Keywords
RMSO craniotomy - cranioplasty - cerebellopontine angle - PMMA cranioplasty - custom
made implant - craniotomy defect
Introduction
Retrosigmoid suboccipital (RMSO) craniotomy is the most commonly used approach for
lesions of cerebellopontine angle (CPA). This approach was first successfully used
by Sir Charles Balance in 1907[1] for the excision of an acoustic tumor. It was later described in detail by Krause[2] and revised by Cushing.[3] The surgical technique was finally refined by Dandy.[4]
The problem with RMSO craniotomy is the risk of injury to the transverse and sigmoid
sinus during the fashioning of the craniotomy. Anatomically, the superior and lateral
borders of the craniotomy are transverse sinuses and sigmoid sinuses, respectively.
Asterion, the junction of lambdoid, temporo-occipital, and occipitomastoid bone sutures,
is not a safe outer landmark to delineate the transition of the transverse sinus to
sigmoid sinus inside the skull since this can be as far as 10 mm in location. So,
to avoid injury to the sinuses, the craniotomy size is usually made small and then
the sinus margins are exposed by trimming the bone margins.
This problem creates a craniectomy defect that is larger than the bone flap removed.
Placement of such a bone flap would cause problems that are similar to the craniectomy
defects. These problems include the risk of cerebrospinal fluid (CSF) leaks, pseudomeningocele,
postauricular cosmetic defects, postoperative incisional pain, and headache.
To counter these problems, it is important for the place cranioplasty flap made from
allogenic sources like titanium, hydroxyapatite (HA), polymethylmethacrylate (PMMA),
and polyetheretherketone. Each of these materials has its advantages and disadvantages.
Also, molding a cranioplasty flap that near perfectly fits into the defect to provide
a superior cosmetic outcome and avoid complications is another daunting task.
In this study, we introduced a novel, low-cost technique for RMSO cranioplasty using
a single standard polyvinyl chloride (PVC) plastic skull model as a guide for cranioplasty
flap development and observed the functional and cosmetic outcomes and compared it
with previous studies through an extensive literature review where craniectomy was
done alone.
Materials and Methods
This observational retrospective cohort study was conducted at the Department of Neurosurgery,
Dr. Ram Manohar Lohia Institute of Medical Sciences, Lucknow, Uttar Pradesh, India.
During 1 year period of this study from May 2021 and May 2022, 25 patients with lesions
of CPA were operated on through RMSO craniotomy followed by PMMA-based retromastoid
cranioplasty in the same sitting. The study complies with human and animals rights.
Preoperative Preparation
All patients underwent routine blood investigations. After the initial neurological
assessment, patients were subjected to audiometry in all cases. An audiometric workup
was done with pure-tone air conduction and pure-tone bone conduction. In all cases,
contrast computed tomography (CT) and magnetic resonance imaging (MRI) of the brain
were performed preoperatively.
Material Details
PMMA is a type of polyester developed from acrylic acid polymerization that was discovered
in 1939 during World War II and the medicinal use of PMMA was done after 1970.[5] Its strength is comparable to that of bone and shows good results in compression
and torsion tests. Furthermore, PMMA is heat resistant, radiolucent, inert, inexpensive,
and readily available. However, this material also has its shortcomings, such as a
high risk of extrusion, decomposition, and infection. Infection rates vary from 5%
in general cases to 23% in patients with previous reconstruction site infection. PMMA
generally adheres to the bone edges, although it sometimes requires titanium mini
plates and screws for anchorage.[6] The preparation of PMMA involves an exothermic reaction with a temperature reaching
as high as 80 to 100°C for 8 to 10 minutes, which can cause burn injuries to the underlying
tissues.[7]
Implant Preparation
A single standard PVC plastic skull model was used as a guide for cranioplasty flap
development in all 25 patients in this study. PMMA (Rapid Repair, PYRAX, Uttarakhand,
India) for commercial use is comprised of two components: a powder and a liquid component
([Fig. 1A]). The powder component contains the acrylic polymer, and the liquid component contains
the solvent, benzoyl peroxide. An adequate amount of powder was placed in a sterile
bowl, and the liquid component was mixed until a semisolid dough was created. We added
Genticyn (Abbott Healthcare Pvt Ltd., Mumbai, Maharashtra, India), a gentamicin solution
to the mixture to prevent infections. The recommended amount is 2.0 g of gentamicin
per 40 g of PMMA.[8] The antibiotic is stable at high temperatures.[8] This dough was then flattened to a thickness of approximately 3 to 5 mm and spread
nearly to the size of the craniotomy defect. An approximately 3 × 2cm cranioplasty
flap was marked on the retromastoid region where the standard RMSO craniotomy is made,
using asterion as a landmark, on the skull model ([Fig. 1B]). When the dough was still in a semisolid phase and shapeable, it was placed on
the life-size three-dimensional (3D) skull model in the marked area of interest, which
acts as a scaffold covered with liquid paraffin and molded according to the standard
contour of the skull model ([Fig. 1C]). This created a precise implant for the patient. The prepared implant was autoclaved
at 121°C and 15 pounds per square inch of pressure for 30 minutes and used under sterile
conditions during surgery.
Fig. 1 (A) Polymethylmethacrylate (PMMA) for commercial use with two components: the powder
component and the liquid component. The powder component contains the acrylic polymer
and the liquid component consists of the solvent, benzoyl peroxide. (B) The defect is marked on the skull model matching the approximate location of the
retrosigmoid suboccipital (RMSO) craniotomy. (C) When the dough is still in a semisolid phase and can be shaped, it is placed on
the life-size three-dimensional model of the skull on which the marking is done that
acts as a scaffold covered with liquid paraffin and molded according to the standard
contour of the skull model. This gives the approximate implant for the patient. The
rest of the fine trimming is done with a high-speed drill during surgery. (D) Case of a 56-year-old male with right cerebellopontine angle acoustic neuroma treated
with gross total excision following right RMSO craniotomy. Intraoperative image showing
a comparison of the craniectomy defect and the craniotomy flap. (E) Intraoperative image after the placement of the PMMA-based cranioplasty flap.
Intraoperative Steps
Anesthesia induction, patient positioning, and part preparation for RMSO craniotomy
were routinely performed. A retroauricular curvilinear skin incision was made in all
patients. Asterion was identified and a burr hole was made inferolateral to it. The
small craniotomy was made and later extended as desired approximately to the size
2 × 3cm to expose the margins of the transverse sinus superiorly and sigmoid sinus
laterally ([Fig. 1D]). The lesion was excised with satisfactory hemostasis. Dura was closed in a watertight
manner using silk 4–0 braided sutures. The uneven edges of the PMMA-based cranioplasty
flap were trimmed off with a motorized drill to facilitate a snug fit within the defect.
The implant was dipped in gentamicin antibiotic solution (4mg Genticyn in 1-L normal
saline) for 5 to 10 minutes before it was placed on the skull. Finally, it was placed
over the defect. The implant was fixed to the skull with titanium mini plates and
screws ([Fig. 1E]).
Postoperative Period
A subgaleal tissue drain was placed under the skin flap for 24 to 48 hours that was
removed on postoperative day 1. A postoperative CT scan was done as a routine to assess
the implant cosmesis. The patients were discharged on postoperative days 5 to 7.
Follow-Up
The average follow-up period was 10 months, ranging from 7 to 16 months. In follow-up,
only a contrast MRI of the brain was performed to document the completeness of tumor
resection. Cosmetic and functional outcomes were assessed according to Honeybul et
al[9] as follows: complete success, partial success, satisfactory, partial failure, and
complete failure.
Results
During the 1-year study period, 25 patients with lesions of CPA were operated on through
RMSO craniotomy followed by PMMA-based retromastoid cranioplasty in the same sitting.
Most of the patients were radiologically diagnosed with acoustic schwannoma (17 patients;
68%), 5 patients had CPA meningioma (20%), and 3 patients had CPA epidermoid (12%).
There was no significant difference in incidence by gender.
The mean surgery duration was approximately 230.00 ± 20 minutes that is a little extra
when compared with surgical time without cranioplasty which is approximately 210.00 ± 15 minutes.
This is an acceptable difference considering the benefits the patient will incur.
Mastoid air cells were opened in three cases during the fashioning of craniotomy that
was sealed with HA bone wax at the end of dural closure.
None of the patients required reoperation, and there was no evidence of CSF leak,
bleeding, infection, or poor scar formation. No patient experienced undue incisional
pain or headache at long-term follow-up. No complications or cosmetic deformities
were observed in any of the patients. The patients and their families were satisfied
with the cosmetic results. According to the Honeybul et al assessment method, 18 patients
(72%) had complete success and 7 patients (28%) had partial success. None of the patients
had a partial or complete failure. This technique was able to provide satisfactory
postoperative cosmesis at follow-up.
Discussion
RMSO craniotomy is the most commonly used approach for lesions of CPA. The problems
encountered with a small craniotomy, to avoid injury to the transverse sinus and sigmoid
sinus, are similar to the craniectomy. These include the risk of CSF leaks, pseudomeningocele,
postauricular cosmetic defects, postoperative incisional pain, and headache. This
necessitates the need for cranioplasty using allografts.
Here, in our study, we analyzed the outcome in terms of postoperative CSF leaks, postoperative
incisional pain and headache, postoperative infection, and cosmesis and compared these
with previous studies where no cranioplasty was performed. It also emphasizes the
use of PMMA as an allograft material for the construction of a cranioplasty flap and
its financial implications in the overall treatment.
The incidence of postoperative incisional discomfort has been reported between 9 and
64%.[10] The cause of this discomfort has been blamed on the scalp to dural adhesion, occipital
nerve injury, neck muscle spasm, leakage of CSF, and aseptic meningitis from bone
dust.[10]
[11] Schessel et al[12] reported pain in 64% of the patient operated by RMSO craniectomy as compared with
the complete absence of similar complaints in patients operated via the translabyrinthine
approach. Since the dura of the posterior fossa is richly innervated, adherence of
the cervical muscles to the dura and subsequent traction is supported to be responsible
for postoperative pain following suboccipital craniectomy.[13] So, interposition of bone or allogeneic material between the muscle and dura has
been suggested to reduce postoperative headache.[10]
[12] In our study, none of the patients had postoperative incisional pain in long-term
follow-up.
Cranioplasty has a theoretically higher risk of infection than the closure of a wound
without cranioplasty due to the introduction of a foreign body. The deep infection
rate of RMSO with titanium or PMMA cranioplasty has been reported between 0 and 8.2%.[14] Infection rates in PMMA cranioplasty flaps vary from 5% in general cases. However,
none of the patients in our study had any deep or superficial infections during the
period of follow-up.
CSF leaks are one of the most common complications of RMSO craniotomy.[15] CSF leak rate in cranioplasty is 1.1 to 14.5%.[14]
[15]
[16] Without cranioplasty, CSF may flow through incompletely closed dura and out of the
wound. As titanium mesh cranioplasty is highly porous, it adds little protection against
CSF leaks and a previous study has shown a CSF leak incidence of 2%.[17] No CSF leaks occurred in our study population. Preventing CSF leakage after retromastoid
craniectomy has been highly emphasized by McLaughlin et al to ensure watertight dural
closure that was followed in every case in our study.[18] Additionally, primary dural closure is the best seal without the introduction of
autologous grafts of fat or artificial dural substitutes.[19] However, this is not always possible due to shrinkage of the dura mater only from
exposure during surgery and/or electrocautery for dural bleeding. Although many dural
replacements have been introduced and used to ensure watertight dural closure, no
substitute has proven to be complication-free in a large clinical trial.[19]
[20] Theoretically, the use of a cranioplasty flap provides added support to the dura
and prevents its bulge and subsequent leak that were supported by the results of our
study.
Postoperative headache following RMSO craniectomy is likely multifactorial and includes
chemical meningitis from blood products and other surgical debris and foreign implants.
Harner et al in reporting the follow-up of 331 acoustic neuroma patients with craniectomy
defects noticed that 23% complained of a headache at 3 months, dropping to 16% at
1 year and 9% at 2 years.[10] Jackler has also reported persistent pain for more than 6 months in 29% of patients.[21] Decreased postoperative headache or relief of chronic pain when a cranioplasty was
used.[22] Treatment considerations should include placing cranioplasty when a bony defect
is present as it is a simple procedure with low morbidity and may help upward of 50%
of patients.[23] Harner et al[10] compared the retromastoid craniectomy to the cranioplasty using PMMA in 24 patients
within each group followed for at least 3 months. They found a 4% incidence of postoperative
headache after cranioplasty versus a 17% incidence of headache after craniectomy.
In another study by Rhoton et al,[24] they reported a significant reduction in postoperative headache by using cranioplasty
alone or in combination with PMMA filled in the bony defect after craniotomy because
replacing the small craniotomy flap still resulted in some residual deformity at the
operative site. This was the problem in our study that was solved with the use of
PMMA cranioplasty. Also, Soumekh et al[25] found cranioplasty to help reduce the incidence of debilitating postoperative headaches.
The histological section of dural removal from a patient who underwent reoperation
for tumor recurrence after excision of a CPA neurofibroma several years previously
stained for elastin. This demonstrates adherence of the nuchal muscle to the dura.
This patient suffered from postoperative headaches after the first operation and was
free of complaint after reoperation with cranioplasty using PMMA.[26] Modification of skin incision did not alter the incidence of headaches.[12] None of the patients in our study had a headache in long-term follow-up.
PMMA is heat resistant, radiolucent, inert, inexpensive, and readily available. This
makes PMMA our preferred choice of material. PMMA can be contoured to the natural
shape of the patient's skull easily which provides an additional watertight and strong
barrier over the craniectomy site immediately following the operation. Cranioplasty
does result in a much more pleasing contour and less noticeable cosmetic defect. All
of the patients in our study were happy with the appearance of the surgical site.
Custom-made commercial implants are costly and account for 64% of the total cost of
cranioplasties.[27] However, PMMA implants are equally effective as their commercial counterparts but
at a fraction of their cost. The cost of making this implant comes out to approximately
USD (US Dollar) 2 in India that is significantly lower when compared with others.
3D modeled titanium cranioplasty flap of similar measurements will cost around USD
500, PMMA-based 3D flap around USD 350, and nonremodeled titanium mesh around USD
100. This makes more sense in developing countries like India where 6.7% of the population
(80 million people) is below the poverty line. A brief comparison of commonly used
cranioplasty materials along with their advantages and disadvantages is cited in [Table 1].
Table 1
Comparing advantages and disadvantages of common cranioplasty materials
|
Material
|
Advantages
|
Disadvantages
|
Cost (USD)
|
Infection rate
|
Postoperative headache
|
|
Autologous bone
|
Low fracture rate, biocompatible, osteointegration, good cosmesis
|
Bone resorption, infection
|
0
|
Moderate
|
Low
|
|
HA
|
Noninflammatory, good cosmesis, osteointegration
|
Low tensile strength, brittle, infection
|
50
|
Low-moderate
|
Moderate
|
|
PMMA
|
Strong, heat resistant, inert, low cost, wide availability, ease of use
|
Infection, fracture, low osteointegration
|
2
|
Moderate
|
Low
|
|
PEEK
|
Radiolucent, inert, strong, heat resistant
|
High cost, need 3D printing, infection, low osteointegration
|
500
|
Moderate
|
Low
|
|
Titanium
|
Noninflammatory, noncorrosive, strong, low infection, malleable, good cosmesis
|
High cost, artifact on imaging, implant exposure, low osteointegration
|
150
|
Low-moderate
|
Low
|
Abbreviations: 3D, three-dimensional; HA, hydroxyapatite; PEEK, polyetheretherketone;
PMMA, polymethylmethacrylate; USD, US dollar.
Cranioplasty using PMMA is not advisable in the case of opened mastoid air cells because
of their close relation to the middle ear and the risk of infection.[28] Though in three of our cases, the exposed mastoid air cells were sealed with HA
bone paste followed by placement of the cranioplasty flap. HA bone cement has been
successfully used for the obliteration of mastoid air cells and prevention of CSF
otorrhea especially in transmastoid approaches. No incidence of CSF leak, otorrhea,
or infection was seen in follow-up.
Conclusion
RMSO craniotomy is the most commonly used approach for lesions of CPA. We have introduced
a new technique for the closure of the craniectomy defect following this approach,
in which a single standard PVC plastic skull model was used to make cranioplasty flaps
in all patients with satisfactory cosmetic outcomes. This technique has been proven
to be not only cost-effective but also time-saving and easily reproduced, which may
be significantly relevant in countries such as India, where the financial burden of
healthcare is very high. This study identifies long-term improvement in CSF leaks
and the long-term advantage of PMMA closure with less postoperative incisional pain
and headache and improvement in retroauricular cosmesis. Hence, PMMA-based cranioplasty
for the RMSO approach is highly recommended.
