Christopher M. Bono

Description

Direct bone grafting and stabilization of spondylolytic (pars) lesions.

Key Principles

Unhealed fractures of the pars interarticularis, also known as isthmic spondylolysis, can be chronically painful despite appropriate nonoperative treatments. Although patients with slippage (i.e., isthmic spondylolisthesis) are usually treated with fusion, direct repair of the nonunion without intersegmental lumbar fusion may be effective in some cases of spondylolysis without spondylolisthesis. Several techniques have been described, which generally involve bone grafting (usually autogenous) of the defect followed by some form of stabilization of the posterior elements. Methods of stabilization have varied from wire loop fixation, to interfragmentary compression screw, to pedicle screw-hook constructs. Though a clear clinical advantage of one technique over another has yet to be demonstrated, biomechanical studies suggest that a pedicle screw-hook construct offers the greatest stability at the nonunion site. As a basic tenet of nonunited fracture treatment, greater stability is usually desirable for better healing.

Expectations

To decrease low back pain from an unhealed spondylolytic (pars) defect by maintaining motion and avoiding intervertebral fusion.

Indications

• Direct pars repair is indicated for those patients with a clearly identified, painful spondylolytic defect.
  
• Clinically, the patient should have low back pain that is localized to the lumbosacral (L5 pars is most common) or lower lumbar region (L4 pars is second most common) without lower extremity radiation.
  
• The neurologic exam should be normal.
  
• Symptoms are typically exacerbated by extension and relieved by flexion.
  
• Ideally, physical examination should reveal tenderness with palpation of the spinous process of the level in question, though paraspinal tenderness over the defects themselves is not uncommon.

Contraindications

Relative

• Grade I slips: It is often not desirable to perform a direct pars repair in patients with spondylolisthesis, though some have described good results in select patients with mild (grade I) slips.
  
• Mild to moderate disk degeneration

Absolute

• Slips of grade II or higher
  
• Advanced, painful disk degeneration (pain more with flexion than extension)

Special Considerations

• Flexion-extension radiographs can be obtained to rule out dynamic subluxation, which is another contraindication to the procedure.
  
• A magnetic resonance imaging (MRI) study should demonstrate minimal to no evidence of degeneration, desiccation, or height loss at the involved disk space.
  
• Temporary pain relief from an intralesional anesthetic injection may be a prognostic indicator of a good response to pars repair.

Special Instructions, Position, and Anesthesia

The patient is positioned prone on a radiolucent operating table. Using a lateral view, the proximal and distal extents of the incision are marked, with the former being the level of the cranial facet joint (usually L4-L5 for a planned L5 repair), and the latter being the superior aspect of the S1 lamina. In addition, a separate incision is marked over the iliac crest bone-graft harvest site. Alternatively, bone graft may be harvested through the same midline lumbar incision.

It is important to understand the anatomy of the two fragments of the affected vertebra. The superior fragment consists of the superior articular processes, which are connected to the pedicles and transverse processes by a small portion of the pars (above the lesion). The proximal fragment is in continuity with the vertebral body. The distal fragment consists of the majority of the pars interarticularis (distal to the lesion), the laminae, the spinous process, and the distal articular processes.

Difficulties Encountered

Adequately seating the hook at the lower border of the lamina.

Key Procedural Steps

General Technique and Bone Grafting