**5. Influence of axial rotation of the pelvis and the lumbopelvic region**

Hip surgeons define a "normal" pelvic posture for imaging work-ups of subjects in a strictly anatomic position, with the two iliac wings projecting exactly and symmetrically, compared with the longitudinal axis of the trunk. Spine surgeons have been made aware of the threedimensional view of the spine and the phenomenon of vertebral rotation that disturbs analysis of the lateral view. In this framework, the concept of the pelvic vertebra leads us to

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

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

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

**5. Influence of axial rotation of the pelvis and the lumbopelvic region** 

Hip surgeons define a "normal" pelvic posture for imaging work-ups of subjects in a strictly anatomic position, with the two iliac wings projecting exactly and symmetrically, compared with the longitudinal axis of the trunk. Spine surgeons have been made aware of the threedimensional view of the spine and the phenomenon of vertebral rotation that disturbs analysis of the lateral view. In this framework, the concept of the pelvic vertebra leads us to

extension **(Fig. 20)**.

al., 2009; Tang et al., 2007).

coxofemoral joints towards flexion

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 superimposed **(Fig. 21).** 

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 (Tannast et al., 2005).

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 91

concept is not yet well defined, but our data suggest that taking spinal flexibility into account is important when planning a THA implantation, or at least identifying the unusual

Knowledge of the biomechanics of the lumbosacral joint is relevant for the hip surgeon performing hip replacements in elderly subjects or in those with abnormal sagittal, frontal or rotational posture and/or a large reduction in functional range of motion. Analysis of sagittal balance must therefore be individual and integrated into the comprehensive evolution of the subject over time, because the phenomenon of an aging spine is frequently

The analysis of acetabular orientation cannot be limited to the frontal orientation of the acetabular cup on the AP view and the lateral view of the hip should be considered: standing, sitting, and squatting positions correspond to changes in spinal orientation and acetabular sagittal tilt. The relation between the position of the spine and the acetabulum has a direct influence on the real functional range of motion of the hips. Anterior pelvic plane, pelvic tilt

The mobility of the lumbosacral junction is a crucial parameter in the mechanical function and the stability of THAs, especially in elderly populations. With the increase of the survival of THA, spinal aging and progressive pelvic posterior version must now to be taken into account. Late dislocations of mechanical origin have been reported: modifications of the biomechanics of the spine, transmitted to the hip, may be responsible. The standard measurement of anatomic acetabular anteversion on CT scan images in a supine position should be carefully interpreted as it can induce a false or approximate analysis of the acetabular cup position from a functional standpoint. The analysis of pelvic morphology based on the incidence angle may provide new information about THR dysfunction and

Ackland, M.K., Bourne, W.B., & Uhthoff, H.K. (1986). Anteversion of the acetabular cup. Measurement of angle after total hip replacement. *J Bone Joint Surg Br,* 68, 409-413. Anda, S., Svenningsen, S., Grontvedt, T., & Benum, P. (1990). Pelvic inclination and spatial

Aubry, S., Marinescu, A., Forterre, O., Runge, M., & Garbuio, P. (2005). [Definition of a

Bolger, C., Kelleher, M.O., McEvoy, L., Brayda-Bruno, M., Kaelin, A., Lazennec, J.Y., et al.

Boulay, C., Tardieu, C., Hecquet, J., Benaim, C., Mouilleseaux, B., Marty, C., et al. (2006).

Chanplakorn, P., Wongsak, S., Woratanarat, P., Wajanavisit, W., & Laohacharoensombat, W.

orientation of the acetabulum. A radiographic, computed tomographic and clinical

reproducible method for acetabular anteversion measurement at CT]. *J Radiol,* 86,

(2007). Electrical conductivity measurement: a new technique to detect iatrogenic

Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard

(2011). Lumbopelvic alignment on standing lateral radiograph of adult volunteers and the classification in the sagittal alignment of lumbar spine. *Eur Spine J,* 20, 706-

and sacral slope variations are relevant parameters for planning and navigation.

patients who have an abnormal pelvic (i.e. acetabular) posture.

abnormal wear, especially in patients with unusual postures.

investigation. *Acta Radiol,* 31, 389-394.

initial pedicle perforation. *Eur Spine J,* 16, 1919-1924.

values and prediction of lordosis. *Eur Spine J,* 15, 415-422.

associated with the process of aging hips.

**7. References** 

399-404.

712.

The impact of pelvic rotation and of the pelvic tilt on acetabular orientation raises the question of the choices of guidelines for pre- and postoperative evaluation in cases of hip replacement surgery. The use of guidelines depending on the pelvic bone (anterior pelvic plane, sacral transverse plane, axis through the femoral heads) neglects these two essential phenomena (Lazennec et al., 2007; Tannast et al., 2005). The use of the horizontal transverse plane (or the horizontal plane in space) makes it possible to integrate pelvic tilt into the assessment of acetabular anteversion, which can thus be envisioned "functionally" and not only restrictively as a simple fixed morphologic parameter. Pelvic rotation must be considered and assessed just like vertebral rotation, as analyzed by spine surgeons, compared with a vertical plane of reference perpendicular to the horizontal transverse plane.

Fig. 22. Pelvic rotation is pushed to extremes in cases of scoliosis with the pelvic vertebrae included in the deformity
