Terminology

Definitions

• Discectomy or partial discectomy
  • Surgical removal of herniated portion of intervertebral disc
• Laminectomy
  • Removal of lamina to decompress spinal canal
    • Unilateral (hemilaminectomy) or bilateral
    • Partial removal of lamina and ligamentum flavum is technically laminotomy, although terminology sometimes used interchangeably
• Interbody fusion
  • Disc removal by posterior [posterior lumbar intervertebral fusion (PLIF)] or anterior [anterior lumbar intervertebral fusion (ALIF)] approach
  • Insertion of bone graft &/or fusion hardware
  • Goal is arthrodesis (fusion) across disc space
• Posterior instrumentation
  • Including pedicle screws, paraspinous rods, transverse rods, laminar hooks
    • Translaminar or facet screws can be inserted with minimally invasive techniques
• Posterolateral fusion
  • With severe loss of disc height, in lieu of interbody fusion
  • Lateral bone graft placement, fusion of transverse processes
  • Usually supplemented by posterior instrumentation
• Disc replacement
  • Variety of intervertebral hardware
  • Preserves motion across segment: Arthroplasty rather than arthrodesis
• Pseudarthorosis
  • Failure to obtain bony union after fusion
  • Ideally, fused segment will show
    • Mature bridging bone on radiographs, CT
    • Cold on bone scan 6-12 months postoperatively
    • Resolution of type I endplate marrow or conversion to type II
    • No motion on flexion/extension radiographs
  • However
    • Geometry of pseudarthorosis may be complex and difficult to appreciate on radiographs, CT
    • Bone scan performs poorly with significant false-negative and false-positive findings
      • Somewhat better performance at 12 months relative to 6 months postoperatively
    • Stability provided by instrumentation may prevent motion on flexion/extension films, despite pseudarthorosis
      • 2-3° motion may be present with fusion due to compliance of normal bone
    • Pedicle screw fracture can be seen with pseudarthorosis
      • Instrumentation temporizing mechanism until bony union can occur
• Malpositioned pedicle screw
  • Pedicle screw should transverse pedicle and be securely positioned within vertebral body
  • Malpositioning includes
    • Perforation of anterior cortex of vertebral body
    • Perforation of cortex of pedicle
      • ± compromise of intervertebral neural foramen or spinal canal
• Lucency around pedicle screw
  • Clear zones (≥ 1 mm) around pedicle screws may be encountered on postoperative radiographs
    • Traditionally, concerning for loosening or infection
  • 1 longitudinal series describes majority (2/3) of clear zones as resolving over several years
    • Persistence 2 years postoperatively predictive of pseudarthorosis
• Peridural fibrosis
  • Some degree of peridural fibrosis along margin of thecal sac is typical finding following discectomy
    • Edema and tissue disruption at discectomy site particularly conspicuous in first 6 weeks postoperatively
      • Can simulate disc residual/recurrent disc herniation
  • Scarring has been implicated with nerve root irritation in failed back surgery syndrome (FBSS)
• Postoperative fluid collection
  • Fluid collection in operative bed is common in immediate postoperative setting
    • May have complex signal with fluid-fluid levels
    • Will demonstrate peripheral enhancement
  • Can be difficult to differentiate from postoperative hematoma, pseudomeningocele, infected collection
  • Severity of compression by epidural fluid does not predict patient's neurologic status
    • Poor neurologic exam after decompressive surgery may require immediate return to operating room for exploration since immediate postoperative imaging not sufficiently sensitive to pathology
• Nerve root clumping, enhancement
  • Transient nerve root clumping can be identified in early postoperative period, often resolves spontaneously
  • Solitary nerve root enhancement may be seen perioperatively; inflammation related to compression, manipulation