Sonolucent Cranial Implants: Cadaveric Study and Clinical Findings Supporting Diagnostic and Therapeutic Transcranioplasty Ultrasound Micah Belzberg, BA,  Netanel Ben Shalom, MD, y Edward Yuhanna, BA, RDMS, z Amir Manbachi, PhD, z§ Aylin Tekes, MD, ô Judy Huang, MD, y Henry Brem, MD, y and Chad R. Gordon, DO y Background: Previously, sonographic evaluation of the intracra- nial contents was limited to intraoperative use following bone flap removal, with placement of the probe directly on the cortical surface or through a transsulcal tubular retractor. Cranioplasty with sono- lucent implants may represent a postoperative window into the brain by allowing ultrasound to serve as a novel bedside imaging modality. The potential sonolucency of various commonly used cranial implant types was examined in this study. Methods: A 3-phase study was comprised of cadaveric evaluation of transcranioplasty ultrasound (TCU) with cranioplasty implants of varying materials, intraoperative TCU during right-sided cranioplasty with clear implant made of poly-methyl-methacrylate (PMMA), and bedside TCU on postoperative day 5 after cranioplasty. Results: The TCU through clear PMMA, polyether-ether-ketone, and opaque PMMA cranial implants revealed implant sonoluceny, in contrast to autologous bone and porous-polyethylene. Intraoperative ultrasound via the clear PMMA implant in a single patient revealed recognizable ventricular anatomy. Furthermore, postoperative bedside ultrasound in the same patient revealed comparable ventricular anatomy and a small epidural fluid collection corresponding to that visualized on an axial computed tomography scan. Conclusion: Sonolucent cranial implants, such as those made of clear PMMA, hold great promise for enhanced diagnostic and therapeutic applications previously limited by cranial bone. Furthermore, as functional cranial implants are manufactured with implantable devices housed within clear PMMA, the possibility of utilizing ultrasound for real-time surveillance of intracranial pathology becomes much more feasible. Key Words: Cranioplasty, implant, poly-methyl-methacrylate, sonolucent, ultrasound (J Craniofac Surg 2019;00: 00–00) L arge-sized cranial defects are repaired with either autologous or synthetic materials. 1,2 Until recently, autologous bone has been considered the ‘‘gold standard’’ due to patient preference for their own tissue, availability, and cost. 1–7 However, over the past decade, mounting reports of bone flap sterile resorption and infection have prompted the widespread use and acceptance of customized cranial implants (CCIs). 5,7–10 The CCIs offer additional benefits over bone stored for prolonged time periods, such as sterility and design shape to reliably address coexisting hard and soft-tissue deficiencies, thus correcting and/or preventing postoperative tem- poral hollowing. 11,12 In parallel, both noninvasive and invasive transcranial ultra- sound have demonstrated numerous therapeutic/diagnostic applica- tions including neuromodulation for movement disorders, magnetic resonance imaging (MRI)-guided lesion ablation, and local drug delivery via blood brain barrier disruption. 13–16 Unfortunately however, these emerging technologies remain limited by the acous- tic properties of cranial bone causing ultrasonic wave attenuation, scattering, and absorption. 13–21 In contrast to adults, neonates have multiple open fontanelles which serve as naturally occurring acoustic windows, hence diag- nostic ultrasound is widely employed and often favored. 22–24 Single-stage cranioplasty presents a newfound opportunity for neurosurgeons to create a synthetic acoustic window by replacing normal bone with a cranial implant composed of sonolucent bio- material, a material providing minimal to no obstruction of ultra- sonic waves. A sonolucent cranial implant would thereby permit ‘‘transcranioplasty ultrasound’’ (TCU) for both diagnostic and therapeutic postoperative applications. 25 Of the over 100 cranioplasty surgeries performed at our institu- tion over the past year, the most common biomaterials inserted included poly-methyl-methacrylate (PMMA), polyether-ether- ketone (PEEK), and porous polyethylene. As of just recently, custom cranial implants can be made with a novel clear appearance using PMMA, thereby allowing full transparency to visible light and wireless Bluetooth signal transmission with respect to wireless From the  Department of Plastic and Reconstructive Surgery, Johns Hopkins University School of Medicine, Baltimore, MD; y Department of Neurosurgery, Johns Hopkins University School of Medicine, Balti- more, MD; z Department of Radiology, Johns Hopkins Hospital, Balti- more, MD; § Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD; and ô Division of Pediatric Radiology and Pediatric Neuroradiology, The Russell H. Morgan Department of Radi- ology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD. Received December 19, 2018. Accepted for publication February 9, 2019. Address correspondence and reprint requests to Chad R. Gordon, DO, FACS, Director, Neuroplastic and Reconstructive Surgery, Associate Professor of Plastic Surgery and Neurosurgery, Johns Hopkins University School of Medicine, JHOC, 8th Floor, 601 N Caroline St, Baltimore, MD 21287; E-mail: cgordon@jhmi.edu CG is a consultant for Stryker and Longeviti Neuro Solutions. JH and CG are stockholders of Longeviti Neuro Solutions. The remaining authors report no conflicts of interest. This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal. Copyright # 2019 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of Mutaz B. Habal, MD. ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000005454 ORIGINAL ARTICLE The Journal of Craniofacial Surgery  Volume 00, Number 00, Month 2019 1