In this prospective study, the mean radiographic inclination (41°) achieved were as per target of 40° and the variability was in line with previous reports (12°). By standardizing factors that have been shown to influence resultant cup orientation (surgeon, approach, and 3-point supports) we illustrated that differences exist between different pelvic supports that surgeons should be aware off. Furthermore, we tested whether the inclinometer use was associated with improved chances of obtaining target cup orientation when the pelvic is well supported. Contrary to previous reports, highlighting the beneficial effect of the inclinometer, in this study the chances of obtaining optimum orientation was similar regardless of visual aid used; however, this is likely to be the case because the inclinometer did not strictly dictate IOI as in previous studies [7, 12, 13]. Non-optimal cup inclinations were seen in cases that the intra-operative inclinations were above 40° and below 32°, or the ΔPelvicPosition was excessive (> 15°; n = 2). We would thus recommend that the intra-operative cup inclination should be centered about 35°, when using 3-point pelvic supports.
Operating in the lateral decubitus position is associated with greater variability in pelvic position at impaction and an associated increased variability in the cup orientations achieved [19]. In the lateral decubitus position, it is primarily the pelvic position at impaction that is the cause of the great variability seen [7, 12, 13]. Type of pelvic support is associated with degree of intra-operative movement and 3-point positioners are associated with the least intra-operative movement during a THA [6]. However, not all 3-point of pelvic positioners provide similar support. In this study, the Stulberg positioner was associated with smaller ΔPelvicPosition (0° Vs. 3°) compared to the Capello one. This led to significantly closer orientations between the intra-operative (37° vs. 32°) and the resultant radiographic (41° vs. 39°). The resultant orientations were not as different as the ΔPelvicPosition and this was due to the intra-operative orientation differences between positioners; illustrating the surgeon likely accommodated for this effect probably due to anatomical clues used in his practice (e.g. TAL). This led to equivalent chances of achieving the radiographic cup orientation target.
The orientation of the cup at impaction and the ‘offset’ due to angular projections (Murray’s definitions) are also important in determining orientation. Two commonly described methods to assess cup inclination angle at impaction, relative to the horizontal, are mechanical alignment guides and the use of an inclinometer. Contrary to previous reports that only reported on the relationship between the intra-operative and resultant radiographic inclination [7, 12, 13], the current study also took into accounted for the resultant radiographic anteversion as this has a significant effect on the difference in cup inclinations between operative and radiographic values (ΔDefinition). There was no difference in any of the parameters measured (inclination/version/chances of reaching cup orientation targets) between the visual aid groups. This likely occurred because there was no fixed narrow target of IOI with the inclinometer, contrary to previous study [3, 4, 7, 12, 18]. Overall, an intra-operative orientation < 45° was aimed for, as radiographic inclination is greater than operative inclination, but the cups were not impacted within a narrow range as evident by the measured IOI (24–46°) post-implantation. This allowed us to study what contributes to cup mal-orientation and whether too high or too low intra-operative inclination was the primary reason for most of the mal-orientated cases. The proportion of cases with cup orientations within the pre-determined target (84%), seem to be comparable to other large series (50–88%) [3, 20,21,22,23,24]. Several studies have investigated freehand cup-positioning, reporting a high variability of cups within the save zone for inclianation and anteversion ranging from 26 to 71% [25]. However, whether by reducing the intraoperative range would lead to less variability akin to that reported in navigated- (87–93%) [23, 24, 26] and robotically-assisted (98–100%) [24, 27] THA requires further study.
During a posterior approach to the hip, the operated hemi-pelvis at the time of impaction is adducted and internally rotated [5, 14, 15]. Thus, in order to achieve a desired radiographic inclination, the intra-operative inclination (relative to the floor) must be reduced more than the Murray definition differences(3–4°) [3, 4, 7, 12, 18]. Non-optimal cup inclinations were seen in cases that the intra-operative inclinations were above 40° and below 32°, or the ΔPelvicPosition was excessive (> 15°; n = 2). All patients with low inclinations, had higher IOIs recorded; this is likely to have occurred because the operated hemi-pelvis was external rotated (or abducted) at impaction. As radiographic cup orientation is very rarely less than the intra-operative but is commonly greater (5–10°: 40%) and occasionally significantly greater (> 10°: 30%), we would recommend an intra-operative cup inclination of 35°, which is in line with other recommendations [28]. Contemporary alignment guides should therefore be redesigned [17]. Until then, inclinometers should be used as the ability to judge a 5–10° reduction in inclination angle with a rod can significantly vary [10].
This study has several limitations. Firstly, it is a single surgeon series and thus suffers from operator-related biases. However, by assessing a single surgeon we were able to account for surgical-related biases which may relate to defined optimum target, ability to judge a three-dimensional angle, cup preparation and impaction techniques and pelvic set-up [1, 6, 10, 12, 18, 20, 29]. Secondly, cases were randomly assigned to the different types of pelvic supports used according to the hospital site that the operation took place, which was not associated with any patient-related bias. This is a pragmatic situation likely to be encountered by surgeons practicing in different institutions with different resources available. Thirdly, the Capello positioner was further reinforced with additional bolster and hence the results do not apply to those that use simply the pegboard, which is therefore likely to be associated with much greater ΔPelvicPosition as it cannot easily, or securely, support both ASIS. Fourthly, we could not detect a significant difference for optimally positioned cups between the two cup orientation aids or two different positioners, which might have been due to the small study cohort size. Lastly, we did not assess for any spinal pathology (e.g. scoliosis) or degree of pelvic obliquity influencing inclination angles at impaction.
In conclusion, when the pelvis is securely positioned with 3-point supports, optimum cup orientation can be achieved with both alignment guides and inclinometer. Non-optimal cup inclinations were seen when intra-operative inclinations were above 40° and below 32°, or the ΔPelvicPosition was excessive (> 15°; n = 2). We would thus recommend that the intra-operative cup inclination should be centered strictly between 30° and 35° relative to the floor [7, 12]. Small differences exist between different type of pelvic positioners that surgeons need to be aware off and account for when impacting the cup.