Spinal Alignment: Significance and Preoperative Planning

Clinical and Radiographic Evaluation

Any evaluation of a patient’s spinal complaint should begin with a detailed history and physical examination. An understanding of prior surgical procedures and a description of symptomatology are paramount to full comprehension of a patient’s clinical picture. Patients with sagittal imbalance often complain of the inability to stand upright, with worsening pain and fatigue after activity. Evaluation of the patient’s standing posture is critical, and compensatory mechanisms, such has hip extension and knee flexion, must be noted to accurately evaluate the severity of deformity. Full-length 36-inch anteroposterior and lateral scoliosis radiographs are necessary to understand the global alignment picture of a given patient. Once these views are obtained, radiographic parameters can be applied to the films to better understand alignment. Sagittal alignment is assessed by a vertical line drawn from the center of the C7 (C2 has recently been discussed as a better marker of overall spinal alignment) vertebral body down to the floor. This reference line is labeled the C7 plumb line or sagittal vertebral axis (SVA; see Fig. 63.2 ). Digital software exists that helps establish these values automatically. Sagittally-balanced patients demonstrate an SVA that passes through the posterior, superior corner of the S1 vertebral body. If this line falls either anteriorly or posteriorly, the sagittal balance is said to be positive or negative, respectively. An offset greater than 2.5 cm is considered abnormal. A C2 plumb line can be drawn from the centroid of C2 down vertically. This line should bisect the femoral heads in a sagittally-balanced patient. A T1-spinopelvic inclination (T1-SPA) angle defined as the angle between a vertical plumb line and a line from the center of T1 to the center of the bicoxofemoral axis can be used to assess global spinal balance. In realignment surgery, a T1-SPA goal of less than 0 degrees may be used.

Separate from global alignment, each region of the spine is also assessed for its contribution to the overall structure of the spine (thoracic kyphosis, T5–T12, average: 41 ± 12 degrees: LL, T12–S1, average: 60 ± 12 degrees).

Within the lumbar spine, approximately two-thirds of the total lordosis from the superior end plate of L1 to the inferior end plate of S1 is located from L4 to S1. A measurement of the lower arc of lordosis to the total arc of lordosis in the lumbar spine can be calculated as follows: L4‒S1/L1‒S1 × 100. This is defined as the lordosis distribution index (LDI). Average LDI values for asymptomatic individuals have been reported to range from 60% to 65%. ^, Use of LDI with relative LL as parameters for realignment surgery can reportedly improve mechanical complication rates and health-related quality of life (HRQOL) scores. To help further categorize adult spinal deformity, many authors have attempted to develop classification systems that help guide treatment. The Roussouly classification, first discussed in 2005, attempted to categorize patients based on PI into one of four types. A fifth type was later added, in addition to the likely natural history of how each curve degenerated based on a steady PI. Using his classification system as the goal for restoration, Roussouly was able to demonstrate that proper curve restoration, without overcorrection, is key to an ideal outcome.

Pelvic morphology and orientation are essential components of standing alignment. PI is an anatomic parameter that is both constant and specific to each individual. It is independent of spatial orientation of the pelvis. PI is defined as the angle between the perpendicular to the upper sacral end plate at its midpoint and the line connecting this point to the femoral head axis. PT is a compensatory mechanism that is often used to maintain upright posture in a patient with progressive sagittal plane deformity. It is defined by the angle between the vertical axis and the line through the midpoint of the sacral end plate to the femoral head axis. Finally, the SS is defined as the angle between the horizontal and the upper sacral end plate. As patients attempt to correct for positive sagittal plane imbalance, they will often retrovert the pelvis, thus increasing PT and decreasing SS while PI remains constant. Quantifying the degree of pelvic retroversion plays a key role in determining the amount of correction needed in surgical reconstruction.

As in any spinal pathology that involves neurological findings, magnetic resonance imagining (MRI) may reveal areas of compression. Additionally, computed tomography (CT) is often justified to evaluate areas of prior fusion or decompression. CT may be used to help determine whether prior instrumentation was placed with appropriate trajectory. Previous fusions should be evaluated, and any pseudoarthroses noted. Advanced degenerative changes on MRI or CT with evidence of ankylosis are associated with a more rigid deformity. On the CT scan, the presence of a vacuum effect within the disc space may indicate a mobile segment. One should take note of flexibility from standing x-rays to supine advanced imaging. Anterior mobile disc spaces often allow for some flexibility with positioning and for anterior column lengthening with posterior column–based osteotomies. Surgical correction and technique are critically dependent on differentiating rigid from flexible deformities.