Patient monitoring, prosthetics  

Wireless sensors for monitoring healing of fractures and performance of artificial joints in vivo

4 April 2006

A system of wireless sensors implanted in biological tissue is being developed by IntelliJoint for dynamic measurements of displacements in vivo, in real time. The system is designed for clinical use such as monitoring orthopaedic procedures, healing of fractures and fusion, and evaluating the performance of artificial implants of hip, knee and shoulder joints.

Following laboratory evaluation of the wired system, the electromagnetic sensors were tested for efficacy and safety in experimental surgery using animal models.

The sensors are expected to monitor the progress of union in the case of fracture and supply information unobtainable from X-rays. Periodic follow-up provides a graph that shows the gradual decrease of relative motion of the fragments until union takes place.

In joint replacements, the sensors are expected to monitor loosening and progressive instability of the fixation of the implant in bone. Progressive wear of the articulating surfaces of the prosthesis, which precedes the deterioration in their performance, is graphically shown on the surgeon's computer.

Conventional imaging modalities fail to diagnose subtle changes in surgery of prosthetic joints. The resolution of X-rays, computed tomography and magnetic resonance imaging is worse than 1 mm. X-rays can detect changes in bone structure only after loss of about 20% of bone mass, and dual energy X-ray absorptiometry (DEXA) can detect changes in bone density only after loss of about 5% of bone mass.

Bone isotopes have the capacity to provide only qualitative value and are diagnostically non-specific. Special techniques that have been described to improve the resolution of X-rays are not suitable for routine clinical assessment, particularly when the material used for the components of the implants is radio-opaque.

The benefit of close follow-up, is in enabling the physician in taking the appropriate measures by early diagnosis in the event of an imminently failing implant prior to bone loss and gross loosening of the prosthesis. The benefit of monitoring the process of union in cases of complicated fractures is in the case of early diagnosis of deviation from the expected healing course and the accuracy in determining the final stage of the healing period. This clearly has social and financial implications.

The tiny sensor are being developed to be biocompatible and are enclosed within a sealing. The sensors are inert, robust, and contain no source of energy. Furthermore, they do not require maintenance or replacement and are therefore suitable for long-term medical applications. Their shape and size can be modified to fit the required application and can be miniaturized by appropriate technology.

The initial laboratory data of the wired system confirmed the high accuracy and resolution better than 100 microns. Conversion of the wired system into a wireless was achieved and required that refining of the sensors and increase of the transmitter/receiver capacity.

In conclusion, the laboratory data and first stage of animal experimental study showed encouraging results. Further refining of the receiver to improve accuracy is currently underway, as is improvement of the sensor's core. It is anticipated that this unique system will suit further applications into other medical fields as well.

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