Magnetohydrodynamics and viscosity variation in couple stress squeeze film lubrication between rough flat and curved circular plates

A simplified mathematical model has been developed for understanding combined effects of surface roughness, viscosity variation and couple stresses on the squeeze film behaviour of a flat and a curved circular plate in the presence of transverse magnetic field. The Stokes (1966) couple stress fluid model is included to account for the couple stresses arising due to the presence of microstructure additives in the lubricant. In the context of Christensen's (1969) stochastic theory for the lubrication of rough surfaces, two types of one-dimensional roughness patterns (radial and azimuthal) are considered. The governing modified stochastic Reynolds type equations are derived for these roughness patterns. Expressions for the mean squeeze film characteristics are obtained. Numerical computations of the results show that the azimuthal roughness pattern on the curved circular and flat plate results in more pressure buildup whereas performance of the squeeze film suffers due to the radial roughness pattern. Further the Lorentz force characterized by the Hartmann number, couple stress parameter and viscosity variation parameter improve the performance of the squeeze film lubrication as compared to the classical case (Non-magnetic, Newtonian case and non-viscous case).

Load-Bearing Capacity and Design Advantages of a Custom-Made, Thin Pure-Titanium Cranioplasty (CranioTop)

BACKGROUND

Adequate and stable coverage of cranial contour and continuity defects of any origin is a common challenge in neurosurgical clinics. This study presents the results of investigations concerning the mechanical load-bearing capacity and design advantages of custom-made implants made from a thin, pure-titanium sheet (CranioTop) (CLinstruments, Attendorn, Germany) for covering complex cranial defects.

METHODS

In 9 test series, the stability of three differently shaped and sized thin titanium sheet implants was tested using vertical, uniaxial compression with 3 different compression stamps, to investigate the behaviour of these implants in relation to punctiform as well as planar forces.

RESULTS

All 9 model implants showed elastic behavior in the synchronously recorded force/displacement diagrams at an impression of up to 2 mm. The forces at 2 mm deformation were between 170.1 and 702.7 Newton.

CONCLUSION

Cranioplasty using CranioTop is a stable procedure for covering skull defects, even those of large dimensions. An added advantage is the significant reduction in effort required to prepare the area of the bone margins compared to other current techniques of cranioplasty.

Virtual reality-based evaluation of neurovascular conflict for the surgical planning of microvascular decompression in trigeminal neuralgia patients

OBJECTIVE

Trigeminal neuralgia (TN) is a lightning bolt of violent, electrifying, and stinging pain, often secondary to the neurovascular conflict (NVC). The vessels involved in NVC are mostly arteries and rarely veins. Evaluation of NVC in the deep infratentorial region is inseparably connected with cranial imaging. We retrospectively analyzed the potential influence of three-dimensional (3D) virtual reality (VR) reconstructions compared to conventional magnetic resonance imaging (MRI) scans on the evaluation of NVC for the surgical planning of microvascular decompression in patients with TN.

METHODS

Medical files were retrospectively analyzed regarding patient- and disease-related data. Preoperative MRI scans were retrospectively visualized via VR software to detect the characteristics of NVC. A questionnaire of experienced neurosurgeons evaluated the influence of VR visualization technique on identification of anatomical structures involved in NVC and on surgical strategy.

RESULTS

Twenty-four patients were included and 480 answer sheets were evaluated. Compared to conventional MRI, image presentation using 3D-VR modality significantly influenced the identification of the affected trigeminal nerve (p = 0.004), the vascular structure involved in the NVC (p = 0.0002), and the affected side of the trigeminal nerve (p = 0.005).

CONCLUSIONS

In patients with TN caused by NVC, the reconstruction of conventional preoperative MRI scans and the spatial and anatomical presentation in 3D-VR models offers the possibility of increased understanding of the anatomy and even more the underlying pathology, and thus influences operation planning and strategy.

Impact of Virtual Reality in Arterial Anatomy Detection and Surgical Planning in Patients with Unruptured Anterior Communicating Artery Aneurysms

Anterior-communicating artery (ACoA) aneurysms have diverse configurations and anatomical variations. The evaluation and operative treatment of these aneurysms necessitates a perfect surgical strategy based on review of three-dimensional (3D) angioarchitecture using several radiologic imaging methods. We analyzed the influence of 3D virtual reality (VR) reconstructions versus conventional computed tomography angiography (CTA) scans on the identification of vascular anatomy and on surgical planning in patients with unruptured ACoA aneurysms. Medical files were retrospectively analyzed regarding patient- and disease-related data. Preoperative CTA scans were retrospectively reconstructed to 3D-VR images and visualized via VR software to detect the characteristics of unruptured ACoA aneurysms. A questionnaire was used to evaluate the influence of VR on the identification of aneurysm morphology and relevant arterial anatomy and on surgical strategy. Twenty-six patients were included and 520 answer sheets were evaluated. The 3D-VR modality significantly influenced detection of the aneurysm-related vascular structure (p = 0.0001), the recommended head positioning (p = 0.005), and the surgical approach (p = 0.001) in the planning of microsurgical clipping. Thus, reconstruction of conventional preoperative CTA scans into 3D images and the spatial presentation in VR models enabled greater understanding of the anatomy and pathology, provided realistic haptic feedback for aneurysm surgery, and influenced operation planning and strategy.

Normal pressure hydrocephalus as a failure of ICP homeostasis mechanism: the hidden role of Monro-Kellie doctrine in the genesis of NPH

OBJECTIVE

The theme of this paper is to outline that the genesis of normal pressure hydrocephalus (NPH) is governed by the intracranial pressure (ICP) homeostatic principle. The development of this new concept is based mainly on rethinking the well-known Monro-Kellie doctrine in the way that ICP homeostasis mechanism is not only a mechanism that works to prevent pathologically high ICP but also a mechanism that aims to protect from pathologically low ICP.

METHODS

The NPH-related literatures are reviewed and reinterpreted to generate a new paradigm for the cascade of pathophysiological events that leads to the genesis NPH, as well as the mechanism of clinical beneficial effects and complications of the shunting procedure.

RESULTS

According to this new paradigm, the suboptimal cerebral perfusion that is associated with the impairment of the cerebral autoregulation is the initial step in the genesis of NPH. When the overall volume of blood that circulates intracranially is diminished, a chronic low ICP with episodes of pathologically low ICP occurs. Since the cranial vault is not collapsible, those episodes of low ICP are compensated by the accumulation of cerebrospinal fluid (CSF) to keep the ICP in normal ideal range. The impairment of brain toxin-flushing mechanism because of CSF pooling combined with the already-established suboptimal cerebral perfusion leads to functional disinhibition of the cerebral cortex.

CONCLUSION

Recognizing the importance of ICP homeostatic mechanisms in the genesis of the NPH is a simple yet novel view that could change the way we look at NP and can give a basic and fundamental theoretical frame work to achieve better understanding of NPH.

An endoscopic, cadaveric analysis of the roof of the fourth ventricle

We performed endoscopic dissections of the roof of the fourth ventricle in eight fresh human cadaveric heads to characterize the endoscopic anatomy of the roof of the fourth ventricle and the anatomical configuration of the structures forming its roof. We also made three-dimensional (3D) silicone casts of the fourth ventricle in seven formalin-fixed specimens to evaluate the 3D configuration of the structures that create the roof of the fourth ventricle. The roof of the fourth ventricle can be divided into three zones. The upper zone is formed by the superior cerebellar peduncle and superior medullary velum and is associated with the lingula. The middle zone is formed by the inferior cerebellar peduncles and inferior medullary velum and is associated with the nodule in the midline and with the peduncle of the flocculus. The lower zone is formed by the tela choroidea and is associated with the tonsils. The 3D shape of the roof the fourth ventricle resembles that of a rhomboid-based pyramid; the edges of the base represent the borders of the ventricle, and the apex is the cerebellar fastigium. The lateral recess is shaped like a triangular-based pyramid, with its base connected to the cavity of the fourth ventricle and its tip opening into the lateral cerebellomedullary cistern through the foramen of Luschka. Our results may help in the endoscopic exploration of and microsurgical approaches to the fourth ventricle through its roof.

Real time parallel intraoperative integration of endoscopic, microscopic, and navigation images: a proof of concept based on laboratory dissections

Endoscope, microscope, and neuronavigation systems are integrated in neurosurgical procedures mainly by using a serial combination algorithm, where, the user must switch his/her field of view from one platform display to another. The integration of theses devices could be optimized by incorporating different displays into one viewing platform thus achieving a parallel combination. In this study, we investigated the feasibility and the applicability of parallel integration of microscopic, endoscopic, and neuronavigation images by real time displaying the endoscope and neuronavigation image datasets in the main operative microscope oculars. The proposed set-up was effective in displaying the three images dataset in an operationally actionable mode. Ergonomically, the ability of using the different image dataset without the need of taking the eyes off the microscope oculars did not disrupt the flow or the tempo of the operative procedure. However, new endoscopes specific to this application are recommended.

A new methodology for laboratory evaluation of neurosurgical approaches based on the volume and shape of the surgical space with a mathematical model to quantify the surgical maneuverability in laboratory settings

We conducted this study to validate the volume/shape of the surgical exposure and to introduce a mathematical model to quantify the maneuverability in a surgical space. We executed the pterional and lateral supraorbital approach four times in fresh cadavers in skull base laboratory. The surgical volumes were filled with a computed tomography (CT)-imageable mixture; CT scans were obtained to evaluate the volume and shape of the surgical space. The volume of the surgical space was 23.60 and 32.90 mL for the lateral supraorbital and pterional approach, respectively, (p < 0.05). The three-dimensional shape of the lateral supraorbital approach was cylindrical and that of the pterional approach pyramidal. The volume of the surgical approach can be used to define, together with other variables, the maneuverability (maneuvering in a surgical volume) by using the following formula [Formula: see text] where M, A, V, and L represent the maneuverability, the degree of the surgical freedom, the volume of the surgical exposure, and the surgical depth, respectively. Volume and shape of the surgical exposure are two objective parameters that can be used to define and contrast different microsurgical approaches in a laboratory setting. The volume of the surgical exposure may be integrated into a mathematical formula defining maneuverability.

Placement of thoracolumbar pedicle screws using O-arm-based navigation: technical note on controlling the operational accuracy of the navigation system

Suboptimal placements of pedicle screws may lead to neurological and vascular complications. Computer-assisted image guidance has been shown to improve accuracy in spinal instrumentation. Checking the accuracy of the navigation system during pedicle screw placement is fundamental. We describe a novel technique of using continuous accuracy check of the navigation system during O-arm-based neuronavigation to instrument the thoracolumbar region. Forty thoracic and 42 lumbar screws were inserted in 12 patients. The Mirza evaluation system was used to evaluate the accuracy of the inserted screws. There was no neurological injury and no need to reposition any screw. The accuracy of the screws placement was excellent. Our technique of continuous at will operational accuracy check of the neuronavigation system is associated with extreme accuracy of screw placement, no need to bring a patient back to the operating room to reposition a pedicle screw, and with excellent outcome.

Microscopic and endoscopic anterior communicating artery complex anatomy as seen through keyhole approaches

The purpose of this anatomical study is to illustrate the microscopic and endoscopic anatomy of the anterior communicating artery complex as viewed through three keyhole approaches (keyhole pterional, lateral supraorbital and supraorbital) and to compare them. Each approach was carried out once on each side in five glutaraldehyde-fixed human cadaveric heads. A microscope and microscope-endoscope combination were used to grade access and visualization of the structures qualitatively using a previously published surgical exposure grading system. All three approaches provided good access and visualization of ipsilateral structures in the anterior communicating artery complex, but the pterional keyhole and the lateral supraorbital approaches gave the best access. Exposure of contralateral structures was limited overall, but was enhanced somewhat by the use of the endoscope.

Lateral supraorbital approach vs pterional approach: an anatomic qualitative and quantitative evaluation

BACKGROUND

Several minimally invasive modifications of the standard pterional approach have been proposed recently. The lateral supraorbital approach is one of these modifications.

OBJECTIVE

To provide a qualitative and quantitative anatomic comparison of the surgical exposure and the operability afforded by the standard pterional approach and the lateral supraorbital approach.

METHODS

Eight pterional approaches and 8 lateral supraorbital approaches were used in 8 fresh human cadaver heads. One qualitative and 2 quantitative tools were used to assess the surgical exposure.

RESULTS

Qualitatively, the lateral supraorbital approach has the advantages of reduced trauma to the temporalis muscle and exposure that is comparable to that provided by the standard pterional approach to the sellar and suprasellar regions. This approach offers limited exposure of the interpeduncular fossa compared with the pterional one. Quantitatively, the pterional approach provides a greater surgical volume than the lateral supraorbital approach (32.90 mL vs 23.60 mL with P < .05). Also, the pterional approach provides a greater associated surgical operability than the lateral supraorbital approach (exposure score of 66 and 53, respectively). However, the lateral supraorbital approach offers an equivalent access to the anterior communicating artery complex, optic nerve, optic chiasm, and sellar area (the exposure scores were 19 for both approaches).

CONCLUSION

From an anatomic point of view, both approaches provide similar exposure to the sellar, suprasellar, and anterior communicating artery areas. The pterional approach provides better exposure of the retrosellar area. The ability to operate in the retrosellar area, as judged by our model, was higher with the pterional than with the lateral supraorbital approach.

Effect of registration mode on neuronavigation precision: an exploration of the role of random error

The aim of this paper is to analyze the variations in registration accuracy for computer-assisted surgical navigation using three different modes of registration, in order to explore the behavior of random error, and to highlight the precision of neuronavigation as a concept distinct from accuracy. The operational accuracy of three different registration modes (bone fiducials, scalp adhesive fiducials and an auto-registration mask) was evaluated in a total of 20 fresh cadaveric heads. The precision of the neuronavigation system was then assessed by evaluating the variation in the accuracy measurements associated with each registration mode. The coefficient of variation was employed to quantify the degree of variation in the attained accuracy using the following formula: Coefficient of variation = standard deviation/mean * 100. For external targets, the precision of the neuronavigation system was greatest with mask registration (43.75 and 51.41 for anterior and posterior external targets, respectively) and lowest with bone registration (65.30 and 67.17 for anterior and posterior external targets, respectively). For internal targets, the precision of the neuronavigation system was greatest with bone registration (47.69 and 42.6 for anterior and posterior internal targets, respectively) and lowest with mask registration (62.9 and 58.67 for anterior and posterior internal targets, respectively). The precision (reproducibility) of the neuronavigation system is another important quantity besides accuracy that characterizes the performance of the system. Understanding both of these quantities for a given registration mode enhances the use of a neuronavigation system in neurosurgery.

Path to the Interpeduncular Fossa: Anatomical Comparison of Endoscopic-Assisted versus Standard Subtemporal Approach

Objective The aim of this study was to assess the assumed advantage of endoscopic assistance to the standard subtemporal approach. The idea was to measure qualitatively and quantitatively visibility versus operability. Design We performed eight subtemporal dissections on four cadaver heads. Our dissections integrated an operating microscope, endoscope, and neuronavigation. Comparison was made between visibility and operability afforded by the microscope alone or by the microscope-endoscope combination. Visibility was recorded as complete or incomplete and was quantified for key structures using linear measurements taken by the navigation system. Operability was determined by whichever maneuvers could be safely and comfortably accomplished in the space afforded. Results From our survey, the structures whose visibility most benefitted from the addition of the endoscope include: contralateral third nerve, posterior perforated substance, mammillary bodies, and contralateral superior cerebellar artery. With regard to quantitative evaluation, we found increased visibility of both basilar artery and posterior cerebral artery. With regard to the operability, no objective advantage was afforded by the addition of the endoscope. Subjectively, the maneuvers were easier to perform while using the endoscope. Conclusion Using the endoscope as an assistance tool during conducting classical subtemporal approach can help in overcome a lot of the classical subtemporal approach limitations.

Qualitative and quantitative radio-anatomical variation of the posterior clinoid process

This study was conducted to investigate the radiological anatomy of the posterior clinoid process (PCP) to highlight preoperative awareness of its variations and its relationships to other skull base landmarks. The PCPs of 36, three-dimensional computed tomographic cadaveric heads were evaluated by studying the gross anatomy of the PCP and by measuring the distances between the PCP and other skull base anatomical landmarks relevant to transnasal or transcranial skull base approaches. PCP variations were found in five specimens (14%): in two the dorsum sellae was absent, in one the PCP and the anterior clinoid process (ACP) were connected unilaterally and in two bilaterally. The mean distance between the right/left PCP and the crista galli was 45.14 ± 4.0 standard deviation (SD_/46.24 ± 4.5 SD, respectively, while the distance to the middle point of the basion at the level of the foramen magnum was 40.41 ± 5.1 SD/41.0 ± 5.2 SD, respectively. The mean distance between the PCP and the ACP was 12.03 ± 3.18 SD on the right side and 12.11 ± 2.77 SD on the left. The data provided highlights the importance of careful preoperative evaluation of the PCP and of its relationships to other commonly encountered skull base landmarks. This information may give an idea of the exposure achievable through different transcranial and transnasal approaches. This is especially relevant when neuronavigation is not available.

Setting up a microneurosurgical skull base lab: technical and operational considerations

Microneurosurgical cadaveric dissections have become popular due to their usefulness in obtaining a working knowledge of the microneurosurgical anatomy in a controlled environment. This same controlled environment is also conducive to experiment with new surgical approaches. These factors have increased the number of microneurosurgical anatomic laboratories. Despite the increase in microneurosurgical laboratories, there is very little literature regarding the logistics of starting and maintaining a new neurosurgical laboratory. The aim of this paper is to provide a general road map and basic guidelines in starting and running a microneurosurgical dissection laboratory. The information in this paper is based on a review of the literature and on the experience we gained in organizing and managing the Dardinger Microneurosurgical Skull Base Laboratory at The Ohio State University.

Extradural endoscope-assisted subtemporal posterior clinoidectomy: a cadaver investigation study

BACKGROUND

Surgical treatment of distal basilar artery aneurysms is challenging because of the narrow surgical corridor, presence of vital perforating vessels, deep location, and difficulty in obtaining proximal control.

OBJECTIVE

To investigate using a cadaver model the feasibility of performing a transcranial extradural posterior clinoidectomy via a subtemporal route between V2 and V3 using an endoscope-microscope combination.

METHODS

Fourteen dissections were performed in 14 fresh cadaver heads. A standard pterional approach with removal of the zygomatic arch was followed by a 2-stage dissection to remove the posterior clinoid process. In stage 1 (microscopic stage), the area between the second and third trigeminal divisions (V2 and V3) was exposed and the anterior half of the bone between them was drilled to the sphenoid sinus cavity inferior to the carotid sulcus. In stage 2 (endoscopic stage), the drilling was continued to the carotid sulcus. Next, the endosteal layer of the dura lining the carotid sulcus was dissected from the bone that was then removed. At the end of this stage, the dura reflection that forms the posterior part of the pituitary capsule was exposed and the base of the posterior clinoid process was removed using a high-speed drill and curet. Finally, the dura was opened to confirm the removal of the posterior clinoid process.

RESULTS

It was possible to remove the posterior clinoid process in every specimen without any obvious anatomic injury to the surrounding structures.

CONCLUSION

This study demonstrated the feasibility of the resection of the posterior clinoid process extradurally. This maneuver could be incorporated in multiple cranial base approaches to the retrosellar area and interpeduncular cistern region.

Posterior interhemispheric transfalx transprecuneus approach to the atrium of the lateral ventricle: a cadaveric study

OBJECT

The posterior interhemispheric transprecuneus gyrus approach is one of the surgical routes that has been suggested to reach the atrium of the lateral ventricle. It has the advantage of avoiding the disruption of the optic radiations; however, it has a narrow working area that at times makes the execution of this approach rather challenging. The aim of this study was to test a modification of the approach that might create a better surgical angle and a wider corridor by accessing the atrium from the contralateral side after transection of the falx. The authors named this new approach the "posterior interhemispheric transfalx transprecuneus approach."

METHODS

The posterior interhemispheic transfalx transprecuneus approach was performed bilaterally on 6 fresh adult cadaveric specimens for a total of 12 procedures. Every head was held in the semisitting position and a parasagittal parietooccipital craniotomy on the contralateral side of the targeted ventricle was executed. The dura mater was opened and reflected based on the sagittal sinus. Then the falx was cut in a triangular fashion based on the inferior sagittal sinus. Using the parietooccipital artery and sulcus as landmarks, the contralateral precuneus gyrus was indentified, and a small area of the gyrus was transected to gain access to the atrium. A neuronavigational system was also used to conduct this approach. The working angle of this approach and other distances were measured.

RESULTS

The authors were able to visualize the ventricular atrium, posterior part of the temporal horn, pulvinar, and choroid plexus in all specimens. The temporal horn could be exposed for a length of 20-30 mm from the atrium. The working angle of the approach was better than that of the classic posterior interhemispheric transprecuneus approach with a mean value of 44.5° as opposed to 25.8°. The distance from the middle point of the corticotomy to the splenium ranged from 11 to 16 mm (mean 13.3 mm); the distance to the torcula, from 34 to 53 mm (mean 41.3 mm); and the distance to the atrium, from 22 to 31 mm (mean 25.7 mm).

CONCLUSIONS

Results of this study suggested that the proposed approach can expose the atrium and the posterior part of the temporal horn of the lateral ventricle with a wider surgical angle compared with the conventional homolateral posterior interhemispheric transprecuneus gyrus approach. Moreover, by minimizing the amount of brain retraction homolateral to the target, this approach could make navigation more accurate.