Radiographic Parameters of Adult Lumbar Scoliosis

Fig. 3.1

Schematic diagram for sagittal vertical axis (SVA)

The incorporation of pelvic parameters led to a fuller understanding of sagittal alignment and its contribution to quality of life outcomes. In 1998, Legaye and Duval-Beaupere et al. proposed pelvic incidence (PI), a measure quantifying the interface between the spine and the pelvis [4, 16]. Defined as the angle between the line from the femoral head axis to the midpoint of the superior S1 end plate and the line perpendicular to the S1 end plate, PI is morphologically unique to each individual and is independent of postural changes. PI, a fixed value, correlated well with LL; patients with a high PI were also likely to have a high LL. They postulated that a chain of interdependence existed between the pelvic and spinal parameters. Other parameters proposed by Legaye include sacral slope (SS), defined as the angle between the S1 end plate and the horizontal on a lateral standing x-ray, and pelvic tilt (PT), defined as the angle between the line from the mid-axis of the femoral heads to the midpoint of the superior S1 end plate and the vertical on a lateral standing x-ray (Fig. 3.2).

Fig. 3.2

Schematic diagrams for pelvic parameters

Attention to the pelvic parameters revealed the importance of pelvic compensation for sagittal malalignment. Earlier papers had characterized the effect of small, angular changes in posture around the hip axis on the SVA, but in the late 1990s and early 2000s, efforts were made to quantify this compensation [1, 12].

Pelvic Parameters and the Sagittal Plane

The high degree of patient-to-patient variability in spinal sagittal alignment complicates the study of pathologic malalignment. Roussouly et al., in 2005, published a classification system describing categories of lumbar lordosis in relation to curve apices and spinopelvic relationships in 160 normal subjects [25]. In addition to describing an association between PI and LL, they found a reciprocal relationship between the sacral slope and pelvic tilt and established the equation: SS + PT = PI. Relating spinal sagittal curves to pelvic parameters lends meaning to these measurements that otherwise vary so wildly as to make radiographic identification of pathology, in many cases, difficult if not impossible.

Spinopelvic alignment criteria have been shown to correlate with patient-reported outcomes. Previous studies sought to delineate, without success, a relationship between coronal deformity and clinical outcomes. However in the sagittal plane, Glassman et al. demonstrated that positive sagittal malalignment is predictive of poor clinical health status; their two studies revealed that symptom severity increased linearly with worsening positive sagittal malalignment and that restoring normal sagittal alignment improved clinical symptoms [7, 8].

The identification of sagittal alignment as a primary driver in adult scoliosis patient satisfaction, both pre- and post-op, set the stage for the establishment of the SRS-Schwab classification system, which has undergone several iterations since the early 2000s. Based originally on a prospective analysis of 95 patients, the initial study in 2002 identified L3 and L4 end plate obliquity in the frontal plane, lateral olisthesis, lumbar lordosis, and thoracolumbar kyphosis as radiographic parameters that correlated with increased pain [29]. This led to the first SRS-Schwab classification system, which grouped patients into three categories based on lumbar lordosis and L3 coronal obliquity. The system was then expanded; the curves were further characterized by their coronal deformity apex, degree of lordosis, and intervertebral subluxation. Coronal curve categories were prescriptive—different curve types demanded tailored surgical approaches—while the lordosis and subluxation modifiers stratified patients into clinical groups, with higher grades indicating worsening HRQOL.

The work of Glassman et al. led to the inclusion of a global sagittal balance modifier in later iterations [8]. Ultimately, outcome-driven criteria led to refining the SRS-Schwab classification system to include a coronal curve modifier and three sagittal alignment modifiers: PI-LL, SVA, and pelvic tilt (Fig. 3.3). The coronal modifier describes the coronal curve type: T for thoracic only, L for thoracolumbar or lumbar only curves, D for double curves (T and TL/L curves both >30°), and N for no coronal curves>30°. The three sagittal modifiers, stratifying patients by clinical symptomatology, were established based on HRQOL studies:

Fig. 3.3

SRS-Schwab classification for adult spinal deformity

PI-LL, calculated by subtracting the lumbar lordosis from pelvic incidence: 0 (non-pathologic) for PI-LL < 10°, + (moderate deformity) for PI-LL between 10° and 20°, and ++ (marked deformity) for PI-LL>20°

Global alignment, assessed by measuring the translational distance from the posterior superior S1 body to a plumbline dropped from the middle of the C7 vertebral body: 0 (non-pathologic) for SVA< 4 cm, + (moderate deformity) for SVA between 4 and 9.5 cm, and ++ (marked deformity) for SVA<9.5 cm

Pelvic tilt, measured as the angle between the line from the mid-axis of the femoral heads to the midpoint of the S1 plate and a vertical line: 0 (non-pathologic) < 20°, + (mild deformity) between 20° and 30°, and ++ (marked deformity) > 30°

The SRS-Schwab classification provides a framework for interpreting radiographic parameters by incorporating the current base of knowledge regarding sagittal alignment, spinopelvic parameters, and compensatory measures [27]. The classification has been validated using patient-reported outcomes for both operative and nonoperative patients [30, 31]. When combined with clinical judgment, the SRS-Schwab classification can guide treatment in adult scoliosis patients. Prospective studies have validated the classification in follow-up studies, relating improvement in SRS-Schwab classification with higher HRQOL scores [32].

Future Directions

Sagittal alignment and spinopelvic parameters have allowed surgeons to pursue evidence-based radiographic goals anchored in patient-reported outcomes. Still, complications persist, and outcomes are not perfect. Several parameters show promise with regard to predicting complications and patient dissatisfaction beyond those described by the SRS-Schwab classification. Patients with severe sagittal malalignment, unsurprisingly, have poorer outcomes than those with mild or moderate deformities. High preoperative PT and SVA have been specifically shown to increase the risk of poor surgical outcomes. Poor postoperative alignment is a common cause of patient dissatisfaction and low HRQOLs; careful and adequate planning is critical in providing the proper degree of sagittal correction tailored to each individual patient. Postsurgical reciprocal changes, e.g., alterations in TK after lumber realignment surgery, have been observed. Surgical planning will need to account for these changes, although they are currently still difficult to predict.

Staying true to the global nature of malalignment, concomitant cervical deformity is also not uncommon in adult thoracolumbar disease. 53% of thoracolumbar deformity patients have cervical deformity, either as a compensatory mechanism or as a primary disease process [33]. New cervical deformity has also been found in 48% of post-op patients, as has improvements in preoperative cervical deformity following thoracolumbar realignment [19, 20, 22, 34]. This is a logical extension of the chain of interdependence connecting the pelvis and thoracolumbar spine. Radiographic parameters to quantify and predict cervical deformity are currently being studied, including T1 angle, T1 spinopelvic inclination, C2-T1 SVA, and cervical lordosis. T1 spinopelvic inclination also correlates with HRQOL outcome scores in adult scoliosis patients [23, 26]. Caudal to the spinopelvic axis, studies are being directed at knee flexion, another compensatory mechanism with similar biomechanics to pelvic tilt.

Predicting outcomes from adult scoliosis surgery has proven difficult. Patients on either end of the disease spectrum tend to improve after surgery; it is those who fall between the extremes—the majority of patients—that have mixed results. Poor outcomes occur even after a successful sagittal realignment. This emphasizes the need for further studies to determine if there are radiographic parameters that can be further optimized to increase chances of obtaining good clinical results.

Conclusion

Radiographic parameters, clinically backed with patient-reported outcomes, are both useful in the baseline evaluation of and the treatment selection for adult spinal deformity patients. With the spinopelvic parameters and the SRS-Schwab classification in mind, a framework has been established to deliver a more personalized surgical approach, resulting in better clinical outcomes and greater patient satisfaction.

References

Berthonnaud E, Dimnet J, Roussouly P, Labelle H. Analysis of the sagittal balance of the spine and pelvis using shape and orientation parameters. J Spinal Disord Tech. 2005;18:40–7. Available: http://www.ncbi.nlm.nih.gov/pubmed/15687851.

Boulay C, Tardieu C, Hecquet J, Benaim C, Mouilleseaux B, Marty C, et al. Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard values and prediction of lordosis. Eur Spine J. 2006;15:415–22. Available: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3489325&tool=pmcentrez&rendertype=abstract. Accessed 18 Nov 2013.

Dubousset J. Three-dimensional analysis of the scoliotic deformity. In: Weinstien SL, editor. The pediatric spine: principles and practices, vol. 1994. New York: Raven Press; 1994. p. 479–96 .Available: http://scholar.google.com/scholar?hl=en&btnG=Search&q=intitle:Three-Dimensional+Analysis+of+the+Scoliotic+Deformity#0. Accessed 5 Dec 2014.

Duval-Beaupère G, Schmidt C, Cosson P. A barycentremetric study of the sagittal shape of spine and pelvis: the conditions required for an economic standing position. Ann Biomed Eng. 1992;20:451–62. Available: http://www.ncbi.nlm.nih.gov/pubmed/1510296. Accessed 17 Nov 2014.CrossRefPubMed

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