**6. Discussion**

Worldwide, the majority of primary THA is still performed in the lateral position [44] although it has been shown that this position is associated with the greatest degree of unintended intra-operative pelvic movement [74, 78, 92, 102]. Accurate acetabular cup orientation is critical in THA for good clinical results [103, 104] and most authors acknowledge that this can often be a difficult task [47]. Pelvic tilt alters apparent cup position [74] and may subsequently result in suboptimal placement [100]. While the operative approach itself whilst in the decubitus orientation is also an independent consideration for movement (more so with posterior versus anterolateral approaches [76]), failure to recognise changes in pelvic position introduces the potential for erroneous cup placement [76], compounding surgeon insertion errors. Poor acetabular component placement has been linked to a number of post-operative adverse outcomes [49, 94] including accelerated bearing wear [23, 45, 70, 91, 105, 106] and dislocation risk [45, 70, 91, 92, 105, 107, 108] mechanical impingement [108], decreased functional range-of-movement [57, 70, 92, 105], component migration [91], poor joint function [106], and metal ion toxicity [106]. Regardless of the target orientation, the ability to reliably and predictably achieve the desired acetabular component position is crucial to successful THA [80, 83, 109].

As discussed previously herein, final cup position is substantially influenced by the 'on table' patient positioning [45, 77], including the initial set up [44, 76, 80]. Despite the best efforts of surgeons/theatre teams, it remains the case that a pelvis will often move unintendedly during the performance of a THA [46, 76]—in spite of seemingly well applied and tensioned positioning devices. Surgeons must remain cognisant to this reality [84]. While several novel devices have been proposed, to date, there exists no mainstream, low-risk accepted method for ensuring a 'true' lateral position at the start

#### *Advanced, Imageless Navigation in Contemporary THA: Optimising Acetabular Component… DOI: http://dx.doi.org/10.5772/intechopen.105493*

of each case [68]. Statistically, a pelvis is (far) more likely to roll anteriorly (p < 0.001) during a THA in the decubitus set up [46] and this forward tilt is likely progressive across the operation [82]. It has been demonstrated that the greatest source of error occurs when the PSP is no longer horizontal at time of cup insertion [110]. While it has been proposed that such sequential loss of starting position likely progresses until at least the point of definitive cup and liner insertion, few quantitative data support this at this stage—another inviting opportunity for future research. Pure anterior pelvic roll has been shown to influence cup anteversion to a greater extent than inclination [75]. It is accepted that major pelvic movement may have an effect on the final cup insertion position [85] through surgeon perceptual error. Given the common anterior roll mechanism seen, this consequently leads to an underestimation of cup anteversion [82], with the degree of error directly related to the magnitude of pelvic tilt [49, 78, 107].

How far does an average pelvis move during a routine, primary, THA? Several previous authors have attempted to quantify 'normal' ranges of unintended pelvic movement during THA [74, 76, 82, 85] and then to propose acceptable 'cut offs' to define clinically-important variation [75]. Anterior (or posterior) pelvic tilt alters the position of the cup in the sagittal plane [111] which has a direct impact on version perception. In case series' including 67–100 hips [74–76, 82, 85] previous works have reported median pelvic tilt values during surgery of >4° [46, 82], however mean values and maximum observed tilts ranged broadly between studies—often approaching 20° for the latter [82]. Such studies show 41–57% of cases rolling anteriorly >5° [74, 75, 80], with 21–38% by >10° [46, 75, 83]. Otero's paper reported 15.4% of cases with 10–20° of tilt and 2.8% with >20° [83]. In interpreting these errors, Grammatopoulos et al. suggested that a > 10° anteversion error had a 3.5 odds ratio of the final cup position falling outside of the target safe zone [46]. Using widely accepted mathematical conversion factors [111, 112], 1° of pelvic tilt results in a 0.7–0.8° change in final anteversion. Given the longstanding surgical goal of achieving target anteversion +/− 10° (see Lewinnek and others [113]), an unappreciated intra-operative pelvic tilt of just 13° would therefore be enough to see an otherwise perfectly centred cup fall outside of the 'safe' anteversion range.

Inconsistency in initial patient set up [76] (i.e. with non-perpendicular 'true' lateral decubitus positioning) linked with a subsequent change in the pelvic position during the operative process (i.e. movement) likely contributes a substantial burden of the variation seen in final cup position [76] despite otherwise technically sound surgical technique. Uniaxial pelvic tilt has been specifically associated with unintended errors in cup version [112]. A high correlation between direct pelvic tilt and version angle (R2 = 0.995, p < 0.001) [112] has been confirmed and is intrinsically linked to the fact that the negative impact of pelvic tilt can be corrected with relative ease using simple (validated) mathematical algorithms with very high precision [112]. Until recently, the challenge however has remained the ability to *recognise* intra-operative pelvic tilt and to accurately quantify its magnitude. While the most common historical methods for determining implantation parameters for acetabular components have included mechanical alignment guides and reference against the TAL [77], both methods have been shown to be unreliable [103] and hinge on precise judgement 'as per the surgeons eye' [114]. Accurate determination of anteversion during conventional hip replacement surgery can be difficult [90], even in experienced hands. Technology-assisted surgical options—such as computer-navigation—however may provide a solution to the limitations of visual human assessment.

Using standard navigation, it is possible to determine pelvic inclination and tilt by calculating the angular difference between the anatomic frontal plane and true

horizontal (i.e. floor) [87]. Modern navigation systems—especially those using accelerometer-based technologies—provide the valuable added benefit of measuring the relative change in the pelvic position independently from data captured from the fixed pelvic tracker(s). Measurement of pelvic tilt during THA allows corrective algorithms to re-calculate the cup insertion angles to correct for the error introduced by pelvic movement and have been shown to improve the accuracy of component placement as per the intended target [42, 111]. The large 2010 study by Zhu and colleagues explored the quantitative value of navigation during THA in a cohort approaching 500 hips [111]. While these authors reported a mean intra-operative tilt of just under 5°, the observed range was from 25° of posterior tilt through to 20° of anterior (i.e. a 45° unintended error range) [111]. Over 25% of patients rolled 6–9°, while over 16% moved more than 10°. It has not yet been definitively established what the perceptual tolerances of visual assessment of pelvic tilt may be by surgeons (or varying levels of experience) although it seems clear that deficiencies in this key skill likely have a negative influence on intended cup implantation position [57].

While much research, attention and interest has centred around pelvic tilt during surgery, the important role of pelvic adduction is rarely assessed or considered [82, 115]. Given that the acetabular cup is a 3D element, inserted with intended orientation goals in 3D, it is conceivable that unintended pelvic movement in any direction may have negative consequence on final cup position [86]. Mathematically, unappreciated pelvic adduction can increase radiographic inclination [114] which may have consequences for final bearing stability [115]. In a routine posterior approach to the hip (in a lateral decubitus position) the relatively wider pelvis as compared to the lower limbs tends to see the uppermost hemipelvis drift into adduction [114]. The previous work of O'Neill and colleagues (2018) assessed the pelvic movement in 270 consecutive primary THAs suggesting that none of their cases showed pelvic abduction with a mean adduction change of 4.4° [115]. This finding was similar to other authors who reported average adduction angles of 2.5–6.7° [82, 84]. It is generally felt that these smaller magnitude changes have a lesser impact on inclination than do comparable movements involving pelvic tilt.

Current research would support the notion that anterior pelvic roll occurs incrementally across the case from set up to definitive implant insertion. The descriptive work of Grammatopoulos et al. suggested a mean angular movement from set up to implant insertion of 9° (sd 6) [76]. Others have suggested similar changes [85]. The later work of Schloemann and colleagues suggested that more than just 5° of change may be 'clinically significant' [89] supporting the suggestion that such unaccounted for angular change may facilitate introduction of critical errors in target cup placement [75, 89]. Several authors have recommended that the highest (and most consistent) level of attainment of target cup position may perhaps be achieved using the combination of an assistive anatomical plane (pelvic) positioner and navigation [100, 103]. Iwakiri and colleagues suggested such an approach was reliably simple, consistent, economical and non-invasive [103].

Despite the focus of hip navigation on radiographic outcomes and intra-operative changes, the critical consideration of patient body habitus must be considered. Most authors agree that increasing patient BMI influences the likelihood of unintended pelvic positional change during surgery itself [84] and strongly correlates with subsequent errors in target cup orientation attainment [80, 85]. The extended size of bariatric tissue retractors for surgical exposure [84, 85] (and sometimes the force applied to them) and direct soft tissue impingement can worsen the magnitude of positional movement [116]. However, high BMI alone cannot be blamed for all

*Advanced, Imageless Navigation in Contemporary THA: Optimising Acetabular Component… DOI: http://dx.doi.org/10.5772/intechopen.105493*

of the issues noted with unintended pelvic positional change—the 2019 work of Schloemann et al. showed clinically-relevant anterior pelvic roll in a cohort with a mean BMI of just 20 [89]. Similarly, other authors have suggested no clear association between BMI and pelvic movement [75, 82, 117]. Regardless, obesity is just one factor so far linked to pelvic movement during THA surgery—with evidence to show that low volume surgeons and the surgical approach employed are also recognised cofactors [30, 118].
