Sagittal Alignment

An understanding of normal sagittal parameters is helpful in assessing patients with sagittal imbalance. Numerous authors have reported on ranges of sagittal alignment in asymptomatic volunteers. Bernhart and Bridwell² studied 102 asymptomatic adolescent volunteers and described wide ranges of thoracic kyphosis (9 to 53 degrees, SD ± 10 degrees) and lumbar lordosis (−14 to −69 degrees, SD ± 12 degrees). The average age of patients in this study was 12.8 years. Stagnara and colleagues³ reported similar measurements in a group of 100 French volunteers aged 20 to 29 years. One hundred asymptomatic subjects and 100 patients with mechanical low back pain of slightly older age (mean age 39 years) were compared by Jackson and McManus.⁴ The differences were statistically significant as a result of the large study size; the absolute numbers were similar. The total lordosis averaged 60.9 degrees in the asymptomatic group versus 56.3 degrees in the symptomatic group. The authors reported that patients who were symptomatic were able to compensate for distal lumbar hypolordosis with exaggerated lordosis in the proximal lumbar spine. In general, all studies noted that approximately two thirds of the total lumbar lordosis is attained distally between L4 and the sacrum.

Standing sagittal alignment is typically measured from the middle of the C7 vertebral body on a 36-inch-long cassette radiograph. The spine is considered to be in neutral sagittal alignment when a gravity plumb line extending from the center of the C7 vertebral body bisects the S1 endplate. When the C7 plumb line passes anteriorly to the sacrum, the sagittal balance is considered positive; sagittal balance is negative when the C7 plumb line falls posteriorly to the sacrum.⁵ Recent studies have demonstrated that the C7 plumb line migrates more anteriorly with advancing age.⁶ This is due in large part to the loss of lumbar lordosis that occurs with disc degeneration.⁷ Pelvic parameters such as pelvic tilt and pelvic incidence and the relationship between the gravity line and heel position are additional methods to further the understanding of sagittal alignment and relationships with age.^8,9

Etiology

Sagittal imbalance can have a variety of causes. One of the most common causes is the prior placement of Harrington distraction instrumentation for the treatment of scoliosis.^10–12 A typical example would be a patient with a double major curve type pattern treated in the 1970s or 1980s with Harrington instrumentation extending from T4 to L3 or L4. If the spine was fused in proximal lumbar hypolordosis, maintenance of normal overall lumbar lordosis was often achieved with increased lordosis of the remaining normal discs—L3-4, L4-5, and L5-S1. If, over time, these discs degenerate, the patient will develop the progressive inability to compensate for the prior hypolordotic fusion and will develop sagittal imbalance. Moe and Denis¹³ have demonstrated the loss of lumbar lordosis after these procedures that results in the forward tilting of the spine. Patients who underwent these procedures 20 to 40 years ago are now becoming symptomatic.¹⁴

A second common scenario is termed degenerative sagittal imbalance.¹⁵ In this case, an adult patient has had numerous lumbar spinal fusion procedures, each with subsequent loss of lower lumbar lordosis. The incremental segmental changes gradually result in the patient’s inability to stand erect.

Another scenario involves the post-traumatic patient. An injury, typically a fracture at or around the thoracolumbar junction that heals in kyphosis, may present with a sharp, angular deformity.¹⁶ These deformities may be the sequelae of either operatively or nonoperatively treated unstable injuries. The patient typically complains of pain around the kyphos, as well as cephalad or caudad to the apex of the deformity. Compensation for the angular kyphotic deformity occurs at the suprajacent and subjacent disc spaces. With time, the discs can degenerate and lead to worsening of the deformity and increasing pain.

A fourth condition is the patient with ankylosing spondylitis. These patients will present with a lumbar hypolordosis and thoracolumbar kyphosis or cervicothoracic kyphosis.¹⁷ Patients with ankylosing spondylitis often have a stiff deformity and, as a result, are unable to compensate for it, leading to difficulties with forward gaze and mobility. Osteotomies in the lumbar spine or at the cervicothoracic junction are often necessary to correct the deformity.^18,19

Patient Evaluation

Physical Examination

The second aspect of the patient evaluation is the physical examination. Particular attention is paid to standing balance and gait. The overall postural balance in the coronal and sagittal planes and the pattern of gait should be examined. A patient with sagittal imbalance may often appear to have normal balance, only for the examiner to find that he or she is compensating with knee flexion. The apparent hip flexion is also often seen—in fact, femoral angulation relative to the vertical axis due to continued pelvic retroversion despite maximal hip extension. Lagrone and colleagues²¹ reported that almost 95% of patients with sagittal imbalance are unable to stand erect; nearly 90% had back pain with prolonged activity, and 27% had to flex their knees to stand erect. It is important to examine patients standing normally and with legs as straight as possible at the knee. The abdomen and back should be examined specifically for prior surgical incisions and postural creases and for a rotational deformity indicating a coronal spinal deformity. Examination of the patient in the supine and prone positions also helps to determine the flexibility of any spinal deformity, as evidenced by whether or not the kyphosis corrects with recumbent positioning. The supine position also eliminates the contributions of hip and knee flexion contractures to sagittal imbalance. Finally, a thorough neurologic examination is critical in all patients, but especially those who report deficits after prior surgery or whom upon history have suggestion of possible spinal cord level stenosis.

A thorough neurologic examination and a brief, directed musculoskeletal evaluation are also part of the complete physical examination. Signs of cervical or thoracic myelopathy such as a Hoffmann sign, inverted radial reflex, abdominal cutaneous reflexes, or a Babinski sign should be sought. Lumbar myotomes and dermatomes should be individually tested for any signs of radiculopathy. The patient’s hips and knees are also examined for contractures or the presence of significant symptomatic degenerative disease. The clinical outcome of sagittal imbalance correction is likely to be compromised in the presence of significant untreated hip pathology.

Radiographic Evaluation

Once the history and physical are complete, an assessment of the radiographic imaging is the next step in evaluation. Standing long cassette radiographs (14″ × 36″) are taken to include the entire spine from the base of the occiput to the distal sacrum and the femoral heads. The global spinal sagittal balance is determined using the C7 plumb line dropped vertically from the center of the body of C7. This may also be referred to as the sagittal vertical axis (SVA). The Scoliosis Research Society defines positive sagittal balance as the SVA passing anterior to the L5-S1 disc on the standing long cassette film. In general, any deviation of the SVA greater than 5 cm anterior to the normal position of the C7 plumb line is considered positive sagittal imbalance.¹⁴ Significant positive sagittal imbalance is defined as the C7 sagittal plumb line falling anterior to the femoral heads.

Other parameters that assist in the radiographic evaluation of these patients include measurements of the regional thoracic kyphosis (Cobb angle measured from T5-T12 on the lateral radiograph), lumbar lordosis (sagittal Cobb angle measured from the inferior endplate of T12 to the superior endplate of S1), thoracolumbar alignment (sagittal Cobb angle from T10-L2), and the pelvic incidence. The pelvic incidence assesses the relationship of the distal lumbar spine and sacrum to the pelvis and hip joints. The measurement is defined by the angle between the perpendicular to the sacral endplate and its midpoint and the line connecting this L point to the midpoint of a line connecting the femoral heads (Fig. 74–1).^22,23

FIGURE 74–1 Method of measuring pelvic incidence (PI). PI is the angle subtended by a line perpendicular from the cephalad endplate of S1 and a line connecting the center of the femoral head to the center of the cephalad endplate of S1.

(Reproduced with permission from Rose PS, Bridwell KH, Lenke LG, et al. Role of pelvic incidence, thoracic kyphosis, and patient factors on sagittal plane correction following pedicle subtraction osteotomy. Spine [Phila Pa 1976] 34:785-791, 2009.)

In addition to the long cassette standing radiographs, the authors routinely obtain focused studies of the lumbar and/or thoracic region. This helps in better defining local anatomy in a “coned-down” manner. Other useful studies include recumbent long cassette radiographs, either supine or prone. These radiographs help to elucidate the degree of flexibility within a deformity. The prone lateral view is similar to, but not exactly like, what the patient’s spine may look like on the operating table. Another useful radiograph is the supine hyperextension lateral, in which a bolster is placed under the apex of a kyphosis and, after some relaxation, a radiograph is taken to determine flexibility.²⁴

Surgical Decision Making

Not all patients with sagittal imbalance require surgery. In large part, this assessment is based on the combination of the patient’s symptoms, the findings from the physical examination, and the radiographic evaluation. If the patient is minimally to mildly symptomatic, surgery for sagittal imbalance may be unnecessary. Nonoperative management options include physical therapy, anti-inflammatory medications, and lifestyle modifications.

For those patients with debilitating symptoms due to significant sagittal imbalance, surgical management generally involves spinal instrumentation and fusion in combination with an osteotomy or osteotomies to allow the surgeon to restore a more normal spinal alignment. The types of osteotomies include the Smith-Petersen osteotomy (SPO), the pedicle subtraction osteotomy (PSO), and the vertebral column resection (VCR).^25,26 The key criterion for determining the necessity of an osteotomy is a partially or completely fixed deformity based on the physical examination and radiographic evaluation.²⁷ Spinal malalignment that corrects entirely with prone positioning (seen on the recumbent lateral radiograph) is flexible; in this case, osteotomies for spinal realignment are unnecessary.

Sagittal imbalance may be divided into two types, I and II.²⁵ Type I refers to a patient with a segmental problem, usually hyperkyphosis or limited lordosis. The segments above and below the problem can compensate for the local deformity, though, thus maintaining normal global sagittal spinal alignment. One example of a type I sagittal deformity is a patient with a post-traumatic kyphosis at the thoracolumbar junction with hyperlordosis of lumbar spine. Another example is a patient with Scheuermann kyphosis. Although the magnitude of the thoracic or thoracolumbar kyphosis can vary, and may be more than 100 degrees, the C7 plumb line remains normal.

A patient with type II sagittal imbalance cannot compensate for the segments of hypolordosis or hyperkyphosis and therefore have global sagittal imbalance. An example of this is a patient with ankylosing spondylitis with lumbar hypolordosis and thoracolumbar ankylosis. An additional example is a patient who has undergone Harrington instrumentation T4-L4 and subsequently degenerated the L4-5 and L5-S1 discs, resulting in progressive forward imbalance. A final common example is the adult patient who has undergone multiple lumbar fusion procedures and with each has lost an increasing amount of lumbar lordosis, ultimately resulting in grossly positive sagittal balance.

Smith-Petersen Osteotomy

The original description of the SPO was given by Smith-Petersen and colleagues²⁸ in 1945. It was described as a posterior wedge resection of the spine through the region of the facet joints in patients with ankylosing spondylitis with a subsequent controlled fracture of the ossified anterior longitudinal ligament. The ideal candidate for an SPO has a long, rounded kyphosis over multiple segments (e.g., a patient with Scheuermann kyphosis). The resection typically achieves about 10 degrees of correction per level, roughly correlating to one degree of lordosis for each millimeter of bone resection.²⁹ The osteotomy requires removal of the interspinous ligaments, the ligamentum flavum, and the superior and inferior articular processes bilaterally. One of the requirements for an SPO is a mobile disc anteriorly because this osteotomy involves resection of posterior elements only, followed by compression posteriorly (Fig. 74–2). Compression of the remaining posterior elements can be achieved through gravity from appropriate intraoperative positioning, hinging of the operating table, or gentle compression against pedicle screw fixation, although the latter is not advisable in osteoporotic patients.³⁰

FIGURE 74–2 A, Standing preoperative and -year postoperative coronal and sagittal radiographs of a 49-year-old female patient who presented having had Harrington fusion from T4 to L4 as an adolescent. She has now developed severe disc degeneration and spinal stenosis at L4-L5 and L5-S1. She has a mild to moderate sagittal imbalance. She was treated with decompression at L4-L5 and L5-S1 and extension of fusion to the sacrum. To restore sagittal balance, she also had two Smith-Petersen osteotomies (SPOs) performed through the prior fusion mass at L2-L3 and L3-L4 with local bone graft and biologics. B, Standing preoperative and -year postoperative coronal and sagittal clinical photos of the patient.

Pedicle Subtraction Osteotomy

The pedicle subtraction osteotomy is the “workhorse” procedure for correcting fixed sagittal imbalance. It allows for a three-column correction of the spine through an entirely posterior approach. More importantly, it does so without lengthening the spinal column.¹⁵ It is completed entirely through one vertebral body, typically in the lumbar spine (Figs. 74-3 and 74-4).

FIGURE 74–3 A, Standing preoperative and 2-year postoperative radiographs of a 48-year-old female patient who presented as an adolescent/young adult with a spondyloptosis at L5-S1. More than 20 years ago she was treated with a partial reduction of the spondyloptosis, fusion, and Harrington instrumentation from the distal thoracic spine to the sacrum. The Harrington implants were removed. She presented to us with dramatic sagittal imbalance and a solid fusion from the distal thoracic spine to the sacrum. She was treated with a pedicle subtraction osteotomy at L3. The most distal screws are transsacral screws from the sacrum into L5. B, Standing preoperative and postoperative coronal and sagittal clinical photos of the patient.

FIGURE 74–4 A, Standing preoperative and 5-year postoperative radiographs of a 69-year-old female patient with fixed degenerative sagittal imbalance. As a relatively young adult she had a fusion at L5-S1 without instrumentation. Subsequent to that she presented with degenerative spondylolisthesis at L3-L4 and L4-L5. She was treated with decompressions and pedicle screw implants L3 to L5. The fusion settled and healed in some kyphosis. The resultant kyphosis from L3 to the sacrum produced sagittal imbalance. Note the C7 plumb is substantially in front of the sacrum. She was treated at our institution with a pedicle subtraction osteotomy at L3 and extension of fusion to L2. B, Standing preoperative and 5-year postoperative sagittal clinical photos of the patient.

The procedure proceeds in the following manner¹⁶:

FIGURE 74–5 A, Surgical technique for pedicle subtraction osteotomy. (Left) The initial resection of the posterior elements and surrounding of the pedicles. (Right) decancellation of the pedicles and the vertebral body. B, (Left) Resection of the pedicles flush to the posterior vertebral body. (Right) Greensticking and resecting of the posterior vertebral cortex. C, (Left) Resection of the lateral walls. Note the V-shaped wedge. (Right) Central canal enlargement. Closure of the osteotomy and final instrumentation.

(Reproduced with permission from Bridwell KH, Lewis SJ, Rinella A, et al. Pedicle subtraction osteotomy for the treatment of fixed sagittal imbalance: Surgical technique. J Bone Joint Surg Am 86A:44-50, 2004.)

The authors recommend widely opening the canal centrally at the osteotomy site to be able to inspect the dura for buckling and to probe the neural foramina with a Woodson elevator to ensure the absence of any neural compression. If a fusion construct is extended distally to the sacrum, as would be the case for a PSO at L3 or below, the authors recommend the addition of iliac screws to protect against S1 pedicle screw pullout or failure.^31,32

A variation of the PSO that approaches the next osteotomy to be discussed is the asymmetric PSO. This can be performed to address sagittal imbalance with a coexistent coronal deformity. More bone along the lateral wall of the vertebral body is resected on the side of the convexity compared with the amount of bone resected along the concavity. This often involves removing the disc above and below the body along the convexity of the deformity. This allows not only for restoration of sagittal alignment but also for translation, which helps to restore coronal alignment (Fig. 74–6).

FIGURE 74–6 A, Standing preoperative and 4-year postoperative radiographs of a 45-year-old female patient who had had eight previous spine surgeries before presenting to us, starting with Harrington instrumentation to the midlumbar spine and then attempts at extension and decompression to the sacrum. She ultimately presented to our institution with nonunion at L5-S1 and fixed coronal and sagittal imbalance. She was treated with an asymmetric pedicle subtraction osteotomy at L2, which amounts to a standard pedicle subtraction procedure on the concavity with an extended pedicle subtraction procedure and, in essence, a vertebral column resection on the convexity. On closure, the coronal and sagittal imbalances were corrected. The nonunion at L5-S1 was treated with sacropelvic fixation and additional bone grafting with biologics. B, Standing preoperative and 4-year postoperative coronal and sagittal clinical photos of the patient.

Outcomes following this procedure have not been reported extensively. A recent study is one of the first to report 5- to 8-year outcomes in patients undergoing PSO. Kim and colleagues³³ reported on 35 consecutive patients undergoing PSO for sagittal imbalance, a follow-up from the initial 2-year study. Scoliosis Research Society outcomes remained similar in all domains at 5 years including very good patient satisfaction (87%), good self-image (76%), good function (69%), and fair pain subscales (66%) at latest follow-up.

Vertebral Column Resection

A vertebral column resection (VCR) is reserved for patients with fixed coronal and sagittal imbalance or complex severe deformities in the thoracolumbar spine.³⁴ It is a complex osteotomy that includes resection of the entire vertebral body and the intervertebral discs above and below. Deformities that may be appropriate for VCR include a sharp, angular kyphosis in the thoracic spine or a severe thoracic or thoracolumbar kyphoscoliosis. A hemivertebrectomy is a form of vertebral column resection, as the hemivertebra is resected with the discs above and below.³⁵ It is a powerful technique in deformity correction. Once completed, the spinal column can be shortened and the combined sagittal and coronal deformity corrected via a combination of translation and compression. An anterior structural cage is often necessary to bridge the defect left by resection of the vertebral body. Because this procedure circumferentially disconnects the spinal column, obtaining a fusion at this level is paramount.

The procedure involves resection of the posterior elements of the spine at the apex of the deformity. The laminae and facets proximal and distal to the level of the VCR are resected. In the thoracic spine, a costotransversectomy is necessary.²⁵ Disarticulation of the rib head affords access to the lateral vertebral body. Once the rib is removed, a temporary rod is placed opposite the working side to stabilize the spine and protect the neural elements. Sacrifice of exiting nerve roots in the thoracic spine is tolerated and makes exposure much easier for working entirely via the posterior approach. In the lumbar region, the nerve roots cannot be sacrificed and thus VCR may require separate anterior and posterior approaches. If possible, a malleable retractor is placed lateral to the vertebral body in the subperiosteal plane. The pedicle, vertebral body, and discs are then removed in piecemeal fashion. Osteotomy closure is accomplished slowly and steadily using two temporary rods.³⁶ It is usually necessary to place an anterior structural cage within the defect before complete closure in order to avoid shortening the spine excessively.

Complications

The surgical treatment of sagittal imbalance can result in significant complications. Osteotomies to restore sagittal alignment may be associated with significant intraoperative blood loss, lengthy operative times, and neurologic deficits. A decision is often made to stage such lengthy procedures. Suk and colleagues³⁶ reviewed 70 patients undergoing VCR for severe spinal deformities. The average intraoperative blood loss was 2333 mL. In this same series, two patients had complete spinal cord injuries, although neither patient was neurologically intact before surgery. Neither patient had evidence of compressive lesion on re-exploration. Buchowski and colleagues³⁷ reported on a consecutive series of 102 patients undergoing 108 PSOs. Twelve patients (11.1%) were found to have a neurologic deficit of two or more motor grades immediately following the procedure. For three patients (2.8%), the deficits were permanent. All three of these patients were able to ambulate. Neurophysiological monitoring did not detect any of the deficits intraoperatively. In all cases, the deficit occurred distal to or at the osteotomy site and was always unilateral. Factors involved include residual dorsal impingement, dural buckling, and osteotomy subluxation. The authors recommended careful closure of the osteotomy site, frequent attention to the neural elements via the central decompression, and an intraoperative wake-up test as neurophysiological monitoring did not detect any deficits.

At five years, Kim and colleagues³⁸ reported on 10 pseudarthroses in 8 patients undergoing PSO. None of the pseudarthroses were at the osteotomy site. Five of the pseudarthroses were discovered after 2 years. The patients tended to be older (>55 years) and have fusions that extended to the sacrum (6 of 8). Nine of the 10 pseudarthroses were at the thoracolumbar junction. The tenth was at L5-S1.

Another complication seen in patients undergoing surgery for sagittal realignment is an acute Fracture at or just above the Proximal aspect of the pedicle Screw Construct (termed FPSC). This appears to be a phenomenon associated more commonly with pedicle screw fixation. O’Leary and colleagues³⁹ reviewed a consecutive series of 13 patients at one institution who sustained acute fracture above the proximal aspect of long pedicle screw constructs (FPSCs). Ten out of 13 patients had significant preoperative positive sagittal imbalance, averaging 13 cm. On average, the SVA of these patients was 3 cm anterior to the posterior superior corner of the sacrum after surgery. When a matched cohort analysis was performed comparing patients with significant sagittal imbalance with and without FPSC, patients sustaining acute proximal junctional fractures tended to be older (average age 66) females, obese (BMI 32), and more osteopenic. A thorough radiographic analysis was completed for the cohort and the FPSC patients. After analyzing sagittal parameters including lumbar lordosis, thoracic kyphosis (T3-T12), sagittal balance (C7 plumb line), pelvic incidence, and a sagittal plumb line from the upper instrumented vertebrae (UIVpl), no differences could be determined between the FPSC group and the cohort group.

A potentially severe consequence of acute junctional fracture is neurologic deficit. There is a shear component to these fractures, and any subluxation or dislocation that causes acute stenosis may result in profound neurologic deficits. In the series mentioned previously, two patients developed acute neurologic deficits that were severe (defined as inability to ambulate). Both patients were treated with revision of fixation, decompression through a posterior approach, and extension of instrumentation and fusion proximally. Both patients have regained the ability to ambulate.

The authors recommend a thorough preoperative evaluation of patients with sagittal imbalance who are older, obese females. Bone mineral density data should be obtained preoperatively. In cases of significant osteopenia or osteoporosis, consideration should be given to appropriate elemental supplements and possibly to pharmacologic management with parathyroid hormone analogs or bisphosphonates before proceeding with major spinal reconstruction. Postoperatively, the authors recommend the use of a walker for 3 to 6 months following surgery, with particular attention to gait training and fall avoidance. No biomechanical data exist to suggest the optimal instrumentation construct for minimizing the risk of this complication.