Abstract
INTRODUCTION
Mal-positioning of the acetabular component in total hip replacement (THR) could lead to edge loading, accelerated component wear, impingement and dislocation [1,2]. In order to achieve a successful position for the acetabular component, the assessment of the acetabular orientation with reference to different coordinate systems is important [3]. The aims of the present study were to establish a pelvic coordinate system and a global body coordinate system, and to assess the acetabular orientations of natural hips with reference to the two coordinate systems.
METHODS
Three-dimensional (3D) computed tomographic (CT) images of 56 subjects (28 males and 28 females) lying supine were obtained from a public image archive (Cancer Image Archive, website: www.cancerimagingarchive.net). 3D solid models of pelvis and spine were generated from the CT images. Two coordinate systems, pelvic and global body coordinate systems, were established. The pelvic coordinate system was established based on four bony landmarks on the pelvis: the bilateral anterior superior iliac spines (RASIS and LASIS) and the bilateral pubic tubercles (RPT and LPT). The global body coordinate system was generated based on the bony landmarks on the spine: the geometric centers of five lumbar vertebrae bodies and the most dorsal points of five corresponding spinous processes, as well as the anterior sacral promontory (Fig 1a and 1b). The acetabular rim plane was obtained by fitting a set of point along the acetabular rim to a plane using least squares method. The acetabular orientation was defined as the three coordinate components (x-, y- and z- components) of the unit normal vector of the acetabular rim plane in the two coordinate systems (Fig. 1c).
RESULTS
Statistically significant differences of y- and z- components of the unit normal vector of the acetabular rim plane were calculated in the two coordinate systems (p<0.05). However, no significant difference of x- components was found (p=0.22) (Fig. 2). The differences of y- and z- components of the unit normal vector between the two coordinate system measurements were positive for most subjects. In addition, the differences and their standard deviations were larger for females compared to those for males (Fig. 3).
DISCUSSION
Significantly different acetabular orientations were measured in the two coordinate systems, with larger variations in the global body coordinate system. The statistical analysis indicates that the different orientations measured in the two coordinate systems are primarily attributed to the pelvic tilt in the sagittal plane. The results also indicates that there was a trend of forward inclination of pelvis for most subjects considered in the present study and that the females generally have larger forward inclination and greater variation of pelvic tilt compared to males.
SIGNIFICANCE
The study suggested that the consideration of pelvic tilt in THR placement is necessarily required in order to achieve a successful positioning of THR component with respect to the biomechanical axis of the body.
