Due to its complex shape and its tightness, it is difficult to prepare the subtalar joint for fusion by means of a minimally invasive approach and conventional straight instruments. The preparation implies the establishment of bleeding contact surfaces. It is desirable to preserve the joint’s shape to keep congruent and smooth contact surfaces for optimal fusion. A compliant instrument was designed to facilitate the preparation by starting the design process from a clinically driven approach. The concept consists of a drill/mill unit at the tip that is steered through the joint by means of a passive automatic control. Since the joint will be fused, the cartilage and the subchondral bone layers should intentionally be damaged. This allows the use of the subtalar joint surfaces themselves as a guiding tunnel to preserve the joint’s shape. Thereto, a guidance frame was placed at the tip. The instrument is equipped with a special shaft that is compliant in one direction (perpendicular to the joint surfaces), and stiff in the two other directions to resist and transmit machining forces. The dimensioning of the instrument was performed by taking into account the specific shape of the subtalar joint. A prototype of the instrument was manufactured. The prototype is powered by a commonly used shaver system. The compliant instrument was tested in cadaver material, and gives promising results. In practice, the difference of the stiffness in y- and z-direction is at least a factor 100.

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