Spongy material helps repair the spine

Scientists have developed biodegradable polymer grafts that, when surgically placed in damaged vertebrae, can grow to be just the right size and shape to fix the spinal column.
"The overall goal of this research is to find ways to treat people with metastatic spinal tumours," said Lichun Lu from the Mayo Clinic in US.
"The spine is the most common site of skeletal metastases in cancer patients, but unlike current treatments, our approach is less invasive and is inexpensive," said Lu.
Often, removing extensive spinal tumours requires taking out the entire bone segment and adjacent intervertebral discs from the affected area.

In this case, something must fill the large void to maintain the integrity of the spine and protect the spinal cord.
There are typically two surgical choices in cases of extensive spinal metastases. In the more aggressive and invasive option, the surgeon opens the chest cavity from the front of the patient, which provides enough room to insert metal cages or bone grafts to replace the missing fragment.

The other approach is less invasive, requiring just a small cut in the back or posterior, but only offers enough space for the surgeon to insert short expandable titanium rods, which are costly.

To develop a less expensive graft compatible with the posterior spinal surgery option, Lu and her postdoctoral fellow, Xifeng Liu, sought a material that could be dehydrated down to a size compatible with posterior spinal surgery, and then, once implanted, absorb fluids from the body, expanding to replace the missing vertebrae.
Researchers started by cross-linking oligo[poly(ethylene glycol) fumarate] to create a hollow hydrophilic cage - the scaffold of the graft - which could then be filled with stabilising materials, as well as therapeutics.
"When we designed this expandable tube, we wanted to be able to control the size of the graft so it would fit into the exact space left behind after removing the tumour," Lu said.

The researchers also needed to control the kinetics of the expansion, because if the cage expands too quickly, a surgeon may not have enough time to position it correctly, while a slow expansion could mean a longer-than-necessary surgery.
"By modulating the molecular weight and charge of the polymer, we are able to tune the material's properties," Liu said.
Researchers studied the effects of these chemical changes by observing the polymer grafts' expansion rates under conditions that mimic the spinal column environment in the lab.
This information is key for determining the optimal size of a spinal implant for use in restorative surgery. The team identified a combination of materials that are biocompatible in animals and that they believe will work in humans.