**5. Intra-operative execution of the surgical plan**

During the THA operation itself, three considerations become important with respect to the accuracy of definitive cup placement. Firstly, is the original position of the patient (as a surrogate for the true pelvis position). In the lateral (i.e. decubitus) set up, the surgeon/surgical team endeavour to ensure the patient's pelvic sagittal plane (PSP) is horizontally orientated. In most practical senses, this refers to this key alignment

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

plane being parallel to the theatre floor [67, 68]. Classically, surgeons have relied on palpation of key bony landmarks (i.e. ASIS and pubic symphysis) [69–71] to determine if the vertically-orientated APP [72] is indeed perpendicular to the flat level surface of the operating table (**Figure 2**). Direct and accurate localisation of the contralateral ASIS for APP determination can be challenging in the lateral position [72], especially with increasing BMI. Unsurprisingly, there is considerable inaccuracy in this subjective process [73] which assumes both landmark symmetry and an ability of the surgeon to accurately appreciate the location of such landmarks. An array of commonly-used positioning aids are employed for achieving and maintaining the true lateral orientation for THA. These usually involve some combination of posterior sacral block [74] and an anterior symphyseal bolster or ASIS post [74]—the latter of which may involve single or paired extensions. More proprietary universal lateral positioners [75] or peg boards [76] are also used with reasonable quoted effect. Interestingly, in a 2019 UK nationwide investigation however, Rutherford and colleagues explored surgeons' sentiment towards current positioning tools [77]. More than 35% of respondents were 'unhappy' with their current supports, while less than a third (31%) felt their current positioning supports were rigid and reliably stable [77]. The need for better positioning tools and supports is almost unanimously championed [74, 76].

Previous authors have proposed customised pelvic orientation devices for use during initial set up claiming simplicity of use and improved accuracy and reproducibility in achieving a pelvis horizontal in the sagittal plane [67]. To date, despite the potential value of such positioning aids, they have failed to attract mainstream uptake and use. Iwakiri and colleagues [78] reported a custom variation of an existing positioning device with the addition of an extra compression pad [78]. Described as 'simple, minimally invasive and cost effective' [78] the authors were able to show significant reductions in intra-operative sagittal pelvic tilt. Other studies have shown highly significant differences (p < 0.001) in the maintenance of pelvic position during surgery when comparing the type of mechanical support used [76].

Beyond blaming the tools, multiple studies have shown significant variation in the ability of surgeons/surgical teams to accurately position the pelvis for THA surgery [76]. The 2011 work of Nishikubo and colleagues used in-theatre fluoroscopy to check for pelvic positioning errors prior to commencement of surgery [79]. With a pelvis orientated with 0° of tilt versus the horizontal sagittal plane as the target standard, they reported a mean positioning error of nearly 6°—this before the operation had even begun [79]. The later study of Lambers et al. reported a more modest error of closer to 3° but a wider range of recorded starting errors as high as 13° [80]. In a subanalysis the same authors suggest that malpositioning is indeed a common occurrence in everyday practice and is more likely with increasing patient body mass index (BMI) [80, 81]. Increased BMI is independently linked to increased rates of post-operative arthroplasty complications, compounded by errors in component placement resulting in suboptimal positioning [81].

The second critical element is the ability of the set up to *maintain* consistently the position of the patient during the operation itself. This too is a multi-factorial consideration. Pelvic movement affects perceived cup inclination and version and may lead to an unintended cup implantation error [82]. While the recommendations are not uniformly agreed, Otero and co-authors suggested that 'proper' (acceptable) positioning could perhaps be defined whereby there was <10° of subsequent pelvic positional change during the THA procedure itself [83]. The initial patient position is maintained by positioning devices presumed to be rigid and stable—in many instances however this is not the case. Especially with increasing BMI [84] and general patient size, the effectiveness with which these one-size-fits-all devices secure and maintain the position of the pelvis is poor. Further undermining this consideration, Milone and colleagues suggested that rigid positioning alone was an unreliable way of ensuring accurate final cup placement [75]. While almost all commonly employed such equipment is designed to provide stable support against unyielding bony landmarks (ASIS etc.), simply tightening them further to increase the rigidity of support is also not without risk. In their recent 2020 publication, Ueno and colleagues demonstrated a 2.64% rate of medically-important soft tissue ulceration secondary to pelvic positioner use for routine primary THAs [85].

The pelvis is exposed to many discrete deforming forces during a conventional THA which may result in iatrogenic pelvic tilt [3, 84]. The process of mechanical cup reaming and implant impaction are obvious examples [84], but 'strong' traction from exposure-permitting retractors are also a recognised culprit [84, 86]. While traction for safe exposure may be an unavoidable evil during primary THA, authors who have considered this important force mechanism recommend releasing or 'backing off' retractor tension during the critical stage of definitive cup impaction [85], which may permit some measure of tilt correction [84]. The 2019 work of Della Valle et al. however suggests that retractor removal is unlikely to facilitate complete correction of anterior roll which had been induced earlier during the case [82].

Thirdly, the surgeon must be able to accurately, consistently and reproducibly introduce the acetabular component with the correct intended 3D orientation and then impact it whilst precisely maintaining this. As a fundamental tenant of the assumption that surgeons can reliably perform this task Somerville et al. [86] explored the accuracy with which a cohort of experienced trauma and arthroplasty surgeons visually assessed cup anteversion and inclination insertion angles [86]. There was great variability amongst the group with results ranging from 'very poor' to 'very good' with only moderate inter-observer reproducibility [86]. There have been many proposed methods for improving the precision and/or reproducibility of cup insertion. Such measures have included: following anatomical landmarks [83], the use of intra-operative imaging [79], manual instrumentation jigs and alignment guides [67] or the use of computer-assisted navigation [87]. The most commonly cited anatomic landmark for cup insertion remains the transverse acetabular ligament (TAL) [44]. The value of this local feature has been questioned however, the earlier work of Epstein et al. suggesting the TAL was only appreciably present in 47% of osteoarthritic hips [88] and that, even when it was identified, its presence and recognition did not improve the attainment of target cup position [88]. They concluded that cup orientation using the TAL was no more accurate than an unassisted freehand insertion technique [88], with the subsequent work of Beverland et al. actively recommending against using the TAL to determine final cup inclination [44].

While the use of real-time imaging has been suggested as a potentially useful step to improve the accuracy of final cup position although this too is not without its inherent challenges. Difficulty in the physical process of introducing imaging equipment into sterile fields and capturing meaningful images (i.e. accurately perpendicular to the long axis of the pelvis), concerns regarding radiation exposure and fundamental problems with the interpretation of an image captured in a decubitus position (as compared to the 'routine' AP supine or standing states) are all noteworthy considerations. While some authors advocate the use of imaging routinely [89, 90] (most often fluoroscopy [80]) as an intra-operative aid—especially in the setting of a high BMI patient [80]—others have suggested limited utility through such means citing mismatch between apparent 'during surgery' and post-operative radiographic

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

cup orientation [68]. Hayakawa et al. suggested mean errors of >5° in both cup inclination and anteversion perception using intra-operative radiographs versus the post-operative gold standard [91] concluding that in-theatre determinations may not reflect post-operative targets.

Using conventional instrumented cup implantation techniques there are many proprietary differences between implant systems which cloud comparability. In essence, the AP inclination angle (i.e. 'lateral opening' or 'abduction' angle) is visually-appreciated as the angle between the cup insertion handle and the sagittal plane [67] (**Figure 3**). Critically, if at the time of cup insertion the PSP is not (or is no longer) parallel to the floor, an error of component placement will occur [67]. Body axis alignment (or appreciation thereof) directly influences cup version, as can changes in pelvic flexion and extension.

Whilst a relatively recent addition to the arthroplasty surgeon's armamentarium in many parts of the world, the use of intra-operative computer-assisted hip navigation provides another means for aiding cup insertion [75, 92]. In its two most basic forms, such systems use either pre-operative 'imaging informed' or 'imageless' [73, 93] approaches. As with accepted total knee arthroplasty (TKA) applications, both portable (i.e. 'mini-nav' [94, 95]) and 'full navigation' systems are available for use during hip surgery. Whilst large volume data are still pending, early applications of navigated THA suggest consistent improvements in achieving the desired insertion orientation [93, 94, 96] with significantly less (p < 0.001) deviations from target [92, 97]. Most commercially-available navigation systems reference the APP which

**Figure 3.** *Cup insertion angles using instrumented alignment towers.*

provides the frame-of-reference orientation for later angular measurement [51, 73] although the paper by Vigdorchik and colleagues suggests that the perpendicular hip-shoulder-axis may actually be the more accurate and consistent registration plane [98]. The well-performed 2020 prospective randomised control trial by Tanino et al. compared the accuracy of a portable, accelerometer-based navigation system with that of conventional instrumented techniques [92]. While adding an average of 10 operative minutes to each case [92], the use of navigation was associated with significant improvements in attainment of target cup position [92]—a sentiment supporting the landmark earlier work of Jolles et al. in 2004 [99]. One of the key benefits of contemporary THA navigation [75, 92] likely lies in the ability of such systems to track pelvic movement during surgery and provide 'corrective' measurements [100]. In many cases, the system has the ability to recognise the occurrence and magnitude of pelvic positional changes, even when such movement is below the threshold of unaided surgeon perception. While many authors (and users) feel that intra-operative navigation stands as the best widely available tool for accurate cup implantation [73], such systems do have their own inherent shortcomings including a user learningcurve, system failures, loss of tracker position and poor reliability with increasing pelvic tilt [95, 101]. The integration of biplanar EOS-based imaging methods with existing navigation applications (NAVEOS; VA, USA) is an exciting novel pairing [69] which has been touted to further simplify cup placement with increased 3D accuracy in a lateral decubitus position [69] however, this technology needs further, rigorous, validation before wider adoption can be championed.
