Lower back pain is an agonising problem that affects millions of people all over the world.
Now, scientist believe the debilitating pain may be caused by cartilage in the spine turning into bone that looks like Swiss cheese.
Cartilage should be soft and firm, protecting the vertebrae – the 33 pieces of bone that make up the spine.
But tests on mice showed their tissue turned in to a hardened structure with holes as the rodents aged.
The porous gaps allow pain-sensing nerves to grow and spread deep into the spinal bones, in turn causing agony.
Johns Hopkins University experts hope the discovery could pave the way for better treatment, with patients currently reliant on standard painkillers.
An estimated 80 per cent of people worldwide will experience low back pain in their lifetime, sometimes owing to strain or injury, such as a slipped disc.
But the vast majority of back pain emerges without an injury, especially in older age when it has worn down.
Researchers experimented on mice genetically engineered to be at least 20 months old – the equivalent of 70-80 in humans.
They looked at layers of cartilage between each vertebrae, called endplates, which cushion the bones and protect them from the weight of the body.
Dr Xu Cao, lead researcher, said: ‘The cartilage endplate is the cushion on a seat that makes it more comfortable.
‘But, like similar tissue in knee and hip joints, it succumbs to wear and tear over time.’
He added: ‘When cartilage becomes a porous bony structure with growth of nerve fibers, it could be the source of back pain.’
The researchers found that the soft tissue in the mice’s spines became hardened and resembled diffuse bone. It had a Swiss cheese-like structure.
Dr Cao and his team suspected that the holey structure would provide fertile ground for abnormal nerve growth
This extension of pain sensing nerves deep into the spinal bones could be the cause of agony, the researchers suggested.
The team have previously shown that a cell type, called osteoclasts, create the porous bone structure where the cartilage should be.
Osteoclasts are specilised cells which remove old bone and replace it with new bone. But in old age, the process is believed to be imbalanced, causing conditions such as osteoporosis.
The team wanted to explore the idea that osteoclasts may also invite nerve cells to grow by analysing tissue samples from the spines of old mice under the microscope.
They labelled all the hole-building osteoclasts as well as nerve fibers with fluorescent tags, according to the findings published in Nature Communications.
Osteoclasts and nerve fibers typically sprung up in the same areas, suggesting that the cells could be triggering the abnormal nerve growth.
Dr Cao and colleagues believed it may be a signalling molecule, netrin-1 – secreted by osteoclasts, at the root of the problem.
The team investigated whether inhibiting osteoclasts from forming would prevent the whole process.
Mice genetically engineered to lack the gene that codes for osteoclast formation did not have Swiss cheese-like endplates.
Separate tests also showed they had fewer pain-sensing nerves in their endplates, compared to mice with the gene.
Dr Cao said the findings are an important lead in understanding how unexplained low back pain develops.
His team next plans to conduct laboratory experiments using compounds that slow the abnormal bone growth to test their potential to treat low back pain.
Some sufferers of back pain may never be given a reason for why their condition flares up because the cause cannot always be identified.
Painkillers, hot or cold compression packs, swimming and physical therapy is the standard advice given by the NHS.
Lower back pain is the leading cause of work absence throughout much of the world, according to the World Health Organization.
More than 100million work days are lost in the UK per year because employees are in too much pain to work, and 150million in the US.