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Optimal Intramuscular Needle-Penetration Depth

^a School of Public Health and Tropical Medicine, Tulane University, New Orleans, Louisiana
^b Division of Orthopaedic Surgery, Cincinnati Children's Hospital Medical Center, Cincinnati, Ohio

	ABSTRACT

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OBJECTIVE. The US Centers for Disease Control and Prevention has needle-length recommendations for intramuscular vaccinations in the thigh and shoulder on the basis of the age of the child. Underpenetration of the intramuscular layer with short needles has been documented; however, few studies have focused on the risk for overpenetration of the intramuscular level with needles that are too long. The purpose of this study was to determine the optimal needle length for intramuscular vaccination of children of various ages and sizes at the shoulder and thigh levels by using MRI and computed tomography scan measurements.

METHODS. A total of 250 MRI and computed tomography scans of shoulders and thighs of children who were 2 months to 18 years of age at a large children's hospital were reviewed. The thicknesses of the subcutaneous fat tissue and muscle layers were measured. Measurements were correlated with age and weight, and regression analysis was performed.

RESULTS. Use of the Centers for Disease Control and Prevention's recommended 1- and 1 -in needles for intramuscular vaccination in the thigh of children 1 year of age would result in 11% (11 of 100) and 39% (34 of 88) overpenetration, respectively, with a minimal risk for underpenetration at 2% (2 of 100). Patients with vaccinations in the shoulder with the Centers for Disease Control and Prevention–recommended -, -, and 1-in needles would experience 11% (16 of 150), 55% (83 of 150), and 61% (92 of 150) overpenetration, respectively.

CONCLUSIONS. There is a substantial risk for overpenetration of the intramuscular layer when using current Centers for Disease Control and Prevention recommendations for vaccination needle lengths. We recommend a revision of the needle-length guidelines for thigh and shoulder injections to minimize the risk for needle overpenetration on the basis of the variability observed in the fat thickness.

Key Words: vaccine • intramuscular injection • anterolateral thigh • deltoid

Abbreviations: CDC—US Centers for Disease Control and Prevention • CT—computed tomography

Intramuscular injection is recommended for most childhood vaccines. The thigh (vastus lateralis muscle) is the preferred intramuscular injection site for infants up to 12 to 18 months. Intramuscular injections are typically administered in the shoulder (deltoid muscle) for children beyond this age. Although there is some age overlap between the 2 sites, the deltoid muscle is often favored in children who are older than 1 year because it is associated with less pain in the affected extremity when ambulating and results in fewer and milder adverse reactions.^1,2 Intramuscular injection has several advantages over subdermal and intradermal delivery. Vaccines are preferably given at the intramuscular level to minimize local irritation, inflammation, granuloma formation, and tissue necrosis.^1–13 Furthermore, intramuscular vaccines, when properly delivered to the intramuscular level, increase immunogenicity and produce a better immune response than when they are delivered at the subcutaneous or intradermal level.^{1,2,4–8,10–14}

Several research groups have studied the risk for short needles' inadvertently underpenetrating the intramuscular level and delivering the vaccine in the subcutaneous fat tissue^{1,3–7,9,10,14,15}; however, the potential for needle overpenetration of the intramuscular level into the bone or periosteum has been given little examination because of its perception as being a minor problem. Overpenetration can cause pain and/or damage to the bone or periosteum in the patient who receives the vaccination. It can also cause the needle to detach from the syringe.

The February 2007 US Centers for Disease Control and Prevention (CDC) Pink Book recommended a 1-in needle for all thigh intramuscular immunizations in infants aged 1 to 12 months and; a 1-in to 1-in needle for all thigh intramuscular immunizations in toddlers aged 12 to 24 months; and a -in to 1-in needle for all deltoid (shoulder) intramuscular immunizations of children of all ages from 1 to 18 years.^16,17 All recommendations from the CDC are outlined in Table 1.

	METHODS

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After institutional review board approval was obtained, a total of 250 patients from a large children's hospital were included in this study. We collected data on patients who had an MRI or a CT scan of their normal thigh between the ages of 2 months and 6 years and patients who had an MRI or CT scan of their normal shoulder between the ages of 12 months and 18 years.

One-hundred patients between the ages of 2 months and 6 years had an MRI or a CT scan of their thigh. We measured the subcutaneous fat and muscle in the middle third of the vastus lateralis (anterolateral of the thigh) at a 90° angle to the skin, which is the recommended CDC site and technique for intramuscular injection (Figs 1 and 2).

View larger version (100K): [in this window] [in a new window]   FIGURE 1 Fat and muscle thickness measurements in the thigh using MRI.

View larger version (66K): [in this window] [in a new window]   FIGURE 2 Fat and muscle thickness measurements in the thigh using a CT scan.

View larger version (115K): [in this window] [in a new window]   FIGURE 3 Fat and muscle thickness measurements in the deltoid using MRI.

View larger version (80K): [in this window] [in a new window]   FIGURE 4 Fat and muscle thickness measurements in the deltoid using a CT scan.

	RESULTS

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Thigh
Thigh measurements were taken from MRI or CT scans of the thigh of 100 children aged 0 to 6 years; 12 were aged 0 to 1 year, and 88 were aged 1 year to 6 years. Using the CDC recommended needle lengths for the thigh muscle, overpenetration would occur in 11% (11 of 100) of patients who received an injection with a 1-in needle and 39% (34 of 88) of patients (age 1 year) who received an injection with a 1-in needle (Figs 5–7). Underpenetration would occur by using the 1-in needle for patients aged 1 year only 2% (2 of 100) of the time. The -in needle for intramuscular injection in the thigh (not recommended by the CDC) would result in a 4% (4 of 100) overpenetration rate and a 2% (2 of 100) underpenetration rate.

View larger version (18K): [in this window] [in a new window]   FIGURE 5 Depth from skin to bone in patients from 2 to 12 months of age who would receive an intramuscular vaccination in the thigh on the basis of recommendations from the CDC. The age is in months, and the tissue (skin-to-bone) depth is in millimeters. The skin/subcutaneous fat thickness is represented by the white area inside the dotted lines; the muscle thickness is represented by the shaded region; and the bone is represented by the white area above the shaded region.

View larger version (31K): [in this window] [in a new window]   FIGURE 6 Depth from skin to bone in patients 1 to 2 years of age who would receive an intramuscular vaccination in the thigh on the basis of recommendations from the CDC. The age is in months, and the tissue (skin-to-bone) depth is in millimeters. The skin/subcutaneous fat thickness is represented by the white area inside the dotted lines; the muscle thickness is represented by the shaded region; and the bone is represented by the white area above the shaded region.

View larger version (41K): [in this window] [in a new window]   FIGURE 7 Depth from skin to bone in patients 3 to 6 years of age who would receive an intramuscular vaccination in the thigh on the basis of recommendations from the CDC. The age is in months, and the tissue (skin-to-bone) depth is in millimeters. The skin/subcutaneous fat thickness is represented by the white area inside the dotted lines; the muscle thickness is represented by the shaded region; and the bone is represented by the white area above the shaded region.

View larger version (22K): [in this window] [in a new window]   FIGURE 8 Depth from skin to bone in patients from 1 to 2 years of age who would receive an intramuscular vaccination in the deltoid on the basis of recommendations from the CDC. The age is in months, and the tissue (skin-to-bone) depth is in millimeters. The skin/subcutaneous fat thickness is represented by the white area inside the dotted lines; the muscle thickness is represented by the shaded region; and the bone is represented by the white area above the shaded region.

View larger version (46K): [in this window] [in a new window]   FIGURE 9 Depth from skin to bone in patients from 3 to 18 years of age who would receive an intramuscular vaccination in the deltoid on the basis of recommendations from the CDC. The age is in months, and the tissue (skin-to-bone) depth is in millimeters. The skin/subcutaneous fat thickness is represented by the white area inside the dotted lines; the muscle thickness is represented by the shaded region; and the bone is represented by the white area above the shaded region.

	DISCUSSION

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Conversely, the results were not as significant for the thigh muscle. Our results showed that overpenetration of the thigh muscle would occur in 11% (11 of 100) of patients who were administered an injection with a 1-in needle and 39% (34 of 88) of patients (age 1 year) who were administered and injection with a 1-in needle (Figs 5–7). Hicks and colleagues^2,6,11,16,17 found subcutaneous fat thickness measurements in the thigh similar to ours and agreed with the CDC recommendation that a 1-in needle should be used when vaccinating at a 90° angle a child, aged 1 to 12 months, in the thigh. Consequentially, we recommend a -in or longer needle for all children up to age 6 years to ensure proper intramuscular injections in the thigh muscle (Table 4). With these recommendations, 90% of all children who receive a vaccination in the thigh will be vaccinated into the intramuscular level, in comparison with the 64% intramuscular delivery rate if using the current CDC recommendations (Figs 5–7).

A standard needle size for a particular weight, age, or gender will not guarantee successful intramuscular injection in all patients as the CDC and other institutions advise because of the significant anatomic variability, especially in the deltoid muscle layer. This is confirmed through the regression analysis, because a linear trend between skin-to-bone thickness, fat thickness, and muscle thicknesses with age in the 1- to 2-year age group was not observed. Also, a linear trend was not seen between fat thicknesses with age in the 3- to 18-year age group. Similar results occurred in the thigh muscle for the 3- to 6-year age group (Table 3). The sample sizes for the 3- to 18-year age group in the deltoid and 3- to 6-year age group in the thigh are substantial enough to suggest that the subcutaneous fat tissue at these injection sites is the possible cause for the variability seen in the overall results. If the other age groups for the thigh and deltoid muscles had larger sample sizes, then the same trend may be witnessed. In addition, the results of the 2-sample t tests show that the boys and girls measured at the deltoid muscle site show significant variations in fat and muscle thicknesses for the 3- to 18-year age group. A significant variation in the 1- to 2-year age group of the deltoid muscle between boys and girls was seen only in the fat thickness. Along with the variability between fat thickness and age, the variability between genders may have contributed to the strong variability in the deltoid muscle. Consequentially, we recommend that clinical judgment be used and an assortment of different needle lengths be readily available for vaccine injection. The Groswasser et al⁴ and Zuckerman⁵ studies made similar recommendations, as well as promoted the use of clinical judgment in selecting appropriate needle length during vaccine administration to optimize the success of intramuscular delivery. An ultrasound may be used to determine the skin-to-bone thickness before administering the vaccine, but this adds a level of complexity to the process. The variability in our data does not represent the entire variability seen in the general population. Thus, the CDC may want to alter their recommendations to include using clinical judgment, because a standard needle size for a certain age group or weight class will never guarantee proper intramuscular administration in all patients. The pinch technique, in which the vaccinator pinches a fold of thigh or arm tissue, may decrease the risk for overpenetration, but this technique may increase the risk for underpenetration. The accuracy of the pinch technique has not been validated. Typically, the pinch technique is used when administering subcutaneous vaccines to avoid the possibility of injecting the vaccine at the intramuscular level.¹⁷ The spreading technique is executed to ensure an intramuscular injection, because it reduces the thickness of the subcutaneous layer.¹⁷ We suggest that a future study be conducted to assess the pinch technique on a wider range of US children by using ultrasound. The pinch technique may increase vaccine safety with the current CDC needle-length recommendations. Given that this study has yet to be conducted, skinfold thickness measurement with calipers may be a simple way to improve the success of intramuscular vaccine delivery.

Our study has several limitations because of its retrospective nature. Although this is one of the largest studies to report on vaccination anthropometric data, the weight and age of the patients are not evenly distributed throughout the large range. Our sample from a large Midwestern city may not accurately represent the general population of the United States. Because of wide geographic variation in infant and child weight, it is difficult to make universal needle-length recommendations. Obesity in children and adolescents has increased tremendously in many Western countries, especially in the United States, and can depend on ethnic, socioeconomic, and religious status.^18–20 There are a high number of undernourished children who are younger than 5 years in India and China,^21,22 which likely skew the results of the Lo et al and Chugh et al studies. The tremendous variability in fat and muscle thickness found in our study makes universal intramuscular vaccine needle-length recommendations difficult. The discrepancy in the results attained from our study in comparison with the Chugh et al,⁹ Cook and Murtagh,¹³ Groswasser et al,⁴ Hick et al,⁶ and Lo et al¹⁰ studies could be because we used MRIs and CT scans, which are deemed more accurate in measuring and showing the distinction between muscle and fat layers because they do not use a probe that touches or compresses the skin.²³

We used the healthy shoulder and thigh on MRIs and CT scans that were obtained for other reasons, and this population may not exactly represent the vaccination population at large. We used the normal side of a bilateral shoulder or thigh MRI or CT scan from which to make measurements. Most of the MRI or CT scans for the young shoulders were performed for brachial plexus palsy on the opposite side, and these children are known to have higher birth weights than a typical newborn.²⁴ This would tend to underestimate the risk for overpenetration. The measurement accuracy in Picture Archiving Communications System for MRI and CT scans was another possible limitation. The measurement error was ±1 pixel when the image was scaled to its absolute size or original resolution (1:1).^25,26 Furthermore, the spacing of the pixels was not a standard size for all patients, and the average measurement of each pixel in all the patients was ±0.63 mm.

In addition, the results of our study assume that the needle is fully inserted so that the hub touches the patient's skin. Typically, overpenetration has not been perceived as a major problem in comparison with underpenetration because vaccinators are advised simply to pull back before depositing the vaccine if they sense that the needle has hit the bone¹²; however, striking the bone with the needle is liable to cause pain, damage to the bone or periosteum, and/or breakage of the needle.

	CONCLUSIONS

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Our study supports the conclusion that a - or 1-in needle is excellent for thigh intramuscular injections in both genders of infants and children up to age 6 years, which is similar to the CDC recommendations; however, our data suggest that the CDC recommendations for needle length for intramuscular injection in the shoulder will overpenetrate the muscle layer and strike bone or periosteum in 11%, 55%, and 61% of patients who receive a -, -, or 1-in needle. This could cause severe pain and also impair delivery to the intramuscular level. This has prompted the following recommendations for vaccination in the shoulder: a -in needle for any girl who weighs 70 kg and any boy who weighs 75 kg; a -in needle for any girl who weighs between 70 and 115 kg and any boy who weighs between 75 and 140 kg; and a - to 1-in needle for any girl who weighs 115 kg and any boy who weighs 140 kg to ensure proper intramuscular injection (Table 4).

	ACKNOWLEDGMENTS

 
We thank Sara Stark, Carrie Schmitt, and the Orthopaedic Clinic Staff at the Cincinnati Children's Hospital Medical Center for assistance. We also thank Dr Janet Rice (Department of Biostatistics, Tulane University School of Public Health and Tropical Medicine) for assistance with data analysis.

	FOOTNOTES

 
Accepted May 30, 2008.

Address correspondence to Eric J. Wall, MD, Cincinnati Children's Hospital Medical Center, 3333 Burnet Ave, MLC 2017, Cincinnati, OH 45229. E-mail: eric.wall{at}cchmc.org

The authors have indicated they have no financial relationships relevant to this article to disclose.

What's Known on This Subject During vaccine administration, overpenetration of the intramuscular layer causing the needle to strike the bone often has been overlooked. Authors who have focused on overpenetration have deemed it better than underpenetration.

 

What This Study Adds The CDC guidelines for vaccine needle lengths should be revised. We found a significant risk for overpenetration by using the guidelines. A universal needle length according to certain age, weight, or gender groups is unacceptable because of the tremendous variability present.

 

	REFERENCES

TOP ABSTRACT METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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This article has been cited by other articles:

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