Acetabular Impingement Syndrome

Overview

Acetabular impingement syndrome is more commonly termed femoroacetabular impingement syndrome (FAIS). It is a motion-related clinical disorder in which symptomatic contact between the proximal femur and acetabular rim causes hip or groin pain, restricted movement and impaired function.

Cam and pincer morphology are common in people without hip pain. These findings describe bone shape; they constitute a syndrome only when they correspond with the patient’s symptoms and examination findings.

Definition

Femoroacetabular impingement syndrome: A motion-related clinical disorder involving symptoms, clinical signs and imaging findings associated with premature contact between the femur and acetabulum.
Cam morphology: Loss of the normal concavity or sphericity at the femoral head–neck junction, producing a bony prominence that may enter the acetabulum during hip movement.
Pincer morphology: Focal or global overcoverage of the femoral head by the acetabulum.
Acetabular labrum: A fibrocartilaginous ring attached to the acetabular rim that contributes to hip stability, load distribution and maintenance of the joint-fluid seal.
FADIR test: Passive hip flexion, adduction and internal rotation used to reproduce symptoms associated with intra-articular hip pathology.

Anatomy & Physiology

The hip is a ball-and-socket synovial joint formed by the femoral head and acetabulum.

The acetabulum is formed by the ilium, ischium and pubis. Its articular surface is lined by hyaline cartilage, while the acetabular labrum attaches to its rim and deepens the socket.

The proximal femur consists of:

The femoral head
The femoral neck
The greater and lesser trochanters
The femoral head–neck junction

The normal femoral head is approximately spherical. The concave femoral head–neck junction provides clearance between the femur and acetabular rim during hip flexion and rotation.

The acetabular labrum:

Deepens the acetabular socket
Assists joint stability
Distributes load around the acetabular rim
Maintains a negative-pressure fluid seal
Contributes to proprioception

Hip movement is controlled by coordinated activity of the gluteal muscles, deep external rotators, iliopsoas, adductors and trunk musculature. Pelvic position and femoral rotation influence how the femoral head–neck junction approaches the acetabular rim during movement.

Aetiology & Risk Factors

Aetiology

FAIS results from an interaction between hip morphology and movement. The morphology may be femoral, acetabular or mixed.

Cam morphology may develop during skeletal maturation, particularly in adolescents exposed to intensive loading through high-impact or multidirectional sport. The anterosuperior femoral head–neck junction becomes less concave, reducing clearance during hip flexion.

Pincer morphology results from acetabular overcoverage. This may be:

Focal, such as with acetabular retroversion
Global, with excessive coverage of the femoral head

Many individuals have combined cam and pincer features.

Morphology alone does not inevitably cause symptoms. The development of FAIS depends on the interaction between bone shape, activity demands, movement patterns, tissue sensitivity and associated labral or cartilage injury.

Risk Factors

Factors associated with FAIS include:

Participation in high-impact sport during adolescence
Sports involving repeated deep hip flexion, rotation or directional change
Cam, pincer or mixed hip morphology
Reduced hip internal rotation
High sporting or occupational load
Previous hip or groin symptoms
Movement patterns that repeatedly reproduce impingement
Associated acetabular labral or chondral pathology
Participation in sports such as football, hockey, dancing and martial arts

Radiographic morphology is common among asymptomatic people, especially athletes.² It should therefore be interpreted as a potential contributing factor rather than an isolated cause of pain.

Pathophysiology

During hip flexion and rotation, reduced clearance may cause the femoral head–neck junction to contact the acetabular rim.

In cam-type impingement, the non-spherical portion of the femoral head–neck junction enters the acetabulum. This can generate shear forces at the anterosuperior chondrolabral junction, producing:

Separation between the labrum and adjacent cartilage
Acetabular cartilage delamination
Labral tearing
Progressive chondral injury

In pincer-type impingement, acetabular overcoverage causes the femoral neck to contact the acetabular rim. This primarily compresses the labrum and may produce a lever effect with posteroinferior cartilage injury.

Repeated mechanical contact may lead to:

Labral irritation or tearing
Chondrolabral separation
Articular cartilage damage
Pain and movement restriction
Progressive loss of hip function

FAIS is associated with the development of hip osteoarthritis, particularly when cam morphology and cartilage injury are present. However, progression is not inevitable, and the effect of treatment on the long-term risk of osteoarthritis remains uncertain.¹

Why do deep squatting and prolonged low sitting commonly provoke symptoms?

Hip flexion brings the femoral head–neck junction closer to the acetabular rim. Adding adduction or internal rotation further reduces clearance, increasing contact at the anterosuperior joint in a susceptible hip.

Clinical Manifestations

Symptoms usually develop gradually and may be intermittent initially.

Common features include:

Anterior hip or groin pain
Pain described using a “C-sign”, with the hand cupped around the anterior and lateral hip
Pain during deep squatting, lunging or pivoting
Pain after prolonged sitting
Pain when entering or leaving a car
Symptoms during running, kicking or directional change
Reduced sporting performance
Hip stiffness or restricted movement
Clicking, catching or locking
Lateral hip, buttock, thigh or referred knee pain

Pain is often motion related or position related. Clicking and catching may suggest associated labral pathology but are not diagnostic by themselves.

Clinical Examination

Reduced hip flexion
Reduced internal rotation, particularly in flexion
Pain with passive hip flexion
Pain reproduced by the FADIR test
Pain with the FABER test
Reduced hip muscle strength
Altered pelvic or lower-limb control
Pain during a squat or single-leg squat
Antalgic gait in more symptomatic disease

The FADIR test is commonly used to reproduce familiar symptoms. A negative test makes FAIS less likely, but a positive test is not specific and may occur with other intra-articular hip disorders.¹

The lumbar spine, sacroiliac region, abdomen and surrounding hip structures should also be assessed when clinically indicated.

Diagnosis

FAIS is diagnosed when the patient has the recognised triad of symptoms, clinical signs and imaging findings.¹ No single symptom, examination manoeuvre or radiographic measurement is independently diagnostic.

Cam or pincer morphology on imaging is not synonymous with FAIS.

The diagnosis requires concordant symptoms, clinical signs and imaging findings. Treating an asymptomatic radiographic shape is not supported.

Plain Radiography

Plain radiographs are commonly the first-line imaging investigation.

Radiographs assess:

Cam morphology
Acetabular overcoverage or retroversion
Hip dysplasia
Joint-space narrowing
Osteophytes or other features of osteoarthritis
Alternative bone or joint pathology

MRI and MR Arthrography

MRI provides further assessment of:

Acetabular labral injury
Articular cartilage damage
Chondrolabral separation
Bone marrow abnormality
Synovitis
Alternative soft-tissue pathology

CT

CT, including three-dimensional reconstruction, may be used when detailed assessment of femoral or acetabular morphology is required for operative planning. It is not routinely required for every patient.

Diagnostic Injection

An image-guided intra-articular local anaesthetic injection may help determine whether pain arises from the hip joint when the clinical picture is uncertain. A response supports an intra-articular pain source but does not independently establish FAIS.

Mechanical symptoms such as clicking or catching do not automatically indicate a labral tear requiring surgery. These symptoms must be interpreted alongside pain behaviour, examination findings and imaging.

Classification

FAIS is classified according to the predominant underlying morphology.

Cam Morphology

A bony prominence or reduced concavity at the femoral head–neck junction enters the acetabulum during movement. This pattern is particularly associated with shear injury at the anterosuperior chondrolabral junction.

Pincer Morphology

Focal or global acetabular overcoverage causes contact between the femoral neck and acetabular rim. Labral compression is more prominent.

Mixed Morphology

Cam and pincer features coexist. Mixed morphology is common in symptomatic clinical practice.

These categories describe morphology rather than symptom severity. Patients with similar radiographic appearances may have markedly different symptoms and functional limitations.

Treatment

Treatment aims to reduce pain, restore function and enable participation in desired activities. Management should be individualised according to symptom severity, functional goals, morphology, associated tissue injury and the presence of osteoarthritis.

Non-operative Management

Non-operative treatment is generally the initial approach and includes:

Education about the condition
Temporary modification of aggravating activities
Avoidance of repeated painful end-range hip positions
Progressive hip and lower-limb strengthening
Trunk and lumbopelvic strengthening
Movement-pattern retraining
Improvement of balance and neuromuscular control
Gradual return to sport or occupational activity
Analgesia or NSAIDs when clinically appropriate

Manual therapy may be used as an adjunct when pain or soft-tissue and capsular restriction limits movement. It should not be used to force the hip repeatedly into painful impingement positions.

Injections

An intra-articular corticosteroid injection may provide temporary symptom relief in selected patients but does not correct the underlying morphology. Evidence supporting other injectables is variable, and injections should not replace an appropriate rehabilitation programme.

Surgical Management

Referral for specialist surgical assessment may be considered when:

Symptoms remain functionally limiting
The diagnosis is secure
Imaging findings correspond with symptoms and clinical signs
An adequate course of structured non-operative care has not produced acceptable improvement
There is no advanced osteoarthritis that would make joint-preserving surgery inappropriate

Randomised trials in selected patients without established osteoarthritis have found that both structured physiotherapy and arthroscopic surgery can improve symptoms. Arthroscopy produced greater average short-term improvement in some trials, but not every patient benefited and surgery carries procedural risks.^5,6 Shared decision-making should therefore consider symptom burden, prior rehabilitation, imaging, cartilage status, patient goals and the uncertainty around long-term disease modification.

Surgery treats symptomatic FAIS—not an incidental radiographic abnormality.

Appropriate surgical selection requires concordant symptoms, clinical signs and imaging findings, persistent functional limitation, and consideration of joint degeneration and previous non-operative treatment.

Complications & Prognosis

Complications

Potential consequences of untreated or persistent FAIS include:

Recurrent hip or groin pain
Reduced hip movement
Impaired sporting or occupational function
Acetabular labral injury
Chondrolabral separation
Progressive articular cartilage damage
Hip osteoarthritis

Prognosis

Many patients improve with education, activity modification and a structured rehabilitation programme. Recovery may take several months and should be assessed through pain, function, strength, movement quality and participation rather than imaging appearance alone.

Patients undergoing appropriately selected arthroscopic treatment may experience meaningful improvement in pain and function.

Neither conservative treatment nor surgery has been conclusively shown to prevent future hip osteoarthritis.¹ Early identification of cartilage damage and appropriate patient selection are important when considering joint-preserving surgery.