**2. Anatomic bases of lumbopelvic balance: The importance of sagittal spinopelvic balance in standing position**

The most important radiographic parameters of spinopelvic balance in upright posture have been well defined (Tassin, 2004; Vaz et al., 2002; Vialle et al., 2005). The sacral slope (SS, 41° ± 8.4°) is the angle between the horizontal line and the cranial sacral endplate tangent (Vialle et al., 2005).

The pelvic tilt (PT, 13° ± 6°) is the angle between the vertical line and the line joining the middle of the sacral endplate and the center of the bicoxofemoral axis (the line between the geometric center of both femoral heads) (Vialle et al., 2005).

The pelvic incidence (I, 55° ± 10.6°) is the angle between the line perpendicular to the middle of the cranial sacral endplate and the line joining the middle of this endplate to the center of the bicoxofemoral axis (Vialle et al., 2005).

Degeneration of the spinopelvic balance, often associated with spinal aging or hip dysfunction, can generate a cascade of mechanical events that involve the relation between the hips and spine and in particular the adaptive capacities of the hips (Itoi, 1991). These problems have become more acute today, with patients' ever greater functional demands after hip arthroplasty (**Fig. 1**). Hip-spine relations must therefore be better investigated in planning total hip arthroplasties (THAs), as lumbosacral orientation plays a critical role in

Fig. 1. Degeneration of the sagittal spinopelvic balance : loss of spinal curvatures and

**2. Anatomic bases of lumbopelvic balance: The importance of sagittal** 

The most important radiographic parameters of spinopelvic balance in upright posture have been well defined (Tassin, 2004; Vaz et al., 2002; Vialle et al., 2005). The sacral slope (SS, 41° ± 8.4°) is the angle between the horizontal line and the cranial sacral endplate tangent (Vialle

The pelvic tilt (PT, 13° ± 6°) is the angle between the vertical line and the line joining the middle of the sacral endplate and the center of the bicoxofemoral axis (the line between the

The pelvic incidence (I, 55° ± 10.6°) is the angle between the line perpendicular to the middle of the cranial sacral endplate and the line joining the middle of this endplate to the center of

the function of the hip joints.

modifications of pelvis orientation

et al., 2005).

**spinopelvic balance in standing position** 

the bicoxofemoral axis (Vialle et al., 2005).

geometric center of both femoral heads) (Vialle et al., 2005).

Each subject is characterized by this "morphologic" parameter, which schematically represents pelvic thickness. The adaptation of other functional factors, such as pelvic tilt, and the spinal parameters (sacral slope, lumbar lordosis, and thoracic kyphosis) makes it possible to position the center of gravity of the component specifically so that it is supported by the femoral heads relative to the pelvic base, to maintain balance with only a minimal muscular effort (Duval-Beaupere et al., 1992; Legaye et al., 1998; Rillardon et al., 2003; Roussouly et al., 2005) (**Fig. 2).**

Fig. 2. Definition of sacral slope (S.S.) and pelvic incidence angle ( i )

Pelvic incidence (I, a morphologic and anatomical parameter) is associated with the sacral slope (SS) and pelvic tilt (PT) (positional parameters) in a geometric relation: I = SS + PT (1) (**Fig. 3).** A sequence of significant correlations has been reported between pelvic incidence and the functional indicators, sacral slope (r = 0.98) and lordosis (r = 0.9) (Boulay et al., 2006; Rillardon et al., 2003; Vialle et al., 2005). A high pelvic incidence corresponds to a sharp sacral slope and strong lordosis, while a small pelvic incidence corresponds to a low sacral slope and flatter lordosis. The borderline balance conditions and spinopelvic balance in seated positions are less known, in particular, for patients with hip prostheses.

Fig. 3. Pelvic incidence ( I ) is associated with the sacral slope ( S.S. ) and pelvic tilt (PT) in a geometric relation: I = SS + PT

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 73

Fig. 5. The standing position **cor**responds to a forward tilt of the pelvis**:** the upper endplate of S1, viewed laterally, makes an angle of approximately 35° to 45 between the sacral slope

When the lower limbs are extended, the sacral slope is often greater than in standing position. The measurement of its angle with the vertical often exceeds 45° (Lazennec et al., 2011a; Lazennec et al., 2004). This can be seen on scout views of pelvic CT scans. This additional pelvic tilt may not be well tolerated in the case of a stiff or deformed spine since the dorsal decubitus position deepens lumbar lordosis. Sometimes a limitation of the available extension of the hips or posterior osteoarthritis makes a strict decubitus position

unbearable, at least without a slight compromise flexion of the coxofemoral joints.

and the horizontal

**2.2 In the supine position** 

The pelvis moves, rotating around the bicoxofemoral axis, leading to both anterior tilt (where the upper portion of the pelvis tips forward) and posterior tilt (upper portion of the pelvis tips backward). The variations in the angles of the sacral slope determine the range of this pelvic tilt, and its variations in turn influence the orientation of the anterior pelvic plane or the Lewinnek plane classically used as a reference for navigation of the acetabular components in THAs **(Fig. 4)** (Lewinnek et al., 1978). The surprising consequences of some spinal arthrodeses or stiffening on the hips and the difficulties of adjusting prostheses below stiffened spines illustrate the importance of these anatomicfunctional parameters.

Fig. 4. The pelvis moves, rotating around the bicoxofemoral axis, the anterior tilt (where the upper portion of the pelvis tips forward) is observed in standing position and posterior tilt (upper portion of the pelvis tips backward) in seated position.

### **2.1 The standing position**

Fig. 5 corresponds to a forward tilt of the pelvis as a whole. In this situation, the upper endplate of S1, viewed laterally, makes an angle of approximately 35° to 45° between the sacral slope and the horizontal (Lazennec et al., 2000; Lazennec et al., 2011c). Some subjects have a small sacral slope angle in standing position: we talk then about posterior pelvic tilt (or pelvic retroversion or pelvic extension) and the sacrum seen on a lateral image appears more vertical than usual (Fig. 6a). On the contrary, other subjects have a very horizontal sacrum in standing position with a sacral slope angle sometimes much greater than 50° (anterior pelvic tilt, pelvic anteversion or pelvic flexion) (Fig. 6b).

The pelvis moves, rotating around the bicoxofemoral axis, leading to both anterior tilt (where the upper portion of the pelvis tips forward) and posterior tilt (upper portion of the pelvis tips backward). The variations in the angles of the sacral slope determine the range of this pelvic tilt, and its variations in turn influence the orientation of the anterior pelvic plane or the Lewinnek plane classically used as a reference for navigation of the acetabular components in THAs **(Fig. 4)** (Lewinnek et al., 1978). The surprising consequences of some spinal arthrodeses or stiffening on the hips and the difficulties of adjusting prostheses below stiffened spines illustrate the importance of these anatomic-

Fig. 4. The pelvis moves, rotating around the bicoxofemoral axis, the anterior tilt (where the upper portion of the pelvis tips forward) is observed in standing position and posterior tilt

Fig. 5 corresponds to a forward tilt of the pelvis as a whole. In this situation, the upper endplate of S1, viewed laterally, makes an angle of approximately 35° to 45° between the sacral slope and the horizontal (Lazennec et al., 2000; Lazennec et al., 2011c). Some subjects have a small sacral slope angle in standing position: we talk then about posterior pelvic tilt (or pelvic retroversion or pelvic extension) and the sacrum seen on a lateral image appears more vertical than usual (Fig. 6a). On the contrary, other subjects have a very horizontal sacrum in standing position with a sacral slope angle sometimes much greater than 50°

(upper portion of the pelvis tips backward) in seated position.

(anterior pelvic tilt, pelvic anteversion or pelvic flexion) (Fig. 6b).

**2.1 The standing position** 

functional parameters.

Fig. 5. The standing position **cor**responds to a forward tilt of the pelvis**:** the upper endplate of S1, viewed laterally, makes an angle of approximately 35° to 45 between the sacral slope and the horizontal

### **2.2 In the supine position**

When the lower limbs are extended, the sacral slope is often greater than in standing position. The measurement of its angle with the vertical often exceeds 45° (Lazennec et al., 2011a; Lazennec et al., 2004). This can be seen on scout views of pelvic CT scans. This additional pelvic tilt may not be well tolerated in the case of a stiff or deformed spine since the dorsal decubitus position deepens lumbar lordosis. Sometimes a limitation of the available extension of the hips or posterior osteoarthritis makes a strict decubitus position unbearable, at least without a slight compromise flexion of the coxofemoral joints.

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 75

any associated spinal disease, we observe pelvic posterior tilt (pelvic retroversion or pelvic extension) more or less accentuated with a sacrum more or less vertical. The difference in the values of the sacral slope between standing and seated positions makes it possible to define the available flexion associated with the lumbosacral joint (extrinsic available pelvic flexion) by opposition to the potential for flexion associated with the coxofemoral joint

Fig. 7. In sitting position, the pelvis tilts backwards. The sacral slope diminishes. The difference

Causes considerable modification in the orientation of the anterior pelvic plane, today considered the reference for adjusting the acetabular cups (D'Lima et al., 2000; Herrlin et al., 1988; Rousseau et al., 2009). These variations must concern us. Lewinnek's plane (the anterior pelvic plane) is evaluated in the dorsal decubitus position to collect morphologic data for planning the hip prosthesis (Lewinnek et al., 1978). The information obtained must extrapolated prudently because it is not necessarily vertical in standing positions, and its tilt is highly variable in sitting positions (Philippot et al., 2009; Rousseau et al., 2009). Accordingly, for mean values of 3° in standing positions, the values reported while sitting

in the values of the sacral slope between standing and seated positions is about 20°

**2.4 Progression from standing to sitting positions** 

were 17.5° (Lazennec et al., 2011c) **(Fig. 8).** 

(intrinsic available pelvic flexion).

Fig. 6. a: Some subjects have a small sacral slope angle in standing position: the sacrum appears more vertical than usual (posterior pelvic tilt , pelvic retroversion or pelvic extension), 6. b: Some subjects have a very horizontal sacrum in standing position with a greater sacral slope angle (anterior pelvic tilt, pelvic anteversion or pelvic flexion)

### **2.3 In a sitting position**

The phenomenon is inversed **(Fig. 7)**. The pelvis tilts backwards as it progresses toward a sitting position. The sacral slope diminishes, to values of 20° to 25° on average (Lazennec et al., 2011a; Lazennec et al., 2011c). This slope may still be slightly positive (by 5 to 10°) or even sometimes negative. As a function of the height of the seat, the subject's morphology or

a b

Fig. 6. a: Some subjects have a small sacral slope angle in standing position: the sacrum appears more vertical than usual (posterior pelvic tilt , pelvic retroversion or pelvic extension), 6. b: Some subjects have a very horizontal sacrum in standing position with a

The phenomenon is inversed **(Fig. 7)**. The pelvis tilts backwards as it progresses toward a sitting position. The sacral slope diminishes, to values of 20° to 25° on average (Lazennec et al., 2011a; Lazennec et al., 2011c). This slope may still be slightly positive (by 5 to 10°) or even sometimes negative. As a function of the height of the seat, the subject's morphology or

greater sacral slope angle (anterior pelvic tilt, pelvic anteversion or pelvic flexion)

**2.3 In a sitting position** 

any associated spinal disease, we observe pelvic posterior tilt (pelvic retroversion or pelvic extension) more or less accentuated with a sacrum more or less vertical. The difference in the values of the sacral slope between standing and seated positions makes it possible to define the available flexion associated with the lumbosacral joint (extrinsic available pelvic flexion) by opposition to the potential for flexion associated with the coxofemoral joint (intrinsic available pelvic flexion).

Fig. 7. In sitting position, the pelvis tilts backwards. The sacral slope diminishes. The difference in the values of the sacral slope between standing and seated positions is about 20°

### **2.4 Progression from standing to sitting positions**

Causes considerable modification in the orientation of the anterior pelvic plane, today considered the reference for adjusting the acetabular cups (D'Lima et al., 2000; Herrlin et al., 1988; Rousseau et al., 2009). These variations must concern us. Lewinnek's plane (the anterior pelvic plane) is evaluated in the dorsal decubitus position to collect morphologic data for planning the hip prosthesis (Lewinnek et al., 1978). The information obtained must extrapolated prudently because it is not necessarily vertical in standing positions, and its tilt is highly variable in sitting positions (Philippot et al., 2009; Rousseau et al., 2009). Accordingly, for mean values of 3° in standing positions, the values reported while sitting were 17.5° (Lazennec et al., 2011c) **(Fig. 8).** 

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 77

Fig. 9. AP and lateral images provide a global evaluation of the pelvis and cup orientations

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

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

in standing or seated positions

(Lazennec et al., 2007) **(Fig. 10)**.

Fig. 8. Progressio**n** from standing to sitting positions causes considerable modification in the orientation of the anterior pelvic plane (A.P.P. or Lewinnek plane)
