Cam-Type Femoroacetabular Impingement

Cam-type femoroacetabular impingement (FAI) results from an aspherical femoral head with loss of head-neck offset, creating a bump that impinges on the acetabular rim during hip flexion and internal rotation. It predominantly affects young athletic males and causes anterosuperior labral and cartilage damage through an outside-in shear mechanism (acetabular cartilage delamination from the labrum). Diagnosis requires an alpha angle greater than 55° on lateral radiographs or MRI (Dunn view). The "pistol grip" deformity is visible on AP pelvis, and clinical examination reveals a positive FADIR test. Treatment is femoral osteochondroplasty (arthroscopic or open surgical dislocation) to restore head sphericity, with concurrent labral repair. Combined cam-pincer morphology is actually most common (86% of FAI cases).

Femoroacetabular impingement (FAI) is a recognized cause of hip pain and early osteoarthritis, particularly in young adults. Cam-type FAI is the most common form, characterized by an aspherical femoral head.

Epidemiology:

• Prevalence: Cam morphology present in 10-15% of asymptomatic population
• Demographics: Young athletic males predominantly affected
• Age: Typically presents 20-40 years of age
• Sports: High risk in soccer, hockey, martial arts, rowing
• Association: Strong link to early-onset hip osteoarthritis

Cam-type femoroacetabular impingement (FAI) occurs when an aspherical femoral head impinges on the acetabulum during flexion and internal rotation. The "cam" refers to the bump that acts like a cam in an engine, causing abnormal contact.

Pathoanatomy

The cam lesion is an abnormal bony prominence at the anterosuperior femoral head-neck junction, causing loss of normal head-neck offset. This may result from abnormal epiphyseal development, particularly in young athletes during growth.

Mechanism of Damage

During hip flexion and internal rotation, the cam bump is driven into the anterosuperior acetabulum. This causes shear forces that strip the acetabular cartilage from the labrum ("outside-in" damage). The labrum is also crushed. This differs from pincer mechanism.

History

Young athletic males typically present with groin pain. Pain is activity-related, particularly with sports requiring hip flexion (soccer, hockey, martial arts). May describe mechanical symptoms such as clicking or catching. Prolonged sitting may aggravate symptoms.

Examination

FADIR Test (Anterior Impingement Test): Flexion, Adduction, Internal Rotation reproduces groin pain. This is the most sensitive test for FAI.

FABER Test: Flexion, Abduction, External Rotation assesses SI joint but also stresses the hip.

Range of Motion: Internal rotation in flexion is typically reduced and painful.

Gait: Usually normal unless advanced damage.

ImagingClassification

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AP Pelvis: May show "pistol grip" deformity (loss of concavity at head-neck junction). Assess for coxa profunda or other acetabular abnormalities suggesting combined morphology.

Lateral View (Dunn, Cross-Table, Frog Lateral): Essential for measuring alpha angle. Shows anterosuperior bump.

Alpha Angle: Angle between the femoral neck axis and a line from the center of the femoral head to the point where the head becomes aspherical. Greater than 55° indicates cam morphology.

MRI/MRA: Shows labral and cartilage damage. Radial sequences best for assessing cam lesion circumferentially. Arthrogram improves sensitivity for labral tears.

Alpha Angle Classification:

• Normal: Less than 50°
• Borderline: 50-55°
• Cam morphology: Greater than 55°

Combined FAI: Most common presentation (approximately 86%). Both cam and pincer elements present.

Severity: Based on cartilage and labral damage on MRI/arthroscopy.

Differential Diagnosis of Young Adult Hip / Groin Pain

A negative response to an intra-articular local anaesthetic injection (less than 50% pain relief) should prompt reconsideration of an extra-articular or referred cause before any joint-preserving surgery.

A focused, stepwise work-up confirms the diagnosis, grades joint damage and excludes mimics.

• Plain radiographs (first line) — AP pelvis (pistol-grip deformity, acetabular over-cover, Tonnis grade) plus a lateral (Dunn 45/90, cross-table or frog-leg) to measure the alpha angle and assess head-neck offset.
• Alpha angle — over 55 degrees supports cam morphology; cohort data suggest 60 degrees for a definite deformity and around 78 degrees for a pathological deformity.
• MRI / MR arthrography — assesses the labrum, chondral surfaces (Outerbridge grade) and the cam lesion circumferentially (radial sequences); arthrography improves sensitivity for labral tears.
• CT (3D reconstruction) — best for precise bony mapping of the cam lesion and surgical planning, and for assessing version.
• Diagnostic intra-articular local anaesthetic injection — confirms the hip joint as the pain source (expect over 80% temporary relief) and helps exclude extra-articular causes; under 50% relief should prompt reconsideration of the diagnosis.

Disease-related

• Progressive anterosuperior chondrolabral damage and labral tearing
• Secondary osteoarthritis, particularly with persistently elevated alpha angles

Surgery-related (arthroscopic osteochondroplasty)

• Under-resection — residual cam morphology is the commonest cause of persistent symptoms and revision arthroscopy
• Over-resection — risk of femoral neck stress fracture and iatrogenic hip instability
• Iatrogenic chondral/labral injury during portal placement or traction
• Traction-related neurapraxia (pudendal, lateral femoral cutaneous, sciatic) — usually transient
• Heterotopic ossification (reduced with NSAID prophylaxis), adhesions, and rarely deep infection or venous thromboembolism
• Progression to total hip arthroplasty — markedly more likely when pre-existing OA (Tonnis 2 or more, Outerbridge III-IV) is present

Open surgical dislocation

• Trochanteric osteotomy non-union or hardware irritation
• Avascular necrosis is rare when the medial femoral circumflex artery is protected

• Surgery vs conservative care. UK FASHIoN (Lancet 2018) favoured arthroscopy over personalised physiotherapy at 12 months by a clinically significant margin, whereas the smaller military FAIT trial (Am J Sports Med 2018) found no difference at 2 years with very high crossover. The two best Level I trials therefore point in different directions, making patient selection and shared decision-making central.
• Which alpha-angle threshold? Commonly quoted cut-offs range from 50 to 60 degrees for "cam morphology," while cohort data (Agricola, CHECK/Chingford) suggest 60 degrees for presence of a deformity and around 78 degrees for a pathological deformity that predicts end-stage OA. There is no single agreed number; the morphology must be symptomatic.
• Morphology is not disease. Cam morphology is present in 10-25% of asymptomatic adults and is far higher in adolescent athletes. The Warwick Agreement defines FAI syndrome as the triad of symptoms, clinical signs and imaging findings — imaging morphology alone is never an indication for surgery.
• Does correction prevent OA? Cam morphology is a strong risk factor for hip OA, but there is no high-quality evidence that prophylactic osteochondroplasty in an asymptomatic hip prevents future OA, so prophylactic surgery is not recommended.
• Adequacy of resection. Both under-resection (residual cam, the commonest reason for revision arthroscopy) and over-resection (risk of femoral neck fracture, iatrogenic instability) are recognised failure modes; intra-operative imaging and dynamic assessment are used to titrate resection.
• Labrum: repair vs debridement vs reconstruction. Repair is generally favoured over debridement where tissue quality allows; reconstruction is reserved for irreparable or deficient labra, with evolving evidence.

Global epidemiology

• Cam morphology is found in roughly 10-25% of asymptomatic adults and is markedly over-represented in adolescent and adult athletes in flexion-loading sports (football/soccer, ice hockey, basketball), where prevalence in some male cohorts approaches or exceeds 30%.
• It develops during skeletal maturation, more in males, and high-impact sport during the growth spurt is associated with larger cam morphology.
• Symptomatic FAI presents typically in active 20-40 year olds; cam morphology is a recognised risk factor for progression to hip osteoarthritis.

Side-by-side guidance

Registry & long-term data

• Hip-preservation procedures are not as comprehensively captured by joint registries as arthroplasty, but conversion to total hip arthroplasty (THA) is the key registry-relevant outcome: pre-existing osteoarthritis (Tonnis 2 or more, joint space under 2 mm, Outerbridge III-IV) strongly predicts early conversion to THA after arthroscopy.
• For patients who ultimately undergo THA, large national registries (NJR — UK, AJRR — USA, AOANJRR — Australia, Swedish and Norwegian registries) consistently show higher revision rates in younger, more active patients, reinforcing the value of delaying arthroplasty in this young population.

High- vs limited-resource practice variation

• In well-resourced settings, MR arthrography, dedicated hip arthroscopy and open surgical dislocation are available, and management is individualised with formal physiotherapy programmes.
• In limited-resource settings, diagnosis relies on plain radiographs (AP pelvis plus a lateral such as Dunn or cross-table for the alpha angle) and clinical examination; structured activity modification and physiotherapy are first-line, with surgery reserved for clear, refractory cases at referral centres.