ELECTROMAGNETICALLY COUPLED NON-INVASIVE EXTENDING PROSTHESIS

Abstract

For the treatment of malignant bone tumours in immature patients, extending prostheses are used to maintain growth in the affected limb. This new prosthesis allows the implant to be lengthened by using electromagnetic coupling that is simple and easy to use.

Because of bone tumour, fourteen patients between the age of 8 and 15 years underwent bone replacement treatment and a further two patients, both male 18 and 61 years, received the same treatment to restore limb length discrepancy. These implants consisted of a telescoping shaft where the expansion is done by a power screw driven by a gearbox connected to a NdFeB magnet. This prosthesis is capable of being extended under an axial load of up to 1350N. This is in line with 76 distraction force measurements taken in 43 patients with growing prosthesis where extension was achieved by invasive procedure and where a force of up to 1513N for an extension of 6mm was recorded. Once implanted, the non-invasive prosthesis is extended by placing the limb through an external drive unit. As the drive unit is turned on, it produces a rotating magnetic field capturing the implant magnet causing it to rotate in synchronisation. At full speed, the implant grows at a rate of 0.23mm per minute.

Of the sixteen patients, seven have been extended with one to its full capacity of 63mm. During extension, the patients have no sensations of vibration, heat, stretching or any other kind although the faint vibrations could be heard by placing a stethoscope on bony protrusions such as greater trochanter. At each sitting, the patients were extended by approximately 4mm during normal outpatient clinics and were able to walk as before immediately after the treatment. Patients with knee joint were functionally assessed before and after the treatment and showed approximately 10° to 15° of reduction in knee flexion/extension.

This new extending mechanism in these prostheses has provided the patients a treatment, which reduces trauma infection and discomfort. The mechanism of extension is reliable and effective.