The paper describes a multimode-interference-effect-based single-mode-multimode-single-mode (SMS) fiber optic sensing system for wide-range displacement monitoring on an Ilizarov orthopedic external fixation device. Displacement measurement is implemented as the uniform extension of a spring, which is connected to an SMS optical fiber displacement sensor, allowing full displacement characterization. SMS fiber structures are used together with a macrobending-fiber-based interrogation system to measure temperature perturbation and hence eliminate its effect on the displacement measurement. Transmission spectra are measured, achieving a displacement measurement range of 110 mm with maximum sensitivity of -53 pm/mm and a resolution of 500 μm. The all-fiber sensing system has facilitated a clinical device with a wide displacement range operating in real-time when attached to the Ilizarov apparatus.