The Normal Knee X-ray: What Are the Different Views?
What Does a Normal Knee X-ray Look Like?
In order to understand problems related to our knees, we must know what is normal. Many people return from their doctors with X-rays of their knees after being told they have arthritis. Most find it very difficult to actually see the arthritis, as they have nothing normal to use for comparison.
Generally, when an X-ray technician takes images of a knee, they take images from multiple points of view. Later when looking at the X-rays, it can be confusing to work out which way is up and which way is down.
This article will discuss and show example images of the three most common knee X-ray views, and describe the features of a normal knee.
The Three Different Knee X-ray Views
The three most common views used during a knee X-ray are:
- anteroposterior (known as AP)
The AP view is looking at the knee, held straight, from directly in front. This view is generally the easiest to understand as it looks like the skeleton we are familiar with. The kneecap is difficult to see in this view as it is overlapping the thigh bone. It can only be seen as a faint outline. The AP view is helpful in diagnosing arthritis in the knee joint between the thigh and shin bones. It can also show whether there is arthritis in the inside or outside of the knee.
The lateral view is taken perpendicular to the AP, showing the side of the knee. It clearly displays the kneecap and is most useful for diagnosing arthritis between the kneecap and the thigh bone.
The skyline view looks between the kneecap and the thigh bone. It is taken with the knee bent (about 30 degrees) and is used to diagnose arthritis. It is much less common. Only about 30% of orthopedic surgeons use this view during a routine examination.
A Normal Knee, AnteroPosterior (AP) View
This X-ray shows the four bones of the knee. The femur is the thigh bone. There are two bones below the joint line. The tibia is the larger bone we know as the shin. The smaller bone on the outside of the leg is the fibula, seen on the left in the above image.
The kneecap can be seen as a faint circular outline overlapping the thigh bone, centered at the widest part of the femur.
Knee joints are covered with smooth cartilage. This does not appear on X-rays, so although the bones are touching, they appear to have a gap between them. In a normal knee X-ray, the gap is easily seen and of an even height. The gap appears here as a black line through the middle of the knee.
A Normal Knee, Lateral View
This view clearly shows the four knee bones: femur, tibia, fibula, and patella. The patella, or kneecap, is seen sitting in front and to the left of the femur.
This X-ray shows a healthy joint with nice, sharp, well-defined edges at the joint margins.
This view is useful for assessing the patellofemoral joint, which is located between the kneecap and the thigh bone.
The hazy outline of the calf and thigh muscles can also be seen in this view.
A Normal Knee, Skyline View
This view is similar to the angle you see when sitting with you leg out in front of you, slightly bent. Only two bones are visible in this skyline view X-ray: the patella and the femur. In a healthy knee, the groove in the femur should match the shape of the patella and there should be a nice gap between the two of a consistent width.
Preparing Yourself for Knee Treatment
With an understanding of healthy knees as a comparison, it may be easier to understand any features of arthritis or trauma. Reading X-rays is a complicated skill that requires years of experience. Armed with a little basic knowledge, however, you may have an easier time understanding what is happening to your knee so you can make good decisions for treatment.
There is no substitute for the professional opinion of an experienced orthopedic surgeon. Educate yourself so you can ask the right questions in the precious few minutes you have with your doctor.