A hip X-ray is a common, non-invasive imaging procedure used as the initial step in evaluating hip pain, injury, or mobility problems. This test uses a small amount of radiation to create a radiograph of the dense structures within the body. X-rays are effective at visualizing bone, which absorbs the electromagnetic waves and appears white on the film. The speed and accessibility of this procedure make it an invaluable tool for quickly assessing the underlying skeletal framework of the hip joint.
Understanding the Bony Structures Captured
A standard hip X-ray provides a clear picture of the ball-and-socket joint, the intersection of the pelvis and the thigh bone. The image shows the rounded top of the femur, the femoral head (the “ball”), fitting into the cup-shaped indentation in the pelvis called the acetabulum (the “socket”).
The space between the femoral head and the acetabulum is known as the joint space. Although the soft cartilage lining the joint surfaces is not visible on the X-ray, its presence is inferred by this clear space. The X-ray also captures the surrounding pelvic bones, including the ilium, ischium, and pubis. A radiologist examines the symmetry and contours of this bony architecture to establish a baseline before looking for signs of disease or injury.
Detecting Sudden Injuries and Traumas
X-rays are the primary diagnostic tool for assessing acute injuries and traumas. They detect fractures, which appear as a break or crack in the bright white line of the bone. A common injury in older adults is a hip fracture, typically occurring in the neck or upper shaft of the femur.
Displaced fractures, where the broken bone segments are separated, are readily identifiable on the radiograph. Less obvious issues, such as non-displaced or subtle stress fractures, can sometimes be detected by careful inspection of the bone alignment and texture. The X-ray is also used to diagnose a joint dislocation, visible when the femoral head is displaced from its normal position within the acetabulum. In a dislocated hip, the smooth, curved line formed by the upper femur and the pelvis, known as Shenton’s line, is disrupted.
Revealing Long-Term Wear and Alignment Problems
Chronic conditions causing gradual pain and stiffness are often diagnosed using a hip X-ray. Degenerative joint disease, commonly known as osteoarthritis, is characterized by several distinct features visible on the radiograph:
- Joint space narrowing: The progressive loss of cartilage leads to a decrease in the joint space.
- Osteophytes (bone spurs): Bony growths form along the joint margins as the body attempts to stabilize the area.
- Subchondral sclerosis: The bone directly beneath the cartilage becomes thicker and denser due to friction.
- Subchondral cysts: Fluid-filled pockets may form within the bone near the joint surface.
The X-ray can also reveal underlying structural issues that predispose a person to chronic problems. Conditions like hip dysplasia, where the acetabulum is too shallow to fully contain the femoral head, are visible. Signs of femoroacetabular impingement (FAI), involving abnormal contact between the ball and socket, can be identified by subtle changes in the shape of the femoral head or acetabulum. These findings help explain long-term, unexplained hip pain.
When Further Imaging is Required
X-rays primarily provide detailed images of dense, calcified structures like bone. Soft tissues, such as cartilage, ligaments, tendons, muscles, and the labrum, are not clearly visible. Therefore, a physician may order follow-up imaging if the X-ray is normal but symptoms suggest soft tissue damage.
Magnetic Resonance Imaging (MRI) is often the next step, as it uses strong magnetic fields and radio waves to generate detailed cross-sectional images of soft tissues. An MRI is useful for detecting labral tears, tendon inflammation, or subtle fractures missed on the initial X-ray. For complex bone abnormalities, such as detailed acetabular fractures or when precise measurements are needed for surgical planning, a Computed Tomography (CT) scan provides three-dimensional views of the bony geometry.