Pelvis and Hip X-Ray

Pelvic and hip radiography shows the pelvic ring (sacrum, ilia, ischia, pubic rami, symphysis pubis), the sacroiliac joints, the acetabula, the proximal femora and the hip joints. The basic film is a supine anteroposterior (AP) view of the pelvis with the legs internally rotated 15 degrees to show the femoral necks in profile.

For a symptomatic hip, an AP view of the hip and a lateral view (cross-table lateral, or a frog leg lateral in children and in non-traumatic disease) are added. Judet oblique views (45-degree obturator and iliac obliques) show acetabular fractures, and inlet and outlet views assess pelvic ring displacement.

The effective dose is about 0.6 mSv for an AP pelvis and 0.3 mSv for a hip, and gonadal doses are relatively high, with shielding where it does not hide anatomy.

Plain films are the first test for hip pain, hip fracture, pelvic trauma, osteoarthritis, inflammatory disease, developmental and paediatric hip disorders, and bone tumours. Hip fractures are common (over 1.5 million a year worldwide) with a 1-year mortality of 20 to 30% in older adults, and early recognition and surgery within 24 to 36 hours improve survival.

Occult fractures are missed on initial films in 2 to 10% of cases, and MRI (sensitivity near 100%) or CT is needed when a fracture is suspected but films are normal. In the polytrauma patient, a pelvic film is part of the primary survey in many trauma services, though CT is used when the patient is stable.

Indications. Hip or groin pain after a fall, or inability to bear weight. Pelvic trauma. Chronic hip pain (osteoarthritis, avascular necrosis, inflammatory arthritis, tumour). Limp or hip pain in a child (slipped capital femoral epiphysis, Perthes disease, developmental dysplasia, septic arthritis, transient synovitis). Suspected sacroiliitis (spondyloarthropathy).

Follow up of arthroplasty and fracture fixation (loosening, infection, periprosthetic fracture). Suspected metastases or myeloma. Paget disease. Screening in infants for developmental dysplasia after 4 to 6 months of age (ultrasound is used before this). Not for routine imaging of mild chronic hip pain without functional impairment.

Contraindications and precautions. Pregnancy: avoid unless essential; MRI or ultrasound is preferred. Trauma patients with suspected pelvic fracture should not be rolled or log rolled repeatedly, and a pelvic binder is applied before further movement if there is haemodynamic instability. Do not force internal rotation of the limb when a hip fracture is suspected, and use a cross-table lateral rather than a frog leg view.

Preparation and technique. AP pelvis supine, with the beam centred midway between the anterior superior iliac spines and the pubic symphysis, and symmetrical positioning (no rotation or tilt). Cross-table lateral of the hip with the unaffected leg raised out of the way. In children, a frog leg lateral with both hips abducted, and gonad shields are used only where they do not obscure the area of interest.

Normal appearances. The pelvic ring is symmetrical and continuous, with equal obturator foramina and sacroiliac joints. Shenton line (a smooth continuous arc from the inferior border of the superior pubic ramus along the femoral neck) and the iliopectineal and ilioischial lines are unbroken. The femoral neck shaft angle is about 125 degrees (120 to 135).

The joint space is uniform (3 to 5 mm), the acetabular roof is smooth, and the lateral centre edge angle is 25 to 40 degrees. The femoral head is spherical and covered, and the sacroiliac joint is 2 to 4 mm wide with smooth sharp margins.

Interpretation.

• Femoral neck fracture: break in the cortex or trabecular pattern, with loss of Shenton line, shortening and external rotation. Classified by displacement: Garden I (incomplete or valgus impacted), II (complete, undisplaced), III (partially displaced) and IV (fully displaced); displaced fractures carry high avascular necrosis risk and are treated by arthroplasty in older patients.
• Intertrochanteric and subtrochanteric fractures: oblique or comminuted fractures between the trochanters, with varus angulation, assessed for stability (lesser trochanter and posteromedial cortex involvement) and fixation planning.
• Occult hip fracture: subtle sclerosis line, cortical irregularity or trabecular interruption, or negative films with persisting pain and inability to bear weight; MRI, CT or bone scan follows.
• Pelvic ring fractures: disruption of the ring in two or more places, with pubic symphysis widening over 2.5 cm and sacroiliac joint widening suggesting an unstable "open book" injury. Acetabular fractures show breaks in the iliopectineal (anterior column) and ilioischial (posterior column) lines, the teardrop, the roof and the wall; Judet views and CT are needed.
• Hip dislocation: posterior (about 90%, with the femoral head smaller than the opposite side and higher and behind the acetabulum, the limb shortened and internally rotated) or anterior; check for associated fracture of the head or acetabulum.
• Osteoarthritis: superolateral joint space narrowing, osteophytes, subchondral sclerosis and cysts, and femoral head migration.
• Inflammatory arthritis: uniform (axial) joint space loss, erosions, protrusio acetabuli (rheumatoid arthritis), sacroiliitis (blurred margins, erosions, sclerosis, ankylosis) in spondyloarthropathy.
• Avascular necrosis: early sclerosis and subtle lucency, the "crescent" sign (subchondral lucency), and later flattening and collapse of the femoral head with secondary osteoarthritis (staged by the extent of collapse). Normal films occur in early disease and MRI is required.
• Slipped capital femoral epiphysis: widening of the physis and displacement of the epiphysis relative to the neck (Klein line drawn along the superior femoral neck does not intersect the epiphysis), best seen on the frog leg lateral; common in overweight adolescents.
• Perthes disease: small, sclerotic, fragmented and flattened femoral epiphysis in a child aged 4 to 8 years, with widening of the joint space early on.
• Developmental dysplasia: shallow acetabulum with an acetabular index over 30 degrees at age 1 year, an upward and lateral femoral head (outside the Perkin square) and a broken Shenton line; ultrasound is used under 4 to 6 months of age. In adults, a lateral centre edge angle under 20 degrees indicates dysplasia and 20 to 25 degrees borderline.
• Femoroacetabular impingement: cam deformity (reduced head neck offset, alpha angle over 55 degrees) or pincer deformity (over coverage, crossover sign).
• Infection and tumour: lytic or sclerotic lesions, periosteal reaction, joint destruction and sequestra; Paget disease shows cortical thickening and coarse trabeculae; tuberculosis of the hip shows osteopenia and slow joint space loss with erosion (Phemister triad).
• Arthroplasty: component position, lucent lines around components (loosening, infection), periprosthetic fracture, wear and dislocation.

Critical values and findings needing immediate action.

• Displaced femoral neck fracture or any hip fracture in an older adult: analgesia, resuscitation, orthogeriatric review and surgery within 24 to 36 hours.
• Unstable pelvic ring fracture or pelvic fracture with haemodynamic instability: pelvic binder, massive transfusion as required, interventional embolisation or pre-peritoneal packing, and urgent trauma team review.
• Posterior hip dislocation: urgent reduction under sedation or anaesthesia within 6 hours to reduce the risk of avascular necrosis, with a CT after reduction.
• Slipped capital femoral epiphysis: immediate non-weight-bearing and urgent orthopaedic surgical fixation.
• Painful hip with fever and inability to bear weight in a child, with raised inflammatory markers: suspected septic arthritis, an emergency for urgent ultrasound, aspiration and surgical washout.
• Suspected hip fracture with normal films: MRI or CT within 24 hours, and protected mobilisation until excluded.
• Periprosthetic fracture or infected arthroplasty: urgent orthopaedic review.

Limitations and artefacts.

• Plain films miss 2 to 10% of hip fractures (undisplaced or impacted fractures), early avascular necrosis, stress fractures, early sacroiliitis, occult pelvic ring injury and early infection; negative films with persistent symptoms need further imaging.
• Rotation and tilt of the pelvis alter symmetry, apparent leg length, acetabular measurements and the appearance of the obturator foramina and the sacroiliac joints; check symmetry before interpreting.
• Overlying bowel gas, faeces and contrast can hide fractures of the sacrum, and the sacroiliac joints are poorly seen on AP films.
• The cross-table lateral is more reliable than the frog leg view for fracture, and the frog leg view can displace an unstable fracture.
• Mimics of fracture include vascular channels, nutrient canals, Mach bands and the normal sutures of the acetabulum in children; normal variants (os acetabuli, bone islands) are common.
• Hip osteoarthritis severity on films does not reliably predict pain or function.
• Gonadal radiation is not trivial, especially in children, and follow up pelvis films should be limited to clinical need.

Special groups.

• Children: use ultrasound for developmental hip dysplasia before the ossification of the femoral head (4 to 6 months); AP and frog leg views for limp, with MRI to detect early Perthes disease and for septic arthritis; shield the gonads when appropriate.
• Older adults with a fall and hip pain: low threshold for MRI or CT when films are normal.
• Patients with sickle cell disease, corticosteroid use, alcohol excess or HIV: avascular necrosis is common, and early MRI is needed.
• Patients with cancer or myeloma: pathological fracture risk, and MRI or CT for staging and fixation planning.
• Pregnancy: avoid; ultrasound or MRI instead.
• Regions: tuberculosis of the hip, paediatric septic arthritis and slipped epiphysis, sickle cell avascular necrosis, and hip fractures in older people are frequent causes of presentation; plain films with high suspicion are the primary tool, and delays to surgery are a major determinant of outcome.

Follow up tests. MRI of the hip or pelvis for occult fracture, avascular necrosis, infection, tumour and sacroiliitis, CT pelvis with three dimensional reconstruction for acetabular and pelvic ring fractures, ultrasound for developmental dysplasia and effusion, hip aspiration with cell count, Gram stain and culture for septic arthritis, full blood count, C-reactive protein,

erythrocyte sedimentation rate, HLA-B27, bone density scan, serum calcium, vitamin D and parathyroid hormone, bone scintigraphy, and orthopaedic review.