Using Body Mass Index to Predict Needle Length in Fluoroscopy-Guided Lumbar Punctures

The authors evaluated patients who underwent oblique interlaminar-approach fluoroscopy-guided lumbar punctures and had cross-sectional imaging of the lumbar spine within 1 year of the lumbar puncture to devise a formula for the appropriate needle length based on BMI. They determined the formula to predict the needle length as Skin-Canal Distance (inches) = 0.077 x BMI + 0.88. BACKGROUND AND PURPOSE: Predicting the appropriate needle length to use in oblique interlaminar-approach fluoroscopy-guided lumbar punctures in patients with a large body mass index is difficult. Using the wrong needle length can lead to an increased radiation dose and patient discomfort. We hypothesized that body mass index could help determine the appropriate needle length to use in patients. MATERIALS AND METHODS: We randomly selected patients who underwent oblique interlaminar-approach fluoroscopy-guided lumbar punctures and had cross-sectional imaging of the lumbar spine within 1 year of imaging (n = 50). The distance from the skin to the midlumbar spinal canal (skin-canal distance) at the level of the lumbar puncture was measured by using an oblique angle of 8.6 degrees, which is an average of angles most often used to perform the procedure. A formula was devised using the skin-canal distance and body mass index to predict the appropriate needle length, subsequently confirmed in 45 patients. RESULTS: The body mass index and skin-canal distance were significantly higher (P < .001) in patients who underwent fluoroscopy-guided lumbar puncture with 5- or 7-inch needles (n = 22) than in patients requiring 3.5-inch needles (n = 28). Using linear regression, we determined the formula to predict the needle length as Skin-Canal Distance (inches) = 0.077 x Body Mass Index + 0.88. We found a strong correlation (P < .001) between the predicted and actual skin canal distance in 45 patients, and our formula better predicted the skin-canal distance than others. CONCLUSIONS: We designed a formula that uses body mass index to predict the appropriate needle length in oblique interlaminar-approach fluoroscopy-guided lumbar punctures and validated it by demonstrating a strong correlation between the predicted and actual skin-canal distance.