3D printing of ceramic implants
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Introduction The primary function of the skeleton is its role as supporting tissue in the human body. The specific hierachic structure of bone (Figure 1) is essential for the unique mechanical properties of bone. The combination of organic and inorganic components provides bone with an extreme resistance against tension, compressive, and bending stress, as well as torsion. The architecture of bone tissue comprises the outer periosteum followed by the dense cortical bone and the inner porous cancellous bone. Bone tissue consists of bone cells (osteoblasts, osteoclasts, and osteocytes) embedded into the bone matrix, which is composed of approximately 60–70% inorganic hydroxyapatite, 20% organic compounds (collagen, osteocalcin, proteoglycans), and 10% water.1 Bone is a slow-growing tissue with only a limited self-healing capacity. Bone defects, caused by disease or trauma, which extend to a critical size >10 mm show no structural bone regeneration but ingrowth of fibrous tissue.2 To avoid this, bone grafts have to be implanted in such defects to fill the space during the healing and regeneration process. Diverse clinical pictures like infantile craniofacial anomalies, trauma, or cancer can cause cranial or maxillofacial bone defects, which often further extend this critical size. Such bone grafting procedures are being performed worldwide for several million patients per year,3 at a cost of more than $2 billion.4
Although the gold standard in terms of healing success is still the transplantation of autologous grafts obtained from the same individual5 (for non-load-bearing implants), this is associated with several disadvantages, such as donor site morbidity, limited availability, and the risk of unpredictable resorption (dissolution of bone structure) without bone defect regeneration. Non-autologous bone grafts obtained from a donor have potential infectious and immunological risks. In contrast, alloplastic bone replacement materials (e.g., polymeric or ceramic cements) have the advantage of unlimited availability and consistent material quality; the materials predominantly used in craniomaxillofacial surgery are titanium alloys,6 calcium phosphate cements, and ceramics7 as well as different polymers, such as acrylic bone cement and polyetheretherketones.8 At the moment, only 15% of the global bone graft market consists of synthetic materials, but due to its high growth rate, the commercial significance of this market segment is expected to increase approximately 15% p.a.9 While small bone defects can often be treated by intraoperatively formed alloplastic materials (e.g., polymeric or ceramic cements), large and/or geometrically complex defects often require pre-formed implants made by using an individual life-size model or by using layer-by-layer additive manufacturing or three-dimensional (3D) printing techniques. Engineering approaches to create individualized patient-specific implants
Elke Vorndran, Department for Functional Materials in Medicine and Dentistry, University of Würzburg, Germany; [email protected]
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