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2 Advances in Orthopedics Figure 1: CT scan showing bilateral first-stage revision THR prostheses with right-sided pelvic discontinuity and a severe left- sided posterosuperior acetabular deficiency. anatomical appreciation, but they must still be viewed on a flat 2D computer screen. With the use of modern rapid pro- totyping techniques, 3D models of actual osseous anatomy can be manufactured from these 3D reconstructed images [9, 10]. Rapid prototyping, or 3D printing, is a term used to describe a group of techniques used to quickly fabricate a scale model of a physical part or assembly using three- dimensional computer aided design (CAD) data. The origins of this technique can be traced back to the 1960s when Pro- fessor Herbert Voelcker described theories and algorithms for 3Dmodel fabrication. Carl Deckard developed a technique to bind metal powders to create a 3D model in the University of Texas in 1987, before Charles Hull patented the first 3D printer in 1988 in California [8, 11]. Rapid prototyping has been used in the medical industry since the early 2000s, initially in the production of dental implants and patient- specific prostheses [12]. Since then, the use of 3D printing in the field of medicine and surgery has been rapidly expanding to include the development of soft tissue, organs, blood vessels, implants, and anatomical models [13]. Also within orthopaedic surgery, 3D printed models have been shown to improve the preoperative understanding of complicated structures in neurosurgery, liver transplant surgery, and vascular aortic surgery [11, 12, 14]. Revision hip arthroplasty is one of the most complex orthopaedic disciplines. Each case provides the surgeon with a challenge specific to the patient’s unique anatomy. Often, 3D images are studied closely, but, as mentioned above, appreciation of the abnormality in question may not always be obtained on a 2D screen. The individual variances of the human body make the use of 3D printed models a valuable asset to surgeons when preparing for a complex procedure [14]. A 3D printed model provides visual and tactile reproduction of the deficient pelvic bony anatomy.This enables an improved understanding of the anatomy prior to surgery and facilitates enhanced preoperative planning [15]. Figure 2: 3D CT reconstruction showing a dislocated left-sided THR secondary to a posterior acetabular wall deficiency. 2. Methods and Results Life-size 3D models were manufactured from the computed tomography (CT) scans of two patients with complex acetab- ular defects waiting for second-stage THRs. 2.1. Case 1. The first patient had a background of multiple bilateral hip arthroplasties for what was thought to have been aseptic loosening. Surgical intervention to date, however, had resulted in minimal symptomatic relief. Bilateral hip aspirations grew Staphylococcus epidermidis on enriched cultures. Bilateral first-stage hip revisions were subsequently performed, with bilateral antibiotic-coated spacers inserted. The postoperative CT scan of the pelvis showed right-sided pelvic discontinuity and a severe left-sided posterosuperior acetabular deficiency (see Figure 1). A six-week course of intravenous vancomycin and rifampicin was completed, as per the recommendation of the hospital’s microbiology department. 2.2. Case 2. The second patient had undergone a first-stage hip revision, after preoperative aspiration confirmed an indo- lent Staphylococcus epidermidis periprosthetic joint infection, which had provoked multiple THR dislocations. Similar to the above, an antibiotic-coated spacer was inserted. A left- sided posterior acetabular wall deficiency was noted on the postoperative CT pelvis that was carried out (see Figure 2). Six weeks of intravenous daptomycin therapy was completed on consultation with the microbiology department. Both patients were listed for elective second-stage THRs as men- tioned. 2.3. Rapid Prototyping. Specific metal reduction protocols were used to reduce artefact on the two mentioned CT scans, with the slice thickness set to 1 mm to improve the image quality. The CT scans obtained were converted to DIACOM images and were then imported into Materialise MIMICS 14.12, medical imaging processing software, in the Mechanical Engineering Department of the Institute of Technology Tallaght, in Dublin. Image thresholding was | The Surgical Technologist | JUNE 2018 254