The goal of any ambulatory patient is to maintain a horizontal gaze with the least amount of energy expenditure. With progressive deformity, and in particular sagittal malalignment, significant compensatory mechanisms must be used to achieve this goal. Each pelvis dictates the amount of lumbar lordosis required through its morphometric parameter pelvic incidence. The pelvis may compensate for decreasing lumbar lordosis (eg, age, flat back deformity) by retroverting and increasing pelvic tilt and decreasing the sacral slope. Underappreciation for these spinopelvic compensatory mechanisms leads to surgical under-correction, iatrogenic flat back and poor clinical outcomes.
Key points
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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.
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Pelvic incidence is a morphometric parameter and is both constant and dictates the amount of lumbar lordosis needed to achieve a globally aligned spine.
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Pelvic tilt and sacral slope are compensatory mechanisms that reflect pelvic rotation in space (sagittal) and reflect efforts to maintain global alignment in the setting of spinal deformity.
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Pelvic parameters play a critical role in heath-related quality of life outcome measures and must be accounted for when planning spinal surgery.
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Underappreciation of the compensatory mechanisms in the pelvis may lead surgeons to poor surgical planning and corresponding poor clinical outcomes.
Introduction
Adult spinal deformity is a broad category that encompasses a diverse group of spinal malalignment patterns. The range of diseases extends from simple biplanar deformity to a more complex three-dimensional deformity with significant loss of coronal and sagittal alignment. With an aging population, the prevalence of adult spinal deformity is increasing in the United States and may be approaching rates as high as 70% among elderly individuals. Although most patients are able to tolerate mild to moderate deformities without significant dysfunction, there is a subset of patients who experience significant back pain, root compression, and loss of acceptable standing global alignment. Whereas historically, treatment focused on scoliosis correction and coronal plane deformity, more recent data have shown the impact of the sagittal plane, and lumbopelvic parameters on health-related quality of life (HRQoL). These parameters are therefore critical to understand and integrate for successful surgical planning in the setting of adult spinal deformity.
Introduction
Adult spinal deformity is a broad category that encompasses a diverse group of spinal malalignment patterns. The range of diseases extends from simple biplanar deformity to a more complex three-dimensional deformity with significant loss of coronal and sagittal alignment. With an aging population, the prevalence of adult spinal deformity is increasing in the United States and may be approaching rates as high as 70% among elderly individuals. Although most patients are able to tolerate mild to moderate deformities without significant dysfunction, there is a subset of patients who experience significant back pain, root compression, and loss of acceptable standing global alignment. Whereas historically, treatment focused on scoliosis correction and coronal plane deformity, more recent data have shown the impact of the sagittal plane, and lumbopelvic parameters on health-related quality of life (HRQoL). These parameters are therefore critical to understand and integrate for successful surgical planning in the setting of adult spinal deformity.
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. Dubousset was the first surgeon to consider the pelvis as a critical structure in the setting of global alignment and introduced the concept of a cone of economy ( Fig. 1 ). The cone is defined as originating from the feet and projected upwards to define the range of mobility that the body can achieve with a minimum of energy expenditure, without external support. If the spinal alignment is ideal, the patient is able to maintain an upright posture in the zone of economy and allow for painless upright posture at rest and during motion. If the body is forced out of this cone (spinal deformity), the energy expenditure increases with the need for compensatory mechanisms (ie, bending of the knee, or pelvic retroversion). The patient may require assistive devices if internal compensation is insufficient or requires excessive energy. The degree of global alignment can be determined by radiographic parameters that assess the patient in a static position. Such an approach allows the physician to determine where the trunk is in relation to the pelvis and to determine some of the compensatory mechanisms that are being used. For optimal radiographic analysis, it is essential that patients stand upright in a free-standing position and then 91.44-cm (36-in) cassette films are obtained in both the coronal and sagittal planes.
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 but ignores the cervical alignment. Similarly, the S1 vertebral body serves as a surrogate for where the feet are in space but ignores the contribution of the foot, knees, hips, and pelvis. Although this global assessment gives the surgeon a general idea of the global sagittal deformity, it underappreciates the role of the pelvis and compensatory mechanisms used.
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 ). 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 maintain spinal balance, and resultant pain and disability. In addition, TK can be determined by measuring the angle of the thoracic spine from the superior vertebral end plate of T2 and the inferior end plate of T12. TK also increases with age and influences global spinal alignment.
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. 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 alignment to describe the synergistic relationship between pelvic morphology and spinal curvature. To understand the spinopelvic alignment, it is critical to understand the static morphometric pelvic parameters (pelvic incidence [PI]) and the dynamic parameters (sacral slope [SS], pelvic tilt [PT]), and how these interact with spinal regional curvatures (LL, TK).
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 ). 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. PI is therefore a static morphologic parameter that is consistent throughout a patient’s life time, with only slight changes that occur during growth. After this time, PI is a fixed parameter (minimal sacroiliac motion), which reliably reproduces the relationship of the sacrum to the pelvis. The constant PI has a profound effect on the spinal parameters. With a large PI, the sacrum is more vertical and thus requires a larger LL to maintain proper global sagittal alignment of the trunk. Conversely, with a low PI, the more horizontal sacrum requires a smaller LL to achieve a balanced posture. Schwab and colleagues reported on the role of PI in determining the degree of LL and developed a formula, based on the work of Boulay and colleagues, in which ideal LL = PI + 9° (±9°).
Related posts:
Clinical and Radiographic Evaluation of the Adult Spinal Deformity Patient
Spinal Deformity Surgery
Coronal Realignment and Reduction Techniques and Complication Avoidance
Spinal Osteotomies for Rigid Deformities
Treatment Algorithms and Protocol Practice in High-Risk Spine Surgery
Management of High-Grade Spondylolisthesis
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