http://casereports.bmj.com/content/2018/bcr-2018-226768.full.pdf?sid=9a86bd4a-40be-4ae7-93bf-9da59f9c560a

Doctors at Pondicherry Institute of Medical Sciences have published an odd case of a woman from India, who can rotate her arm 180 degrees. The strange case has been published in the prestigious British Medical Journal.
According to the report, a 37 year-old-woman, whose name is anonymous, sought the help of doctors after feeling difficulty in using her left arm and pain in the shoulder for three months.
After close examinations, the doctors have found that her shoulder is soft. The medics also noticed that she can rotate her arm 180 degrees, whereas the normal rotation is 90 degrees. Again, a close look at the x-rays of the hand revealed the absence of the humeral head. Explaining the condition further, Dr Pasupathy Palaniappan wrote that her bones were reduced to a small sliver. To get a clear idea regarding her condition, doctors conducted several blood tests and imaging techniques to test the infections, hormonal or metabolic problems. But everything was normal. The doctors then checked for the nerve problems in the affected area, but she had no symptoms of neuropathy either.
The absence of nerve symptoms and the presence of pain lead to the suspicion of the Gorham-Stout syndrome(GSS), which is characterised by the disappearing of bones. But the lady refused to undergo corrective surgery since her condition created a little discomfort. Hence, she was given some shoulder strengthening exercises.
GSS is a very rare disease, with only 70 reported cases so far. People with this condition experience an abnormal growth of blood vessels, which can gradually damage the bone and break it. The Gorham-stout disease most commonly affects the ribs, spine, pelvis, skull, collarbone and jaw. The symptoms vary from person to person. People experience rapid pain and swelling in the area or a dull ache and weakness that gets worse over time. Since the symptoms appear to be different, the treatment also differs according to the symptoms.

Source: 1. Suranigi SM, Kerketta AH, Palaniappan P, et al. BMJ Case Rep Published Online First: August 2018. doi:10.1136/bcr-2018- 226768

2. https://www.thesun.co.uk/news/7136872/woman-rotate-shoulder-180-degrees/
3. http://www.dailymail.co.uk/health/article-6117929/Woman-37-rotate-shoulder-180-degrees-bones-joint-VANISHED.html