title: Investigation of the hydrodynamic properties of a new MRI-resistant programmable hydrocephalus shunt.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Shunt Evaluation Laboratory & Academic Neurosurgical Unit, Addenbrooke's Hospital, P,O, Box 167, Hills Road, Cambridge CB2 2QQ, UK. mc141@medschl.cam.ac.uk.
authors.forename: David M
authors.initials: DM
authors.lastname: Allin
authors.forename: Marek
authors.initials: M
authors.lastname: Czosnyka
authors.forename: Hugh K
authors.initials: HK
authors.lastname: Richards
authors.forename: John D
authors.initials: JD
authors.lastname: Pickard
authors.forename: Zofia H
authors.initials: ZH
authors.lastname: Czosnyka