The Effectiveness of a Non-Invasive Shot Blocking Device for Reducing Pain of In-office Injections in Hand Surgery

Abstract

Background: The gate control theory asserts that non-painful stimuli can block pain perception. The ShotBlocker™ device is a plastic disk with blunt projections that rests on the skin, and we hypothesize that it will reduce pain during hand injections. Methods: This is a prospective randomized trial of 117 patients undergoing injections for common hand conditions. Patients were randomized into 3 groups: device, placebo (device with projections removed), and control. Patients recorded on an analog pain scale the pain severity of the injection, as well as their most recent tetanus shot. A normalized pain score was obtained from the difference between the injection and tetanus shot pain scores. The mean non-normalized and normalized scores for each treatment group were compared to the control group using the Wilcoxon signed rank test. Results: There were 91 women and 26 men. Common diagnoses included trigger finger (n = 53), DeQuervain’s tendonitis (n = 33), and basal joint arthritis (n = 22). The groups did not differ significantly in age, gender, or diagnosis. Mean pain score in the device group was 5.2 out of 10, and it was 5.7 for the control group. The normalized pain score in the device group was significantly lower than the control group. Normalized and non-normalized pain scores for the placebo group were not significantly lower than the control group. Conclusions: The shot blocking device effectively reduced pain of injection versus controls when pain scores were normalized for pain tolerance. The modified device did not reduce the pain of injection, suggesting that gate control is the mechanism of action.

Introduction

The gate control theory of pain was proposed by Ronald Melzack and Patrick Wall ¹ of McGill University in 1965, and it had a profound impact on the understanding of pain perception. They hypothesized that input from non-painful stimuli acts upon inhibitory interneurons in the dorsal horn of the spinal cord to prevent painful sensations from traveling to the brain. According to gate control theory, large fibers (mediating touch, pressure, and vibration) excite the inhibitory interneurons, whereas thin fibers (mediating pain) impede these inhibitory cells. Therefore, the greater the large fiber activity reaching the spinal cord relative to thin fiber activity, lesser the pain felt. ¹ The gate control theory explains why rubbing a site of injury makes it feel better.

The ShotBlocker™ (Bionix, Toledo, Ohio) is a U-shaped soft plastic disk with multiple blunt projections on 1 side (Figure 1). It is a sterile, single use, disposable device, and it is currently marketed at a price of 35USD for 100 devices. The disk rests with the projections on the skin prior to and during the administration of an injection, and the injection is given through the opening in the device. (Figure 2) The multiple projections are designed to stimulate the touch receptors in the area, thus inhibiting the perception of pain through a gate control mechanism. The device has been shown in prior prospective studies to be effective in reducing pain of immunization in neonates and children,^2,3 as well as intramuscular injections in adults. ⁴

Figure 1.

The ShotBlocker™ device is a U-shaped plastic disk with multiple blunt projections on 1 side.

Figure 2.

The device is placed with the projections against the skin prior to the injection procedure.

There are an estimated 300,000 corticosteroid injections to the hand performed annually in the United States. ⁵ Injections to the hand are typically quite painful, and fear of painful injections is a major contributor to lack of patient compliance with treatment plans for inflammatory conditions of the upper extremity. ⁶ The use of ethyl chloride as a topical anesthetic prior to injection is a technique which has found broad application, but it has not been shown to be consistently effective in clinical trials.^7,8 An inexpensive and reliable method for reducing the pain of in-office steroid injections to the hand would be of enormous benefit to hand surgeons and their patients. The purpose of this prospective, randomized, placebo-controlled study is to determine if the use of the ShotBlocker™ device is effective in reducing the pain of in-office steroid injections to the hand.

Materials and Methods

The study received the approval of the University of Kentucky institutional review board on studies utilizing human subjects (IRB #13-0432-P1H). All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008. Informed consent was obtained from all patients for being included in the study.

An a priori power analysis was performed which indicated a 90% chance of detecting a large effect size and a 65% chance of detecting a medium effect size (defined as .8 and .5 of a population standard deviation between the means, respectively) ⁹ among the 3 groups as significant at the 5% level (2-tailed), with a sample size of n = 30 in each group.

Subjects were recruited from patients presenting to the office of the senior surgeon with upper extremity inflammatory conditions. Eligible patients were adults (age greater than 18 years) requiring injection of a corticosteroid in either hand. Proficiency in the English language was required in order to understand the study procedures and scripts. Patients who had received narcotic pain medication within 6 hours prior to injection were excluded, as were patients with medical conditions precluding a safe in-office injection.

After completion of informed consent, a standardized script was read to each patient explaining the ShotBlocker™ device, its purpose, and use. Patients were randomized using Research Randomizer (www.randomizer.org) to 1 of 3 treatment groups: shot blocking device, placebo, and control, but they were not told in which group they were placed. Age, gender, diagnosis, medical history, and pain medication history were recorded. Patients were asked to rate the severity of pain of their most recent tetanus vaccination on a continuous analog pain scale. On this scale, a score of 0 was assigned to “no pain,” and a score of 10 was assigned to “pain as bad as it can possibly be,” and patients marked a point along this line to rate their pain severity. Patients in the shot blocking device group were given their injection using the device per the manufacturer’s guidelines. The device was placed over the injection site with the fenestration directed proximally. This was done to allow the device to be held securely in the non-dominant hand while allowing good visualization of the injection site. The device can be used with the fenestration oriented in any direction as long as the projections are in contact with the skin in the same dermatome as the injection site. For patients in the placebo group, a modified shot blocking device was used, which had the projections removed. The modified device was held in the non-dominant hand in an elevated position approximately 0.5 cm above the skin, and not in contact with it, while the injection was performed through the fenestration. For patients in the control group, the injection was given without a shot blocking device. No oral or topical analgesic agents were administered. In all cases, the skin was prepared with alcohol. Each injection had a volume of 3 mL and consisted of 1 mL of 10 mg/mL triamcinolone acetonide and 2 mL of 1% lidocaine. In each instance, a 25-gauge needle was used. All injections were performed by the same senior hand surgeon. Immediately following the injection procedure, patients were asked to rate the severity of pain of the injection on the analog pain scale. There was no time delay between injection and rating, and this was standardized across all patients in the study. The 3 treatment groups were compared for homogeneity. Distributions of age, gender, and diagnosis were compared among the 3 groups using the Kruskal–Wallis test by ranks.

For each group, a mean pain severity score was calculated. A normalized pain score was calculated for each subject from the difference between the injection pain score and the tetanus shot pain score. The mean non-normalized and normalized scores for each treatment group were compared to the control group using the Wilcoxon signed rank test. To evaluate the relative pain of each injection site, a mean normalized pain score was calculated for each diagnosis, and these were compared using the Wilcoxon signed rank test. Significance was defined as P < .05.

Results

The study group consisted of 91 women and 26 men. The mean age was 51 years (range: 20-78 years). The most common presenting diagnosis was trigger finger (n = 53), followed by DeQuervain’s tendonitis (n = 33), and basal joint arthritis (n = 22). (Table 1) The remaining 10 patients had a variety of diagnoses, including scapholunate synovitis, scaphotrapezial synovitis, and flexor carpi radialis tendonitis. There were no injections for carpal tunnel syndrome in the study group, as such injections are not a typical part of the senior surgeon’s practice.

Table 1.

Presenting Diagnoses.

Diagnosis	n
Trigger finger	53
DeQuervain’s tendonitis	33
Thumb carpometacarpal arthritis	22
Scapholunate synovitis	3
Lateral epicondylitis	2
Metacarpophalangeal synovitis	2
Other	3

There were 34 patients randomized to the shot blocking device group, 41 in the placebo group, and 42 in the control group. The 3 groups did not differ significantly in age, gender, diagnosis, or pain medication history.

The mean non-normalized pain score in the shot blocking device group was 5.2 ± 3.1, compared to 5.7 ± 2.9 for the control group. This difference was not statistically significant (P = .53). The mean non-normalized pain score in the placebo group was 6.1 ± 3.0, which was likewise not significantly different from the control group (P = .52). The mean normalized pain score in the shot blocking device group was 1.4 ± 3.4, compared to 3.0 ± 2.8 for the control group. This difference was statistically significant (P = .03). The mean normalized pain score in the placebo group was 2.2 ± 2.5, which was not significantly different from the control group (P = .21). (Figure 3)

Figure 3.

Mean non-normalized and normalized pain scores by treatment group.

Overall, injection of the basal joint was rated as more painful (mean normalized score = 3.0) than injection for trigger finger (score = 2.1) or DeQuervain’s tendonitis (score = 2.5). However, these differences were not statistically significant.

Discussion

Injections of the hand and wrist are painful, and the fear of injection pain is a major barrier to care in patients with upper extremity inflammatory conditions.^10,11 This has led surgeons to seek a way to alleviate pain during these procedures. Topical anesthetics have been shown to be effective in painful pediatric procedures,^12-14 but they require a time delay, add as much as $10 per application,^12,15 and are associated with allergic reactions and other complications. ¹⁶ Furthermore, little support exists in the literature for their use in hand surgery.

The use of ethyl chloride vapocoolant anesthesia is a common practice among hand surgeons, with a recent survey revealing that 59% of hand surgeons use the technique “often or always.” ⁸ Vapocoolant agents add expense and risk to procedures, and the limited clinical data on ethyl chloride do not support its effectiveness in analgesia during hand injection procedures. ⁸ Nerve blocks at the wrist level are occasionally used prior to injections of the palm, ⁶ but it is unclear whether the benefits of avoiding the pain of a palm injection justify the additional risk of a wrist injection, which may be just as painful.

The gate control theory of pain was proposed by Melzack and Wall ¹ in a seminal 1965 publication. This article was a refutation of the 2 conflicting previous theories: specificity theory, whereby pain is a specific modality with its unique apparatus, such as vision or hearing, and the pattern theory where pain is produced by the intense stimulation of nonspecific receptors. Neither of these theories explained how the same stimuli can be alternately non-painful or painful, depending on the circumstances. The gate control theory proposed that there exist cells within the substantia gelatinosa in the dorsal horn of the spinal cord which exert an inhibitory effect on afferent sensory fibers. In turn, these inhibitory cells are stimulated by large-diameter fibers mediating touch and vibration and inhibited by small-diameter fibers mediating pain. ¹ The authors speculated further that the brain could exhibit control over the sensory input through the gate control system, explaining why pain can be modified by past experience and emotional states. ¹ The gate control theory has been supported by voluminous experimental data since its initial description. In an early study, Wall and Sweet ¹⁷ reported on 8 patients who experienced pain relief during and after high-frequency stimulation of the large fibers in the same distribution as their pain. A similar protocol was attempted in a group of patients suffering from post-herpetic neuralgia and some success was observed in a subset whose pain was moderate.^18,19 The relief from pain continued for up to 2 hours after the stimulation was stopped. These studies formed the basis for clinical trans-cutaneous electrical nerve stimulation which is still in broad use today.

The ShotBlocker™ is a drug-free plastic device that is pressed against the skin during injection procedures. Its cost is approximately 0.70 USD per application. It requires no wait time and is associated with no known side effects. The device has numerous blunt projections which are designed to stimulate the large-diameter afferent fibers surrounding the injection site, thus inhibiting pain perception. The ShotBlocker™ has been tested in previous clinical trials, with mixed results. In 2009, Drago et al ² published the results of a prospective, randomized trial of 165 children undergoing intramuscular injections. Nurses and caregivers rated the “perceived pain” of injection, which was significantly less in the shot blocking device group than in controls. However, the nurses and caregivers were not blinded, and there were no differences seen when children were allowed to rate their own pain. ² Cobb and Cohen presented a randomized controlled trial of 89 children between 4 and 12 years of age receiving either subcutaneous and submuscular immunizations. ²⁰ This study included a placebo group, where the device was reversed with the smooth side resting against the skin. No differences were seen among the 3 groups when pain was rated by the children, parent, or caregiver. ²⁰ The authors speculated that the mix of subcutaneous and submuscular injections as well as having a large number of young children with little prior injection experience for internal comparison may have limited the validity of their findings. ²⁰ A randomized placebo-controlled study evaluating the use of the ShotBlocker™ device in adults was published by Celik and Khorshid in 2015. ³ The study group consisted of 180 patients receiving intramuscular injections of diclofenac sodium. As in the prior study, a placebo group was included where the shot blocking device was reversed. The subject-rated pain scores were significantly decreased in the ShotBlocker group, compared to the placebo and control groups, but no differences in anxiety ratings or pulse rate were observed. ³

The present study differs from previous studies in that it focuses exclusively on common injections of the hand and wrist, which are known in clinical practice to be particularly painful. A placebo arm was included in the study, but the shot blocking device was held above, and not in contact with, the skin in the placebo group. In prior studies with a placebo group, the device was reversed with the smooth side against the skin and the projections facing outward.^3,20 We feel this does not represent a true placebo, as there is still some stimulation of the large-diameter afferent fibers mediating touch sensation when the device is in contact with the skin. It is reasonable to assume that some placebo effect may be present if the patient sees a device at the injection site, which is clearly labeled “Shot Blocker” and the purpose of which is to reduce pain. However, in the present study, the placebo group did not report statistically lower pain scores than the control group, suggesting that more than a placebo effect is responsible for the significantly lower pain scores seen in the ShotBlocker™ treatment group.

The subjectivity of pain perception and pain expression is a major challenge to researchers using pain as an endpoint. Pain tolerances vary widely, as do the definitions of “slightly painful,” “moderately painful,” and “very painful.” ²¹ In order to standardize these definitions, we asked patients to rate the pain of their last tetanus shot. A tetanus injection was chosen, because it is relatively uniform, and all patients have experienced it. This baseline score was subtracted from the procedural injection score to produce a normalized pain score. This method of normalization has limitations, however, as there is an inherent recall bias in asking patients to rate the pain of a past injection. There are significant age-related and gender-related differences in pain perception as well. ²² However, the treatment groups in our study did not differ significantly in composition with regard to age or gender.

This study has some limitations. Unlike previous studies looking at immunization, all of the patients in this study were seeking treatment for inflammatory conditions and were experiencing varying degrees of extremity pain as a baseline, which could confound the pain scoring. Although standardized scripts were used to limit bias, the surgeon performing the injections was not blinded to treatment group. Likewise, the patients were not truly blinded, as they knew whether or not they were in the control group. Finally, the patients had a variety of diagnoses which could increase the variability of the results.

Conclusion

The ShotBlocker™ device effectively reduced the pain of injection versus controls when scores were adjusted to account for pain tolerance. When the device was modified and held above the skin, it did not reduce the pain of injection, suggesting that gate control, rather than distraction or placebo, is the mechanism of action.