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. 2025 Sep;19(5):1317-1325.
doi: 10.1177/19322968241231996. Epub 2024 Feb 22.

The Needle Shield Size and Applied Force of Subcutaneous Autoinjectors Significantly Influence the Injection Depth

Anne-Sofie Madsen Staples  1   2 , Mette Poulsen  2 , Kezia Ann Friis Præstmark  3 , Thomas Sparre  3 , Marie Sand Traberg  2   4
Affiliations

The Needle Shield Size and Applied Force of Subcutaneous Autoinjectors Significantly Influence the Injection Depth

Anne-Sofie Madsen Staples et al. J Diabetes Sci Technol. 2025 Sep.

Abstract

Background: This study examines how shield-triggered autoinjectors (AIs), for subcutaneous drug delivery, affect injection depth. It focuses on shield size and applied force, parameters that could potentially lead to inadvertent intramuscular (IM) injections due to tissue compression.

Method: A blinded ex-vivo study was performed to assess the impact of shield size and applied force on injection depth. Shields of 15, 20, and 30 mm diameters and forces from 2 to 10 N were investigated. The study involved 55 injections in three Landrace, Yorkshire, and Duroc (LYD) pigs, with injection depths measured with computed tomography (CT). An in-vivo study, involving 20 injections in three LYD pigs, controlled the findings, using fluoroscopy (FS) videos for depth measurement.

Results: The CT study revealed that smaller shield sizes significantly increased injection depth. With a 15 mm diameter shield, 10 N applied force, and 5 mm needle protrusion, the injection depth exceeded the needle length by over 3 mm. Injection depth increased with higher applied forces until a plateau was reached around 8 N. Both applied force and size were significant factors for injection depth (analysis of variance [ANOVA], P < .05) in the CT study. The FS study confirmed the ex-vivo findings in an in-vivo setting.

Conclusions: The study demonstrates that shield size has a greater impact on injection depth than the applied force. While conducted in porcine tissue, the study provides useful insights into the relative effects of shield size and applied force. Further investigations in humans are needed to confirm the predicted injection depths for AIs.

Keywords: applied force; autoinjectors; injection depth; injection technology; intramuscular risk; subcutaneous tissue behavior.

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Conflict of interest statement

Declaration of Conflicting InterestsThe author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Anne-Sofie Madsen Staples is employed as Industrial Ph.D. student at Novo Nordisk A/S. Mette Poulsen, Kezia Ann Friis Præstmark, and Thomas Sparre are full-time employees at Novo Nordisk A/S.

Figures

Figure 1.
Figure 1.
Overview of needle shield geometries.
Figure 2.
Figure 2.
(a) Photo taken during the CT ex-vivo blinded injection study. (b) Photo taken during the in-vivo C-arm fluoroscopy study. Abbreviation: CT, computed tomography.
Figure 3.
Figure 3.
Examples of CT images of injections. Same letter means that images are from the same tissue sample: (a1) slice A (Ø15 shield/6 N); (a2) slice B (Ø15 shield/6 N); (b1) slice A (Ø20 shield/10 N); (b2) slice B (Ø20 shield/10 N); (c1) slice A (Ø30 shield/2 N); (c2) slice B (Ø30 shield/2 N). The orange dotted line indicates the skin surface. Abbreviation: CT, computed tomography.
Figure 4.
Figure 4.
Mean injection depth and SD whiskers from the CT study. The dashed line indicates the needle length (5 mm). The first 2 mm of the graph are colored dark gray to illustrate the skin layer, and the remainder is colored light gray for SCT. Abbreviation: CT, computed tomography; SCT, subcutaneous tissue; SD, standard deviation.
Figure 5.
Figure 5.
Shields with descending LSM. The shield LSM analysis includes data from all applied forces. Shields not connected by the same letter caused significantly different injection depth (P < .05). LSM and 95% CI are of the mean injection depth in mm. Abbreviation: CI, confidence interval; LSM, least square means.
Figure 6.
Figure 6.
Least square means with 95% CI for the injection depth. The applied forces LSM analysis includes data from all shields. Forces not connected by the same letter caused significantly different injection depth (P < .05). Least square means and 95% CI are of the mean injection depth in mm. Abbreviation: CI, confidence interval; LSM, least square means.
Figure 7.
Figure 7.
Example of FS image taken from in-vivo live videos of injections. The orange dotted line indicates the skin surface. Abbreviations: FS, fluoroscopy.
Figure 8.
Figure 8.
Mean injection depth and SD whiskers from the FS study. The dashed line indicates the needle length (4.5 mm). The first 2 mm of the graph is colored in a darker shade of gray to illustrate the skin layer, and the remainder is in a lighter shade to illustrate the SCT layer. Abbreviations: FS, fluoroscopy; SCT, subcutaneous tissue; SD, standard deviation.
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