**7. Looking to the future …**

The future for hip arthroplasty appears exciting, especially as appropriatelyemployed technologies facilitate further improvements in planning, precision and intra-operative execution. Historical two dimensional (2D) templating and planning has already been shown to be far less accurate than modern 3D equivalents [119–121]. The evolution to more universal 3D standards is likely to incrementally improve surgical planning [122–124] as such technologies become more mainstream. The cutting edge integration of artificial intelligence algorithms into the pre-operative decision making pathways may represent further advancement still [124]. Similarly, as realtime computer navigation is taken up more broadly many anticipate improved attainment of target cup placement [87] (in a similar fashion to accuracy improvements that were seen during the evolution of TKA navigation). Despite great enthusiasm in some spheres, navigated arthroplasty is not without its inherent problems and limitations. Tracker pin site placement and loosening [102] continue to undermine case-bycase precision with only small positional changes resulting in magnified degradation in accuracy. As with other bony-mounted navigation applications in other parts of the body, site fractures, wound and pin site issues post-operatively also plague use and present technique-specific challenges [101].

Some supporters of technology have suggested that formal ('full') hip navigation may be unnecessary, suggesting that less invasive and less time consuming alternatives are already available to improve operative precision. Using a simple off-the-self smartphone with basic accelerometer capability, Peters et al. in 2012 reported a series of 50 THAs suggesting their novel technique was simple, 'quick and accurate', reporting that 'all' cases were able to achieve less than 5% deviation from the intended pre-operative plan [47]. Similar work by O'Neill et al. using a simple digital inclinometer reported achieving target cup position within 2.5° in 88% of cases [110] and showed positive statistically-significant differences as compared to conventional instrumented approaches. Contrasting CT-based full navigation with 'imageless' accelerometer (mini) navigation however, the recent work by Testsunaga et al. suggested the latter lacked the accuracy of image-based techniques [102] but it was unclear whether the precision-versus-target cup position translated to meaningful clinical benefit. Equally, the potential improvement in accuracy must be weighed against the time, expense and radiation exposure associated with CT-based pre-op imaging. As point-of-care image registration approaches continue to improve with software and algorithm refinement, ease of use and reliable user accuracy will likely improve in parallel.

Regardless of the fundamental imaging method employed (i.e. plain X-ray, augmented X-ray, CT or MRI based), the concept of 'fusing' advanced or even 3D preoperative templating with highly-precise intra-operative navigation means poses an exciting state-of-the-art possibility. Such novel approaches—already in clinical use in some domains—exploit the optimal elements of contemporary planning and surgical case execution. Some authors feel this may represent the best of both considerations [43].

Opponents of navigation frequently cite the 'is the extra angular precision actually worth it' argument. The now standard use of larger heads, and with increasingly-common selection of dual mobility bearings [59], has arguably improved the stability and mechanical characteristics in many instances perhaps negating the need for such high levels of cup orientation accuracy. Indeed, in their 2013 paper Eilander and colleagues suggested that hip navigation may be an 'unnecessary' technical burden, claiming that 82% of the hips included within their comprehensive study had cups within radiographic safe zones using conventional free hand techniques [105]. So far however, this has not been the sentiment shared by most. Finally, the progression to robotassisted THA surgery [75]—arguably an evolutionary extension of computer-based navigation—may offer further clinical advantages with early science suggesting value, especially in complex cases [125]. This area too requires further research to ensure the evidence base underpinning wider uptake stays ahead of the enthusiastic hype.
