"A hormone most commonly produced during pregnancy could be used to treat a painful joint condition according to new research," the Mail Online reports.

The research involved rats who'd undergone surgery to replicate what's commonly called frozen shoulder in humans.

This is where scar tissue builds up around the ball and socket joint, causing pain and restricted range of movement.

The condition is not usually serious, but can persist for months, or even years, and impact quality of life.

The rats received injections of the naturally occurring human hormone relaxin into the shoulder.

Relaxin is released by the ovaries and placenta, helping the ligaments relax and preparing the body for birth during pregnancy.

The researchers found multiple injections restored range of movement and reduced scar tissue in the rats' limb joints, and wonder whether this could be a potential new treatment for frozen shoulder.

But to date there's been very little research into using relaxin injections in humans.

The findings seem encouraging, but we're a long way off knowing whether this will ever translate into an effective, and safe, treatment for frozen shoulder in humans.

Frozen shoulder is usually treated through a combination of physiotherapy and painkilling medicines.

Where did the story come from?

This study was conducted by researchers from Boston University and Harvard Medical School in the US and Yerevan State Medical University in Armenia.

Financial support was provided by Mr and Mrs Tom and Phyllis Froeschle and the Boston University Ignition Award.

The article was published in the peer-reviewed journal PNAS.

Mail Online's coverage was accurate and made it clear that the study was conducted in rats and cells in a laboratory, not humans.

What kind of research was this?

This was laboratory research involving the study of human cells and an animal model to explore a potential treatment for frozen shoulder.

The researchers use this term to refer to 2 slightly different medical diagnoses: adhesive capsulitis and arthrofibrosis.

Adhesive capsulitis is the classic frozen shoulder where scar tissue has built up in the connective tissue capsule that surrounds the shoulder joint. It's often not clear why this happens.

Arthrofibrosis can affect various joints like the shoulders, wrists or knees. It's where scar tissue has built up in the joint, but usually follows trauma or injury.

It's not classically given the term frozen shoulder, but essentially causes the same symptoms of pain and restricted range of movement when it affects the shoulder.

What did the research involve?

The research involved 20 rats who'd had a surgical procedure to immobilise the shoulder joint and cause the production of scar tissue.

They were then put into 4 groups of 5 assigned to have:

no treatment

a single injection of relaxin into the shoulder joint

multiple injections of relaxin into the shoulder joint

multiple injections of relaxin into the bloodstream (intravenous)

Over the following 8 weeks, the effects on range of movement were tested using a special apparatus, which did not cause any injury to the animal.

The shoulder joints were later examined after the animals died.

In another part of the study, the researchers also looked at the effect of adding different doses of relaxin to human cells taken from the lining of the shoulder joint.

What were the basic results?

There was no change in the range of limb movement throughout the follow-up period for either the untreated rats or those that received the intravenous relaxin injections.

There was a temporary improvement in the rats who received the single relaxin injections into the joint, but this returned to restricted movement by 2 weeks.

But rats given multiple joint injections had a significant improvement in their range of movement and actually returned to the mobility level they'd had prior to surgery.

In the human cells, they also found relaxin given at higher doses reduced production of the connective tissue protein collagen. Overproduction of collagen can lead to frozen shoulder.

How did the researchers interpret the results?

The researchers concluded that multiple joint injections of human relaxin restore range of movement and eliminate fibrosis of the shoulder joint capsules.

They suggested these encouraging results support the further development and clinical study of this as a new therapy for treating frozen shoulder, specifically referring to arthrofibrosis in this case.

Conclusion

There are various treatments available to help frozen shoulder-like conditions, such as anti-inflammatories, physiotherapy and occasionally steroid injections, and most cases will get better with time.

The research shows that joint injections of relaxin at high or repeated doses may prevent the build-up of scar tissue in the joint.

But it's currently not certain whether this could lead to a safe and effective new treatment for humans.

The study involved rats who'd had a surgical procedure to restrict their range of limb movement.

This is not necessarily the same as a person who's developed frozen shoulder after an injury or for an unknown reason.

And the study involved a small sample of rats: only 5 rats in each treatment group.

We do not know whether repeated relaxin injections into the joint may cause any unpleasant side effects in rats, much less in people.

Even well-established treatments for arthritic conditions, like steroid injections, can cause side effects.

Just because this is a naturally occurring reproductive hormone does not mean we know for sure that it would be safe to repeatedly inject high doses into the joint.

If you think you may have a frozen shoulder or any other frozen joint, see a GP. They can refer you to a physiotherapist if needed.

Analysis by Bazian

Edited by NHS Website