Which statement best describes the mechanical axis in knee arthroplasty alignment?

• A. The line from the hip to the ankle passing through the knee, ideally within ±3 degrees of neutral
• B. The line along the femur's axis only
• C. The curve of the knee's joint line
• D. The distance between the centers of two implants

Answer: (A)

The mechanical axis is the weight-bearing line of the leg, defined by a straight line from the center of the hip to the center of the ankle as it passes through the knee. In knee arthroplasty, aiming for this line to be as close to neutral as possible ensures that the load is distributed evenly across the prosthetic components. A neutral alignment typically means only a small deviation, often about 3 degrees or less. This balanced load reduces wear on one side of the knee, lowers the risk of loosening, and improves implant longevity. If the line shifts inward or outward, it concentrates forces in one compartment, which can hasten wear and lead to complications. The other descriptions—focusing only on the femur’s axis, the curved joint line, or the distance between implants—do not represent the overall weight-bearing alignment of the leg, which is why they aren’t the best description of the mechanical axis.