Regional variations in pain to controlled mechanical skin traumas from automatic needle insertions and relations to ultrasonography

Background/aims: Variations in pain from needle insertions in different anatomical regions have not previously been investigated in relation to the mechanical parameters of needle insertions. The objective of the present study was to investigate these relations and furthermore to investigate possible relations between the skin parameters measured by high‐frequency ultrasound and the mechanical parameters of needle insertion. Methods: Forty‐eight needle insertions were performed in a randomised and blinded design into the deltoid region, the abdominal region, and the lateral region of the thigh in 30 healthy volunteers. The insertions were performed with hypodermic insulin needles 27 gauge (G) and 30G, at 19 mm/s, 90 degree angle, and 8 mm along the insertion axis. From each needle insertion the occurrence, intensity, and quality of pain as well as bleeding, and mechanical insertion parameters [maximum mechanical needle penetration force (N), indentation work load (10 ‐3 Nm), and total mechanical work load (10 ‐3 Nm)] were registered. Also, the skin of each anatomical region was characterised by 20‐MHz ultrasonographic examination. Results: The maximum mechanical penetration force was highest in the thigh and lowest in abdominal region ( P <0.01). The indentation work load was lowest in the thigh ( P <0.001). The total mechanical work load was highest in the deltoid region and lowest in the abdominal region ( P <0.001). The occurrence of bleeding from the needle insertions was lowest in the abdominal region ( P =0.01). The occurrence of pain and the pain quality from needle insertions did not differ between the anatomical regions. However, the intensity of pain in the needle insertions reported to be painful was lowest in the deltoid region ( P <0.01). Positive relations between higher mechanical parameters of insertions with 30G needles and the occurrence of pain were demonstrated. Higher total mechanical work load was related to increased pain intensity. Ultrasonographically, the thickness of the epidermal entrance echo was highest in the thigh ( P <0.001), and the thickness of both the dermal echo and the total skin echo thickness were highest in the abdominal region ( P <0.001). The mean ultrasonographic density of the dermal echo was lowest in the abdominal region and highest in the thigh ( P <0.001). The thickness of the dermal echo, of the total skin echo and the dermal echo density were all significantly inversely related to the indentation work load ( P <0.001). Conclusions: Specific mechanical parameters of needle insertions could be demonstrated in different anatomical regions. Differences in pain perception from needle insertions in different anatomical regions were not reflected by occurrence of pain but by pain intensity, with the lowest pain intensity registered in the deltoid region. Ultrasonographically, increased skin thickness and decreased density of the dermal echo were related to lower indentation work load during the needle insertions.