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Pelvic Incidence: The Great Biomechanical Effort

Diebo, Bassel G. MD; Lafage, Virginie PhD; Schwab, Frank MD

doi: 10.1097/BRS.0000000000001430
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Medical textbooks present the pelvis and the spine as distinct entities—an unfortunate practice that does not reflect the crucial and critical role that the pelvis plays in regulating spino-pelvic alignment. Researchers are working to delineate this role. Dubousset proposed the concept of the 3-dimensional pelvic vertebra, which suggested that the pelvis is just another caudal vertebra of the spine, and that analysis of the spine requires simultaneous analysis of pelvic morphology.1 To quantify pelvic morphology, Legaye introduced the pelvic incidence angle (PI) and espoused the theory that this angle regulates sagittal curvature of the spine.2 The PI is formed from 2 lines: line 1, perpendicular to the sacrum from the midline of the sacral plate, aims to quantify spatial orientation and dictate the lumbar curve; line 2, extending from the midline of the sacrum to the midpoint between femoral heads, illustrates the importance of sacral position inside the pelvis (SDC Figure 1,

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*Spine Service, Hospital for Special Surgery, New York, NY

NYU Hospital for Joint Diseases, New York, NY.

Address correspondence and reprint requests to Bassel G. Diebo, MD, Spine Service, Hospital for Special Surgery, 535 E. 70th Street, New York City, NY 10021; E-mail:

Received 4 January, 2016

Revised 5 January, 2016

Accepted 5 January, 2016

The manuscript submitted does not contain information about medical device(s)/drug(s).

No funds were received in support of this work.

No relevant financial activities outside the submitted work.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Website (

The PI, which increases significantly during childhood as the spine adapts to changing physiological needs, and stabilizes after adulthood, shows wide variation in normality. The PI dictates ideal sacral orientation and, therefore, ideal lumbar lordosis (LL): higher PI means greater ideal LL; lower PI means lesser ideal LL. Schwab et al 3 demonstrated better clinical outcomes of spinal realignment when LL was kept within 10° of the PI. With increased PI, sacral spatial orientation and acetabular position change. Thus, the surgeon cannot reconstruct the spine without analyzing the sagittal plane, starting with the PI (SDC Figure 2, The pelvis compensates for spinal malalignment by rotating around the femoral heads. Pelvic rotation is measured by pelvic tilt (PT), which shares line 2 with the PI, but PT is determined by the PI vector versus the vertical. PT influences the sacral slope; thus, clinicians use the morphologic PI when determining ideal LL.

The concept of pelvic translation was introduced by Lafage to explain involvement of the pelvis in spinal alignment.4 Anterior truncal shift leads to PT, and in advanced stages to knee flexion; both mechanisms lead to posterior shift of the pelvis with regard to the feet. This phenomenon aims to counteract anterior shift of the trunk to maintain the line of gravity passing between the feet. The global sagittal axis (GSA) is a simple measurement for quantifying PT, pelvic shift, and truncal inclination for quick assessment of full body sagittal alignment (Figure 1). Findings show good correlation of GSA with quality of life and other patient-reported outcomes.

Figure 1

Figure 1

Today's spine care provider knows that assessment of the spine must include a check of the sagittal plane and its foundation—the pelvis. The behavior of the pelvis is an important determinant of involvement of other joints in compensating for spinal pathologies.

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1. Dubousset J. Weinstein SL. Three-dimensional analysis of the scoliotic deformity. The Pediatric Spine: Principles and Practice. New York, NY: Raven Press; 1994. 479–496.
    2. Legaye J, Duval-Beaupère G, Hecquet J, et al. Pelvic incidence: a fundamental pelvic parameter for three-dimensional regulation of spinal sagittal curves. Eur Spine J 1998; 7:99–103.
    3. Schwab F, Patel A, Ungar B, et al. Adult spinal deformity—postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery. Spine (Phila Pa 1976) 2010; 35:2224–2231.
    4. Lafage V, Schwab FJ, Skalli W, et al. Standing balance and sagittal plane spinal deformity: analysis of spinopelvic and gravity line parameters. Spine (Phila Pa 1976) 2008; 33:1572–1578.

    pelvic incidence angle; pelvic tilt; pelvic translation

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