**2. Recognising the importance of optimised acetabular component orientation**

Optimal insertional orientation of the acetabular component during THA is a critical determinant of many tangible outcomes, including construct stability [43]. At its extreme, malpositioning may lead to prosthetic dislocation. With the majority of THAs currently still being performed in a lateral decubitus patient position [43, 44], factors which introduce inconsistency or error in achieving the desired final cup position have been extensively explored. Sound previous research has confirmed the following: 1. there is great inconsistency and often poor reproducibility in the accuracy with which a true decubitus position is achieved during the 'set up' phase of a THA operation; 2. conventional positioning devices perform poorly in maintaining the initial set up position during the performance of a THA; 3. there is considerable patient loss-of-position during the operation itself (i.e. the position of the pelvis changes during surgery); 4. an erroneous pelvic position (from the start of the operation) and/or a loss of position during the procedure introduces a substantial potential for error in the ultimate insertional orientation of the cup; 5. suboptimal cup position has been strongly associated with a number of poor outcome measures, including wear, increased revision rate and dislocation.

A number of previous investigations have attempted to quantify the 'average' amount of unintended pelvic movement which occurs during the performance of a routine primary THA. These results are discussed in more detail later herein. Interpretation of such information has however—in many instances—been clouded by inconsistent data collection methods or by unreliable measurement approaches.
