What is a knee x-ray?
A knee x-ray (or knee radiograph) is the black and white image produced by sending an x-ray beam through the area of interest (the knee in this case) and getting the images on a film. It is usually the first type of imaging done when a knee problem is suspected. The bones absorb the x-ray beam more than the other surrounding soft tissues and therefore they are usually well seen on x-rays. Therefore, x-rays are usually used to assess bone problems.
Normal knee X-ray
What will a knee x-ray show?
Knee x-rays are readily available and provide the doctor with valuable information about the condition of the knee joint. The main indications for a knee x-ray are:
Knee arthritis: A knee x-ray will assess if there is any arthritis and the degree of changes. In this case, knee x-rays are usually taken while you are standing (called weight bearing). Usually two views are obtained; one from the front to assess the main knee joint for arthritis changes. This is the joint located between the thigh bone (called femur) and the shin bone (called fibula). A second view is taken from the side of the knee to assess for patellofemoral (knee cap) joint arthritis. This is the joint located between the knee cap (patella) and the leg bone (femur). An x-ray will also help to assess for presence of any extra fluid within the knee joint (called joint effusion). This can be seen in knee arthritis and may require drainage under ultrasound guidance.
Suspected knee fracture: When there is suspicion of any bony injury, then a knee x-ray is routinely performed to assess for any fractures. It can also assess for presence of fluid within the knee joint (joint effusion) which can be an indirect sign of significant soft tissue injury to the knee. It is also useful to follow up fractures to assess for fracture healing.
When there is suspicion of bone problem (like bone infection, mass coming from the bone or bone thinning).
Knee arthritis. Note the loss of the joint space (the space between the bones) at the inside of both knee joints. This is because there is complete loss of the cartilage here.
Fracture of the thigh bone (femur) just above the knee joint.
What are the limitations of a knee x-ray?
A knee x-ray cannot assess the knee soft tissues adequately. Therefore, when there is suspicion of injury to the knee soft tissue structures (like ligaments, cartilage, or menisci, then further imaging is usually required).
Knee x-ray vs. Ultrasound and MRI
Ultrasound is high frequency sound waves produced by a special machine and can provide very useful images of the tissues within the body. To find out more, please see our article about knee ultrasound.
An MRI examination uses strong magnetic fields and radio waves and produces detailed images of a certain body part. Knee x-rays are the first choice to assess for knee joint arthritis (wear and tear) and also in suspected fractures around the knee joint. It is readily available and very quick to perform. It can be used to guide knee joint injections but ultrasound is preferred as it does not involve any radiation. MRI is preferred when there is a significant knee trauma with suspicion of multiple injuries to the ligaments and internal structures. MRI however is more expensive and require you to stay still for 20-30 minutes inside a special machine. To find out more, please see our article about knee MRI.
A knee ultrasound examination is very useful to assess the ligaments and tendons that are close to the skin surface (the patellar tendon, the pes anserine tendon and the collateral ligaments). It is also very useful for the assessment when there is swelling around the knee joint (like Baker’ cyst, joint effusion, and pre patellar bursitis). It cannot assess the very deep structures inside the knee joint (the menisci, the cruciate ligaments and the articular cartilage). The main strength of ultrasound is to guide knee injections and treatment. Ultrasound allows for direct visualisation of the needle to ensure medicines are injected into the exact targeted site. There is a significant evidence that ultrasound guided injections provide better levels of pain relief and longer effects compared with injections done without imaging guidance. Performing the injection under ultrasound guidance allows for visualisation of the adjacent structures and reduces the risk of tissues injury at the injection site. To find out more, please see our article about the different types of scans used for knee problems.
Knee conditions and treatment