Spinopelvic Parameters: Lumbar Lordosis, Pelvic Incidence, Pelvic Tilt, and Sacral Slope: What Does a Spine Surgeon Need to Know to Plan a Lumbar Deformity Correction?

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Key points

  • A thorough history and physical examination are essential to successfully treat patients with lumbar spinal deformity.

  • In younger patients without spinal deformity, lumbar lordosis (LL) should be within 11° of pelvic incidence (PI).

  • A normal pelvic tilt is less than 15°.

  • PI-LL mismatch of greater than 11° and a pelvic tilt of greater than 22° is strongly correlated with an Oswestry Disability Index score greater than 40.

  • Increasing pelvic tilt is a limited compensatory mechanism to maintain normal

Introduction and historical context

Observations of spinal deformity date to antiquity. Hippocrates described both the normal contours of the spine as well as deformities of the spine and their causes, grouping abnormal spinal alignments under the umbrella term scoliosis.1 Galen of Pergamum defined the terms kyphosis, scoliosis, and lordosis; their use continues as Galen described them to this day.1 In 1935 Bohler2 described compensatory mechanisms, including pelvic retroversion, for maintaining an upright posture in patients

Imaging

Preoperative evaluation of patients with adult spinal deformity begins with full-length standing 36-in posteroanterior and lateral radiographs (Fig. 1). From the lateral image, the physician then measures the patients’ LL, PI, sacral slope (SS), pelvic tilt (PT), as well as the sagittal vertical axis (SVA) (Fig. 2). The recent introduction of biplanar low-dose radiography (EOS images, EOS Imaging, Inc. Cambridge, MA) allows for improved visualization of the entire spine, skull, and lower

Lumbar lordosis

LL is defined as the sagittal Cobb angle measurement from the superior end plate of L1 to the sacral end plate (see Fig. 2). There is significant variability in what is considered normal LL, and it is more useful to think of the LL as patient specific rather than population based. Specifically, in the well-aligned spine, LL will match PI within 11°. However, as a general rule, between 20° and 70° of LL is considered normal on reaching maturity. Different patterns of lordosis have been reported.

Pelvic incidence

PI is equal to the sum of the PT and SS (PI = SS + PT). Geometrically, this is the angle formed between a line from the center of the femoral head to the midpoint of the sacral end plate and a line orthogonal to the sacral end plate (see Fig. 2). PI generally increases during skeletal growth but becomes a fixed, patient-specific quantity at skeletal maturity.13

Roussouly and colleagues14 described how a patients’ unique PI influences not only their LL but also their thoracolumbar thoracic and

Pelvic tilt

PT is the angle formed by a vertical line through the center of the femoral heads and the line from the center of the femoral axis and the midpoint of the sacral end plate (see Fig. 2). A normal PT is less than approximately 15°. In response to a loss of LL, patients will increase their PT via hip extension, which is variable and genetically determined, if possible, to achieve normal sagittal balance. Increasing PT is also termed pelvic retroversion. There are limits to the amount of PT that

Sacral slope

SS is defined as the angle formed between the horizontal and the sacral end plate (see Fig. 1). SS is not a fixed angle, rather it relies on the position of the pelvis relative to the hip axis. SS is inversely correlated with PT; as discussed previously, the sum of the SS and PT is the PI. The lateral radiograph of patients with sagittal imbalance will reveal a horizontal sacrum, increased PT, and loss of LL.

Like PT, lumbar spine surgery indirectly affects SS; however, unlike PT, there is no

What does a surgeon need to know to plan a deformity correction?

Although radiographic measurements of spinal deformity, such as PI, LL, and PT, are easy to measure and communicate, they leave out essential information when planning a deformity correction. When evaluating patients with lumbar spinal deformity, a surgeon must first take a history. A thorough understanding how pain interferes with activities of daily living versus activities of recreation allows the surgeon to better interpret self-reported scores, such as the ODI and SRS-22r.

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    Citation Excerpt :

    Lumbar fusion as an intervention for correcting lumbar deformity with associated symptomology has not only increased in volume over recent years but has also exhibited substantial advancement in achieving optimal spinal alignment and favorable outcomes.1-8 There exists a host of investigations exploring the use of corollary markers for spinal alignment and whether attaining “optimal” radiographic positioning correlates to an improvement in PROs.5-7,9-17 The latter research question has resulted in equivocal results to date.

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Disclosure Statement: Nothing to disclose (P. Celestre). Medronic: consulting, royalties; Norton Hospital: speaking, research funding; DePuy: consulting; Scoliosis Research Society: board member, Education Council Chair; Federation of Spine Associations: board member (J.R. Dimar). Medronic: intellectual property royalties, consulting; Scoliosis Research Society: board member (S.D. Glassman).

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