MRI Hip

MRI of the hip is the best test for the femoral head, acetabular labrum, articular cartilage, bone marrow, tendons and muscles around the hip. It is the most sensitive test for early osteonecrosis, occult fractures and labral injury.

The study uses a 1.5 or 3 tesla scanner with a body or flexible surface coil. For a single hip, a small field of view in coronal, sagittal and oblique axial planes shows the joint in detail.

For the pelvis and both hips, a larger field of view is used. Slices are 3 to 4 mm thick. The examination takes 20 to 30 minutes.

The core sequences are coronal T1-weighted, fat suppressed or short tau inversion recovery coronal images, and proton density fat suppressed images in sagittal, coronal and oblique axial planes.

Direct MR arthrography (diluted gadolinium injected into the joint under image control) improves detection of labral tears and chondral defects. Radial sequences perpendicular to the femoral neck show the labrum in detail.

Hip pain is common in adults. Femoroacetabular impingement is found in 10 to 15% of the general population by imaging. Osteonecrosis of the femoral head affects 10 000 to 20 000 people each year in the United States, and is a major cause of hip replacement in people under 50.

MRI has a sensitivity of over 95% for early osteonecrosis, and over 90% for occult femoral neck fractures. For labral tears, sensitivity is 60 to 70% without arthrography and 90% with it.

Indications.

• Hip or groin pain with normal radiographs, especially suspected osteonecrosis or occult fracture.
• Suspected femoroacetabular impingement, labral tear and chondral injury.
• Hip dysplasia in adolescents and young adults.
• Stress and insufficiency fractures (femoral neck, pubic rami, sacrum).
• Septic arthritis, osteomyelitis, soft tissue abscess and pyomyositis.
• Inflammatory arthritis, transient bone marrow oedema, and synovial disease.
• Bone and soft tissue tumours, metastases and marrow disease.
• Tendon and muscle injury (gluteal tears, iliopsoas, hamstring origin, adductor), trochanteric pain syndrome and bursitis.
• Sacroiliac joint disease and sports hernia (pubic symphysis, adductor attachment).
• Painful hip arthroplasty: loosening, infection, metal reaction.
• Children with limp: osteonecrosis in children, slipped femoral epiphysis, infection and tumours.

Not for classic osteoarthritis on radiographs, or for isolated acute fracture clearly seen on radiograph.

Contraindications and precautions.

• The usual MRI safety screen. Most hip prostheses are safe but cause artefact.
• Gadolinium: renal precautions. Intra-articular injection needs a sterile technique.
• Pregnancy: non-contrast MRI is acceptable.
• Claustrophobia and large body size.
• Patients unable to lie flat because of severe hip pain: analgesia first.

Preparation and technique.

No preparation is needed for non-contrast MRI. The patient lies supine with the feet together and slightly internally rotated, and the hips are held still.

A coronal T1 and fat suppressed series covers both hips and the pelvis to find marrow disease and avoid missing a contralateral lesion (osteonecrosis is bilateral in over half of cases). A dedicated small field series of the symptomatic hip is then taken.

Metal artefact reduction sequences are used after arthroplasty. Direct arthrography uses injection under fluoroscopy or ultrasound with 10 to 15 mL of dilute gadolinium and local anaesthetic.

Normal appearances.

The femoral head is round and the marrow is bright on T1, with a thin subchondral dark line. The acetabulum is cup shaped, with a dark triangular labrum that is continuous around the rim except at the transverse ligament inferiorly.

Hyaline cartilage is thin, about 2 to 3 mm, and smooth. A small amount of fluid in the joint is normal. The femoral neck has a normal junction with the head with a concave contour.

Gluteal, iliopsoas, adductor and hamstring tendons are dark and continuous. The bursae contain little or no fluid. Muscle bulk is symmetrical.

Interpretation.

• Osteonecrosis: a serpiginous low signal line (double line on T2) in the anterosuperior femoral head, surrounding the necrotic segment, with early marrow oedema. The subchondral fracture appears as a low signal line parallel to the surface, and collapse follows. The extent and the proportion of the head involved predict collapse.
• Transient bone marrow oedema: diffuse oedema of the head and neck without a discrete line, a self-limiting condition in men in middle age and in pregnancy. Differentiate from early osteonecrosis.
• Occult fracture: a linear low signal line with surrounding oedema in the femoral neck, trochanter, pubic rami or acetabulum.
• Stress fractures: periosteal and marrow oedema, with a fracture line in the compression side of the femoral neck.
• Femoroacetabular impingement: cam morphology (loss of the normal concavity at the head neck junction, bump), pincer morphology (over coverage of the acetabulum, deep socket), labral tears, and anterosuperior chondral damage.
• Labral tear: contrast or fluid extending into or through the labrum, detached or irregular labrum, and paralabral cyst. Degenerative labral signal is common with age.
• Cartilage injury: thinning, fissures, delamination and full thickness defects, best seen with arthrography.
• Dysplasia: shallow acetabulum with reduced lateral coverage of the head, labral hypertrophy, and cartilage damage.
• Osteoarthritis: cartilage loss, osteophytes, subchondral cysts and bone marrow lesions.
• Septic arthritis and osteomyelitis: effusion, synovitis, marrow oedema, abscess and soft tissue collections, with rim enhancement.
• Inflammatory arthritis: synovitis, erosions and marrow oedema. Pigmented villonodular synovitis causes blooming low signal.
• Tendon and bursa: gluteus medius and minimus tendinosis and tears at the greater trochanter, trochanteric bursitis, iliopsoas tendinosis and bursitis, hamstring origin tears, and adductor injuries.
• Muscle injury: strain (feathery oedema), partial or complete tear, haematoma, and myositis ossificans.
• Sports hernia and pubic symphysis: parasymphyseal oedema, a cleft at the symphysis, and adductor aponeurosis tears.
• Tumours: osteoid osteoma (nidus with surrounding oedema), chondroblastoma, giant cell tumour, chondrosarcoma, metastases, myeloma, and soft tissue sarcomas. Marrow replacement shows low T1 signal.
• Arthroplasty: periprosthetic osteolysis, loosening, infection (fluid collection, synovitis), and adverse reaction to metal debris (cystic or solid pseudotumour).
• Children: epiphyseal osteonecrosis (femoral head flattening, widened joint space), slipped capital femoral epiphysis (widening and displacement of the physis, oedema), and transient synovitis.

Critical values and findings needing immediate action.

• Septic arthritis or osteomyelitis: urgent aspiration, drainage and antibiotics.
• Displaced femoral neck fracture: urgent orthopaedic fixation or replacement. An occult neck fracture also needs protected weight bearing and fixation.
• Slipped capital femoral epiphysis: non-weight bearing and urgent pinning.
• Pre-collapse osteonecrosis with high risk: orthopaedic referral for head preserving surgery.
• Aggressive bone or soft tissue tumour: sarcoma centre referral before biopsy.
• Pseudotumour or large periprosthetic collection: revision surgery review.

Limitations and artefacts.

• Labral tears, cartilage change and cam morphology are common in people without symptoms, so findings must match the pain.
• Non-arthrographic MRI has limited sensitivity for labral and cartilage lesions. Direct arthrography is more accurate.
• Normal variants mimic tears: the labral sulcus at the anterior and posterior edges, perilabral recess, and the stellate crease of cartilage at the acetabulum.
• Bone marrow oedema is non-specific: osteonecrosis, fracture, transient oedema, infection, tumour and arthritis all cause it. Correlate with the line pattern.
• Early osteonecrosis (within days) may be missed, and the first MRI can be negative in very early fractures. Repeat after 3 to 7 days if suspicion persists.
• Hip arthroplasty causes major artefact. Metal artefact reduction sequences help but limit accuracy.
• Red marrow in children and young adults mimics marrow infiltration.
• Motion and hip pain limit quality.

Special groups.

• Children: slipped femoral epiphysis, osteonecrosis, infection, and tumours. Sedation may be needed.
• Pregnant patients: transient oedema and stress fractures are more likely. MRI without contrast is used.
• Athletes: labral, cartilage, tendon and pubic injuries, with early arthrography.
• Patients on corticosteroids or with sickle cell disease, alcohol excess or HIV: high osteonecrosis risk, with imaging of both hips.
• Older adults with a suspected neck fracture and normal radiographs: MRI is the best test, and CT is the alternative.
• Regions: septic arthritis, tuberculosis of the hip, sickle cell osteonecrosis, HIV-related osteonecrosis, neglected fractures and developmental dysplasia are frequent. MRI is scarce, so radiographs, ultrasound and CT are used, with aspiration for infection.

Follow up tests.

• Radiographs of the pelvis and hip (anteroposterior and lateral views).
• Ultrasound for effusion, tendon, bursa and injection under ultrasound.
• CT for bone detail, morphology and surgical planning.
• Bone scan or SPECT if MRI is not possible.
• Joint aspiration for cell count, crystals, Gram stain and culture.
• Blood tests: full blood count, inflammatory markers, calcium, vitamin D, alkaline phosphatase, sickling status, HIV test, lipid profile.
• Bone density scan for insufficiency fractures.
• Referral to orthopaedics, sports medicine, rheumatology or oncology.