Inferior shoulder dislocation (luxatio erecta) can occur from an axial load to an outstretched arm. Imagine a swimmer hitting the side of a pool with his arm in front. Also this can occur from forced hyperabduction (grabbing something while falling). Clinically, you will notice the patient with their arm over their head and the elbow bent (fixed abduction). You may feel the humeral head in the axilla/lateral chest wall.
X-rays will show the humeral head inferior to the glenoid.
To reduce these dislocations, after procedural sedation, apply traction upwards and laterally (axis of the humerus), with counter-traction. Then, put on a splint and all patients get ortho follow-up. Do not attempt reduction if there is accompanying humeral fracture or vascular injury. A great reduction video by my former attending Dr.Mellick.
Although inferior dislocations are rare (<1%), they have the greatest incidence of neurovascular injury. Axially nerve palsy is frequently present but resolves after reduction. If vascular compromise is suspected, emergent ortho and vascular consultation is indicated. Other complications include inferior capsule tear and rotator cuff disruption.
Sources
Ortho Bullets; Luxatio Erecta
WikiEM: Inferior Shoulder Dislocation
Medscape: Shoulder Dislocation
Radiopaedia: Inferior shoulder dislocation
Rosh Review