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What is femoroacetabular impingement?

What is femoroacetabular impingement?

Femoroacetabular hip impingement (FAI) usually refers to a bony impingement that can happen at the front and outer aspect of the hip joint. FAI usually results from altered bony anatomy (namely a pincer or cam-type bony morphology). These can predispose to injury of an important cartilaginous structure within the hip joint “called the labrum”

Relevant anatomy for femoroacetabular impingement

The hip joint is a ball and socket type joint formed by the articulation between the femoral head (the head of the long thigh bone) and the acetabulum (a cup-shaped bone that is part of the pelvis). It forms the socket that accommodates the ball (the femoral head).

Both the femoral head and the acetabular surfaces are covered by "articular cartilage". This is a special tissue that covers the bone surfaces within our joints to ensure smooth and frictionless movement. The labrum is another important structure within the hip joint. It is a special type of cartilage that forms a ring along the acetabular margin. This functions to increase the hip joint stability by deepening the socket, and at the same time allows for reasonable movement of the joint. The hip joint is surrounded by a thin layer of tissue (called the joint capsule) and it contains a special viscous fluid (called synovial fluid) to help in joint lubrication and smooth movement.


What are the causes of femoroacetabular impingement?

Femoroacetabular impingement is usually seen in young and middle-aged people. It is more common in females. FAI usually results from altered bony anatomy, resulting in the femur and acetabulum coming in contact with each other during hip movements (particularly hip flexion). Mainly, two types of variation in hip anatomy can predispose to femoroacetabular impingement:

  1. Cam lesion - refers to a focal bony prominence at the femoral head-neck junction.

  2. Pincer lesion - refers to a focal bony prominence at the outer acetabular margin.


In both conditions, the excess bony tissue will cause impaction on hip flexion. This is referred to as “femoroacetabular impingement”.

It is not uncommon to have mixed-type FAI with both cam and pincer lesions. Hip impingement is a painful condition. The cartilage at the edge of the acetabulum is particularly susceptible to injury secondary to FAI.


An important finding that can be associated with femoroacetabular hip impingement is the presence of a labral tear. This refers to an injury to the special cartilage that forms a ring along the acetabular margin. A labral tear can be painful and interfere with daily activities and exercise.


The labrum is an important structure for hip joint stability as well. Labral tears and FAI can lead to abnormal loading on the hip joint, which can predispose to early thinning of the articular cartilage with time. This is known as hip osteoarthritis. Fortunately, evidence shows that maintaining a good range of movement, muscle strength, and flexibility in the hip is effective in preventing or delaying this process.


How would you know if you have femoroacetabular impingement?

The main symptoms of femoroacetabular impingement are:

  • Hip pain- the pain can be sharp, or a deep and dull ache usually located at the front of the hip or the groin. Less commonly, the pain is felt at the outer aspect of the hip.

  • The pain can become worse after periods of prolonged standing, walking, or even sitting.

  • Hip pain during sports activities that involve rotational movement and sudden change in direction (like football, basketball, and rugby).

  • On examination, the pain can be brought by hip flexion, adduction, and internal rotation.

  • The pain can result in a feeling of joint stiffness and a reduced range of joint movement.

  • Clicking sensation and giving way are also reported in patients with femoroacetabular hip impingement. This could indicate the presence of a labral tear.

What conditions can mimic femoroacetabular hip impingement?

Femoroacetabular hip impingement can be confused with other hip problems. These include:

The above conditions can result in pain at the front of the hip and, therefore, can be mistaken for FAI. Detailed clinical assessment and imaging are very useful in establishing the correct diagnosis.

  • Hip Trochanteric bursitis. This is likely to be confused with FAI as it usually results in pain at the outer aspect of the hip.

Femoroacetabular hip impingement vs hip osteoarthritis

Both conditions can result in hip pain and stiffness. The symptoms are usually more gradual in hip osteoarthritis. Detailed clinical assessment and imaging are very useful in differentiating the two conditions. A major differentiating factor between the two is the patient's age. FAI tends to involve younger patients between 20 and 40, while hip osteoarthritis is more prevalent in people over 50.

How is femoroacetabular hip impingement diagnosed?

Detailed clinical assessment by a specialist orthopaedic doctor is essential. Usually, the diagnosis is suspected clinically and confirmed on imaging. Imaging is almost always required to confirm the diagnosis, determine the type of impingement and assess for the presence of any labral tear.

X-ray is usually the first imaging modality used. It is helpful to assess the bony morphology of the hip joint for any pincer or cam variation. It will also assess for any arthritic changes within the joint.


The femur and acetabulum come in contact with each other during hip flexion.

Ultrasound can assess excess fluid within the hip joint, "called joint effusion," and inflammation. It can only visualise the part of the labrum at the hip's front. To have a more detailed assessment, a hip MRI examination is recommended. The MRI will assess the bony morphology, the labrum, the articular cartilage, and the tendons and soft tissues around the hip joint.

Ultrasound guidance is instrumental if an ultrasound-guided hip joint injection is indicated. Plenty of evidence shows that injections done under ultrasound guidance result in better outcomes, with better pain relief and improved function.

What is the treatment for femoroacetabular hip impingement?

Management usually starts with conservative measures, including physiotherapy, patient education, and exercise modification. Physiotherapy usually consists of hip stretching and muscle-strengthening exercises. If the response from physiotherapy is not satisfactory, and the imaging has shown the presence of a labral tear, then having a surgical consultation would be helpful. The surgeon will discuss the surgical treatment options with you, including minor keyhole surgery to repair the labral tear. Ultrasound-guided hip joint steroid or local anaesthetic injection might be appropriate for the management.

When to consider a hip joint injection for femoroacetabular hip impingement?

An ultrasound-guided steroid injection might be considered part of managing femoroacetabular hip impingement, especially if your pain significantly interferes with daily activities, exercises, and rehabilitation. A steroid is a potent anti-inflammatory medicine and can provide effective pain relief by reducing the inflammatory process within the hip joint. The injection aims to provide a window of opportunity for you to achieve more effective rehabilitation.

Sometimes, we perform an ultrasound-guided local anaesthetic injection. This involves injecting a numbing medicine only into the hip joint. This is considered more like a test. If you notice significant pain relief following the injection, this will confirm that the injected hip is the cause of your pain. The result from the injection will be valuable for further management decisions, especially if surgical treatment is contemplated. It is essential to use ultrasound guidance when performing such injections.

Should hip injections be done under ultrasound guidance?

Yes. This is our routine practice, as plenty of evidence supports ultrasound guidance when performing musculoskeletal injections. The hip joint is a deep structure, and ultrasound is essential to ensure the correct needle placement into the hip joint. Doing injections under ultrasound/imaging guidance allows for direct visualisation of the needle to ensure accurate placement into the area of pain/inflammation (like a bursa, an arthritic joint, or an inflamed tendon sheath). Ultrasound guidance results in more accurate, less painful, and faster procedures, with better outcomes than these injections without guidance. Ultrasound guidance avoids sensitive structures (like nerves and vessels) during the procedure.

What are the possible side effects of a steroid injection?

Generally, these injections are very safe and routinely done in our practice. There is a minimal risk of infection (about 1:10.000). The injected area may feel sore for the first few days. This is referred to as (steroid flare) and can be seen after a steroid injection. The procedure will be explained in detail during your appointment, and all your questions will be addressed. To find out more about cortisone injection in general, please see our FAQs.

How long will the effect of a hip cortisone injection last?

Current evidence suggests that cortisone can improve pain and function for up to 3 months, but in some cases, it can last longer. The injections usually also contain a local anaesthetic that provides immediate pain relief lasting a few hours.

How soon will a steroid injection start to work?

A steroid injection usually takes a few days (1-3) before you notice the effect, although sometimes the pain relief can start on the same day. The injected area will often feel sore for the first few days. This is referred to as (steroid flare) and can be seen after a steroid injection.

How many steroid injections can I have for femoroacetabular hip impingement?

The injection aims to suppress the inflammation within the joint, providing a window of opportunity for you to manage the underlying condition and undergo effective rehabilitation. We recommend reducing the number of cortisone injections by combining any injection therapy with an effective physiotherapy program to address the underlying cause. Repeated steroid injection into the same area should be avoided if the previous injection was less than 3-4 months ago.​

How long should I rest after a hip injection?

Usually, we advise patients to rest for 48-72 hours after having a hip injection. This can vary depending on the type of treatment and severity of the condition. Usually, patients who have a steroid injection are usually advised to rest for 48 hours.

Do steroid injections just hide/mask the pain?

Steroid injections do not just mask or hide the pain, but they act by reducing the inflammation in the targeted area, thus providing a solid and local anti-inflammatory effect to help control the symptoms and allow the patient to manage the condition, usually by undergoing effective rehabilitation.

Causes of femoroacetabular impingement
Clinical featurs femoroacetabular impingement
Conditions that can mimic FAI
Diagnosis of hip FAI
Treatment of hip impingement
Ultrasound-guided injections in hip impingement
FAQs about injections in hip impingement
FAI vs. hip osteoarthritis

Specialist Consultant Musculoskeletal Radiologist Doctor with extensive experience in image-guided intervention

To book a consultation:

Call us on 020 3058 909or Book online

The Musculoskeletal Ultrasound & Injections clinic

Healthshare West London (The Riverside) Clinic
Unit 3, Brentside Executive Park

Brentford, TW8 9DR

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