Intramuscular (IM) injections are usually used to administer high-viscosity or large-volume medications and long-term injections such as biological agents, hormones, corticosteroids, and antibiotics [1-4]. IM injection sites usually use the deltoid, gluteal muscles, and vastus lateralis. The gluteal region, one of three IM injection sites, is a frequent target of IM injection for high volumes because it comprises three large muscle groups [2, 4].

According to the World Health Organization (WHO), 16 billion injections administered every year and approximately 90% among them are given into intramuscular or subcutaneous or intradermal [5]. IM injections to the gluteal region usually use two regions: dorsogluteal and ventrogluteal. The dorsogluteal site is known as the “traditional” site for IM injection into the posterior gluteal region and is located in the upper outer quadrant of the gluteal region [1, 4, 6]. The ventrogluteal site is called the “V method” and is located using some bony landmarks such as the greater trochanter, anterior superior iliac spine, and the iliac crest. It is injected via the V space created between the second and third fingers as described above [1, 7, 8]. There has been much controversy as to which of these two IM injection sites is more successful or safer. However, the ventrogluteal site is claimed to be preferable to the dorsogluteal site [9, 10]. Korean Accreditation Board of Nursing Education has suggested to learn both the ventrogluteal and dorsogluteal regions as the IM injection site in the gluteal region. Nevertheless, in hospitals, only the dorsogluteal region used for IM injection in Korea until now. Newly graduated nurses and health provider feel be confused when injecting gluteal intramuscularly due to this difference.

Several studies have been conducted to define the factors that influence successful IM injection outcomes. Unsuccessful IM injections are mostly the result of injection into fat, not muscle, or those causing nerve damage. Physical characteristics such as the patient’s sex, subcutaneous fat and muscle thicknesses, body mass index (BMI), body shape, nurse’s skills, and the size of syringes and needles appear to be influencing factors. Studies have shown that subcutaneous fat thickness and BMI are important factors in selecting the IM injection site and size of the needle [1, 8, 9, 11]. Previous studies reported that subcutaneous fat thickness and BMI are also related to sex and population groups. Therefore, suitable IM injection sites and techniques such as the size of syringes should be targeted for such groups [3, 12-14].

The aims of this study were to compare the subcutaneous fat and muscle thicknesses at the dorsogluteal and ventrogluteal sites, to determine the influence of sex and BMI on subcutaneous fat and muscle thicknesses, and to determine the most suitable site for IM injection in not only Koreans but also Asian-Pacific populations. In addition, it is possible to provide evidence-based practice for conducting education that can help not only undergraduate students but also health providers, including new nurses, for IM injection in the gluteal region.

Eleven fresh cadavers of known sex, age, height, and weight were obtained from the anatomy laboratory. Height and weight of cadavers were measured in an anatomy laboratory. The institutional review board of the Catholic University of Korea ruled that a cadaveric study is beyond its review authority. The average age of cadavers was 81.7 years (female 85.7 years and male 78.4 years). The average weight was 53.7 kg, height was 158.6 cm, and BMI was 21.1 kg/m². For male cadavers, these were 60.5 kg, 164.5 cm, and 22.3 kg/m², respectively and for female cadavers, they were 45.6 kg, 151.6 cm, and 19.8 kg/m². According to BMI category, under BMI was 2 of 11 cadavers, 6 normals, 3 obesities, respectively.

Gelatin was completely liquefied by heating and was then mixed with two different color paints for injection. Colored gelatin using 3 ml syringes (23 gauge, 2.5 cm), which are usually used in hospitals, was injected into the dorsogluteal and ventrogluteal sites bilaterally for all cadavers by one registered nurse. The dorsogluteal site is identified superior to a line extending from the posterior superior iliac spine to the superior border of the greater trochanter. By it can be divided into quadrants and is injected the upper lateral quadrant area of the gluteal region [15]. The ventrogluteal site is determined by placing the heel to the bed and the palm on the greater trochanter of the femur, the index finger pointing to the anterior superior iliac spine, and the third finger is extended along the iliac crest. This is known as the “V” method involving an injection into the space created between the second and third fingers of the injecting individual [16]. After the IM injection of colored gelatin, the injection sites were marked by stiches and cadavers were placed in a freezer at –50°C for one week. Using an electric chainsaw, they were cut in the transverse plane at the marked injection sites and at intervals of 1 cm cranial and caudal to them. The cut parts of the gluteal region were photographed using a digital camera (EOS 5D Mark IV; Canon Inc., Tokyo, Japan). All measurements were made using photographs and performed by the same registered nurse who carried out the injections (Fig. 1). All procedures such as injection, freezing, cutting, and taking photographs, were carried out with the cadavers in the prone position.

Four variables were measured using graphic analysis software (Photoshop CC 2018; Adobe, San Jose, CA, USA) (Table 1). First, the body part in a photograph was arranged in a prone position and then horizontal and vertical lines were drawn through the colored gelatin core for measurements. Four measured variables were taken on both the dorsogluteal and ventrogluteal sites twice. There were no statistically significant differences between the values measured twice, so the mean of the two values was used for subsequent analysis. Correlation analysis was used to ascertain the relationship between each measurement and the cadaver’s BMI. Data were analyzed using IBM SPSS Statistics for Windows (version 26.0; IBM Corp., Armonk, NY, USA).

All variables—four direct measurements, two ratios and one percentile—did not show a statistically significant difference between male and female cadavers in two regions (Table 2). Four of the variables showed statistically significant differences between the ventrogluteal and dorsogluteal regions: namely fat thickness (P=0.001), muscle thickness (P=0.017), the ratio of fat thickness to total length (P=0.006), and the ratio of muscle thickness to total length (P=0.006).

This study was examined to identify the most suitable IM injection site in the gluteal region for Koreans using cadaveric injections. There were statistically significant differences in four measurements related to the fat and muscle between the ventrogluteal and dorsogluteal regions. The position of the injected gelatin core had a statistically significant correlation with BMI category for both the ventrogluteal and dorsogluteal regions, but only in male cadavers.

It means that successful or safe IM injection is injected into the muscle and there are no adverse side effects. Recently, studies have been conducted to measure the fat thickness at the injection site or to determine the relationship with BMI for safer IM injection [8, 9, 13, 17]. These found that sex and BMI influenced subcutaneous fat thickness and muscle thickness and also affected successful IM injection. According to these studies, total tissue and fat thicknesses were thicker at the dorsogluteal region than at the ventrogluteal region. In this study, the total length and muscle thickness at the dorsogluteal region were greater than at the ventrogluteal region, but the fat thickness was thicker in the ventrogluteal region.

There were statistically significant differences between the ventrogluteal and dorsogluteal regions not only measurements related to the fat and muscle but also the position of the injected gelatin core. The position of the injected gelatin core from the bone surface was at an average of 78.0 percentile points in the ventrogluteal region and at 74.8 percentile points in the dorsogluteal region. As per BMI categories, the injected core positions it in the ventrogluteal region were at 64.5, 67.1, and 109.0 percentile points in order from the underweight to the obese categories, and at 74.3, 73.0, and 78.9 percentile points in the dorsogluteal region, respectively. For the obese cadavers, it is possible that successful IM injection in such individuals might be less effective in the ventrogluteal region than in the dorsogluteal region.

With the rapidly increasing prevalence of obesity in Asia, including Korea, the BMI indirectly represents the degree of obesity and has been used as a disease predictor [18, 19]. Previous studies have shown that BMI and fat thickness have an effect on each other, the probability of successful IM injection may be relatively low in such cases if of the BMI is high [1, 9, 11, 17]. However, because the BMI categories proposed by the WHO are based on non-Asians, and Asian populations have a higher percentage of body fat at a low BMI than other population groups, a new standard has been proposed for such populations [3, 18]. In this study, the Asia–Pacific BMI categories were used to classify the cadavers from underweight to obese. Among them, the risk of IM injection failure was high only in cases classified as obese, and in cases classified as underweight being at risk of bone contact with the ventrogluteal IM injection. Consistent with other studies, this study also found that cadavers with a high BMI had a thicker fat layer. For such cases, there is a high risk of IM injection failure into the ventrogluteal region.

In conclusion, muscle or fat thicknesses are factors that affect the success of IM injection, and it should be possible to predict in which subjects one, neither, or both of the gluteal injection regions are suitable. However, comparing the total length of the gluteal IM injection site and the position of the injected gelatin core was a more accurate method to do this. This study might be suggested to determine the injection site according to the BMI rather than sex when injecting gluteal intramuscularly. As a result of this study, in the case of obesity, the fat is thicker in the ventrogluteal region than in the dorsogluteal region. Although the number of cadaver used in this study was small, it might be meaningful that both regions were injected at the same time, and all of the injected sites were directly checked and compared. In particular, it may be suggested that the using traditional method is better than the ventrogluteal region in the ethnic groups in the Asia-Pacific region including Koreans. Educators in universities and hospitals should educate students and health providers on selecting the IM gluteal injection site in consideration of the BMI in the Asia-Pacific population group.