Three-dimensional (3D) printing is a recent advance that offers unique opportunities for biomedical applications. As opposed to traditional “manufacturing by subtraction,” whereby stock undergoes machining to remove material and attain a desired shape, additive manufacturing creates new objects by bonding layer upon layer until the desired shape has been achieved. In 1987, stereolithography apparatus was created with liquid photopolymers cured with ultraviolet (UV) laser; the UV laser draws a desired section, and the photopolymer hardens. This process is repeated by constant lowering of the build platform until the 3D object is created. Choice of polymers is limited, but this process is widely used, particularly for rapid prototype design.
Two more traditional methods of 3D printing were developed through laser printing technology. The first method uses extrusion, whereby layer upon layer of substrate is placed on the build platform, usually with added energy that can cause melting, fusion, or polymerization. The second method is powder-based and consists of layering of a substrate in powder form, followed by addition of energy to cause polymerization or other chemical processes such as melting, sintering, or crystallization to the desired shape. This process is repeated layer by layer.
Several techniques of 3D printing are used for biomaterials. Selective laser sintering, a powder-based method, can achieve high resolution with materials including ceramics, polymers, and metals. Fused deposition modeling uses wires or filaments of material extruded through a polyjet and melted, then polymerized or fused on the plate build, layer by layer, similar to cake decorating. Metal devices can be 3D printed via direct laser deposition, shaped metal deposition, and electron beam melting. The first two processes leave internal stresses and currently are not suitable for medical applications. However, electron beam melting, an FDA-approved process, is used to make orthopedic devices, such as hip acetabular cups and interbody devices. Advantages include optimization of porous surfaces that can be fused to the entire device, rather than sintered on, as is typically done.
For a typical medical device, an object must be created through 3D printing, most commonly with computed tomography or magnetic resonance imaging through segmentation and isolation of a 3D object such as bone. For a biologic implant, a 3D drawing is made, and the region of interest is segmented, often requiring optimization and conversion to STL (stereolithography) file format. The STL is then printed on the 3D printer and the object created.
Advantages of 3D printing include customizable results, speedy design, low cost, availability in a variety of materials, optimized mechanical properties, and low barriers facilitating performance in Third World countries. Disadvantages include altered mechanical properties compared with general manufacturing techniques, unknown performance in vivo, concerns regarding dimensional accuracy, and unknown sterility of metallic devices, as pores created may not be accessible with a gas autoclave. Long-term data on this process are not yet available.
Potential applications for 3D printing include education and simulation training, prosthetics or orthotics, preoperative planning, design and rapid prototyping, manufacture of instruments customized to patients, custom implants, routine manufacture of medical devices, and tissue-engineering applications.
Medical school curricula are de-emphasizing anatomic dissection, making models increasingly important. McMenamin et al1 created 3D complex anatomy models that act as prosected dissection, are relatively low in cost, and can be based on abnormal or normal patient anatomy. Madrazo et al2 used 3D models to enhance patients’ understanding of their medical condition to guide decision making.
Orthotic devices, such as a scoliosis brace, can be customized. Patients’ dimensions are digitized and a brace is created through a process that allows patients to participate in the design of the brace, leading to greater patient satisfaction and improved compliance. Application of 3D models to prosthetics is even more important. Amputations are a worldwide problem, particularly in Third World countries savaged by war or earthquakes. With 3D printing, customizable prosthetic devices such as Willie Raptor, Cyborg Beast, and eNable prosthetic limbs can be assembled in less than 30 minutes and produced locally in a nonsterile environment, at costs less than $50.
For surgical planning in patients with complex deformities, 3D-printed models have been used to prebend plates for clavicle fixation, to act as drill guides for placement of pedicle screws, and to prepare sterilizable models in complex deformities such as congenital scoliosis to improve the accuracy of pedicle screw insertion. Custom metal and ceramic implants can be created for complex reconstruction of bone defects. In one case, a custom prosthesis was 3D printed from titanium alloy to reconstruct C2 after resection of Ewing sarcoma.3
Patient-specific implants are gaining in popularity, although long-term outcomes have not been improved. In the current paradigm for total knee arthroplasty, the implant of closest appropriate size is selected and manufactured by traditional techniques, but custom patient-specific instruments are created from CT through 3D printing technology, which requires lower storage costs because the whole implant system uses only a single set of instruments and is customizable.
3D printing is an intriguing idea for standard manufacturing processes. The initial cost for electron beam melting is high, but marginal costs for production of implants may be lower. Advantages include stronger material properties in plates manufactured with the titanium alloy and important surfaces fused to the implant rather than centered and placed.
A variety of matrices used in tissue engineering can be created via 3D printing, including hydrogels, ceramics such as hydroxyapatite, and bioactive glasses of many materials. Cells and proteins can be added to create an implantable tissue-engineered device. Advantages of these methods include production of customizable shapes and material properties that promote the desires of biological processes.
In conclusion, 3D printing is transforming technology that can be used in many aspects of surgical care, including education, surgical planning, design, customization, and manufacturing, and as a distributive technology for worldwide use with very few barriers.