'Frozen shoulder' is a common condition in which the shoulder is painful and stiff, decreasing range of motion in the joint. It happens when when the tissue around the shoulder joint becomes inflamed, perhaps because of injury, overuse or immobilisation.
Treatment often includes pain relief and gentle movement, leading to shoulder exercises once the joint is less painful. Steroid injections are sometimes recommended to bring down the swelling.
But now a new discovery could lead to a more effective treatment.
It comes after orthopaedic surgeon Edward Rodriguez, from Beth Israel Deaconess Medical Center (BIDMC) in Boston, noticed that some of his patients experienced lasting relief from their frozen shoulders after becoming pregnant.
To find out whether biochemical changes related to pregnancy could be responsible for melting away a frozen shoulder, he approached Boston University scientist Mark Grinstaff and his research group.
As Boston University reports, the team's interest was piqued by a natural hormone called 'relaxin', which helps a woman's body adjust to pregnancy and prepare for childbirth.
Relaxin is present in all men and women at a low level, but when a woman becomes pregnant her body begins producing much more of the hormone.
The researchers conducted a study in rats with stiff shoulder joints, which showed that injections of relaxin directly to the shoulder joint could restore the shoulder's full range of motion.
The findings have been published in the journal Proceedings of the National Academies of Science.
Known medically as arthrofibrosis, frozen joints affect more than 5% of the general population. The condition is particularly common among people who have undergone ACL reconstruction surgery, one-third of whom will go on to develop arthrofibrosis in their affected knee.
"While more research is needed, repurposing this pregnancy hormone as a treatment for arthrofibrosis could provide an unprecedented opportunity," said Ara Nazarian from BIDMC, one of the study's co-corresponding authors.