Sagittal Spinopelvic Parameters

Historically, scoliotic deformities were evaluated and treated primarily as coronal-plane entities, although appreciation of the three-dimensional nature of scoliosis is increasing, most specifically in the sagittal plane. Additionally, recognition that the spinal axis is only the proximal link in the entire global chain of mechanical alignment of the human standing posture has led to significant work directed toward understanding the role of the pelvis in this alignment and how it relates to global sagittal standing balance. Further research is needed to define the axial-plane component of standing alignment and optimal, patient specific, three-dimensional spinopelvic alignment, although this section of the present chapter is aimed at providing an outline of the important sagittal spinopelvic parameters appreciated to date and the observations made during investigations of the asymptomatic “normal” population.

Sagittal Spinal Parameters

It can be understood without great intuition that the spine in the sagittal plane differs vastly from the “straight spine” in the coronal plane. The sagittal spine includes four curvatures, two “kyphotic” primary curves at the levels of the thoracic and sacral spine, respectively, and two secondary “lordotic” curvatures at the respective levels of the cervical and lumbar spine. Although specific regions of the spine may be labeled as kyphotic or lordotic, variability exists as to the nature of alignment at the individual vertebral levels, most notably at the junctional levels between regional curvatures.2

Radiographic analysis of the standing sagittal alignment in the asymptomatic population has demonstrated a broad range of normal values of thoracic kyphosis and lumbar lordosis. These values are listed in Table 6.1 and provide a basic guideline of normal ranges and what would be considered abnormal. Additionally, reports suggest that sagittal spinal curvatures and alignment vary with age. Cil et al3 conducted a radiographic analysis of the sagittal alignment of 151 asymptomatic children grouped by age (age range: 3 to 15 years). Significant differences in numerous parameters were identified among age groups. Older children stood with a more negative (backward) sagittal vertical axis (SVA). With an increased (positive) SVA in younger children, there is a greater L1 offset and more distal thoracic apex, resulting in a forward-leaning posture. With growth, the regional curvatures of both thoracic kyphosis (TK) and lumbar lordosis (LL) increase in angulation; the thoracic apex moves upward. As observed by Voutsinas and colleagues,4 and more recently by Mac-Thiong et al,5 TK and LL tend to increase during childhood, although a longitudinal study of normal subjects is required to fully validate these observations ( Table 6.1 ).

Sacropelvic Parameters

Since the work published by Legaye and Duval-Beaupére and coworkers,6 several studies5,7–12 have emphasized the importance of pelvic morphology with regard to sagittal alignment during standing in both children and adults, particularly through its effect on LL. Three main parameters are utilized to define the morphology and positional characteristics of the pelvis.6

Pelvic Incidence

Pelvic incidence (PI) is a morphological parameter described as the angulation joining the bicoxofemoral axis to the mid-sacral endplate and the perpendicular. PI has been suggested to remain set during adulthood, with a wide range of what are considered normal curves (40 to 65 degrees), although changes in PI during growth have been reported by several authors. Mangione and colleagues13 demonstrated that PI tends to undergo a linear increase during childhood after the initiation of walking. Descamps et al14 suggested that PI is relatively stable before the age of 10 years and then increases significantly until reaching its maximum at skeletal maturity. More recently Mac-Thiong et al5 in a prospective radiographic study including 180 asymptomatic children, found a significant positive correlation between age and PI. They hypothesized that an increasing PI during childhood was a necessary mechanism for maintaining an adequate sagittal alignment during growth.

Sacral Slope and Pelvic Tilt

Sacral slope (SS) and pelvic tilt (PT) are positional pelvic parameters that remain variable with changes in alignment, position, and posture. Significant variation has reported in the normative values of these parameters for adults (SS: 30 to 50 degrees, PT: 10 to 25 degrees). During adulthood, when PI remains stable, changes in one of these parameters negatively affects the other, such that PI = SS + PT. SS, the angulation of the sacral endplate with the horizontal, carries the strongest correlation with lumbar lordosis. A vertical SS is typically met with a large lordotic angulation, whereas the reverse holds true for lower values of SS. It has also been demonstrated that SS remains constant with growth, whereby SS through childhood does not differ significantly from that in childhood for a given individual.5 Conversely, PT describes the position (rotation) of the pelvis centered on the hip joint. PT has been found to increase during childhood with increases in PI.5 Positive changes in global spinal alignment typically lead to compensatory changes in PT; As the spine moves forward, increasing the SVA, PT increases (undergoes retroversion) to maintain ergonomic posture with spinal alignment over the pelvis.

Lafage et al15 conducted a recent investigation of pelvic parameters and their impact on measures of health-related quality of life (HRQOL). This prospective study involved 125 adult patients (mean age: 57 years) who had spinal deformities. Full-length radiographs of patients in the freestanding position, and including the spine and pelvis, were available for all patients. Instruments for measuring HRQOL included Oswestry disability index (ODI), Health Outcome Short Form-12 (SF-12), and the Scoliosis Research Society (SRS-22) questionnaire. A correlation analysis of radiographic spinopelvic parameters with measures of HRQOL did not reveal any significant associations pertaining to coronal-plane parameters. However. significant sagittal-plane correlations were identified. Following SVA and truncal inclination, PT was the next most highly correlated parameter with patient-reported measures of HRQOL (0.28< r <0.42) ( Fig. 6.1 ).