**1. Introduction**

Three-dimensional (3D) printing, also known as 'Additive Manufacturing' (AM), of titanium orthopaedic implants has revolutionized the treatment of massive bone defects in the pelvis due to their ability to be customized with complex shapes, size and surface geometries; this is more complicated to achieve with conventional manufacturing (i.e., non-3D printing) methods, such as drop forging and machining, which are commonly used to produce orthopaedic implants. The greatest impact of 3D printing of orthopaedic implants is, however, still to come: the mass production of millions of off-the-shelf (non-personalized) implants.

Every year, over 600,000 total hip arthroplasty (THA) procedures are performed in Europe and 1.4 million worldwide; these numbers are expected to grow significantly by 2030 [1–3]. The main clinical rationale for the use of 3D printed off-the-shelf implants is achieving a successful long-term fixation with bone to restore biomechanical function of the joint.

The process of 3D printing has many adjustable variables which, taken together with the possible variation in designs that can be printed, has created even more variables in the final product that must be understood if we are to predict the safety and performance of 3D printed implants [4–7]. The regulatory approval systems have not ye<sup>t</sup> completely caught up with the change in technology [8]; surgeons prefer to use implants that have been followed up for several years and have been highly rated by systems such as the Orthopaedic Data Evaluation Panel (ODEP, UK) [9]. Orthopaedics has already shown cases of unpredictable outcomes with design solutions that were thought to be revolutionary, such as metal-on-metal hip replacements [10,11].

This review aims to describe the role of 3D printing of orthopaedic implants, focusing on acetabular components used in THA. To achieve this, we (1) explain the rationale for 3D printed acetabular cups, (2) describe the variables and the limitations involved in the 3D printing manufacture, and (3) sugges<sup>t</sup> a classification for these cups, presenting also the investigation methods and the clinical outcomes of 3D printed cups.

#### **2. Rationale for 3D Printing in Orthopaedics**

Although the majority of THA procedures are still performed using conventionally manufactured cups, acetabular components produced using 3D printing technologies are being increasingly used for primary and revision hip surgeries. There are advantages and disadvantages of each production techniques (Table 1). In terms of manufacturing, 3D printing enables complex porous structures with specifically designed pores shapes to be produced, unlike conventional technologies, where the control over the final architecture of the porous backside coating is limited. Furthermore, customization of implants can be achieved more easily using 3D printing. In terms of clinical outcomes and investigation of the implants, the conventional cups have a long-track record, with long-term clinical results, unlike 3D printed; however, aseptic loosening (i.e., loss of fixation without infection) is still one of the most common reasons for revision [12]. Independent investigations of full-post production 3D printed acetabular components are currently missing.

**Factor 3D Printing Conventional Manufacturing Advantages** • Complex and easily adjusted porous structure for enhanced fixation • Cup size optimization • Easily customized/personalized • Widespread clinical use • Long-term clinical outcomes **Disadvantages** • Potential risks and clinical impact poorly understood • Absence of implants investigations Fewreportedclinicaloutcomes• Poor fixation still an issue in THA • Limited design freedom • Customization limited

**Table 1.** Summary of advantages and disadvantages of 3D printing and conventional manufacturing for acetabular components in hip arthroplasty [6,7,12,13].

#### *2.1. Clinical Rationale for 3D Printed Cups*

•
