Life Science: Healing broken bones

To fix a broken bone, doctors often harvest another bone from the patient’s  body or  from someone else.  It is not always easy to find the right bone, so scientists have been exploring synthetic material — such as calcium phosphate and titanium — for bones.

Now, a team led by Prof Mukhesh Doble of the biotechnology department of IIT-Madras has come up with a new material — magnesium alloy.

While small fractures can be treated with plates and rods, ‘segmental defects’ or fractures longer than 5 cm in long bones (such as the ones in the forearm, leg or thigh that are about a foot long) take a long time to heal and need some sort of support. A titanium mesh cage is placed in the gap, which stabilises the bone and helps it heal. But titanium does not degrade — it remains in the body permanently, causing other problems.

Magnesium’s capability to heal bones has been known, but the problem is the opposite of titanium — it degrades. Prof Doble’s team has tackled this by coating magnesium with slow degrading polymers called ‘polycaprolactone’ and ‘hydroxyapatite’, using a coating technique known as electrospinning.

“Hydroxyapatite is a ceramic material naturally present at the bone extracellular matrix. It is the same as the bone material, so it doesn’t cause any toxicity and integrates with the bone,” says Prof Doble.

The team used AZ31, an alloy of magnesium, for the magnesium mesh cage implants. They then coated the implants with polycaprolactone and nano hydroxyapatite.

This nano-coated magnesium mesh was then used to heal bone defects in the femur (thigh bone) of rabbits. The researchers found that the  femur  showed bone formation and also bridged the defect. This was possible due to the biocompatibility  of polycaprolactone and nano-hydroxyapatite, which ensured  good recovery without  adverse reactions such as fibrosis.

“Current materials like titanium do not degrade, so they remain in the body forever,” Prof Doble tells Quantum. “Also they have a much higher strength than  bone, which is not desired. It leads to something called ‘stress shielding’. Such strong material weakens the bone,” he explains.

Magnesium, on the other hand, gets absorbed. It is part of the body, so  does not create toxicity. With  a mechanical strength close to  bone,  it has low stress shielding, he adds.

The nano coating provides ‘osteo-conductive aligned structures’ —  on which bone can grow. This serves as a pathway for bone cells that divide and ultimately form bones to travel to the defect area and generate new bone.

This  new way for making implants leads to better in-vivo bone formation.

“This work opens the way to the generation of clinically useful biodegradable bone implants with improved bone formation ability due to biomimetic strategies introduced in the implant design,” writes Dr Ander Abarrategi, an expert in the  field and Principal Investigator at the Spain-based CICbiomaGUNE Research Institute,  in IIT-M Tech Talk.

Prof Doble says  there is now a need to increase the number of tests on rabbits and  larger animals. In-vivo studies also require  large funds, which  academic institutions cannot muster easily.