Spinopelvic Parameters and Sagittal Balance
Preoperative attention to sagittal alignment is a clinical necessity in the longer stability constructs of degenerative deformity/scoliosis surgery. For shorter constructs in SFC surgery on the PDLS, the kinematics of junctional stress may provide less of an imperative for aggressive sagittal correction. Though adjacent segment disease (ASD) is a realistic concern in these cases, more precise indications for sagittal correction have not yet been established. And individual variances of other pertinent clinical factors may not allow a logrhythmic approach to care for this group of patients. But the degree of sagittal misalignment is an important factor in the over-all individual equation defining the need for, and extent of, sagittal correction.
Sagittal alignment parameters are skeletal measurements, taken in the standing position, usually describing some relationship between the spine and the pelvis. As true global alignment is in reference to whole-body balance around the feet (i.e. Dubousset’s “cone of economy”) these spinopelvic parameters have some limitation in defining direct gravitational stress on the lumbar spine.
The most common spinopelvic measurements are based in plumb-line variance. The sagittal vertical axis (SVA) for instance measures the distance between the plumb-line (vertical) from the centroid of C7 and the posterior/superior corner of S1. The accuracy of plumb-line measurements will be distorted when compensatory mechanisms are used to achieve a better balance of the upper body over the pelvis; the rotation of the hip on the femoral axis and knee-bending are the main such mechanisms. Other non-plumb-line spinopelvic parameters may be independent of such compensation. These may accurately reflect gravitational stress of the upper body on the lumbar spine and thus useful to the PDLS surgeon. The measured reciprocal difference between the pelvic incidence and lumbar lordosis (PI-LL) is commonly used to calculate sagittal correction needs. The T1 pelvic angle is another such parameter which obviates the need for compensatory measurement (e.g., pelvic tilt) when deciding on the need for surgical sagittal correction.
Keywords: cone of economy, gravity line, spinopelvic parameters, pelvic incidence, pelvic tilt, sacral slant, sagittal balance, sagittal vertical axis, T1pelvic angle, lumbar lordosis
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In major scoliosis surgery, recent data have demonstrated the significance of sagittal misalignment in lumbar pain/disability as well as a causative factor in kinematic and degenerative changes of the lumbar spine. 1, 2, 3 In the more limited and segmental PDLS surgery of symptom-focused care (SFC), sagittal balance evaluation may have relevance to surgical planning in certain scenarios. These pertain to cases where a short-segment stabilization procedure may be indicated as integral to symptomatic therapy and the potential/degree of postoperative junctional stress is assessed. There is evidence that the development of adjacent segment disease in short-segment stabilization has a strong correlation with regional spinopelvic imbalance. 4 Thus, evaluation of sagittal parameters in these instances may influence the extent of sagittal correction needed, and thus the techniques adjusted accordingly. There is also evidence that sagittal imbalance may have relevance to the outcome of multisegment decompression for stenosis. 5 (These considerations will be examined in more detail in Chapter 4.)
Specific indications of sagittal balance evaluation and correction, in these instances, need further elucidation. Furthermore, patient-specific modifiers will influence the surgeon’s evaluation as to the relevance of these considerations. However, it is important that the PDLS surgeon understands these metrics as, assuredly, they will be factorial in the surgical decision-making as relevant to outcome in certain patients.
3.1.1 Spinopelvic Parameters
The bony pelvis is a unified structure (minimal sacroiliac joint movement) and functions as the platform foundation on which the spine is based. Its rotary function on the femoral head is essential for use of hands at the ground level, for sitting, for re-erection to the upright position, and for bipedal ambulation. The pelvic structure has individual morphologic variation. Such variation has effect on the standing spinopelvic relationship (as well as with locomotion). Specifically, during growth/development, the platform morphology of the pelvis will have formative influence on the spine’s ultimate shape, as it responds to the constant force of gravity in maintaining a balanced position. 6, 7 As overall (global) balance is established to the feet, through the rotational fulcrum at the hips, the relationship between the pelvic platform and the femoral heads also has influence on the morphologic development of the spinal structure.
The pelvic incidence (PI) represents the fixed inclination of the axis of the sacrum relative to the pelvic acetabula. In adults, it is a morphological parameter inherent to each individual. It is measured as the angle formed by the line from the midfemoral axis (bicoxofemoral axis) to the center of the sacral end plate and the other line from this point perpendicular to the end plate. Thus, the greater this angle, the more inclined the sacral axis is relative to the overall pelvis, as represented by a greater slope of the sacral end plate (▶ Fig. 3.1). The slope of the S1 end plate may vary (slightly) from the perpendicular of the sacral axis, also having effect on the measured PI; a third factor in determining the PI, inherent to any given patient, is the location of the S1 end plate along the sacral axis. Thus, if the sacral end plate is higher and more anterior, the PI will be represented as a smaller angle, and the reverse if it is lower/posterior. With normal segmental anatomy, this end plate variation is negligible. However, in the situation of a transitional level with complete or partial sacralization of L5 (four lumbar vertebrae) or lumbarization of S1 (six lumbar vertebrae), the level of the end plate can create a significant variation in measured PI (▶ Fig. 3.2).
Lumbar lordosis (LL) is a relatively fixed parameter measured as the Cobb angle from the superior end plate of T12 (or L1) to that of S1. The relationship between the PI and the LL will establish a segmental sagittal alignment from T12 to the acetabula. The PI and the LL would be close to equal to establish T12 neutrally above the acetabula (normal variance, ± 10 degrees). It should be noted that, normally, the LL is predicted by the PI. At low PI (30–40 degrees = flatter sacral end plate), the normal curve at the L4 and L5 motion segments provides adequate lordosis, with either a neutral or a kyphotic upper lumbar curve. As the PI increases, the higher lumbar motion segments are progressively recruited, with the consequence of moving the apex of the lordotic curve cranially. In effect, the L1 and L2 motion segments crucially fine-tune the LL according to the PI. The understanding of this relationship may have consequence in segmental reconstruction 8 (▶ Fig. 3.3).
The sacral slant (SS) refers to the angle formed by the sacral end plate and the horizontal. It is unfixed in that it varies as to any rotation of the pelvis over the femoral heads (▶ Fig. 3.4).
The pelvic tilt (PT) measures the rotation of the pelvis at the hip. Thus, anterior rotation is the primary mechanism of movement in bending forward. However, posterior rotation (retroversion) has a much more limited range. It is represented by a hip extension. The PT measures this rotation as the angle formed by the anterior PI line (femoral heads to central sacral end plate) and a vertical plumb line. Hence, as the pelvis rotates anteriorly, this PI line will rotate similarly relative to the fixed plumb line and the PT will become smaller in degree (and becomes a negative value when it passes the plumb line). In retroversion, PT becomes larger. The SS necessarily varies inversely with PT in pelvic rotation: with anterior rotation (PT decreasing), the sacral end plate inclines more vertically (relative to the horizontal) and thus SS increases. In retroversion, this angle lessens as PT increases (▶ Fig. 3.5).
PI = SS + PT: As PI is a fixed value for any individual, the inverse relationship between PT and SS establishes this equation. Thus, a major compensatory mechanism (PT) is established as the measureable difference between PI and SS (PI – SS = PT). An elevated PT (either directly measured or calculated) is a sign of sagittal imbalance compensation by pelvic restoration.
Fig. 3.1 Pelvic incidence (PI): a fixed individual angle describing inclination of sacral axis within skeletal pelvis.
Fig. 3.2 PI is variable individually dependent on level of sacral end plate.
Fig. 3.3 Lumbar lordosis.
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