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 catheter.

The tension of the flat spring can be adjusted non-invasively by moving the rotor with the external programming 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 hydrocephalus-related 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 11, 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 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 test protocol.

The shunt was tested under two different regimes:

(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 (pressure-flow, 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 x-axis, drawn between pressure (independent variable) and flow (dependent variable) for flow from 0.2 to 0.05 ml/min.

• 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 opened shunt.

• 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 valve)

Additionally parameters were compared at three temperatures (30, 36 and 40°C, 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 so-called 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. 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 parameters.

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 statistical significance.

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 2–3 times lower than physiological resistance 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 30°C to 40°C. 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.

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 was found.

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 catheter.

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).

Opening and closing pressures displayed very limited variation during all the tests. The changes were not time-related.

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 mmHg.

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 mmHg).