Evaluation of a Simple Method of Locating Femoral Block Puncture Site using Finger Width Measurements and Correlation with Ultrasonographic Nerve Artery Distance: A Prospective Cohort Study

Background: Femoral nerve block, a relatively easy and safe peripheral nerve block, provides prolonged and effective perioperative analgesia in lower limb surgeries. A good landmark technique avoids unnecessary needle punctures and complications during block performance. We decided to find a simple technique to locate block puncture site and confirm it with ultrasonography. Materials and Methods: The aims were to find the correlation between finger width and femoral nerve-artery distance with ultrasonographic imaging, BMI, hip and waist circumferences and femoral nerve depth in Indian population. 360 subjects with planned femoral nerve block or healthy volunteers were studied. Patient’s age, height, weight, waist circumference, and hip circumference were noted. Finger width of both hands at distal inter-phalangeal joint was measured using Vernier’s calliper. Femoral nerve-artery distance and nerve depth were measured using ultrasonography. Results: Finger widths ranged from 1.24± 0.0481 cm to 1.42± 0.0623 cm. The femoral nerve-artery distances were in the range of 1.14 mm to 1.46 mm with median of 1.24 mm. Correlation coefficients between nerve-artery distance and finger width were 0.36 (Right little finger), 0.61 (Right middle finger), 0.44 (Left little finger) and 0.65 (Left index finger). Regression analysis yielded the model NA = 0.18 + 0.86 d with NA as the femoral nerve-artery distance and d the distal inter-phalangeal joint finger width. On simplification owing to the negligible constant (0.18) and multiplier (0.86) being close to one, femoral nerve – artery distance can be said to correspond to distal inter-phalangeal joint finger width of index finger of left hand. An association between BMI>25 kg/m2 and femoral NA distance (P value =0.0000) and hip circumference and femoral NA distance in males (P value =0.017) was found. Conclusion: A simplified, individualized landmark approach not requiring any additional, costly technical aids for needle insertion site for femoral nerve block was found. Puncture site should be lateral to the lateral most point of pulsating femoral artery at a distance equivalent to the width of the patient’s index finger of the non-dominant hand measured at the distal inter-phalangeal joint. It is especially useful for patients with intermediate BMI values.