All of us will have a mental picture of the deltoid muscle, by far the most common site chosen for intramuscular vaccinations. We can envisage its triangular shape, the base arising from the bony outer edges of the shoulder girdle (clavicle, scapula), with its downward pointing tip attaching halfway along the lateral side of the humerus.
We know it to be powerful, enabling us to raise the arm out sideways from the chest wall. Underneath it lies a capsule enclosing the ball and socket shoulder joint, the bones themselves, and the muscles and tendons which perform its actions.
Directly under the deltoid itself, is a delicate fluid-filled pocket (the subdeltoid bursa), almost a secondary joint capsule, which permits these components to slip and slide effortlessly over one another (Codman, 1984).
All of these structures under this muscle have been injured from time to time as a result of vaccinations, and damage to the shoulder mechanism can be very debilitating, so taking steps to avoid injecting them seems logical (Barnes et al, 2012; Cook, 2015; Martín Arias et al, 2017). Figure 1 shows the left shoulder seen from behind.
Figure 1. Left shoulder seen from behind.
Almost as frequently damaged as the parts mentioned above, are two nerves, the radial and the axillary (Choi et al, 2001; Tak et al, 2008; Imran and Hayley, 2013; Kim et al, 2017; Harsh et al, 2020).
Both arise in the armpit (axilla) and travel forwards from the back of the humerus under cover of the deltoid.
The axillary nerve runs round the neck or upper shaft of the humerus with its ‘at risk’ branches passing on its undersurface to control the middle and front parts of the muscle (Fujimoto, 2007; Uz et al, 2007; Hachadorian et al, 2020).
The radial nerve, by far the most commonly affected, is additionally hidden by a flap of the triceps muscle (the edge of its lateral head). It travels more vertically down the arm and on a level with the deltoid's triangular insertion, parallels the posterior edge of the tendon as it winds forwards next to the bone in its ‘spiral groove’. Most injuries are inflicted above this groove. Because of these positions, injections should not be given in the posterior part of the deltoid.
If a nerve is hit by a needle going in, it causes a sharp pain, often likened to an electric shock; the procedure must be stopped if this happens, as repositioning is required (Choi et al, 2001). Both nerves have accompanying blood vessels, creating a bigger target for injury.
The shelf of bone projecting over the ball and socket joint is called the acromion, and is marked ‘A’ in our diagrams (Figure 1 and 2). It forms part of the scapula, and the middle section of the deltoid muscle fibres arise from this.
Figure 2. The shelf of bone projecting over the ball and socket joint is called the acromion. A = acromion seen from the side.
It is used as a reference point for measuring how far structures lie below it, notably the axillary nerve.
With the arm hanging loosely down alongside the chest wall, our two nerves lie almost vertically below the posterior corner of the acromion, as seen with this skeleton (Figure 2).
The bursa (‘B’ in Figure 1) projecting between the muscle layers is squashy and mobile, but its lower limit never extends beyond the axillary nerve and its accompanying vessels (Beals et al, 1998).
The distance from the edge of the acromion to the axillary nerve does reflect the subject's stature to a degree, as compared to arm span (Burkhead et al, 1992), humeral length (Cetik et al, 2006; Samart et al, 2014; Ikemoto et al, 2015), and overall height (Hachadorian et al, 2020), but it is not a strictly linear relationship.
It has been measured in several ways – on dissected bodies most commonly – but more recently in the living with the subjects upright, using ultrasound and MRI scans. Results from these studies show a wide range from the closest position to the most distant: presently for women 3.1–7.7 cm, and for men 4.1–9.2 cm (Burkhead et al, 1992; Nakatani et al, 2000; Klepps et al, 2004; Gurushantappa and Kuppasad, 2015; Nakajima et al, 2017).
Locating your target
Finding the important landmarks of the deltoid in the living is not difficult and can soon be learned with a little practice on yourself, your partner and patients; having a clear mental picture enables a safe injection site to be found confidently.
Start with the acromion, with its edge above the head of the humerus. Under your finger tips it will be found to have distinct front and back corners, but may take a bit of prodding to identify, especially if the patient is muscular or plump. Shrugging the shoulder up and down will help (Figure 3a).
Figure 3. a) Locating the acromion; b) and deltoid insertion
Next, with your hand on the subject's elbow, ask them to push out sideways. You will then be able to feel the apex of the muscle where it inserts halfway down the bone (Figure 3b).
The anterior and posterior portions of the muscle arise from the outer third of the clavicle and from the spine of the scapula, respectively. Their edges are outlined on the subject with dotted lines in subsequent photos (Figures 4–6).
Figure 4. Dotted deltoid outline, with yellow markers showing the lower limit of the axillary nerve, and the emerging path of the radial nerve. 
Figure 5. Below the level of the armpit folds, the deltoid becomes more fibrous. 
Figure 6. Between the patient's little finger and the deltoid insertion is safe to inject - see text.
In a slim subject you may be able to roll the radial nerve under your fingers as it travels round the side of the arm and on towards its destination on the back of the forearm. If you compress your own radial nerve you may feel discomfort along this course. If the nerve is injured the muscles here may be paralysed and the wrist drops forwards resulting in a weak grip.
The axillary nerve is smaller, and cannot be felt, as it lies under the deltoid. If permanently damaged it results in inability to raise the arm from the body (abduct).
In Figure 4 the yellow spots have been added to show where the axillary and radial nerve may lie (as evidenced by the most distally measured estimates from the ultrasound studies mentioned before).
Towards its point, the muscle is composed of increasingly tough fibres which blend into the bone and surrounding muscle coverings … it is not a good target for your needle. (Lorne et al, 2001; Klepps et al, 2004; Rispoli et al, 2009).
The best immune response to vaccines is from the fleshy intramuscular tissue, probably because of its good blood supply (Zuckerman, 2000; Cook, 2021).
An imaginary line drawn between the front and back skin folds of the armpit (antero-posterior axillary line) defines the point of the deltoid attachment as a triangular tip to the muscle (Rispoli et al, 2009). It is emphasised in Figure 5 by the sheet of paper, and subsequently by a line of plastic, which you can envisage in your mind's eye on your patient.
Choosing a suitable target from the different governmental immunisation guidelines, or the many books and websites, is confusing. They vary from advising specific defined points to vague triangles of differing sizes over the deltoid, some even with the axillary nerve shown lying provocatively in the middle (Bancsi et al, 2018; Centers for Disease Control and Prevention, 2020; New Zealand Ministry of Health, 2020; Public Health England, 2021a; Australian Government, 2021). Levels are described from 2.5 cm and 2–4 fingers' breadths from the acromial edge, some seeming to aim above or below the nerve, and risking the shoulder (Cook, 2011; Davidson and Rourke, 2013). The use of finger widths is also controversial … in bygone days when medical texts were written for a predominantly male audience, a width of 2 cm (Becker et al, 1971) might have been acceptable, but nowadays with a wide variety of vaccinators, this is no longer the case. It may be wise to measure your own hands if you intend to use them as a ruler (Davidson and Rourke, 2013).
Busy front-line staff are hardly likely to use a tape measure on each shoulder, and as there is at least some proportionality between patient size and nerve position (Hachadorian et al, 2020), the best advice must be to use the patients' own fingers for reference, and not yours (Saunders and Longworth, 2019).
In this illustration (Figure 6), our female subject (at 5 foot 4 inches, average height for the UK) has swung her opposite arm across and aligned her index finger with the tip and lower edge of the acromion. In this position her hand covers the lowest recorded level of the axillary nerve in any study, and all that lies above: joint, bursa, tendons etc. As demonstrated, there is still an adequate safe target site in the anterior/central deltoid lying between her little finger and the muscle insertion.
This is a simple manoeuvre provided the area is suitably exposed and identified; a basic requirement when doing a proper job.
Which needle?
In order to achieve the best immune response, the vaccine needs to be deposited in muscle. This has been suggested for all vaccines, with a couple of exceptions, BCG and rotavirus (Cook, 2021).
It is also important to try not to get it into the subcutaneous layer as this can cause local reactions and complications (including nodules, fat loss, cellulitis and sterile abscesses) (Cook, 2015).
While these effects may not be as severe as nerve and joint damage, they are far more common.
Needle length recommendations across the world (Centers for Disease Control and Prevention, 2020; New Zealand Ministry of Health, 2020; Australian Government, 2021; Public Health England, 2021a) all seem to refer to conclusions drawn from one seminal study of the deltoid area (Poland et al, 1997). This used skin fold callipers and ultrasound to measure from the skin surface to the muscle layer, and ultrasound to record the depth of muscle to bone. The findings were then related to the weight, body mass index (BMI), and sex of the participants.
The measurements showed consistently that the fat layer is deeper in women, and the muscle thickness greater in men, but that passing a needle tip to a depth of at least 5 mm into muscle could be achieved by using an identical length needle in both sexes.
Their recommendation of employing a 25 mm needle for all adults weighing between 60 kg (9 st 6 lb/approx 130 lb) and 90 kg (14st 2lb/approx 200 lb) has been adopted internationally (23 gauge [blue hub], or 25 [orange] for these or shorter needles).
In women weighing more than 90 kg, the fat layer was too deep for a 25 mm needle, and a 38 mm was suggested; this length is now also standard for men over 118 kg (18 st 8 lb/approx 260 lb) (21 gauge [green] or 22 gauge [black] usually).
In women lighter than 60 kg, a shorter needle (16 mm) was suggested, an idea no longer emphasised or even mentioned in some current guidance (Public Health England, 2021a).
It is notable that the key study was based on 220 healthy American health workers aged 18–59 years, whereas most routine vaccinations are given to the elderly – 80% of UK over 65 year olds had flu jabs in winter 2020/2021 (Public Health England, 2021b).
There have been subsequent studies on other populations. An Australian group of subjects aged 65 years and over showed a reduced muscle depth and tendency to a thicker fat pad (Cook et al, 2006), so that the recommendation remained a 25 mm needle, but the study failed to mention that in their males and females with a BMI below 20, that needle depth would reach bone. Leaner populations, eg one from India (Shankar et al, 2014), showed that 25 mm needles could reach bone in nearly 50% of cases, and in a cohort of Japanese 20 year olds, those with BMI <20, 33% fell into the same category (Nakajima et al, 2017).
Clearly being of low body weight increases the risk of over penetration with 25 mm needles. Should only a fixed size needle be available, then consideration of how far to insert is essential.
The thickness of the skin itself is quite consistent over the deltoid at about 2 mm (Laurent et al, 2007), but the subcutaneous tissue is much more variable. Fat cells are more tightly bound closer to the surface, but are generally more free to move and slide immediately above the tough muscle coverings (deep fascia) (Stecco, 2015).
This allows skin folds to be raised by a pinching action, viewed, and their thickness quantified with calipers. The depth of the skin and subcutaneous tissue will be half that of the total skin fold measurement (Figure 7). In some studies the depth to muscle has been measured again by ultrasound and compared for accuracy. These findings show that the estimates made from skin fold thickness are very close to ultrasound values below about 1 cm, but with decreasing precision above that (Poland et al, 1997; Selkow et al, 2011; Nakajima et al, 2017).
Figure 7. Raising a skin fold can be used to estimate the depth to muscle, but does not work well with greater fat deposits.
For our lean, frail patients, a simple pinch will be a guide for the depth down to the deep fascia, but it should also be noted that muscle thickness decreases with ageing (Volpi et al, 2004), and the deltoid can be as little 4–5 mm thick in the very elderly (Komatsu et al, 2014). In such cases inserting just a small part of a 25 mm needle will be required, or reverting to a shorter length.
With experience, it may be possible to detect when the advancing needle has passed through subcutaneous tissue and reaches the muscle's fascial layer.
Patient positioning
In all the photographs and recommendations in this article, the position adopted is with the subject sitting allowing the whole arm to hang loosely down against the chest wall. When done comfortably, the palm faces in towards the body and the thumb forwards.
The needle should be inserted perpendicular to the skin, and not pushed in to the hilt, ie not flattening the skin, nor pinched into a ridge.
All advice and measurements are based on these fundamentals.
[Some authorities suggest different positioning (eg Cook, 2011), with the hand resting on the hip, fingers on the abdomen and thumb pointing backwards. This alters the targeting significantly… the nerves move closer to the acromion, the arm lies at an angle to the trunk so the level of the armpit relative to the deltoid insertion is altered, and if the needle is inserted horizontally rather than perpendicular to the skin, an increased length is required to reach muscle. This position also rotates the humerus on its axis and will tend to bring the nerves further forward.]
Conclusion
By applying current knowledge of shoulder anatomy, and carefully allowing for the patient's weight and build, it should be possible to vaccinate into the deltoid muscle with a minimum of risk.
KEY POINTS:
- Key landmarks of the shoulder should be clearly identified
- The patient's own hand is guided as shown, to align with the lower edge of the acromion
- This manoeuvre should shield the joint structures and the axillary nerve and its accompanying vessels
- The safe injection site lies between the covered area and the deltoid insertion, keeping away from the posterior part of the muscle
- An appropriate needle length should be chosen for the patient's weight and build
CPD reflective practice:
- How confident are you with the anatomy of the deltoid? Take some time to consider your own body, and locate the important landmarks identified here
- Why is it important to inject a vaccine into the muscle (in most cases)?
- What factors affect the selection of needle size for an individual patient?