**3.1 Anatomic foundations**

Most surgeons use only AP X radiograms of the pelvis to assess the cup orientation in standing position, measuring the frontal inclination angle or abduction angle (AA). The lateral view is little used in the literature. Acetabular tilt is assessed by the SIA (sagittal inclination angle between the horizontal and the axis of the cup ellipse, also called the sagittal tilt (Lazennec et al., 2011a) **(Fig. 9)**.

Fig. 8. Progressio**n** from standing to sitting positions causes considerable modification in the

Most surgeons use only AP X radiograms of the pelvis to assess the cup orientation in standing position, measuring the frontal inclination angle or abduction angle (AA). The lateral view is little used in the literature. Acetabular tilt is assessed by the SIA (sagittal inclination angle between the horizontal and the axis of the cup ellipse, also called the

**3. Influence of sagittal posture on the frontal and sagittal acetabular** 

orientation of the anterior pelvic plane (A.P.P. or Lewinnek plane)

**orientation in functional situations** 

sagittal tilt (Lazennec et al., 2011a) **(Fig. 9)**.

**3.1 Anatomic foundations** 

Fig. 9. AP and lateral images provide a global evaluation of the pelvis and cup orientations in standing or seated positions

Because of basic trigonometric relations in three dimensions, the SIA has the same value as the operative anteversion described by Murray (Murray, 1993), although it is a different angle. But sitting and even squatting are critical positions, as they address most of instability or impingement situations. The abduction and sagittal inclination angles of the cup are functional parameters with significant variations between the standing, sitting, and supine positions (Lazennec et al., 2011b). The interdependence between the sagittal tilt or **AST** of the cup and the sacral slope is obvious when reading lateral images, seated or standing, of the lumbosacral joint (Bolger et al., 2007). This interrelation is expressed by the value of the sacroacetabular angle **(SAA)**, defined by the axis of the acetabular ellipse viewed laterally (which defines the acetabular sagittal tilt, **AST**, with the horizontal) and the tangent to the sacral endplate. This is a fixed angle, imposed by the surgeon empirically at implantation (Lazennec et al., 2007) **(Fig. 10)**.

In a standing position, the value of the sacral slope is high and the angle of the acetabular tilt is small. Inversely, in a seated position, the sacral slope diminishes and the acetabular tilt increases. From both AP and lateral positions, the THA cup appears more vertical in seated than in standing positions. Observational series of THA from diverse institutions (Lazennec et al., 2011a; Lazennec et al., 2011c) report mean values of 49° to 52° for the frontal cup inclination in standing position and 57° to 64° while seated. At the same time the sagittal inclination is 36° to 47° standing and 51° to 58° seated. These variations in acetabular tilt

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 79

Fig. 11. b: The variations in acetabular tilt in standing and seated positions contribute to modifying the "anterior opening" of the acetabulum and therefore the orientation of the

Modifications in the acetabular orientation produce consequences for the stability of hip arthroplasties and for all aspects of their tribology in the intermediate and long term (Lazennec et al., 2007; von Knoch et al., 2002; Woo & Morrey, 1982; Yamaguchi et al., 1997). The acetabular parameters in a supine position are highly correlated with those in a standing position but poorly correlated with those in a sitting position (Lazennec et al., 2011a). These correlations underline the importance of specific studies of the seated position in work-ups of prostheses that malfunction and of screening subjects at risk before surgery.

These considerations illustrate the consequences of spinal fusions regarding hip function and the possible difficulties in adjusting acetabular cup implantation in cases of an unusually stiff or unbalanced spine (Watanabe et al., 2002). Excess posterior tilt of the pelvis is often associated with postural imbalance to compensate for a forward tilt of the entire trunk. This postural adaptation verticalizes the cup from both the AP and lateral views and places the hips in a hyperextended position when standing. This solicitation of the available hip extension can lead to a posterior impingement. A typical example is posterior impingement of the hip prosthesis in standing position even though the cup was placed perfectly in accordance with the pelvic bone landmarks **(Fig. 12).** This situation can be encountered in native hips in the case of excessive posterior tilt of the pelvis (abnormal posture, trunk aging and induced posterior coxarthrosis) (Hammerberg & Wood, 2003; Itoi,

functional "mobility cone" generated by the coxofemoral joint

**3.2 Disruptions can come from atypical postures** 

1991; Lafage et al., 2009; Offierski & MacNab, 1983).

contribute to modifying the "anterior opening" of the acetabulum and therefore the orientation of the functional "mobility cone" generated by the coxofemoral joint **(Fig. 11 a and b)** (Kummer et al., 1999; Lazennec et al., 2011a) (Pradhan, 1999).

Fig. 10. The interdependence between the sagittal tilt or AST of the cup and the sacral slope is expressed by the value of the sacroacetabular angle (SAA)

Fig. 11. a: The variations in acetabular tilt in standing and seated positions contribute to modifying the "anterior opening" of the acetabulum and therefore the orientation of the functional "mobility cone" generated by the coxofemoral joint

contribute to modifying the "anterior opening" of the acetabulum and therefore the orientation of the functional "mobility cone" generated by the coxofemoral joint **(Fig. 11 a** 

Fig. 10. The interdependence between the sagittal tilt or AST of the cup and the sacral slope

Fig. 11. a: The variations in acetabular tilt in standing and seated positions contribute to modifying the "anterior opening" of the acetabulum and therefore the orientation of the

functional "mobility cone" generated by the coxofemoral joint

**and b)** (Kummer et al., 1999; Lazennec et al., 2011a) (Pradhan, 1999).

is expressed by the value of the sacroacetabular angle (SAA)

Fig. 11. b: The variations in acetabular tilt in standing and seated positions contribute to modifying the "anterior opening" of the acetabulum and therefore the orientation of the functional "mobility cone" generated by the coxofemoral joint

Modifications in the acetabular orientation produce consequences for the stability of hip arthroplasties and for all aspects of their tribology in the intermediate and long term (Lazennec et al., 2007; von Knoch et al., 2002; Woo & Morrey, 1982; Yamaguchi et al., 1997). The acetabular parameters in a supine position are highly correlated with those in a standing position but poorly correlated with those in a sitting position (Lazennec et al., 2011a). These correlations underline the importance of specific studies of the seated position in work-ups of prostheses that malfunction and of screening subjects at risk before surgery.

### **3.2 Disruptions can come from atypical postures**

These considerations illustrate the consequences of spinal fusions regarding hip function and the possible difficulties in adjusting acetabular cup implantation in cases of an unusually stiff or unbalanced spine (Watanabe et al., 2002). Excess posterior tilt of the pelvis is often associated with postural imbalance to compensate for a forward tilt of the entire trunk. This postural adaptation verticalizes the cup from both the AP and lateral views and places the hips in a hyperextended position when standing. This solicitation of the available hip extension can lead to a posterior impingement. A typical example is posterior impingement of the hip prosthesis in standing position even though the cup was placed perfectly in accordance with the pelvic bone landmarks **(Fig. 12).** This situation can be encountered in native hips in the case of excessive posterior tilt of the pelvis (abnormal posture, trunk aging and induced posterior coxarthrosis) (Hammerberg & Wood, 2003; Itoi, 1991; Lafage et al., 2009; Offierski & MacNab, 1983).

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 81

We also observed a mechanism of specific adaptation in hips that could not be fully extended secondary to coxarthrosis. The loss of range of motion of the abnormal hip results in a forward tilt of the pelvis when the patient tries to straighten up. When possible, the spine adapts by increasing lumbar lordosis, thus causing low back pain. Frequently, only one hip is involved. The test for available extension allows us to assess the phenomenon

Fig. 13. EOS**®** technology makes it possible to individualize the available extension associated with the lumbosacral joint (the extrinsic available extension) as well as the intrinsic available extension of each coxofemoral joint. Each hip is assessed in its maximum range of extension in standing position, placing the contralateral lower limb on a step such that it is in a position of maximum flexion of the coxofemoral joint. The extrinsic available extension is measured by the capacity to increase the sacral slope (SS 2 – SS 1). The intrinsic available extension is measured by the variation of the sacro-femoral angle (SFA). In this

example: the available extension is 10° for the 2 hips.

selectively.

Analysis of these situations can be difficult because they involve simultaneously diseases of the spine and hips. Assessment of the available hip extension is thus essential to distinguish true and false inability to extend the hip fully **(Fig. 13).** This analysis can be performed easily with EOS**®** technology, which makes it possible to individualize the available extension associated with the lumbosacral joint, that is, the extrinsic available extension, as well as the intrinsic available extension of each coxofemoral joint. Inversely, excess anterior tilt of the pelvis in a seated position can cause anterior impingement **(Fig. 14)**. In some cases, the anterior tilt of the acetabulum is also excessive in a standing position: the acetabulum is "horizontalized" from both AP and lateral views, as if the hips are permanently flexed when the subject is standing. This type of mechanism is suggested on native hips in anterior impingements, especially in some repetitive athletic or occupational movements.

Fig. 12. Excess posterior tilt of the pelvis is often associated with postural imbalance to compensate for a forward tilt of the entire trunk. A typical example is posterior impingement of the hip prosthesis in standing position even though the cup was placed perfectly in accordance with the pelvic bone landmarks ( no impingement in sitting position).

Analysis of these situations can be difficult because they involve simultaneously diseases of the spine and hips. Assessment of the available hip extension is thus essential to distinguish true and false inability to extend the hip fully **(Fig. 13).** This analysis can be performed easily with EOS**®** technology, which makes it possible to individualize the available extension associated with the lumbosacral joint, that is, the extrinsic available extension, as well as the intrinsic available extension of each coxofemoral joint. Inversely, excess anterior tilt of the pelvis in a seated position can cause anterior impingement **(Fig. 14)**. In some cases, the anterior tilt of the acetabulum is also excessive in a standing position: the acetabulum is "horizontalized" from both AP and lateral views, as if the hips are permanently flexed when the subject is standing. This type of mechanism is suggested on native hips in anterior impingements, especially in some repetitive athletic or

Fig. 12. Excess posterior tilt of the pelvis is often associated with postural imbalance to compensate for a forward tilt of the entire trunk. A typical example is posterior

impingement of the hip prosthesis in standing position even though the cup was placed perfectly in accordance with the pelvic bone landmarks ( no impingement in sitting

occupational movements.

position).

We also observed a mechanism of specific adaptation in hips that could not be fully extended secondary to coxarthrosis. The loss of range of motion of the abnormal hip results in a forward tilt of the pelvis when the patient tries to straighten up. When possible, the spine adapts by increasing lumbar lordosis, thus causing low back pain. Frequently, only one hip is involved. The test for available extension allows us to assess the phenomenon selectively.

Fig. 13. EOS**®** technology makes it possible to individualize the available extension associated with the lumbosacral joint (the extrinsic available extension) as well as the intrinsic available extension of each coxofemoral joint. Each hip is assessed in its maximum range of extension in standing position, placing the contralateral lower limb on a step such that it is in a position of maximum flexion of the coxofemoral joint. The extrinsic available extension is measured by the capacity to increase the sacral slope (SS 2 – SS 1). The intrinsic available extension is measured by the variation of the sacro-femoral angle (SFA). In this example: the available extension is 10° for the 2 hips.

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 83

Fig. 15. In subjects with a high angle of pelvic incidence, lumbar lordosis is greater and the coxofemoral joints have a greater theoretical available extension and a better ability to adapt.In subjects with a low pelvic incidence angle, there is less lumbar lordosis and the

**4. Influence of sagittal posture on cross-sectional acetabular cup orientation:** 

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

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

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

adaptability of the sacral slope and pelvic tilt may be more limited

axis and the midsagittal plane of the pelvis (Murray, 1993).

**Anatomic and functional anteversion** 

pelvic orientation in the lying position.

**4.1 Standard data** 

Fig. 14. Excess anterior tilt of the pelvis in a seated position can cause anterior impingement; no impingement in standing position
