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Background: Modern extendable endoprostheses allow for non-invasive extension procedures to maintain limb length equality. However, these devices are incompatible with normal diagnostic techniques due to ferromagnetic materials and require physicianfacilitated extensions. The aim, therefore, is to develop an extendable implant, capable of carrying out daily minor extensions comparable to natural growth, as well as permitting monitoring of the surrounding soft tissue site through MRI or CT. Methods: A biocompatible, non-ferromagnetic prototype device has been constructed for in-vitro testing. The titanium body of the device encloses a piezoelectric motor, a gearbox, a lead screw/telescopic shaft and an electronic circuit board. Testing has been performed to quantify the linear force developed by the device, the electronics’ survival and behaviour after gamma radiation, the permeability of water through the seals, the rate of extension and the suitability for MRI and CT imaging. In addition, an external control unit was manufactured, and designed to be programmable by the physician, to control both the amount of daily lengthening and the number of extensions until the next assessment. This device transmits power to the receiver of the extendable device (designed for transcutaneous placement), by means of inductively coupled coils. Results: The prototype device, through its driving mechanism of the piezoelectric motor (a non-ferromagnetic motor), gearbox and a lead screw, produced an extension at a rate of 5 μm/min and generated the required force of 500 N for limb extension under non-invasive external control. Testing under a strong magnetic field of 3 Tesla, the MRI imaging produced a void in the area where the extendable device was placed, but it did not produce artefacts typical of those observed with ferromagnetic materials. The electronic circuitry, within the prototype, withstood the effects of 25 kGy gamma sterilisation (required for biomedical implants prior to implantation), and the lead cable and inductive coil maintained their flexibility. The device was submerged in water at 37 °C and 10 kPa pressure for a period of 72 hours and no fluid passed its seals into the motor and gearbox chamber. After concluding the mechanical testing, as well as the MRI and gamma radiation, the external control unit successfully transmitted (via the inductively coupled coils) the signals to the piezoelectric motor, which functioned normally. Conclusions: The preliminary testing proved that the prototype could be developed into a suitable extendable implant delivering sufficient force for limb lengthening. The external electronic control unit can be programmed by the physician to allow daily extensions of the limb at a rate of 5 μm/min. At this rate a maximum daily extension of 30 μm will take 6 min. Although MRI testing at 3 Tesla magnetic field strength was inconclusive, but  produced no artefacts or scattering, literature suggests that at the recommended strength of 1.5 Tesla, the MRI will produce a sharp and clear image of the device in the patient’s limb. Level of evidence: Level 5

A new prototype of a non-ferromagnetic non-invasive femoral extendable prosthesis for skeletally immature patients