In an effort to study the causes of postoperative instability of ankle joint endoprostheses, local mechanical properties of the osseous tissue of tibia and talus were studied in terms of physiological loading on 19 cadaverous preparations in the zones, subjected to resection with the purpose of implant setting. High sensitivity of spongiosa tissue to the local mechanic action and significant reduction of the strength characteristics to the depth from subchondral layer have been noted. It has been ascertained that the main cause of the postoperative instability is the excessively voluminous bone resection, dictated by the structural limitations of the used endoprostheses. Admissible tibia and talus resection zones in case of ankle joint prosthetics have been determined.