Muscles that work with the deltoid include the supraspinatus, infraspinatus, teres minor and the subscapularis. These muscles help initial abduction from 0 to 15 degrees, but also provide stabilization to the glenohumeral joint when the deltoid works to abduct past 15 degrees.

Although rare, anatomical variants of the deltoid have been described in the literature. Several case reports have reported the posterior part of the deltoid having separate facial sheaths.[15] One case report described a complete separation of the posterior deltoid fibers with the rest of the muscle.[16] It is important to acknowledge these variants as they may generate confusion during posterior deltoid flap procedures. The surgeon may confuse this variant with the teres minor because of the location and distinct fascia.

Other noted anatomical variations include abnormal insertion into the medial epicondyle of the humerus. Here, the fibers inserted in a way that passed superficially to the brachial artery, ulnar nerve, and median nerve. Therefore, surgeons should be mindful if this variation appears and appreciate the neurovascular proximities. The literature has also reported aberrant straps of the deltoid. These straps traveled perpendicular to the posterior deltoid fibers. Surgeons should likewise know aberrant muscles as they can confuse them with other muscle fibers.

Variations of the thoracoacromial artery are also reported in the literature. As previously described, the thoracoacromial artery supplies the deltoid muscle as it runs within the deltopectoral groove with the cephalic vein. Two types of variations have been noted. In type I, it crosses the interval and tunnels into the deltoid muscle. In type II, it crosses the interval and runs with the cephalic vein. However, it will then cross back and toward the pectoralis major.[17]

The deltoid is a significant factor when considering the anterior surgical approach to gain access to the shoulder joint.  Some of these technical procedures include, but are not limited to the following:

• Open Bankart repair/capsular reconstructions
  • indicated in the setting of recurrent anterior (or other directional) instability of the shoulder
• Shoulder arthroplasty
  • indicated for cases of post-traumatic deformity, advanced degenerative arthritis, and/or avascular necrosis
  • includes hemiarthroplasty, total shoulder arthroplasty (TSA), reverse total shoulder arthroplasty (rTSA)
• Long head of the biceps tendon (LHBT) repair versus tenotomy versus tenodesis procedures
  • indicated in the setting of either LHBT/bicipital groove instability or advanced/end-stage LHBT tendinopathy and degeneration[18][19]
• Rotator cuff repair
  • contemporary indications remain somewhat controversial although most of these procedures are now being performed arthroscopically 
  • popular approaches (as opposed to the deltopectoral approach) include the mini-open approach (lateral deltoid-splitting approach)

Deltopectoral (DP) approach

When marking out the anatomic landmarks for the DP approach, the coracoid process is marked on the skin to plan out the trajectory for the surgical incision. From there, an incision is made which follows over the deltopectoral groove. The deltoid and pectoralis major muscle fibers are appreciated, and this most often includes direct visualization of a "fat stripe" which includes the cephalic vein in the center of the incision/approach. 

The deltoid is retracted laterally while the pectoralis major muscle is retracted medially. The cephalic vein is retracted either laterally or medially (depending on surgeon preference).[20] 

Other approaches

Other surgical approaches that involve the deltoid are the anterolateral and direct lateral approach to the shoulder.  A modified anterolateral approach can be preferred over the more anterior approach for humeral fractures because this may facilitate access to the specific fracture fragments.[21]

The anterolateral approach to the acromioclavicular joint and subacromial space is used primarily for repair of the rotator cuff and anterior decompression of the shoulder joint. This incision uses the coracoid process and the acromion as landmarks. After the acromioclavicular joint is exposed, retraction of the deltoid is required. If the surgery requires repairs of the rotator cuff, splitting of the deltoid muscle is required. According to a 2018 study, the anterolateral surgical is especially useful when exposing the posterior aspect of the shoulder.[22]

The lateral approach also requires the splitting of the deltoid. It involves a 5 cm longitudinal incision from the acromion to the lateral aspect of the arm. Division of the deltoid is necessary, which includes splitting it downwards for 5 cm. Like the anterolateral approach, it requires a suture at the apex of the incision for inadvertent splitting and axillary nerve damage. Minimally invasive approaches to the shoulder also involve splitting of the deltoid. The lateral minimally invasive technique requires making two incisions of the deltoid. One will be proximal, and one will be distal with the axillary nerve running in between the two incisions. The minimally invasive anterolateral part involves pressing a guide wire through the substance of the deltoid. Radiographic imaging will confirm the placement, which subsequently leads to splitting of the deltoid. 

Complications

Complications relating to these surgical approaches include deltoid injuries. One of these complications involves detachment of the deltoid from the clavicle. Reattachment of the deltoid would require full thickness sutures and transosseous sutures along with 4-6 weeks of complete healing. The deltoid is also at risk for being denervated. The axillary nerve runs under the deltoid muscle and travels posterior to anterior. It is at risk if the anterior deltoid is vigorously retracted laterally or if retractor placement is under the deltoid. Denervation can cause loss of arm abduction past 15 degrees and loss of sensory over the deltoid. Incorrect retraction of the deltoid also might also lead cephalic vein rupture which will lead to upper limb edema.

The deltoid's function is testable in the clinical setting. It is done by manually raising the patient’s arm 15 degrees. After this, ask the patient to abduct the arm against resistance. In patients who have normal deltoid function, the arm will contract, and the lateral deltoid contraction will be appreciated via palpation. In patients who have a dysfunctional deltoid, these clinical findings will not be present. A deltoid dysfunctional problem commonly arises due to axillary nerve palsy. Common etiologies of this condition include overuse of a crutch, surgery, or posterior shoulder dislocation from severe trauma.

The inability to abduct the arm does not specifically indicate that there is a dysfunction with the deltoid. The inability to abduct the arm presents during proximal neuromuscular disorders. Neurological problems include Lambert Eaton disease.[23]  Muscular inflammatory processes polymyositis/dermatomyositis,[24] polymyalgia rheumatica (stiffness rather than weakness), and as a side effect of aluminum hydroxide containing vaccines.[25] These conditions could all present with proximal muscle weakness or with the inability to abduct the arm. Other differential diagnoses for patients with deltoid weakness include cachexia from chronic disease or malnourishment. All clinicians keep these differentials in mind upon finding the patient's inability to abduct the upper limb.