Smith-Peterson-Type Osteotomy

Vincent J. Devlin

The Smith-Peterson-type osteotomy is a posterior wedge osteotomy centered between the vertebral pedicles. A posterior wedge is resected in such a manner that the axis of rotation is located at the anterior border of the intervertebral foramen at the posterior aspect of the disc space. Closure of the posterior wedge osteotomy is accompanied by opening of the anterior and middle spinal columns (Fig. 48–1). The Smith-Peterson-type osteotomy can be performed at a single spinal level or at multiple adjacent spinal levels. This type of osteotomy can be performed either through a previous posterior fusion mass or through a nonfused spinal region.

Goals of Surgical Treatment

1. Restoration of sagittal and coronal spinal balance in a patient with fixed spinal deformity

2. Achievement of solid arthrodesis

3. Rigid internal fixation to permit brace-free mobilization

4. Relief of axial and radicular pain

Diagnosis

Patients who are candidates for spinal osteotomy procedures present with varied symptoms that may include back pain, spinal fatigue, progressive spinal deformity, and the inability to stand erect with the knees fully extended. A comprehensive musculoskeletal examination includes consideration of the following questions:

1. Is the most severe spinal deformity located in the sagittal plane or coronal plane? Is a complex deformity involving multiple planes present?

2. Is the spinal deformity balanced or unbalanced? This is assessed by the relationship of the C7 plumb line to the sacrum in the coronal and sagittal planes.

3. Is the spinal deformity flexible or rigid? Are flexible nonfused spinal segments present above or below a previously fused spinal region?

4. What is the relationship of the shoulders and pelvis to the spinal deformity? Factors such as shoulder imbalance, pelvic obliquity, and hip flexion contractures require consideration when planning osteotomy procedures.

Radiographic Assessment

Appreciation of normal three-dimensional spinal alignment is essential when analyzing spinal radiographs. Spinal alignment is assessed on a global, regional, and segmental basis using standing posteroanterior (PA) and lateral radiographs taken on a 36-inch cassette (Fig. 48–2). Specialized radiographs including supine anteroposterior (AP) bending views, traction views, and hyperextension lateral radiographs are obtained as indicated.

1. Global assessment: In the coronal plane, a plumb line suspended from C7 on a PA radiograph will bisect the vertebra below and pass through the center of the sacrum in the absence of spinal deformity. Sagittal plane balance is assessed by suspending a plumb line from the center of C7. This global measurement is termed the sagittal vertical axis (SVA) and normally falls anterior to the thoracic spine, through the center of the L1 vertebral body, posterior to the lumbar spine and through S1.

2. Regional assessment: Cervical lordosis (occiput-C7) averages 40 degrees. In the thoracic region, normal kyphosis (T1-T12) ranges from 20 to 50 degrees with a tendency to increase slightly with age. The thoracolumbar junction is essentially straight and serves as the transition area between the relatively stiff kyphotic thoracic region and the relatively mobile lordotic lumbar region. Normal lumbar lordosis (L1-S1) ranges from 30 to 80 degrees with a mean lordosis of 60 degrees.

3. Segmental assessment: The majority of cervical lordosis occurs at the C1-C2 motion segment. The kyphosis in the thoracic spine usually starts at T1-T2 and gradually increases at each level toward the apex (T6-T7 disc). Below the thoracic apex, segmental kyphosis gradually decreases until the thoracolumbar junction is reached. The thoracolumbar junction is essentially straight. Lumbar lordosis generally begins at L1-L2 and gradually increases at each distal level toward the sacrum. The apex of lumbar lordosis is normally located at the L3-L4 disc. Normally two thirds of lumbar lordosis is located between L4 and S1 and one third between L1 and L3. Eighty percent of lumbar lordosis occurs through wedging of the lumbar intervertebral discs and 20 % is due to the lordotic shape of the vertebral bodies. It has been shown that the wedge shape of the lowest three discs is responsible for one half of the total lumbar lordosis.

Indications for Surgery

Patients with symptomatic fixed sagittal and/or coronal spinal deformity merit consideration for surgical treatment with spinal osteotomy. Conditions for which spinal osteotomies are most commonly indicated include:

1. Ankylosing spondylitis

2. Postsurgical flat-back syndrome

3. Iatrogenic spinal deformities arising after scoliosis fusion

4. Posttraumatic kyphotic deformity

5. Transition syndromes (proximal or distal) following degenerative lumbar spinal procedures

Contraindications

1. Spinal deformities that can be treated by less extensive procedures such as multiple anterior discectomies and fusion followed by posterior segmental spinal instrumentation and fusion.

2. Patients with severe degrees of fixed decompensated spinal deformities, in whom spinal balance would not be achieved despite multiple Smith-Peterson-type osteotomies. This situation may occur when (1) greater than 6 cm of fixed coronal imbalance exists; (2) a fixed upper thoracic curve and pelvic obliquity coexist; or (3) asymmetric length exists between the convex and concave sides of the spinal column. In these cases, a vertebral column resection procedure is considered.

Advantages

1. May be used to treat coexistent sagittal and coronal spinal deformity

2. May result in long harmonious sagittal curves if multiple osteotomies are performed over adjacent levels

Disadvantages

1. This osteotomy requires the anterior structures of the spine to be flexible enough to allow the osteotomy gap to completely close posteriorly. If the anterior disc spaces are narrow or have been previously fused, anterior surgery including discectomy and/or anterior osteotomy may be necessary prior to posterior osteotomy. In this situation, a pedicle subtraction osteotomy may be preferable and allow correction with a single posterior procedure.

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