Principle drawing of the bending stiffness measurement device. The measurement device is adjusted to the external fixator, aligning the longitudinal rod of the device with the neutral axis of the bone, and the strain gauges (rotational center) positioned at the fracture site. The longitudinal rod of the external fixator is then removed. When applying compressive or distractive forces between the clamps on each side, the measurement device bends through the center of the fracture, producing a four-point-bending moment between the inner pins. The angular deflection is measured by means of strain gauges.