3D printing, or additive manufacturing, is revolutionising medical practice. With an expanding range of materials to print from, including plastics, metals, and even biodegradable materials, this technology has the potential to rapidly change the surgical landscape. Here we review the use of additive manufacturing across various surgical specialities.
Uncharacteristically, given the multi-dimensionality of medical applications, AM technologies for medical applications extend across various polymer-based techniques.
AM Technologies for medical applications
Selective laser sintering (SLS) and melting (SLM) involves a fine powder bed of varied materials such as nylon or metals, such as titanium and stainless steel. A focused energy source (laser or electron beam) is used to sweep the powder bed, tracing out the shape of a 2D slice, thus melting and fusing areas of powder to form the geometry of each layer.
Fused deposition modelling (FDM) is where layers are created by the deposition of a heat softened polymer with the use of a computer controlled extrusion nozzle.
Stereolithography (SLA) uses an optical light energy source to scan over a vat of light curable resin, solidifying specific areas on the surface of the liquid. The floor of the fluid container gradually descends, which increases the depth of material as the model grows and successive layers of resin are cured on top of each other.
Three main themes characterise surgery applications of additive manufacturing :
1. Anatomical models to be used for 1) pre-operative planning and 2) education and training 
2. Surgical instruments used in 1) pre-operative planning and 2) during operations
3. Implants and prostheses, organ and tissue printing.
Let’s focus here on the 3rd topic.
Implants and prostheses
The fields of maxillofacial, cardiothoracic and orthopedic surgery seem to be the greatest innovators in the use of additive manufacturing technologies.
As well as guides and templates that may assist with implant insertion or surgical technique, AM can produce implantable materials that can be manufactured in-house and left inside the patient as part of the surgical repair or fixation. The technology lends itself well to the production of precise and often bespoke implants.
In cranioplastic surgery (surgical intervention to repair cranial defects) , bespoke plates have been created based on CT imaging (http://www.insidemetaladditivemanufacturing.com/blog/non-destructive-testing-of-additively-manufactured-components-challenges-ahead) of a specific lesion. Using computer-aided design, well-fitting, more esthetically pleasing implants have been produced, which maintain anatomical symmetry based on the patient’s own anatomy; thus avoiding the need to further modify the patient’s anatomy to fit off-the-shelf prostheses.[3,4]
Cosmetic and plastic surgery
In cosmetic surgery (surgical specialty dedicated to reconstruction of facial and body defects) “virtual “noses can be trialed on digital models of patient’s faces.  With the patient’s input, the future prosthesis can be digitally manipulated into a more preferable shape and, once the design is complete, it can be printed and fitted onto the patient. This can be corrected as per conventional methods by an anaplastologist (health care professional that creates custom made facial, ocular, and non-weight bearing somatic prostheses). In addition, 3DP has been utilized to produce complete nasal prostheses. [6,7,8]
In thoracic surgery (surgical repair of congenital and acquired conditions of the heart), a biodegradable airway splint was created for a child with tracheobronchomalacia, to keep the left main bronchus from collapsing.9 This demonstrates the scope for novel patient-specific treatments.
Implants can be printed from a variety of materials. For example, precontoured plates or titanium meshes may be used in maxillofacial surgery, to ‘patch’ or provide structural support to lesions of the facial skeleton. Methods have been utilized which involve pre-forming a titanium mesh sfter preoperative 3D printed patient anatomy, in order to create a bespoke implant to repair a significant skeletal lesion around the nose.
Even printed scaffolds are now being developed, into which cells may infiltrate that can be used for implants [10, 11]. Similar work also shows promising results, with construction of bespoke scaffolds to perfectly reproduce healthy bone structure to counter pathological defects. New composite materials for 3D printing of implants are currently being developed. These have been found to be a viable alternative to patient specific ceramic bone substitutes.
With the right materials, 3D printing technologies can be used to manufacture patient-specific implants or prostheses that can be built in-house and left in-situ. Provided safety and sterility issues are addressed, AM can reduce delivery time of components that can usually take weeks to get fabricated. Much of the current work that is being done in 3D printing for medical application revolves around bone-related applications, however, as the technology advances with the increased use of silicon, gels, tissues and bio-absorbable materials, an increasing number of specialties will be able to harness the utility of additive manufacturing in their daily practice.
 Malik HH, Darwood ARJ, Shaunak S, Kulatilake P, El-Hilly AA, Mulki O, Baskaradas A, Three-Dimensional Printing In Surgery: A Review Of Current Surgical Applications, Journal of Surgical Research (2015), doi: 10.1016/j.jss.2015.06.051
 Dean D, Min KJ, Bond A. Computer aided design of large-format prefabricated cranial plates. Journal of Craniofacial Surgery 2003;14(6): 819-832.
 Kim B-, Hong K-, Park K-, Park D-, Chung Y-, Kang S-. Customized cranioplasty implants using three-dimensional printers and polymethyl-methacrylate casting. Journal of Korean Neurosurgical Society 2012;52(6): 541-546.
 Reitemeier B, Gotzel B, Schone C, Stockmann F, Muller R, Lexmann J, et al. Creation and utilization of a digital database for nasal prosthesis models. Onkologie 2013;36(1-2): 7-11. Available from: doi: http://dx.doi.org/10.1159/000346668
 Ciocca L, De Crescenzio F, Fantini M, Scotti R. CAD/CAM and rapid prototyped scaffold construction for bone regenerative medicine and surgical transfer of virtual planning: A pilot study. Computerized Medical Imaging and Graphics 2009;33(1): 58-62.
 fripp design http://www.frippdesign.co.uk/problems-solved/the-wellcome-trust/
 Zopf DA, Hollister SJ, Nelson ME, Ohye RG, Green GE. Bioresorbable airway splint created with a three-dimensional printer. New England Journal of Medicine 2013;368(21): 2043-2045. http://dx.doi.org/10.1056/NEJMc1206319
 Lalan S, Pomerantseva I, Vacanti JP. Tissue engineering and its potential impact on surgery. World journal of surgery 2001;25(11): 1458-1466.
 Wang X, Schroder HC, Grebenjuk V, Diehl-Seifert B, Mailander V, Steffen R, et al. The marine sponge-derived inorganic polymers, biosilica and polyphosphate, as morphogenetically active matrices/scaffolds for the differentiation of human multipotent stromal cells: Potential application in 3D printing and distraction osteogenesis. Marine Drugs 2014;12(2): 1131-1147.
 Khalyfa A, Vogt S, Weisser J, Grimm G, Rechtenbach A, Meyer W, et al. Development of a new calcium phosphate powder-binder system for the 3D printing of patient specific implants. Journal of Materials Science: Materials in Medicine 2007;18(5): 909-916.
We provide practical and actionable info dedicated to additive manufacturing of high-value metal components