Femoral nerve blocks : a comparison of neonatal and adult anatomy

Abstract

The femoral nerve (FN) block is regarded as the quickest and most effective method of pain relief for femoral shaft fractures when general anaesthesia is contra-indicated. Correct placement of the needle relies heavily on the pulse of the femoral artery (FA) however, and in times where this pulse is absent, a lack of well-defined and constant bony landmarks increases the chance of a failed block. This study aimed to determine the position of the FN using constant bony landmarks, the anterior superior iliac spine (ASIS) and pubic tubercle (PT). Dissections were performed on both sides of a sample of 54 neonatal (50 left and 50 right sides; mean length: 0.44m±0.08m; mean weight: 1.96kg±1.57kg) and 77 adult (70 left and 68 right sides; mean age: 61.6±20.1 years; mean length: 1.69m±0.09m; mean weight: 56.43kg±15.53kg) cadavers. No significant difference was found when comparing the left and right sides of either sample. The data for the left and right sides of both samples were therefore combined, i.e., a total of 100 neonatal and 138 adult femoral triangles were examined. The distances from the ASIS to the PT (ASIS-PT) were measured as well as the distance of the FN and FA to the ASIS and the FN to the FA. These measurements were converted to a percentage of the ASIS-PT distance and the neonatal and adult data was compared using a paired t-test. In neonates the FN lies between 33.38% and 36.30% of the ASIS-PT distance from the ASIS. In adults it lies between 48.06% and 49.96% from the ASIS. A significant difference was also found between the distances of the ASIS to the FN (p=0.000) and FA (p=0.000) when comparing the two samples. Two separate linear regression models, both with a strong correlation to the ASIS-PT distance (R=0.806 for the neonatal and R=0.784 for the adult sample), was developed for the two samples. The data also proves that neonatal and adult anatomy differs appreciably from each other. It is also important to have proper knowledge of neonatal anatomy before attempting to perform regional nerve blocks on paediatric patients.