Because of its ball and socket formation, the shoulder joint is the most mobile of any joint in the body. That mobility also makes it the most unstable joint as well. The online orthopedic journal Maitrise Orthopédique explains that the bony incongruence of the joint—the failure of the shallow socket to fully cover the ball of the upper arm bone—forces the shoulder to depend on soft tissue to hold it in place. When the humerus, or upper arm, comes partially out of the shoulder socket beyond normal joint mobility, it's called a subluxation.
Popping and Looseness
When a shoulder subluxes, the patient usually feels a popping as the ball joint pushes out of the socket then bursts back in. Sometimes the popping is audible. "Ramamurti's Orthopaedics in Primary Care" indicates that patients with subluxation may say that their shoulder feels like it's slipping or catching, or that their arm goes dead from time to time. Also, many patients express a feeling of looseness in their shoulder. According to the Sanders Clinic, patients may be especially reluctant to raise their arms overhead for fear that the shoulder will pop out.
Pain, Numbness and Weakness
When shoulder subluxation occurs, pain is centered directly in the shoulder joint. The motions that trigger the pain are partially dependent on the type of subluxation. The American Academy of Orthopaedic Surgeons states that the majority of shoulder subluxations and dislocations are anterior, in which the shoulder slips forward. Throwing motions may exacerbate pain with an anterior subluxation. Pain leads to weakness, because the patient avoids uncomfortable motions. The shoulder itself will often feel numb, and numbness and tingling may travel down the arm.
According to the American Academy of Orthopaedic Surgeons, visual signs often accompany shoulder subluxations. Both bruising and swelling are common over the spot where the injury occurred. When compared to the healthy shoulder, the subluxed shoulder often appears less rounded. The sulcus test, which checks for shoulder instability, would likely show a positive result—dimpling below the bony protuberance, or protrusion, at the top of the shoulder, known as the acromium.