The knee operation is not considered a risky operation and does not last long, however it does require extensive preparation. Namely, the positioning of the tool for bone surface preparation takes longer than the endoprosthesis installation itself. For the operation to be successful it is necessary to achieve neutral alignment of the foot axis with adequate tension within the joint. Furthermore, exposure of the wound increases the risk of infection, thus it is very important that the operation is performed as soon as possible. The paper is divided into four parts. The first part is consisted of an overview of endoprosthesis and anatomy of the knee with a review of biomechanics and emphasis on the desired geometric and kinematic characteristics. After introduction of the problem, market research was conducted in the second part deriving the requirements that the instrument should meet, followed by development goals. In the design part, the product was functionally shaped, followed by concepts created with the aim to propose an improvement in regards to the existing solutions, which were then evaluated. Based on the aforementioned, the structural development of the final solution was performed. In this paper, an instrument construction was made, the usage of which shortens the duration of the operation by facilitating the evaluation of alignment of lower and upper leg axis. Also, the goal of reducing the human factor impact when determining joint tension was achieved by limiting the applied force with construction. In today's practice, joint tension is determined on the basis of surgeon experience, which often leads to errors that cause pain after implantation, disturbed motion kinematics and endoprosthesis instability as the end result.