Flat Back and Sagittal Plane Deformity

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CHAPTER 288 Flat Back and Sagittal Plane Deformity

History, Etiology, Incidence, and Rate of Progression

Sagittal plane deformity as described by Doherty1 in 1973 is a fixed forward inclination of the trunk because of loss of normal lumbar lordosis after posterior spinal fusion for scoliosis. The term flat back syndrome is also known as kyphotic decompensation syndrome and flat buttock syndrome.2,3 There are other causes of sagittal plane deformity as defined by Errico and colleagues, such as ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis, Scheuermann’s disease, congenital kyphosis, postlaminectomy kyphosis, and kyphosis secondary to radiation therapy and trauma.4

Sagittal balance is present when a plumb line dropped from C2 or C7 on a lateral radiograph falls within approximately 2.5 to 4 cm of the posterosuperior corner of S1. Lines that fall more anterior are said to exhibit “positive” sagittal balance; those more posterior exhibit “negative” sagittal balance. In most clinical situations such as those discussed here (e.g., flat back deformity), the problem is positive sagittal balance.

Iatrogenic flat back syndrome is classified into two types. Type 1 is segmental (previous fusion levels) hypolordosis or kyphosis of the lumbar spine with the body of the C7 vertebral body remaining centered over the lumbosacral disk (i.e., a compensated deformity). A defining characteristic of type 1 on standing lateral radiographs is that anterior disk height is 5 mm greater than posterior disk height because of compensatory hyperextension to maintain sagittal balance. A noteworthy goal on postoperative radiographic assessment is to have the anterior disk height be reduced to less than 2 mm greater than posterior height on standing radiographs.5 Type 2 flat back syndrome is present when the plumb line from C7 falls more than 5 cm anterior to the lumbosacral disk.6

Clinical Findings

On clinical examination, patients literally look like they have a “flat back,” hence the term flat back deformity. With time, these patients progress and tilt more forward as their center of gravity is shifted anterior to the sacrum.

When evaluating a patient with a kyphotic deformity, it is important to determine which area is contributing to the sagittal imbalance. For instance, in ankylosing spondylitis it is sometimes difficult to establish whether the cervicothoracic junction, the thoracolumbar junction, the lumbar region, or the hips are contributing to the kyphotic deformity. This may be determined clinically by obtaining dynamic films and evaluating the patient standing, sitting, and lying supine. When sitting, if the deformity is not very pronounced, it is probably predominantly located in the thoracic and lumbar region. If the deformity appears to be severe on sitting films and the patient has difficulty looking up to the ceiling, it probably involves the cervicothoracic region. If the posterior aspect of the patient’s thigh does not touch the table when lying down, a hip flexion deformity may be associated.

Forward inclination of the trunk because of loss of lumbar lordosis and difficulty extending the knees when standing erect subject most patients to pain in the lower part of the back. (Patients with flat back deformity have pain in the lower part of their backs because of muscle fatigue resulting from forward inclination of the trunk secondary to loss of lumbar lordosis. This sagittally imbalanced posture results in the need for continual hip and knee flexion to maintain an upright stance.)

Lee and colleague7 and Hasday and associates8 discussed the significance of hip contractures in these patients leading to an abnormal pelvic tilt. This abnormal tilt increases the chance for a suboptimal postoperative result despite correction of the lordosis and should therefore be assessed preoperatively.

Accelerated degenerative changes from a chronically abnormal posture can also increase the incidence of radicular and claudication symptoms as a result of stenosis, so evidence for these concomitant problems must be assessed.

Positive sagittal balance is the most reliable predictor of clinical symptoms in patients with spinal deformity.9,10 Sagittal imbalance greater than 4 cm results in deterioration of pain and function scores over time in most unoperated patients. Restoration of normal sagittal balance should therefore be one of the main goals of any deformity reconstruction procedure.10

These patients also exhibit decreased step, stride length, and gait velocity. Sarwahi and coworkers prospectively analyzed the gait function of 21 patients with postsurgical flat back deformity.11 The authors concluded that the patients’ gait was slower than that of normal controls and that several compensatory mechanisms were used by the patients. The compensatory mechanisms adversely affected the hip and knee joints.11

Prevention of Flat Back and Sagittal Plane Deformity

A common cause of flat back syndrome is previous spinal fusion. Potter and associates mentioned four essential methods for prevention of this iatrogenic condition12: (1) thorough preoperative assessment of sagittal alignment, (2) limitation of the caudad extent of fusion when possible, (3) use of segmental instrumentation and avoidance of distraction with preservation or improvement of physiologic lumbar lordosis and sagittal balance, and (4) intraoperative positioning of the hips in an extended fashion.

Preoperatively, the surgeon must thoroughly evaluate the patient’s deformity and its overall impact. Multiple clinic visits are recommended to fully evaluate gait, pain levels, the severity of the deformity, and radiographic evidence. Maintaining the current normal curves of the patient while addressing correction of the deformity should be a leading consideration in formulating the surgical plan. For degenerative short-segment fusion in the lumbar spine, increasing lordosis in anticipation of loss of lordosis over time as a result of ongoing degenerative changes is a preferred strategy.13,14

Limitation of caudal fusion levels to L3 or lower to avoid decompensation or progression of the curve during scoliosis surgery and thus prevent flat back syndrome should be evaluated on a case-by-case basis. Although several studies have indicated benefit,1517 it must be noted that these studies were undertaken with nonsegmental and currently rarely used Harrington rod instrumentation.

Most early cases of flat back syndrome secondary to loss of lumbar lordosis arose from the use of distraction via Harrington instrumentation in the lumbar spine.1,1517 The evolution of posterior instrumentation to segmental pedicle screws from Harrington instrumentation and Luque segmental wiring has produced greater correction of curves with better construct rigidity and has thus decreased the incidence of sagittal plane deformity.12

Intraoperative positioning is vital in preventing sagittal plane deformity during long-segment spinal fusion. In a study of 13 anesthetized patients, Benfanti and Geissele demonstrated that 95% of lordosis was maintained on a Wilson frame when the patients were positioned with the hips in full extension.18 With the use of a Jackson table (our preferred choice for instrumented fusion), Stephens and colleagues proved that positioning patients in hip extension resulted in a minor increase in lumbar lordosis.19 It is therefore prudent to position patients for prone posterior lumbar fusion with the hips in extension to preserve physiologic lumbar lordosis.

Radiographic Findings

Global assessment of spinal balance is required in all patients with deformity, particularly if any surgical intervention is anticipated. This radiographic work-up begins with upright, full-length 36- by 14-inch posteroanterior and lateral scoliosis films to determine coronal and sagittal balance. This may be complemented with additional studies, including supine, flexion, extension, and side-bending radiographs, which can be used to better evaluate dynamic pathologies such as degenerative spondylolisthesis. The “clavicle position” should optimized to visualize the entire spine on the lateral scoliosis radiograph. In this position, the patient fully flexes the elbows with the hands in a relaxed fist, wrists flexed, and the proximal interphalangeal joints placed comfortably up into the supraclavicular fossa while passively flexing the humerus forward. This maneuver produces an overall visualization of critical vertebral landmarks that is significantly better than that achieved with positions in which the arm is either positioned straight out or partially flexed. Ideally, on the lateral radiograph one should be able to visualize C2 to the pelvis, including the femoral heads, to assess the global sagittal balance of the spinal column (Fig. 288-1A to D). Similarly, on the posteroanterior view, the margins of the rib cage and the pelvis along with the femoral heads can be clearly visualized. Assessment of the hips helps determine whether a leg length discrepancy, hip arthritis, or pelvic pathology is present. Visualization of the ribs helps in diagnosing the presence of any associated thoracic cage deformity. Either congenital fusion of the ribs or a significant chest wall deformity can be associated with rigid or fused spinal segments. After assessing spinal balance in the sagittal and coronal planes, Cobb’s measurements are obtained for each area of the spine, including the cervical, proximal thoracic, main thoracic, thoracolumbar, and lumbar areas. Vertebral body rotation at the apex of the coronal plane is a factor in determining the rigidity of the curve. The greater the vertebral body rotation, the greater the rigidity of the coronal curve. If the curve and clinical characteristics warrant serious consideration of surgical intervention, the flexibility of the curve can be further assessed with dynamic side-bending radiographs.

Sagittal balance is determined by examining the vertical axis with a line drawn through the middle of the C7 vertebral body and projecting inferiorly to intersect a horizontal line through the L5-S1 disk space. In a balanced spine, this line passes through the posterior third of the L5-S1 disk space, although a line up to 4 cm anterior to the L5-S1 disk space may be considered relatively normal in elderly patients. Computed tomography (CT) myelography is frequently used for the evaluation of adults with scoliosis when surgery is planned. The CT myelogram provides intimate details of bone anatomy and is helpful in identifying areas of lateral recess and far lateral stenosis. The greater the spinal deformity, degenerative changes, or number of previous surgeries, the greater the diagnostic value of the CT myelography. The bone anatomy will dictate the instrumentation options available for the patient.

Magnetic resonance imaging of the spine can provide additional detailed information about the neural elements, vasculature, and soft tissues, such as hydration of the disks. The degenerative status of the lower lumbar disks is a factor to be considered when deciding the lowest segment to be instrumented.

Operative Treatment of Sagittal Plane Deformity

The goal of surgery for correction of deformity is to achieve a stable, well-balanced spine centered over the pelvis by fusing as few motion segments as possible. A balanced spine is created by a close interplay of the patient’s spinal anatomy, the biomechanical properties of the spine, and the corrective capabilities of surgical techniques and instrumentation.

The goals of surgery are (1) to restore sagittal balance so that the patient can stand erect without having to flex the hips or knees and (2) to reduce pain. Frequently, patients who have previously undergone surgical procedures have multiple abnormalities that are contributing to both the pain and the sagittal imbalance. An error is to treat a portion of the global condition and neglect to fully address the sagittal imbalance. For instance, if a patient has pseudoarthrosis and significant sagittal imbalance, simply repairing the pseudarthrosis will not improve the patient’s posture and will rarely improve the pain. In this case, the lumbar pain is related in part to fatigue of the spinal extensor muscles and to coexistent pseudarthrosis.

A patient with decompensated sagittal balance stands with the knees flexed and the hips in extension to maintain an upright posture. The sagittal and coronal profiles demonstrate the true extent of the sagittal imbalance, pelvic obliquity, and coronal plane deformity. Any neurological deficit, truncal deformity, and muscle contractures should be carefully noted, as well as the patient’s overall posture. Previous surgical scars or a scar over the ilium may provide valuable information with regard to planning appropriate wound closure and the probability of autologous bone harvesting and help in the selection of instrumentation techniques.

Surgical Options

Lumbar pedicle subtraction osteotomy (PSO) has been used with increasing frequency for the management of either sagittal plane deformity or a combination of coronal and sagittal plane deformity. Treatment of fixed sagittal imbalance involves performing osteotomies to shorten the posterior elements of the spine or performing anterior column osteotomy to release or lengthen the anterior column. Surgical options include performing one or more Smith-Petersen osteotomy (SPO) procedures, one or more polysegmental osteotomy procedures, or PSO. Both SPO and polysegmental osteotomy do not require the surgeon to osteotomize the anterior column. Another viable option is to perform PSO, which usually achieves greater angular correction by removing a posterior wedge of corticocancellous bone from the vertebral body. This procedure is highly effective in restoring sagittal balance in patients with a fixed sagittal plane deformity.

Pedicle Subtraction Osteotomy

Pedicle resection plus transpedicular wedge resection of the vertebral body to restore sagittal balance was first reported in patients with ankylosing spondylolysis in 1985.20 Since Thomasen’s initial description, the procedure has been used for the management of flat back deformity and lumbar kyphosis secondary to other causes. PSO directly involves performing two Smith-Petersen–type (extension) osteotomies, as well as resection of the intervening pedicles and a portion of the vertebral body from a posterior approach. It accomplishes approximately as much correction as three SPO procedures. This aggressive osteotomy results in removal of the posterior elements, including the pedicle and transverse process. With this technique, removal of up to 6 cm of bone is possible with resultant sagittal plane correction of up to 60 degrees. By performing asymmetrical removal of the posterior elements, correction of both sagittal and coronal plane deformities can be achieved. PSO is a technically demanding procedure, and substantial blood loss can occur from the epidural venous plexus or from cancellous bone. With removal of the pedicle bilaterally, two nerve roots exit through the reconstructed neural foramina at the level of the osteotomy and are at risk for injury. During closure of the osteotomy, care must be taken so that impingement of the thecal sac or nerve roots does not occur.