A patient with a right above-knee prosthesis will be able to maintain the knee in extension while weight bearing if the center of gravity falls which side of the axis?

A patient with a right above-knee prosthesis will be able to maintain the knee in extension while weight bearing if the center of gravity falls which side of the axis?

• A. Posterior to the axis of the right knee joint
• B. Lateral to the axis of the right knee joint
• C. Anterior to the axis of the right knee joint
• D. Medial to the axis of the right knee joint

Answer: (C)

The key idea is how gravity creates a turning effect (external moment) about the knee axis. If the center of gravity lies in front of the knee axis (anterior to the axis), gravity pushes the limb in a way that produces an external knee extension moment, helping to hold the knee in extension during weight bearing. In a transfemoral prosthesis, this forward (anterior) line of gravity makes the knee more stable in extension as you bear weight. If the center of gravity were behind the axis, gravity would tend to flex the knee and could cause buckling. Lateral or medial positions would create sideways moments, which aren’t the primary factor for maintaining sagittal-plane knee extension.

The key idea is how gravity creates a turning effect (external moment) about the knee axis. If the center of gravity lies in front of the knee axis (anterior to the axis), gravity pushes the limb in a way that produces an external knee extension moment, helping to hold the knee in extension during weight bearing. In a transfemoral prosthesis, this forward (anterior) line of gravity makes the knee more stable in extension as you bear weight. If the center of gravity were behind the axis, gravity would tend to flex the knee and could cause buckling. Lateral or medial positions would create sideways moments, which aren’t the primary factor for maintaining sagittal-plane knee extension.