Abstract
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
Highlights
Modern extendable endoprostheses allow for non-invasive extension procedures to maintain limb length equality
This approach was not feasible due to potential neurovascular damage and, as a result, a large variety of extendable endoprostheses have been developed, revised and improved on over the past 30 years to accommodate a gradual limb lengthening over the remaining duration of skeletal growth
Counting the pulses to the stepper motor, the rotation is monitored and the linear extension can be calculated. That for both the Stanmore Mark V and the Implant Cast extendable devices, the ferromagnetic influence on MRI and CT imaging would not provide a clear picture of any secondary tumour developing in the nearby bone and soft tissues. This project aims to develop a prototype extendable endoprosthesis, which would consist of parts that would not interfere with MRI and CT imaging and that would provide feedback, from which small daily extensions could be monitored and measured
Summary
Modern extendable endoprostheses allow for non-invasive extension procedures to maintain limb length equality. 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. The motivation for development has been the improvement of patient quality of life through the reduction in revision surgeries for prosthesis extension, which increases both the risk of infection,[1] and financial implications This improvement has resulted in producing minimally invasive and non-invasive extendable implants allowing a lengthening of 6 to 10 mm per procedure to avoid joint stiffness.[2] Minimally invasive extendable implants require open surgery, during which the lengthening mechanism of the implant is activated. Lengthening of these implants avoids major soft tissue dissection, the patient is still subjected to a procedure and risk of infection, twice yearly until completion of growth
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