Hip Radiography

Hip radiography (or hip X-ray) uses a small amount of radiation to produce images of the hip joints, which attach the legs to the pelvis. A radiographic examination is the most basic and critical method of diagnosing various hip disorders⁽¹⁾.

An appropriate image testing and thorough understanding of standard radiographic techniques can help radiologists achieve better diagnostic accuracy.

Hip radiography can help determine the cause of common signs and symptoms, such as pain, limping, swelling, tenderness, or deformity in the hip area⁽²⁾. 

This imaging technique can also detect a dislocated joint, broken bones, bone cysts, tumors, and hip joint infections⁽³⁾. If a patient needs hip surgery, an X-ray may be taken before and after the surgery. 

How Does Hip Radiography Work?

During the radiographic test, an X-ray machine sends a radiation beam through the pelvic bones and hip bones to record the X-ray images. 

The images are recorded on a computer program or special film and show the bones and soft tissues of the hip joints and pelvis. The X-ray image appears in black and white.

Dense body parts, like bones, block the passage of the X-ray beam through the body and appear white on X-ray images. Softer body tissues, like skin and muscles, allow the X-ray beam to pass through the body and appear darker on X-ray images⁽⁴⁾.

Hip Radiography Techniques

Hip joint images can be taken from various angles. A standard hip radiographic examination generally includes an anteroposterior (AP) image and a lateral image (also known as “frog leg”).

The anteroposterior hip image shows both hip joints to allow comparison with the other hip. The distance between the film and the X-ray tube should be 1.2 meters⁽⁵⁾. 

Patients lie on their back, and the X-rays pass through the hip joint from anterior to posterior. The leg is then internally rotated 15˚ to 20˚ to achieve femoral anteversion, extending the femoral neck and improving its evaluability⁽⁶⁾. 

One of the most common mistakes in AP hip radiographs is taking the images in a supine position (lying on one’s back with the face upward). The image becomes distorted as the hip is externally rotated⁽⁷⁾.

The image is taken in a supine position towards the middle of the line connecting the upper symphysis pubis and anterior-superior iliac spine. 

There are various imaging techniques for lateral hip radiography, such as the frog-leg lateral view, cross-table lateral view, and Löwenstein view.

In the frog-leg lateral view, images show both sides of the hips. The X-ray passes through the hip joint from medial to lateral. 

The knee joint is flexed 30° to 40° in a supine position. Meanwhile, the hip is externally rotated by 45°⁽⁸⁾.

Frog-leg lateral images are particularly useful in evaluating the femoral head shape and head or neck transitions to confirm conditions, like epiphysiolysis and avascular necrosis⁽⁹⁾. 

In the cross-table lateral view, the lower extremity is internally rotated by 15° to 20° in a supine position⁽¹⁰⁾. The flexing of hip and knee joints on the other side prevents interference in radiographic projection. 

A radiographic cassette is positioned on the side of the hip to project toward the groin region at 35° to 45°⁽¹¹⁾.

In the Löwenstein view, patients are turned onto the affected hip at least 45° with the hip flexion angle of 90° and internal rotation angle of 45° in a supine position. Hip images on each side are taken vertically from the groin region⁽¹²⁾. 

Pathology

Proximal Femoral Fractures

Proximal femoral fractures occur regularly in elderly patients with osteoporosis. These fractures may be caused by a simple fall or high-energy trauma in younger people⁽¹³⁾. 

The shortening and exorotation of the affected leg is a classical clinical presentation of proximal femoral fractures⁽¹⁴⁾.

Intracapsular Fractures

Intracapsular fracture of the hips includes the femoral head and neck. The femoral neck is the proximal femur’s weakest point. 

The intracapsular fracture is associated with an increased risk of intracapsular vascular damage. This fracture may cause devascularization of the femoral head and eventually avascular necrosis⁽¹⁵⁾.

Due to interrupted blood supply and limited intrinsic stability, the intracapsular fracture also entails a higher risk of nonunion or malunion fractures⁽¹⁶⁾.

Avascular Necrosis

Avascular necrosis is a notorious complication of femoral neck fractures. Traumatic damage to supplying arteries can lead to osteonecrosis (bone infarction)⁽¹⁷⁾.

The non-traumatic form of avascular necrosis may have various causes, including hematologic disorders (sickle cell disease and thalassemia), chronic renal failure, and chronic corticosteroid use⁽¹⁸⁾. 

Radiologic characteristics of the femoral head’s avascular necrosis are⁽¹⁹⁾:

• Stage I: Normal or mild osteopenia as a sign of bone resorption (bone tissue destruction)
• Stage II: Mixed picture of osteopenia, sclerosis, or subchondral cysts
• Stage III: Crescent sign (linear subchondral lucency) or cortical collapse
• Stage IV: Hip joint’s secondary osteoarthritis

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