
==== Front
Interv Pain Med
Interv Pain Med
Interventional Pain Medicine
2772-5944
Elsevier

S2772-5944(22)00162-5
10.1016/j.inpm.2022.100164
100164
Original Article
Comparison of the spread pattern of medial-to-lateral and lateral-to-medial rotator interval injections: A cadaveric study
Kozlowski Benjamin J. a
Tran John a
Peng Philip W.H. b
Agur Anne M.R. ac
Mittal Nimish MBBS, MD, MSc Nimish.Mittal@uhn.ca
bcd∗
a Temerty Faculty of Medicine, Division of Anatomy, Department of Surgery, 1 Kings College Circle, University of Toronto, Toronto, Ontario, M5S 1A8, Canada
b Department of Anesthesiology and Pain Management, Toronto Western Hospital, University of Toronto, 399 Bathurst Street, Toronto, Ontario, M5T2S8, Canada
c Temerty Faculty of Medicine, Division of Physical Medicine and Rehabilitation, 27 King's College Circle, University of Toronto, Toronto, Ontario, M5S 1A8, Canada
d Faculty of Kinesiology and Physical Education, University of Toronto, Canada
∗ Corresponding author. Temerty Faculty of Medicine, Division of Physical Medicine and Rehabilitation, 27 King's College Circle, University of Toronto, Toronto, Ontario, M5S 1A8, Canada. Nimish.Mittal@uhn.ca
30 11 2022
12 2022
30 11 2022
1 4 10016421 9 2022
8 11 2022
11 11 2022
© 2022 The Authors
2022
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Objective

Rotator interval (RI) corticosteroid injections are used to alleviate pain associated with adhesive capsulitis, though the pattern of injectate spread remains unclear. The purpose of this anatomical study was to assess the staining patterns of intra-articular, intracapsular/extrasynovial, and pericapsular structures of the glenohumeral joint following medial-to-lateral and lateral-to-medial RI injections.

Design

Ten cadaveric specimens were injected with a methylene blue dye injectate: five using a medial-to-lateral RI injection technique and five using a lateral-to-medial RI injection technique. Serial dissection was performed to assess the staining of intra-articular, intracapsular/extrasynovial, and pericapsular structures. The frequency of capture and degree of staining were compared between injection groups.

Results

The lateral-to-medial injection resulted in the capture of all intra-articular; intracapsular/extrasynovial; and pericapsular structures, whereas the medial-to-lateral injection did not consistently stain all structures. Intracapsular/extrasynovial structures (superior glenohumeral ligament and the long head of biceps tendon) were more darkly stained in the lateral-to-medial group, and pericapsular structures (supraspinatus tendon and coracohumeral ligament) were more darkly stained in the medial-to-lateral group.

Conclusion

The frequency of capture and degree of staining of intra-articular, intracapsular/extrasynovial, and pericapsular structures of the glenohumeral joint differed between medial-to-lateral and lateral-to-medial RI injection techniques, which may influence outcomes in pain management for adhesive capsulitis.

Highlights

• The lateral-to-medial injection group had a higher frequency of staining of anatomical structures affected in adhesive capsulitis

• The medial-to-lateral injection group had an inconsistent frequency of staining compared to the lateral-to-medial injection group.

• The medial-to-lateral injection group had darker staining of pericapsular structures, whereas the lateral-to-medial group had darker staining of intracapsular/extrasynovial structures.

Keywords

Rotator interval injection
Glenohumeral joint
Adhesive capsulitis
Long head of biceps tendon
Ultrasound
Cadaveric
Abbreviations

LHBT long head of bicep tendon

SC subscapularis

SS supraspinatus
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pmc1 Introduction

Adhesive capsulitis, also known as frozen shoulder, is a common shoulder condition that causes pain and range of motion limitations [1,2]. Historically, adhesive capsulitis was thought to be a pathology giving rise to inflammatory changes and fibrosis of the glenohumeral joint capsule and subacromial bursa [3,4]. Patients with adhesive capsulitis usually progress through three sequential phases, which begins with the ‘freezing’ phase that is characterized by progressive increase of pain and mild restriction of range of motion. Next, the ‘frozen’ phase has the greatest restriction of range of motion; and lastly the ‘thawing’ phase, which is characterized by the gradual recovery of normal range of motion and resolution of pain. It has been shown that corticosteroid injections are more effective during the ‘freezing’ and ‘frozen’ phases of adhesive capsulitis [5]. More recently, arthroscopic studies have reported the involvement of rotator interval structures, including the intra-articular portion of the long head of biceps tendon (LHBT) and the coracohumeral ligament, which were found to be thickened and fibrotic [6,7]. Depending on the intra-articular and pericapsular structures involved in the pathology of adhesive capsulitis, various sites of corticosteroid injections in the frozen shoulder have been proposed, and include the intra-articular joint space; subacromial bursa; and bicipital groove [[8], [9], [10]]. However, there has been growing interest in the use of rotator interval injections to treat adhesive capsulitis [[11], [12], [13], [14], [15]].

A recent randomized controlled study by Elnady et al. reported that an ultrasound guided anterior rotator interval injection was more effective in providing pain relief than the conventional posterior intra-articular injection [11]. The needle trajectory for the anterior rotator interval injection was from lateral-to-medial, with the final needle tip placement in the sheath of the LHBT between the LHBT and anterior boarder of the supraspinatus tendon, deep to the lateral portion of the coracohumeral ligament. Similarly, a recent study by Sun et al. reported that a lateral rotator interval corticosteroid injection along the coracohumeral ligament, used to treat adhesive capsulitis, resulted in a significantly greater reduction in pain at 4 weeks compared to intra-articular and subacromial injections [12]. Alternative to the lateral-to-medial needle trajectory used in these studies, a recent publication described a rotator interval injection (Gaurav-Botchu technique) that uses a medial-to-lateral needle trajectory to position the needle tip between the LHBT and superior border of the subscapularis tendon, deep to the medial portion of the coracohumeral ligament [13]. Owing to the complex and intricate anatomy of the rotator interval [16], different needle trajectories may influence the spread of the injectate -which may ultimately influence patient outcomes. To date, no anatomical study has investigated whether the final needle placement, relative to the intra-articular part of the LHBT, influences the injectate spread following rotator interval injections. A cadaveric injectate spread study would provide a better understanding of the spread pattern and staining of anatomical structures following rotator interval injections. Therefore, the objective of this anatomical study was to investigate and compare the spread and staining patterns between the lateral-to-medial and medial-to-lateral rotator interval injection techniques.

2 Materials and methods

2.1 Cadaveric specimens

Ten lightly embalmed cadaveric specimens with a mean age of 87 ​± ​11 years (8 males, 2 females; 5 ​L/5 ​R) were used for this study. Each specimen was in the anatomical position. There were two study groups of 5 specimens each. One group was injected using an ultrasound guided medial-to-lateral rotator interval injection (injection site between the LHBT and subscapularis [SC] tendon) and the other group using a lateral-to-medial injection (injection site between the LHBT and supraspinatus [SS] tendon). All specimens had no visible signs of previous surgery, pathology, or trauma. No further demographic data were available. All specimens were obtained from the Willed Body Program of the Division of Anatomy at the University of Toronto. Ethics approval was received from the University of Toronto Health Sciences Research Ethics Board (No. 27210).

2.2 Ultrasound scanning protocol

All injections were performed by one of the authors (N.M.), a physiatrist with over ten years of experience in ultrasound guided pain procedures. A Fujifilm Sonosite M-Turbo ultrasound machine (Bothell, WA) with a linear 10–15 ​MHz probe was used. A 22-gauge 2-inch needle with 5 ​mL of injectate (1 ​mL of methylprednisolone, 4 ​mL of 5% methylene blue dye) was used for all injections. The choice of injectate volume was based on two previous studies that reported good pain relief outcomes following 5 ​mL injections to treat adhesive capsulitis [9,17].

Each specimen was prescanned to localize relevant structures for both rotator interval injections techniques. With the probe oriented transversely, the LHBT was located within the intertubercular groove and followed to the anterosuperior surface of the humerus, where the groove and tendon flattened. The tendon continued to be scanned as proximal as possible at the rotator interval. Next, the SC tendon and SS tendon, forming the anterior and posterior boundaries of the rotator interval, respectively, were identified. The coracohumeral ligament, glenohumeral joint capsule, anterior and posterior edges of the LHBT, and the superior glenohumeral ligament were identified.

2.3 Ultrasound guided medial-to-lateral rotator interval injection

For the medial-to-lateral rotator interval injection, the ultrasound probe was placed in a transverse oblique orientation, just lateral to the tip of the coracoid process (Fig. 1 A). The needle tip was advanced from medial to lateral, through the coracohumeral ligament and capsule of the glenohumeral joint, to reach the rotator interval between the anterior margin of the tendon of the LHBT and the superior margin of the SC tendon (Fig. 1 B). The site of injection was approximately at the 2 o'clock position in relation to the center of the LHBT. After confirmation of the location of the needle tip, the entire injectate was slowly deposited in the rotator interval.Fig. 1 Needle entry point, position of the ultrasound probe, and the corresponding ultrasound scan showing the needle trajectory for medial-to-lateral (A and B) and lateral-to-medial (C and D) rotator interval injections. A sample analysis for the supraspinatus and infraspinatus tendons: red-green-blue raw images were converted into raw grayscale images, and a 100 by 100 homogenously stained (yellow box) section for each structure was selected for analysis. The stained samples of each structure were normalized to an unstained portion of the acromioclavicular ligament of the same specimen. Histograms represent the relative distribution of grayscale values within each sampled section (E). CP ​= coracoid process; CHL ​= coracohumeral ligament; HH ​= ​head of humerus; IS ten. ​= ​infraspinatus tendon; LHBT ​= long head of biceps tendon; SC ten. ​= ​subscapularis tendon; SGHL ​= superior glenohumeral ligament; Spine (cut) ​= spine of the scapula (cut); SS ten. ​= ​supraspinatus tendon. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 1

2.4 Ultrasound guided lateral-to-medial rotator interval injection

For the lateral-to-medial rotator interval injection, the ultrasound probe was placed in a transverse oblique orientation and the needle inserted anterior to the acromion process (Fig. 1 C). The needle tip was advanced from lateral to medial, through the coracohumeral ligament and capsule of the glenohumeral joint, to reach the rotator interval between the posterior margin of the tendon of the LHBT and the anterior margin of the SS tendon (Fig. 1 D). The site of injection was approximately at the 10 o'clock position in relation to the center of the LHBT. After confirmation of the location of the needle tip, the entire injectate was slowly deposited in the rotator interval.

2.5 Dissection protocol

All specimens were dissected by one of the authors (B.J.K.), who was blinded to the ultrasound approaches adopted for the injections. Following injection, each specimen was dissected using the same protocol. First, the skin; subcutaneous tissue; trapezius; pectoralis major and minor; deltoid; teres major; inferior belly of omohyoid; and axillary contents were excised. Next, the SS; infraspinatus; teres minor; SC; coracohumeral ligament; coracoacromial ligament; and transverse humeral ligament were exposed in their entirety. The LHBT was followed extracapsularly to the transverse humeral ligament, and distally as it coursed within the intertubercular groove.

To enable assessment of staining of the glenohumeral joint capsule and intra-articular structures, further serial dissection was necessary. First, the clavicle was disarticulated at the acromioclavicular joint and the tendons of the rotator cuff muscles were reflected laterally, revealing the glenohumeral joint capsule. The presence and location of staining of the joint capsule were recorded. Next, the anterior joint capsule was incised laterally and reflected to reveal the internal surface of the joint capsule; superior, middle, and inferior glenohumeral ligaments; head of the humerus; glenoid fossa; glenoid labrum; and intra-articular part of the LHBT. The frequency and degree of staining of intra-articular (head of humerus and glenoid fossa/labrum), intracapsular/extrasynovial (superior, middle, and inferior glenohumeral ligaments; LHBT), and pericapsular (coracoacromial ligament, coracohumeral ligament, SS tendon, and SC tendon) structures were documented. All specimens were photographed following each step of the serial dissection under the same environmental conditions.

2.6 Data and statistical analysis

The frequency of staining of intra-articular, intracapsular/extrasynovial, and pericapsular structures following lateral-to-medial and medial-to-lateral rotator interval injections were compared between injection groups. The frequency of staining was visually confirmed and defined as present (stained) or absent (not stained). A Fisher exact test was used to assess the differences in the frequency of staining of structures between the two injection groups.

The degree of stained structures was classified based on the dye staining intensity. The degree of staining was compared between stained structures only. As each specimen was photographed in the same lighting conditions and with the same equipment, differences in the stain intensity of structures can be attributed to the saturation resultant from the dye. In order to assess the saturation from the dye, each image was converted from red-green-blue into 8-bit grayscale and calibrated to the red channel, which was chosen based off the most pronounced distinction in saturation. The staining intensity for each structure was quantified by determining the average grayscale value from a homogeneously stained 100 by 100-pixel field (Fig. 1 E). To account for possible interspecimen variation in how the dye reacted with structures of interest, each grayscale value was normalized to an unstained portion of the acromioclavicular ligament of the same specimen, analogous to a similar tissue composition to the structures of interest, by taking the difference between the stained grayscale value and the unstained grayscale value. The absolute value of the grayscale difference for each structure was calculated as: | grayscalestained – grayscalenot stained |. An independent sample t-test was used to compare the mean grayscale difference of each structure between the two injection groups. Grayscale values are presented as mean ​+ ​standard error. The level of statistical significance was set at α ​= ​0.05 (2-tailed) for all analyses. Grayscale values were obtained from ImageJ [18], and statistical measurements calculated with jamovi software (Version 1.6.23, Sydney, AUS).

3 Results

The frequency of stained structures between medial-to-lateral and lateral-to-medial injections is found in Table 1. Seventy-eight percent of structures (intra-articular, intracapsular/extrasynovial, and pericapsular) analyzed in this study were stained using the medial-to-lateral injection, and 92% using the lateral-to-medial injection. The frequency of staining of structures affected in adhesive capsulitis (head of the humerus, glenoid fossa/labrum, superior; middle; and inferior glenohumeral ligaments, LHBT, and coracohumeral ligament) was significantly greater (P ​< ​.001) in the lateral-to-medial injection group (100%) compared to the medial-to-lateral injection group (82%). Although the medial-to-lateral injection group had a greater percentage of unstained structures (22%) compared to the lateral-to-medial injection group (8%), the difference was not statistically significant (P ​= .071). No staining of the head of the humerus, glenoid fossa/labrum, middle glenohumeral ligament, inferior glenohumeral ligament, SC tendon, and SS tendon was found following the medial-to-lateral injection in a total of 8 structures. In the lateral-to-medial injection group, the only structure that was not stained was the LHBT distal to the bicipital groove. As a whole, the mean grayscale difference (ie, staining intensity) of stained structures analyzed in this study was not significantly different (P ​= ​.674) between medial-to-lateral and lateral-to-medial injection groups (Fig. 2).Table 1 Frequency of staining of intra-articular, intracapsular/extrasynovial, and pericapsular structures following medial-to-lateral and lateral-to-medial rotator interval injections.

Table 1Injection group	Medial-to-Lateral
% stained (n/5)	Lateral-to-Medial
% stained (n/5)	
Staining of Structures	Yes	No	Yes	No	
Intra-articular	
 Head of the humerus	60% (3)	40% (2)	100% (5)	0%	
 Glenoid fossa/labrum	80% (4)	20% (1)	100% (5)	0%	
Intracapsular/Extrasynovial	
 Superior glenohumeral lig.	100% (5)	0%	100% (5)	0%	
 Middle glenohumeral lig.	80% (4)	20% (1)	100% (5)	0%	
 Inferior glenohumeral lig.	60% (3)	40% (2)	100% (5)	0%	
Pericapsular	
 Coracohumeral lig.	100% (5)	0%	100% (5)	0%	
 Coracoacromial lig.	100% (5)	0%	100% (5)	0%	
 Subscapularis tendon	100% (5)	0%	100% (5)	0%	
 Supraspinatus tendon	80% (4)	20% (1)	100% (5)	0%	
 Infraspinatus tendon	80% (4)	20% (1)	100% (5)	0%	
 LHBT (distal to bicipital groove)	0%	100% (5)	0%	100% (5)	
LHBT (intra-articular part)	100% (5)	0%	100% (5)	0%	
All structures	78% (47)	22% (13)	92% (55)	8% (5)	
Abbreviations: lig. ​= ​ligament; LHBT ​= ​long head of biceps tendon.

Fig. 2 Mean grayscale difference values, indicative to the intesntiy of dye staining, for stained structures between medial-to-lateral and lateral-to-medial injection groups. Grayscale values are presented as mean ​+ ​standard error. ∗indicates significant difference (P ​< .05). CAL ​= coracoacmoial ligament; CHL ​= coraohumeral ligament; GF/GL ​= ​glenoid fossa/glenoid labrum; HH ​= ​head of the humerus; IGHL ​= inferior glenohumeral ligament; LHBT ​= long head of biceps tendon; SC ​= ​subscapularis tendon; SGHL ​= ​superior glenhumeral ligament; SS ​= supraspinatous tendon.

Fig. 2

3.1 Intra-articular structures

The entire surface of the glenoid fossa/labrum was stained in all specimens using the lateral-to-medial injection, and in 80% of specimens using the medial-to-lateral injection. Additionally, the medial-to-lateral injection resulted in 20% of specimens with staining of only the inferior two-thirds of the glenoid fossa/labrum and 20% with no staining. The head of the humerus was completely stained in 40% of specimens in both medial-to-lateral and lateral-to-medial injections. Only the superomedial aspect of the head of the humerus was stained in 20% of specimens using the medial-to-lateral injection, and in 60% using the lateral-to-medial injection. Staining was not present in 40% of specimens using the medial-to-lateral injection (Table 1).

Although there was no significant difference for the mean grayscale difference for both the glenoid fossa/labrum (P ​= .242) and head of the humerus (P ​= ​.212) between medial-to-lateral and lateral-to-medial injection groups, the head of the humerus had a greater mean grayscale difference using the medial-to-lateral injection, and the glenoid fossa/labrum using the lateral-to-medial injection (Fig. 2). Staining patterns of intra-articular structures are found in Fig. 3.Fig. 3 Dye stain distribution patterns following medial-to-lateral and lateral-to-medial rotator interval injections. A - F, medial-to-lateral injection. G - I, lateral-to-medial injection. AC ​= ​anterior capsule; CP ​= ​coracoid process; GT ​= greater tubercle; HH ​= ​head of the humerus; IS ​= ​infraspinatus; PC ​= ​posterior capsule; SCt. ​= ​subscapularis tendon; SSt. ​= ​supraspinatus tendon; TM ​= ​teres minor; 1 ​= ​long head biceps tendon (extra-articular); 2 ​= ​glenoid fossa; 3 ​= ​glenoid labrum; 4 ​= ​long head of biceps tendon (intra-articular); 5 ​= ​superior glenohumeral ligament; 6 ​= ​middle glenohumeral ligament; 7 ​= ​inferior glenohumeral ligament.

Fig. 3

3.2 Intracapsular/extrasynovial structures

The lateral-to-medial injection resulted in staining of all three glenohumeral ligaments and the intra-articular part of the LHBT in all specimens. Similar to the lateral-to-medial injection, the medial-to-lateral injection resulted in staining of the superior glenohumeral ligament and the intra-articular LHBT in all specimens. However, the middle glenohumeral ligament was stained in 80% of specimens and the inferior glenohumeral ligament in 60% using the medial-to-lateral injection (Table 1).

There was a significant difference in the mean grayscale value for the superior glenohumeral ligament (P ​= .028) and intra-articular part of the LHBT (P ​= .030), which were more darkly stained in the lateral-to-medial injection group than the medial-to-lateral injection group. There was no significant difference in mean grayscale value for the middle (P ​= .330) and inferior (P ​= .288) glenohumeral ligaments between injection groups (Fig. 2). Staining patterns of intracapsular/extrasynovial structures are found in Fig. 3.

3.3 Pericapsular structures

Using the medial-to-lateral injection, the stain infiltrated the tendons of the SS, teres minor, and SC in 80% of specimens from the musculotendinous junction to their attachment site to the greater or lesser tubercles. In contrast, the lateral-to-medial injection resulted in staining of these pericapsular structures in all specimens. No staining of the LHBT distal to the bicipital grove was found in any specimen (Table 1).

There was a significant difference in the mean grayscale value for the coracohumeral ligament (P ​= .019) and SS tendon (P ​= .040), which were more darkly stained in the medial-to-lateral injection group than the lateral-to-medial injection group (Fig. 2). There was no significant difference in the mean grayscale value of the coracoacromial ligament (P ​= .089) and SC tendon (P ​= .763) between injection groups (Fig. 2). Staining patterns of pericapsular structures are found in Fig. 3, Fig. 4.Fig. 4 Dye stain distribution patterns of the LHBT in the bicipital groove (top row, anterolateral views) and pericapsular structures (bottom row, superior views). A, B, D; medial-to-lateral injection. C, E; lateral-to-medial injection. AC ​= anterior capsule; CAL ​= coracoacromial ligament; CP ​= ​coracoid process; CHL ​= ​coracohumeral ligament; GT ​= greater tubercle; LHBT ​= long head of biceps tendon; LT ​= lesser tubercle; SS ten. ​= supraspinatus tendon.

Fig. 4

4 Discussion

Rotator interval injections have been clinically used to treat adhesive capsulitis, with evidence supporting its greater effectiveness in providing symptom relief as compared to current posterior intra-articular glenohumeral joint approaches [11,19]. To optimize rotator interval injection techniques, a detailed knowledge of the spread patterns of injectate is critical. In the current study, the lateral-to-medial injection resulted in staining of all intra-articular and pericapsular structures, whereas the medial-to-lateral injection resulted in staining of 80% of intra-articular structures or 85% of combined intra-articular or pericapsular structures. Moreover, the staining intensity varied between the injection groups. Structures with the greatest intensity of staining following the lateral-to-medial injection included intracapsular/extrasynovial structures. Following the medial-to-lateral injection, pericapsular structures were most darkly stained. Adhesive capsulitis has been associated with pathologies related to the glenohumeral joint capsule including the glenohumeral ligaments, surrounding ligamentous structures, synovium, and the SC bursa [[20], [21], [22]]. Therefore, the dye spread/staining results of the current study suggest that the lateral-to-medial rotator interval injection provides greater frequency of staining of pathological structures affected in adhesive capsulitis when compared to the medial-to-lateral injection. Potentially, this greater staining frequency and intensity of pathological structures following lateral-to-medial rotator interval injection may translate into better clinical outcomes, pending further clinical investigation.

From an anatomical perspective, the lower staining intensity and frequency of capture of intra-articular and intracapsular/extrasynovial structures in the medial-to-lateral injection group could be attributed to inconsistencies in needle tip placement within the glenohumeral joint space. For the medial-to-lateral injection, the needle tip was advanced from medial to lateral, piercing the coracohumeral and superior glenohumeral ligaments, to terminate intra-articularly within the rotator interval. Due to the orientation of the fibers of the superior glenohumeral ligament, it appears hypoechoic compared to the coracohumeral ligament in the sonographic view, increasing the possibility that needle advancement may be incomplete. As a result, the needle tip may be located within the substance of the superior glenohumeral ligament, in the synovial membrane surrounding the LHBT, or in the capsuloligamentous “sling” formed by the oblique and transverse fibers of the superior glenohumeral ligament [23,24]. This could lead to incomplete or no staining of some intra-articular and intracapsular structures. Thus, even though the final needle placement should be intra-articular, there is a possibility of inadvertent pericapsular injection. The differences in medial-to-lateral and lateral-to-medial injections may also be explained by the needle trajectory. In the lateral-to-medial injection, the needle tip pierced the coracohumeral ligament to terminate intra-articularly and posterior to the LHBT. In contrast, the medial-to-lateral injection had a needle trajectory anterior to the LHBT, and for the reasons explained above, the final needle tip placement may be extra-articular, within the superior glenohumeral ligament, or with the synovial membrane of the LHBT. The anatomical considerations discussed here, in addition to the dye staining patterns, provide support favoring the use of the lateral-to-medial rotator interval injection over the medial-to-lateral technique pending future clinical investigation. In most cases, a linear probe is sufficient for visualizing all pertinent rotator interval structures; however, a curvilinear probe may be necessary for individuals with larger body habitus. Identification of the intra-articular portion of the biceps tendon is an important sonoanatomical landmark for targeting the rotator interval space and administering an accurate injection. In patients with a previous history of proximal long head of biceps tenotomy, the anterior rotator interval approach may not be appropriate.

Ultrasound imaging has become a standard of practice that has led to a trend in reducing the volume of injectate [[25], [26], [27]], as the visualization of targets has enable more accurate deposit of anesthetic. Previous clinical literature has reported the use of large and small injectate volumes to treat adhesive capsulitis, which include injections of 10–15 ​mL [28], 5 ​mL [9], and 3 ​mL [12]. Based on the current study, following an injection volume of 5 ​mL using two different ultrasound guided rotator interval injections, extensive dye spread was observed. This anatomical observation suggests that a relatively lower injection volume of 5 ​mL can result in the complete coverage of relevant intra-articular, intracapsular/extrasynovial, and pericapsular structures implicated in adhesive capsulitis. Furthermore, several clinical studies have found that adhesive capsulitis can result in contracture of the glenohumeral joint capsule [29,30], and diminished volume of the rotator interval; bicipital groove; and the glenohumeral joint [31,32]. The smaller joint space may hinder the retention of steroid using larger injectate volumes, which may lead to unwanted clinical outcomes such as rupture of the subacromial bursae and LHBT sheath, and/or extravasation of injectate [33,34]. Additionally, owing to the possible reasons for the incomplete penetration of the joint capsule using a medial-to-lateral rotator interval injection (noted above), excessive injectate may harbor outside of the joint space if larger volumes are used, which can cause adverse effects such as weaking of tendons and ligaments, and atrophy of adipose tissue [31,32]. The current anatomical study suggests that a 5 ​mL injection volume can provide adequate coverage of implicated structures in adhesive capsulitis. Therefore, future studies should compare the clinical efficacy between different volumes with rotator interval injections.

This study has a few limitations. Firstly, the sample size was small; however, this is a pilot study that can be expanded, anatomically and clinically, to corroborate findings. Also, as with all anatomical studies, there may be differences between in vivo and lightly embalmed tissues, although it has been reported that lightly embalmed tissues are comparable to unembalmed, fresh tissues [35].

5 Conclusions

The lateral-to-medial rotator interval injection resulted in a greater capture frequency of intra-articular, intracapsular/extrasynovial, and pericapsular structures of the glenohumeral joint than the medial-to-lateral injection. Additionally, the current study findings suggests that the use of a 5 ​mL rotator interval injection volume may provide adequate staining of anatomical structures relevant to adhesive capsulitis. Further clinical studies are necessary to assess the efficacy and analgesic effectiveness of the two injection techniques in treating the pain associated with adhesive capsulitis.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

Dr. Anne Agur is an Anatomy Faculty with the Allergen Academy of Excellence. Dr. Philip Peng received equipment support from SonoSite Fujifilm Canada. Partial results of this study were presented by Benjamin Kozlowski in an abstract at the American Association for Clinical Anatomist (AACA) Virtual Conference 2021.

Acknowledgment

The authors would like to thank those who donated their bodies for the advancement of this research.
==== Refs
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