The invention relates to a method for predicting kinking, breaking or fracturing of flexible medical instruments 100 such as guidewires. Fiber optic methods for determining curvature are known. However, according to this invention such optic methods can be utilized for determining bending of flexible medical instrument, for example by determining when the curvature of the flexible medical instruments becomes smaller than a given curvature threshold. The flexible medical instruments may kink, break or fracture due to material fatigue. To predict failure of the instrument due to fatigue, the bending actions on the instrument may be monitored during the entire lifetime or during several uses of the instrument for determining a warning of a possible risk of instrument failure.
