Anatomy and role of the labrum
The acetabulum (acetabulum) is formed by the ilium, ischium, and pubis. Its depth is increased by the fibrocartilaginous labrum (fibrocartilaginous labrum). The labrum forms a ring, increasing joint stability.
Labral injuries are a frequent cause of intra-articular problems. Tears can be traumatic or degenerative. The gold standard for diagnostics is high-field MRI (3T) or MR arthrography (intra-articular contrast injection). Normally, on MRI, the labrum has a triangular shape and low signal intensity. In a tear, contrast or fluid extends beneath the labrum or into its substance. The anterosuperior portions are frequently affected, which can be accompanied by paralabral cysts and adjacent cartilage damage.
Femoroacetabular impingement (FAI)
FAI (femoroacetabular impingement) is a conflict between joint structures leading to labral and cartilage tears. There are two main types that often coexist:
- Cam impingement (cam type): Aspherical femoral head with a bone bump (cam deformity) at the head-neck junction. Creates shearing forces during rotation and abduction. Characteristic of young athletes. Diagnosed via the alpha angle (>55°). A specific sign is the "carpet sign" (detachment of the acetabular cartilage).
- Pincer impingement (pincer type): Excessive coverage of the femoral head by the acetabulum (deep socket, rim hypertrophy, retroversion). More common in middle-aged women. Pelvic radiographs measure the center-edge angle (>40° indicates pathology). A severe form is protrusio acetabuli.
Soft tissue injuries and impingement syndromes
Muscle and tendon injuries (acute and chronic) are well visualized on MRI and ultrasound. In young athletes, hamstrings and adductors are more commonly affected, whereas in older adults, the gluteus medius muscle.
Ischiofemoral impingement: Compression of the quadratus femoris muscle between the ischial tuberosity and the lesser trochanter (space narrowing <15 mm). Leads to edema, muscle atrophy, and sometimes sciatica. Diagnosed via MRI.
Snapping hip syndrome (snapping hip): Can be intra-articular (loose bodies, labral tear) or extra-articular. External type involves friction of the iliotibial tract over the greater trochanter. Internal type involves friction of the iliopsoas tendon over the pubic bone. Extra-articular causes are excellently diagnosed using dynamic ultrasound.
Bone pathologies: AVN and transient osteoporosis
Avascular necrosis (AVN): Has multiple causes (trauma, steroids, alcohol, etc.). Often bilateral. Early diagnosis is only possible via MRI. A specific sign is the double line sign on T2-weighted images, where an outer dark line (sclerosis) borders an inner bright line (granulation tissue).
Transient osteoporosis: A self-limiting syndrome characterized by extensive bone marrow edema of the femoral head and neck. Etiology is unknown. Resolves spontaneously within 6–12 months. Requires differential diagnosis with osteonecrosis of the femoral head (ONFH), fractures, osteomyelitis, and tumors.
Briefly: answers to topical questions
What MRI features are characteristic of different stages of avascular necrosis of the femoral head?
In the early stages of avascular necrosis, MRI allows the detection of diffuse bone marrow edema and subchondral changes. These signs are identified even before the development of cortical collapse, with the presence of edema directly correlating with pain syndrome. MRI has high sensitivity for diagnosing early necrosis, although it is considered less informative for detecting osteochondral fractures. Specific MRI features for other stages of the disease are not described in the materials.
Which ligaments strengthen the fibrous capsule of the hip joint?
The fibrous capsule of the hip joint is strengthened by the following ligaments:
- Iliofemoral ligament (Lig. iliofemorale) — located on the anterior aspect of the joint; limits extension and prevents the body from falling backward during bipedal standing.
- Ischiofemoral ligament (Lig. ischiofemorale) — originates posteriorly to the joint; restricts internal rotation of the femur and, together with the lateral part of the iliofemoral ligament, limits adduction.
- Pubofemoral ligament (Lig. pubofemorale) — blends into the capsule; restricts abduction and limits external rotation.
Class and exam questions
What is the difference between cam and pincer impingement?
Cam impingement is caused by a pathological bone prominence on the femoral neck/head, while pincer impingement is caused by excessive head coverage by a deep acetabulum.
Which method is best suited for diagnosing labral tears?
The gold standard is MR arthrography (or 3T MRI), which allows visualization of small lesions and fluid extravasation beneath the labrum.
What is the "double line sign" on MRI?
This is a specific sign of avascular necrosis, where T2-weighted images reveal a dark line of sclerosis and an adjacent bright line of granulation tissue.
What else to study on this topic
- Radiological criteria for FAI: measurement of the alpha angle and center-edge angle
- Avulsion fractures of pelvic apophyses in children
- Differential diagnosis of femoral head bone marrow edema
- Anatomy and imaging appearance of synovial bursae (trochanteric and iliopsoas bursitis)
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