Lower-Limb Alignment: What Is the Mechanical Axis and Why Does It Matter?

What is the mechanical axis of the leg?

The mechanical axis is a straight line from the centre of your hip joint to the centre of your ankle joint. In a normally aligned leg, this line passes through, or very close to, the centre of the knee. This means your body weight is spread roughly equally across the inner and outer halves of the knee joint.

When the mechanical axis drifts to one side of the knee, the load is no longer shared evenly. One compartment of the knee carries more stress than the other. Over months and years, that imbalance can speed up cartilage wear and contribute to osteoarthritis.

Varus vs valgus alignment

You may have heard the terms "bow-legged" and "knock-kneed." Doctors call these varus and valgus alignment.

• Varus (bow-legged): The mechanical axis falls to the inner side of the knee. The inside of the joint is overloaded.
• Valgus (knock-kneed): The mechanical axis falls to the outer side. The outside compartment takes more stress.

Neither is always painful. Many people live with mild varus or valgus alignment and have no problems at all. But when it comes with cartilage wear, an old injury or more activity, even a small shift in the axis can become significant.

How do you know if your leg alignment is off?

You may not notice mild malalignment yourself. Often the first clue is a pattern of symptoms that does not respond to the usual treatments.

What causes lower-limb malalignment?

Malalignment can develop for several reasons. Sometimes it is the way your bones naturally grew. Sometimes it follows an injury or gradual wear.

Your body is a connected chain from feet to head. A tilt at the hip or a problem in the ankle can change the knee's position, and the other way round. That is why a scan of a single area sometimes misses the bigger picture. The pain you feel may come from your body compensating somewhere else in the chain.

How is leg alignment measured when you are standing?

To measure the mechanical axis accurately, you need a full-length image of the leg taken while you stand and bear your own weight. This is the only way to see how gravity and your muscles affect the position of your joints.

Ordinary X-rays are usually taken lying down. When you lie flat, your muscles relax, your joints decompress, and the true alignment under load disappears. A knee X-ray taken lying down can look normal even when your standing alignment is significantly off.

An EOS scan captures your legs from hip to ankle in a single standing image, with about 90% less radiation than a standard X-ray, around ten times less. Because the image is taken while you bear weight, it shows your joints exactly as they work in real life. The scan also captures both legs at the same time, so your clinician can compare one side with the other.

What can be done about lower-limb malalignment?

The right approach depends entirely on the cause and severity of the malalignment. ScanAlign is a diagnostic imaging clinic. We provide the detailed measurements your clinician needs to make that decision. We do not provide treatment ourselves.

In general, clinicians use alignment data to guide options such as targeted physiotherapy, orthotic insoles or surgical planning. For example, a surgeon planning a knee replacement needs precise axis measurements to place the new joint correctly. A surgeon considering an osteotomy (a procedure that re-angles the bone) relies on the same data to work out exactly how much correction is needed.

The most important step is getting an accurate picture of your alignment today, under real weight-bearing conditions. That is what a standing full-length scan provides.

Before you start a treatment, or change the one you are on, it is worth getting a detailed diagnosis. Knowing whether the cause sits in the pelvis, the legs or the spine changes what will actually help.

Frequently asked questions about lower-limb alignment