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
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
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
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.