What causes foot drop in lumbar spine surgery?

Causes of Foot Drop in Lumbar Spine Surgery

Foot drop following lumbar spine surgery is most commonly caused by injury to the L5 nerve root or common peroneal nerve, with multiple mechanisms including direct nerve damage, compression, traction, or ischemia during or after the procedure.

Primary Mechanisms of Foot Drop in Lumbar Spine Surgery

Direct Nerve Root Injury

• Injury to the L5 nerve root during decompression or instrumentation is the most common cause of foot drop, as L5 nerve root compression was observed in 93.3% of patients with foot drop due to lumbar degenerative disease 1
• Mechanical trauma can occur during manipulation of neural elements, particularly during decompression procedures for stenosis 2
• Sustained neurotonic discharges in response to retraction of the thecal sac can lead to nerve injury, as observed in patients undergoing interbody fusion 2

Pedicle Screw Malposition

• Breach of the medial pedicle wall by misplaced screws can directly compress or injure the L5 nerve root 2
• Stimulation thresholds less than 11 mA during intraoperative EMG monitoring can predict pedicle breach with 100% accuracy, suggesting a mechanism for nerve injury 2
• Patients with pedicle breaches are at higher risk of postoperative radiculopathy and foot drop 2

Postoperative Complications

• Epidural hematoma formation can cause delayed compression of nerve roots 2
• Postoperative edema around nerve roots can lead to compression and ischemia 2
• Seromas, pseudomeningoceles, and epidural abscesses can develop throughout the postoperative course and may cause extrinsic compression of the neural elements 2

Extrinsic Causes of Foot Drop After Lumbar Surgery

Common Peroneal Nerve Compression

• Compression of the common peroneal nerve at the fibular head by tight-fitting below-knee compression stockings can cause progressive foot drop even weeks after surgery 3
• Improper patient positioning during surgery can lead to compression of the common peroneal nerve at the fibular head 4
• This cause is particularly important to recognize as it is preventable and may be mistaken for an L5 radiculopathy 3

Surgical Approach-Related Factors

• Extensive or wide decompression has been shown to have a positive correlation with increased risk of nerve injury 2
• Aggressive facetectomy performed during decompression can result in iatrogenic destabilization and subsequent nerve compression 2
• Multilevel laminectomies increase the risk of nerve root injury and subsequent foot drop 2

Prognostic Factors for Recovery

Duration of Symptoms

• Duration of palsy is a significant factor influencing recovery, with shorter duration (less than 3 months) associated with better outcomes (p=0.0360, OR=2.543) 1, 5
• Improvement of tibialis anterior muscle strength typically stabilizes at the 6-month follow-up 1

Preoperative Muscle Strength

• Preoperative muscle strength of the tibialis anterior is a significant predictor of recovery (p=0.0064, OR=5.528) 1
• Only 15.6% of patients with foot drop recover to a muscle strength grade ≥4 after surgery 1

Age

• Younger age is associated with better recovery from foot drop (p=0.0309, OR=3.208) 1
• Elderly patients may have less favorable outcomes due to reduced neural regenerative capacity 1

Prevention Strategies

Intraoperative Monitoring

• Electromyographic monitoring during lumbar fusion procedures can help identify potential nerve injury 2
• Mechanically elicited and electrically stimulated EMG can detect nerve root irritation, allowing for immediate corrective measures 2
• Patients who had corrective actions taken following positive EMG responses showed better outcomes with no postoperative deficits 2

Surgical Technique

• Careful visualization and palpation of the medial pedicle wall during screw placement can help prevent breaches 2
• Avoiding excessive retraction of neural elements, particularly during interbody fusion procedures 2
• When planning procedures near the common peroneal nerve (such as microphlebectomy), care or avoidance of the region near the fibular head is recommended to prevent injury 2

Postoperative Care

• Proper fitting of compression stockings to avoid compression of the common peroneal nerve at the fibular head 3
• Early recognition and intervention for postoperative hematomas or seromas that may cause neural compression 2
• Regular neurological assessment to identify early signs of nerve compression 4

Clinical Pearls and Pitfalls

• Foot drop may have a delayed presentation after lumbar spine surgery, so ongoing neurological assessment is crucial 3
• Not all cases of foot drop after lumbar surgery are due to L5 nerve root injury; consider peripheral nerve compression as a potential cause 3, 4
• The prognosis for foot drop is better when the causative lesion is at the thoracolumbar spine compared to the lumbar spine 5
• Severe preoperative paralysis is not necessarily a poor prognostic factor for recovery 5