**4.2 The concept of functional anteversion**

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

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 87

Fig. 19. In standing positions, there is less cross-sectional anteversion than in sitting positions, where the cup opens completely forward, thus favoring flexion of the hip and

In standing positions, there is less cross-sectional anteversion than in sitting positions, where the cup opens completely forward, thus favoring flexion of the hip and avoiding impingement at the femoral neck **(Fig. 19).** In supine positions, with the lower limbs extended, the pelvic tilt is often greater than in standing positions, lordosis more accentuated and anteversion still more reduced than when standing. In a series of 328 THAs, we reported a mean anteversion of 31.7° standing, 38.8° seated , and 24.2° supine (Lazennec et al., 2011a). The strong correlation observed between the supine and standing measurements suggests that the classic CT assessment of the anatomic acetabular anteversion is still a relevant source of information in cases of anterior THA dislocation and subluxation. But poor correlations have been reported between the supine and sitting positions; they demonstrate that the usual CT scan protocol is biased and not fully appropriate in an investigation of posterior THA dislocation, which occurs in hip flexion

Globally, the "standard" CT scan measurements tend to overestimate anteversion in standing positions and to underestimate it in seated positions. More or less, 1° of pelvic tilt affects acetabular anteversion by 0.5 to 1°, depending on the specific study (Anda et al., 1990; Lazennec et al., 2004). Some subjects have completely stiff lumbosacral joints, and this significantly reduces the variations in acetabular anteversion between seated and standing positions. Such stiffening of the pelvis, in either a relative anterior or posterior tilt, may lead to a reproducible impingement situation. This phenomenon has been documented in the literature for deformities of the sagittal plane in spondylarthritis, in poorly adjusted lumbosacral arthrodesis (flat back), and especially in spinal aging (During et al., 1985; Fogel & Esses, 2003; Hammerberg & Wood, 2003; Itoi, 1991; Lazennec et al., 1997; Offierski &

avoiding impingement at the femoral neck

(Lazennec et al., 2011a)

Fig. 18. a: The horizontal transverse plane can be used as a reference plane; the relation with the acetabulum is different according to the standing or seated positions. In current practice, anteversion is measured on CT scans but the angle value depends on the pelvic orientation in the lying position. Fortuitously, the cross-sectional slices, perpendicular to the plane of the examination table, can also be strictly perpendicular to the longitudinal axis of the pelvic bone frame. But, most of the time, these slices do not correspond to this anatomic plane because the position of the supine subject has a more or less marked sagittal pelvic tilt

It is calculated from the CT scan slices taken as a function of the sacral slope measured on the initial standard lateral image. In a plane with the upper sacral endplate, these slices reconstitute the sacral tilt angle, measured in a standing, sitting, or supine position. They make it possible to observe important changes in the cup orientation and improve our understanding of some coxofemoral joint dysfunctions.

Fig. 18. b: 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**.** The measurement of the functional anteversion corresponding to standing and sitting positions can be done using CT scan : it requires an adjustment of the sectional plane following the value of the SS on lateral radiographs with the patients in standing and sitting positions

Fig. 18. a: The horizontal transverse plane can be used as a reference plane; the relation with the acetabulum is different according to the standing or seated positions. In current practice, anteversion is measured on CT scans but the angle value depends on the pelvic orientation in the lying position. Fortuitously, the cross-sectional slices, perpendicular to the plane of the examination table, can also be strictly perpendicular to the longitudinal axis of the pelvic bone frame. But, most of the time, these slices do not correspond to this anatomic plane because the position of the supine subject has a more or less marked sagittal pelvic tilt

It is calculated from the CT scan slices taken as a function of the sacral slope measured on the initial standard lateral image. In a plane with the upper sacral endplate, these slices reconstitute the sacral tilt angle, measured in a standing, sitting, or supine position. They make it possible to observe important changes in the cup orientation and improve our

Fig. 18. b: 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**.** The measurement of the functional anteversion corresponding to standing and sitting positions can be done using CT scan : it requires an adjustment of the sectional plane following the value of the SS on

lateral radiographs with the patients in standing and sitting positions

understanding of some coxofemoral joint dysfunctions.

Fig. 19. In standing positions, there is less cross-sectional anteversion than in sitting positions, where the cup opens completely forward, thus favoring flexion of the hip and avoiding impingement at the femoral neck

In standing positions, there is less cross-sectional anteversion than in sitting positions, where the cup opens completely forward, thus favoring flexion of the hip and avoiding impingement at the femoral neck **(Fig. 19).** In supine positions, with the lower limbs extended, the pelvic tilt is often greater than in standing positions, lordosis more accentuated and anteversion still more reduced than when standing. In a series of 328 THAs, we reported a mean anteversion of 31.7° standing, 38.8° seated , and 24.2° supine (Lazennec et al., 2011a). The strong correlation observed between the supine and standing measurements suggests that the classic CT assessment of the anatomic acetabular anteversion is still a relevant source of information in cases of anterior THA dislocation and subluxation. But poor correlations have been reported between the supine and sitting positions; they demonstrate that the usual CT scan protocol is biased and not fully appropriate in an investigation of posterior THA dislocation, which occurs in hip flexion (Lazennec et al., 2011a)

Globally, the "standard" CT scan measurements tend to overestimate anteversion in standing positions and to underestimate it in seated positions. More or less, 1° of pelvic tilt affects acetabular anteversion by 0.5 to 1°, depending on the specific study (Anda et al., 1990; Lazennec et al., 2004). Some subjects have completely stiff lumbosacral joints, and this significantly reduces the variations in acetabular anteversion between seated and standing positions. Such stiffening of the pelvis, in either a relative anterior or posterior tilt, may lead to a reproducible impingement situation. This phenomenon has been documented in the literature for deformities of the sagittal plane in spondylarthritis, in poorly adjusted lumbosacral arthrodesis (flat back), and especially in spinal aging (During et al., 1985; Fogel & Esses, 2003; Hammerberg & Wood, 2003; Itoi, 1991; Lazennec et al., 1997; Offierski &

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 89

integrate pelvic rotation into the analysis of the overall trunk posture (Dubousset et al., 2007). The use of standing and seated EOS® images in the subject's "usual" position is particularly instructive. Our database of complete EOS® acquisitions, both standing and seated, from more than 2500 patients reveals the frequency of cases involving a forward hemipelvis and therefore a backward contralateral hemipelvis. This is expressed on the AP image by asymmetry of the projection of the iliac wings; the "forward" wing appears thinner than the other. Laterally, the two femoral heads and the two iliac wings are not

Fig. 21. This "twisting" phenomenon of the pelvis is expressed on the AP image by

asymmetry of the projection of the iliac wings; the "forward" wing appears thinner than the other. Laterally, the two femoral heads and the two iliac wings are not superimposed

This "twisting" phenomenon is difficult to quantify with standard radiography because of the cone-shaped distribution of X-rays, which distorts interpretation of the image of the femoral head furthest from the scanner source. On the other hand, it is well analyzed by EOS® images in both the standing and seated positions, and 3D visualization of the pelvic position is possible. This situation of pelvic rotation is pushed to extremes in cases of scoliosis with the pelvic vertebrae included in the deformity **(Fig. 22).** The consequences on cup orientation can be significant, in particular for patients with THA, because of the induction of changes in functional anteversion, in both standing and seated positions

superimposed **(Fig. 21).** 

(Tannast et al., 2005).

MacNab, 1983). In these cases, we observed a reduction in lordosis and the appearance of an adaptive posterior pelvic tilt. The patient is standing, as if he were seated. This phenomenon reduces adaptation in the lumbosacral area and deviates the functional mobility cone of the coxofemoral joints towards flexion. The cup is permanently in hyper-anteversion, which is not bothersome during hip flexion but creates a problem of posterior impingement, especially in a standing position: the person progressively loses his or her available extension **(Fig. 20)**.

The compensating hyperextension of the hips often has a limited impact in these patients, especially those who are elderly or have hip disorders. The last adaptation to attempt to improve balance is thus flexion of the knees, which enables additional posterior tilt of the pelvis. This phenomenon explains prosthesis dislocation after several years, following an overall sagittal disruption of the lumbo-pelvic-femoral complex (Legaye, 2009; Rousseau et al., 2009; Tang et al., 2007).

Fig. 20. Reduction in lordosis and the appearance of an adaptive posterior pelvic tilt has been documented in deformities of the sagittal plane in spondylarthritis, in poorly adjusted lumbosacral arthrodesis (flat back), and especially in spinal aging : This phenomenon reduces adaptation in the lumbosacral area and deviates the functional mobility cone of the coxofemoral joints towards flexion
