**4. Pre-operative assessment: setting a target position**

Respecting that the optimal prosthetic position for an acetabular component is likely to be subtly (or not so subtly) different for each individual patient, establishing a clear target position for the acetabular component of a THA is of critical importance. Even once a 'target' is defined, attainment of this can be a challenging process. As discussed later herein, inaccurate patient set up, loss of pelvic position during the procedure (i.e. patient movement) and errors in intended implantation angles can all undermine the achieved outcome [2, 43]. Langston et al. [57] suggested that a change in pelvic tilt of 13° or more on pre-operative assessment may be deemed unfavourable as this will result in a change in the functional anteversion of the acetabulum of 10°. This has the potential to place even a well-orientated component outside of a +/−10° target safe zone [57]. In the same work, the authors suggested that unfavourable pelvic mobility was independently associated with limited lumbar flexion, a more posterior standing pelvic tilt and increasing age [57, 58]. Unsurprisingly, they strongly advocated for pre-operative functional X-ray imaging [57]. It is noteworthy that none of the three associated factors they determined are immediately amendable to perioperative correction prior to elective THA and may also thus be considered immutable (albeit important perhaps to recognise and consider). In extreme such cases,

there is already a trend for some surgeons to move towards large head and/or dual mobility bearings in an attempt to increase the functional safe range-of-movement [59] in instances whereby concerns regarding spinopelvic stiffness have been raised. Even with such informed pre-surgical patient data, what best to 'do' with this information is less clear. Simply centring the implanted cup to the middle of the functional movement range has inherent risk and may not necessarily result in the perfect construct orientation to accommodate the rigours of daily activity. Recognising that final construct stability is a composite of optimised component mechanics AND the concurrent effect of the static and dynamic elements of the surrounding soft tissue envelope is a far too often under-acknowledged reality.

The process of high precision data capture with pre-operative functional imaging is also not without its challenges. True lateral pelvic X-rays (required for accurate conventional angular measurement) can be technically challenging to obtain and under present conditions—at best reflect a series of pre-determined static captures of the bony relationship between the lower lumbosacral spine, pelvis and proximal femur [43]. These are not dynamic measures and do not directly take into account the critical impact of the surrounding soft tissue envelopes. Using the more commonly employed proprietary functional x-ray series, the relationship of the key bony elements in the extremes of motion are not represented—likely the positions most vulnerable to permit prosthetic dislocation [43].

Undoubtedly, an awareness of spinopelvic movement parameters also allows informed consideration of customised/patient-specific cup implantation targets [50, 60–63]. Many centres now incorporate pre-operative spinopelvic movement assessment into routine work up pathways [64]. While it is clear that fundamental clinical assessment alone is insufficient to fully appreciate the linked movement characteristics of the human spine and pelvis on a patient-by-patient basis [64], how best to interpret often complex pre-operative imaging data and how to best apply this information to target cup planning [51] remains unclear and represents an opportunity for future investigation.

Recent work has suggested potential enhanced value with pre-operative simultaneous biplanar imaging [37, 64], as compared to conventional plain film X-rays. The proprietary EOS imaging system (Euronext: EOSI; Paris, France) is touted to reduce the radiation dose by two thirds as compared to equivalent plain X-ray imaging [64, 65]. Such technologies permit simultaneous capture of precisely orthogonal X-ray images in an upright, physiological load-bearing position and are claimed to be more accurate and less dependent on patient positioning [64]. Given that some have suggested limited practical utility of plain film X-rays in judging sagittal pelvic tilt [66] consideration of EOS (or other high precision imaging modalities) may hold merit. However, while the current science may suggest a role for EOS (or EOS-like imaging means) in replacing pre-operative radiographic assessment [64], the technology is not universally available and carries associated expense [43].
