When we look at the leg, it's easy to assume it's a simple pillar. But from a biomechanical standpoint, it has two key axes, and the difference between them is crucial for understanding joint health. The first is the anatomical axis, which is simply a line drawn straight down the shaft of a bone, like the femur. It follows the shape of the bone itself.

The second, and more important for joint loading, is the mechanical axis. This is the true line of force transmission, running straight from the centre of the femoral head (the ball in your hip socket) to the centre of the ankle joint. In a perfectly aligned leg, this line passes directly through the centre of the knee.

In genu valgum, or knock knees, the knee joint deviates inward, shifting the mechanical axis so it passes lateral to the centre of the knee. This change concentrates weight and pressure on the outer (lateral) compartment of the joint, leading to uneven wear on the cartilage, ligament strain, and pain. Assessing the location of this axis is the first step in quantifying the severity of the condition.

Before turning to imaging, clinicians often use simple physical measurements to get a clear picture of alignment. One of the most common is the intermalleolar distance (IMD). The patient stands with their knees just touching. The distance between the medial malleoli, the bony prominences on the inside of the ankles, is then measured. A gap of more than 8 centimetres is generally considered pathological and indicates significant genu valgum.

Another critical measurement is the Q-angle, which helps explain why knee pain, particularly around the kneecap, is so common with knock knees. This angle reflects the line of pull of the quadriceps muscle on the patella (kneecap).

A larger Q-angle indicates a greater lateral pull on the kneecap. This can lead to patellar maltracking, where the patella doesn't glide smoothly in its groove on the femur. Instead, it's pulled outwards, causing friction, inflammation, and pain—a condition known as patellofemoral pain syndrome.

For a definitive diagnosis and surgical planning, we need to look inside with X-rays. Radiographs allow for precise measurement of the angles that define the limb's alignment. Two key angles are the mechanical Lateral Distal Femoral Angle (mLDFA) and the Medial Proximal Tibial Angle (MPTA). These angles measure the orientation of the knee joint relative to the mechanical axes of the femur and tibia, respectively.

The mLDFA is the angle formed by the mechanical axis of the femur and the joint line of the knee. A normal mLDFA is about 85-90°. In femoral-based genu valgum, this angle becomes smaller as the distal femur tilts inward.

The MPTA is the angle between the mechanical axis of the tibia and the knee joint line. A normal MPTA is also around 85-90°. If the tibia is the source of the valgus, this angle will be greater than normal.

By measuring these angles, an orthopaedic surgeon can pinpoint the exact location of the deformity. This precision is essential for planning corrective procedures, ensuring that the mechanical axis is restored to pass through the centre of the knee, thereby balancing the load across the joint.

Understanding these axes and angles is the foundation for accurately assessing and treating genu valgum.