Apparatus and method for monitoring bone-fracture union

A system for monitoring bone union at a bone fracture (13) comprises a fracture cast sleeve (15) to the external surface of which is bonded a transducer (16). Transducer (16) is electrically-conductive and has an electrical resistance which varies with elastic extension and contraction of the transducer. The output of transducer (16) is applied via a resistance-sensitive network (18) to a data evaluating device (20) arranged to normalize signals received from transducer (16) with respect to the time interval during which that signal appears since the signal is of an impulse nature depending upon the rate at which the force is applied to the fracture (13). The normalized signal is further normalized having regard to the mass of the limb concerned and a plurality of such signals are stored in store (30) together with a data log from date data device (32) and these signals are fed to a trend evaluator (34) in order to predict the time interval before bone union occurs.

This invention relates to a system for monitoring union of bone fractures. 
Hitherto monitoring of bone fracture union has been undertaken primarily on 
a manipulative basis by an experienced clinician who by manual means 
manipulates the two portions of the fractured bone and assesses the degree 
of movement achievable within the comfort level of the patient. This 
technique however is non-quantitative and does not lend itself easily to 
any assessment of when complete union will occur. 
In order to obtain an assessment of when complete union will occur it has 
previously been proposed to affix pins to the two portions of the 
fractured bone and to mount a strain gage on a side bar interconnecting 
these pins externally of the limb containing the bone, and thereafter to 
effect repetitive monitoring of the strain when the limb is subjected to 
loading. However this proposal has not found favour amongst clinicians 
because it is invasive, requiring insertion of pins directly into the bone 
portions, and because the healing rate is affected by the rigidity of the 
pin and bar structure. 
It has previously been proposed to utilise X-ray inspection of the bone 
fracture site but it has been found that the soft bone-forming tissue or 
callus is not reflective to X-rays and therefore until after union is 
complete and the callus has transformed itself into actual bone, no 
assessment of bone union or of the rate of union has been possible by this 
technique. 
According to the present invention there is provided a bone-fracture 
union-monitoring system comprising a fracture cast sleeve shaped and 
dimensioned to fit a patent's limb which contains a bone fracture, the 
union of which is to be monitored, an elastomeric transducer bonded to the 
external surface of said sleeve, said transducer being electrically 
conductive and having an electrical resistance which varies with elastic 
extension and contraction of the transducer, means for applying a force to 
said limb to effect a bone movement at the fracture site whereby 
elastically to extend the transducer in order to vary the electrical 
resistance thereof, a resistance sensitive electrical network connected to 
the transducer, and data evaluating means connected to said network and 
arranged to evaluate the extent of said bone movement at time intervals 
during progressive union of the bone and consequentially to predict the 
time lapse until substantially zero bone movement at the fracture site 
when bone union is completed. 
Preferably, the data evaluating means comprises first means for normalising 
a transducer signal with respect to the time interval during which that 
signal appears. It will be understood that the transducer tends to provide 
a signal of an impulse nature depending upon the rate at which the force 
is applied by the force applying means. 
Preferably also, the data evaluating means comprises second means by 
normalising the signal output by the transducer with respect to the mass 
of the limb being monitored. 
Preferably the data evaluating means comprises a trend analyser adapted to 
collate previous transducer readings and to evaluate the trend of such 
readings in order to predict the time interval until substantially zero 
bone movement at the fracture site or to identify the absence of any such 
trend which thereby is indicative of an absence of bone union. 
It will be understood that a fracture cast sleeve is a known device which 
is currently fitted to the limb of a patient for the purpose of providing 
limited elastic support at the fracture site but more particularly for 
isolating the fracture site from a conventional encasement means such as a 
plaster of paris cast or a rigid plastics brace. The fracture cast sleeve 
is a woven elastomeric/fabric which is clinically sterile and is shaped to 
conform with the limb in question. One example of such a sleeve is 
marketed by Unied States Manufacturing Company, of U.S.A. 
The present invention is applicable to uniform monitoring of fractures in 
long bones in particular, namely, those of the upper and lower extremities 
of the human body. The invention is however equally applicable to 
monitoring of fractures in bones of the extremities in animal bodies. 
The elastomeric transducer is preferably a strip of electrically-conductive 
elastomer sandwiched between strips of non-electrically-conductive 
elastomer which thereby encase the electrically-conductive elastomer and 
render it capable of sterilisation. By way of example the conductive 
elastomer may be U-shaped with electrical leads connected to the free ends 
of the U-shape. It is preferred that the transducer is made of the 
materials and manufactured as described in U.S. Patent Specification No. 
4,444,205.

As is shown in FIG. 1 a patient's leg 10 having a tibia bone 11 and a fibia 
bone 12 contains a fracture 13 in the tibia 11 and the leg 10 is encased 
in a plaster of paris cast 14, the fracture 13 having been appropriately 
set. FIG. 2 shows the plaster cast 14 partly cut away to reveal a fracture 
cast sleeve or sock 15 to the front external surface of which is bonded a 
transducer 16 which is releasably connected by electrical leads 17 to a 
resistance-sensitive electrical network 18. In the interests of clarity 
the patient's leg is not shown in FIG. 2, but it will be appreciated that 
transducer 16 is located on axis 9 on the front surface of sleeve 15 so 
that it is separated from tibia 11 by a minimal amount of skin and muscle 
tissue, in preference to being located on a lateral or rear surface of the 
sleeve 15. Furthermore, in FIG. 2 only one transducer 16 is illustrated 
and in this case the fracture location longitudinally of the leg 10 
requires to be identified in the first instance (for example by X-ray 
techniques) and a sleeve 15 with an appropriately located transducer 16 
selected thereafter for application to the patient's leg 10. In this 
connection it has been found that only three prepared sleeve types are 
adequate in practise. Namely with the transducer 16 bonded in an upper 
region, in a central region (i.e. as shown in FIG. 2) and in a lower 
region of the sleeve 15. 
In order to monitor and assess union of the tibial bone fracture 13 
transducer 16 is connected to network 18 and a force is applied 
longitudinally (in this instance) to the leg 10 by means of transferral of 
the patient's weight onto leg 10 as quantitatively identified by a weigh 
cell 21 located beneath the patient's foot. This reading is transferred by 
leads 22 to an evaluating device 20 shown in FIG. 3, together with the 
output of network 18. Evaluator 20 comprises a first normalising device 24 
which is arranged to receive the pulse-like signal from network 18 and to 
normalise that signal with respect to the time duration of the pulse. This 
is effected by preprogramming normaliser 24 with a time interval value 
representative of the mean pulse duration for a statistically 
significantly large group of patients undertaking a similar process. The 
time-normalised peak value of the impulse signal is stored in store 26 as 
is a signal delivered by weigh cell 21 and representative of the mass of 
the limb 10 which conveniently is represented by the entire body weight of 
the patient concerned. A second normaliser 28 is arranged to evaluate from 
the data stored in store 26 the anticipated time-normalised signal to be 
expected at a predetermined force level measured in terms of percentage 
body weight and this data is stored in store 30 together with the date of 
the reading which is input by means of device 32. 
After a suitable time interval within the anticipated time scale of the 
fracture union, which in the case of a tibial fracture is of the order of 
10 weeks after the fracture has been set and the plaster of paris cast 14 
applied, further sets of readings are taken and stored in stores 26 and 
30. When store 30 has accumulated 3 or more readings these are fed to 
trend analyser 34 which analyses the readings on a time scale determined 
by the date data contained in store 30 and if any trend is apparent (as 
determined by a conventional curve fitting programme) identifies that 
trend and outputs a reading being a prediction of the time lapse until 
substantially zero bone movement at the fracture 13. 
A typical set of data showing progressive union is illustrated in the graph 
of FIG. 4 from which it will be seen that initially when the fracture is 
set and cast 14 applied the transducer output signal is of the order of 90 
units but as time elapses the transducer output signal drops progressively 
until readings of less than 10 units are achieved as union becomes 
practically complete. At complete union, as diagnosed clinically by a 
conventional technique, the transducer output signal is of the order of 
one or two units only. It will be observed that the curve 40 is continuous 
nad it has been found over a statistically significantly large group of 
patients to be of consistent shape and it is on this basis that the trend 
analyser 34 operates to predict the time interval to union from only a 
small number of individual sampling points on the curve for any particular 
patient within a short time interval after bone setting. That is to say, 
the data collected may be represented in FIG. 4 by the points A,B and C 
and trend analyser 34, in accordance with its curve fitting programme, 
predicts point D and more particularly the time interval between points C 
and D. 
It will be observed that the system according to the present invention is 
non-invasive and is dependent only upon a level of force applied to the 
limb in accordance with the comfort level of the patient in that it is the 
patient himself who applies the force to the limb by weight transferral. 
By way of example this weight transferral may be of the order of 20-40% of 
the patient's body weight in the case of the tibial fracture 13. FIG. 5 
illustrates the output waveform of a transducer 16 as a result of a 
patient with an unfused fracture of the left tibia moving into four 
different positions. Portion W of the waveform results with the patient 
sitting in a chair; portion X which occurs twice results from the patient 
standing on both legs but it will be noticed that the signal amplitude is 
greater in the first occurrance of X than in the second occurrance 
indicating a greater weight transferral to the left leg on the first 
occurrance of X; portion Y which occurs with the patient standing only on 
his right leg (i.e. the non-fractured leg); and portion Z which results 
with the patient standing solely on his left leg (i.e. the fractured leg). 
It will be seen that the differing peak amplitudes of portions X,Z and X 
is consequential on the differing percentages of body weight applied to 
the left leg by this patient and it is for this reason that normaliser 28 
is required, whilst the time durations of portions X, Z and X are 
substantially the same because there is only one patient involved. A 
different patient would display different time durations on the X, Z and X 
portions of the comparable waveform and it is for this reason that 
normaliser 24 is required. 
It is also to be understood that the effect of applying a compressive force 
to tibia 11 as is illustrated in the embodiment, is to cause a lateral 
movement at the site of fracture 13 which lateral movement effects 
extension of transducer 16 by elongation of the transducer between its 
upper and lower ends both of which remain substantially immobile. In other 
words the transducer extension is caused by a bulging of a central portion 
of the transducer. The transducer 16 is preferably composed of a U-shaped 
strip 2 (FIG. 6) of electrically-conductive elastomer sandwiched between 
strips 1 of non-electrically-conductive elastomer effecting an encasement 
of the conductive strip. The U-shape may lie in the general plane of the 
front surface of sleeve 15 but preferably lies perpendicularly to that 
plane so that transducer bulging caused by bone movements at fracture 13 
strains both limbs of the U-shape, thereby enhancing signal strength from 
the transducer 16. 
In accordance with the present invention the transducer may also take the 
form of a three separate transducers 16 simultaneously bonded to sleeve 15 
along line 9 (FIG. 7), one such transducer being located in the upper leg 
portion, one in the central leg portion, and one in the lower leg portion. 
With this arrangement only the one transducer 16 overlying the site of the 
fracture is connected to the normaliser 24 via the network 18 for the 
purpose of providing bone movement data whilst either or both of the other 
two transducers are connected into network 18 to provide muscle-noise 
signals for reduction of the bone-movement data signal on a common-mode 
rejection basis. This arrangement in addition to providing for data signal 
enhancement simplifies storage by reducing the inventory of sleeves 15. 
Each transducer 16 referred to previously is preferably provided with 
low-extension/high signal output characteristics and low tear strength 
having regard to the required duty cycle of elastic extensions which are 
only of the order of 2% or less. 
In accordance with the present invention the force applying means may be 
compressive as previously described or tensile or torsional. For example, 
in the case of a fractured tibia, the force may simply be applied at the 
site of fracture 13 by raising the leg from the vertical position to a 
horizontal position. Alternatively the weigh cell may be arranged 
vertically in order to measure a horizontal force generated by the 
patient. 
It will be understood that although the invention has been described in 
particular terms with reference to a fractured tibia its application is 
not limited either to tibial fractures or even to leg fractures and whilst 
it is desirable to have the limb containing the fracture encased either by 
a plaster of paris cast or by a brace this is not necessary for the 
purposes of applying the monitoring system of the present invention. Such 
encasement means however does limit the maximum bone movement at the site 
of the fracture and is therefore a safety feature. Furthermore because the 
present invention does not utilise reported doses of X-rays, there is no 
need for the patient to enter a hazardous environment. Furthermore because 
in the preferred arrangement the sensor is made of elastomeric materials 
it is unaffected by the environmental conditions which pertain inside the 
encasement means. The transducer which is primarily made of elastomeric 
material is of course bonded to the fracture cast sleeve by an elastomeric 
bonding agent such as a silicone rubbber extending throughout the areal 
extent of the transducer.