Spinal Pedicle-Screw Fixation

Biomechanics

• Increased strength with:
  • Fit & fill of screw in isthmus of pedicle with increased screw diameter correlating favourably with pullout strength
  • Converging screws

• Patients with multiple spontaneous compression fractures are poor candidates for pedicle-screw based internal fixation because of poor bone-mineral density
• Body weight is a major determinant affecting structural survival of rods used for scoliosis correction

Technique of Safe Insertion (Funnel technique)

• Dorsal projection of pedicle localized
• 1cm-diameter section of cortical bone removed over top of pedicle with a burr or rongeur
• Cancellous bone within pedicle visualized & removed with curette until pedicle cortical wall felt & visualised, followed by going deeper into pedicle toward isthmus
• Kerrison rongeur used to remove cortical bone peripherally so that isthmus of pedicle can be seen
• Once isthmus of the pedicle is directly palpated, a small (2mm) pedicle probe is passed through isthmus into vertebral body
• Larger (5mm) probe then used to enlarge path through isthmus of pedicle
• Small pins placed into probed pedicles as radiographic markers
• AP & lateral images confirm pedicle path & length of screw to be used (depth of each pin measured after removal)
• Threads cut into pedicle with progressively larger taps until firm cortical purchase achieved to determine screw diameter
• Ball-tip probe used to feel pedicle wall viability in all directions
• Screw inserted into pedicle
• AP & lateral images confirm proper positioning after all of screws, rods, & connectors are inserted

 Supplemental Fixation

• Polymethymethacrylate may be utilised to improve fixation, esp in osteoporotic bone
• Bicortical purchase routinely utilised at 1^st sacral level but not at any other level

 Assessment of Fusion

• Radiographic demonstration of trabeculation across intertransverse (lateral) or interbody area to determine presence or absence of solid union of a spinal fusion

 Union Rate

• 90 to 95% with pedicle-screw-based posterolateral fusion alone, without cages, using only autograft obtained from laminectomy

Complications

• Nerve-root &/or cauda equina injury (5%)
• Dural penetration (4%)
• Deep infection (2%)
  • Prompt wound debridement & administration of antibiotics, with preservation of implant & subsequent delayed primary closure
• Screw breakage (5%)
• Screw pull-out & screw-connector disengagement
• Implant-related pain

 Indications

• Scoliosis
• Spondylolisthesis:
  • Vertebrectomy at 5^th lumbar level with reduction of 4^th lumbar onto 1^st sacral vertebra with use of single-level instrumentation & fusion may be used for spondyloptosis
• Spinal fracture:
  • 1 vertebra cephalad to the damaged vertebra to 1 vertebra caudal to it
• Lumbar degenerative disc disease
• Spinal osteotomy
• Spina bifida
• Neoplasms:
  • Post-total vertebrectomy or radical resection
• Lesions of cervical spine & cervicothoracic junction:
  • Traumatic & developmental lesions
• Spinopelvic trauma:
  • Traumatic spinopelvic disruption & vertical fractures of sacrum