**4.1 Standard data**

The transverse orientation of the acetabulum is expressed by its anterior opening angle or anteversion. Anatomical acetabular anteversion is a restrictive concept as it is defined as the anterior opening according to the pelvic frame. According to Murray, "anatomical" or "morphological "anteversion is the angle between the AP pelvic axis and the acetabular axis when this is projected on to the transverse reference plane perpendicular to the longitudinal axis and the midsagittal plane of the pelvis (Murray, 1993).

In current practice, anteversion is measured on CT scans: this angle is often considered to be anatomical anteversion, leading to some confusion because the angle value depends on the pelvic orientation in the lying position.

That is, the orientation of the cross-sectional slices in relation to the sagittal plane is left to radiologists to assess without any specific criterion for standardization, although it has a critical effect on the values of the angles measured. Fortuitously, they can be strictly

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 85

Fig. 17. Anatomic or morphologic anteversion can be measured reliably by using the reference transverse plane through the upper sacral endplate (the sacral transverse plane): the anteversion value refers exclusively to the pelvic bone landmarks without taking into

account the variations of position. The anteversion value is the same in all of the

An accurate understanding of lumbosacral posture and its influence on cup tilt shows us that the measurement of anteversion must not be linked to an arbitrary orientation of the slices to the machine stand (Anda et al., 1990). This point is essential for assessing the real acetabular anteversion of THA in terms of instability, especially when the lumbosacral joint is stiffened or in an atypical position (Eddine et al., 2001; Lembeck et al., 2005; Terver et al., 1982). The concept of functional standing and sitting anteversion has been defined to address this problem. The functional anteversion values are the projected angles measured in the horizontal transverse plane, for any of the positions of the pelvis, reflecting the different amount of anterior opening which varies as a function of pelvic tilt

positions

**(Fig. 18a and b).** 

**4.2 The concept of functional anteversion** 

perpendicular to the longitudinal axis of the pelvic bone frame. Most of the time these slices, perpendicular to the plane of the examination table, do not correspond to the anatomic plane because the position of the supine subject has a more or less marked sagittal pelvic tilt (Lazennec et al., 2011a). Therefore this angle is only a "functional" supine anteversion, reflecting the projected anterior opening of the acetabulum in a specific position.

The variation in the anteversion measured is approximately 0.5° for 1° of rotation of the plane of the slice in relation to the pelvis (Anda et al., 1990; Lazennec et al., 2004; Muller et al., 2005). The literature also reports measurements of anatomic acetabular anteversion on CT, according to a plane perpendicular to the anterior pelvic plane (Lewinnek's plane)(McKibbin, 1970). In this case, the cut plane analyzes the acetabulum according to the same orientation, regardless of the subject's position. The anteversion value measured refers exclusively to the pelvic bone landmarks without taking into account the variations of position in standing, seated and squatting positions. It is fixed, regardless of the subject's position **(Fig. 16).** 

Anatomic or morphologic anteversion can also be measured reliably relative to the table or machine stand by using the reference transverse plane (Aubry et al., 2005; Lazennec et al., 2004; Lazennec et al., 2007) through the upper sacral endplate (the sacral transverse plane). This technique is easier to implement than the preceding method, but the acetabulum is not analyzed according to the same angle, and the angle values are different **(Fig. 17).** 

Fig. 16. Anatomic acetabular anteversion can be measured on CT scans according to a plane perpendicular to the anterior pelvic plane (Lewinnek's plane): the cut plane analyzes the acetabulum according to the same orientation, regardless of the subject's position. The anteversion value is the same in all of the positions

perpendicular to the longitudinal axis of the pelvic bone frame. Most of the time these slices, perpendicular to the plane of the examination table, do not correspond to the anatomic plane because the position of the supine subject has a more or less marked sagittal pelvic tilt (Lazennec et al., 2011a). Therefore this angle is only a "functional" supine anteversion,

The variation in the anteversion measured is approximately 0.5° for 1° of rotation of the plane of the slice in relation to the pelvis (Anda et al., 1990; Lazennec et al., 2004; Muller et al., 2005). The literature also reports measurements of anatomic acetabular anteversion on CT, according to a plane perpendicular to the anterior pelvic plane (Lewinnek's plane)(McKibbin, 1970). In this case, the cut plane analyzes the acetabulum according to the same orientation, regardless of the subject's position. The anteversion value measured refers exclusively to the pelvic bone landmarks without taking into account the variations of position in standing, seated and squatting positions. It is fixed, regardless of the subject's

Anatomic or morphologic anteversion can also be measured reliably relative to the table or machine stand by using the reference transverse plane (Aubry et al., 2005; Lazennec et al., 2004; Lazennec et al., 2007) through the upper sacral endplate (the sacral transverse plane). This technique is easier to implement than the preceding method, but the acetabulum is not

Fig. 16. Anatomic acetabular anteversion can be measured on CT scans according to a plane perpendicular to the anterior pelvic plane (Lewinnek's plane): the cut plane analyzes the acetabulum according to the same orientation, regardless of the subject's position. The

anteversion value is the same in all of the positions

analyzed according to the same angle, and the angle values are different **(Fig. 17).** 

reflecting the projected anterior opening of the acetabulum in a specific position.

position **(Fig. 16).** 

Fig. 17. Anatomic or morphologic anteversion can be measured reliably by using the reference transverse plane through the upper sacral endplate (the sacral transverse plane): the anteversion value refers exclusively to the pelvic bone landmarks without taking into account the variations of position. The anteversion value is the same in all of the positions
