Background: The deltoid muscle’s morphology and functional subdivisions have been extensively
studied, yielding various models and classifi cations. Understanding the anatomy of deltoid muscle origin
is crucial for accurate diagnosis and treatment.
Methods: This research employs ultrasound imaging to systematically examine the anatomical features of
the deltoid muscle origin. Landmarks and scanning techniques are described for identifying the different
functional segments of the deltoid muscle.
Results: Five patients with deltoid muscle tendinous injuries were enrolled, highlighting the signifi cance
of accurate diagnosis in the context of rotator cuff injuries. These patients demonstrated the diversity
of deltoid injuries and their association with rotator cuff pathology. Deltoid injuries, although relatively
rare, exhibit increased prevalence in individuals with concurrent rotator cuff injuries. Ultrasound imaging
emerges as a valuable tool for assessing deltoid muscle injuries, offering real-time dynamic assessments
and aiding in treatment decisions.
Conclusion: Accurate assessment of deltoid muscle origin and function is crucial for diagnosing and
treating deltoid injuries, particularly in individuals with rotator cuff pathology. Ultrasound imaging offers
a cost-effective and accessible means of evaluating these injuries, facilitating tailored rehabilitation
strategies. Further research may explore the functional characterization of deltoid muscle units and their
force outputs across different planes.

deltoid, shoulder, ultrasound, sonography, pain

The morphology of the deltoid muscle has been extensively studied with various models proposed. Leijnse et al. [1] described the deltoid muscle using a tendon(origin)-muscle-tendon(end) model (Figure 1), where the anterior, lateral, and posterior parts of the deltoid all assist in shoulder abduction. However, the anterior part primarily assists in shoulder flexion while the posterior part aids in shoulder extension.

Alternatively, Brown et al. [2] divided the deltoid muscle into seven segments using the surface anatomy and findings of electromyogram during muscle activation. These segments were referred to as “muscles within muscles,” and their categorization provides a detailed understanding of the functional subdivisions of the deltoid muscle.

Sakoma et al. [3] validated the concept of seven functional segments (A1, A2, A3, M1, P1, P2, and P3) by using magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography. These segments provide insights into the functional diversity of the deltoid muscle. Notably each deltoid origin tendon functions differently depending on the angle of horizontal abduction and adduction. Therefore, identifying each origin tendon is crucial for a comprehensive survey of the deltoid muscle.

Ultrasound examination is a readily accessible tool in clinical practice compared with other expensive techniques. Moreover, as the origin tendons of the deltoid muscle are superficially located in the body, they are less likely to be obstructed by other structures. Herein, we would like to revisit the anatomy of the deltoid muscle origin as well as a systematic ultrasound scanning approach. The sonographic and MRI features in patients with deltoid muscle/tendon complex injuries would also be presented.

Figure 1. Tendon(origin)- Muscle-Tendon(end) Model
(A) The tendon (origin) to muscle to tendon (end) model reveals that the end tendons on either side are associated with muscles distinct from those linked to the origin tendons. (B) The illustration depicts findings from cadaveric research, which identified that the four longest origin tendons on the lateral side originate from the acromion and establish muscle connections with unique end tendons. S stands for Scapula, A stands for Acromial, and C stands for Clavicle.

This study employs ultrasound imaging as a pivotal tool for conducting a systematic exploration of the anatomical characteristics pertaining to the origin of the deltoid muscle. We provide a comprehensive description of landmarks and scanning techniques essential for identifying the various functional segments within the deltoid muscle. All patient images used in this study were retrieved retrospectively from our clinic’s registry, eliminating the need for informed consent.

The inclusion criteria for participants were as follows: (1) age greater than 20 years, (2) experiencing at least one painful shoulder, and (3) having undergone ultrasound or MRI examination of the affected shoulder. Exclusion criteria encompassed: (1) prior surgeries on the affected shoulder and (2) receiving shoulder injections within the past six months.

Several landmarks can be used to distinguish each functional segment of the deltoid muscle Sakoma et al. [3]. The A1 segment is adjacent to the pectoralis major and is separated from the pectoralis major muscle by the cephalic vein. The border between the A1 and A2 segments is marked by a line approximately 5-mm medial to the medial border of the acromioclavicular (AC) joint. The A1 segment lies within the lateral third of the clavicle and typically contains 2–3 origin tendons (Figure 2).

Moving laterally towards the AC joint, the A3, M1, and P1 segments can be distinguished by their respective origin tendons. For example, the A2 origin tendon separates the A2 and A3 muscles, while the A3 origin tendon separates the A3 and M1 muscle bellies. The M1 tendon is more prominent, indicating the anterior location of the muscle, whereas the tendon itself lies more posteriorly between the M1 and P1 muscles. Similarly, the P1 origin tendon separates the P1 and P2 muscles. However, the origin tendons for P2 and P3 are less noticeable and resemble intramuscular tendons (Figure 3).

Figure 2. Illustration of the A1 and A2 Units of the Deltoid Muscle, and the Pectoralis Major Muscle

Figure 3. Illustration of Different Bundles of the Deltoid Muscles in the Axial Plane

To identify the A1 segment, a practical approach is to begin from the lateral border of the pectoralis major, approximately 4 cm below the clavicle. The cephalic vein serves as a clear landmark of the boundary between the A1 segment and the pectoralis major. By laterally tracing toward the conoid tubercle of the clavicle, two to three origin tendons are observed to attach to the clavicle. Rotating the probe allows visualization of the long axis of the origin tendons from the A1 segment (Figure 4).

To examine the A2, A3, M1, and P1 segments, the probe can be positioned parallel to the disc of the AC joint to laterally trace this. The first origin tendon that appears, attached to the acromion, belongs to the A2 segment. The long axis of the A2 origin tendon, is inclined at an angle of approximately 15°–30° laterally to the sagittal plane (Figure 5).

Resetting the probe parallel to the AC joint disc and performing lateral tracing reveals the attachment of the A3 origin tendon to the lateral facet of the acromion. The A3 tendon is distinctly connected to both the superficial and deep parts of the acromion, setting it apart from the A2 origin tendon. When tracing along the long axis of the A3 origin tendon, it deviates approximately 45° from the sagittal plane (Figure 5).

Alternatively, the A2 and A3 origin tendons can be differentiated by positioning the patient’s elbow at a 90° flexion with the forearm in supination. Thus, while tracing the long axis of the A2 tendon, the long head of the biceps will be visible below it. Similarly, when tracing the long axis of the A3 tendon, the supraspinatus tendon can be seen below this (Figure 5).

To perform a scan of the M1 origin tendon, begin by positioning the probe to bridge the AC joint disc. Then, gently shift it in a more lateral and posterior direction as compared to the A3 attachment point. This maneuver enables the visualization of the M1 muscle in the front and the M1 tendon in the back. Notably, several muscle fibers may be observed running above the tendon, and by tracing its long axis, the M1 origin tendon can be thoroughly examined (Figure 6).

After examining the A2, A3, and M1 muscles and tendons in sequence, the probe is moved posteriorly to visualize the P1 tendon attached to the posterior facet of the acromion. We’ll see the oblique view of P1 bundle first, and then we will pivot our probe posteriorly to align it parallel with the long axis of the P1 bundle (Figure 7). The P1 tendon is also noticeably large and can be traced down to the middle of the deltoid muscle.

No distinct large origin tendons are present in the P2 and P3 muscles, and these have multiple small intramuscular tendons instead, which can be traced in a short axis until they reach the scapulae (Figure 7).

The scanning technique described above is summarized in Table 1.

Figure 4. Schematic Drawing for the Transducer Position and Ultrasound Imaging for the Short and Long Axis View of A1 Bundle

Schematic drawing for the transducer position (A) and ultrasound imaging (B) for the short axis view of A1 bundle. The scan is bottomed up to the clavicle, whereas the intramuscular tendon from A1 are visible inside the deltoid muscle (arrow). Schematic drawing for transducer position (C) and ultrasound imaging (D) for the long axis view of A1 bundle. A total of 2–3 intramuscular tendon would be visible (empty arrow).

Figure 5. Schematic Drawing for Transducer Position and Ultrasound Imaging for Bundles

Schematic drawing for transducer position (A) and ultrasound imaging (B) for A2 bundle. Schematic drawing for transducer position (C) and ultrasound imaging (D) for A3 bundle. Note, the A3 origin tendon covers the superficial and deep part of the acromion. Positioning the patient’s forearm in supination with the elbow flexed at 90° allows for clear visualization of the A2 origin tendon (indicated by the arrow), which runs atop the biceps long head. In contrast, the A3 origin tendon (indicated by the empty arrow) courses over the supraspinatus tendon. Bic_LH: long head of biceps tendon, SS: supraspinatus tendon.

Figure 6. Schematic Drawing for the Transducer Position and Ultrasound Imaging for the Short and Long Axis View of M1 Bundle
Schematic drawing for transducer position (A) and ultrasound imaging (B) for the short axis of M1 bundle. Schematic drawing for transducer position (C) and ultrasound imaging (D) for the long axis of M1 bundle. The M1 origin tendon inserted to the superficial and deep part of the clavicle (arrow). OT: origin tendon.

Figure 7. Schematic Drawing for the Transducer Position and Ultrasound Imaging for the Oblique, Short, and Long Axis View of P1 Bundle
Schematic drawing for the transducer position (A) and ultrasound imaging (B) for the oblique view of P1 bundle. Schematic drawing for the transducer position (C) and ultrasound imaging (D) for the long axis view of P1 bundle. Schematic drawing for the transducer position (E) and ultrasound imaging (F) for short axis view of P1 bundle. Please take note that the fi brillar pattern of the P1 tendon may become less distinct when we position our probe in an oblique view (arrow).

Table 1. Ultrasound Scanning Protocols for the Deltoid Muscle/Tendon Complex

Given the limited number of patients included in this study, we primarily employed descriptive statistics to present the demographics and ultrasound findings of the participants. For continuous variables, we reported their mean and standard deviation. If a comparative analysis was necessary, we used the independent Student’s t-test for normally distributed data or the Mann-Whitney U test for non-normally distributed data. To assess data normality, we applied the Shapiro-Wilk test.

Regarding categorical data, we presented the number and corresponding proportion. Similarly, if a comparative analysis was required, we utilized the Chi-square test for non-sparse data or Fisher’s exact test for sparse data. All analyses were conducted using MedCalc Statistical Software version 19.2.6 (MedCalcSoftware Ltd, Ostend, Belgium; https://www.medcalc. org; 2020), and statistical significance was defined as a P-value less than 0.05.

The current research included five patients, all of whom had been diagnosed with deltoid muscle injuries, either through ultrasound imaging or MRI scans. The gender distribution among these patients was 2 females to 3 males. Their average age was 53.8 ± 11.5 years. Importantly, none of these individuals reported the presence of chronic systemic diseases such as hypertension, diabetes mellitus, or hyperlipidemia.

A 35-year-old man (Case 1) came in with a complaint of experiencing painful weakness while performing lateral raises with his left shoulder during workouts, specifically when reaching the end of the movement. Interestingly, the patient did not report any pain when undergoing the painful arc examination. Pain was elicited when resisting shoulder elevation in the horizontal plane, but this pain was not experienced during movements in the scapular plane or the vertical plane. Upon further investigation, we followed the M1 tendon’s path and identified an avulsion fracture, accompanied by fibrotic changes in the muscular segment (Figure 8).

A 55-year-old woman (Case 2) encountered a partial tear of her supraspinatus tendon half a decade ago, for which she received platelet-rich plasma injection and actively engaged in rehabilitation. Nonetheless, challenges endured when the patient performed shoulder elevation exercises, as the patient communicated heightened soreness and reduced strength on the affected side in contrast to the unaffected side. Our assessment of the A1 tendon unveiled no abnormalities. However, the A2 and A3 tendons displayed cortical irregularities and thickening, accompanied by indications of fatty atrophy in the A3 muscle region. (Figure 9).

Figure 8. Ultrasound Imaging for the Short and Long Axis View of M1 Bundle With Different Status

Sonograms depicting the short-axis view of the injured M1 bundle (A), the healthy M1 bundle (B), and the long-axis view of the injured M1 bundle (C). The tendon insertion site reveals an avulsion fracture in both the short and long axis (arrow). Additionally, the muscle portion exhibits hyperechoic changes in the injured site (empty arrow) when compared to the healthy site. OT: origin tendon.

Figure 9. Ultrasound Imaging for the Short and Long Axis View of A2 Origin Tendon With Different Status
These sonograms illustrate the long-axis views of the healthy A2 origin tendon (A) and the injured A2 origin tendon (B), as well as the healthy A3 origin tendon (C) and the injured A3 origin tendon (D). It is important to observe the irregularities in the cortex (arrow) and the atrophy of the A3 muscle (empty arrow). OT: origin tendon, Bic_LH: biceps long head, SS: supraspinatus tendon

We noted a notable increase in occurrences of deltoid injuries among individuals with rotator cuff injuries. To further investigate this trend, we conducted a retrospective analysis of patients who exhibited evidence of rotator cuff injuries in their MRI scans.Intriguingly, many of these patients also displayed simultaneous deltoid injuries in various locations.

In Case 3, a 65-year-old man has a complete supraspinatus tendon injury with retraction observed. The MRI findings indicated damage to the A1 and A2 muscle-tendon complex (Figure 10). In Case 4, a 61-year-old woman has a partial tear identifi ed in the supraspinatus tendon, alongside a minor disruption in the A2 muscle fi bers (Figure 11). In Case 5, a 53-yearold man has a chronic supraspinatus tendon injury.

The MRI displayed heightened signal intensity in the P1 bundle muscle area (Figure 12).

Figure 10. Magnetic Resonance Imaging for the Axial View and Coronal View of the Shoulder
(A) In the axial view, hyperintensity is evident in the A1 and A2 muscle areas, suggesting a deltoid injury. (B) In the coronal view, a complete tear of the supraspinatus tendon with retraction, classified as Patte stage 3 (arrow), is observed, along with an associated injury to the A2 muscle and tendon (empty arrow).

Figure 11. Images Revealing Hyperintense T2 Signal and Slight Disruption of Muscle Fibers in the A2 Muscle
(A) Oblique coronal view (arrow) and (B) axial fat-suppressed T2-weighted view (empty arrow). (C) Oblique sagittal fat-suppressed T2-weighted
image displaying an articular-sided partial-thickness tear of the supraspinatus tendon (arrowhead).

Figure 12. Images Revealing a Hyperintense T2 Signal in the P2 Muscle Without any Apparent Fiber Disruption
(A) Axial view (arrow) and (B) oblique sagittal fat-suppressed T2-weighted view (empty arrow) of the P2 muscle. (C) Oblique sagittal fat-suppressed T2-weighted image displaying an intermediate T2 signal in the supraspinatus tendon, indicative of chronic tendinopathy (arrowhead).

Deltoid tears are relatively uncommon in routine clinical practice. For instance, Ilaslan et al. [4] documented 24 instances of deltoid tears out of 8,562 shoulder MRI studies, resulting in a prevalence of 0.3%. However, when there is a concurrent rotator cuff issue, the occurrence of deltoid tears increases to 7%. Moreover, in cases of large or massive full-thickness rotator cuff tears, this incidence escalates significantly to 16.7% and 42.3%, respectively [5]. Alenabi et al. [6] highlighted distinct behaviors among the anterior, middle, and posterior deltoid muscles across various planes of shoulder elevation. 

Ultrasound imaging is a highly accessible, well-tolerated, swift, and cost-effective alternative in comparison to MRI for examining the musculoskeletal injury. Ultrasound imaging offers real-time dynamic assessments to allow practitioners to track changes in imaging observations pre- and post-isometric contractions, providing valuable insights into the dynamic characteristics of the condition. Chang et al. [7] comprehensively outlined how ultrasound examinations effectively illustrate muscle, tendon, and muscle-tendon junction injuries, along with their grading.

The diverse grades and lesions identified in ultrasound imaging may warrant distinct medical recommendations. Minor or lower-grade tears may be treated through oral medications and physical therapy. Interventions for Grade III tears or complete ruptures could range from hematoma aspiration with bandaging to even platelet rich plasma injections, and surgical procedures may be necessary.

Concerning the rehabilitation protocol for deltoid muscle injuries, physicians may consider referring to a protocol akin to that used for post-arthroscopic rotator cuff repair. In a study conducted by Chang et al. [8], involving an examination of 6 randomized controlled trials, it was suggested that early engagement in range of motion exercises could expedite the recovery of individuals following arthroscopic rotator cuff repair. However, it is important to acknowledge that this approach might potentially compromise tendon healing, especially in cases involving large-sized tears in the shoulders [8]. In a separate study by Edwards et al. [9], a variety of exercises were recommended for individuals with non-surgically managed rotator cuff tears. These exercises were tailored to specifically target the deltoid muscles and played a significant role in the comprehensive active rehabilitation process [9].

Therefore, for patients that have rotator cuff tears or that have incurred deltoid muscle injuries, such as individuals with AC joint injuries or bodybuilders, complaints of shoulder pain at specific angles of elevation should be addressed by a thorough assessment of the deltoid muscle. This approach can help prevent misdiagnosis of deltoid injuries and ensure accurate treatment.

This study has several limitations, including a relatively limited number of cases and the retrospective nature of data collection. Additionally, we focused on describing the origin tendons of the deltoid muscles, omitting any mention of the end tendons. Furthermore, a detailed functional characterization for the various deltoid functional units is absent. We anticipate that future research will incorporate surface electromyography to access the precise force outputs of each functional unit across varying planes.

In conclusion, deltoid injuries are uncommon in the general population, although the frequency of these injuries has noticeably increased in people with rotator cuff injuries. Most rehabilitation and strength training exercises used during the recovery process for rotator cuff injuries can be tailored for deltoid muscle injury. When patients have persistent shoulder pain with or without prior rotator cuff injury, a special focus on the imaging/function assessment of individual deltoid muscle units will be necessary.

The study was made possible by (1) the research funding (11202 and 112-BH002) of National Taiwan University Hospital and its Bei-Hu Branch, Taipei, Taiwan; (2) Ministry of Science and Technology (MOST 106-2314-B-002-180-MY3,109-2314-B-002-114-MY3, 109-2314-B-002-127, 110-2314-B-002-069 and 111-2314-B-002-161) and (3) Taiwan Society of Ultrasound in Medicine.

The authors of this manuscript declare no relationships with any companies, whose products or services may be related to the subject matter of the article.