Sagittal alignment
An overview of sagittal alignment and its key measurements.
Sagittal allignment
This describes the alignment of the skeletal system in the sagittal plane. Both the global and regional levels of sagittal alignment should be considered:
- Global alignment is the sagittal profile of the skeletal system: head, spine, pelvis, and legs.
- Regional alignment is the sagittal profile of the spine and pelvis.
It is also important to recognize the difference between sagittal alignment and sagittal balance:
- Alignment describes static, upright posture (standing).
- Balance implies dynamic upright motions (gait and other activities of daily living).
Alignment of the skeletal system in the sagittal plane can be affected by several factors, including pelvic anatomy, body mass, degeneration, instability, deformity, and previous surgeries. Normal alignment is patient-specific.
Sagittal alignment of the spine may be restored through increasing segmental lordosis. In OLIF procedures, a large lordotic cage is placed on the vertebral apophysis, which is the strongest part of vertebral body, minimizing the risk of graft subsidence.
Impact of sagittal alignment
Sagittal malalignment can cause abnormal posture and motion, including loss of horizontal gaze, center of gravity away from feet, loss of capacity to react to disturbances in balance, and increased energy expenditure leading to fatigue, disability, and pain. To compensate for the malalignment and restore horizontal gaze, patients often tilt their pelvis forward (increasing pelvic tilt) and flex their knees.
Sagittal alignment key concepts
Pelvic incidence (PI) is the +angle between the perpendicular line to the sacral plate at its midpoint and the line connecting this point to the biocoxofemoeral axis. It is a patient- specific, anatomical parameter related to the shape of the pelvis and is fixed from skeletal maturity. PI value is constant1 and not affected by patient position or age after maturity. It is typically about 40° to 60°.2,3
Pelvic tilt (PT) is "defined by the angle between the vertical line through the midpoint of the sacral plate" and the line connecting this point to biocoxofemoeral axis. It is a positional parameter representing the position of the pelvis relative to a vertical reference. PT value varies depending on the extent of pelvic retroversion (compensation) and is typically 5° to 20°.2,3
Sacral slope (SS) is the angle of the sacral plate in relation to horizontal line. It is a positional parameter indicating the slope of the sacral endplate (base of the spine) relative to a horizontal reference. The value of SS varies, changing inversely with pelvic tilt and is typically 30° to 50°.2,3
Assessment of sagittal alignment
Spinopelvic mismatch (PI-LL mismatch) provides an indicator for a patient’s optimal lumbar curvature and can be measured from standard lateral lumbar films including femoral heads. Sagittal alignment is considered optimum when spino-pelvic mismatch (PI-LL) is less than 9° (LL= PI +-9°).3,4
Pelvic Tilt (PT) is a spinopelvic parameter that represents the relationship of the femoral heads to the sacrum in sagittal plane. A spinal realignment surgery should aim to achieve PT of less than 20°.4
Sagittal vertical axis (SVA) is measured as the distance between the C7 plumb line and the posterior-superior corner of S15,6 and is optimum when less than 5 cm (SVA < 5 cm).5,6 SVA is commonly used to assess deformity. A 36" x-ray of the full thoracolumbar spine is used to measure SVA.
Both T1-Spinopelvic Incidence (T1-SPI) and SVA represent the truncal inclination in reference to the pelvis. Global spinal realignment should aim to achieve T1-SPA of less than 0° and SVA less than 50 mm.4
Clinical studies about sagittal alignment
MIN ET AL, J SPINAL DISORD TECH. 2008⁷
In this retrospective study, n=48 patients with a L4-L5 fusion and minimum year follow-up (mean follow-up of 44.6 months) were evaluated and divided into two groups based on the development of adjacent segment disease (ASD, as defined by radiographic criteria) at final follow-up. 62.5% (30/48) of patients were in Group 1 (developed ASD), and n=18 patients were in Group 2 (no ASD).
There were no statistically significant differences in sex, weight, original diagnosis, bone mineral density, preoperative segmental lordosis, preoperative lumbar lordosis (LL), and preoperative clinical outcome score between the groups. There was a significant difference in LL improvement between the groups (-5.8° in Group 1 vs. 0.6° in Group 2, p=0.0228). Patients who developed ASD had decreased LL postoperatively.
FARSHAD-AMACKER ET AL, E SPINE J. 2014⁸
In this retrospective cohort study, the MRI scans of n=90 subjects without spine surgery were analyzed at two different time intervals for the presence or absence of disc degeneration (DD) using the Pfirrmann grade classification.
The two MRIs had a minimum of four year intervals. 38% (34/90) of subjects had no DD progression compared to 62% (56/90) with DD progression. Subjects with no DD progression had a significantly higher lumbar lordosis than subjects with DD (49°vs. 43°, p=0.017). Authors concluded that lumbar hypolordosis could be predictive of progression of degenerative changes in the disc.
ROTHENFLUH ET AL, E SPINE J. 2014⁹
In this retrospective study, the radiographic parameters of n=45 patients with symptomatic adjacent segment disease (ASD) that required surgery were compared to n=39 matched controls who had no signs of symptomatic ASD upon last follow-up (mean follow-up of 84 months and minimum of five year follow-up).
Controls had similar preoperative grades of disc degeneration on MRI. The pelvic incidence (PI) and pelvic tilt (PT) were significantly higher in the ASD group (PI: 60.9° vs. 51.7°, p=0.001 and PT: 22.4° vs. 18.8°, p=0.012). The lumbar lordosis was significantly lower in the ASD group (48.1° vs. 53.8°, p=0.012).
1
Barrey C, Jund J, Noseda O, Roussouly P (2007) Sagittal balance of the pelvis-spine complex and lumbar degenerative diseases. A comparative study about 85 cases. Eur Spin J 16 (9): 1459-67.
2
Vaz G, Roussouly P, Berthonnaud E., Dimnet J (2002) Sagittal morphology and equilibrium of pelvis and spine. Eur Spine J 1(11):80- 87.
3
Schwab F, Lafage V, Patel A, Farcy JP (2009) Sagittal plane considerations and the pelvis in the adult patient. Spine 34 (17): 1828-33.
4
Schwab F, Patel A, Ungar B, Farcy JP, Lafage V (2010) Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery. Spine 35 (25):2224-31.
5
Jackson RP, McManus AC (1994). Radiographic analysis of sagittal plane alignment and balance in standing volunteers and patients with low back pain matched for age, sex, and size: a prospective controlled clinical study. Spine 19:1611-8.
6
Gelb D, Lenke L, Bridwell K, Blanke K, McEnery K (1995). An analysis of sagittal spinal alignment in 100 asymptomatic middle and older aged volunteers. Spine 20 (12): 1351-8.
7
Min, J. H., Jang, J. S., Jung, B., Lee, H. Y., Choi, W. C., Shim, C. S., Lee, S. H. The clinical characteristics and risk factors for the adjacent segment degeneration in instrumented lumbar fusion. J Spinal Disord Tech, 2008. 21(5): p. 305-9.
8
Farshad-Amacker, N. A., Hughes, A. P., Aichmair, A., Herzog, R. J., Farshad, M. Determinants of evolution of endplate and disc degeneration in the lumbar spine: a multifactorial perspective. Eur Spine J, 2014. 23(9): p. 1863-8.
9
Rothenfluh, D. A., Mueller, D. A., Rothenfluh, E., & Min, K. Pelvic incidence-lumbar lordosis mismatch predisposes to adjacent segment disease after lumbar spinal fusion. Eur Spine J, 2014. (Epub)