Buckling of prosthetic knee after transfemoral amputation; most likely cause?

• A. Prosthesis too long
• B. Mechanical knee with too much friction
• C. Socket with a high medial wall
• D. Knee axis anterior to the trochanteric-knee-ankle line

Answer: (D)

Stability of a transfemoral prosthetic knee during stance relies on where the knee joint axis sits relative to the line from the greater trochanter down to the knee (the trochanter–knee–ankle line). If the knee axis is placed too far forward (anterior) to that line, the body's weight and ground reaction force create a knee flexion moment in early stance that the prosthetic knee may not resist, causing the knee to buckle. Moving the axis posteriorly toward the trochanter–knee–ankle line increases the extension moment during stance and improves stability, reducing the tendency to buckle.

Other factors listed are less likely to cause buckling: being too long mainly affects limb length and toe clearance, not sagittal-plane stability; too much friction in the knee would make the knee too stiff rather than buckle; and a socket with a high medial wall affects medial-lateral alignment and comfort rather than the sagittal stability that leads to buckling.