Design of a lengthening element for a modular femur endoprosthetic system

A malignant tumour may develop around the knee joint of a child. In the majority of cases it will then be necessary to resect the involved bone with adjacent tissue. A joint team of Groningen University Hospital and University of Twente is currently working on the project of developing a modular endoprosthetic system to bridge the defect resulting from the resection. Since the other, normal, leg continues to grow, the endoprosthetic system will have to include an element the length of which can be adjusted non-invasively. The main conditions to be met by the lengthening element are non-invasive continuous adjustability and a maximum total lengthening of 114 mm. This was achieved by using an external magnetic field. Animal experiments showed that the lengthening element worked well, although moisture infiltrated the telescopic tubes and the lengthening element was covered by proliferating bone at an early stage. Also, the necessary magnetic field proved to be larger than calculated. In a revised design, these problems are resolved. In vitro tests show that the new lengthening element meets all requirements.