The anterior and posterior branches of the PHCA supply the shoulder muscles, which include the deltoid muscle, the teres major, and teres minor muscles.[6][7] 

Anatomical variants of the PHCA are very rare, but there are reports in some studies. Multiple case reports have documented the unusual origin of the PHCA coming off the subscapular artery as either a branch of the first or the second part of the axillary artery compared to the normal anatomy in which it branches off of the third part of the axillary artery with the subscapular trunk as an individual branch.[14][15][16] Another study describes a case where the PHCA arises from the third part of the axillary artery, but as a branch of the deep brachial artery rather than its normal variant of being a direct branch itself.[17] Interestingly, most of these studies have identified these anomalies as phenomena that occur unilaterally.

Aside from variations in its origin, other studies have reported variations in its course and the branches that come off the PHCA. One study reported a case in which the PHCA, accompanied by the axillary nerve and its associated vein, passes through the lower triangular space to reach the scapular region rather than its normal descent through the quadrangular space. The same study also reported an unusual origin of the radial collateral artery arising from the PCHA that can mimic symptoms of quadrangular space syndrome.[18]  Another study discussed the variations in the number of terminal branches that come off the PHCA. The study revealed that 92% of patients, out of a total of 92 patients, had a terminal branch that crossed the space between the deltoid and the proximal humerus. The majority of patients were found to have a single vessel branch variant (75%), followed by double vessel variants (16%), and last, triple vessel variants. This finding has important implications during a deltopectoral surgical approach to the shoulder as these vessels are vulnerable to tearing and can cause persistent bleeding leading to prolonged surgery and postoperative hematoma and infection.[19] 

Quadrilateral space syndrome (QSS)

QSS occurs secondary to compression or mechanical injury to the axillary nerve and/or PHCA.  Involvement of the PHCA results in vascular QSS, which most often occurs secondary to repetitive mechanical trauma in a predisposed or already compromised tight quadrangular space as the PHCA wraps around the humeral neck during abduction and external rotation shoulder movements.  Patients develop PHCA thrombosis and/or aneurysm with distal embolization and digital ischemia[20].  The vascular form of QSS is treated with PHCA ligation to prevent distal embolization.  Acute thrombotic embolization management is with thrombolytic modalities.  

Proximal humerus fractures

In the case of managing proximal humeral fractures, one study identified anatomic landmarks for establishing quick access to the PHCA and AHCA as an effort to aid protection of the arteries during surgical fixation. The same study reported that the mean distances from the origin of PHCA arising from the third part of the axillary artery to the infraglenoid tubercle, the coracoid, the acromion, and the midclavicular line were 27.7 mm, 50.2 mm, 68.4 mm and 75.8 mm, respectively. Similarly, the mean distances from the origin of ACHA to the same landmarks were 26.9 mm, 49.2 mm, 67.0 mm and 74.9 mm.[21]

Injuries to the PHCA are uncommon but may occur as a result of compression or disruption secondary to trauma.

• Vascular quadrangular space syndrome (vQSS) is an underdiagnosed cause of ischemia in the upper extremities of overhead throwing athletes less than 40 years old who are otherwise healthy. The mechanism of vQSS is still unclear but is thought to result from impingement of the neurovascular structures within the quadrangular space due to trauma, fibrous bands, or hypertrophy of a muscular border. Other rare causes include labral cysts, hematomas secondary to fracture, osteochondroma, lipomas, axillary schwannomas, the existence of anatomic variations and accessory muscles, and as a rare complication of thoracic surgery. Axillary nerve compression or repeated arm abduction and external rotation from overhead activities (i.e., swimming, baseball, volleyball) may also cause distraction injury of the PHCA as it courses through the quadrangular space causing dissection and aneurysm formation. Embolism may occur as a thrombus from this arterial injury travels down the arm and results in the symptomatic presentation of ischemia which includes pain, pallor, paresthesias, diminished or absent distal pulses, cyanosis, and coolness of the digits and hand. Later stages of presentation may include ischemic ulceration and gangrene. Early recognition and awareness of this syndrome among coaches and athletic trainers are essential to optimal treatment. Initial therapy involves physical therapy, physical activity modification, NSAID use, and maybe even perineural steroid injections. Surgical decompression is recommended when patients are unresponsive to conservative measures for at least 6 months and depends on the extent of damage to the PHCA. Patients with a PHCA aneurysm undergo surgical resection. Patients with a thrombus undergo PHCA ligation and division with or without thrombolysis. Last, patients who have a thrombus in their PHCA and digital emboli usually undergo thromboembolectomy. When treated early and promptly, athletes may return to baseline.[2][22][23]
• The PHCA may also suffer disruption in the case of a proximal humeral fracture and dislocation, which can lead to a life-threatening hemorrhagic complication during surgical fixation. Obtaining earlier angiographic studies is imperative in the course of management and treatment of this condition.[24]