**1. Introduction**

Total hip replacement remains a tried-and-true method for managing hip pain and dysfunction resultant from end-stage degenerative disease and a number of other medical conditions [1–3]. It has a long-standing proven clinical track record with strong evidence to support consistent improvements in patient function and satisfaction—indeed primary total hip arthroplasty (THA) has been claimed as one of the most significant surgical advances of the 20th century [4].

In the late 1970s, Lewinnek and colleagues generated the landmark paper proposing the acetabular 'safe zone'—an idealised target orientation for component placement—suggested to be associated with decreased risk of prosthetic dislocation [5]. Nearly 45 years later, the paper stands as one of the most cited in the orthopaedic

literature [6]. The '40/15 safe zone' of Lewinnek (inferring a target acetabular component insertion position of 40° of abduction and 15° of anteversion, each +/− 10°) (**Figure 1**) has largely become the 'ideal' cup orientation for hip replacement surgeons and forms the basis of most conventional implantation tools/aids. This fundamental premise of THA surgery has however been challenged extensively in the contemporary literature with many authors suggesting limited value for these targets on a patient-by-patient level—several larger, reputable, papers have shown large proportions of post-operative dislocations occurring well within the defined 'safe zone' [7, 8]. The suggestion that 'one size does *not* fit all' is gaining wider acceptance and a move towards 'functional' safe zones and/or 'patient-specific' acetabular component orientation is gaining momentum [9–12].

Acetabular prosthesis implantation angles have been shown to affect peri-articular muscle mechanical advantage, rates of dislocation, gait and gait efficiency, limb lengths, impingement, noise generation, loosening, postoperative range-of-movement, liner wear and overall revision rates [13–23]. Balanced biomechanical and anatomical reconstruction of the joint is therefore critical to achieve function, enduring longevity and prevention of avoidable complications following surgery [15, 16, 20]. Dislocation rates following primary THA are acknowledged to occur in 1–4% of cases, with 'instability' accounting for approximately 23% of all revisions and remains the most common reason for such surgery in the United States [24, 25]. Preoperative templating from a plain anterior-posterior radiograph is the primary method for initial evaluation and forms the cornerstone of pre-operative prosthesis position planning, however the value of such images are subject to degradation due to uncompensated patient pelvic malposition. Suboptimal acetabular component position can significantly negatively impact the results of a hip arthroplasty, including increased risk of instability, impingement, dislocation and cup failure [12, 14, 16, 19, 26–30]. Correct template positioning influences the accuracy of acetabular cup placement planning and hence the long-term success of the THA.

Traditional freehand THA techniques rely heavily upon surgeon judgement to manually place acetabular components accurately. Computer navigation to reduce acetabular malpositioning has been used for more than 20 years, demonstrating improved attainment of target cup placement and variable reports of improvement in clinical outcomes, including reducing rates of revision [31–33]. By comparison, the prestigious Australian Orthopaedic Association National Joint Replacement

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

Registry (AOANJRR) began collecting data on the use of computer-navigated total *knee* arthroplasty (TKA) in 2003 and has previously reported on the outcomes [31] which show clear outcome benefit in several patient demographics [34, 35]. The use of computer navigation has steadily increased in that setting for TKA, from 2.4% in 2003 to 33.2% in 2018. However, by comparison the AOANJRR shows that <2% of THAs recorded to date have utilised navigation-assistance [31, 36].

Historically two separate means of informing computer-assisted navigation systems have existed—those reliant upon pre-operative imaging and 'imageless' systems. Although plain X-ray-based renditions do exist [37], most 'image driven' systems rely on images generated from pre-operatively obtained computer tomography (CT) scans using proprietary image reconstruction and feature recognition. Common to 'imageless' systems is some method of anatomic feature recognition determined during the surgery itself which informs the surgical navigation plan. The use of CT-based navigation has been shown to be highly accurate, however it is burdened by the associated cost, the need for dedicated pre-operative imaging, and incumbent radiation exposure risk—all of which have been linked to low levels of clinical utilisation [38–41].

Imageless intra-operative navigation systems allow real-time, surgeon-controlled, determination of leg length and offset changes, and three-dimensional (3D) cup position [42]. The key features of these commercially-available systems are intended to overcome some of the recognised barriers to uptake associated with 'imagingbased' navigation, and are already in widespread use [3]. This chapter aims to review the rationale for, evolution of, and current evidence base supporting the use of such 'imageless' navigation tools and also provide understanding as to why pelvic positional variability makes such systems of high value, as well as exploring some of the exciting cutting-edge extensions of such technology into the foreseeable future.
