Investigation of the hydrodynamic properties of a new MRI-resistant programmable hydrocephalus shunt

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.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.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 [2]. Also, some shunts may present with reflux at low flow [3]. 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 s