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Year : 2021  |  Volume : 69  |  Issue : 2  |  Page : 311--314

Bone Wax as an Intra-Cavitary Fiducial for Intra-Operative Magnetic Resonance Imaging Guided Resections of Eloquent Cortex Glioma

Kirit Arumalla, Harsh Deora 
 Department of Neurosurgery, National institute of Mental Health and Neurosciences (NIMHANS), Bangalore, India

Correspondence Address:
Harsh Deora
Department of Neurosurgery, National Institute of Mental Health and Neurosciences (NIMHANS), Bangalore - 560 029, Karnataka


Background: The extent of resection of eloquent cortex low-grade glioma is improved by intraoperative magnetic resonance imaging (IOMRI) which is faced with challenges such as difficulty in automated registration after the MRI and prolongation of operative time. Objective: We describe an easier and foolproof method of resection using bone wax as fiducial. Surgical Procedure: A young male with right-frontal low-grade glioma anterior and superficial to the motor fibers was operated using IOMRI and awake craniotomy under neuromonitoring. Motor cortex mapping using phase reversal and continuous motor evoked potential (MEP) monitoring was used initially followed by continuous sub-cortical suction stimulation for deeper decompression until stimulation was seen. Bone wax in a triangular shape was placed at the suspicious margin and IOMRI was done. Results: The tumor residue was seen anterior and deeper to the bone wax and removed subsequently. Conclusion: The use of bone wax as a fiducial can aid in IOMRI-guided resection of eloquent cortex glioma in awake patients.

How to cite this article:
Arumalla K, Deora H. Bone Wax as an Intra-Cavitary Fiducial for Intra-Operative Magnetic Resonance Imaging Guided Resections of Eloquent Cortex Glioma.Neurol India 2021;69:311-314

How to cite this URL:
Arumalla K, Deora H. Bone Wax as an Intra-Cavitary Fiducial for Intra-Operative Magnetic Resonance Imaging Guided Resections of Eloquent Cortex Glioma. Neurol India [serial online] 2021 [cited 2021 Jun 16 ];69:311-314
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Full Text

The usage of intraoperative magnetic resonance imaging (IOMRI) has been shown to increase the extent of resection of gliomas.[1],[2] The usage of IOMRI has also shown an improved survival advantage in low-grade glioma resection.[3],[4],[5] However, the margin of resection is affected by brain shift due to arachnoid dissection[6] and automatic registration is difficult in awake craniotomy patients[7] and typically requires at least 12 of the 20 fiducials to be present in the field of view. We propose the use of bone wax which appears hypointense in T2 images and is easy to interpret, and thus, can be used as an intra-cavitary fiducial. We present a low-grade glioma that was resected using the same to the best of our knowledge, this is the first video demonstration of this technique in literature.

 Case Illustration

A young male presented with generalized tonic seizures for two years. He was evaluated with MRI imaging which showed a T1 isointense, T2 hyperintense, non-enhancing lesion in the right frontal precentral gyrus anterior and superficial to motor cortex with fMRI suggesting the motor fibers posterior and deeper to the tumor.

 Surgical Approach

Preoperative imaging using functional MRI for motor cortex and diffusor tensor imaging are fused with the neuro-navigation [Figure 1]. Since a low-grade glioma was suspected fluid attenuated inversion recovery (FLAIR) images were used for navigation here.We use the complete awake technique with the patient remaining awake throughout the surgery, breathing spontaneously.[2] Oxygen is supplemented using nasal cannulas.After scalp block, patient is positioned using a modified high-field MR imaging-compatible head holder. The surgical drapes are arranged in an “open tent fashion” such that the face is visible to the anesthesiologist under the drapes.Patient is positioned supine with a head tilt of the 45-60° with a pillow under the ipsilateral shoulder such that the lateral surface of the frontal lobe is superior most.Before incision, local anesthetic is infiltrated along the incision line and after craniotomy cottons soaked in 2% lignocaine are kept on the dura along the incision line.After dural opening, the motor cortex mapping is done by checking for phase reversal via a 4-contact strip electrode and the tumor location with respect to motor cortex is mapped using an ultrasound.Tumor decompression is then done in a standard fashion with the help of suction-sub-cortical electrode and the neurosurgeon, neuropsychologist/neurophysiologist and anesthesiologist maintaining verbal contact with the patient and checking for deficits.Once the resection is deemed complete either via neuro-navigation or monitoring shows stimulation via the sub-cortical at 5 mA (= 5 mm) the resection is stopped.Bone wax fashioned as a triangle is inserted near the area of probable residue as deemed by the neurosurgeon. If no area is suitable, it is usually placed in the deepest portion of the cavity so that it does not move during the shifting of the patient.'Soft-closure' using only skin continuous sutures is done and patient is shifted to the MRI suite after checking motor responses. At this point, the corkscrew and stimulation electrodes need to be removed to prevent burn risks.[5]Since this was a low-grade glioma on frozen section, FLAIR/T2 images are taken to determine the extent of resection and this also allows the clear intra-cavitary fiducial (hypointense bone wax) to be appreciated clearly.Residue is present anterior and deeper to the position of the bone wax for approximately 1 cm depth [Figure 2].The patient is again shifted back and skin sutures opened after applying fresh drapes. Since automated registration is not required, valuable time can be saved. Also, during the MRI, the exact location of the residue was noted with respect to the bone wax, and thus, immediate 1 cm resection is carried out anterior to the bone wax which is seen in situ.Immediate post-operative MRI shows complete resection of the 1 cm residue [Figure 3] with no deficits on post-operative examination.{Figure 1}{Figure 2}{Figure 3}

 Methods to Prevent Movement of Bone Wax

There are some methods to prevent displacement of bone wax which may be useful, especially in large cavities:

Cottonoids can be placed to pack the cavity while placing the bone wax. This prevents displacement of the wax but needs to be clearly differentiated during MRI done intraoperatively.The bonewax as shown is made in a triangle or oval shape which allows it to be “inserted” rather than placed at the suspicious margin and prevents displacement.The bonewax is usually placed at the most dependent portion and the suspicious margin can be seen in relation to the same.

Video link:

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Video timeline with audio transcript :

0.00-0.11 s: Today we are going to be demonstrating the technique of bone being used as an intra-cavitary fiducial with an IOMRI in eloquent cortex gliomas as an example.[Video 1]

0.11-0.20 s: IOMRI has been used in the resection of all eloquent cortex gliomas and have resulted in better resection rates.

0.21-0.36 s: However, loss of time and poor quality of intraoperative images lead to difficulty in interpreting the images, especially in cases where automatic registration is not possible, due to minimum 12/16 fiducials needed to be imaged. Thus, this technique will serve as a fail-safe.

0.37-0.42 s: This case of low-grade glioma can be seen anterior to the motor strip and superficial to the motor fibers.

0.43-0.58 s: Supine position with head tilt is preferred for this case. The motor cortex is mapped using strip electrodes with the help of phase reversal and after that the strip electrode is placed on the motor cortex to allow for continuous MEP monitoring.

1.02- 1.10 s: Ultrasound to confirm the location of the tumor under continuous saline irrigation.

1.11-1.20 s: Once the gyrus involved is delineated, the decompression proceeds in a standard manner. We use dynamic retraction in place of fixed retraction in these cases.

1.21-1.45 s: In my left hand I hold the sub-cortical continuous suction stimulator which stimulates and suctions at the same time and allows for tumor decompression as is seen. This allows for demonstration of the eloquent cortex below it.

1.45-1.53 s: Once tumor decompression is complete, we reach the pial surface.

1.53-1.57 s: Bone wax in triangular shape inserted at the suspicious margin

2.00-2.10 s: Temporary closure of the wound done. Since the patient is awake, the responses can be checked and the patient shifted to intraoperative MRI.

2.10-2.20 s: The coil is placed above the head of the patient and all the electrodes have been removed and IOMRI done.

2.20-2.43 s: Bone wax seen distinctly as a hypointense part of tumor resection cavity and residual tumor made out deep and anterior to the same. One patient brought back bone wax seen “not displaced” and removed.

2.43-3.05 s: We exactly know that the tumor residue is anterior and deep to the location of the bone wax. Once the bone wax is removed resection is thus done only anterior and deeper to it, carefully removing only 1 cm of the tumor as no electrodes are now available.

3.06-3.11 s: After resection, hemostasis is achieved. Post-operative MRI indicating complete resection of the tumor

3.11-3.25 s: This technique has certain limitations that the bone wax cannot be placed at the superolateral margin, or if ventricle has been opened, the bone wax can get displaced and needs to be supported by the gelfoam.

3.26-3.40 s: Initial resection needs to be as complete as possible. Unnecessary sequences need to be avoided.

3.41-4.08 s: Eloquent cortex gliomas need planning and fMRI and diffusor tensor imaging (DTI) need to be reviewed by the neurosurgeon themselves. Awake surgery is helpful in such cases. If automatic registration fails, the bone wax is an inert substance which allows delineations of the residue. Intraoperative monitoring is essential to prevent deterioration. Avoiding delay in imaging and ultrasound can help in further resection. These are my references.


The patient was discharged in a stable condition with no motor deficit.

Pearls and pitfalls

This technique can be used in any eloquent cortex glioma resection where margin of resection needs to balance between complete tumor decompression and neurological deficit.[1],[3]Loss of time and poor quality of intraoperative MRI images can be limiting factors for automatic registration using fiducials as intraoperative MRI may not cover all the fiducials (min 12 out of 16 needed for automated registration).This method serves as a fail-safe in such cases and is completely safe and easily available in every neurosurgery theater.The distinct hypointense nature of the bone wax can allow easy identification in intraoperative MRI images, and thus, help identify location of the residue.Due to tumor decompression, the opercula may collapse in the cavity and during re-resection retraction of these opercula again causes the navigation to shift. This is crucial in areas with less than 1 cm residue. Since bone wax is placed at the deepest portion, its position remains stable and the residue location is known before retracting the opercula.Placing the bone wax at superior lateral boundaries can cause it to move and thus alter interpretation.[3]In any surgery where ventricle has been opened, either the opening needs to be plugged with gelfoam or bone wax use needs to be avoided.Initial resection needs to be as complete as possible as monitoring is usually not available after intraoperative MRI.Image sequences needed for residue determination should be asked. Unnecessary sequences increase operative time and patient fatigue.[6],[7]


Level I evidence for better resection rates with the use of IOMRI in gliomas has been published.[1] The goal of any glioma surgery is maximum safe resection and differentiating glioma from normal brain solely with the help of visualization under a microscope may not be ideal.[2] High-grade gliomas may still be easier due to the presence of necrosis and high vascularity. Low-grade gliomas are difficult as they mimic the cortex in terms of consistency and vascularity.[3],[4],[5] To target this issue IOMRI was introduced. Studies demonstrate that the introduction of IOMRI improved the extent of resection, thus, overall survival in low-grade glioma patients, while decreasing mortality and recurrence rates.[3] The resection of the tumor in the eloquent cortex must be performed under neuromonitoring to decrease the post-operative neurological deficits. Before initiation of resection, an intraoperative ultrasound can be used to determine the relation of the tumor with the gyrus. After resection of the bulk of the tumor, identification of the margin from the normal functional cortex is difficult. The opening of the arachnoid during surgery can lead to cerebrospinal fluid (CSF) leak and brain shift and decompression of the tumor internally can lead to distorted margins seen on neuro-navigation.[6] Automatic registration may present problems with 12 out of 20 fiducials in the intraoperative MRI picture needed for registration (may need multiple sequences or re-runs of the same sequence) which is difficult in cases of awake craniotomy.[7] Registration using face and nose in a case with open wounds and drapes is difficult. The bone wax as a triangular-shaped structure was used as fiducial at the suspicious margin with the imaging suggestive of tumor anterior and deep to it on MRI which was resected. Since we neurosurgeons can interpret these images easily it allows for better resection and greater confidence without the risk of brain shift or retraction changing the anatomy while doing resection. The placement of the bone wax at the superficial and lateral end of the resection cavity is tricky as it can be displaced during shifting, thus, altering the interpretation.[3] This can be avoided by completing maximum safe resection before the MRI. The other limitation is the difficulty in usage when the ventricle is opened, which either should be plugged with gelfoam or bone wax avoided in usage.

Failure of automatic registration or poor-quality images are no longer barriers in IOMRI-guided removal as bone wax can be seen as a clear hypointense structure inside the resection cavity.


Bone wax can be used as a safe and convenient method to circumvent poorer image quality and delay in re-registration. Fiducials are placed around the skull in fixed positions for registration. It makes sense that an “intracavitary-fiducial” which shifts with the brain will read to better resection efficacy and use of intraoperative MRI. To our knowledge, this is the first video demonstration of this technique.[8]

Declaration of patient consent

A full and detailed consent from the patient/guardian has been taken. The patient's identity has been adequately anonymized. If anything related to the patient's identity is shown, adequate consent has been taken from the patient/relative/guardian. The journal will not be responsible for any medico-legal issues arising out of issues related to patient's identity or any other issues arising from the public display of the video.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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