Acta Neurochir (2014) 156:1351–1356
DOI 10.1007/s00701-014-2076-y
CLINICAL ARTICLE - PEDIATRICS
The Frankfurt horizontal plane as a reference
for the implantation of gravitational units: a series of 376 adult
patients
Florian Baptist Freimann & Marie-Luise Luhdo &
Veit Rohde & Peter Vajkoczy & Stefan Wolf &
Christian Sprung
Received: 15 January 2014 / Accepted: 19 March 2014 / Published online: 4 May 2014
# Springer-Verlag Wien 2014
Abstract
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.
F. B. Freimann (*) : V. Rohde
Department of Neurosurgery, Universitätsmedizin Göttingen,
Georg-August-Universität Göttingen, Germany, 37099 Göttingen,
Germany
e-mail: florian.freimann@med.uni-goettingen.de
F. B. Freimann : M.<L. Luhdo : P. Vajkoczy : S. Wolf : C. Sprung
Department of Neurosurgery, Charité - Universitätsmedizin Berlin,
Germany, Berlin, Germany
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.
Keywords Frankfurt horizontal plane . Gravitational unit .
Hydrocephalus . Ventriculoperitoneal shunt
Introduction
Ventriculoperitoneal shunting is accompanied by risks such as
surgical complications, shunt infections, or dislocations of the
ventricular or peritoneal catheter. Those hazards are not related to the shunt valve. Another non-valve-related shunt malfunction is the underdrainage of cerebrospinal fluid (CSF) due
to mechanical obstacles like kinking of the catheter, catheter
occlusion, or abdominal pseudocysts [3, 4]. In contrast, an
unsatisfying clinical result after shunt implantation without an
obstruction of the catheter may represent a functional
underdrainage of CSF [18]. The functional underdrainage
and the overdrainage of CSF are valve-related issues. The
overdrainage of CSF may lead to orthostatic headache, slitlike ventricles, or subdural effusions. A surgical revision with
a ligation of the shunt, a change of the valve, or the surgical
decompression of subdural hematoma may be necessary.
However, the flow resistance within the shunt system is mainly controlled by the opening pressure of the valve. Both CSF
overdrainage and functional underdrainage can be treated to a
certain extent by valve adjustments. This was demonstrated
especially for modern adjustable and gravity-assisted valves
such as the proGAV valve (Miethke, Potsdam, Germany),
which consists of an adjustable differential pressure (DP)
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valve and a gravitational unit with a fixed opening pressure [6,
7, 9, 11, 12, 20].
It has been stated that the gravitational unit of the proGAV
valve is mainly responsible for those beneficial effects. This
device consists of a cylindrical titanium housing in which a
tantalum ball blocks the CSF flow through the device gradually via a position-dependent mechanism [1]. The implantation angle of the gravitational unit is therefore important for
the amount of CSF drainage. Ideally it should be in a vertical
position when the patient is standing upright and in a horizontal position when the patient is lying flat [19]. If the gravitational unit position is suboptimal, in theory, overdrainage or
underdrainage of CSF may occur. A deviation from the vertical body axis of <10° was suggested as acceptable [21].
However, the question of to what degree the implantation
angle of gravitational units is of clinical relevance has not
yet been answered entirely. Furthermore, an ideal reference
plane for the implantation of gravitational units is, to our
knowledge, not defined in the literature.
The Frankfurt horizontal plane (FHP) is defined as a plane
passing through the inferior margin of the orbit and the upper
margin of the external auditory meatus. It is described as a
plane that is the nearest match to the gravity horizontal line
(GHL) [2, 14]. We hypothesize that the FHP is useful to serve
as a reference for the surgeon to identify the optimal angle for
implantation of the gravitational unit. The purpose of the
current study was to investigate the factors contributing to
primary CSF overdrainage and underdrainage with special
attention to the spatial relationship of the gravitational unit
to the FHP.
Patients & methods
Study group
We retrospectively analyzed the postoperative course of 376
consecutive adult patients treated for different entities of hydrocephalus between February 2004 and December 2011. All
patients received a ventriculoperitoneal shunt with a proGAV
valve.
The patients’ general parameters such as age, gender, body
weight, body height, and mobility were evaluated. The implantation site of the gravitational unit and perioperative complications were included in this analysis. The various hydrocephalus entities were defined as primary normal pressure
hydrocephalus (pNPH), secondary NPH with a history of
more than 3 months between causative event and development of hydrocephalus (sNPH), malresorptive hydrocephalus
(MH), hypertensive hydrocephalus (HH), pseudotumor
cerebri (PC), and congenital hydrocephalus (CH) with shunt
dependency since early childhood. The latter patients received
the proGAV valve at an adult age in a revision surgery.
Acta Neurochir (2014) 156:1351–1356
Overdrainage and underdrainage
The initial setting of the valve opening pressure and all documented first valve adjustments were analyzed. An
unsatisfying clinical result with no significant reduction of
the ventricular size in postoperative imaging and no evidence
of a mechanical obstruction in the shunt system resulted in an
initial decrease of the valve opening pressure and was considered to be a primary functional underdrainage. If signs and
symptoms like orthostatic headaches and slit-like ventricles,
subdural hygroma, or hematoma were detected in cranial
imaging, an increase of the valve opening pressure was carried
out, and a primary overdrainage of CSF was assumed.
Valve implantation angle
The FHP and the gravitational unit (SA) were identified on the
lateral scout view of computed tomography (CT) scans conducted subsequent to the implantation of the new shunt system
in all cases. A line was drawn vertical to the FHP (VL), and
another line along the implantation axis of the gravitational
unit. The angle between VL and the axis of the gravitational
unit was measured. A deviation of the gravitational unit from
VL in the anterior direction above the FHP was defined as an
inclination of the gravitational unit. A deviation in the posterior direction above the FHP was defined as a reclination of
the gravitational unit (Figure 1).
Statistical analysis
All statistical analysis was performed using the statistical
environment R 3.0.1 (R Software Foundation, Vienna, Austria, www.r-project.org). Summary data is presented as mean
and standard deviation, or median (25 %/75 % quartile),
whichever is appropriate. Univariate comparisons were
performed with the Wilcoxon test. Multivariate assessment
was modeled with generalized linear models, with all
demographic factors being initially included. To achieve the
most parsimonious model, a step-down approach was used
and the least significant term excluded until only factors
significant at p<0.05 remained in the final model.
Results
General parameters
We included 200 female (53.2 %) and 176 male (46.8 %)
consecutive patients in our analysis. All patients received a
ventriculoperitoneal shunt for different hydrocephalus entities
(99 pNPH, 57 sNPH, 133 MH, 37 HH, 16 PC, and 27 CH)
with a retroauricularly implanted proGAV valve. The opening
pressure of the gravitational unit was +20 cm H2O in the vast
Acta Neurochir (2014) 156:1351–1356
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Fig. 1 Lateral scout view of the postoperative CT imaging of three different patients with (a) reclined, (b) vertical, and (c) inclined implantation angles
of the gravitational unit (SA) in relation to the vertical line (VL) to the Frankfurt horizontal plane (FHP)
majority of cases. The mean age at shunt implantation was
55.6 (±18.2) years. The mean body weight was 75 (±16) kg,
while the body height was 170 (±11) cm. We considered 304
(80.9 %) patients of the study population to be “mobile”,
while 72 (19.1 %) patients were “bedridden” due to various
neurological disorders. The median follow-up period of the
analysis was 20 (15/30 quartile) months (Table 1).
Surgical revisions
A total of 61 shunt revisions were carried out during the study
period. In 48 cases (12.8 %), revision surgery was indicated
due to non-valve-related reasons. We observed 14 shunt infections with explantation of the shunt (3.7 %), 10 superficial
wound infections (2.7 %), 7 revisions of the ventricular catheter (1.9 %), 12 revisions of the abdominal catheter (3.2 %), 2
mechanical obstructions of the catheter (0.5 %), and 2 secondary disconnections (0.5 %). An injury of the intestines
occurred during the placement of the abdominal catheter in
one case (0.3 %). In 13 cases (3.5 %), revision surgery was
indicated for valve-related reasons. A functional
underdrainage led in 12 cases (3.2 %) after a stepwise downregulation of the opening pressure to 0 cm H2O to the assumption of presence of a dysfunction and surgical revision of
the valve. In six of these cases (1.6 %), the gravitational unit
was removed. One patient suffering from MH after posttraumatic bilateral cranial decompression developed a subdural
hematoma subsequent to the shunt implantation and reimplantation of a bone flap in a combined procedure. The
hematoma was evacuated and the shunt was ligated
transiently.
Overdrainage and underdrainage
The opening pressure of the DP unit of the proGAV valve was
set to a median of 5 (5/5 quartile) cm H2O at the time of
implantation. Valve adjustments were carried out in 154 patients (41 %). A primary upregulation of the valve opening
pressure was carried out in 41 of the cases (10.9 %) for the
treatment of conditions related to CSF overdrainage. These
conditions were orthostatic headache in 7 cases (1.9 %), slitlike ventricles in 18 cases (4.8 %), subdural hygroma in 13
cases (3.5 %), and subdural hematoma in 3 cases (0.8 %). All
overdrainage-related events could be sufficiently treated by
valve adjustments (excluding the case mentioned above). A
primary downregulation of the valve opening pressure for the
treatment of a functional underdrainage of CSF was carried
out in 113 of the cases (30.1 %).
We found a significant correlation between patient age and
CSF overdrainage and underdrainage in our series. Patients
suffering from primary overdrainage of CSF had a mean age
of 47.1 (±18.3) years (p<0.01). In contrast, patients having
signs and symptoms which were considered as a primary
underdrainage had a mean age of 59.4 (±17.8) years
(p<0.001) (Table 1). The incidence of CSF underdrainage
was found to increase with age (0.4 % increase per age year)
(p=0.05).
No significant correlation of patient gender, body height,
body weight, mobility, and the initial valve setting with the
occurrence of overdrainage or functional underdrainage was
found in a univariate analysis (Table 1). Multivariate analysis
revealed that underdrainage was significantly less common in
mobile patients (p<0.01) and more likely with higher initial
valve settings (4 % increase per cm H2O) (p=0.001).
Hydrocephalus entities were found to play a significant
role in valve-related complications. Only five atients with
pNPH were treated for overdrainage (p<0.05), while in 50
(of 99) cases a primary downregulation of the valve opening
pressure for the treatment of a functional underdrainage was
carried out (p<0.001). No gender-specific preponderance for
this observation was found. The mean age in this subpopulation was 70.7 (±9) years; the median initial opening
pressure of the DP valve was 5 (5/5 quartile) cm H2O
(Table 1). Patients with MH had significantly fewer signs and
symptoms of overdrainage than the remaining study population did (26 patients out of 133)(p<0.001). The mean age in
the MH sub-population was 54 (±14.7) years; the median
initial opening pressure of the DP valve was 5 (5/5 quartile)
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Acta Neurochir (2014) 156:1351–1356
Table 1 Patients’ general parameters, mobility, and hydrocephalus entities in regard to the primary overdrainage and underdrainage of CSF. Level of
significance *=p<0.05, **=p<0.01, ***=p<0.001 compared to the study group
Age (mean years ± SD)
Gender
Female
Male
Body height (mean cm ± SD)
Body weight (mean kg ± SD)
Mobility
Mobile
Bedridden
Primary valve setting (median and 25 %/75 % quartile)
Hydrocephalus entities
Primary NPH
Secondary NPH
Malresorptive
Hypertensive
Pseudotumor
Congenital
All patients
Primary overdrainage
Primary underdrainage
55.6 (±18.2)
47.1 (±18.3)**
59.4 (±17.8)**
200
176
26
15
63
50
170 (±11)
75 (±16)
167 (±16)
71 (±12)
171 (±13)
73 (±13)
304
72
5 (5/5)
32
9
5 (5/5)
90**
23
5 (5/5)
99
57
133
37
16
27
5*
8
13
6
1
8**
50***
15
26***
8
3
9
cm H2O (Table 1). In contrast, the patients with CH who
received the proGAV valve in a revision surgery had a significantly higher risk for overdrainage-related events and corresponding valve adjustments (8 patients out of 27)(p<0.01).
The mean age in this sub-population was 27.4 (±7.2) ye;rs, the
median initial opening pressure of the DP valve was 5 (5/6
quartile) cm H2O (Table 1).
Valve implantation angle
A deviation of the implantation angle of the SA in relation to
the VL was measured in 360 patients of the whole study group
(95.7 %)(Fig. 1). The absolute mean deviation from the VL
was 10° (±7.8). 95 % of the patients had a deviation of less
than 25°. No correlation was found between the implantation
angle in relation to the VL of the FHP and the occurrence of
overdrainage or underdrainage (all p >0.4) (Table 1). A
reclination of the gravitational unit was found in 185 cases,
with a mean deviation of 9.7° (±7.7); while an inclination was
found in 175 cases, with a mean deviation of 10.3° (±8).
Neither the reclination nor the inclination of the gravitational
unit in relation to the VL was observed to predispose significantly for overdrainage or underdrainage of CSF (all p>0.3)
(Table 2). No effect of the hydrocephalus entity, the mobility
of the patients, or other parameters on the significance of the
implantation angle for the overdrainage or underdrainage of
CSF was found in our study group. Furthermore, no threshold
value for the deviation from the VL of the FHP could be
determined to predispose for overdrainage or underdrainage.
Discussion
The augmentation of adjustable DP valves with antisiphoning devices (ASD) is advocated in the literature. Beneficial effects for the prevention and the treatment of
overdrainage and functional underdrainage in patients with a
ventriculoperitoneal shunt have been reported, especially for
the use of gravitational valves [6, 7, 12]. The opening pressure
of gravitational units depends on their spatial positioning. A
suboptimal placement of the gravitational unit might therefore
predispose patients, along with other factors such as patients’
age, gender, body weight, body height, hydrocephalus entity,
and the initial setting of the DP valve opening pressure, for
overdrainage or underdrainage of CSF.
Park et al. [16] described this phenomenon in a retrospective series of 24 adult patients shunted with a gravitational
valve with a fixed opening pressure (GAV) (Miethke, Potsdam, Germany). They found a positive correlation of the valve
inclination relative to the patients’ vertical position with the
extent of ventricular volume changes after shunting. It seems
notable that the radiographic images for this analysis were
taken in the patient standing position. An inclination of more
than 20° to a patient’s vertical position was described as a
predisposition to functional underdrainage in the sub-group of
non-ambulatory patients [16]. Deininger and Weyerbrock [5]
and Kaestner et al. [8] also emphasized the relevance of the
spatial positioning of gravitational units, especially for bedridden patients in smaller retrospective series [5, 8].
But to what extent is the implantation angle of the gravitational unit relevant for the clinical course of patients with a
Acta Neurochir (2014) 156:1351–1356
Table 2 The deviation angles of
the gravitational units from VL in
regard to the primary
overdrainage and underdrainage
of CSF
1355
Deviation from VL of FHP (mean ± SD)
Number of inclination
Degree of inclination (mean ± SD)
Number of reclination
Degree of reclination (mean ± SD)
ventriculoperitoneal shunt? We aimed at providing data on
that question by retrospectively analyzing a series of 376
consecutive adult patients who received a proGAV valve
(Miethke, Potsdam, Germany), which consists of an adjustable DP unit and a gravitational unit with a fixed opening
pressure, for the treatment of various hydrocephalus entities.
The spatial positioning of the gravitational units of the
proGAV was evaluated on lateral scout CT images obtained
from the patients while in a horizontal body position. A deviation from the vertical axis in the lateral direction is not detectable on two-dimensional lateral plane images; this is a limitation of our retrospective study design. However, a relevant
deviation of the implantation angle from the VL is only possible in the anterior-posterior direction due to the skull shape.
A deviation in the anterior-posterior direction requires a
reference plane, which represents the gravity horizontal line
(GHL), as is, in fact, the case for the intraoperative situation
when the gravitational unit is implanted. The vertical axis to
the FHP (VL) was used for the anterior-posterior direction as
the reference for the implantation angle of gravitational units
in regard to the patient’s vertical axis. The FHP is defined as a
plane between the inferior margin of the orbit and the upper
margin of the ipsilateral external auditory meatus. It was
historically introduced in 1884 at the World Congress on
Anthropology in Frankfurt am Main, Germany, and is nowadays still accepted as a plane that nearly matches the gravity
horizontal line (GHL) [10, 13]. Nevertheless, some authors
challenge the congruence of the FHP with the GHL, and a
mean deviation of the FHP from the GHL of approximately 5°
has been reported [14, 17].
The state of CSF overdrainage or underdrainage was evaluated by analyzing the initial DP shunt opening pressure, the
first valve adjustments, and the corresponding clinical and
radiological findings after shunt implantation. All subsequent
valve adjustments were not included in this analysis. We
regard those following adjustments as being mainly the result
of the preceding adjustment. A clear relationship to the implantation angle of the gravitational unit or other independent
factors contributing to CSF overdrainage or underdrainage
cannot be abstracted from those actions.
We observed a correlation between patient age and
overdrainage and underdrainage. Patients of younger age
had thereby a significantly higher risk for signs and symptoms
All patients
Primary overdrainage
Primary underdrainage
10 (±7.8)
175
10.3 (±8)
185
9.7 (±7.7)
10 (±7.5)
22
10.8 (±7)
19
9.1 (±8.2)
10.4 (±6.9)
54
11.2 (±6.4)
58
9.6 (±7.2)
indicating ovedrainage, while older patients had significantly
more frequent valve adjustments for the treatment of a functional underdrainage. Furthermore, patients suffering from a
hydrocephalus entity typical for those of younger age had a
significantly higher risk for overdrainage-related events. We
interpret this finding to reflect an assumedly higher grade of
activity and mobility in younger patients. This is in agreement
with our finding that underdrainage is less common in mobile
patients. In contrast, patients suffering from hydrocephalus
entities typical for advanced age were found to have a functional CSF underdrainage significantly more often. The phenomenon of a stepwise downregulation of the valve opening
pressure was described in particular during the long-term
treatment of pNPH [7].
The recently published data from a prospective and
multicentric trial on "Shunt Valves plus shunt Assistant versus
Shunt valves alone for controlling Overdrainage in idiopathic
Normal pressure hydrocephalus in Adults" (SVASONA) indicate inter alia an association of female sex with a higher risk
of clinical signs and symptoms suggestive of overdrainage
[12, 15]. We also expected to find correlations between gender
and other parameters, such as body height and body weight,
and the overdrainage or underdrainage. The reason why no
such correlation was found remains speculative, but we assume that these parameters were overrated compared to other
less obviously influential variables within our large and inhomogeneous study group.
We did not observe a significant correlation of the implantation angle of the gravitational unit in regard to the VL of the
FHP. The heterogeneity of our study group and small numbers
of patients of single hydrocephalus types might have masked
an effect of the implantation angle in this particular subgroup.
We hypothesize that the habitual head position and the need
for CSF drainage varies between individuals and during daily
activity. Furthermore, a deviation of the FHP from the GHL
was reported in the literature. These factors may have eliminated possible effects resulting from different implantation
angles in regard to the FHP on CSF overdrainage and
underdrainage. We found a mean deviation from the VL of
10°; in 95 % of the patients the deviation was less than 25°.
We therefore conclude that an implantation of the gravitational
unit of the proGAV within this range is safe, as it was suggested previously [21]. Nevertheless a more precise
1356
orientation of the spatial positioning of gravitational units still
has to be sought after. The FHP appears as a useful reference
plane for the implantation of gravitational shunt valves.
Conclusion
The retroauricular implantation of the gravitational unit of the
proGAV within a range of at least 10° deviation from the VL
in reference to the FHP may not predispose for CSF
overdrainage or underdrainage. The FHP may serve as a
useful reference for the surgeon’s orientation.
Conflicts of interest FBF, MLL, VR, PV, and SW have no conflicts of
interest to disclose. CS is supported by Aesculap AG to deliver presentations at scientific meetings but has no financial interest in the product
this article discusses.
References
1. Allin DM, Czosnyka ZH, Czosnyka M, Richards HK, Pickard JD
(2006) In vitro hydrodynamic properties of the Miethke proGAV
hydrocephalus shunt. Cerebrospinal Fluid Res 3:9
2. Barbera AL, Sampson WJ, Townsend GC (2009) An evaluation of
head position and craniofacial reference line variation. Homo 60(1):
1–28
3. Browd SR, Gottfried ON, Ragel BT, Kestle JRW (2006) Failure of
cerebrospinal fluid shunts: part II: overdrainage, loculation, and
abdominal complications. Pediatr Neurol 34(3):171–176
4. Browd SR, Ragel BT, Gottfried ON, Kestle JRW (2006) Failure of
cerebrospinal fluid shunts: part I: Obstruction and mechanical failure.
Pediatr Neurol 34(2):83–92
5. Deininger MH, Weyerbrock A (2009) Gravitational valves in supine
patients with ventriculo-peritoneal shunts. Acta Neurochir (Wien)
151(6):705–709, discussion 709
6. Freimann FB, Sprung C (2012) Shunting with gravitational valvescan adjustments end the era of revisions for overdrainage-related
events? J Neurosurg. doi: 10.3171/2012.8.JNS1233
7. Freimann FB, Vajkoczy P, Sprung C (2013) Patients benefit from
low-pressure settings enabled by gravitational valves in normal pressure hydrocephalus. Clin Neurol Neurosurg. doi: 10.1016/j.clineuro.
2013.06.010
Acta Neurochir (2014) 156:1351–1356
8. Kaestner S, Kruschat T, Nitzsche N, Deinsberger W (2009)
Gravitational shunt units may cause under-drainage in bedridden
patients. Acta Neurochir (Wien) 151(3):217–221, discussion 221
9. Kiefer M, Eymann R, Meier U (2002) Five years experience with
gravitational shunts in chronic hydrocephalus of adults. Acta
Neurochir (Wien) 144(8):755–767, discussion 767
10. Leitão P, Nanda RS (2000) Relationship of natural head position to
craniofacial morphology. Am J Orthod Dentofac Orthop 117(4):406–
417
11. Lemcke J, Meier U (2010) Improved outcome in shunted iNPH with a
combination of a Codman Hakim programmable valve and an
Aesculap-Miethke ShuntAssistant. Cent Eur Neurosurg 71(3):113–116
12. Lemcke J, Meier U, Müller C, Fritsch MJ, Kehler U, Langer N,
Kiefer M, Eymann R, Schuhmann MU, Speil A, Weber F,
Remenez V, Rohde V, Ludwig HC, Stengel D (2013) Safety and
efficacy of gravitational shunt valves in patients with idiopathic
normal pressure hydrocephalus: a pragmatic, randomised, open label,
multicentre trial (SVASONA). J Neurol Neurosurg Psychiatr 84(8):
850–857
13. Lundström A, Lundström F (1995) The Frankfort horizontal as a
basis for cephalometric analysis. Am J Orthod Dentofac Orthop
107(5):537–540
14. Madsen DP, Sampson WJ, Townsend GC (2008) Craniofacial reference plane variation and natural head position. Eur J Orthod 30(5):
532–540
15. Meier U, Stengel D, Müller C, Fritsch MJ, Kehler U, Langer N,
Kiefer M, Eymann R, Schuhmann MU, Speil A, Weber F, Remenez
V, Rohde V, Ludwig HC, Lemcke J (2013) Predictors of subsequent
overdrainage and clinical outcomes after ventriculoperitoneal
shunting for idiopathic normal pressure hydrocephalus.
Neurosurgery 73(6):1054–1060
16. Park J, Kim G-J, Hwang S-K (2007) Valve inclination influences the
performance of gravity-assisted valve. Surg Neurol 68(1):14–18,
discussion 18
17. Petricevic N, Celebic A, Celic R, Baucic-Bozic M (2006) Natural
head position and inclination of craniofacial planes. Int J Prosthodont
19(3):279–280
18. Sprung C, Miethke C, Schlosser H-G, Brock M (2005) The enigma
of underdrainage in shunting with hydrostatic valves and possible
solutions. Acta Neurochir Suppl 95:229–235
19. Stockhammer F, Miethke C, Knitter T, Rohde V, Sprung C (2013)
Flow-related noise in patients with ventriculoperitoneal shunt using
gravitational adjustable valves. Acta Neurochir (Wien). doi: 10.1007/
s00701-013-1876-9
20. Toma AK, Tarnaris A, Kitchen ND, Watkins LD (2011) Use of
proGAV® shunt valve in normal pressure hydrocephalus.
Neurosurgery. doi: 10.1227/NEU.0b013e318214a809
21. Vienenkötter B, Unterberg A, Aschoff A (2009) Implantation failures
and suboptimal positions of gravitational valves - with massive
impacts on shunt dysfunction. Cerebrospinal Fluid Res 6(Suppl 2):20