RECONSTRUCTION OF AN EXTENSIVE MIDFACIAL DEFECT USING ADDITIVE MANUFACTURING TECHNIQUES

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1 RECONSTRUCTION OF AN EXTENSIVE MIDFACIAL DEFECT USING ADDITIVE MANUFACTURING TECHNIQUES JH van den Heever 1, CF Hoogendijk 2, SJP Botha 2, GJ Booysen 3, J Els 3, M Truscott 3 and JG van der Walt 3 1: Department of Prosthodontics, Faculty of Health Sciences, University of Pretoria 2: Department of Oral and Maxillofacial Surgery, Faculty of Health Sciences, University of Pretoria 3: Centre of Rapid Prototyping and Manufacturing, Central University of Technology Free State Introduction: Malignant peripheral nerve sheath tumours (MPNST s) are extremely rare sarcomas arising from the Schwann cells of peripheral nerves [1,2,3]. MPNST s of the trigeminal nerve are exceedingly rare [1] with only 17 cases published in English medical literature [2,5,6]. These tumours carry a poor prognosis [1] with a 5 year survival rate of 20% in the head and neck region [1,8]. The local recurrence rate ranges from 30% to 60%; even in cases of complete resection [1,3]. MPNST s have been shown to metastasize to the lungs, soft tissue, bone, liver, and/or brain in almost 65% of cases [1,2,3,9,10]. Due to the aggressive highly infiltrative nature [1], the high rate of recurrence [1,3], and poor prognosis, aggressive radical resection is the recommended treatment for these tumours [1,2,5,11]. Complete removal of the lesion with tumour-free margins [3,4,10] of 2 cm in all directions is seldom possible in the head and neck region [3] and therefore radiation therapy after resection could offer increased rates of tumour control and long-term survival [1,2,8,11]. Case report: We present the case of a 33 year old female patient who presented in 2011 with a slow growing exophytic soft tissue mass of the anterior maxilla in the region the 13, 12, 11, 21 spreading to the palate. A biopsy of the lesion was performed and diagnosed as Kaposi sarcoma. The patient was referred for chemotherapy but however never received the treatment and no follow up appointments were scheduled. She returned in 2012 and presented with an anterior palatal tumour measuring 12x10x8 mm infiltrating the underlying maxilla. The lesion appeared to be secondarily infected with continuous bleeding and occasional pain. An incisional biopsy was submitted for histology and the lesion was diagnosed as a malignant peripheral nerve sheath tumour. The anterior part of the maxilla was resected. This resection included all the anterior teeth and premolars as well as the body of the maxilla up to about 2 cm above the nasal floor. The patient disappeared again and returned in 2013 with tumour infiltration of the left and right alae, the right cheek as well as an ulcerated lesion of the upper lip. Histological examination was diagnosed as malignant peripheral nerve sheath tumour recurrence (Fig 1).

2 Figure 1: Malignant peripheral nerve sheath tumour of the face A maxillectomy, rhinectomy, resection of the upper lip and aspects of the left and right cheeks were performed. Histologic examination found all the surgical margins to be free from tumour (Fig 2). Figure 2: Specimen consisting of the nose, cheeks, upper lip, and maxilla including three molar teeth Four oncology implants (Southern Implants, Irene. South Africa) were placed at time of resection and an impression of the implants and the defect was made (Fig.3). A modified provisional implant supported obturator prosthesis as well as a provisional silicone facial prosthesis were manufactured and placed three days after resection in order to establish separate oral and nasal cavities to improve swallowing, facilitate salivary control, and enhance speech articulation. (Fig 4 & 5). Figure 3: Oncology implants placed at time of resection Figure 4: Provisional implant supported acrylic obturator Figure 5: Provisional facial prosthesis

3 Due to parafunctional activities caused by the psychological side effects of the disease and the resection, the implants were lost after 4 weeks and the obturator had to be removed. This posed a major complication as effective retention of the prosthesis by engaging undercuts was impossible because the tissues bordering the defect did not have any bony foundation and could easily be compressed and distorted by the impression material during the taking of the impression (Fig 6). Figure 6: Defect after loss of implants Due to the extent and complexity of the defect it was decided to fabricate an anatomical model of the hard tissue. This model was used for planning the design of a titanium laser sintered prosthetic frame for the patient (Fig. 8). Computer Tomography (CT) was used as a starting platform and the Digital Imaging and Communications in Medicine (DICOM) files from the scanner were converted to Standard Triangulation Language (.stl) format using dedicated software namely Mimics from Materialise, Belgium. The software allows one to alter the grey scale values from the DICOM images to differentiate between soft tissue and bone. Figure 7: Planning model. Green part resected maxilla The region of interest was then masked and calculated as a 3D model which was exported as a.stl file, sliced using RP Tools (EOS, GmbH) and sent to the Additive Manufacturing (AM) or 3D printing machine to manufacture the planning models. The Centre for Rapid Prototyping and Manufacturing (CRPM) at the Central University of Technology, Free State did the CT segmentation of the skull and produced the 3D model on an EOS P385 Laser Sintering machine in PA2200 Polyamide material at 150 µm layer thickness (Fig 8).

4 a b Figure 8: Front (a) and side (b) view of PA2200 Laser sintered planning model indicating the severity of the defect The 3D model was used to identify the boundaries and fixation areas of the planned implant. The 3D skull data were imported into 3-MATIC (Materialise, Belgium) in order to design the implant (Fig. 9). The areas indicated on the physical planning models were used as starting points to indicate anatomical areas on the computer model. The fixation areas as indicated in Fig. 9 were extruded from the surface and would later form the areas of the titanium implant that will be in direct contact with the skull. This would ensure a perfect fit of the implant on the anatomical features of the skull. The implant design was next printed in resin on an Objet Connex 350 machine and positioned onto the PA2200 scull to test fitment. Figure 9: Final 3D CAD design of titanium implant The implant design was exported as a.stl file which was converted to a slice file and transferred to an EOSINT M280 Direct Metal Laser Sintering (DMLS) machine that manufactured the implant from a biocompatible Titanium-6Al-4V powder of sub-40 µm particle size (Fig. 10).

5 Figure 10: Polished midfacial titanium implant Figure 11: Implant fitted to pre-operative model to confirm the correct fit 3-MATIC was furthermore utilised to design a custom drill guide following a similar design process as the implant design to aid the surgeon in drilling the fixating holes at the correct angles to ensure that the cortical screws are place in areas where the most bone is located (Fig. 12 and 13). Figure 12: 3D model of drill guide with drilling angles indicated by the red lines Figure 13: Drill guide manufactured by means of 3D printing in PA2200 polyamide material The anatomical data were also used to produce a mask from PA2200 polyamide in order to aid the production of a soft tissue prosthesis that would cover the defect post-operatively. The implant was trimmed and polished where after the fitting surfaces of the implant were treated in the same way as dental implants in order to achieve possible cortical osseointegration of the implant. The procedure included some post machining and thread cutting done by Southern Implants, Irene, South Africa.The implant was placed and fixated using 21 cortical screws. An impression was made for the manufacturing of a maxillary denture as well as a magnet retained facial prosthesis. The facial mask was used as a special tray to determine the vertical dimension of the face as all landmarks had been lost due to the loss of the provisional implant supported prosthesis (Fig. 14).

6 Figure 14: The facial mask The fitting surface of the maxillary denture was lined with a soft denture base material. The purpose of the soft lining was to act as a shock absorber during function (Fig. 15 a, b, c, d). Figure 15a: Implant immediately post-surgery Figure 15b: Maxillary denture with soft base fitting surface Figure 15c: Maxillary denture with soft base fitting surface and magnets for retention of facial prosthesis Figure 15d: Provisional facial prosthesis

7 FINITE ELEMENT ANALYSIS: A finite element analysis (FEA) analysis was conducted on the implant (fixed onto the skull) in order to determine the maximum displacement and stress induced [12]. The full model was analysed and the material properties of titanium (Table 1) was applied to the implant and the properties of zygomatic bone (Table 2) was applied to the whole skull. Table 1: Properties of titanium Titanium Density 4430 kg/m 3 Modulus of Elasticity MPa Poisson s Ratio 0.33 Table 2: Properties of bone Bone Density 1700 kg/m 3 Modulus of Elasticity MPa Poisson s Ratio 0.4 Due to limited time only one 2400 N point load was applied to a concentrated area at the centre of the upper jaw to simulate a worst-case scenario. The Von Mises stresses in the skull and the implant for the applied load are shown in figures 16 & 17 [13]. Figure 16: Von Mises stress in the skull and implant Figure 17: Von Mises stress in the implant When a 2400 N point load is applied, a stress concentration occurs at the applied point of the point load. This stress concentration amounts to 3000 MPa at the applied region. This is not a realistic stress as the load would be distributed over a larger area in reality reducing the stress dramatically. The remainder of the stresses in the implant (maximum 400 MPa) and the skull are well below the yield stress of the materials.

8 CONCLUSION: MPNST s of the trigeminal nerve are exceedingly rare and to our knowledge this is the first case where additive manufacturing techniques were used in the manufacturing of a titanium implant for an extensive midfacial reconstruction. The finite element analysis has shown that the implant and the attachment to the bone can withstand the occlusal forces as the stresses are well within the yield stress of the different materials. The patient has adapted well to the implant and the prosthesis and has been recurrence free for 6 months. ACKNOWLEDGEMENTS: The authors would like to thank: Southern Implants, Irene, South Africa for surface enhancement and sterilizing of the implant. Me. A. Olwagen, Central University of Technology, Bloemfontein, South Africa for the Finite Element Analysis. The South African National Research Foundation for financial support in the acquisition of the Objet Connex 350 machine that was used to print the initial designs of the titanium implant in resin. Materialise, Belgium for their continued support to the research team at the CRPM. REFERENCES: [1] Schmidt RF, Yick F, Boghani Z, Eloy JE, Liu JK: Malignant peripheral nerve sheath tumors of the trigeminal nerve: a systematic review of 36 cases. Neurosurg Focus., 34:1-10, [2] Bowers CA, Taussky P, Duhon BS, Chin SS, Couldwell WT: Malignant peripheral nerve sheath tumour of the trigeminal nerve: case report and literature review. Br J Neurosurg.,25: , [3] Gupta G, Mammis A, Maniker A: Malignant peripheral nerve sheath tumors. NeurosurgClin N Am., 19: , [4] Hajdu SI: Peripheral nerve sheath tumors. Histogenesis, classification, and prognosis. Cancer., 72: , [5] Chibbaro S, Herman P, Povlika M, George B: Malignant trigeminal schwannoma extending into the anterior skull base. ActaNeurochir., (Wien)150: , [6] Sheitnauer BW, Erdogan S, Rodriquez FJ, et al: Peripheral nerve sheath tumors of cranial nerves and intracranial contents: a clinicopathologhic study of 17 cases. Am J SurgPathol., 22: , [7] MacNally SP, Rutherford SA, Ramsden RT, Evans DG, King AT: Trigeminal schwannomas. Br J Neurosurg., 22: , [8] Minovi A, Basden O, Hunter B, Draf W, Bockmühl U: Maligmant peripheral nerve sheath tumors of the head and neck: management of 10 cases and literature review. Head Neck., 29: , [9] Ducatman BS, Scheithauer BW, Piepgras DG, Reiman HM, Ilstrup DM: Malignant peripheral nerve sheath tumors. A clinicopathologic study of 120 cases. Cancer., 57: , [10] Stark AM, Buhl O, Hugo HH, MehdornHM:Malignant peripheral nerve sheath tumours-report of 8 cases and review of the literature. ActaNeurochir., (Wien) 143: , [11] Zaidi A, Saliba I: Malignant peripheral nerve sheath tumor of intracranial nerve: a case series review. AurisNasus Larynx 37: , [12] An YH, Draughn RA: Mechanical testing of Bone and the Bone Implant Interface. CRC Press:2000. [13] Olwagen A: Report of preliminary Finite Element Analysis on the Skull Prosthesis. Central University of Technology:2013.