Sagittal Spinal Pelvic Alignment

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Key points

  • The spine and pelvis have robust compensatory mechanisms to achieve a globally well-aligned spine (center of mass over pelvis, horizontal eye gaze) to allow for minimum energy expenditure.

  • Pelvic incidence is a morphometric parameter and is both constant and dictates the amount of lumbar lordosis needed to achieve a globally aligned spine.

  • Pelvic tilt and sacral slope are compensatory mechanisms that reflect pelvic rotation in space (sagittal) and reflect efforts to maintain global alignment in

Cone of economy

The ability to maintain an upright posture and horizontal eye gaze is fundamental to normal activities of daily living. Spinal deformity creates suboptimal spinal alignment, and may result in increased energy requirements to maintain appropriate posture and balance. The spine is not the only structure that is involved in the standing axis, and any analysis that attempts to understand global alignment must involve the pelvis and lower extremities.9, 10, 11, 12, 13, 14, 15 Dubousset16 was the

Sagittal vertical axis

Global spinal alignment is most often assessed by determining the sagittal vertical axis (SVA). The SVA is determined by measuring the horizontal distance from the C7 plumb line and the posterior superior aspect of the S1 vertebral body (Fig. 2). Positive and negative values are defined depending whether the C7 plumb line falls anterior or posterior, respectively. Normative values have been established as being less then 5 cm. The C7 plumb serves as a surrogate for where the head falls in space

Spinal alignment

Global spinal alignment may be subdivided into its component parts, namely the thoracic kyphosis (TK) and lumbar lordosis (LL). LL is measured from the superior end plate of L1 to the superior end plate of S1 and plays an important role in maintenance of upright posture (see Fig. 2).14 Normative values of 40° to 60° have been described in the adult population; however, that number seems to decrease with aging. Decreasing lordosis or flatback deformity has been associated with inability to

The pelvis

The pelvis is the base of the spine. Its morphology determines the foundation on to which the spine is seated. Although the morphology is relatively constant in adulthood, the mobile spine may adapt to the sacral position, adjusting the degree of curvature to achieve a mechanically efficient posture.19 In order for the body to achieve an efficient upright posture, the spine must be in harmony with the pelvis. This harmony has been coined the spinopelvic alignment20 to describe the synergistic

PI

PI is defined as the angle subtended by a line drawn between the center of the femoral heads and the sacral end plate and a line drawn perpendicular to the sacral end plate (Fig. 3).21 This angle represents the sacral relationship to the acetabulum and assumes limited motion through the sacroiliac joints. PI normative values of 50 to 55 are typically found; however, the individual reported range may vary substantially from 28 to 84.6 PI is therefore a static morphologic parameter that is

PT

PT is defined as the angle created by a line from the midpoint of the sacral end plate to the center femoral heads and a vertical plumb line (see Fig. 3).4 This is a dynamic pelvic parameter, which can increase or decrease through rotation about the hip axis and can also change over time.4, 14 With age, the hip joints may become arthritic, and thereby lose range of motion, most notably the ability to recruit hip extension in the setting of spinal deformity. This situation may in turn limit the

SS

SS is defined as the angle created by a line drawn parallel to the end plate of the sacrum to a horizontal reference line (see Fig. 3).24 By performing simple geometry, PI is defined as the sum of PT and SS (PI = PT + SS).4, 6, 8, 9, 24 According to the formula, as PT increases (pelvic retroversion), the SS decreases and the sacral end plate becomes more horizontal.14

Spinopelvic alignment

Historically, the focus of surgical treatment of scoliosis has been on the coronal alignment and less on the sagittal parameters. However, several studies3, 6, 25 have shown that proper sagittal alignment is the single most important factor that determines the outcome for adults undergoing spinal deformity surgery. Patients with spinal deformity and undercorrected sagittal alignment with decreased LL have significantly worse HRQoL scores for physical and social function, self-image, and pain.3

Surgical planning

Understanding and measuring the global spinal parameters, as well as the regional specific parameters, allows the surgeon to plan more effectively when determining the amount of correction necessary to achieve a good outcome. A high PT indicates pelvic retroversion, which is caused by an attempt to compensate for decreased LL.14 Correction of deformity requires correction of not only the LL but must do so in the context of understanding the impact on PT. Failure to recognize an elevated PT

Summary

Many of the current sagittal plane radiographic parameters (SVA, TK, LL) do not account for compensatory mechanisms above and below the measured segments. Compensation may occur through the feet, knees, hips, pelvis, and cervical spine to restore horizontal eye gaze and maintain proper truncal posture. Excessive compensation leads to pain and disability. The pelvis plays a critical role in the maintenance of global spinal alignment. Restoration of spinopelvic harmony allows the spine and pelvis

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