Hip flexion and lumbar puncture: a radiological study

When lumbar puncture is performed in the sitting position, the patient's thighs are usually at an angle of approximately 90 degrees to the trunk, whereas in the lateral position, hip flexion is employed by flexing the patient's knees to the chest. We measured the presumed but hitherto unquantified widening of lumbar interspinous spaces resulting from hip flexion. Lumbar spine lateral radiographs were taken in volunteers in the sitting position with and without hip flexion, and interspinous space width was measured and compared. Mean lumbar interspinous space width at L2-3, L3-4 and L4-5 increased by 7%, 11% and 21%, respectively, with the hips flexed. Hip flexion in the sitting position will anatomically optimise lumbar interspinous space width for needle passage, and statistically significant increases in space width have been demonstrated increasing progressively from L2-3 to L4-5.