Category: anatomy

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Microsurgical anatomy of the subthalamic nucleus: correlating fiber dissection results with 3-T magnetic resonance imaging using neuronavigation

J Neurosurg 130 (3): 716-732

Despite the extensive use of the subthalamic nucleus (STN) as a deep brain stimulation (DBS) target, unveiling the extensive functional connectivity of the nucleus, relating its structural connectivity to the stimulation-induced adverse effects, and thus optimizing the STN targeting still remain challenging. Mastering the 3D anatomy of the STN region should be the fundamental goal to achieve ideal surgical results, due to the deep-seated and obscure position of the nucleus, variable shape and relatively small size, oblique orientation, and extensive structural connectivity. In the present study, the authors aimed to delineate the 3D anatomy of the STN and unveil the complex relationship between the anatomical structures within the STN region using fiber dissection technique, 3D reconstructions of high-resolution MRI, and fiber tracking using diffusion tractography utilizing a generalized q-sampling imaging (GQI) model.

METHODS
Fiber dissection was performed in 20 hemispheres and 3 cadaveric heads using the Klingler method. Fiber dissections of the brain were performed from all orientations in a stepwise manner to reveal the 3D anatomy of the STN. In addition, 3 brains were cut into 5-mm coronal, axial, and sagittal slices to show the sectional anatomy. GQI data were also used to elucidate the connections among hubs within the STN region.

RESULTS
The study correlated the results of STN fiber dissection with those of 3D MRI reconstruction and tractography using neuronavigation. A 3D terrain model of the subthalamic area encircling the STN was built to clarify its anatomical relations with the putamen, globus pallidus internus, globus pallidus externus, internal capsule, caudate nucleus laterally, substantia nigra inferiorly, zona incerta superiorly, and red nucleus medially. The authors also describe the relationship of the medial lemniscus, oculomotor nerve fibers, and the medial forebrain bundle with the STN using tractography with a 3D STN model.

CONCLUSIONS
This study examines the complex 3D anatomy of the STN and peri-subthalamic area. In comparison with previous clinical data on STN targeting, the results of this study promise further understanding of the structural connections of the STN, the exact location of the fiber compositions within the region, and clinical applications such as stimulation-induced adverse effects during DBS targeting.

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Anatomic Characteristics of Intrapetrous Carotid Artery: A 3-Dimensional Segmentation Study on Head Computed Tomography Scan

World Neurosurg. (2019) 121:e419-e425

The intrapetrous carotid artery (IPCA) is one of the most unexplored anatomic regions, and its 3-dimensional reconstruction in living subjects is still missing. This study aims to describe the IPCA on 3D models extracted from head computed tomography (CT) scans.

METHODS: The intrapetrous carotid artery was manually segmented on head CT scans of 100 healthy patients free from vascular and neurologic pathologies (50 men and 50 women; age range, 18e91 years). Angles of the posterior and anterior genu, diameter and length of the horizontal portion, and volume of the entire canal were calculated through 3D analysis software. Statistically significant differences according to sex and side were assessed through 2-way analysis of variance (P < 0.05). Correlation of each measurement with age was calculated as well.

RESULTS: On average, the angles of the posterior and anterior genu were 120.1±2.4 and 118.0±10.0 in men and 119.5±9.2 and 117.6±10.3 in women, respectively, without statistically significant differences according to sex or side (P > 0.05). The average length and diameter of the horizontal part were, respectively, 25.5±2.9 and 5.8±0.8 mm in men and 24.0±2.3 and 5.3±0.8 mm in women. The volume of the IPCA was 0.941±0.215 cm3 in men and 0.752±0.159 cm3 in women. The length and diameter of the horizontal portion and the volume of the IPCA showed statistically significant differences according to sex (P < 0.05). No correlation with age was found.

CONCLUSIONS: This study provides data concerning not only linear and angular measurements, but also volumes of the IPCA, which are useful in planning surgical interventions of the cranial base.

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Anterior clinoidectomy using an extradural and intradural 2-step hybrid technique

J Neurosurg 130:238–247, 2019

Anterior clinoidectomy is a difficult yet essential technique in skull base surgery. Two main techniques (extradural and intradural) with multiple modifications have been proposed to increase efficiency and avoid complications.

In this study, the authors sought to develop a hybrid technique based on localization of the optic strut (OS) to combine the advantages and avoid the disadvantages of both techniques. Ten cadaveric specimens were prepared for surgical simulation. After a standard pterional craniotomy, the anterior clinoid process (ACP) was resected in 2 steps. The segment anterior to the OS was resected extradurally, while the segment posterior to the OS was resected intradurally. The proposed technique was performed in 6 clinical cases to evaluate its safety and efficiency.

Anterior clinoidectomy was successfully performed in all cadaveric specimens and all 6 patients by using the proposed technique. The extradural phase enabled early decompression of the optic nerve while avoiding the adjacent internal carotid artery. The OS was drilled intradurally under direct visualization of the adjacent neurovascular structures. The described landmarks were easily identifiable and applicable in the surgically treated patients. No operative complication was encountered.

A proposed 2-step hybrid technique combines the advantages of the extradural and intradural techniques while avoiding their disadvantages. This technique allows reduced intradural drilling and subarachnoid bone dust deposition. Moreover, the most critical part of the clinoidectomy—that is, drilling of the OS and removal of the body of the ACP—is left for the intradural phase, when critical neurovascular structures can be directly viewed.

 

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Utility of the Vidian Canal in Endoscopic Skull Base Surgery: Detailed Anatomy and Relationship to the Internal Carotid Artery

World Neurosurg. (2019) 121:e140-e146

OBJECTIVE: To investigate key anatomic features of the vidian canal that have a critical role in planning and performing endoscopic skull base surgeries.

METHODS: We reviewed skull base computed tomographic images of 640 consecutive subjects. Studies were analyzed in axial, coronal and sagittal planes. –

RESULTS: The mean (SD) length of the vidian canal was 15.4±2.0 mm in female subjects and 16.6±1.7 mm in male subjects, and the difference between genders was statistically significant (P < 0.001). The most common rostral-caudal course of the vidian canal was medial to lateral and was followed by the straight course, tortuous course, and lateral-to-medial course. The frequency of pneumatization pattern from most common to least common was types 0, III, II and I. Of 342 evaluated sides, the vidian canal was located below the level of the anterior genu of petrous ICA in 303 (89%) sides, at same level with the anterior genu of petrous ICA in twenty-five(7%) sides, and above the level of the anterior genu of petrous ICA in fourteen(4.1%) sides.

CONCLUSIONS: A variety of previously undefined features of the vidian canal that can alter the course of surgical procedure were defined. The position of the vidian canal with respect to the petrous internal carotid artery (ICA) was extensively described. From a surgical standpoint, a working room inferior and medial to the vidian canal might not always be a safe approach, because the vidian canal could be located superior to the level of the anterior genu of petrous ICA according to our findings in the present study.

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The microneurosurgical anatomy legacy of Albert L. Rhoton Jr., MD: an analysis of transition and evolution over 50 years

J Neurosurg 129:1331–1341, 2018

Dr. Albert L. Rhoton Jr. was a pioneer of the study of microneurosurgical anatomy. Championing this field over the past half century, he produced more than 500 publications.

In this paper, the authors review his body of work, focusing on approximately 160 original articles authored by Rhoton and his microneuroanatomy fellows.

The articles are categorized chronologically into 5 stages: 1) dawn of microneurosurgical anatomy, 2) study of basic anatomy for general neurosurgery, 3) study for skull base surgery, 4) study of the internal structures of the brain by fiber dissection, and 5) surgical anatomy dealing with new advanced surgical approaches.

Rhoton introduced many new research ideas and surgical techniques and approaches, along with better microsurgery instruments, through studying and teaching microsurgical anatomy, especially during the first stage. The characteristic features of each stage are explained and the transition phases of his projects are reviewed.

 

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Robot-assisted multi-level anterior lumbar interbody fusion: an anatomical study

Acta Neurochirurgica (2018) 160:1891–189

Minimally invasive surgical approaches still provide limited exposure. Access to the L2–L5 intervertebral discs during a single procedure is challenging and often requires repositioning of the patient and adopting an alternative approach.

Objectives Investigate the windows to the L2–L5 intervertebral discs to assess the dimensions of the interbody implants suitable for the procedure and evaluate the feasibility of multi-level lumbar intervertebral disc surgery in robot-assisted surgery (RAS)

Methods Sixteen fresh-frozen cadaveric specimens underwent a retroperitoneal approach to access the L2–L5 intervertebral discs. The L2–L3 to L4–L5 windows were defined as the distance between the left lateral border of the aorta (or nearest common iliac vessel) and the medial border of the psoas, measured in a static state and after gentle medial retraction of the vascular structures. Two living porcine specimens and one cadaveric specimen underwent da Vinci robot-assisted transperitoneal approach to expose the L2–L3 to L4–L5 intervertebral discs and perform multi-level discectomy and interbody implant placement.

Results The L2–L3 to L4–L5 intervertebral disc windows significantly increased from a static to a retracted state (p < 0.05). The mean L2–L3, L3–L4, and L4–L5 windows measured respectively 20.1, 21.6, and 19.6 mm in the static state, and 27.2, 30.9, and 30.3 mm after gentle vascular retraction. The intervertebral windows from L2–L3 to L4–L5 were successfully exposed through an anterior transperitoneal approach with the da Vinci robot on the cadaveric and living porcine specimens, and interbody implants were inserted.

Conclusion RAS appears to be feasible for a mini-invasive multi-level lumbar intervertebral disc surgery. The RAS procedure, longer and more expensive than conventional MIS approaches, should be reserved for elective patients.

 

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Veins of the Cerebellopontine Angle and Specific Complications of Sacrifice, with Special Emphasis on Microvascular Decompression Surgery

World Neurosurg. (2018) 117:422-432

Good knowledge of the anatomy of veins is of crucial importance for the functional surgery of cranial nerve (CN) disorders, especially microvascular decompression for trigeminal neuralgia (TN), hemifacial spasm (HFS), and vagoglossopharyngeal neuralgia (VGPN). Although controversial, veins may be involved in neurovascular conflicts and may constitute dangerous obstacles to access to the CNs. With the aim of estimating the implications of veins in those diseases and evaluating the linked surgical difficulties, we carried out a review of the literature from 2000 to the end of February 2018.

For this review, articles found on PubMed that gave enough precision about veins were retained (39 articles on TN, 38 on HFS, 8 on VGPN, and 26 on complications related to venous sacrifices). Before this review, we described a simplified anatomic classification of veins, amenable to easing the surgical approach to CNs.

Access to the trigeminal nerve, via the infratentorial-supracerebellar route, is almost always affected by the superficial superior petrosal venous system, whereas access to the facial and cochleovestibular complex as well as to the lower CNs, through the infrafloccular trajectory, is almost always exempt of important venous obstacles. Respective incidences of venous compression at the origin of hyperactive CN syndromes are given. The percentages of a venous conflict alone were calculated at 10.8% for TN, 0.1% for HFS, and 2.9% for VGPN.

We review the complications considered in relation with venous sacrifices. Precautions to minimize these complications are given.

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Microsurgical anatomy and approaches around the lateral recess with special reference to entry into the pons

J Neurosurg 129:740–751, 2018

The lateral recess is a unique structure communicating between the ventricle and cistern, which is exposed when treating lesions involving the fourth ventricle and the brainstem with surgical approaches such as the transcerebellomedullary fissure approach. In this study, the authors examined the microsurgical anatomy around the lateral recess, including the fiber tracts, and analyzed their findings with respect to surgical exposure of the lateral recess and entry into the lower pons.

METHODS Ten cadaveric heads were examined with microsurgical techniques, and 2 heads were examined with fiber dissection to clarify the anatomy between the lateral recess and adjacent structures. The lateral and medial routes directed to the lateral recess in the transcerebellomedullary fissure approach were demonstrated. A morphometric study was conducted in the 10 cadaveric heads (20 sides).

RESULTS The lateral recess was classified into medullary and cisternal segments. The medial and lateral routes in the transcerebellomedullary fissure approach provided access to approximately 140°–150° of the posteroinferior circumference of the lateral recess. The floccular peduncle ran rostral to the lateral recess, and this region was considered to be a potential safe entry zone to the lower pons. By appropriately selecting either route, medial-to-lateral or lateral-to-medial entry axis is possible, and combining both routes provided wide exposure of the lower pons around the lateral recess.

CONCLUSIONS The medial and lateral routes of the transcerebellomedullary fissure approach provided wide exposure of the lateral recess, and incision around the floccular peduncle is a potential new safe entry zone to the lower pons.

 

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The subatlantic triangle: gateway to early localization of the atlantoaxial vertebral artery

J Neurosurg Spine 29:18–27, 2018

Exposure of the vertebral artery (VA) between C-1 and C-2 vertebrae (atlantoaxial VA) may be necessary in a variety of pathologies of the craniovertebral junction. Current methods to expose this segment of the VA entail sharp dissection of muscles close to the internal jugular vein and the spinal accessory nerve. The present study assesses the technique of exposing the atlantoaxial VA through a newly defined muscular triangle at the craniovertebral junction.

METHODS Five cadaveric heads were prepared for surgical simulation in prone position, turned 30°–45° toward the side of exposure. The atlantoaxial VA was exposed through the subatlantic triangle after reflecting the sternocleidomastoid and splenius capitis muscles inferiorly. The subatlantic triangle was formed by 3 groups of muscles: 1) the levator scapulae and splenius cervicis muscles inferiorly and laterally, 2) the longissimus capitis muscle inferiorly and medially, and 3) the inferior oblique capitis superiorly. The lengths of the VA exposed through the triangle before and after unroofing the C-2 transverse foramen were measured.

RESULTS The subatlantic triangle consistently provided access to the whole length of atlantoaxial VA. The average length of the VA exposed via the subatlantic triangle was 19.5 mm. This average increased to 31.5 mm after the VA was released at the C-2 transverse foramen.

CONCLUSIONS The subatlantic triangle provides a simple and straightforward pathway to expose the atlantoaxial VA. The proposed method may be useful during posterior approaches to the craniovertebral junction should early exposure and control of the atlantoaxial VA become necessary.

 

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The Surgical White Matter Chassis: A Practical 3-Dimensional Atlas for Planning Subcortical Surgical Trajectories

Operative Neurosurgery 14:469–482, 2018

The imperative role of white matter preservation in improving surgical functional outcomes is now recognized. Understanding the fundamental white matter framework is essential for translating the anatomic and functional literature into practical strategies for surgical planning and neuronavigation.

OBJECTIVE: To present a 3-dimensional (3-D) atlas of the structural and functional scaffolding of human white matter—ie, a “Surgical White Matter Chassis (SWMC)”—that can be used as an organizational tool in designing precise and individualized trajectorybased neurosurgical corridors.

METHODS: Preoperative diffusion tensor imaging magnetic resonance images were obtained prior to each of our last 100 awake subcortical resections, using a clinically available 3.0 Tesla system. Tractography was generated using a semiautomated deterministic global seeding algorithm. Tract data were conceptualized as a 3-D modular chassis based on the 3 major fiber types, organized along median and paramedian planes, with special attention to limbic and neocortical association tracts and their interconnections.

RESULTS: We discuss practical implementation of the SWMC concept, and highlight its use in planning select illustrative cases. Emphasis has been given to developing practical understanding of the arcuate fasciculus, uncinate fasciculus, and vertical rami of the superior longitudinal fasciculus, which are often-neglected fibers in surgical planning.

CONCLUSION: A working knowledge of white matter anatomy, as embodied in the SWMC, is of paramount importance to the planning of parafascicular surgical trajectories, and can serve as a basis for developing reliable safe corridors, or modules, toward the goal of “zerofootprint” transsulcal access to the subcortical space.

 

 

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Symmetry of the arcuate fasciculus and its impact on language performance of patients with brain tumors in the language-dominant hemisphere

J Neurosurg 127:1407–1416, 2017

Cerebral damage in frontal, parietal, and temporal brain areas and, probably more importantly, their interconnections can lead to deficits in language. However, neural plasticity and repair allow the brain to partly compensate for neural injury, mediated by both functional and structural changes. In this study, the authors sought to systematically investigate the relationship between language performance in brain tumor patients and structural perisylvian pathways (i.e., the arcuate fasciculus [AF]) using probabilistic fiber tracking on diffusion tensor imaging. The authors used a previously proposed model in which the AF is divided into anterior, long, and posterior segments. The authors hypothesized that right-handed patients with gliomas in the language-dominant (left) hemisphere would benefit from a more symmetrical or right-lateralized language pathway in terms of better preservation of language abilities. Furthermore, they investigated to what extent specific tumor characteristics, including proximity to the AF, affect language outcome in such patients.

METHODS Twenty-seven right-handed patients (12 males and 15 females; mean age 52 ± 16 years) with 11 low-grade and 16 high-grade gliomas of the left hemisphere underwent 3-T diffusion-weighted MRI (30 directions) and language assessment as part of presurgical planning. For a systematic quantitative evaluation of the AF, probabilistic fiber tracking with a 2 regions of interest approach was carried out. Volumes of the 3 segments of both hemispheric AFs were evaluated by quantifying normalized and thresholded pathways. Resulting values served to generate the laterality index of the AFs.

RESULTS Patients without language deficits tended to have an AF that was symmetric or lateralized to the right, whereas patients with deficits in language significantly more often demonstrated a left-lateralized posterior segment of the AF. Patients with high-grade gliomas had more severe language deficits than those with low-grade gliomas. Backward logistic regression revealed the laterality index of the posterior AF segment and tumor grade as the only independent statistically significant predictors for language deficits in this cohort.

CONCLUSIONS In addition to the well-known fact that tumor entity influences behavioral outcome, the authors’ findings suggest that the right homologs of structural language-associated pathways could be supportive for language function and facilitate compensation mechanisms after brain damage in functionally eloquent areas. This further indicates that knowledge about preoperative functional redistribution (identified by neurofunctional imaging) increases the chance for total or near-total resections of tumors in eloquent areas. In the future, longitudinal studies with larger groups are mandatory to overcome the methodological limitations of this cross-sectional study and to map neuroplastic changes associated with language performance and rehabilitation in brain tumor patients.

 

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The superior fovea triangle approach: a novel safe entry zone to the brainstem

J Neurosurg 127:1134–1138, 2017

The authors describe a safe entry zone, the superior fovea triangle, on the floor of the fourth ventricle for resection of deep dorsal pontine lesions at the level of the facial colliculus.

Clinical data from a patient undergoing a suboccipital telovelar transsuperior fovea triangle approach to a deep pontine cavernous malformation were reviewed and supplemented with 6 formalin-fixed adult human brainstem and 2 silicone-injected adult human cadaveric heads using the fiber dissection technique to illustrate the utility of this novel safe entry zone.

The superior fovea has a triangular shape that is an important landmark for the motor nucleus of the trigeminal, abducens, and facial nerves. The inferior half of the superior fovea triangle may be incised to remove deep dorsal pontine lesions through the floor of the fourth ventricle.

The superior fovea triangle may be used as a safe entry zone for dorsally located lesions at the level of the facial colliculus.

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Quantitative anatomical analysis and clinical experience with mini-pterional and mini-orbitozygomatic approaches for intracranial aneurysm surgery

J Neurosurg 127:646–659, 2017

The aim of this investigation was to modify the mini-pterional and mini-orbitozygomatic (mini-OZ) approaches in order to reduce the amount of tissue traumatization caused and to compare the use of the 2 approaches in the removal of circle of Willis aneurysms based on the authors’ clinical experience and quantitative analysis.

METHODS Three formalin-fixed adult cadaveric heads injected with colored silicone were examined. Surgical freedom and angle of attack of the mini-pterional and mini-OZ approaches were measured at 9 anatomical points, and the measurements were compared. The authors also retrospectively reviewed the cases of 396 patients with ruptured and unruptured single aneurysms in the circle of Willis treated by microsurgical techniques at their institution between January 2006 and November 2014.

RESULTS A significant difference in surgical freedom was found in favor of the mini-pterional approach for access to the ipsilateral internal carotid artery (ICA) and middle cerebral artery (MCA) bifurcations, the most distal point of the ipsilateral posterior cerebral artery (PCA), and the basilar artery (BA) tip. No statistically significant differences were found between the mini-pterional and mini-OZ approaches for access to the posterior clinoid process, the most distal point of the superior cerebellar artery (SCA), the anterior communicating artery (ACoA), the contralateral ICA bifurcation, and the most distal point of the contralateral MCA. A trend toward increasing surgical freedom was found for the mini-OZ approach to the ACoA and the contralateral ICA bifurcation. The lengths exposed through the mini-OZ approach were longer than those exposed by the mini-pterional approach for the ipsilateral PCA segment (11.5 ± 1.9 mm) between the BA and the most distal point of the P2 segment of the PCA, for the ipsilateral SCA (10.5 ± 1.1 mm) between the BA and the most distal point of the SCA, and for the contralateral anterior cerebral artery (ACA) (21 ± 6.1 mm) between the ICA bifurcation and the most distal point of the A2 segment of the ACA. The exposed length of the contralateral MCA (24.2 ± 8.6 mm) between the contralateral ICA bifurcation and the most distal point of the MCA segment was longer through the mini-pterional approach. The vertical angle of attack (anteroposterior direction) was significantly greater with the minipterional approach than with the mini-OZ approach, except in the ACoA and contralateral ICA bifurcation. The horizontal angle of attack (mediolateral direction) was similar with both approaches, except in the ACoA, contralateral ICA bifurcation, and contralateral MCA bifurcation, where the angle was significantly increased in the mini-OZ approach.

CONCLUSIONS The mini-pterional and mini-OZ approaches, as currently performed in select patients, provide less tissue traumatization (i.e., less temporal muscle manipulation, less brain parenchyma retraction) from the skin to the aneurysm than standard approaches. Anatomical quantitative analysis showed that the mini-OZ approach provides better exposure to the contralateral side for controlling the contralateral parent arteries and multiple aneurysms. The mini-pterional approach has greater surgical freedom (maneuverability) for ipsilateral circle of Willis aneurysms.

 

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The paramedian supracerebellar infratentorial approach

Acta Neurochir (2017) 159:1529–1532

Lesions of the superior cerebellar surface, pineal region, lateral and dorsal midbrain and mesial temporal lobe are challenging to treat and often require neurosurgical intervention.

Methods The paramedian variation of the supracerebellar infratentorial approach utilizes the downward slope of the cerebellum to facilitate exposure and the lower density of cerebellar bridging veins away from the midline decreases the need to sacrifice larger venous channels. We also discuss our experiences with the approach, and some of the drawbacks and nuances that we have encountered as it has evolved over the years.

Conclusions This approach is versatile and effective and the authors’ surgical approach of choice for resecting these challenging lesions.

 

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Perpetuation of errors in illustrations of cranial nerve anatomy

J Neurosurg 127:192–198, 2017

For more than 230 years, anatomical illustrations have faithfully reproduced the German medical student Thomas Soemmerring’s cranial nerve (CN) arrangement. Virtually all contemporary atlases show the abducens, facial, and vestibulocochlear nerves (CNs VI–VIII) all emerging from the pontomedullary groove, as originally depicted by Soemmerring in 1778.

Direct observation at microsurgery of the cerebellopontine angle reveals that CN VII emerges caudal to the CN VIII root from the lower lateral pons rather than the pontomedullary groove. Additionally, the CN VI root lies in the pontomedullary groove caudal to both CN VII and VIII in the vast majority of cases.

In this high-resolution 3D MRI study, the exit location of CN VI was caudal to the CN VII/VIII complex in 93% of the cases. Clearly, Soemmerring’s rostrocaudal numbering system of CN VI-VII-VIII (abducens-facial-vestibulocochlear CNs) should instead be VIII-VII-VI (vestibulocochlear- facial-abducens CNs). While the inaccuracy of the CN numbering system is of note, what is remarkable is that generations of authors have almost universally chosen to perpetuate this ancient error. No doubt some did this through faithful copying of their predecessors. Others, it could be speculated, chose to depict the CN relationships incorrectly rather than run contrary to long-established dogma.

This study is not advocating that a universally recognized numbering scheme be revised, as this would certainly create confusion. The authors do advocate that future depictions of the anatomical arrangements of the brainstem roots of CNs VI, VII, and VIII ought to reflect actual anatomy, rather than be contorted to conform with the classical CN numbering system.

 

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Probabilistic versus deterministic tractography for delineation of the cortico-subthalamic hyperdirect pathway in patients with Parkinson disease selected for deep brain stimulation

J Neurosurg 126:1657–1668, 2017

Diffusion-weighted MRI (DWI) and tractography allows noninvasive mapping of the structural connections of the brain, and may provide important information for neurosurgical planning. The hyperdirect pathway, connecting the subthalamic nucleus (STN) with the motor cortex, is assumed to play a key role in mediating the effects of deep brain stimulation (DBS), which is an effective but poorly understood treatment for Parkinson disease. This study aimed to apply recent methodological advances in DWI acquisition and analysis to the delineation of the hyperdirect pathway in patients with Parkinson disease selected for surgery.

METHODS High spatial and angular resolution DWI data were acquired preoperatively from 5 patients with Parkinson disease undergoing DBS. The authors compared the delineated hyperdirect pathways and associated STN target maps generated by 2 different tractography methods: a tensor-based deterministic method, typically available in clinical settings, and an advanced probabilistic method based on constrained spherical deconvolution. In addition, 10 highresolution data sets with the same scanning parameters were acquired from a healthy control participant to assess the robustness of the tractography results.

RESULTS Both tractography approaches identified connections between the ipsilateral motor cortex and the STN. However, the 2 methods provided substantially different target regions in the STN, with the target center of gravity differing by > 1.4 mm on average. The probabilistic method (based on constrained spherical deconvolution) plausibly reconstructed a continuous set of connections from the motor cortex, terminating in the dorsolateral region of the STN. In contrast, the tensor-based method reconstructed a comparatively sparser and more variable subset of connections. Furthermore, across the control scans, the probabilistic method identified considerably more consistent targeting regions within the STN compared with the deterministic tensor-based method, which demonstrated a 1.9–2.4 times higher variation.

CONCLUSIONS These data provide a strong impetus for the use of a robust probabilistic tractography framework based on constrained spherical deconvolution, or similar advanced DWI models, in clinical settings. The inherent limitations and demonstrated inaccuracy of the tensor-based method leave it questionable for use in high-precision stereotactic DBS surgery. The authors have also described a straightforward method for importing tractography-derived information into any clinical neuronavigation system, based on the generation of track-density images.

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Surgical Approaches to the Temporal Horn: An Anatomic Analysis of White Matter Tract Interruption

Operative Neurosurgery 13:258–270, 2017

Surgical access to the temporal horn is necessary to treat tumors and vascular lesions, but is used mainly in patients with mediobasal temporal epilepsy. The surgical approaches to this cavity fall into 3 primary categories: lateral, inferior, and transsylvian. The current neurosurgical literature has underestimated the interruption of involved fiber bundles and the correlated clinical manifestations.

OBJECTIVE: To delineate the interruption of fiber bundles during the different approaches to the temporal horn.

METHODS:We simulated the lateral (trans-middle temporal gyrus), inferior (transparahippocampal gyrus), and transsylvian approaches in 20 previously frozen, formalin-fixed human brains (40 hemispheres). Fiber dissection was then done along the lateral and inferior aspects under the operating microscope. Each stage of dissection and its respective fiber tract interruption were defined.

RESULTS: The lateral (trans-middle temporal gyrus) approach interrupted “U” fibers, the superior longitudinal fasciculus (inferior arm), occipitofrontal fasciculus (ventral segment), uncinate fasciculus (dorsolateral segment), anterior commissure (posterior segment), temporopontine, inferior thalamic peduncle (posterior fibers), posterior thalamic peduncle (anterior portion), and tapetum fibers. The inferior (transparahippocampal gyrus) approach interrupted “U” fibers, the cingulum (inferior arm), and fimbria, and transected the hippocampal formation. The transsylvian approach interrupted “U”fibers (anterobasal region of the extreme capsule), the uncinate fasciculus (ventromedial segment), and anterior commissure (anterior segment), and transected the anterosuperior aspect of the amygdala.

CONCLUSION: White matter dissection improves our knowledge of the complex anatomy surrounding the temporal horn. Identifying the fiber bundles at risk during each surgical approach adds important information for choosing the appropriate surgical strategy.

 

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Three-Dimensional Printed Modeling of Diffuse Low-Grade Gliomas and Associated White Matter Tract Anatomy

Neurosurgery 80:635–645, 2017

Diffuse low-grade gliomas (DLGGs) represent several pathological entities that infiltrate and invade cortical and subcortical structures in the brain.

OBJECTIVE: To describe methods for rapid prototyping of DLGGs and surgically relevant anatomy.

METHODS: Using high-definition imaging data and rapid prototyping technologies, we were able to generate 3 patient DLGGs to scale and represent the associated whitematter tracts in 3 dimensions using advanced diffusion tensor imaging techniques.

RESULTS: This report represents a novel application of 3-dimensional (3-D) printing in neurosurgery and a means to model individualized tumors in 3-D space with respect to subcorticalwhite matter tract anatomy. Faculty and resident evaluations of this technology were favorable at our institution.

CONCLUSION: Developing an understanding of the anatomic relationships existing within individuals is fundamental to successful neurosurgical therapy. Imaging-based rapid prototyping may improve on our ability to plan for and treat complex neuro-oncologic pathology.

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The white matter tracts of the cerebrum in ventricular surgery and hydrocephalus

J Neurosurg 126:945–971, 2017

The relationship of the white matter tracts to the lateral ventricles is important when planning surgical approaches to the ventricles and in understanding the symptoms of hydrocephalus. The authors’ aim was to explore the relationship of the white matter tracts of the cerebrum to the lateral ventricles using fiber dissection technique and MR tractography and to discuss these findings in relation to approaches to ventricular lesions.

METHODS Forty adult human formalin-fixed cadaveric hemispheres (20 brains) and 3 whole heads were examined using fiber dissection technique. The dissections were performed from lateral to medial, medial to lateral, superior to inferior, and inferior to superior. MR tractography showing the lateral ventricles aided in the understanding of the 3D relationships of the white matter tracts with the lateral ventricles.

RESULTS The relationship between the lateral ventricles and the superior longitudinal I, II, and III, arcuate, vertical occipital, middle longitudinal, inferior longitudinal, inferior frontooccipital, uncinate, sledge runner, and lingular amygda-loidal fasciculi; and the anterior commissure fibers, optic radiations, internal capsule, corona radiata, thalamic radiations, cingulum, corpus callosum, fornix, caudate nucleus, thalamus, stria terminalis, and stria medullaris thalami were defined anatomically and radiologically. These fibers and structures have a consistent relationship to the lateral ventricles.

CONCLUSIONS Knowledge of the relationship of the white matter tracts of the cerebrum to the lateral ventricles should aid in planning more accurate surgery for lesions within the lateral ventricles.

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Occipitocervical Instability After Far-Lateral Transcondylar Surgery

Neurosurgery 80:140–145, 2017

After a far-lateral transcondylar approach, patients may maintain neutral alignment in the immediate postoperative period, but severe occipitoatlantal subluxation may occur gradually with cranial settling and possible neurological injury. Previous research is based on assumptions regarding the extent of condylar resection and the change in biomechanics that produces instability.

OBJECTIVE: To quantify the extent of bone removal during a far-lateral transcondylar approach, determine the changes in range of motion (ROM) and stiffness that occur after condylar resection, and identify the threshold of condylar resection that predicts alterations in occipitocervical biomechanics.

METHODS: Nine human cadaveric specimens were biomechanically tested before and after far-lateral transcondylar resection extending into the hypoglossal canal (HC). The extent of condylar resection was quantified using volumetric comparison between pre- and postresection computed tomography scans. ROM and stiffness testing were performed in intact and resected states. The extent of resection that produced alterations in occipitocervical biomechanics was assessed with sensitivity analysis.

RESULTS: Bone removal during condylar resection into the HC was 15.4%-63.7% (mean 35.7%). Sensitivity analysis demonstrated that changes in biomechanics may occur when just 29% of the occipital condyle was resected (area under the curve 0.80-1.00).

CONCLUSION: Changes in occipitocervical biomechanics may be observed if one-third of the occipital condyle is resected. During surgery, the HC may not be a reliable landmark to guide the extent of resection. Patients who undergo condylar resections extending into or beyond the HC require close surveillance for occipitocervical instability.

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