Lumbar spinal fusion is used to treat a wide variety of spinal disorders, including degenerative spinal pathologic processes, degenerative conditions that result in segmental instability, spinal deformities that lead to alterations of normal sagittal and coronal plane alignment, instability due to pathologic bone destruction (tumors and trauma), and acute traumatic lesions. Lumbar fusions are often performed in concert with neural decompression procedures. The development of pedicle-based spinal instrumentation and its widespread use starting in the 1980s has led to significant improvement in fusion rates compared with in situ noninstrumented fusion. These devices make ideal spinal anchors for vertebral segments requiring laminectomy. In addition, pedicle screw instrumentation gives the surgeon segmental control of vertebral position in all three planes.
The determination of fusion levels is based on the underlying pathologic condition, the number and location of levels requiring decompression, and abnormalities in adjacent spinal segments and spinal regions.
Single-level pathologic changes that result in segmental instability, such as spondylolisthesis, can be managed by addressing the single segment. But lesions that involve multiple segments may require more extensive constructs covering multiple spinal segments ( Figure 19-1 ). Degenerative changes of the lumbar spine are often concentrated in the lower three or four segments. Such changes can include disk degeneration, spondylolisthesis, or postlaminectomy instability, among others. Certain pathologic conditions affect the entire lumbar spine. These include lumbar scoliosis and kyphosis. Multisegmental lesions typically require inclusion of all the pathologic segments. Short segmental fusions usually cover one or two segments. Three- to four-segment fusions are usually considered moderate in length. Fusions of five or more levels are defined as long posterior (lumbar) fusions. Fusions extending from L1 to S1 or even further proximally may occasionally be required to adequately address the pathology, provide enough implant fixation to ensure maintenance of spinal construct stability, correct spinal deformity, and address pathologic changes in adjacent segments.
A 65-year-old practicing dentist underwent a minimally invasive posterior lumbar interbody fusion (PLIF) procedure at L2 and L3 for radicular pain. The operating surgeon ignored adjacent-segment pathologic changes and did not recognize the severe regional and global sagittal plane imbalance that the patient’s history clearly described.
PMH: Generally healthy and active, with a long history of lumbar pain symptoms and neurogenic claudication associated with forward leaning posture
PSH:“Minimally invasive” L2-3 PLIF procedure that did not address adjacent segment compressive pathology, or positive sagittal balance
Exam: Forward-leaning posture secondary to lumbar hypo-lordosis; compensations to maintain vertical truncal position included hip/knee flexion position. Neural exam was normal, but history supported diagnosis of neurogenic claudication.
Imaging: Analysis of the patient’s standing anteroposterior (AP) and lateral radiographs revealed severe spinal stenosis at L1-2 and L3-4, with kyphotic disk degeneration at L4-5 and L5-S1. Sagittal vertical axis (SVA) imbalance was +10 cm ( Figure 19-2 , A and B ).
Further proximal surgery without addressing the lumbar spine alignment would result in a worsening of SVA and flat back syndrome symptoms. The surgical strategy must address the compressive pathology at L1-2 and L3-4, and the multi-level sagittal balance problem. This requires a multi-stage approach.
Stage 1/day 1: The patient underwent posterior laminectomy with prior instrument removal and Smith-Petersen osteotomies from L1 to L4, excluding the solidly fused level L2-3, with insertion of pedicle screws at L1 to S1 and bilateral iliac screws ( Figure 19-2 , D and E). Surgical time was 4 hours.
Stage 2/day 2: An anterior spinal osteotomy and fusion procedure at L3-L4, L4-L5, and L5-S1 was performed. Titanium mesh and carbon fiber implants were placed and autogenous and allogeneic bone graft and bone morphogenetic protein (BMP; 6 mg per level) were inserted via a retroperitoneal straight anterior approach to address segmental lumbar kyphosis. Surgical time was 3 hours.
Pathologic Conditions That Require Long Fusion and Instrumentation
Sagittal plane abnormalities. Sagittal plane imbalances can occur on a segmental, regional, or global level, resulting in serious problems in the individual’s ability to maintain an upright posture with an erect and vertical trunk. It has been clearly demonstrated that significant sagittal plane alterations require greater energy expenditure, result in lower quality-of-life scores, and are associated with an increased risk of spinal pain and malalignment (kyphosis)—flat back syndrome ( Figure 19-3 ). Patients with this latter condition frequently complain of a fatiguing spinal pain syndrome affecting the lumbar and thoracic regions, as well as buttock and thigh pain due to the recruitment of the hip extensors and knee extensors to compensate for the forward-leaning posture ( Figure 19-4 ). Patients experience a decrease in walking tolerance, but can perform sitting exercise programs with reasonable facility.
Coronal plane deformities. These deformities include many variants of scoliosis. The Scoliosis Research Society classification of adult spinal deformity acknowledges the following coronal plane deformities seen in adults: (1) de novo degenerative scoliosis; (2) adult idiopathic scoliosis, which is seen in a group of patients who had adolescent scoliosis in whom the lumbar curve demonstrates degenerative progression; (3) degenerative lumbar kyphosis, a condition that is seen more commonly in Asian countries but is increasingly being recognized in the United States; and (4) primary lumbosacral scoliosis, which results in severe coronal plane decompensation. Lower lumbar disk degeneration is the proximate cause of the latter, but this deformity may be associated with a preexisting congenital lumbosacral obliquity.
Combined sagittal and coronal plane deformities
Fixed sagittal or coronal plane deformities owing to the following:
Spondyloarthropathy with ankylosis
Previous fusion procedures
Multilevel lumbar spinal instability, which can involve three or more levels
Multilevel spondylolisthesis, usually combined with degenerative retrolisthesis or deformity
Biomechanical Differences between Short (Two or Fewer) and Long (Five or More) Spinal Constructs
The biomechanical demands on spinal implants, the bone-implant interface, and the adjacent spinal segments are affected by the number of lumbar segments included in the construct. Biomechanical and anatomic differences exist between the lumbosacropelvic junction, the midlumbar spine, and the thoracolumbar junction. These differences can explain the variation in fusion and pseudarthrosis rates and in implant failure rates reported for these different regions. Mechanical failures in the early postoperative period are most often due to a failure of the bone-implant interface. Although the adjacent segment is not actually considered part of the construct, it is exposed to increased forces that intensify as the length of the construct increases. This can lead to adjacent-segment failures such as vertebral body fracture or posterior ligament failure. Pseudarthrosis rates are directly related to the number of segments stabilized, and pseudarthrosis most commonly occurs at the lumbosacral junction. This can result in late instrument failure, usually around 6 months after surgery.
The surgical decision depends on the extent of the pathologic changes, the spinal alignment on standing spinal radiographs, and the findings on dynamic radiographs, which may include left- and right-bending radiographs, flexion-extension lateral radiographs, and traction radiographs. Careful attention to the patient’s hip and knee joints is warranted to assess for concomitant appendicular abnormalities that may impact the clinical and radiographic outcome and be a source of pain.
The clinical and radiographic analysis is augmented by neuroimaging studies. These should include magnetic resonance imaging (MRI) scans of the thoracic and lumbar spine and, if necessary, dynamic gravity-loaded myelography followed by computed tomographic (CT) scanning. The myelography may reveal pathologic changes not picked up on a supine MRI. The CT scan gives the surgeon a clear view of the posterior elements and the health of the facet joints. CT without myelography may also be useful for the latter purposes.
Not uncommonly, the pathologic changes are multisegmental but the primary abnormality involves the lumbosacral junction, and one may be able to concentrate on the lumbosacral defects without needing to address the more proximal pathologic elements. This situation is commonly seen in patients with a combination of L4-L5 degenerative spondylolisthesis and degenerative lumbar scoliosis. If the neural compressive syndromes are related to lumbosacral pathologic changes, one may address the lumbosacral segments primarily. (See case 1.)
It is not uncommon to see significant degeneration and coronal and/or sagittal deformity extend through the entire lumbar spine.