In this work, a magnetic measurement system was designed and tested to estimate the Internal-External rotation between the Tibial and Femural parts of a knee prosthesis. To obtain this, the sensors were inserted in polyethylene part while a permanent magnet was placed below the Tibial plate. The configuration was designed to keep the intrinsic unbiassness to the positive and negative internal-external angles. A linear regression model was used to map magnetic measurements of the sensors to the angles. For validation the prosthesis was placed in a mechanical simulator equipped with reflective markers tracked by optical motion capture system