**1. Introduction**

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The sagittal balance of the trunk is considered to be a key point for spinal fusion and disc replacement. This balance is the manifestation of a postural strategy conditioned by anatomic and functional characteristics that can differ greatly from one person to another (Duval-Beaupere et al., 1992). The role of the pelvic area is evident for spinal surgeons in their planning and analyses; they pay special attention to the sacral slope, which is closely related to the pelvic tilt (Chanplakorn et al., 2011; Dorr et al., 1983; Jackson & McManus, 1994; Kobayashi et al., 2004; Murray, 1993; Vialle et al., 2005). Dubousset underlined this concept as early as 1984, in treating the pelvis as a "pelvic vertebra" (Dubousset, 1984).

Although the concept of spinopelvic balance is well accepted today (Lazennec et al., 2004), conventional imagery underestimates the influence of the coxofemoral joint and explains it inadequately. The EOS imaging system (Dubousset et al., 2008), by optimizing radiologic data in functional situations, is revolutionizing our understanding of hip-spine relations (Lazennec et al., 2011b).

These anatomic and functional considerations often remain foreign to hip surgeons, who focus on the bone landmarks of the pelvis for their navigation, individualized adjustments, and failure analyses (Woo & Morrey, 1982; Woolson & Rahimtoola, 1999). Although the hip is a highly mobile joint, surgical concepts are still based on the static AP view of the pelvis in standing or supine position to assess cup orientation, which is an essential risk factor for prosthesis instability and wear (Kennedy et al., 1998). Influenced by the classic anatomic culture of cross-sectional slices, they consider the CT scan a reference tool for the "horizontal" assessment of the coxofemoral joints (Ackland et al., 1986; Dorr et al., 1983; Kennedy et al., 1998; Murray, 1993; Seki et al., 1998; Wan et al., 2009). Nonetheless, as this chapter will show, lateral views of the hips and the evaluation of sitting position provide new information about "normal" hip function and THR failures.

The concept of cumulative anteversion between the acetabular cup and the femur is considered a key factor in the stability of the hip prosthesis. This concept, however, is based on the measurements taken by computed tomography in a supine position, which do not take into account the functional dimension of the problem.

Hip-Spine Relations: An Innovative Paradigm in THR Surgery 71

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;

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

Fig. 3. Pelvic incidence ( I ) is associated with the sacral slope ( S.S. ) and pelvic tilt (PT) in a

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

seated positions are less known, in particular, for patients with hip prostheses.

Roussouly et al., 2005) (**Fig. 2).**

geometric relation: I = SS + PT

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 the function of the hip joints.

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