femur, often resulting from predisposing Cam or Pincer hip morphologies³. This condition leads to a characteristic triad of groin or hip pain, restricted range of motion, and distinctive imaging findings⁴. Over time, FAIS can contribute to early-onset osteoarthritis (OA) of the hip, particularly in active individuals⁴.

Athletes are uniquely predisposed to FAIS, especially those with Cam morphology, due to repetitive, high-impact activities that push the hip joint beyond physiological limits. In particular, weight-bearing sports that involve deep flexion and rotational hip movements exacerbate the risk of developing hip morphologies associated with FAIS^5-7. Furthermore, altered hip anatomy in these individuals lowers the threshold for injury during intense physical activity, accelerating degenerative changes. In contrast, individuals with lower activity levels or athletes with normal hip anatomy can typically tolerate these activities without significant joint damage until much later in life. Without timely diagnosis and intervention, FAIS can compromise athletic performance and, over time, result in chronic pain and reduced hip mobility⁴.

FAIS is a dynamic condition that extends beyond a static diagnosis based solely on imaging. While specific bony morphologies are essential for diagnosing FAIS, many athletes with these anatomical features remain asymptomatic. This underscores the importance of a comprehensive approach combining clinical evaluation and imaging to distinguish clinically significant cases from incidental findings³. Advances in imaging techniques have played a transformative role in this process, enabling detailed visualization of bone morphology, addressing chondral and labral lesions, soft tissue damage, and assessing differential diagnosis. Evaluating specific morphological features, such as Pincer and Cam, is crucial for tailoring imaging and management strategies to the unique biomechanical demands of athletes, enabling early detection and optimized treatment planning^8-10.

This article examines the role of imaging in diagnosing and managing FAIS in athletes, highlighting diagnostic approaches and imaging protocols.      

HIP AND GROIN INJURIES IN ATHLETES

Hip and groin pain may result from an acute injury or overuse/chronic trauma and is particularly prevalent in sports involving rapid acceleration and deceleration, sharp directional changes, and kicking motions. Common examples include football, ice hockey, basketball, and dance. These complaints are often linked to adductor strains, hamstring, or muscle-tendon injuries (see supplemental material).  For instance, in soccer, hip and groin injuries constitute approximately 15% of all reported injuries, among these, adductor injuries are the most prevalent, representing about two-thirds of cases, followed by hip flexor or iliopsoas lesions, which account for 8%¹.

Interestingly, intra-articular hip injuries are far less common, making up only about 6% of all hip and groin injuries in athletes¹.

Of note, overuse hip injuries are significantly more common than acute injuries, particularly in professional soccer players, where they occur three times more frequently. These injuries represent up to 18% of all time lost due to injury at the professional level. Alarmingly, up to one in every three athletes will experience persistent symptoms lasting longer than six weeks, and in fact, this chronicity leads to significant challenges in daily activities, reduced quality of life, and impaired athletic performance¹.

Complexity of Diagnosis in Hip and Groin Pain

A key challenge in managing hip and groin pain lies in its complex etiology. It is very important to notice that many distinct and often coexisting clinical entities can contribute to symptoms. That is why meticulous assessment of symptoms, clinical signs, and imaging findings is paramount to reach an appropriate diagnosis¹¹.

The Doha Agreement Meeting on Terminology and Definitions in Groin Pain in Athletes¹¹ has provided a valuable framework, categorizing the causes of groin pain into three major groups:

1. Defined clinical entities for groin pain, which include four primary subgroups (adductor-related, iliopsoas-related, inguinal-related, and pubic-related groin pain);
2. Hip-related groin pain;
3. Other causes of groin pain in athletes.

Hip-Related Groin Pain and Hip Impingement

Within the category of hip-related groin pain, one of the key mechanisms involves hip impingement, a condition in which abnormal contact between anatomical structures of the hip leads to restricted movement and pain^1,3,11. Hip impingement can be broadly classified into two main types:

1. Intra-articular impingement: The most recognized form is FAIS, which occurs due to abnormal morphology of the femoral head or acetabulum, leading to pathological contact during hip movement.
2. Extra-articular impingement: This category includes several types of mechanical conflicts involving structures outside the joint capsule, such as soft tissues, tendons, or bony prominences.

FEMOROACETABULAR IMPINGEMENT SYNDROME

From a radiological perspective, FAIS can be classified into two primary subtypes based on the underlying morphological abnormality: Cam and Pincer (Table 1)³.

Cam morphology is further categorized into primary Cam morphology (PCM), which occurs in individuals without prior hip disease, and secondary Cam morphology, which develops due to an underlying primary hip condition.

PCM is mostly a benign bony prominence around the femoral head and neck (FHN) junction, most commonly in the anterolateral region. It results in a loss of the typical concave shape, leading to a flattened or convex contour³. This progression, commonly seen with age, likely begins in early adolescence during skeletal maturation and reflects a physiological adaptation to mechanical loading. Additionally, childhood physical activity and sports play a significant role in developing this condition^2,3,12.

This “bony bump” is often a natural adaptation to training and most athletes with this morphology remain asymptomatic. However, for some, it may lead to symptoms (FAIS) and, over time, increase the risk of developing hip OA³.

In the few athletes with symptomatic PCM, abnormal contact between the femoral head and acetabular rim, particularly during hip flexion and internal rotation, generates shearing forces across the acetabular cartilage. These forces primarily affect the anterosuperior quadrant and chondrolabral junction, often leading to early, silent cartilage damage. As the condition advances, symptoms worsen due to increasing labrum and cartilage involvement. Such lesions are associated with a reduced range of motion, mainly restricted internal hip rotation and hip pain^7-10.

Pincer morphology refers to either global or focal overcoverage of the femoral head by the acetabulum, leading to labrum compression between the acetabulum and femoral neck. Its association with pain or OA is unclear, and prevalence is similar in athletes and symptomatic patients. Repeated impingement can lead to labral injury, ossification, and chondral damage^7-10.

Some patients exhibit features of both subtypes, termed FAIS with mixed morphology, which is considerably more prevalent among athletes than asymptomatic individuals⁷.

Association between FAIS and Sports Activities

The likelihood of developing Cam morphology is shaped by factors like genetics, sex, and physical activity. Male athletes at higher levels of competition are 2 to 8 times more likely to develop this condition. Cam morphology is especially common in sports that involve repetitive hip impact, with rates as high as 89% in athletes, while it’s only around 9% in non-athletic populations. Importantly, arthroscopic surgery to correct FAIS with Cam morphology is one of the most frequently performed hip procedures in professional and Olympic athletes, representing 36% of all hip surgeries and 81% in hockey players¹³.

Although this morphology is not exclusive to athletes, recognizing its association with specific sports allows us to explore preventive strategies and potentially mitigate the risk of subsequent hip OA. However, predicting which athletes will be affected remains challenging, as most individuals with PCM remain asymptomatic and never develop hip disease. Bone is a dynamic tissue, and the impact of mechanical loading on the epiphyseal growth plate is highly complex, influenced by factors such as the type of load and the athlete’s physical and physiological maturity (see supplemental material). What constitutes a standard load for one athlete may be excessive for another, underscoring the need for individualized approaches in training and monitoring².

While many athletes may exhibit structural alterations associated with impingement on imaging, not all experience symptoms or functional limitations. The FAIS framework ensures a comprehensive approach by requiring the presence of three key elements: symptoms, clinical signs, and imaging findings (Figure 1). This triad helps clinicians distinguish clinically significant cases from incidental findings, enabling more accurate diagnoses and tailored treatments^3,8-10.

IMAGING TECHNIQUES IN FAIS

This section outlines key imaging modalities and parameters for assessing suspected FAIS based on expert consensus recommendations^3,8–10.

Radiographs are recommended as the initial diagnostic tool for evaluating patients with potential impingement (see supplemental material). The standard protocol includes an anteroposterior (AP) pelvic radiograph and a lateral hip view, crucial for assessing the pelvis and hip structures, excluding other potential causes of symptoms, and supporting the diagnosis of FAIS^3,8,10. The AP view evaluates the shape and orientation of the acetabulum, while the lateral view assesses proximal femoral morphology. FHN asphericities are better visualized with a Dunn 45° view. This view, performed with the hip flexed at 45° and abducted at 20°, enhances the detection of deformities and facilitates measurements such as the alpha angle and head-neck offset. Therefore, when Cam morphology is suspected, combining the AP radiograph with the Dunn 45° view offers the most effective initial evaluation strategy (Figure 2)¹⁰.

While essential to the diagnostic process, radiographs only partially represent the hip’s three-dimensional anatomy. They are not adequate for evaluating chondral or labral pathology and provide only indirect evidence of damage, such as joint space narrowing or secondary OA changes. When FAIS is clinically suspected, further assessment with advanced imaging is typically performed to obtain a more comprehensive evaluation^3,8-9.

Magnetic resonance imaging (MRI) is the gold standard imaging technique for hip evaluation. MRI is ideal for characterizing bone morphology (particularly Cam morphology), addressing chondral and labral lesions, and assessing differential diagnosis. A minimum field strength of 1.5T is recommended for accurate FAIS assessment^8-9.

In a young patient with hip pain, the recommended protocol typically includes unilateral small field-of-view (FOV) high-resolution sequences of the symptomatic hip, radial imaging, femoral torsion assessment, and a fluid-sensitive sequence covering the whole pelvis (Figure 3)^8-9.

1. Radial imaging, performed around the femoral neck axis, provides circumferential visualization of the hip joint in a clockwise fashion and is particularly effective for identifying FAIS related morphologies. These images can be obtained through either 3D volumetric reconstructions or high-resolution 2D radial sequences based on sagittal oblique localizers. Radial MRI sequences offer improved correlation with increased alpha angles observed in the Dunn 45° radiographic view, enhancing diagnostic accuracy. The radial sequence should include at least 12 slices to meet the minimum standard for adequate imaging^8-9.
2. Fast axial sequence of the distal knee (femoral condyles) and the proximal femoral neck for femoral torsion evaluation should be a standard practice in MRI studies for FAIS, as this critical osseous abnormality is one of the three major osseous abnormalities involved in the development of impingement^8-9.
3. Additionally, a fluid-sensitive sequence covering the entire pelvis, in the axial or coronal planes, should be incorporated to screen for pathologies beyond the hip joint and to address potential diagnostic oversights from clinical assessment alone, such as to screen for soft-tissue and bone marrow edema beyond the hip. While whole-pelvis imaging is valuable for broader screening, it should not replace targeted imaging of specific regions such as the sacroiliac joint or pubic symphysis^8-9.

MRI plays a pivotal role in diagnosing intra-articular lesions, as in cases without overt OA can identify focal cartilage lesions despite minimal or absent radiographic findings, which may significantly influence the choice between surgical and non-surgical management. Direct MRA (dMRA) is generally considered superior to standard non-contrast MRI, particularly for detecting labral tears and chondral injuries. Evidence suggests that non-contrast 3T MRI offers a viable alternative with similar sensitivity to 1.5T dMRA for diagnosing labral tears and cartilage delamination. Indirect MRA is generally not indicated. For improved visualization, dMRA can be performed with leg traction to distract the hip joint and uncover cartilage flaps, which has shown promising results^8-9.

Ultrasound has a limited role in FAIS and it is mainly reserved for onsite diagnosis/differential diagnosis and guidance of diagnostic/therapeutic injections^8-9.

Unenhanced Computed Tomography (CT) is not widely established as a primary imaging modality for FAIS. While volumetric CT effectively identifies osseous morphologies and osteoarthritic changes and supports 3D motion simulations, it cannot detect chondrolabral changes and involves radiation exposure, especially concerning younger patients. Although sometimes combined with MRI for comprehensive evaluation or pre-surgical planning, volumetric MRI offers a radiation-free alternative, simplifying the diagnostic process and reducing costs and time^8-9.

IMAGING FINDINGS AND INTERPRETATION IN FAIS

Specific imaging parameters should be considered to assess the hip and pelvis in patients with hip pain and suspected FAIS (Table 2).

Femoral parameters

The evaluation of femoral head sphericity and joint congruency is crucial in diagnosing morphologies associated with FAIS. One of the key areas of assessment is the FHN junction, as abnormalities in this region often indicate the presence of Cam morphology^8,10.

Defining Cam morphology (Table 3):

• The main imaging criteria is an alpha angle (reflects the extent of asphericity) > 60° at any location around the anterosuperior FHN junction, most commonly in the anterosuperior quadrant (3 o’clock to 12 o’clock). This threshold is strongly associated with symptomatic hips, mid-term progression to OA, and the differentiation between FAIS patients and asymptomatic individuals, particularly at 1:30 to 2 o’clock. However, values below 60° do not exclude the possibility of symptoms, and higher values do not always indicate clinical issues^8,10.
• Femoral offset (FO) and offset ratio, are used to a lesser extent to define Cam morphology. The FO measures the narrowing of the femoral neck relative to the head, while the offset ratio measures the proportion of the anterior offset to the diameter of the femoral head; they are primarily used at the anterior FHN junction (3 o’clock) as its application in other location lacks strong evidence. An anterior offset below 8 mm and offset ratio ≤0.15 is typically considered abnormal and indicative of Cam morphology^8,10.

Neck orientation in the coronal (neck-shaft angle (NSA)) and axial (torsion) planes, provide further insights into hip morphology^8,10.

1. The NSA, assessed using standardized radiographs or coronal imaging planes, typically ranges from 120–135° on radiographs and 120–140° on CT/MRI. Values below 120° indicate coxa vara, while values above 135° suggest coxa valga. Variability in NSA measurements can occur due to differences in imaging techniques, hip rotation, and femoral torsion, but the NSA remains an important parameter in understanding proximal femur structure^8,10.
2. Femoral torsion is an important anatomical factor that influences hip biomechanics, describing the intrinsic rotation of the femur along its length, extending from the hip to the knee. In adults, normal femoral antetorsion averages approximately 13° (±10°) based on Reikerås’ method. The variability in torsion highlights the need for consistent measurement methods and careful interpretation. Abnormal femoral anteversion has been linked to the development of FAIS, with reduced femoral anteversion commonly associated with FAIS with Cam morphology^8,10.

These measurements offer a comprehensive framework for analyzing hip morphology. However, their moderate reproducibility and limited discriminatory power highlight the importance of a multidimensional diagnostic approach^8,10.

Acetabular parameters

The evaluation of acetabular morphology, including coverage, depth, and orientation, is a fundamental step in diagnosing morphologies associated with FAIS and should be routinely assessed. Pincer morphology arises from acetabular retroversion, overcoverage, or a combination of both^8,10.

Defining Pincer morphology is identified through specific imaging criteria that ensure a structured and consistent diagnostic approach (Table 4).

Acetabular coverage is typically assessed using the center-edge angle (CEA) of Wiberg, the acetabular index (AI), and the presence of protrusio acetabuli^8,10.

1. The CEA of Wiberg (W-CEA) categorizes acetabular coverage as undercoverage when <20°, borderline undercoverage 20–25°, normal coverage 25–39°, and overcoverage  ≥40°. It is essential to specify whether the measurement refers to the W-CEA, which assesses anterosuperior coverage, or the lateral CEA (L-CEA), which evaluates superolateral coverage, as both provide complementary information.
2. The AI, which measures acetabular inclination, defines values below 0° as indicative of overcoverage and values above 13° as suggestive of undercoverage.
3. Protrusio acetabuli, a qualitative marker of global overcoverage, should always be documented.

Acetabular version, a key feature of pincer morphology, is typically identified using radiographic markers such as the cross-over sign (COS), posterior wall sign (PWS), and ischial spine sign (ISS). These markers are valuable for surgical planning, as they help distinguish between focal cranial retroversion (indicated by an isolated COS), and global retroversion (characterized by the presence of COS, PWS, and ISS together). Such distinctions are essential for tailoring treatment strategies, particularly in cases where retroversion patterns influence the range of motion and predisposition to impingement^8,10.

Interpreting these markers requires caution, as their reliability can be influenced by pelvic tilt and rotation, particularly in assessments of AP coverage and retroversion. Although radiographs remain the standard initial imaging modality, their accuracy in detecting pincer morphology is debatable. Cross-sectional imaging provides greater precision and may play an increasingly important role in the diagnostic process^8,10.

Labrum

Imaging should detail labral lesions (Table 5 and Figure 4), as they may impact athletic biomechanics and performance. Labral pathologies are common even in asymptomatic individuals, so their clinical relevance depends on patient history and examination^8,10.

Cartilage

Evaluate the pattern, location, and extent of cartilage lesions, which influence joint mechanics, surgical planning and prognosis (Table 6 and Figure 4-6)^8,10.

CONCLUSION

FAIS is a critical condition in sports medicine that requires timely diagnosis and intervention to prevent long-term complications. A combination of clinical evaluation and advanced imaging plays a vital role in accurately diagnosing and effectively managing FAIS in athletes.

How to Optimize Imaging for FAI: Top 5 Tips

1. Radiographs are the minimum requirement for diagnosing suspected FAIS (AP pelvis and Dunn 45º view).
2. MRI, utilizing a dedicated protocol, is considered the gold standard for a detailed assessment of FAIS and differential diagnosis.
3. An effective MRI protocol for FAIS should include a unilateral small FOV with radial imaging, evaluation of femoral torsion, and fluid-sensitive sequences encompassing the entire pelvis.
4. Key parameters to evaluate the acetabular side include acetabular coverage (e.g., Wiberg’s angle, acetabular index) and version (e.g., crossover sign, posterior wall sign, ischial spine sign). For the femoral side, assessment should focus on the head-neck junction (α angle, offset), neck-shaft angle, and torsion.
5. Always correlate imaging findings with clinical symptoms to avoid misdiagnosis.

Teresa B. Rodrigues M.D.

Radiology Department

Imaging Center

Hospital da Luz

Grupo Luz Saúde

Lisbon, Portugal

Vasco V. Mascarenhas M.D., M.Sc., MBA

Musculoskeletal Imaging Unit

 Radiology Department, Imaging

Center

Hospital da Luz

Grupo Luz Saúde

Lisbon, Portugal

Católica Medical School

Universidade Católica Portuguesa

Rio de Mouro, Portugal

Contact: vmascarenhas@me.com

References

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