The Frankfurt horizontal plane as a reference for the implantation of gravitational units: a series of 376 adult patients.

Background: The in-line combination of adjustable differential pressure valves with fixed gravitational units is increasingly recommended in the literature. The spatial positioning of the gravitational unit is thereby decisive for the valve opening pressure. We aimed at providing data on factors contributing to primary overdrainage and underdrainage of cerebrospinal fluid (CSF), with special attention paid to the implantation angle of the gravitational unit.

Methods: Weretrospectively analyzed the postoperative course of 376 consecutive patients who received a ventriculoperitoneal shunt with a proGAV valve. The incidence of both primary CSF overdrainage and underdrainage was correlated with the implantation angle of the gravitational unit in regard to the Frankfurt horizontal plane and the patients' general parameters.

Results: Primary overdrainage was found in 41 (10.9 %) patients. Primary underdrainage was found in 113 (30.1 %) patients. A mean deviation of 10° (±7.8) for the gravitational unit in regard to the vertical line to the Frankfurt horizontal plane was found. In 95 % of the cases the deviation was less than 25°. No significant correlation between the implantation angle and the incidence of overdrainage or underdrainage of CSF was found. The patients' age and having single hydrocephalus entities were identified as factors significantly predisposing patients to overdrainage or underdrainage.

Conclusion: The implantation of the gravitational unit of the proGAV valve within a range of at least 10° in regard to the vertical line to the Frankfurt horizontal plane does not seem to predispose patients to primary overdrainage or underdrainage in ventriculoperitoneal shunting. The plane may serve as a useful reference for the surgeon's orientation.