Deltoid Intramuscular Injections: A Systematic Review of Underlying Neurovascular Structures to the Muscle and Proposing a Relatively Safer Site

The deltoid is the preferred site for intramuscular injection (IMI) because of its easy accessibility for drug and vaccine administration. Government immunization advisories, standard anatomy textbooks, and researchers have proposed various injection techniques and sites, but specific guidelines are lacking for the administration of IMIs in the increasingly used deltoid site. This study analyzes the procedures of administering IMIs in the deltoid related to the neurovascular network underlying the muscle and proposes a preferred site with the least chance of injury. The review protocol was submitted with PROSPERO (ID: 319251). PubMed, Google Scholar, and Websites of National Public Health Agencies were searched from 1950 up to 2022 for articles, advisories, and National Immunization Guidelines using Medical Subject Headings (MeSH) terms, including IMIs, deltoid muscle, safe injection sites, to identify recommendations for safer sites and techniques of administering deltoid IMIs. All the authors strictly adhered to a well-developed registered review protocol throughout the study and followed the risk of bias in systematic reviews (ROBIS) guidance tool. The proposed sites and landmark data were tabulated, and each site was analyzed based on the underlying neurovascular structures. Data were depicted by self-generated images. The initial search identified 174 articles. After applying the inclusion and exclusion criteria, 57 articles were shortlisted. Out of the 39 selected articles, 18 focused on the administration of deltoid IMIs, whereas seven focused on the variations in the underlying neurovascular structures in proximity to the deltoid muscle. The remaining 14 articles were the immunization guides issued by the National Public Health Agencies of the Government of India and abroad, whose data was used for comparison. Twelve deltoid IMI sites and techniques were identified. A site 1-3 fingerbreadths/5 cm below the mid-acromion point (7 studies); mid-deltoid site/densest part of the deltoid (1 study); a site at the middle third of the deltoid muscle (1 study); triangular injection site (1 study). Limitations included the unavailability of free access to complete text in many articles resulting in exclusion. The area around the shoulder joint and up to the lower level of the intertubercular sulcus is highly vascularized by the presence of many anomalous arterial patterns. To avoid injury, a safer site is proposed of 5 fingerbreadths/10 cm below the midpoint of the lateral border of the acromion. The authors received no specific funding for this study except for the journal publication charges.


Introduction And Background
Intramuscular injections (IMIs) are among the most common medical procedures performed in any healthcare center [1]. Globally, the deltoid is the preferred IMI site in clinical practice [1]. Many other IMI sites have been considered over the deltoid based on the risk of injury to the underlying vessels and nerves. However, a paucity of uniform guidelines and algorithms persists for IMI administration by healthcare professionals [2]. This study analyzes the procedures of IMI administration in the deltoid in relation to the arterial network underlying the muscle. Our study proposes a site preferred to the deltoid for IMIs with the least chance of injury to neurovascular structures. Figure 1 and Table 1

Review Methods
To reduce the risk of bias in the study, a systematic review protocol was prepared and submitted with PROSPERO at the Centre for Reviews and Dissemination, University of York (ID: 319251). The review protocol can be accessed from https://www.crd.york.ac.uk/prospero/.

Eligibility criteria
The articles which satisfied the inclusion and exclusion criteria were eligible for review. Inclusion criteria were complete articles published between January 1, 1950, and January 31, 2022, authorship by both foreign and Indian authors, immunization guidelines and advisories issued by national public health agencies, and all articles related to deltoid IMIs. Excluded articles were published before January 1, 1950, and after January 31, 2022, those focusing on topics other than deltoid IMIs, and those accessible through abstracts only. There was a restriction for the non-English language of publications.

Search strategy
A scoping review was conducted of articles published from 1950 to 2022 on PubMed, Google Scholar, and National Immunization Guidelines using these Medical Subject Headings (MeSH) terms: IMIs, deltoid muscle, safe site for injection, axillary nerve, needle depth, nursing practice, post-injection complications, posterior circumflex humeral artery, and anterior circumflex humeral artery. The citation search was carried out for all the selected articles in the study.

Study selection
After applying inclusion and exclusion criteria, two authors (SC and SS) independently assessed all of the titles, abstracts, articles, and guidelines found during the initial search, and relevant publications were shortlisted. All of the shortlisted full publications were downloaded and independently examined for relevant data using the data extraction checklist prepared by both authors. The authors and the methods of the investigations were not hidden from the reviewers. Any differences were settled through conversation or the involvement of a third reviewer (VM). The special data needed for the review is mentioned on the checklist ( Table 2). We chose and incorporated the articles that contained this information.

Data extraction process
The authors created a data extraction form that they used to collect data from any two papers they chose, and it was verified after the pilot trial. Two reviewers (SC and SS) worked separately to gather data from all of the studies that were included. Disagreements were addressed through dialogue and the participation of a third independent reviewer (VM). The following features of the study were gathered: (i) the research author; (ii) the study design; (iii) the country of publication; (iv) the number of participants; (v) the participants' age group; (vi) the participants' gender; and (vii) the participants' ethnicity. The following information was gathered about the IMI location in the research and control groups: any new suggested deltoid IMI location; (ii) the distance between the recommended IMI site and the mid-acromial point; (iii) any underlying neurovascular structures to the recommended deltoid IMI site; (iv) any post-injection complications occurring after the deltoid IMI at the recommended site; (vi) any site not recommended or marked as high risk for deltoid IMI; (vii) any underlying neurovascular structures to the not-recommended deltoid IMI site; and (vii) any underlying neurovascular structures to the not-recommended deltoid IMI.

Risk of bias
Two reviewers (SC and SS) independently conducted the risk of bias assessment that was included in the data extraction form. The risk of bias in systematic reviews (ROBIS) tool was used to assess the risk of bias in our review study [4].

Literature Search
There were 174 published papers found after the initial search. After filtering for the English language, original content, and human involvement, 97 articles remained. Duplicate articles (n=19) were deleted, and two reviewers independently assessed 97 publications. The same two independent reviewers independently examined 12 papers found through citation searches for eligibility against the pre-specified inclusion criteria. Disagreements were settled by conversation. After applying the inclusion and exclusion criteria, 57 articles remained. Thirty-one publications were excluded due to irrelevant text and two publications due to unavailability of the full text. Based on each author's appraisal and cross-verification, 39 papers were selected for synthesis. Eighteen papers were eligible for inclusion and exclusion based on the inclusion and exclusion criteria but were eliminated after reviewers screened them for irrelevant data that was outside the scope of the current review. They are namely Wempe 1961 [5], Taylor 2021 [6], and Micallef et al. 2020 [7]. Figure 2 shows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flowchart, which shows the step-by-step literature search and consideration/rejection procedure.   In about 33% of cases, the subscapular artery can arise from a common trunk with PCHA.
Occasionally the subscapular, circumflex humeral, and profunda brachii arteries arise from a common trunk. In some cases, the PCHA may arise from profunda brachii artery.  The student nurse should use four fingers, placing the little finger on the acromion process, and three fingers below   Site is at the middle third of the deltoid muscle, with acromion as the origin of the deltoid and the deltoid tuberosity as the insertion of the deltoid muscle. This site is the densest part of deltoid. The site is shown in a selfgenerated image in Figure

Characteristics of Included Studies
Tables 3-5 summarize the characteristics of the selected studies. Of the selected 39 articles, 18 articles focused on the administration of deltoid IMIs (Shown in Table 3). These 18 comprised two public surveys, three cadaveric studies, four case reports, four review studies, one book chapter, and four crosssectional/cohort studies. Of the selected 39 articles, seven articles focused on the variations in the underlying neurovascular structures in proximity to the deltoid muscle (Shown in Table 4). These seven publications comprised three observational studies, one cadaveric study, two book chapters, and one immunization report. The remaining 14 publications were the immunization guides issued by the Indian and international public health organizations and the data obtained from them was used for comparison. Among the 39 reviewed articles, only 12 articles focused specifically on the site of deltoid IMI. Table 5 summarizes the data extracted from these 12 articles regarding the sites, surface landmarks, techniques, and post-injection complications. All the proposed sites in these 12 publications were demonstrated on our study volunteers who underwent surface marking of the bony, soft tissue structures. The results section describes the images that were taken.

Risk of Bias Assessment
The studies selected for this review study used a variety of methodological techniques. All three phases of the ROBIS tool were utilized to assess the risk of bias in our review study's methodology. Except for a few points in Domain 2 of Phase 2, all other requirements were met. As a result, the total risk of bias was determined to be minimal.

Discussion
The proposed techniques and observations in all these studies were considered in the context of the guidelines recommended by several national public health agencies and regulatory bodies across India and the world. The Centers for Disease Control and Prevention (2021) and National Immunization Technical Advisory Groups (NITAGs) in Ireland (2020) and New Zealand (2020) advised that the correct site to insert the needle in the deltoid muscle is in the central and thickest portion of the muscle, which lies in the center of a triangle (Figure 4), the base of which is formed by the lower edge of the acromion process and the apex direct downwards at the crease of the axillary fold/armpit [40]. However, a review of deltoid anatomy and the AXN showed that AXN is usually 5-7 cm below the acromion tip, but the distance was between 4.34 cm and 6.39 cm, with an average distance of 5.58 cm [41]. The nurse who administers a deltoid IMI 5 cm below the acromion can therefore only be millimeters from the AXN.
According to the Canadian Immunization Guide (Modified 2020), issued by the Government of Canada, the deltoid muscle is the preferred site for IMI in adults and adolescents older than 12 years, whereas this site is not recommended for children aged 12 months and younger [42]. As per the guidelines issued by NITAGs in Australia (2018), the anatomic site recommended for deltoid IMI is a smaller triangle-shaped area in the middle of the deltoid, above the deltoid tuberosity. This site is located midway between the acromion and deltoid tuberosity, in the middle of the muscle [43]. McGarvey and Hooper (2005) stated that the subdeltoid bursa extends 5 cm below the acromion process, so the midpoint of the deltoid IMI site may be dangerous [10]. (2017) states that the midpoint of the deltoid is about 2 inches (or 2-3 fingerbreadths) below the acromion process and above the armpit in the middle of the upper arm. This area is the central and thickest portion of the deltoid muscle and is the recommended site for IMI [44]. However, this site could be dangerous due to the risk to subacromial bursa [10]. The National Health and Medical Research Council in the Australian Immunization Handbook (2015) proposed for IMI a region in the middle part of the deltoid muscle with acromion as the beginning of the deltoid muscle and tuberosity of the deltoid muscle as its insertion [45]. However, this site also could be dangerous due to the risk to subacromial bursa [10].

The US Department of Health and Human Sciences Centers for Disease Control and Prevention
Based on the neurovascular network lying underneath the deltoid muscle and in relation to the upper end of the humerus, the proximal humerus is related to a network of arteries arising from the second part of the axillary artery. The PCHA, along with the AXN, is frequently found in the region between 5 and 9 cm below the lateral border of the mid-acromion process. Hence, an alternative site (site "b") is proposed which is 5 fingerbreadths (more than 10 cm) below the midpoint of the lateral border of the acromion (Figures 9-10). This site is far below the surgical neck of the humerus (7 cm), the AXN (7 cm), the subdeltoid bursa (5 cm), and the PCHA (7.6 +/-1.0 cm); therefore, the chance of injury to blood vessels and nerves is the least. The intersection of the anteroposterior axillary line (the line connecting the upper end of the anterior axillary line and the upper end of the posterior axillary line) and the perpendicular line of the middle acromia is like the site proposed by a study in Japan [1] as the most suitable site for IMI. Further cadaveric and ultrasonographic studies are needed to study the neurovascular profile in relation to safer areas for IMI in the deltoid muscle.

Limitations
Free access to complete text was unavailable for several articles; thus, a review of such articles could not be performed because abstracts were excluded from our study. This limitation shall be overcome in our future review articles.

Conclusions
Based on findings from our literature review for deltoid IMI sites and techniques, we conclude that the area around the shoulder joint and up to the lower level of the intertubercular sulcus is highly vascular, and the presence of many anomalous patterns of arteries in this area is not rare. We propose an alternative site (site "b") that lies 5 fingerbreadths/10 cm below the midpoint of the lateral border of acromion as the safest site to avoid injury to the AXN, PHCA, subacromial and subdeltoid bursae, shoulder joint, and radial nerve. We believe that our proposed site for IMI can be useful for clinicians in a daily clinical practice setting.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.