Creating high-resolution 3D cranial implant geometry using deep learning techniques

Creating a personalized implant for cranioplasty can be costly and aesthetically challenging, particularly for comminuted fractures that affect a wide area. Despite significant advances in deep learning techniques for 2D image completion, generating a 3D shape inpainting remains challenging due to the higher dimensionality and computational demands for 3D skull models. Here, we present a practical deep-learning approach to generate implant geometry from defective 3D skull models created from CT scans. Our proposed 3D reconstruction system comprises two neural networks that produce high-quality implant models suitable for clinical use while reducing training time. The first network repairs low-resolution defective models, while the second network enhances the volumetric resolution of the repaired model. We have tested our method in simulations and real-life surgical practices, producing implants that fit naturally and precisely match defect boundaries, particularly for skull defects above the Frankfort horizontal plane.

Outcomes of Cranioplasty Reconstructions: Review of Cranioplasty Implants and Free Flap Coverage Variables that Affect Implant Exposure

BACKGROUND

Complex scalp wounds with cranial/dural involvement are challenging to reconstruct. Successful reconstruction can be achieved with cranial implants/hardware and free flap coverage. Wounds can breakdown and require revision procedures. We addressed reconstructive outcomes of different implants requiring free flaps.

OBJECTIVE

To determine the factors associated with implant exposure.

DESIGN

Multi-institutional retrospective review of 82 patients, 2000-2020, repaired with cranial implants and free flap coverage.

RESULTS

Implant exposure occurred in 13/82 (16%) reconstructions. Flap atrophy or thinning leading to implant exposure occurred in 11/82 (13%) reconstructions, including partial flap atrophy OR 0.05 (95% CI 0.0-0.35) and total flap atrophy OR 0.34 (95% CI 0.02-19.66). Revision surgeries that occurred subsequent to flap reconstruction were also associated with implant exposure (OR 0.02 (95% CI 0.0-0.19)). Implant exposure was not associated with radiation therapy, patient health history, implant type, flap type, or postoperative complications.

CONCLUSIONS

Implant exposure is associated with free flap atrophy, leading to inadequate implant coverage and the need for revision surgeries. Completing reconstruction with adequate soft tissue bulk and coverage and avoiding revision surgery may decrease the risk for implant exposure over time.

LEVEL OF EVIDENCE

4 Laryngoscope, 133:2954-2958, 2023.

Different materials of cranioplasty for patients undergoing decompressive craniectomy: A protocol for systematic review and network meta-analysis

BACKGROUND

Cranioplasty is widely applied on patients who has undergone decompress craniectomy (DC) due to intractable increased intracranial pressure and the cranioplasty materials have been on the bleeding edge of biomolecular and material science. This systematic review and network meta-analysis (NMA) will be conducted to comprehensively evaluate the safety and efficacy of different cranial implants for patients with cranial defects due to various reasons.

METHODS AND ANALYSIS

This protocol has been reported following the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols. The following electronic databases will be searched from the date of database establishment to September 1, 2020: PubMed, Embase, Cochrane Library, Web of Science, China National Knowledge Infrastructure, VIP, and Wanfang. Randomized controlled trials and non-randomized prospective studies focus on cranial implants will be included. Quality assessment will be conducted using Cochrane Collaboration's tool or risk of bias in nonrandomized studies of interventions based on their study designs. The primary outcome will be postoperative early mortality and implant failure while various complications for secondary outcomes. Pairwise and network meta-analysis will be conducted using STATA V.14 (StataCorp, College Station, Texas, USA). Subgroup analyses and sensitivity analyses will be conducted to assess the robustness of the results.

ETHICS AND DISSEMINATION

This systematic review does not require an ethics approval or the need to obtain informed consent. The results will be published in a peer-reviewed scientific journal.

PROTOCOL REGISTRATION NUMBER

INPLASY 202110001.

Targeted ToF-SIMS Analysis of Macrophage Content from a Human Cranial Triphasic Calcium Phosphate Implant

Macrophages play a key role in determining the fate of implanted biomaterials, especially for biomaterials such as calcium phosphates (CaPs) where these cells play a vital role in material resorption and osteogenesis, as shown in different models, including clinical samples. Although substantial consideration is given to the design and validation of different CaPs, relatively little is known about their material-cell interaction. Specifically, the intracellular content of different CaP phases remains to be assessed, even though CaP-filled macrophages have been observed in several studies. In this study, 2D/3D ToF-SIMS imaging and multivariate analysis were directly applied on the histology samples of an explant to reveal the content of macrophages. The cellular content of the macrophages was analyzed to distinguish three CaP phases, monetite, beta-tricalcium phosphate, and pyrophosphate, which are all part of the monetite-based CaP implant composition under study. ToF-SIMS combined with histology revealed that the content of the identified macrophages was most similar to that of the pyrophosphate phase. This study is the first to uncover distinct CaP phases in macrophages from a human multiphasic CaP explant by targeted direct cell content analysis. The uncovering of pyrophosphate as the main phase found inside the macrophages is of great importance to understand the impact of the selected material in the process of biomaterial-instructed osteogenesis.

Group Refractive Index of Nanocrystalline Yttria-Stabilized Zirconia Transparent Cranial Implants

Transparent “Window to the Brain” (WttB) cranial implants made from a biocompatible ceramic, nanocrystalline Yttria-Stabilized Zirconia (nc-YSZ), were recently reported. These reports demonstrated chronic brain imaging across the implants in mice using optical coherence tomography (OCT) and laser speckle imaging. However, optical properties of these transparent cranial implants are neither completely characterized nor completely understood. In this study, we measure optical properties of the implant using a swept source OCT system with a spectral range of 136 nm centered at 1,300 nm to characterize the group refractive index of the nc-YSZ window, over a narrow range of temperatures at which the implant may be used during imaging or therapy (20–43°C). Group refractive index was found to be 2.1–2.2 for OCT imaging over this temperature range. Chromatic dispersion for this spectral range was observed to vary over the sample, sometimes flipping signs between normal and anomalous dispersion. These properties of nc-YSZ should be considered when designing optical systems and procedures that propagate light through the window, and when interpreting OCT brain images acquired across the window.

Biomaterial and implant induced ossification: in vitro and in vivo findings

Material-induced ossification is suggested as a suitable approach to heal large bone defects. Fiber-reinforced composite-bioactive glasses (FRC-BGs) display properties that could enhance the ossification of calvarial defects. Here, we analyzed the healing processes of a FRC-BG implant in vivo from the perspective of material-induced ossification. Histological analysis of the implant, which was removed 5 months after insertion, showed the formation of viable, noninflammatory mesenchymal tissue with newly-formed mineralized woven bone, as well as nonmineralized connective tissue with capillaries and larger blood vessels. The presence of osteocytes was detected within the newly generated bone matrix. To expand our understanding on the osteogenic properties of FRC-BG, we cultured human adipose tissue-derived mesenchymal stromal cells (AD-MSCs) in the presence of two different BGs (45S5 and S53P4) and Al2 O3 control. AD-MSCs grew and proliferated on all the scaffolds tested, as well as secreted abundant extracellular matrix, when osteogenic differentiation was appropriately stimulated. 45S5 and S53P4 induced enhanced expression of COL2A1, COL10A1, COL5A1 collagen subunits, and pro-osteogenic genes BMP2 and BMP4. The concomitant downregulation of BMP3 was also detected. Our findings show that FRC-BG can support the vascularization of the implant and the formation of abundant connective tissue in vivo. Specifically, BG 45S5 and BG S53P4 are suited to evoke the osteogenic potential of host mesenchymal stromal cells. In conclusion, FRC-BG implant demonstrated material-induced ossification both in vitro and in vivo.

Chronic Brain Imaging Across a Transparent Nanocrystalline Yttria-Stabilized-Zirconia Cranial Implant

Repeated non-diffuse optical imaging of the brain is difficult. This is due to the fact that the cranial bone is highly scattering and thus a strong optical barrier. Repeated craniotomies increase the risk of complications and may disrupt the biological systems being imaged. We previously introduced a potential solution in the form of a transparent ceramic cranial implant called the Window to the Brain (WttB) implant. This implant is made of nanocrystalline Yttria-Stabilized Zirconia (nc-YSZ), which possesses the requisite mechanical strength to serve as a permanent optical access window in human patients. In this present study, we demonstrate repeated brain imaging of n = 5 mice using both OCT and LSI across the WttB implant over 4 weeks. The main objectives are to determine if the WttB implant allows for chronic OCT imaging, and to shed further light on the question of whether optical access provided by the WttB implant remains stable over this duration in the body. The Window to the Brain implant allowed for stable repeated imaging of the mouse brain with Optical Coherence Tomography over 28 days, without loss of signal intensity. Repeated Laser Speckle Imaging was also possible over this timeframe, but signal to noise ratio and the sharpness of vessels in the images decreased with time. This can be partially explained by elevated blood flow during the first imaging session in response to trauma from the surgery, which was also detected by OCT flow imaging. These results are promising for long-term optical access through the WttB implant, making feasible chronic in vivo studies in multiple neurological models of brain disease.

Prosthetic Rehabilitation of Cranial Deformity in Hyperostotic Convexity Meningioma

Cranial defects lead to unesthetic appearance and are a constant source of apprehension to the patient. Meningioma with calvarial extension requires the excision of the involved bone for complete excision. Such total excision would leave behind a bony defect which would need reconstruction. Presurgical fabrication of acrylic flap helps in reconstruction of such cranial defect following complete excision in single stage, thereby decreasing the cost and morbidity of surgery. Further, it facilitates the reproduction of the contours, and the tissue bed is not exposed to the heat of polymerization or to the free monomer. The authors report a case of hyperostotic convexity meningioma in a middle-aged female where heat-cured acrylic resin alloplastic implant was prefabricated and used successfully.

The Recent Revolution in the Design and Manufacture of Cranial Implants: Modern Advancements and Future Directions

Large format (i.e., >25 cm) cranioplasty is a challenging procedure not only from a cosmesis standpoint, but also in terms of ensuring that the patient's brain will be well-protected from direct trauma. Until recently, when a patient's own cranial flap was unavailable, these goals were unattainable. Recent advances in implant computer-aided design and 3-dimensional (3-D) printing are leveraging other advances in regenerative medicine. It is now possible to 3-D-print patient-specific implants from a variety of polymer, ceramic, or metal components. A skull template may be used to design the external shape of an implant that will become well integrated in the skull, while also providing beneficial distribution of mechanical force in the event of trauma. Furthermore, an internal pore geometry can be utilized to facilitate the seeding of banked allograft cells. Implants may be cultured in a bioreactor along with recombinant growth factors to produce implants coated with bone progenitor cells and extracellular matrix that appear to the body as a graft, albeit a tissue-engineered graft. The growth factors would be left behind in the bioreactor and the graft would resorb as new host bone invades the space and is remodeled into strong bone. As we describe in this review, such advancements will lead to optimal replacement of cranial defects that are both patient-specific and regenerative.

Prefabricated patient-matched cranial implants for reconstruction of large skull defects

Cranial defects can be caused by injury, infection, or tumor invasion. Large defects should be reconstructed to protect the brain and normalize the cerebral hemodynamics. The conventional method is to cover the defect with bone cement. Custom-made implants designed for the individual patient are now available. We report our experience with one such product in patients with large cranial defects (>7.6 cm in diameter). A CT scan with 2 mm slices and a three-dimensional reconstruction were obtained from the patient. This information was dispatched to the company and used as a template to form the implant. The cranial implant was received within four weeks. From 2005 to 2010, custom-made cranial implants were used in 13 patients with large cranial defects. In 10 of the 13 patients, secondary deep infection was the cause of the cranial defect. All the implants fitted well or very well to the defect. No infections were seen after implantation; however, one patient was reoperated on for an epidural hematoma. A custom-made cranial implant is considerably more expensive than an implant made of bone cement, but ensures that the defect is optimally covered. The use of custom-made implants is straightforward and timesaving, and they provide an excellent medical and cosmetic result.