Investigation of the hydrodynamic properties of a new MRI-resistant programmable hydrocephalus shunt
Cerebrospinal Fluid Research
Investigation of the hydrodynamic properties of a new MRI-resistant programmable hydrocephalus shunt
David M Allin 0
Marek Czosnyka 0
Hugh K Richards 0
John D Pickard 0
Zofia H Czosnyka 0
0 Address: Shunt Evaluation Laboratory & Academic Neurosurgical Unit, Addenbrooke's Hospital , P.O. Box 167, Hills Road, Cambridge CB2 2QQ , UK
Background: The Polaris valve is a newly released hydrocephalus shunt that is designed to drain cerebrospinal fluid (CSF) from the brain ventricles or lumbar CSF space. The aim of this study was to bench test the properties of the Polaris shunt, independently of the manufacturer. Methods: The Polaris Valve is a ball-on-spring valve, which can be adjusted magnetically in vivo. A special mechanism is incorporated to prevent accidental re-adjustment by an external magnetic field. The performance and hydrodynamic properties of the valve were evaluated in the UK Shunt Evaluation Laboratory, Cambridge, UK. Results: The three shunts tested showed good mechanical durability over the 3-month period of testing, and a stable hydrodynamic performance over 45 days. The pressure-flow performance curves, operating, opening and closing pressures were stable. The drainage rate of the shunt increased when a negative outlet pressure (siphoning) was applied. The hydrodynamic parameters fell within the limits specified by the manufacturer and changed according to the five programmed performance levels. Hydrodynamic resistance was dependant on operating pressure, changing from low values of 1.6 mmHg/ml/min at the lowest level to 11.2 mmHg/ml/min at the highest performance level. External programming proved to be easy and reliable. Even very strong magnetic fields (3 Tesla) were not able to change the programming of the valve. However, distortion of magnetic resonance images was present. Conclusion: The Polaris Valve is a reliable, adjustable valve. Unlike other adjustable valves (except the Miethke ProGAV valve), the Polaris cannot be accidentally re-adjusted by an external magnetic field.
New models of hydrocephalus shunts are continuously
being released onto the health-care market [1-5]. Yet these
new designs do not always match the needs of the patient
suffering from hydrocephalus. For example: many valves
have very low hydrodynamic resistance but, without
siphon-preventing mechanisms, they cause over-drainage
. Also, some shunts may present with reflux at low flow
. In some valves adjustable settings can be changed
accidentally [5,6]. One recently raised criticism was that
many of the magnetically-adjusted shunts in use can be
altered by an external magnetic field. This includes weak
sources created by home appliances [6,7], as well as the
more obvious, stronger fields encountered during
magnetic resonance (MR) scanning [8-10]. The Polaris valve is
an adjustable hydrocephalus shunt, which incorporates a
mechanism that allegedly prevents accidental
re-adjustment in a magnetic field of up to 3 Tesla.
The aim of this study was to measure the properties of the
shunt, in order to provide neurosurgeons with
independent, reliable and accurate data about its performance. The
long-term stability of a valve's behaviour was tested in a
laboratory environment that mimics, at least in part,
conditions within the human body. The tests are able to
demonstrate whether the shunt is susceptible to alterations in
CSF drainage caused by postural changes, by external
magnetic fields, by changes in ambient temperature, or by
the presence of a pulsating pattern in the inlet pressure.
Where possible, we have stated whether specific shunt
properties revealed during the tests may be considered
useful or detrimental to the restoration of the CSF
circulation. Such an assessment may encourage or discourage the
use of the shunt in the management of specific types of
The Polaris Programmable Valve (Sophysa Ltd, Orsay,
France) is a differential-pressure valve, the opening
pressure of which can be magnetically adjusted after
implantation using a magnetic tool supplied with the valve.
CSF flows through the inlet which is closed by a ruby ball
(Figure 1: 9) sitting in a cone, and supported by a flat
semi-circular spring (Figure 1: 2). When the inlet pressure
increases, the ruby ball rises out of the cone and CSF flows
into the rigid main fluid container and then into the distal
The tension of the flat spring can be adjusted
non-invasively by moving the rotor with the external programming
SFcigheumreat1ic diagram of construction of Sophysa Polaris Valve (Figure scanned from the leaflet provided by the manufacturer)
Schematic diagram of construction of Sophysa Polaris Valve (Figure scanned from the leaflet provided by the
manufacturer). 1: inlet connector, 2: semicircular spring; 3 & 12: radiopaque setting identification points, 4: rotor, 5: ruby
axis, 6: micromagnet, 7: outlet, 8: fixation holes, 9: ruby ball, 10: adjustment lugs, 11: safety stop.
magnet. The rotor is composed of two permanent
magnets that are not susceptible to demagnetisation. The rotor
shifts the end of the spring between the consecutive
indexing notches, thereby changing the working pressure. The
pre-load or performance level is claimed, by the
manufacturer, to increase the performance pressure in a linear
fashion from 30, 70, 110, 150 to 200 mmH2O.
Manufacturers traditionally use mmH2O as units to express valve
pressure settings, whereas in the most
hydrocephalusrelated publications, pressure is expressed in mmHg. The
pressure settings of the Polaris valve correspond to 2.3,
5.2, 8.1,11.1 and 14.7 mmHg.
In comparison to the older Sophysa programmable valve,
which was susceptible to accidental re-programming by
an external magnetic field (stronger than 40 mT), the
Polaris valve is equipped with a patented self-locking
magnetic system. It contains two mobile micromagnet
shuttles, which attract each other when in the resting
position. In this position the rotor is blocked by the lugs
sliding in between adjustment notches. An adjustment
instrument containing a magnet with a uniquely profiled
magnetic field is required to move the rotor. When the
adjustment tool is in the proper position the shuttles are
pushed outward, unblocking rotor, which can then be
moved to change the performance levels (Figure 1).
The hydrodynamic properties of the shunts are described
by various parameters such as opening pressure, closing
pressure, resistance to fluid flow, pressure-flow
performance, etc. Our testing methods and definitions of the
hydrodynamic parameters have been described in
previous articles and reports , but for better understanding
the description is repeated below (Figure 2). The shunts
under test were submerged in a water bath at a constant
temperature at a defined depth (h). The working fluid,
deionised and de-aerated water, was supplied by the fluid
container or the infusion pump. A pulse pressure of
controlled amplitude created by the pulse pressure generator
FAigduiargera2m of the test rig showing the main components
A diagram of the test rig showing the main components.
could be added to the static pressure. The viscosity and
specific gravity of water reflect the physical properties of
CSF under normal conditions.
In hydrocephalus, the resistance to CSF outflow in the
patient is usually increased and finite. A model of
resistance to CSF outflow could be added to the circuit before
the shunt, to study the shunt's performance in conditions
mimicking the in vivo environment (called 'residual
resistance'). Pressure before the shunt was measured with an
absolute pressure transducer (Gaeltec Luer Lock
transducer, Gaeltec Ltd, Scotland). The fluid flowing through
the shunt was collected in a container placed on the
electronic balance. Measurement of flow was recorded on a
standard IBM compatible personal computer that read
and zeroed the balance to calculate the flow rate every 15s.
By this method the weight of the outflowing fluid was
measured incrementally, which negated any effect of fluid
vaporisation from the outlet container, since vaporisation
over 15s period was considered to be negligible. The
computer also analysed the pressure waveform from the
pressure transducer and controlled the rate of the infusion
pump. The effect of changes in atmospheric pressure was
compensated for by using the reference barometer, such
that the effective pressure was measured as current
pressure minus drift of atmospheric pressure.
The shunt and pressure transducer were placed at the
same height. The water column in the fluid container (H),
the degree of the shunt submersion (h) and the level of
the outlet tubing (O) could be changed according to the
(i) The differential pressure was measured while flow
through the shunt was varied (flow-pressure, Figure 3A).
(ii) The flow through the shunt was measured while the
differential pressure across the shunt was varied
(pressureflow, Figure 3B).
Three Sophysa Polaris shunts (set at 70 mmH2O,
equivalent to 5.2 mmHg) were filled with deionised and
de-aerated water. Air bubbles were gently flushed out, according
to the manufacturer's instructions. The shunts were then
mounted in three identical rigs (Figure 2). Before each
test, the shunts were inspected for air bubbles and gently
flushed if necessary, and the pressure transducers and
reference barometers zeroed.
The following parameters were calculated:
Closing pressure: The differential pressure below which
flow through the shunt ceases. The closing pressure was
measured as the intercept of the regression line with the
xaxis, drawn between pressure (independent variable) and
flow (dependent variable) for flow from 0.2 to 0.05 ml/
Opening pressure: The differential pressure above which
non-zero flow through the shunt was measured during
the ascending ramp of the infusion pump rates.
Hydrodynamic resistance: The change in pressure
divided by the change in flow decreasing from about 1.2
to 0.3 ml/min. The resistance was measured as a linear
regression gradient between pressure and flow. This
parameter describes the resistance of the permanently
Operating pressure at 0.3 ml/min flow: The pressure
measured during infusion of fluid at 0.3 ml/min, which is
approximately equivalent to CSF flow.
The above parameters were measured at all the different
adjustable pressure levels (10 tests at each setting for each
Additionally parameters were compared at three
temperatures (30, 36 and 40C, three repeated tests). Pulse
waveform given from the generator of graded peak-to-peak
amplitude (1 to 50 mmHg, at 60 cycles per minute) was
used to measure influence of proximal pulsations on
operating pressure. Negative outlet pressure of -19 mmHg
was applied to mimic siphoning in upright position.
The magnitude of magnetic field translational attraction
was assessed using the standardized procedure of the
socalled deflection angle test. The valve was suspended by a
piece of lightweight thread that was attached to a plastic
protractor so that the angle of deflection from the vertical
of a line could be measured. Assuming the mass of the
thread to be negligible, a quantitative estimate of the
translational attraction of the valve was calculated.
Artefacts were determined using a spherical gel-filled
phantom. The T1 and T2 values for this gel were similar to
those of grey matter. MRI was performed using a 3 Tesla
MR system (MAGNETOM Tim Trio, SIEMENS, Erlangen,
Germany) and a 12-channel head matrix radio frequency
receiver coil and whole body transmit coil. The following
pulse sequences were used: 1. T1 weighted spin echo, TR/
TE 500/20 ms, matrix size 256 256, 4 mm slice
thickness, 22 cm FOV, 2 excitations; and 2. Gradient echo pulse
sequence, TR/TE 500/20 ms, flip angle 20, matrix size 256
256, 4 mm slice thickness, 22 cm FOV, 2 excitations.
Time (approx. 4.5 hours)
TFiygpuicrael p3lots of flow and pressure over time: A: Flow-pressure test
Typical plots of flow and pressure over time: A: Flow-pressure test. Flow is the controlled variable and pressure is the
measured variable. B: Pressure-flow test. Pressure is the controlled variable and flow the measured variable.
Artefact volume was expressed in cm3. All image display
parameters were carefully selected to facilitate a valid
determination of the artefact size.
Mean values, standard deviations and maximal-minimal
values were used to express average parameters and their
spread for the three shunts. To evaluate fluctuations of
parameters in altered conditions a paired t-test for
parameters at a baseline and in altered conditions was used. The
t-test was used to evaluate sample-related differences in
Analysis of variance (ANOVA), with time as the
independent factor, was used to evaluate the stability of parameters
over time. The level p < 0.05 was used as the limit for
Valve under normal conditions with opening pressure set at
performance level 70 mmH2O (5.2 mmHg)
The mean value of the shunt opening and closing
pressures (these two pressures were identical) was 6.7 mmHg
with a range of 5.0 to 7.1 mmHg (tests repeated 30 times
in 3 valves). The addition of a distal catheter did not
change the opening and closing pressure significantly (10
tests in 3 valves).
A typical pressure-flow curve with pressure plotted along
the x-axis and flow along the y-axis, is shown in Figure 4.
The valve without a distal catheter, and with no pulsatile
pressure wave, had slightly non-linear characteristics. The
hydrodynamic static resistance was equivalent to the
inverse of the gradient. Composite pressure-flow curves
(10 tests in 3 valves) from one valve are presented in
Figure 5 as flow versus pressure.
The hydrodynamic resistance was 2.06 0.41 mmHg/ml/
min (30 tests in 3 valves). This is a low value, and is
approximately 23 times lower than physiological
resistIFnidgiuvirdeua4l pressure-flow curves for the Polaris Valve without a distal catheter and set at 70 mmH2O (5.2 mmHg)
Individual pressure-flow curves for the Polaris Valve without a distal catheter and set at 70 mmH2O (5.2
SFuigpuerriem5posed pressure-flow curves taken from 10 tests utilizing one shunt at 70 mmH2O (5.2 mmHg)
Superimposed pressure-flow curves taken from 10 tests utilizing one shunt at 70 mmH2O (5.2 mmHg). The
scatter of the measurement points indicates good agreement over repeated measurements.
ance to CSF outflow. The resistance increased to 5.12
0.76 mmHg/ml/min after the connection of the distal
catheter (110 cm long; ID 1.1 mm).
Operating pressure was stable and consistent; the mean
value was 6.7 mmHg, range from 6.1 to 8.5 mmHg (30
tests in 3 valves).
A pulse pressure with variable peak-to-peak amplitude of
1 to 50 mmHg produced a significant decrease in
operating pressure of around 3 mmHg (Figure 6; test repeated 3
times in 3 valves). It is predicted that flow through the
valve after implantation may be affected by the pulsatile
component of CSF pressure.
None of the parameters (opening, closing pressure and
resistance) were altered by a temperature change from
30C to 40C. Therefore we would not expect a change in
CSF drainage even during a high fever or when ambient
temperature is low.
The Polaris Valve increases CSF drainage rate when a
negative outflow pressure is applied. By decreasing outlet
level by 20 cmH2O, flow increased by around 4 ml/min.
None of the parameters was altered by changing the
residual resistance to CSF outflow.
Effect of programming
Programming of the valve was checked using both
pressure-flow and flow-pressure tests. Good conformity
between the pressure-flow curves and the nominal data
Operating pressures (for flow of 0.3 ml/min) and 95%
confidence limits at different programming levels are
shown in graphical form in Figure 7 (10 tests in 3 valves
at each setting). Checking the position of the valve,
settings and re-adjustments was easy and reliable. The
hydrodynamic resistance depended on operating pressure and
increased with higher settings. Nominal values are given
in Table 1 for the shunt working without and with distal
(FTxhi-geauxerisfef)e6ct of pulse amplitude: A plot of the relationship between operating pressure (y-axis) and peak-to-peak pulse amplitude
The effect of pulse amplitude: A plot of the relationship between operating pressure (y-axis) and peak-to-peak
pulse amplitude (x-axis). The test was repeated 3 times in 3 valves.
Influence of a magnetic field
The valve cannot be re-programmed by an external
magnetic field of up to 3T (MRI magnet). It does not heat up
when placed within the magnetic field. The maximal
translational force measured was 81G. These values are
still considered as safe after implantation. Distortion of
the MRI scan was significant: gradient echo 954 cm3, and
T1 100 cm3 (Figure 8).
When the valve was unpacked and filled for the first time
with water it worked normally almost immediately,
providing that all air bubbles had been removed.
The hydrodynamic resistance and operating pressure did
not exhibit any time-related trends during the 30 days of
testing. No significant (p > 0.05) differences in measured
parameters were found between the three valves tested. All
the values of closing pressures measured were within the
limits specified by the manufacturer. The valve did not
show any reflux when tested. It did not exhibit reversal of
flow for an outlet-inlet differential pressure of up to 200
Assembled junctions (standard surgical sutures were
applied by a trained neurosurgeon) did not break when a
test specimen was subjected to a load of 1 kg force for 1
minute. All junctions remained free from leakage when
the water pressure was increased to 3 kPa (about 25
The Polaris valve represents the next generation of the
well-known Sophysa programmable valves. First released
onto the market in 1985, these were the first valves in
which the operating pressure could be adjusted
transcutaneously. However, one design flaw that has consistently
been a problem with programmable valves, has been their
tendency to be reset by the external magnetic field created
by domestic appliances [6,7] and by MRI scanners [8,10].
As there is only one other programmable valve that is able
to resist magnetic resetting (Miethke ProGAV) [1,9], the
FMiegaunreva7lues and 95% confidence limits for the valve's closing pressure at different performance levels
Mean values and 95% confidence limits for the valve's closing pressure at different performance levels. The test
was repeated 10 times in 3 valves.
Polaris will hopefully prove to be a welcome addition to
the market. The translational forces observed in a 3T
magnet appear to be safe for the patient. The artifact produced
by the valve is, however, considerable. Implantation on
the chest instead of the head might, therefore, be worth
considering if the patient is to be scanned in the future.
The Polaris Valve is similar to the previously evaluated
Sophysa Valve with regards to its hydrodynamic
properties. The main difference being its aforementioned ability
to resist MR induced resetting.
At the low opening pressure the valve has a low
hydrodynamic resistance and as the valve does not have a
siphoncontrol mechanism, it would be safer if the resistance was
closer to a physiological value of 810 mmHg/ml/min. It
might, therefore, be appropriate to use a thinner distal
catheter when lower performance levels are required .
Shunt with catheter (110 cm long, internal diameter 1.1 mm)
Units are mmHg/ml/min; mean values and standard deviation (in parentheses). Each test was repeated 3 times in 3 valves.
FAirgtiufarcet 8on MR image: The Polaris valve was placed in a water-filled container, with diameter equivalent to adult skull)
Artifact on MR image: The Polaris valve was placed in a water-filled container, with diameter equivalent to
adult skull). A) Gradient echo image; B) T1 image.
The hydrodynamic resistance increases with the pressure
performance level. The resistance at 8.21 14.2 mmHg/
ml/min matches and exceeds the normal CSF outflow
resistance when the shunt is set at the highest pressure
levels (150 to 200 mmH2O). This is consistent with
manufacturer's recommendations to use a high pressure setting
at the initial stage of the treatment, in order to minimize
the risk of overdrainage and shunt dependency.
Even if the test demonstrates a limited decrease of the
differential pressure with CSF pulse amplitude, it seems
hazardous to speculate any clinical consequences without
consideration of brain and distal compliance, and/or the
type of ICP waveform.
As for any adjustable shunt, distortion of the MRI image is
significant when the slice is crossing the Polaris valve. This
should be taken into consideration if MRI examination is
considered necessary for the follow-up after shunt
implantation and should influence the location of the
shunt placement accordingly. It should be remembered
that the performance of the Polaris valve is not affected by
the location of the implant on the skull. Regarding the
suggestion to implant the Polaris valve on the chest rather
than on the skull, the manufacturer is very cautious on
this implantation site for the thickness of the skin
covering the valve is likely to exceed 8 mm, making the
unlocking of the valve impossible.
Laboratory testing of the valve provides knowledge of its
hydrodynamic parameters. This data can be used for
obtaining further functional information when testing the
valve's performance in vivo, either by using an infusion
test or with overnight CSF pressure monitoring [13,14].
Nevertheless, regardless of how well the valve performs in
the laboratory, data should ideally be supplemented by
The Polaris valve performed well in the in vitro testing
protocols; the hydrodynamic properties were good and the
valve pressure settings were MRI resistant. It presented a
significant artifact in MRI scans. The valve should be
closely monitored once used clinically, to ensure that its
function corresponds to pre-clinical data obtained.
This evaluation study was commissioned by Sophysa SA
in, agreement with the University of Cambridge. The
study was conducted independently and the presented
results were not influenced by the manufacturer. MC is on
unpaid leave from Warsaw University of Technology.
DMA analysed the data and wrote the manuscript, ZHC
and MC performed measurements, JDP and HKR
supervised project and helped in data analysis and writing the
manuscript. All authors have read and approved the final
version of the manuscript.
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