**3. Understanding the relationship between pelvic movement and resultant acetabular component position**

While most surgeons agree that accurate implantation of the acetabular component of a THA is important for patient outcomes, the 'ideal' position is far from universally agreed upon [44, 45]. While the historic reference of Lewinnek's safe zone has formed the basis of most 'target' cup positions [46, 47], many contemporary authors suggest that there may be merit in a 'patient-specific' orientation goal and that 'one size' does not fit all [10, 11]. Indeed, many proponents advocate for individualised patient assessment—often in the form of pre-operative functional imaging [48–50]—to inform intra-operative decision making. Deviation from an 'anatomically neutral' starting position can have considerable negative impact on resultant cup insertion decision making. One key determinant 'pelvic tilt' (or 'roll') reflects the divergent angle between the anterior pelvic plane (APP) and a vertical line in the anatomical (standing) position [51] (**Figure 2**). The large recent study of 1517 THAs by Pierrepont and colleagues suggested that in nearly 20% of patients the extent of functional sagittal pelvic rotation (reflecting pelvic tilt) identified could potentially lead to construct instability using historical 'safe zone' targets [50]. This is a staggeringly high proportion.

#### **Figure 2.**

*Pelvic tilt. Measurement of the anterior tilt angle in a lateral decubitus position. Forward tilt is determined as the angle subtended by the difference in degrees from the true APP (i.e. the vertical starting position) to the measured APP as it approaches the MSP (i.e. as the pelvis rolls anteriorly) [43]. APP = anterior pelvic plane; MSP = midsagittal plane; β = anterior tilt angle.*

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

The effects of changing pelvic position on pelvic tilt has been well studied [52–54]. When the pelvis tilts posteriorly during basic pre-operative functional screening, the respective anteversion and abduction angles of the final acetabular implant position increases, which may in turn lead to excessive wear due to neck impingement and edge loading, with an increased risk of dislocation [19]. Prediction of pelvic displacement before surgery has key importance for accurate placement of the acetabular implant despite historically being an under-valued consideration. Objectively, Babisch et al. demonstrated that acetabular cup positions are affected by pelvic tilt on CT models, with good accuracy and reproducibility [55]. Similarly, a previous study by Maratt et al. using a computer-generated 3D model also demonstrated the substantial effect of pelvic tilt on resultant acetabular angles [56]. In a practical sense, the functional angle of the acetabular implant is directly related to the pelvic tilt angle, with the anteversion angle of the acetabular implant changing by approximately 0.7° with every degree of change in pelvic inclination [43]. Therefore, only small linear magnitude changes can significantly affect endpoint cup version and contribute to construct instability.

In contrast, until recently there has been a lack of literature describing the isolated effects of radiographic pelvic rotation (PR) on preoperative acetabular planning angulation of acetabular prostheses. The recent work of Lourens et al. used high resolution 3D CT pelvis models generated from healthy controls and arthroplasty patients to quantify the effects of pelvic rotation on acetabular cup position in various static planes simulating radiographic errors in basic imaging used for component templating [2]. They concluded that pelvic rotation can also significantly impact on the perceived acetabular angles observed on an AP pelvic radiograph used for pre-operative planning, which can in turn result in poor prosthetic placement and subsequent poorer long-term clinical outcomes. Supportive of the reliability of conventional approaches, the presented data indicated that PR of less than 20° however was unlikely to have a clinical impact of preoperative measurements and therefore may serve as a guide for clinical application and operative planning.
