Intended intramuscular gluteal injections: Are they truly intramuscular?L Dayananda, VV Belaval, A Raina, R Chandana
Department of Radiology, Narayana Hrudayala and Muzumdar Cancer Center, Bangalore, Karnataka, India
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0022-3859.132334
Source of Support: None, Conflict of Interest: None
Context: In patients with obesity, intramuscular injections may be deposited subcutaneously due to an increase in gluteal fat. We aimed to use abdominal CT done in our institute for gluteal fat thickness to test our hypothesis. Materials and Methods: After IRB approval, CT scans of the abdomen and pelvis of the past 6 months were analyzed. The thickness of gluteal region subcutaneous fat was measured in a standardized manner. Results: Out of 700 CT scans, studied, 476 were males and 224 were females. The average gluteal fat thickness was 2.34 cm +/- 1 cm. The average fat thickness in males was 1.98 cm +/- 0.98 cm whereas in females was 3.0 cm +/- 1.2 cm. Subcutaneous granulomas were seen in 17 cases and one injection granuloma in the intramuscular plane. Conclusion: A significant number of female patients had increased gluteal fat thickness beyond the reach of routinely used needles. The medications in these patients will thus be unintentionally injected to subcutaneous plane, possibly altering the pharmacokinetics.
Keywords: Fat thickness, gluteal injection, granuloma
Drugs are given via the parenteral route to increase the bioavailability and rapid onset of action. Intramuscular injections deliver drugs into well-perfused muscles, providing the former and also allowing for larger dose deposition. The gluteal region is a common site of administration of intramuscular injections. Plasma concentration of the injected drug varies depending on whether the drug reached muscle or fat. , With an increasing incidence of obesity, there is concern regarding intended intramuscular injections reaching subcutaneous fat. The present study aimed to study the average fat thickness of the population presenting to our institute.
After IRB approval, data was anonymized and analyzed.
We retrospectively analyzed 700 abdominal CT scans done between June and December 2010.
The probable site of gluteal injections weree identified by three different methods. The first two methods of localization are ones developed by the author, based on clinical localization of injection site.  The dorsogluteal site of injection is initially identified on CT scanogram similar to clinical localization [Figure 1]a-c. The line joining the superior trochanter of the femur and superior part of the sacroiliac (SI) joint divides the gluteal region into superior-lateral and inferio-medial quadrants. A line is drawn perpendicular to this imaginary line at the midpoint which divides the supero-lateral quadrant of the gluteal region into upper and lower halves. The center of the upper half of the supero-lateral quadrant was localized on the scanogram. Using the 3D localization software the region was also localized on axial sections. Subcutaneous gluteal fat thickness was measured in the same plane.
The ventrogluteal injection site was also localized on the scanogram in a method similar to clinical localization[Figure 1]d-f. A line was drawn from the center of the head of the femur to the anterior superior iliac spine. Another line was drawn vertically from the head of the femur to the superior most part of the iliac crest, thus forming a triangle. The center of the triangle was considered as the probable site of injection.
The third method was adapted [Figure 2].  In this method a axial section of the CT scan was chosen which was 1 cm above the lowermost part of the SI joint. The subcutaneous fat thickness 5 cm away from the SI joint was considered as a probable site of gluteal injection.
Subcutaneous fat was measured by all three methods. Associated finding like subcutaneous granuloma, abscess or air due to gluteal injections was also noted. The lengths of the commonly used needles in our hospital were taken into consideration.
Statistical analysis: Descriptive statistics were used for demographic data. Measures of central tendency were used for gluteal fat. Agreement between assessments between the three measures was done using the kappa statistic.
Out of 700 CT studied, 476 were males and 224 were females. The age ranged from 3-91 years. These patients underwent CT studies for various abdominal complaints like pain evaluation, mesenteric ischemia, intra abdominal mass ,intra abdominal collection, infection etc.
Gluteal fat thickness
The average gluteal fat thickness was 2.34 cm +/- 1 cm. The gluteal fat thickness ranged from 0.3 mm to 7.3 cm. The average fat thickness in male was 1.98 cm whereas in female was 3.0 cm. Maximum fat recorded in our series was 7.3 cm in a 54-year-old female. Minimum fat thickness was 0.3 cm in a 17-year-old male.
Sub-age group analysis showed that the females between 20 and 40 years of age had thicker gluteal fat pads (3.4 cm) as compared to females of other age groups [Table 1]. It was found that 70% of the females were having gluteal fat thickness above the length of commonly used 20-G and 22-G needles. On the contrary, only 20% of the males were having fat thickness above the length of commonly used needles. No significant variations were seen in gluteal fat thickness in men of different age groups.
Granulomas were seen in 18/ 0.02 % cases. All the granulomas were seen in the subcutaneous plane except for one granuloma which was in the muscular plane [Figure 3] and [Figure 4].
Muscles have rich vascular supply in contrary to subcutaneous fat which is relatively less vascular.  The bioavailability of the subcutaneous injections is different from that of drugs injected intramuscularly. This is because the intramuscularly injected drugs are absorbed better. Subcutaneous root of injection s preferred only when slow and sustained action is desired. , Dorsogluteal injections are associated with increased possibility of damaging the sciatic nerve as compared to the ventrogluteal site. Hence, the ventrogluteal site is preferred. 
The length of the commonly used 22-G and 20-G needle in our hospital is 25 mm. We presume that that a fat thickness more than 25 mm results in subcutaneous deposition, without considering the compressibility of fat into account. Since 70% of female population in our study did have subcutaneous adipose tissue thickness more than 2.5 cm, gluteal injection in these individuals are likely to have been subcutaneous. On the other hand, only in 20% of male population these needles would have reached the intramuscular plane.
The average fat thickness was more in females of 20-40 years age group as compared to other age groups. About 70% of the females between 20 and 40 age group have muscles beyond the reach of the blue and green needles. On the contrary, only 20% of males have fat thickness above the reach of the blue and green needle.
Vukovitch et al. reported sex difference in absorption and bioavailability of the cephadrine injected in the gluteal region.  and concluded that the sex difference was related to greater thickness of the subcutaneous fat in female.
Three different methods were employed in our study to measure the fat thickness in various locations of the gluteal region. There is very good correlation (k +0.61) between the various methods suggesting that the distribution of the fat is more or less uniform all over the posterior quadrants of the gluteal region. In our observation gluteal adipose tissue is thicker in the extreme lateral aspects of the buttock which is not measured by any of the three methods. All patients with granulomas had a history of prior gluteal injections, suggesting that the injections were the likely cause for granulomas. We assume that the absorption of the injected drug plays a major role in granuloma formation and that poor absorption is the cause for granuloma.  Due to lack of patient details, further analysis was not done. Limitations of the study were not taking compressibility of the fat into account, not studying causes of granuloma, not using BMI, and lack of healthy controls. These could be addressed in a prospective study.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]