What space superficial to the ligamentum flavum can cause a false loss of resistance?

False Loss of Resistance During Epidural Placement: The Retrodural Space of Okada

The retrodural space of Okada, located between the ligamentum flavum and interspinous ligament, is the most common anatomical space that can cause a false loss of resistance during epidural placement. 1

Anatomical Considerations

• The retrodural space of Okada is a potential space posterior to the ligamentum flavum that allows communication with bilateral facet joints, creating a false impression of reaching the epidural space 1
• Midline gaps in the ligamentum flavum occur most frequently at the L1-L2 level (22.2%), with decreasing frequency at lower lumbar levels (L2-L3: 11.4%, L3-L4: 11.1%, L4-L5: 9.3%, L5-S1: 0%) 2
• These gaps can lead to false identification of the epidural space when using the loss of resistance technique, as the needle may pass through a gap rather than through the ligamentum flavum 2

Risk Factors for False Loss of Resistance

• Degenerative and pathological facet joint changes significantly increase the risk of inadvertent spread into the retrodural space during epidural injection (OR 4.382; 95% CI 1.160 to 16.558) 1
• Various anatomical communication routes in the posterior ligamentous complex leading to the retrodural space are more commonly observed in specimens with degenerative changes 1
• Operator inexperience significantly increases the risk of incorrect epidural space identification, with low-volume specialists having 3.77 times greater odds of complications compared to high-volume specialists 3

Clinical Implications

• The failure rate of thoracic epidural blocks due to incorrect identification of the epidural space with loss of resistance technique can be as high as 23.1% 4
• When the needle enters the retrodural space rather than the true epidural space, contrast may inadvertently spread into facet joints (observed in 5.9% of fluoroscopy-guided lumbar interlaminar epidural injections) 1
• False identification of the epidural space can lead to ineffective analgesia, as the catheter may be placed in a space that does not allow proper spread of local anesthetic 3

Confirmation Techniques

• Epidural waveform analysis (EWA) provides a valuable adjunct to loss of resistance technique with high sensitivity (91.1%) and specificity (83.8%) for confirming correct needle placement 4
• When the needle is correctly positioned inside the epidural space, measurement of pressure at its tip results in a characteristic pulsatile waveform 4
• The "dripping infusion method" offers a visual technique for confirming epidural space identification with 97.8% success rate, using hydrostatic pressure from a suspended micro-drip IV apparatus 5
• MRI-derived measurements from the skin to the anterior border of the ligamentum flavum show strong agreement with clinical loss of resistance depths (ICC values: 0.88 for lumbar and 0.72 for cervical measurements) 6

Prevention Strategies

• Using combined spinal-epidural technique may be more reliable than epidural alone, as free flow of CSF through the spinal needle confirms correct midline placement 3
• Performing epidural placement during daytime hours when possible, as epidurals placed during night shifts (19:00-08:00) have a 6.33 times higher relative risk of complications 3
• Ensuring proper patient positioning, with some evidence suggesting lateral decubitus position may be preferable to sitting 3

By understanding the anatomical structures that can lead to false loss of resistance, particularly the retrodural space of Okada, practitioners can improve their technique and reduce the risk of ineffective epidural analgesia.