Considering skin-to-muscle depth for successful intramuscular injections in an increasingly obese population

Described as a pandemic, levels of obesity have increased markedly in recent times (Suraweera et al, 2017). According to the World Health Organization (WHO) worldwide obesity has trebled since 1975, and in 2016 more than 1.9 billion adults over 18 years (39%) were overweight with 650 million people (13%) described as obese (WHO, 2021). Overall, there is an increase with age with a peak prevalence worldwide between 40 and 60 years of age in developing countries (Kanter and Caballero, 2012). Box 1 gives an idea of the levels of obesity in the UK. Worldwide, it has been estimated that obesity will apply to 18% of men and more than 21% of women by 2025 (NCD Risk Factor Collaboration, 2016).

Box 1.Obesity and overweight in UK

• Data from the Health Survey for England indicate that 26% of adults aged over 18 are obese, and 38% are overweight
• In Scotland, 67% of those aged 16 and over are overweight or obese, with 30% classed as obese and 4% very obese
• In Wales, 62% of people aged 16 and over are overweight or obese, with 25% obese
• Data from the Health Survey (NI) indicate that 27% of adults in Northern Ireland are obese, with a further 38% classed as overweight

Source: Department of Health, 2020; Scottish Government, 2022; Welsh Government, 2022; Baker, 2023

Obesity is linked strongly with risk for conditions such as type 2 diabetes, cardiovascular diseases, respiratory disorders, infertility, and some forms of cancers (Afshin et al, 2017). Furthermore, statistics show that being overweight or obese in 2010 caused 3.4 million deaths, a decrease in life expectancy of 4 years and a further 4 years of disability-adjusted life years (DALYs) (Lim et al, 2012).

There is a growing recognition in nursing and health care in general that needle length needs further consideration when injecting medicine intramuscularly in people who are overweight or obese (Strofhus et al, 2017; White et al, 2018). Obesity and needle length also is an issue in other injection sites, as discussed by Kehrl et al (2016). Gender is an important consideration because women have more subcutaneous fat, as opposed to muscle, than men. Thus, when considering which needle to use it appears logical to select a longer needle in overweight and obese populations, with an important consideration in all women (Palma and Strohfus, 2013). This issue relating to intramuscular (IM) injection has been identified for some time in nursing literature related to mental health nursing practice (eg Poland et al, 1997; Nisbet et al, 2006) but does not seem to have been yet fully embedded into guidance for the administration of IM injections. For example, there is no guidance in the UK from NICE, or worldwide through WHO. This article presents a review of the evidence considering needle length for patients who are overweight/obese and the impact of gender on accurate IM injection administration.

Intramuscular injections: needle length and current nursing practice issues

An IM injection is the administration of medication through the dermis, cutaneous and subcutaneous tissue layers, into a skeletal muscle. IM injections are used where there is a risk that the medicine or its excipient may cause local, neural, vascular or tissue injury, for medication that requires relatively quick but reliable absorption or where a prolonged therapeutic action is desired (Ogston-Tuck, 2014: Wynaden et al, 2015). In this final example, the muscle acts as a store or ‘depot’, releasing the medicine over time.

There are five main muscle sites used for IM injection: the deltoid, dorsogluteal, ventrogluteal and vastus lateralis. The rectus femoris site was once used but has been removed from recent clinical guidelines due to concerns about risk of damage to the femoral nerve (particularly in young adults) (Nakajima et al, 2020). Product licences may specify which sites or muscles are to be used for particular medicines but where the site is not specified the practitioner is expected to consider patient factors, including individual physiology and preference and adhere to guidance about the recommended maximum volume that can be injected into the particular site chosen.

Needle length is indicated on every needle pack in inches and/or millimetres along with the gauge (a measure of the internal diameter of the needle bore that is usually colour coded). A variety of lengths are available from manufacturers for each specific gauge. If a medicine for IM injection does not include needles and specific instructions about their intended use (in a pack assembled by the manufacturer and part of the product licence) the practitioner is expected to select the appropriate needle from the clinical supplies they have available to them at the time.

An assessment of the length of needle required to reach the muscle for the site selected should be made by an assessment of the individual patient, taking into account the depth of their subcutaneous fat (Lister et al, 2021). However, practitioners may select a standard needle length regardless of the physiology of the patient. For example, a 35 mm length needle is commonly selected for the administration of antipsychotic long-acting injections into the dorsogluteal site because this will be how many practitioners were taught and assessed as competent and confident in the administration procedure (Wynaden et al, 2015). This practitioner may then go on to demonstrate this practice and teach it to others. Another example is the practice of leaving approximately 2-3 mm of the needle length exterior to the skin to allow the needle to be removed if it were to break. Today's single-use hypodermic needles are subject to robust quality control on manufacture and as a result are unlikely to break (Moustafa et et al, 2021). A practice that anticipates the risk of needle breakage appears to be related to a time when needles were used multiple times and autoclaved between use. Several clinical nursing textbooks and authors perpetuate this practice by including it in examples of good IM injection technique, even though in many cases this appears to be based on the opinion of one author commonly cited in support of this technique (Workman et al, 1999).

Where manufacturers have introduced longer needles in an administration pack as part of their product licence, post-marketing surveillance has revealed a reluctance among practitioners to select the longer needle or utilise its full length due to a perception that a longer needle will inflict pain (Kara and Yapucu Güneş, 2016). Selection of a needle length that fails to reach the intended muscle on administration increases the risk of medicines being delivered into adipose tissue (Strohfus et al, 2017), rather than the muscle. Reports of ‘granulomas’ at injection sites are well documented and some methods of IM drug delivery have a known increased risk of abscess if delivered into fat (Palma and Strohfus, 2013). Injection into fat is also likely to alter the intended pharmacokinetics of the medicine, potentially limiting its effectiveness and increasing the risk of adverse events following injection (Nisbet, 2006; Zayback et al, 2007; Correll et al, 2021).

Administering injections is an accepted part of the registered nurse's role (Beyea and Nicoll, 1995; Nursing and Midwifery Council, 2018). Administration of IM injections is taught in nurses' pre-registration education but adherence to guidelines means the choice of needle length is unlikely to be emphasised, risking the perpetuation of ritualised practice that relies on an evidence base that is not revisited (Malkin, 2008; Palma and Strohfus, 2013; Wynaden et al, 2015). Needle length is taught within the university setting through online platforms – for example, using resources from clinicalskills.net – and professional guidance, for example, that developed for the administration of long-acting antipsychotic medication (White, 2022). There also have been some studies starting to address this trenchant issue (Boyd et al, 2013; Strohfus et al, 2017; White et al, 2018). There is no stated national or international guide to needle length based on the body weight of a patient, although it is discussed in good practice guides (Lister et al, 2021; White, 2022). The authors found no national or international (ie from WHO) agreed directions for different body weight requiring different size of needle, and this has been recognised for some time (Elsom and Kelly, 2009). Although reviewed for the deltoid site (Kearns et al, 2023), there is no documented systematic review of the evidence of needle length and whether it reaches the muscle in intended site for people who are obese or very obese for all suitable injection sites.

Aim

The aim was to systematically review the skin-to-muscle depth required to achieve injection into muscle in adults. Objectives were to identify any implications of obesity status when selecting an appropriate needle length and site in clinical practice.

Methods

A systematic review of the literature was carried out according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (Moher et al, 2009). Adapted critical appraisal tools were used to assess the included papers (Critical Appraisal Skills Programme, 2023).

Inclusion and exclusion criteria

The target population consisted of adults above the age of 18 years who had an IM injection. Studies of subjects recruited from any setting were considered (inpatient or community; tertiary, secondary or primary care).

Study designs

There was no set intention to exclude any specific study designs but the authors considered randomised controlled trials (RCTs), observational (cross-sectional, cohort or case-control) and experimental design cohort studies, whether prospective or retrospective in design, and audits were suitable for inclusion. Key textbooks and previously conducted narrative reviews, systematic reviews, and evidence-based clinical practice guidelines relating to the length of needle required for IM injection were considered. Conference papers, poster abstracts, opinion papers and case studies were excluded. No constraints were placed on language or date of publication.

Exposure and outcomes

Clinical exposure was to a medical test that generates images of the structure of the soft tissue (ie ultrasound, magnetic resonance imaging (MRI) or computed tomography (CT)). Studies would be included where the distance from the skin to muscle was measured at a site where an IM injection could be administered (ie, deltoid, ventrogluteal, dorsogluteal, vastus lateralis or rectus femoris muscle), where an established standardised, reliable and validated method of measurement and obesity status was reported.

Search strategy

Embase (1974 to 12.2022), Medline (1946 to 12.2022), CINAHL (1981 to 12.2022) were searched using keywords and medical subject headings (MESH) determined by the authors: (“INTRAMUSCULAR INJECTION” OR “IM”. ti,ab) AND (“BODY MASS INDEX” OR “BMI” .ti,ab) (keyword searches on title and abstract). The study team also searched the reference lists of all the studies that met the selection criteria for the review. Where only abstracts were available, the team attempted to contact the original study authors to identify if the full paper was available. Duplicate studies were excluded.

Study selection

Two reviewers (SH and JW) independently screened titles and abstracts. Full text of all articles that were included at the title/abstract screening stage were retrieved and judged for eligibility by the same two reviewers (SH and JW). Discrepancies were resolved by discussion, involving the third reviewer (SL) for a final opinion where necessary. Reasons for excluding studies were recorded. Reviewers were not blinded to any aspect of the studies' designs or authorship.

Risk of bias in individual studies

The Joanna Briggs Institute (JBI) checklist for the quality appraisal of cross-sectional analytical studies (Moola et al, 2017) was used to summarise the quality of the studies. This checklist includes eight domains where the reviewer is asked to make a yes/no/unclear/not appropriate judgement about the methodology used: definition of inclusion criteria, details of subjects and setting, valid and reliable measurement of the exposure, use of objective standardised measurement criteria, identification of confounding factors, strategies for dealing with confounding factors, valid and reliable outcome measurement, appropriate statistical tests

The study inclusion criteria defined subjects and setting as valid and reliable measurements of tissue. This involved objective standardised measurement criteria to account for confounding factors, including valid and reliable outcome measurements. Finally, appropriate statistical tests also needed to be reported.

Results

The literature search identified 155 published papers, of which 20 were duplicates. Initial screening of the titles and abstracts excluded 104 records. The remaining 31 articles were assessed for eligibility and 13 articles were excluded at this stage, including a duplicate where the same study had been reported in two different publications. A total of 18 articles were appraised using the JBI checklist where more detailed scrutiny of the methodology led to a further 4 exclusions (3 because there was no reliable and valid measurement of the distance to the muscle, one because obesity status was not reported). The final analysis included 14 papers.

Data extraction

A data extraction form was developed to collect data on citation details, study design, setting, sample size, participant characteristics, details of the exposure, any documented adverse effects and the results. The form was pre-piloted on four of the identified articles to make sure both reviewers recorded identical data. All data were then recorded in duplicate (by SH and JW), and any discrepancies resolved by discussion involving all review authors.

Discussion

Study characteristics

The studies identified are shown in Table 1. All included studies explored distance to the muscle at IM sites (deltoid, ventrogluteal, dorsogluteal, vastis lateralis and/or rectus femoris) but all of the sites were not explored in every study. A range of different anatomical landmarking methods was used across the studies to determine the site for measurement, different medical tests (ultrasound, MRI or CT) and measurement methods (eg digital callipers, lasers). Obesity was either reported as BMI, or hip-to-waist ratio. All the included articles used cross-sectional observational designs. One study included an experimental design to test an educational intervention that aimed to improve adherence to guidelines, but only data from the earlier phase where depth to muscle had been measured were included (Boyd et al, 2013).

Design, context and sample

Three of the studies were retrospective in design, all others (n=11) were prospective. All studies reported using cross sectional methods. All the studies included observational (descriptive) designs. In terms of the context, studies were conducted across the globe, in the USA (n=5), Australia (n=2), Turkey (n=3), India, Ireland, UK, and Canada (n=1 each). All studies used a convenience sample, with the exception of Shankar et al (2013), who did attempt a purposeful selection of varying BMI groups. Zayback et al (2015) was the only study that actually targeted a sample of people in the obese category.

Risk of bias in individual studies

The JBI quality appraisal process showed that 10 studies achieved the highest possible score 8/8. One failed to report the context of the research (Chan et al, 2006) and three failed to consider any confounding variables, with only one study (Holliday et al, 2019), reportedly using two clinicians (radiographers) to ratify that skin-to-muscle depth was achieved. Future studies would need to ensure some form of inter-rater reliability to negate such bias.

Measurement of skin-to-muscle depth

The 14 studies identified used medical tests to generate images of the soft tissue enabling measurement of the distance between tissue boundaries. All included studies reported using the BMI criteria to class obesity. All used medical devices to screen the potential needle reaching muscle: ultrasound (n=10), CT (n=3) and MRI (n=1). Although one procedure was used more than the others there was no evidence emerging that this or the other two ways of measuring the skin-to-muscle depth held any advantage over the other two. The need to identify boundaries between tissue types, rather than relying on the observational skill of the radiographer/sonographer is paramount whatever medical device is used. Finding an accurate and inexpensive device is surely something that would help meet the need for truly IM injection and the drug reaching the target area as appropriate when administering to a person defined as obese as per BMI (Soliman et al, 2018). Strothus et al (2022) stated – at least for dorsogluteal injections – that if skin-to-muscle depth is questionable then an ultrasound scan is appropriate, but did not discuss the other alternatives.

Intramuscular administration sites

The reviewed studies had included the vastus lateralis (n=5) although one of these also measured at the dorsogluteal site. The ventrogluteal site (n=6) was also measured also involving or as a stand alone, although one of these also measured at rectus femoris site (Zaybak et al's (2015) study was conducted before current guidelines recommended avoiding this site). The dorsogluteal site also featured three times. The vastus lateralis studies compared depth with autoinjector needle length although in one study, Song et al (2005), the length of autoinjector needle was nearly a whole millimetre shorter than other studies, although this may have been affected by the needle length availability being shorter in 2005 than other studies conducted up to 10 years later. In all studies there was a correlation between obesity status and the distance from skin surface to muscle. In females this exceeded 37 mm at both gluteal sites, independent of obesity status.

The following themes related to obesity and IM injection emerged:

Skin-to-muscle measure considered along the lines of obesity

The included studies support the idea that results in patients with a noted obesity status (BMI ≥30kg/m²) results showed a longer measurement of skin-to-muscle, therefore longer needles are required.

Skin-to-muscle measure considered along the lines of gender

It is clear that from the included studies support the idea that females naturally have more subcutaneous fat than males and therefore a larger skin-to-muscle measure. Bhalla et al (2013) from a 50% split of genders (n=60 females, n=60 males) found that injections in women were 6.4 times more likely not to reach the intended muscle site. Boyd et al (2013) in their sample (n=66 females, n=49 males) calculated that injections were successful for 66% of men, and only 31% of women. Chan et al (2006) in their sample (n=25 females, n=25 males) reported a projected 56% success with males and only 8% females. Cook et al (2006) (n=124 females, n=134 males) found men had lower subcutaneous layers than women and higher muscle layers in the deltoid. Holliday et al (2019), from their study (n=224 females, n=126 males), recommended longer needles were needed in the ventrogluteal site for women with a BMI of 30kg/m² and above, versus a threshold for men of BMI 35kg/m². Johnstone et al (2015) with a small sample and disproportionately more females (n=23 females, n=5 males) showed females had a greater skin-to-muscle depth), whereas the samples for Larkin et al (2017) (female n=83, male n=62) and Zaynbak et al (2007) (n=59 females, n=60 males) had considerably thicker subcutaneous fat at the dorsogluteal and ventrogluteal sites). Poland et al (1997) (n=126 females, n=94 males) and Shankar et al (2014) found that for the deltoid, females had a significantly higher fat pad thickness than males, thus for the deltoid a longer needle would be needed. Song et al (2005), Tsai et al (2014) and Zayback et al (2015) found the distance from skin to muscle in the thigh was greater for women versus men. Ozen et al (2019) (n=27 females, n=33 males) did not report differences between genders but recorded that 38% of their sample (n=23) did not reach the muscle. Therefore, although some samples were small it clearly shows in 13/14 studies a longer needle needs to be considered for females in all common injection sites.

Choice of intramuscular administration sites, gender, obesity and needle site

For the dorsogluteal there was definite evidence that both obesity and the female gender have a high correlation with suboptimal administration of injections. Chan et al (2006) found only 8% (2/25) of the sample of 25 females had an injection intramuscularly versus 56% (14/25) of males. Ozen et al (2019), found that 13/50 of their sample had injections in the subadipose tissue rather than muscle, and that obesity was the key factor interfering with the success of the injection. The success of the injections can be can be improved significantly for both genders with better technique (landmarking, quick needle insertion and use of non-syringe hand to compress the injection site) with an improvement for both genders (66-75% male, 38-75% females) using a 38 mm needle (Boyd et al, 2013).

When comparing dorsogluteal and ventrogluteal, Larkin et al (2017) recommended both gluteal sites should be avoided for people in the obese category, however they suggested a 32 mm injection needle for all males and normal weight females in the dorsogluteal site, and 38 mm needle for all females at the ventrogluteal site, and for all males and 98% of females at the dorsogluteal site. Zaybak et al (2007), had similar results to Larkin and colleagues, concluding that the dorsogluteal site is effective for obese and overweight men, whereas the ventrogluteal site was only effective for men who were in the overweight category; both administration sites would be suboptimal for women.

When comparing dorsogluteal and thigh Zayback et al (2015) found that the stated standard needle length of 38 mm would not reach the required muscle for a person with a BMI of 24.9kg/m², but would reach muscle in both genders in the thigh. Zayback et al (2015) also stated the thigh site was preferable for obese patients (both genders), and a longer needle than 38 mm in females with severe obesity when injecting into the thigh.

One included study focused on the ventrogluteal site (Holliday et al, 2019), and, for a BMI of 30kg/m² in females and 35kg/m² in males a needle length greater than 38 mm would be needed for a successful delivery to the muscle. For studies on administration into the thigh, Bhalla et al (2013) found the standard 15.9 mm autoinjector needle for anaphylaxis was inadequate for 31% of their sample, a higher BMI and female gender being the main reasons; Song et al (2005) (14.3 mm needle), Johnstone et al (2015) and Tsai et al (2014) (15.02 mm needle) came to the same conclusion. For the deltoid site evidence was provided that a 25 mm needle would suffice usually, but a 1.5 inch (38 mm) needle would be needed for women who weighed more than 90 kg (Poland et al, 1997; Shankar et al, 2014).

One issue that could affect the choice of site in the future for nurses is the sexual dimorphism in which men carry more subcutaneous fat around the abdomen and women in the gluteal-femoral region (Palmer et al, 2015). This in turn can affect how a drug is absorbed in the body (Chang et al, 2018), and, importantly, the effectiveness of IM injections (Cooke, 2009). This will affect all sites but is less pronounced in the deltoid region and lateral thigh sites. Added to this, the deltoid site has the smallest skin-to-muscle depth, it would appear logical that in cases where the patient is obese, and especially female, there is an argument to favour the deltoid when injecting intramuscularly. Further research would be needed to assess the administration of IM injections in the deltoid versus thigh versus dorsogluteal and ventrogluteal sites.

Discussion

This review confirms what is already known or stated that people in the obese category (Sakamaki et al, 2013; McWilliam et al, 2014; Dayananda et al, 2015; Painter et al, 2015) and the female gender, of medium BMI (25–29.9kg/m² or ‘overweight’) and above have a greater chance of receiving a suboptimal dose of medicine from an IM injection (Soliman et al, 2018; Strofhus et al, 2022). This has been highlighted in the nursing literature as an issue all nurses need to be aware of (Malkin, 2008; Strohfus et al, 2017) with focus on selection of needle and injection technique (Boyd et al, 2013; Palma and Strohfus, 2013; Wynaden et al, 2015; White et al, 2018).

However, evidence accrued from this review confirms that there needs to be a greater emphasis on obesity status in nursing decision making and in the decision about site of administration, needle size chosen for the person who is obese and female with a BMI of 30kg/m² and above. It is noted that there has been an emphasis on the gluteal muscle and finding ways to improve the potential of the medicine through injection reaching the muscle in the gluteal site (Boyd et al, 2013; White et al, 2018; Sahebkhar et al, 2022; Strohfus et al, 2022). However, from this review it is suggested that the gluteal sites for females of medium BMI and above will be suboptimal. Although that the choice of a 40 mm needle length and correct landmarking can help toward reaching the muscle in the gluteal muscles, it is the vastus lateralis and particularly deltoid sites that need to be used for the injection to have the full potential of being truly IM. This has implications for the licensing of medicines, as in psychiatry, for example, the majority of long-acting IM injections are licensed for administration in the gluteal muscle (Soliman et al, 2018).

A choice of needle sizes also needs to be available so as to allow the nurse to assess, plan and implement an injection fit for purpose – intramuscularly. Nursing needs to move away from rituals when it comes to IM injection and use the latest evidence (Greenway, 2014; White, 2022). This will allow the potential of the medicine administered to be optimised.

Limitations

In conducting the search strategy, the team may not have captured all published evidence (eg ‘grey literature’ evidence). It should also be noted there were different study populations across the studies and all were convenience samples, with patients either with suspected conditions attending an outpatient clinic for investigation and diagnosis, or with a known condition attending for IM treatment, or (healthy) student volunteers. This has implications for the potential validity and reliability of studies included in this review. Three studies included were at least 10 years old, limiting the timeliness of the review. The studies only included descriptive analysis and although some inference was attempted, no real power calculations were undertaken. There was not one gold standard measurement distance between soft tissue boundaries, with different methods used by researchers, thus not providing a standardised procedure to undertake this. There were also methodological reporting problems identified in three studies that highlighted risk of bias. Finally, a major limitation to this review is that the designs were heterogenous to support a meta-analysis.

Conclusion and implications for practice

Although anecdotally there has been some change in practice regarding addressing the needle length in the context of the obese population, a review of studies has suggested IM injections may have not reached the muscle target in many occasions. This has implications for patients who would be classed in the obese category and who most of the time will have a comorbid health problem, if the injection does not reach the intended muscle site and in essence is subcutaneous. First, that the medicine injected does not induce the expected therapeutic response. Second, there may be tissue damage that may lead to pain and discomfort in the short and long term. In conclusion, suboptimal administration leads to reduced likelihood of positive health outcomes for the person receiving the injection, whether obese or not.

Nurses need to assess obesity status before selecting needle length for IM injections in both genders in adults. Weight is not a commonly completed element on observation charts, and BMI is not always documented at admission, in electronic records within hospital settings or in patients' homes. This needs consideration by employing healthcare organisations and individual nurses who administer medication as part of their role. Where female patients have a BMI of 25kg/m² (overweight) or higher a needle length of at least 40 mm should be selected. Consideration needs to be given to the deltoid or vastus lateralis site in obese (BMI ≥30kg/m²) or very obese females (if the product licence allows) as dorsogluteal injections have been shown to end up subcutaneous rather than IM. Deltoid injections are most likely to achieve muscle penetration in both genders. There is a clear need for national and international recognised standards to guide this important area of nursing practice. Finally, high-quality (experimental) research is needed to determine if achieving muscle penetration improves health outcomes.

KEY POINTS

• The distance between the skin surface and penetration of the muscles used for intramuscular (IM) injection is positively correlated with obesity status
• The distance from skin surface to penetration of the gluteal muscle is greater than 37 mm in overweight or obese females
• Obesity status should be assessed before selecting an appropriate length of needle for IM injection (unless needle length is specified in the medicine product licence)
• When injecting into the gluteal a needle length of at least 40 mm should be selected for any female with a BMI ≤25kg/m² (overweight).
• Deltoid injections are more likely to achieve muscle penetration in people who are overweight or obese, independent of their gender

CPD reflective questions

• Obese male adults and overweight and obese female adult women are at risk of receiving intramuscular (IM) injections into fat, rather than the intended target muscle. Reflect on what the possible outcome of this could be for the service users you work with.
• If you have to use your clinical judgement to select an appropriate needle length for IM injection in your practice what lengths of needle are available to you, and do you know how to source longer needles if you decide they are needed?
• Is information about obesity status of service users readily available? Do you have equipment in your area that would enable you to calculate and accurately record body mass index and/or hip-to-waist ratio?