Reading a hip X-ray starts with knowing what normal looks like, then working through the image systematically so you don’t miss anything. Whether you’re a student, a new clinician, or someone trying to understand your own imaging results, the same structured approach applies: check the image quality, trace the key bony landmarks, assess alignment and joint spacing, and then look for specific signs of disease or injury.
Standard Views and What They Show
Most hip imaging begins with an anteroposterior (AP) view, taken from the front with the patient lying flat. This single image shows both hips side by side, which is useful because you can compare one to the other. The AP view is best for evaluating the acetabulum (the socket) and the overall alignment of the joint.
A frog-leg lateral view is the second most common projection. The patient bends their knee and lets it fall outward, which rotates the femur and exposes the femoral neck and head from a different angle. This view is especially helpful for spotting subtle fractures or early bone death that the AP view might hide. A cross-table lateral, where the X-ray beam shoots across the body, serves a similar purpose and is preferred when a fracture is suspected and the leg shouldn’t be moved.
Key Bony Landmarks to Identify
Before looking for problems, you need to orient yourself to normal anatomy. On an AP pelvis X-ray, start by identifying these structures:
- Femoral head: The ball at the top of the thighbone. It should appear round and smooth, with a small divot on the inner surface called the fovea (where a ligament attaches).
- Acetabular roof: The bony overhang of the socket that covers the femoral head. Look at how much of the head it covers.
- Femoral neck: The angled bridge between the femoral head and the shaft. This is the most common fracture site in older adults.
- Greater trochanter: The bony bump on the outer side of the upper femur, where several hip muscles attach.
- Lesser trochanter: A smaller bump on the inner side, lower down. The powerful hip flexor tendon inserts here.
- Iliopectineal line: A curved line tracing the front column of the pelvis. Disruption suggests a pelvic fracture.
- Ilioischial line: A similar line tracing the back column.
Once you can consistently pick out these landmarks, the rest of the evaluation becomes much more intuitive.
Checking Alignment: Shenton’s Line
Shenton’s line is the single most useful alignment check on a hip X-ray. It’s an imaginary curved arc that runs along the inner edge of the femoral neck and continues smoothly into the lower border of the pubic bone. In a normal hip, this arc is one unbroken, gentle curve.
If Shenton’s line is “broken,” meaning the curve doesn’t flow smoothly and there’s a step-off, something is wrong. A disrupted Shenton’s line points to a hip fracture, dislocation, or developmental abnormality. It’s one of the first things to check on every hip X-ray, and it takes only a few seconds.
Measuring the Joint Space
The joint space is the gap between the femoral head and the acetabulum. On X-ray, this gap represents the cartilage lining both surfaces (cartilage itself doesn’t show up on plain film, so you’re seeing the space it occupies). In healthy adults, the hip joint space averages about 3.6 mm, with a normal range of roughly 2.3 to 6.1 mm. Taller individuals and those with larger femoral heads tend to have slightly wider spaces. Interestingly, age alone doesn’t shrink the joint space in healthy hips.
When the space narrows below that range, it typically means cartilage loss, which is the hallmark of osteoarthritis. Narrowing that’s worse on one side of the joint (usually the top or inner portion) is more telling than mild, even narrowing throughout.
Assessing the Femoral Neck Angle
The neck-shaft angle is the angle formed between the femoral neck and the shaft of the thighbone. The global average in adults is about 127 degrees, though normal values range from 120 to 140 degrees. You’ll sometimes see textbooks quote 135 degrees, but large population studies put the true average lower.
An angle below 120 degrees is called coxa vara, which makes the femur appear more vertical and can result from fracture healing, metabolic bone disease, or developmental conditions. An angle above 140 degrees is coxa valga, where the neck is more horizontal. Both can change the mechanics of walking and may contribute to hip pain.
Spotting Osteoarthritis
Hip osteoarthritis has four hallmark signs on X-ray, and they tend to appear in a predictable sequence as the disease progresses:
- Joint space narrowing: The earliest reliable sign. The gap between the femoral head and socket gets thinner as cartilage wears away.
- Osteophytes: Bony spurs that form around the joint margins, especially on the edges of the femoral head and acetabular rim. The body grows these in an attempt to stabilize the joint.
- Subchondral sclerosis: The bone just beneath the cartilage becomes denser and whiter on the X-ray. This happens because the bone is absorbing more stress as cartilage disappears.
- Subchondral cysts: Small, dark, round areas within the bone near the joint surface, formed by fluid being forced into the bone through damaged cartilage.
The Kellgren-Lawrence grading system ranks osteoarthritis from grade 1 to grade 4. Grade 1 is doubtful: maybe some mild narrowing and a possible osteophyte. Grade 2 is definite but mild, with clear narrowing and visible spurs. Grade 3 shows marked narrowing, cysts forming, and early changes in the shape of the femoral head. Grade 4 is severe: the joint space is nearly or completely gone, the femoral head is deformed, and large osteophytes are present. Most people searching for information about hip arthritis on their X-ray will fall somewhere in grades 2 through 4.
Identifying Hip Fractures
Hip fractures fall into three main categories based on location, and the location determines both treatment and prognosis.
Femoral neck fractures occur in the narrow bridge between the femoral head and the trochanters. These are “intracapsular,” meaning they happen inside the joint capsule. This matters because the blood supply to the femoral head runs along the neck, so a displaced fracture here can cut off circulation and lead to bone death. On X-ray, look for a dark line crossing the neck, a change in the angle of the neck, or a disrupted Shenton’s line. Non-displaced fractures can be extremely subtle, sometimes appearing as just a faint haze or slight irregularity in the bone.
Intertrochanteric (pertrochanteric) fractures run between the greater and lesser trochanters, outside the joint capsule. These are generally easier to spot because the fracture line tends to be more obvious, and the fragments often shift apart. You may see the lesser trochanter pulled away as a separate piece.
Subtrochanteric fractures occur in the shaft just below the lesser trochanter. These are less common but important to recognize because they’re subject to high mechanical stress and sometimes require different surgical approaches.
If a fracture is suspected clinically but the X-ray looks normal, an MRI or CT scan is the next step. About 2 to 10 percent of hip fractures are initially invisible on plain X-rays.
Signs of Avascular Necrosis
Avascular necrosis (also called osteonecrosis) happens when the blood supply to the femoral head is disrupted and bone tissue begins to die. Early stages can look completely normal on X-ray, which is why MRI is the gold standard for early detection. But as the condition progresses, X-rays reveal a characteristic pattern.
The first visible sign is a wedge-shaped area of dense, white bone in the upper outer portion of the femoral head, bordered by a hardened (sclerotic) rim. On a lateral view, you may see a “crescent sign”: a thin dark line just beneath the surface of the femoral head, representing a zone where the dead bone is separating from the cartilage above it. The crescent sign is a critical finding because it indicates the femoral head is about to collapse. Once collapse occurs, the normally round femoral head becomes flattened or irregular, and the joint quickly deteriorates.
Pediatric Hips: Growth Plates and Pitfalls
Children’s hip X-rays are fundamentally different from adults’ because much of the skeleton is still cartilage, which doesn’t show up on X-ray. The growth plates (physes) appear as dark lines near the ends of bones, and these can easily be confused with fractures by someone unfamiliar with pediatric anatomy.
The key distinction is that normal growth plates have smooth, regular margins and appear in predictable, symmetrical locations on both sides. Fractures through or near the growth plate may show irregularity, widening, or a slight offset of the bone on one side compared to the other. Some non-displaced growth plate fractures are genuinely invisible on X-ray, making the physical exam just as important as the image. Comparing the injured side to the uninjured side is essential in pediatric hip imaging.
A Step-by-Step Reading Approach
To avoid missing findings, work through every hip X-ray in the same order. One effective sequence is:
- Image quality: Is the pelvis straight? Check for rotation by comparing the obturator foramina (the two large holes in the lower pelvis). They should be roughly symmetrical. A tilted or rotated pelvis distorts every measurement.
- Alignment: Trace Shenton’s line on both sides. Check that both femoral heads are seated in their sockets.
- Bones: Follow the cortex (outer edge) of every bone in a continuous loop. Look for breaks, areas of abnormal whiteness (sclerosis), or dark spots (cysts or lytic lesions). Check the femoral neck carefully.
- Joint space: Compare the joint space width on both sides. Look for narrowing, especially at the top of the joint.
- Soft tissues: Look at the fat planes around the hip. A displaced or bulging gluteal fat stripe (the fat layer over the outer hip muscles) can signal a hidden fracture or fluid in the joint.
This systematic approach works whether you’re looking at a healthy hip or one with advanced disease. The goal is never to rely on a single finding in isolation. A broken Shenton’s line, an abnormal joint space, or an unusual density all mean more when placed in context with the rest of the image.