Utility of PMMA-Based Cranioplasty Flap for the Reconstruction of Retromastoid Craniotomy Using a Single Standard Skull Model: A Low-Cost Alternative

Abstract

Introduction

Retrosigmoid suboccipital (RMSO) craniotomy is the most commonly used approach for lesions of cerebellopontine angle. The problem with RMSO craniotomy is the risk of injury to the transverse and sigmoid sinus during the fashioning of the craniotomy. 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. To avoid this complication, cranioplasty reconstruction is advised.

Objectives

In this study, we introduced a novel, low-cost technique for RMSO cranioplasty using a single standard polyvinyl chloride plastic skull model as a guide for cranioplasty flap development and observed the functional and cosmetic outcomes.

Results

None of the patients required reoperation, and there was no evidence of cerebrospinal fluid (CSF) leak, bleeding, infection, or poor scar formation. No patient experienced undue incisional pain or headache at long-term follow-up. The patients and their family members were satisfied with the cosmetic results of the procedure.

Conclusion

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 polymethylmethacrylate closure with less postoperative incisional pain and headache and improvement retroauricular cosmesis.

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.

Conflict of Interest

None.

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Publication History

Article published online:
26 June 2025