Lumbar Fusion and Instrumentation, Posterior

Common Preoperative Diagnosis

Lumbar segmental instability

Spondylolisthesis - anterior displacement of the vertebra - may cause back pain, stiffness, and tightening of hamstrings 

Iatrogenic lumbar instability - caused by outside influence

Lumbar disc disease

Spondylolysis - defect of vertebra

Mechanical back pain syndrome

Patients age 15 – 85 yrs., male: female 3:2

Various Approaches

Pedicle Screw Stabilization

Provides rigid three-column spinal fixation - the preferred instrumentation mode in lumbar spinal surgery. Pedicle screws are passed after tapping the entry site, fixed with rods or plates on each side of each vertebral segment.

A major risk is screw malposition and nerve-root injury

Pedicle screws may be combined with hooks to provide fixation of the lumbar/thoracolumbar spine, which improves stability and minimizes the risk of instrumentation failure

Posterolateral Fusion (PLF)

Posterior midline incision with the paraspinal muscles retracted. Decompressive laminectomy and discectomy as needed. Posterolateral fusion is performed by decorticating (removing the outer layer), the facet joints and transverse processes. The bone graft is then placed over the decorticated bone. Instrumentation with pedicle screws and plate/rod is often done for stability and to facilitate fusion

Posterior Lumbar Interbody Fusion (PLIF)

Bilateral laminectomy and removal of the inferior facet and the medial portion of the superior facet. The facet joints provide stability to the spine, allow the spine to bend and rotate, and keep the back from slipping forward. There is a thin layer of cartilage between the two joint surfaces to provide smooth and efficient movements.

The dural sac is retracted, and total discectomy with removal of cartilaginous end plates is performed. The anterior disk space is packed with bone graft. Bone graft is inserted into the posterior half of the disc space on both sides to provide structural support close to the center of rotation. Monitor nerve roots closely. Instrumentation with pedicle screws and a rod/plate is often added to facilitate early fusion and ambulation while preventing the extrusion of the graft. This procedure enables the achievement of combined anterior and posterior spinal fusion while avoiding the significant morbidity often associated with anterior lumbar surgery. The disadvantage is the risk of nerve root injury and compromise of facet joint integrity.

Preoperative Considerations

Mortality

0-5%

Morbidity

• Nonunion - 20-30%
• Nerve-root injury
• CSF leak
• Infection
• Significant blood loss

Pain Score

6-10

Surgical Time

2 hrs for single level + 0.5 hr for each additional level

Antibiotics

Cefazolin weight-based

Position

Prone for the posterior approach

This patient category may be difficult to position due to their spine issues

Will have to individualize and improvise to provide the best possible support and avoid pressure point issues

It may require a longer time for positioning

Prone Position

Preop assessment of head, neck, shoulder, and arm mobility

Heart ok as long as avoiding occlusive pressure on the inferior vena cava and femoral veins

Increased SVR, and PVR leading to decreased SV/CO/CI

Abdominal pressure may impede venous return due to compression of the inferior vena cava and aorta and increase surgical bleeding during spine surgery due to engorgement of epidural veins

Increased zone 3 a > v > A  

Decreased TLC, and reduced compliance leading to increased work of breathing

Increased PIP to achieve adequate ventilation as the abdomen pushes up. Monitor for barotrauma

CNS damage is high risk - keep the neck in neutral

If head below heart level = blood/cerebrospinal fluid accumulates (remember no valves, gravitational flow) = increased ICP, decreased CPP

Risk of post-op blindness - lubricate pad eyes with op-site, gauze, and shield. Check eyes every 15 minutes during the case to ensure no pressure

LR max of 40ml/kg for total surgery to possibly minimize postop blindness from ischemic optic neuropathy; if additional fluid is needed, give hetastarch 6% up to 20ml/kg, albumin 5%, or blood.

Pressure Points to be Padded

• Torso typically supported on a frame or with rolls from shoulder to iliac crest/crosswise at pelvis and shoulders
• Lower legs supported with pillows
• UE either tucked or on arm boards with arms flexed at shoulder and elbow (know preop issues with joint mobility)
• The face must be padded and secured
  • Be careful with forehead, eyes, cheeks, and chin
• Pad pressure points at elbows, knees, ankles, and genitalia
  • Ensure limited pressure on nipples

Preoperative Anesthetic Considerations

Neurologic

Patients may present with motor weakness, sensory deficits, or bowel/bladder dysfunction

Make sure you document any issues. The patient may have had a preop MRI

Hematologic

High risk of blood loss. Patients need to have T&C done, with a minimum of 2 units of RPBCs available. Consider using a cell saver. Pts may have been on anticoagulants, which increases the risk of bleeding

Laboratory

Baseline BMP and H&H

Preoperative

More pain than other patient categories; may have had previous back surgeries. They may not have gotten their regular pain medication due to surgery. They may have anxiety due to prior surgeries

Medicate with versed and possibly a small dose of fentanyl

Placement of A-line in preop holding or in OR after induction along with placement of CVP line

Possible use of Jackson table.

Anesthetic Considerations

ERAS protocol, limit NPO duration, alcohol, and smoking cessation. Review nutritional status and discuss post-operative pain management. Consider using goal-directed therapy for fluid management.

Blood conserving strategies

Autologous blood donation before surgery if medically stable

• Still, risk of infection, more costly, may go to waste, risk of perioperative anemia, and an increased risk of transfusion

Cell saver if there is no risk of contamination

• Urine, cancer, bone fragments, bowel contents, infection, malignancy, sickle cell disease, clotting agents

Acute normovolemic hemodilution

If blood transfusion - 1 unit PRBC increases Hb 1g and Hct 2-3%

If mass transfusion, consider the need for FFP, platelets, cryoprecipitate (clotting factors)

• Risk of hypothermia, hypervolemia, elevated K⁺ levels, decreased 2,3 DPG levels (so need increased O₂ delivery), dilutional coagulopathy (due to limited plasma in RPBC), acid-base changes with prolonged storage (reduced pH), and citrate intoxication (CPDA preservative - citrate/phosphate/dextrose/adenine)

Complications

Hypotension - due to vascular injury

Nerve root injury

Lung tissue injury - may need a chest tube

Blindness

Renal compromise

Bowel compromise