• CT is the principal imaging modality used for the pre-operative 3D planning and assessment of total hip arthroplasty (THA).

• The image quality offered by CT has a radiation penalty to the patient. Higher than necessary radiation exposure is of particular concern when imaging young patients and women of childbearing age, due to the greater risk of radiation-induced cancer in this group.

• A harmonised low-dose CT protocol is needed, evidenced by the huge variability in the 17 protocols reviewed. The majority of the protocols were incomplete, leading to uncertainty among radiographers when performing the scans.

• Only three protocols (20%) were optimised for both ‘field of view’ and image acquisition parameters. 10 protocols (60%) were optimised for ‘field of view’ only. These protocols included imaging of the relevant landmarks in the bony pelvis in addition to the knees – the reference for femoral anteversion.

• CT parameters, including the scanner kilovoltage (kV), milliamperage–time product (mAs) and slice thickness, must be optimised with a ‘field of view’ that includes the relevant bony landmarks. The recommended kV and mAs values were very wide ranging from 100 to 150 and from 100 to 250, respectively.

• The large variability that exists amongst the CT protocols illustrates the need for a more consistent low-dose CT protocol for the planning of THA. This must provide an optimal balance between image quality and radiation dose to the patient.

• Current CT scanners do not allow for measurements of functional pelvic orientation and additional upright imaging modalities are needed to augment them.

• Three-dimensional (3D) pre-operative planning in total hip arthroplasty (THA) is being recognized as a useful tool in planning elective surgery, and as crucial to define the optimal component size, position and orientation. The aim of this study was to systematically review the existing literature for the use of 3D pre-operative planning in primary THA.

• A systematic literature search was performed using keywords, through PubMed, Scopus and Google Scholar, to retrieve all publications documenting the use of 3D planning in primary THA. We focussed on (1) the accuracy of implant sizing, restoration of hip biomechanics and component orientation; (2) the benefits and barriers of this tool; and (3) current gaps in literature and clinical practice.

• Clinical studies have highlighted the accuracy of 3D pre-operative planning in predicting the optimal component size and orientation in primary THAs. Component size planning accuracy ranged between 34–100% and 41–100% for the stem and cup respectively. The absolute, average difference between planned and achieved values of leg length, offset, centre of rotation, stem version, cup version, inclination and abduction were 1 mm, 1 mm, 2 mm, 4°, 7°, 0.5° and 4° respectively.

• Benefits include 3D representation of the human anatomy for precise sizing and surgical execution. Barriers include increased radiation dose, learning curve and cost. Long-term evidence investigating this technology is limited.

• Emphasis should be placed on understanding the health economics of an optimized implant inventory as well as long-term clinical outcomes.

Cite this article: EFORT Open Rev 2020;5:845-855. DOI: 10.1302/2058-5241.5.200046

• Three-dimensional printing is a rapidly growing manufacturing method for orthopaedic implants and it is currently thriving in several other engineering industries. It enables the variation of implant design and the construction of complex structures which can be exploited in orthopaedics and other medical sectors.

• In this review, we develop the vocabulary to characterise 3D printing in orthopaedics from terms defined by industries employing 3D printing, and by fully examining a 3D-printed off-the-shelf acetabular cup (Fig. 1). This is a commonly used 3D-printed implant in orthopaedics, and it exhibits a range of prominent features brought about by 3D printing.

• The key features and defects of the porous and dense regions of the implant are clarified and discussed in depth to determine reliable definitions and a common understanding of characteristics of 3D printing between engineers and medical experts in orthopaedics.

• Despite the extensive list of terminology derived here, it is clear significant gaps exist in the knowledge of this field. Therefore, it is necessary for continued investigations of unused implants, but perhaps more significantly, examining those in vivo and retrieved to understand their long-term impact on patients and the effects of certain features (e.g. surface-adhered particles).

• Analyses of this kind will establish an understanding of 3D printing in orthopaedics and additionally it will help to update the regulatory approach to this new technology.